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PREPARATION OF HIGHER ALPHA OLEFIN OLIGOMERS
by
CHANTELLE CRAUSE
Dissertation
submitted in fulfilment of the requirements
for the degree
MASTER OF SCIENCE
in
CHEMISTRY
in the
FACULTY OF SCIENCE
at the
RAND AFRIKAANS UNIVERSITY
Supervisor: PROF. F.R. VAN HEERDEN
DECEMBER 1996
TABLE OF CONTENTS
Synopsis iv
Ops omming vi
List of Schemes viii
List of Figures
List of Tables r xi
Abbreviations xii
Chapter 1
The oligomerization of higher alpha olefins : a literature study
1.1 Alpha olefin beneficiation: the key to value-adding and economic growth 1
1.1.1 The South African perspective 1
1.1.2 Focus on alpha olefins 2
1.1.3 Alpha olefin oligomers as base stocks for synthetic lubricants 3
1.1.4 Use and functionalization of poly(alpha olefins) 4
1.2 General principles of oligomerization 5
1.3 Olefin oligomerization catalysts 6
1.3.1 Friedel-Crafts catalysts 6
Polyalphaolefins 6
Cationic acid catalysed oligomerization 7
BF3-catalysed oligomerization 8
Mechanistic considerations 10
Correlation of paraffin structure with physical
properties 16
Reaction variables 18
1.3.2 Free radical catalysts 19
1.3.3 Transition metal catalysts 20
Heterogeneous Ziegler-Natta catalysts 23
Homogeneous Ziegler-Natta catalysts: Metallocenes 26
(a) Nature and formation of the active species 27
11
1.4 Conclusion
Kinetics and mechanism of alpha olefin
polymerization 29
Stereoregular polymers from chiral metallocene
catalysts 32
Reaction control and product distribution 35
Future developments 36
(iii) Other important transition metal catalysts 36
Supported reduced transition metal oxide catalysts 36
Homogeneous nickel catalysts 37
38
Chapter 2
Discussion
2.1 Aim of research 39
2.2 Synthesis of model oligomers 40
2.2.1 Synthetic target and strategy 41
2.2.2 Synthesis of 1,2-epoxypentane 42
(i) Direct epoxidation 43
Magnesium monoperphthalate 44
Urea hydrogen peroxide 45
Dimethyldioxirane 45
(ii) Indirect epoxidation 46
Iodohydrin 47
Bromohydrin 48
1,2-Epoxypentane 49
2.2.3 Synthesis of the dimer analogue 50
2.2.4 Synthesis of the trimer analogue 57
(i) Bromination of the C 10 alcohol 57
(ii) Synthesis of the C15 alcohol 59
Formation of Grignard reagent 59
Carbon-carbon bond formation 61
(iii) Synthesis of a model trimer 62
2.2.5 Attempted synthesis of a model tetramer 63
111
2.3 Catalyst survey 65
2.3.1 Cationic oligomerization 65
2.3.2 Radical oligomerization 67
2.3.3 Metallocene catalysts 69
Metal 76
Metallocene trichloride 77
Substituted cyclopentadienyl rings 78
Temperature 79
Purity of the alpha olefin monomer 80
2.3.4 Comparison of different catalysts 81
Comparison of viscosity data 81
General comparison 81
2.4 Conclusion 82
Chapter 3
Experimental
3.1 General 84
3.2 Standard chemical methods 86
3.3 Preparation of model oligomer 88
3.4 Oligomerization reactions 95
References 102
Acknowledgments 109
SYNOPSIS
The economic growth of a country can be stimulated by value-adding to basic resources e.g.,
chemical intermediates. Alpha olefins are products of the Sasol Fischer-Tropsch process and
oligomerization is one of the most significant beneficiation processes available for these
alpha olefins.
Alpha olefin oligomers obtained from the cationic oligomerization with BF3/ROH, have
been used mainly as base stocks in synthetic lubricants. These oligomers consist of a
mixture of isomeric products that complicates the investigation and rational modification of
this and other closely related oligomerization processes. A comparison of these isomeric
mixtures to alkane standards could assist in the study of these processes, but almost no
standards are available. This project was aimed, in part, at establishing a methodology for
the synthesis of 1-pentene model oligomers. A model branched 1-pentene dimer and trimer
as well as a precursor to the analogous tetramer were prepared. The syntheses of other
model oligomers in conjunction with the comparison of the properties of model oligomers
with different branching patterns will certainly be of value.
Some of the main classes of catalysts were evaluated for the oligomerization of higher alpha
olefins and especially of 1-pentene. The BF3/n-BuOH cationic oligomerization of 1-pentene
yielded dimers, trimer, tetramers and pentamers consisting of a large number of isomers
and the product distribution peaked at the trimer. This is in agreement with previous
results for other monomers. Radical oligomerization of 1-pentene with organic peroXide
initiators proceeded in low yield. Dimers and trimers consisting of a mixture of isomers
were mainly formed.
Metallocene catalysts offer a convenient route to single component, structurally well
characterized alpha olefin oligomers. A variety of functional group transformations and
consequent applications are possible for these oligomers. The general nature of the
oligomerization of 1-pentene, 1-hexene and 1-octene with metallocene/methylaluminoxane
catalysts was investigated. The influence of different metals, the number of
cyclopentadienyl groups and substitution on the cyclopentadienyl groups in the
metallocene moiety, the reaction temperature and the purity of the monomer were studied.
By employing the appropriate metallocene in combination with methylaluminoxane, higher
V
alpha olefin dimers and trimers could be formed in high yield at ambient temperatures.
Cp2ZrC12 was the most active of the metallocene catalysts and single isomers were formed
with high selectivity.
The metallocene-based oligomerization of higher alpha olefins offers great scope for further
research and promises a new era in olefin oligomerization.
OPSOMMING
Die ekonomiese groei van 'n land kan bevorder word deur waarde-toevoeging tot basiese
hulpbronne, byvoorbeeld chemiese grondstowwe. Alfa-olefiene word in die Sasol Fischer-
Tropsch proses vervaardig en oligomerisasie is een van die belangrikste prosesse vir
waarde-toevoeging tot hierdie alfa-olefiene.
Alfa-olefienoligomere verkry deur die kationiese oligomerisasie met BF3/ROH, word
hoofsaaklik as basisolies vir sintetiese smeermiddels aangewend. Hierdie oligomere bevat
'n mengsel van isomeriese produkte wat die ondersoek en rasionele modifikasie van hierdie
en verwante oligomerisasieprosesse bemoeilik. Die vergelyking van hierdie isomeriese
mengsels met alkaanstandaarde kan waardevol wees in die studie van hierdie prosesse,
maar byna geen standaarde is beskikbaar nie. Hierdie projek was gedeeltelik daarop gemik
om 'n metodologie daar te stel vir die sintese van 1-penteen modeloligomere. 'n Vertakte
model 1-penteendimeer en - trimeer asook 'n voorloper tot die analoe tetrameer is berei. Die
sintese van ander modeloligomere, tesame met die vergelyking van die eienskappe van
modeloligomere met verskillende vertakkingspatrone, sal van verdere waarde wees.
Sommige van die hoofgroepe katalisatore is geevalueer in die oligomerisasie van hoer alfa-
olefiene en spesifiek vir 1-penteen. Dimere, trimere, tetramere en pentamere, bestaande uit
'n groot aantal isomere, is gevorm in die BF3/n-BuOH oligomerisasie van 1-penteen en die
produk verspreiding het gepiek by die trimeer. Dit is in ooreenstemming met resultate
verkry vir ander monomere. 'n Lae opbrengs is verkry in die radikaal oligomerisasie van 1-
penteen met organiese peroksiede as inisieerders. Hoofsaaklik dimere en trimere bestaande
uit 'n aantal isomere, is gevorm.
Metalloseenkatalisatore bied 'n eenvoudige metode om enkel-komponent, struktureel goed-
gekarakteriseerde alfa-olefien oligomere te berei. 'n Verskeidenheid funksionele groep
transformasies en gevolglike toepassings is moontlik vir hierdie oligomere. Die algemene
eienskappe van die oligomerisasiereaksies van 1-penteen, 1-hekseen en 1-okteen met
metalloseen/metielaluminoksaan katalisatore is ondersoek. Die invloed van verskillende
metale, die aantal siklopentadienielgroepe en substitusie op die siklopentadienielgroepe in
die metalloseeneenheid asook die reaksie temperatuur en suiwerheid van die monomeer is
bestudeer. Hoer alfa-olefiendimere en - trimere is gevorm by kamertemperatuur deur die
vii
gepaste metalloseen saam met metielaluminoksaan as katalisator to gebruik. Cp2ZrC12 was
die aktiefste metalloseenkatalisator en enkele isomere is gevorm met 'n hoe selektiwiteit.
Die metalloseengebaseerde oligomerisasie van hoer alfa-olefiene bied ruimte vir verdere
navorsing en lei 'n nuwe era in olefienoligomerisasie in.
vii'
LIST OF SCHEMES
Chapter 1
The oligomerization of higher alpha olefins : a literature study
Scheme 1.1. Kinetics of cationic chain growth oligomerization. 8
Scheme 1.2. Formation of catalyst complex. 9
Scheme 1.3. Conventional cationic mechanism. 11
Scheme 1.4. Shubkin's mechanism. 12
Scheme 1.5. Oligomerization of 1-butene and cis-2-butene. 15
Scheme 1.6. First step in oligomer formation. 16
Scheme 1.7. Free radical polymerization of vinyl monomers. 20
Scheme 1.8. Simplified hydride mechanism. 21
Scheme 1.9. Formation of active heterogeneous Ziegler-Natta catalyst. 24
Scheme 1.10. Monometallic Ziegler-Natta mechanism. 25
Scheme 1.11. Formation of active metallocene/ MAO species. 27
Scheme 1.12. Formation of active MAO-free catalysts. 29
Scheme 1.13. Basic steps in polymerization of alpha olefins using
metallocene-based catalysts. 30
Scheme 1.14. Illustration of agostic interactions. 31
Scheme 1.15. An example of stereocontrol in metallocene/ MAO polymerization. 34
Chapter 2
Discussion
Scheme 2.1. General route to tetraalkylmethanes. 41
Scheme 2.2. Retrosynthetic strategy. 43
Scheme 2.3. MMPP epoxidation of 1-pentene. 44
Scheme 2.4. UHP epoxidation of 1-pentene. 45
Scheme 2.5. Dimethyldioxirane epoxidation of 1-pentene. 46
Scheme 2.6. Formation of iodohydrin. 47
Scheme 2.7. Formation of bromohydrin. 48
Scheme 2.8. Illustration of the preparation of an epoxide from a halohydrin. 49
ix
Scheme 2.9. Preparation of 1,2-epoxypentane. 50
Scheme 2.10. Formation of C10 alcohol. 53
Scheme 2.11. Formation of two stereogenic centres. 56
Scheme 2.12. Bromination of 2-heptanol. 58
Scheme 2.13. Bromination of alcohol 44. 58
Scheme 2.14. Generation of Clo Grignard reagent. 59
Scheme 2.15. Preparation of C10 iodide. 60
Scheme 2.16. Formation of C15 alcohol. 61
Scheme 2.17. Preparation of model trimer. 63
Scheme 2.18. Decomposition of a secondary organic peroxide. 69
Scheme 2.19. Formation of methylated oligomers. 73
Scheme 2.20. Formation of normal dimers and trimers. 74
LIST OF FIGURES
Chapter 1
The oligomerization of higher alpha olefins: a literature study
Figure 1.1. Possible functionalization and application of single
component alpha olefin oligomers. 5
Figure 1.2. Gas chromatogram of typical oligomer. 9
Figure 1.3. Gas chromatogram of 1-decene trimer. 10
Figure 1.4. Bent metallocene. 26
Figure 1.5. Relationship of pro-catalyst symmetry to polypropylene
stereostructure. 33
Figure 1.6. Two types of supported chromium catalysts used in ethylene
polymerization. 37
Chapter 2
Discussion
Figure 2.1. 1-Pentene model trimer. 42
Figure 2.2. Gas chromatogram of two alcohol produdts. 54
Figure 2.3. Gas chromatogram of 1-pentene trimer. 66
Figure 2.4. Structure of oligomers. 75
xi
LIST OF TABLES
Chapter 2
Discussion
Table 2.1. Types of organocopper reagents. 51
Table 2.2. Summary of model C-C bond formation reactions. 52
Table 2.3. Influence of different organometallic nucleophiles on the addition
of a Grignard reagent to an epoxide. 55
Table 2.4. Attempted functional group transformations. 64
Table 2.5. Product distribution of the BF3/n-BuOH oligomerization of 1-pentene. 66
Table 2.6. Radical oligomerization of 1-pentene with organic peroxide initiators. 68
Table 2.7. Oligomerization of alpha olefins with titanocene dichloride/MAO. 71
Table 2.8. Oligomerization of alpha olefins with Cp2ZrC12/ MAO. 77
Table 2.9. Oligomerization of alpha olefins with CpTiC13/ MAO. 78
Table 2.10. Room temperature oligomerization of alpha olefins with
Cp2ZrC12/ MAO. 80
Table 2.11. Viscosity/temperature relationship of 1-pentene trimer fractions 81
xii
ABBREVIATIONS
by boiling point
Bu butyl
CI chemical ionization
Cp cyclopentadienyl
El electron ionization
fg functional group
GC gas chromatograph
MAO methylaluminoxane
MCPBA meta-chloroperoxybenzoic acid
Me methyl
MMPP magnesium monoperphthalate
MS mass spectrometer
Ms mesylate
NMR nuclear magnetic resonance
PAO polyalphaolefin
Pr propyl
psi pounds per square inch
rf radio frequency
rt room temperature
SHOP Shell Higher Olefin Process
THE tetrahydrofuran
Ts tosylate
UHP urea hydrogen peroxide
CHAPTER 1
THE OLIGOMERIZATION OF HIGHER ALPHA OLEFINS:
A LITERATURE . STUDY
1.1 Alpha olefin beneficiation: the key to value-adding and economic
growth
1.1.1 The South African perspective
The value-adding to manufactured goods is one of the pivotal factors governing the
economic growth and competitiveness of a country.. South Africa is blessed with abundant
natural resources. Gold, platinum, diamonds and coal are some examples of South Africa's
mineral wealth. Coal is of particular importance to the local chemical industry since it can
be converted into liquid hydrocarbons. The industrial process utilizing coal derived
mixtures of carbon monoxide and hydrogen to produce hydrocarbons, is based on the work
of Fischer and Tropsch. 1 South Africa, starting in the 1950's, developed an updated
Fischer-Tropsch synthetic fuel oil plant (Sasol) using improved engineering and technology.
These synfuels and synchemical plants are a lucrative means of adding substantial value to
more than 40-million tons of bituminous coal. 2
The Sasol synfuel plants were originally designed to produce mainly fuels, ethylene and
unavoidable byproducts such as ammonia. The Sasol/ Lurgi gasification process uses coal,
water and oxygen to generate synthesis gas (syngas: CO + H2) and a range of hydrocarbon
chemicals. The Fischer-Tropsch high-temperature synthesis reaction employs syngas to
create liquid fuels and a broad spectrum of useful olefins, alcohols and ketones. Market
growths in conjunction with the development of new separation and purification
technologies has realized the opportunity to redirect synfuel feedstocks to petrochemical
feedstocks. The 1994 Sasol alpha olefin plant, producing comonomer quality 1-hexene and
1-pentene, is a prime example of a profitable value-adding expansion.
2
Approximately two million tons of olefins (C2 - Cio range) are produced annually in the
Fischer-Tropsch process, of which about a third is currently recovered for the petrochemical
market. The balance is still being converted into liquid fuels. The new alpha olefin plant has
the potential to yield 450 000 tons per year of high quality 1-pentene and 1-hexene. This
beneficiation project forms part of Sasol's strategy to add greater value to its treasure chest
of petrochemicals. Alpha olefins represent a major niche-market opportunity for Sasol, viz.:
the 1-hexene market has been in tight supply, while the 1-pentene comonomer was not
previously available in commercial quantities. The feasibility of the production of additional
linear alpha olefins in the C2 to Cu range, is under consideration. 3,4
Sasol boasts a competitive advantage in petrochemicals, which stems from its unique one-
step process of producing fuels and petrochemicals without the larger capital investment
and higher operating costs of naphtha crackers. Seen in the light of this competitive edge,
further attention should be focused on in-house alpha olefin beneficiation.
1.1.2 Focus on alpha olefins
"Alpha olefins" is a generic term referring to linear alkenes with the double bond in the first
or alpha position. These olefins are important feedstocks in the petrochemical industry for
they are versatile chemical intermediates. Higher linear alpha olefins (C6 - Go) can be
transformed into biodegradable surfactants, new types of polymers, lubricants and several
industrially useful chemicals.
The widespread commercial usage of alpha olefins can be attributed to their ready
availability, low cost, high reactivity and ease of transformation into a range of valuable
products. Higher linear olefins have been available in commercial quantities since the
1960's. The processes used for the preparation of alpha olefins comprise: thermal and
catalytic cracking of paraffins, oligomerization of ethylene, dehydrogenation of paraffins,
dimerization and metathesis of olefins, dehydration of alcohols and electrolysis of C3 - C30
straight-chain carboxylic acids. The first process is used for commercial production of C2 - C5
olefins and the second for the large scale manufacturing of C9 - C30 alpha olefins.
3 •
The high reactivity of the alpha olefins can be explained in terms of the basic structure of
alkenes. The carbon-carbon double bond is best described by using an sp 2 hybridized
orbital model which predicts a strong c bond and a weaker it bond. The overlapping 7C-TC
bond behaves as a high-electron-density site and can be accessible to electrophilic reagents.
In alpha olefins the double bond is in the terminal position and is more available than
internal alkenes that is more sterically hindered. Therefore, alpha olefins are generally more
reactive than internal olefins. Additionally, functional group modifications often result in
products that are also linear, and this may be advantageous to the industrial user, e.g. the
linear surfactants obtained from alpha olefins are biodegradable. 4,5
1.1.3 Alpha olefin oligomers as base stocks for synthetic lubricants
Oligomerization of alpha olefins to produce feedstocks for synthetic lubricants is one of an
array of processes available for the beneficiation of alpha olefins. In the most fundamental
sense, a lubricant may be defined as a substance that has the ability to reduce friction
between two solid surfaces rubbed against each other. Animal fats, vegetable oils as well as
mineral oils have been used throughout history to achieve this effect. For more than a
millennium petroleum products have been employed in a primitive fashion and to this day,
petroleum based lubricants play a dominant role in the world lubricant market.
The first synthetic hydrocarbon oils were produced in 1877 by Charles Friedel and James
Mason Crafts. Synthetic lubricants are not obtained directly from crude oil and may be
described as blends of base stocks made from low-molecular weight (chiefly petrochemical)
building blocks. These base stocks consist of a limited number of components with uniform
molecular structures and, therefore, well defined properties that can be tailored to specific
applications.
Initial attempts to commercialize synthetic hydrocarbon oils failed due to insufficient
demand. However, this situation is changing. Technological advances in modern machines
and engines have been placing a greater pressure on the performance of existing functional
fluids. Today's lubricants are facing increasingly stringent demands. Some of these
challenges are cost effectiveness, lessened dependance on crude oil reserves,
biodegradability, specialized performance requirements and operation under more severe
4
conditions. These limitations led to the development of synthetic lubricants such as
polyalphaolefins, organic esters, silicones and polyglycols. Although the replacement of
mineral oil by synthetic lubricants seems farfetched, it should be noted that current
estimates predict that we may run out of crude oil in the next century.
Hydrogenated polyalphaolefins form a class of high-performance fluids considered to be the
most versatile of the synthetic lubricants. The term polyalphaolefin (PAO) is commonly
used to designate such fluids derived from the catalytic oligomerization of linear alpha
olefins having six or more (usually ten) carbon atom s . Owing to inherent and highly sought
after characteristics of PAO's these fluids are rapidly gaining ground as high-performance
lubricants. PAO's are currently being used as base stocks in, for example, automatic
crankcase lubricants, hydraulic oils and turbine oils. Synthetic lubricants manufactured
from alpha olefins can improve motor fuel efficiency and prolong oil lifetime. 6,7
1.1.4 Use and functionalization of poly(alpha olefins)
Alpha olefin oligomers and their derivatives are central to modem life, not only as synthetic
lubricants but in a variety of end-uses. Attention will henceforth be focused on the general
oligomerization process, use and functionalization of oligomers and the possible catalyst
systems active in this reaction.
Isomeric mixtures of olefin oligomers accessible via cationic catalysts like the BF3/ROH
system are commonly used in lubricant applications. Metallocene catalysts now offer a
convenient route to single component, structurally well characterized alpha olefin
oligomers. The possible functionalization and potential application of these oligomers are
depicted in Figure 1.1.8
additives for lacquers, / paper and leather industry
fuel additives
OH NH2
5
R OH R'..,. .OH -I. adhesives (Co-)Polymer
lubricants ointment white oil R' CH3
--PI.. fragrances
Figure 1.1. Possible functionalization and application of single component alpha olefin
oligomers. 8
1.2 General principles of oligomerization
According to the IUPAC nomenclature, oligomers 9 are molecules containing a few
constitutional units repetitively linked to each other. No specific molecular weight or degree
of oligomerization is defined. It states, however, that the physical properties of oligomers
vary with the addition or removal of one or several units. Polyolefin chain formation,
depending on a number of n reacting molecules, can in most cases be named as follows:
dimerization (n = 2), oligomerization (2 < n < 100) and polymerization (n > 100). 5
Alpha olefins may be oligomerized thermally or catalytically; the latter having industrial
application. A variety of oligomerization catalysts are known and reactions can be classified
accordingly as acid-, base-, radical- or metal catalysed.
The typical catalytic cycle of these catalysts consists of initiation, propagation and
termination steps. The relative propagation (r e) and termination (1-.1 rates determine the
molecular weight of the obtained product. If r p > rt many propagation steps take place
before termination and a high molecular weight polymer is formed, when rt > r p dimers are
produced, and finally when r t rp oligomers are obtained.5
6
The main oligomerization catalysts will be discussed in the following section, with specific
reference to higher alpha olefin monomers, reaction mechanisms and the factors governing
the product distribution.
1.3 Olefin oligomerization catalysts
1.3.1 Friedel-Crafts catalysts -
As early as 1877 Charles Friedel and James Mason Crafts found that anhydrous aluminium
chloride could be used to catalyse the polymerization of unsaturated hydrocarbons. During
the ensuing years aluminium chloride was shown to be a most versatile catalyst effecting an
almost unlimited number of reactions. Today any substitution, isomerization, elimination,
cracking, polymerization, or addition reaction that takes place under the catalytic effect of
Lewis acid-type acidic halides (with or without cocatalysts) or proton acids, are regarded as
Friedel-Crafts-type reactions. 10
i) Polyalphaolefins
Friedel-Crafts type catalysts are currently the only ones used commercially for producing
PAO base stocks for synthetic lubricants. A few aspects concerning polyalphaolefins should
be highlighted and expanded. Polyalphaolefins are the largest volume synthetic lubricant
base stock worldwide and are considered to have the largest potential for growth. The
combination of their relative low cost and desirable properties make the PAO's the preferred
base stock for a wide variety of synthetic lubricant applications. Among these favorable
properties are:
A wide operational temperature range
High viscosity index
Thermal stability
Oxidative stability
Hydroly tic stabili ty
• Shear stability
Low corrosivity
7
Compatibility with mineral oils
Compatibility with various materials of construction
Low toxicity
Manufacturing flexibility that allows the "tailoring" of products to specific end-use
application requirements.
However, PAO's lack the ability to swell the elastomers used in the seals of many lubricant
systems, and they have limited ability to dissolve the additives needed to enhance critical
properties. As a result, they are often used in combination with petroleum-based oils
and/or esters (primarily dibasic acid esters) to overcome these deficiencies. The synthetic
lubricants compete with petroleum lubricants on a cost-competitive basis, but some key
environmental and quality policy changes could affect future growth positively. 6,7
ii) Cationic acid catalysed oligomerization
Lewis - and Bnansted acids, that is, the common Friedel-Crafts catalysts, may effect the
oligomerization of alkenes. 11 Cationic oligomerization is distinguished by the nature of the
carrier ions on the growing chains: in the process of growth, an ion pair is formed in which
the chains carry carbocations. Initiation occurs by electrophilic attack of the initiators on the
monomers. A general scheme for cationic oligomerization is presented in Scheme 1.1. The
coinitiator is presented as A, the initiator as RH, and the monomer as M.
Generally, cationic oligomerization depends on the initial formation of positive and negative
ions in an organic environment. Initiating systems include: Lewis acid/Bronsted acid
combinations, Brensted acids by themselves, stable cationic salts, some organometallic
compounds, and some cation forming substance's. Propagation takes place through
successive addition of monomeric units to the charged or reactive groups of the propagating
chains. Terminations are the result of unimolecular reactions, or transfers to other
molecules (such as monomers, solvents or impurities like moisture). They can also result
from quenching by deliberate addition of reactive terminating species.'2
8
Cationic Initiation Process :
A + RH K
[H+] [AR- ]
[HM+][AR- ]
[HMnM+][Ak-
Mn+1 + [HM+][AR-
Mn+1 [HI [AR-
Rate of initiation :
Ri = KKi[A][RH][M]
Rate of propagation :
Rp = KpRHM+) (AR - )] [M]
Rate of termination :
Rt = Kt [ (HMnM+)(ARli
where Kt is a combination of
Ktr, Kst and Kr
Ki [H+][AR- ] + M
Cationic Propagation Process :
[HMn+] [AR- ] + M Kp
Cationic Termination :
By transfer :
+ M [HMnM+] [AR- Kt _
By spontaneous termination
[HMnM+] [AR] Kst
By rearrangement of the kinetic chain :
[HMnM+][AR- ] Kr HMnMAR
Scheme 1.1. Kinetics of cationic chain growth oligomerization. 12
iii) BF3-catalysed oligomerization of alkenes
Cationic catalysts, such as AlCb, have been reported for the oligomerization of alkenes as
early as 1931. 13 In 1951 a patent was issued to Gulf oil company describing the use of AlCb
for the oligomerization of 1-octene. 14 Brennan (Mobil Oil) . showed in 1968 that
oligomerization of alpha olefins could be carried out in a reproducible fashion with boron
trifluoride in the presence of a protic cocatalyst. 15 Commercial PAO's are made from 1-
decene. The low viscosity products are made using BF3 catalysts, while the higher viscosity
PAO's are produced with catalysts like a trialkylaluminium or alkylaluminiumhalide in
conjunction with a halogen or halide source. 716
BF3 is the catalyst of choice for the most common grades of PAO's. While incapable of
catalyzing the oligomerization of alpha olefins on its own, BF3 is active when used in
conjunction with a proton-donating cocatalyst (typically a low molecular weight alcohol
9
such as n-butanol). BF3 complexes with proton-donating compounds by one of the
following general reactions:
BF3 (g) + ROH (1) --> BF3.ROH (1)
or
BF3 (g) + 2 ROH (1) ---> BF3.2ROH (1)
Scheme 1.2. Formation of catalyst complex!
Since BF3 is normally used in excess, the BF3 and alcohol cocatalyst react in a 1:1 molar ratio
to form the catalyst complex BF 3.ROH. The BF3.ROH system is unique for two reasons: the
oligomer peaks at the trimer (Figure 1.2) and the products exhibit unexpectedly good
low-temperature viscosity characteristics due to the presence of excess branching in the
molecular skeleton.
TA ImEF,S
TC7.7AmEA S
CMEPS
?ENT AmEqS
HEXAMEAS
10' 1Sr 2d 23. T al. fain)
Figure 1.2. Gas chromatogram of typical oligomer.16
Figure 1.2 is a high resolution GC trace of a typical oligomerization product resulting from
the reaction of 1-decene in the presence of the BF 3.ROH complex. Figure 1.3 shows clearly
that the single oligomer fractions are composed of a very large number of branched isomers.
10
These isomer mixtures form even under tightly controlled reaction conditions.
Notwithstanding, this has the advantage, especially in lubricant applications, of lowering
the melting point of the trimer mixture relative to that of pure compounds of comparable
molecular weight and structure. 17
lOr 12. 14' le 18" 20' 22' 24' 19" 19'
Ti.. (ain)
Figure 1.3. Gas chromatogram of 1-decene trimer.' 6
a) Mechanistic considerations
1H NMR studies of the hydrogenated oligomers indicated about one additional methyl
group compared to that predicted on the basis of the conventional cationic mechanism
(Scheme 1.3). As yet no mechanism that explains this branching has been proven
conclusively in view of the complexity of the products. Various studies of the hydrogenated
and distilled oligomer fractions indicate anomalies with the theoretically expected results.
Skeletal rearrangement at the dimer stage, 18 rearrangement of the monomer 19 as well as
positional isomerization of the monomer 20 have been proposed in order to explain these
discrepancies.
2522a
- H-F
H-F Isomers
11
H+
- H+
H+
- H+
H+
- H+
R— CH-2 CH= CH2
MONOMER
CH3 R— CH-2 CH— CH= CH—CH-2 R
(+ isomers)
DIMER
CH3 R— CH— CI H— CH—C H— CH-2 R 2 2
CH I I
(+ isomers) CH
H2
R- CH2 CH—CH3
R—CH-2 CH=CH2
CH I + R— CH-2 CH— CH-2 2 CH—CH— R
R— CH-2 CH= CH2
CH3 R— CH-2 CH— CH-2 CH— CH-2 R
CH2 +CH
CH2 R R
TRIMER
Scheme 1.3. Conventional cationic mechanism.
Experimental work of Shubkin et al.18 indicated that the conventional mechanism did not
explain the composition of the oligomerization products adequately. Firstly, the unique
product distribution could not be clarified: at lower temperature (30 °C) mainly trimer was
formed, while the product peaked at the dimer at elevated temperature (85 °C). Arguments
based on steric crowding were unconvincing. Secondly, the physical properties of the
oligomerization products differed from the expected: lower viscosities and significantly
lower pour points than would have been predicted from the expected molecular structures,
were found. Further investigation revealed that on average two-thirds of the oligomers had
one more methyl group (in other words one more branch) than expected.
The observed nature of the BF3-catalysed oligomerization reaction, as well as the physical
and chemical properties of the products, led Shubkin 18 to suggest a mechanism that includes
skeletal rearrangement (Scheme 1.4). According to Shubkin, rearrangement occurs only at
the dimer stage, proceeding via a protonated cyclopropyl intermediate.
12
R—CH=CH—CH3 3
H+ Double bond migration
H-F R—CH-2 CH— CH3 2
+ 1
CH H \ 3/
C'■ 2 CH2
R—CH—
4
2 R
R—CH-2 CH=CH2 -
1
CH I 3
R—CH2 CH
Cx, / —CH2—R
H+ CH2 6
1 CH3
R—CH-2 C—CH—CH-2 R
7 CH3 +
H+1[ -H+
ICH3
R—CH=C—CH—CH—R I 2 CH3
+
C H3 I
8
CH3 R—CH-2 CH—CH=CH—CH-2
5
CH3
R—CH-2 C—CH 2—CH—R 1 \
CH2 CH3 + CH
I 9 CH2 I R
+ 1
H+ Double bond R migration - H+
H\14+
—— R—CH2 C= C —CHR I 2
CH3 Tetramer
11
CH3 R—CH2 —C—CH—CH—R
1 \ 2
CH CH3 CH 10
CH2 R
Scheme 1.4. Shubkin's mechanism's
13
Protonation of the olefin (1 ---> 2) is followed by nucleophilic attack by another olefin
molecule to give a protonated dimeric species (4). This species may then deprotonate to
form dimeric product (5) and/or isomerize by double bond migration. Shubkin states,
however, that there is a unique feature about intermediate 4 which is the key to this
reaction: there is a labile, tertiary hydrogen p to the carbonium ion. Skeletal rearrangement
through a protonated cyclopropyl intermediate (6) to give the more stable tertiary
carbocation 7 is thermodynamically favored. Intermediate 7 may deprotonate to 8 and
undergo isomerization by double-bond migration. But the inherent stability of a tertiary
carbocation makes it available for attack by another olefin to produce the trimeric
carbocation 9. This is a secondary carbocation and therefore less stable than 7; furthermore,
it cannot rearrange the way that 4 does. The carbon atoms p to the carbocation either
contain no hydrogen atom or only secondary hydrogen atoms which are not particularly
labile. Thus deprotonation to 10, the trimeric olefin, is favored over reaction with an
additional alpha olefin to give 11, the tetramer.
Onopchenko and coworkers 19 studied the oligomerization of 4-methyl-1-pentene by the BF3
catalyst system to test Shubkin's proposed mechanism. 1H NMR of the dimer and trimer
product fractions showed 5.0 and 6.8 methyl groups, respectively, instead of the expected 6
and 8 predicted by Shubkin's mechanism. According to Onopchenko, this indicated that: (a)
no methyl group rearrangement occurred at the dimer stage or, (b) that the initial secondary
4-methylpentyl carbocation had rearranged at the monomer stage and was behaving as the
tertiary carbocation derived from the vinylidene alkenes.
In order to provide evidence that methyl group migrations can occur at the monomer stage
(and not only at the dimer stage as claimed by . Shubkin et al.18), the composition of
unreacted decenes were determined under varying conditions. Onopchenko et al. 19
confirmed the results of Brennan: 17 At high 1-decene conversion (90%), the unreacted alkene
contained only 60% of 1-decene, 35% of internal alkenes and 5% of methyl nonenes.
Conversely, at low 1-decene conversion (30%), less than 0.5% of methylnonenes was
detected. This data suggested that the BF3 complex is a mild isomerization catalyst, but an
effective oligomerization catalyst.
The complexity of the oligomers formed from 1-decene necessitated the examination of a
simpler system. The structures of the dimers from the oligomerization of 1-butene and
14
cis-2-butene using the BF3/mannitol complex were investigated (theoretically only 18
isomers are possible for C8F112paraffins).
The data obtained showed that all of the isomers had either three, four or five methyl
groups. The similarity of the product isomers from the two butenes suggested that the
same intermediate had to be involved in both cases. The common intermediate is the sec-
butyl carbocation. The only significant difference in the oligomerization products from the
two butenes was the formation of substantially more 3-methylheptane from 1-butene and
3,4-dimethylhexane from 2-butene.
Onopchenko et al.19 assumed that two combinations (sec-butyl 12 and tert-butyl 13 in Scheme
1.5) were involved in the dimerization of the two butenes. This assumption followed from
the postulated mechanism for the Koch reaction, 21 where a protonated cyclopropane is the
key-intermediate in the formation of tertiary alkyl carboxylic acids. Onopchenko and
coworkers used this reaction to predict the nature of the major isomers.
Following the protonation of either 1- or 2-butene, the secondary butyl carbocation
rearranges to the tertiary butyl carbocation, and both then add to an isomerized mixture of
alkenes. The intermediate ions can then undergo deprotonation to afford the corresponding
olefinic dimers and their isomers. Eventually six paraffins (14 - 19) would result from each
butene, but these accounted for less than 70% of the experimentally determined number of
isomers. Eight additional isomers were present and this suggested that some of the
expected intermediates had rearranged further and thus the model used by Onopchenko et
al. was only an approximation. The total percentage of isomers 16 - 19, nevertheless, could
be used to estimate the extent of methyl group migration at the monomer stage - it was in
the range of 14 - 22% for the two butenes considered.
Driscoll and Linkletter 20 used NMR to analyse 1-decene dirner fractions. Their conclusion
was that the identified structures were formed by the attack of secondary carbocations on
secondary olefin. Further studies, incorporating 1-decene selectively enriched in position 2,
suggested that in most cases oligomerizations took place internally and not at the terminal
double bond.
15
C—C—C=C -
- C—C=C—C rearrangement C—c—C
12 13
C C C I I I
C—C—C—C C—C—C—C C—C—C—C C—C—C C—C—C C—C—C + + + + + +
1 C=C—C—C 1C—C=C—C 1C —LC 1C=C—C—C 1C—C=C—C 1C—LC
C—C—C—C
C
+C
C
CI
1- H+ 1- H+ H+
H+ 1- H+
C C C
I I I c—c—c—c c—c—c—c c—c—c—c c—C—C C—C—C C—C—C
CI I I I I I
II c—c C C C—C
c II II II I I II
C C C—C C C C—C I I I I I I c c C C C C
1 i C c
H2 1 H2 "1 H2 I H2 1 H2 1 H2
C C C
I I I C—C—C—C C—C—C—C C—C—C—C C—C—C C—C—C C—C—C
C C I I I
C I
C C—C C—C
15 c1 1
c1
18 c1 i
14 C 16 c—c 17 19 c—c
C I I I I I c c C c C
C C
3-methyl- 3,4-dimethyl- 2,4-dimethyl- 2,2-dimethyl- 2,2,3-trimethyl- 2,2,4-trimethyl-
heptane hexane hexane hexane pentane pentane Total:
1-C4= : (34.4%) (8.6%) (14.3%) (2.2%) (1.8%) (3.8%) 64.1%
(3.0%) 69.7%
Scheme 1.5. Oligomerization of 1-butene and cis-2-butene."
c c c I I I
c—c—c—c C—C—C—C C—C—C C—C—C C—C—C I I I I
C C CI
I C—C C—C
-EC C—C+ I
+C I
+C I
C—C + I
cl
cI
CI
C I
CI
2-C4= : (14.2%) (41.2%) (8.7%) (1.5%) (1.1%)
16
Thus, double bond isomerization took place appreciably faster than oligomerization and the
first step in the mechanism can be depicted as follows:
H3c(042)7&1=cH2
H3C(CH2)mCH=CH(CH2)nCH3 H2C=CH(CH2)6h-I2CH3
Scheme 1.6. First step in oligomer formation 2°
This step embodies the formation of the dimer through the reaction of a secondary
carbocation with a primary or secondary olefin. According to Driscoll and Linkletter 2° linear
olefins such as decene, in all probability have limited thermodynamic preference for the
internal olefin isomers and as a result, carbocation formation will proceed at random along
the chain. The attack on the primary olefin will be preferred sterically, but not statistically.
This led Driscoll and Linkletter to claim that random bonding, primarily by secondary
carbon atoms, would take place to afford a large number of isomers containing the labelled
carbon next to the methyl group. This proposed mechanism explains the presence of more
methyl groups than predicted by the conventional cationic mechanism (attack of a
secondary carbocation on a primary olefin)..
b) Correlation of paraffin structure with physical properties
Before the synthesis of novel fluids with improved physical and thermal properties can be
considered, some understanding of the structure-property relationship is required.
Cupples, Kresge, Onopchenko and Pelligrini 22 related the structure of paraffins to physical
properties such as viscosity, pour point and in some cases thermal stability. The pour point
of a lubricant may be described as the temperature just before the fluid solidifies during
cooling, in other words it is the freezing point.
As the carbon number of the feed decreased, the low temperature viscosity increased; thus
the viscosity-temperature relationship worsened. Likewise, as more branching was
introduced into the molecule, the viscosity of the oligomers deteriorated. Introduction of a
short chain (methyl) in the 2-position of n-alkanes practically had no effect on the viscosity,
but moving the methyl group toward the centre of the alkane backbone showed some
17
deviation. The introduction of a single methyl branch into a linear chain lowered the
melting point significantly, but the effect was most pronounced when the branch was near
the centre of the chain. Lengthening the branch to C4 or longer reduced the melting point
even further. When considering the low temperature performance of fluids, including both
viscosity and pour point, it should be noted that the structural requirements for good low
temperature viscosity and low pour point are conflicting. The low degree of branching of
the linear alpha olefin oligomers which imparts their superior viscosity properties is
precisely what will cause these oligomers to be the first to fail a given pour point. In short,
the preferred structure would appear to be a molecule having linear characteristics but with
sufficient branching to provide the needed pour points.
Thermal stability seemed to be favored by less branched, more linear structures; as the
number of tertiary hydrogens increased, the thermal stability decreased. This, of course, is
consistent with the data of Benson, 23 who reported that the tertiary carbon-hydrogen bonds
have the lowest bond dissociation energies and are therefore considered the weak points to
thermal stability.
Brennanl7 found that the following theoretical molecular structure, in which the
concentration of atoms is very close to the centre of the chain of atoms, is associated with
superior low temperature fluidity:
R1
R— C— R2
R3
where R and R2 are nearly identical in size, R is generally a hydrogen but may be an alkyl
group, and R3 can vary from a methyl group to a group larger than R and R 2.. The carbon
atoms comprising R, R2 and R3 may be arranged in a chain or a ring. Compounds with
carbon numbers up to 36 in which Ri is a hydrogen and R, R2 and R3 are alkyl groups and
nearly identical in size, exhibited very good wide temperature range fluidity. Long chain
linear alpha olefins should yield trimer fractions structurally similar to this ideal and thus
the trimerization of long-chain olefins is a one-step synthesis of a superior lubricant
molecule.17
18
c) Reaction variables
Even though the mechanism of the BF3.ROH catalysed oligomerization remains to be fully
elucidated, researchers have learned how to advantageously control the composition of the
final PAO products in order to tailor the oligomer distribution to fit the requirements of
specialized end-use applications. 16 This customizing may be done by manipulation of the
reaction variables, which include: chain length of olefin raw material, temperature, reaction
time and pressure, catalyst concentration, cocatalyst type and concentration, cocatalyst feed
rate, olefin feed rate, reaction quench and recovery procedures, hydrogenation catalyst and
conditions, and distillation. 6
It is now clear why BF3/decene was adopted as the basis of manufacture by every major
producer of low viscosity PAO's. It is, however, normally not possible to manufacture
higher viscosity products using BF3.ROH technology, but other catalyst systems are known
that can give the desired products. These include alkylaluminium compounds in
conjunction with TiC4 or allcylhalides, peroxides and A1Cb catalysts. 6,16
The function of the cocatalyst is obviously complex. Shubkin and Kerkemeyer 16 showed
empirically that the choice of cocatalyst and the concentration at which it is used plays an
important role in determining the composition of the product PAO. Priola et al.24 observed
that an increased temperature decreased the molecular weight of the oligomers as well as
increased the presence of fragmentation products. Shubkin et al. 16 also established that
higher temperatures increased the branching in the oligomers as well as promoted some
depolymerization. Relative to the reaction time, both the composition and the isomeric
distribution of the product change. This represents a useful opportunity for tailor making
PAO's with special physical properties. 6
In addition to controlling the relative distribution of oligomers by manipulation of the
reaction parameters, major alterations can be made to the product properties by the choice
of starting olefin to better satisfy the requirements for a particular end-use application.
Fluids prepared from short-chain monomers tend to have low pour points and moderately
low viscosity indices, whereas fluids prepared from long chain monomers have moderately
low pour points and high viscosities. 22 A final consideration is the position of the double
bond in the starting olefin. Shubkin et al. 16 established that increased levels of internal
19
olefins in the starting mixture resulted in slower reactions and more dimer in the product.
In addition the low temperature properties of the hinter were influenced: the pour point
decreased and the viscosity increased.
The cationic oligomerization of unrefined C 10 cuts obtained from the Fischer-Tropsch
process have also been researched. 28,26 The raw Cio Fischer-Tropsch products are suitable
for oligomerization and the included alcohols serve as promoters. Oxygenated compounds,
aromatics as well as branched, internal and cyclic olefins were found to be more
problematic. The presence of ketones led to early termination of the oligomerization
reaction and as a result the oligomer distribution was shifted to the lower end.
Additionally, the aromatics caused a lowering of the viscosity indices. Preliminary
purification of the feed enabled better reaction control and product quality could be
improved.
Much of the recent research activity is centred on the development of either new catalysts
that would alleviate the environmental problems associated with BF3, or alternate feedstocks
that are less expensive than 1-decene. 7
1.3.2 Free radical catalysts
The use of organic peroxides as free-radical initiators for polymerization became important
during World War II as a result of the increased demand for synthetic rubber and plastics. 27
It has been reported in a Mobil European patent application that high viscosity PAO's may
be produced by dimerizing lower oligomers with peroxides. 28 In reality this is not currently
economically feasible, since stoichiometric quantities of di-tert-butyl peroxide are used.
The steps in free radical polymerization are illustrated below. The formation of initiating
radicals is the rate-determining step in the initiation reaction. Numerous reactions lead to
the formation of free radicals: thermal decompositions with azo and peroxy groups are
common sources, but "redox" reactions, light induced decompositions and ionizing
radiation can also be used to form initiating radicals. 12 These initiating radicals must be
energetic enough to react with the vinyl compounds. Once the initiating radical is formed,
there is competition between the addition to the monomer and all other secondary reactions.
After the initiating radical has diffused into the proximity of the monomer, the capture of
20
the free radical by the monomer is a straightforward addition reaction and completes the
step of initiation. By comparison to other steps in the polymerization, initiation is slow and
requires high activation energy. The rate of propagation depends upon both the reactivity
of the monomer and the growing radical chain. Steric hindrance, polar effects and
resonance are important factors in this reaction. The termination processes in free-radical
polymerization are: combination, disproportionation or chain transfer.
Initiation :
R—R --I,- 2 R".
R* + CH2= CHX R—CHT CHX •
Propagation :
R— CH-2 CHX " + n CH =---- CHX ---0- R— (—CH-2 CHX—)n— CH-2 CHX • 2
Termination :
By combination
R— (— CH— CHX--)n—CHT CHX • + • XHC— CHT(—XHC— CH2)n—R
R— (—CHTCHX—)n— CHTCHX— XHC— CHT (— XHC— CHT-) n — R
By disproportionation
R— (— CHT CHX — CHT CHX • + • XHC— CHT (— XHC— CHHn—R
R— (— CHT CHX—)n— CHT CH2X + XHC:= CH— (— XHC— CHT-)n— R
By transfer
R— (— CHT CHX--)n— CHT CHX + R'H
-111' R— (— CH2— CHX —)n — CHT CH 2X + R''
Scheme 1.7. Free radical polymerization of vinyl monomers. 12
1.3.3 Transition metal catalysts
In the last two decades, there has been as surge of interest in the chemistry and application
of catalysis by certain transition metal complexes. Polymerization catalysts have been the
focus and examples of industrial application abound. The oligomerization of alkenes by
H
LnIvi—R' LnM—C—C—R
21
transition metal catalysts is however of considerable academic and industrial interest for the
synthesis of linear and branched higher mono-olefins. The oligomerization of ethylene and
propylene are the most well known, e.g. in the formation of higher linear alpha olefins in the
Shell Higher Olefin Process (SHOP). 29,30
A number of catalysts are known for the oligomerization of higher alpha olefins. Catalysts
similar to those used for ethylene and propylene oligomerization and polymerization can be
used for higher alpha olefin oligomerization. It is assumed that the polymerization
mechanisms of these catalysts can be applied to the corresponding oligomerizations of
higher linear alpha olefins. Among the most popular catalysts are titanium, zirconium and
nickel complexes; homogeneous and heterogeneous Lewis and Bronsted acids; and
inorganic oxides. 5 The reactivity of the olefinic substrates can be correlated to their relative
coordination abilities. Olefin reactivity decreases in the following order : ethylene >>
propylene > 1-butene > 1-pentene > 1-hexene. 31,32
In general, two different reaction mechanisms are considered relevant: 29,30 a hydride
mechanism and a mechanism based on metallacyclic intermediates. Keim 29 states that the
`hydride insertion-alkyl migration mechanism is the most generally accepted one, but proof
is still questionable. It is, however, the accepted one for the majority of oligomerization
reactions. Considerable attention has also been attracted by the catalytic sequence involving
metallacyclopentane intermediates. A simplified scheme for the metal-hydride mechanism
is portrayed in Scheme 1.8.
Kp
propagation n C=C—R
Scheme 1.8 Simplified hydride mechanism.29,3°
22
The first step comprises the insertion of an olefin into a metal-hydrogen bond. Further
insertion of olefins into the metal-alkyl bond leads to growth products (propagation step).
The elimination of the thus formed oligomers occurs by a n-hydrogen elimination returning
to Li,M-H which re-enters in the catalytic cycle.
Analyzing the sequence given in Scheme 1.8, the relative propagation and elimination rates
again determine whether polymerization, oligomerization or dimerization will be observed.
Which of these reactions prevail is dependent on the nature of the metal and its oxidation
state, the nature of the ligand and the reaction parameters. These parameters critically
influence the electronic structure of the complex.
According to Keim 29 the following generalizations can be made: Group VIII transition
metals favor 13-hydrogen transfer (elimination) . and thus olefin dimerization and
oligomerization are observed preferentially. In contrast, with Group IV to VI transition
metals propagation prevails and predominately polymerization catalysts are found in this
class. With the same metal, chain propagation decreases with increasing oxidation state.
Furthermore, the metal character can be altered by adding ligands. Donor ligands tend to
increase and acceptor ligands to decrease the chain propagation.
There should be distinguished between the two possible modes of addition of a metal-
carbon or metal-hydrogen bond to an unsymmetrically substituted olefin, namely the
Markovnikov and anti-Markovnikov addition. The direction of addition determines
whether linear or branched products are formed. Higher alpha olefins can form a greater
number of isomers, as a result the selectivity of oligomerization is lower. Highly selective
formation of certain isomers can be achieved by the selection of appropriate ligands.
Homogeneous, transition metal based catalysis has received attention over the past number
of years and a variety of industrial processes eXiSt. 33,34,35 It is broadly assumed that the true
catalysts consist of a metal surrounded by a ligand field. The ligands are essential in
controlling the activity, selectivity and catalyst life. 36 The theoretical interest in
homogeneous catalysts, as opposed to heterogeneous catalysts, stems from the unique
insight into the mechanism of catalytic action. The nature and surfaces of solids are not well
known or understood, but the nature of molecular and ionic species existing in solution has
been relatively well established.
23
For catalyst preparation one of three routes are generally followed. Transition metal salts in
a higher oxidation state are reduced to the catalyst, low valent transition metal salts are
oxidized to the catalyst or organometallic complexes are used as catalyst precursors. 36
Organometallic complex catalyst precursors are favored, because they tend to be highly
active, specific and selective thus minimizing unwanted reactions. 37
i) Heterogeneous Ziegler-Nana catalysts
The Ziegler-Natta catalysts received their initial attention when Ziegler showed that some
transition metal halides, upon reaction with aluminium alkyls, can initiate the
polymerization of ethylene. 38 Simultaneously, Natta demonstrated that similar catalysts can
polymerize various other olefins like propylene, butylene and higher alpha olefins. 39 Higher
molecular weight linear polymers formed as well, and more importantly, stereospecific
ones. These reactions require much lower pressures than do free-radical polymerizations
and form the foundation of today's polyalkene industry. Despite the consequent
development of a wide range of highly reactive and selective catalysts, the complex
multisited nature of heterogeneous Ziegler-Natta catalysts has hindered their detailed
characterization and rational modification. As a result, fundamental questions concerning
the mechanism of monomer addition and the origin of stereochemical control in
heterogeneous Ziegler-Natta catalysts remain unanswered.
0
Ziegler-Natta catalysts are products from reactions of metal alkyls or hydrides of Group I to
III of the Periodic Table with metals, salts or complexes of Group IV to VIII. The catalysts
that originally gained commercial importance were combinations of TiC14 or TiC13 with R 3A1
or R2A1C1. These so-called first generation heterogeneous Ziegler-Natta catalysts exhibited
rather low activity, water-sensitivity and were susceptible to catalyst poisoning. High-
activity-high-yield (second generation) catalysts were developed incorporating trivalent
alkoxy chlorides of transition metals with certain halogen-free organoaluminium
compounds. The third generation comprises supported Ziegler-Natta catalysts prepared by
reacting titanium compounds with hydroxylated magnesium derivatives, magnesium
alkoxides, or magnesium organometallics followed by activation with aluminium alkyls.11,12
24
The Ziegler-Natta catalysts can be subdivided into two groups: (1) heterogeneous insoluble
catalysts, and (2) homogeneous or soluble, ones. Soon after the initial discoveries of Ziegler
and Natta, efforts were made to devise homogeneous models of the heterogeneous catalysts
that would prove more amenable to study. In 1957 Nattao and Breslow 41 reported that
Cp2TiC12 could be activated for alkene polymerization by Et 3A1 or Et2A1C1.
Several mechanisms have been proposed to explain the action of heterogeneous Ziegler-
Natta catalysts. All agree that the polymerization takes place at localized active sites on the
catalyst surfaces by coordinated anionic mechanisms. The organometallic component is
believed to activate the site on the surface by alkylating the transition metal. Some
controversy still exists about the exact mechanism of catalytic action, whether it is
monometallic or bimetallic. The majority of opinion leans to the former. In addition, it is
well accepted that the monomer insertion into the polymer chain is between the transition
metal atom and the terminal carbon of the growing polymeric chain. 42
The role of the metal alkyl in forming the active catalyst is illustrated in Scheme 1.9. After
complexation with TiC14 (or in general, with the transition metal compound), the transition-
metal alkyl is generated through halogen-alkyl exchange. This complex is then reduced in a
subsequent intricate reduction step including dealkylation to yield low-valence titanium.
These reduced metal species are believed to constitute the active catalytic species. 11
TiC14 + AlEt3 TiC14.AlEt3
TiC14.AlEt3 EtTiC13.AlEt2C1
2 EtTiC13.A1Et2C1 2 TiC13.A1Et2C1 C2H4 C2H6
Scheme 1.9 Formation of active heterogeneous Ziegler-Natta catalyst. 11
All evidence to date indicates that chain growth occurs through repeated four-centred
insertion reactions of the monomer into the transition metal-carbon cy-bond. It is still not
established whether the basic metal alkyls serve only to produce the active centres and have
no additional function. This would imply that the mechanism is monometallic. If the active
centres must be stabilized by coordination with base metals, then the mechanism would be
bimetallic.
25
An example of the monometallic mechanism is one proposed by Cossee and Arlman
(Scheme 1.10). 43 This mechanism assumes that the reaction occurs at a transition metal ion
on the surface layer of the metal trichloride (or perhaps dichloride) lattice. Here, the halide
is replaced by an alkyl (R) group. The adjacent chloride site is vacant and accommodates
the incoming monomer molecule. The newly formed transition metal-alkyl bond becomes
the active centre and a new vacant site form in place of the previous transition metal alkyl
bond.
R' a ss
cl al I
R R' Cl 1 I ...-• CH—R
+ CH=CH —110- Cl—Ti--H 2 al 1 CH2 CI
CH—R
R'
a '
,CH, -
al I Cl
four-centre intermediate
R'
CH
/CH— R /
al2
Cl
Scheme 1.10 Monometallic Ziegler-Natta mechanism. 11,12
Terminations in coordination polymers usually occur by chain transfer either by an internal
hydride transfer or by transfer to a monomer. Hydrogen or protonic acids also terminates
the reaction and is often used to control the molecular weights of the products. 12
Ziegler catalysts yield outstanding wide-temperature range hydrocarbon fluids when used
in the oligomerization of alpha olefins. Compared to similar cationic catalysed reaction
products, the Ziegler derived materials exhibit higher oxidative and thermal stability as well
as a slightly higher viscosity index. The pour point is lower due to the presence of highly
linear chain segments in the oligomer. The high quality of these Ziegler prepared fluids can
be ascribed to the little or no isomerization that takes place during oligomerization. The BF3
cationic oligomerization still holds some advantages: while BF3 uses none, the Ziegler
process uses an unspecified amount of solvent; the Ziegler oligomerization takes much
longer; and the conversion for the BF3 process as well as the yield of the trimer are higher.17
26
ii) Homogeneous Ziegler-Natta catalysts: Metallocenes
Forty years after the discoveries of Ziegler and Natta we are witnessing the evolution of new
generations of catalysts and polyolefin materials which originate from studies on
homogeneous, metallocene-based polymerization catalysts. Research on
metallocene-catalysed olefin polymerization derived much of its impetus from the desire to
model the reaction mechanisms of the -heterogeneous polymerization catalysts.
The first soluble catalysts (Cp2TiC12 activated by Et3A1 or Et2A1C1) exhibited lower activities
in ethylene polymerization than their heterogeneous counterparts and were inactive for
propylene. 40,41 This situation changed dramatically in the early 1980's when Sinn and
Kaminsky44• 5 discovered that partially hydrolyzed Me3A1 activated Group 4 metallocenes
catalysed the polymerization of ethylene and other alpha olefins.
Active homogeneous olefin polymerization catalysts are obtained on mixing bent
metallocenes Cp'2MX2 (Cp' = general cyclopentadienyl ligand; M = Zr, Hf, Ti; X = Cl, Me),
with excess methylaluminoxane (MAO) in inert solvents such as toluene (Figure 1.4). Such
uniform site catalysts generally contain a single kind of active site and offer potential
advantages over traditional multi-site Ziegler-Natta polymerization catalysts. The most
important is the ability to control polymer structure and properties by variation of the
catalyst structure, allowing production of a wide range of new polymers, unavailable using
the classic catalysts. 46
M x
where M = Hf, Zr, or Ti
and X= Me or Cl
Figure 1.4 Bent metallocene.
27
a) Nature and formation of the active species
Initial metallocene research was focused on establishing the catalytically active species of the
metallocene/MAO system. Both a cationic monometallic species and a neutral bimetallic
species have been proposed. In 1960, Shilov 47 proposed that the active species was a 14-
electron cationic metal alkyl of the form [Cp2MR] 4- .• The isolation of the cationic insertion
product from the reaction of Cp2TiC12 -/MeA1C12 with an alkyne by Eich48 supported Shilov's
proposal. This result suggested that the metal alkyl cation [Cp2TiMe]+ was the initially
formed catalytically active species for this alkene polymerization system. In addition, the
catalytic nature of discrete do metallocenes (n = 0, 1) substantiated the existence of
a cationic active site. Strong evidence for the involvement of cationic intermediates came in
1986 when Jordan49 isolated and characterized the cationic complex
[Cp2ZrMe(THF)]+[BPh4] , which polymerized ethylene in the absence of a cocatalyst.
Controlled hydrolysis of Me3A1 produces a mixture of methylaluminoxanes (MAO) thought
to have the general formula [Al(Me)O]n (5 < n , 20). 50 MAO plays at least three roles in
polymerization: as a alkylating reagent for the generation of transition metal-alkyl adducts;
as a Lewis acid for anion abstraction from the complex generating an electrophilic species;
and as a scavenger for the removal of impurities, particularly water in the olefin and
solvent.46
Highly active alkene polymerization catalysts are formed when a large excess of MAO is
mixed with Group 4 metallocenes (Scheme 1.11). It is thought that the excess of MAO is
required to shift an equilibrium involving a metal alkyl (20) and MAO to the active cationic
species (21). 51 Initially a fast ligand exchange takes place between Cp2MC12 and MAO to
generate primarily the monomethyl complex Cp2M(Me)Cl; excess MAO leads to
Cp2M(Me)2.52
---(- Cp2MC12 + jid—C) --IP- Cp2M(Me)C1 + MAO' [Cp2MMer[(MA0 1)Clr CH3 n
20 21.
Scheme 1.11 Formation of active metallocene/MAO species.
28
It is generally assumed that some of the Al centres in MAO have an exceptionally high
propensity to abstract a CH3 ion from Cp2M(Me)2 and to sequester it in a weakly
coordinating ion CHs—MAO-. High catalyst activity suggests that this MAO derived anion
is labile and easily displaced by the monomer prior to insertion. This weak association gives
way in the presence of substituted olefins, to olefin-separated ion pairs
[Cp2MR(olefin)]+CH3—MAO-, the presumed prerequisite for olefin insertion into the M-R
bond (where R is the growing polymeric chain). This hypothesis - that the unusually low
coordinating capability of the anion AT in [Cp2M(Me)]+A - is crucial for catalytic activity 53 -
led to the discovery of a series of highly active cationic metallocene catalysts for the
polymerization of propylene and higher alpha olefins.
The key to the synthesis and activity of such active cationic metallocenes is the utilization of
robust and poorly coordinating anions showing good solubility in inert aromatic solvents.
Early attempts to use the traditional 'non-coordinating anions', such as BF3 and PF6, were
unsuccessful as fluorine transfer to the cation led to deactivation. 54 Even for large, weakly
coordinating anions such as (C6H5)413 - and C2B9Hu- fairly strong interactions have been
observed with cationic (alkyl)zirconocene species and as a result propylene was
polymerized only at low rates, if at all. 55'56'57 A breakthrough in this regard was the
introduction of perfluorinated tetraphenylborate as counterion by Hlatsky and Turner 58 and
by Marks et al.56 These was the first well-defined metallocene catalysts capable of
polymerizing propylene and higher olefins at high rates without addition of further
activators. Similar activities for propylene polymerization were subsequently observed with
other base-free or weakly stabilized (alkyl)metallocene cations. Tetraphenyl borates, alkyl
triphenylborates and, to a lesser extent, carborane anions have found wide use in cationic
metallocene chemistry. 54,55,56,59,60
The formation of these highly active, base-free cationic metallocene alkyls is represented in
Scheme 1.12. Usually an ion pair such as [Cp'2ZrMe]f(C6F5)13 - (where Cp' is some
substituted Cp or indenyl ligand) is formed in which residual coordinative contacts between
the cationic Zr centre and its counterion exists. This contact appears to resemble those yet
unidentified interactions that stabilize an (alkyl)metallocene cation in contact with a
CH3—MAO- counterion. In both cases, a fast ligand exchange occurs between the cationic
and anionic complex moieties; most importantly, bo. th types of contact appear to be weak
HIGH ACTIVITY
Me /
Zr (C6F5)3
` Me
ZrMe2 B(C6F5)3
29
enough to allow an alpha olefin to displace the anion from its coordination site at the metal
centre. 59,61
ZrMe2 + [Bu3NFIRB(C6F5)4]
- Bu3N
Zr+ - B(C6F5)4
`Me
ZrMe2 + [Ph3C][B(C6F5)4]
MODERATE ACTIVITY
Scheme 1.12 Formation of active MAO free catalysts. 46
Of even greater simplicity are catalysts based on an (alkyl)metallocene complex containing a
Scm, Yifi or a trivalent lanthanide centre. Both Ballard 62 and Watson63 found that neutral
complexes of the type Cp2MEHR acted as single component catalysts for the oligomerization
of alpha olefins. 64,65 These catalysts are however, more difficult to prepare than the Group 4
metallocene systems.
b) Kinetics and mechanism of alpha olefin polymerization
The availability of MAO-free polymerization catalysts has allowed the study of the olefin
insertion and termination (chain transfer) steps in the catalytic cycle. Important aspects of
30
alpha olefin insertion are the regioselectivity (1,2-insertion is normally found for simple
olefins), the stereospecificity and the mode of stabilization of the insertion product
(presumed to be via weak secondary bonding interactions). In Scheme 1.13 the basic steps
believed to be involved in polymerization using metallocene-based catalysts are shown.
n R
Scheme 1.13. Basic steps in polymerization of alpha olefins using metallocene-based
catalysts.66
The reactive complex, [Cp2MR(olefin)]+, is firstly formed by the displacement of an anion
CH3—MAO- from its [Cp2MR]+ counterion by an olefin molecule. Coordination of the
alkene followed by migratory insertion generates a coordinatively unsaturated species (22)
that can repeat this propagation sequence. For alpha olefins the insertion step is typically
highly regioselective, with greater than 98% of the monomer units undergoing primarily
1,2-cis insertion. 67
Kinetic isotope studies by Bercaw 68 and Brintzinger, 69 as well as molecular modeling"
suggested that the insertion process can be accompanied by an agostic interaction between
the metal centre and one of the a-hydrogen atoms of the growing polymer chain. According
to Morokuma et al.71 this transition state for alkene insertion into [M]—R+ exhibits an
interaction of an a-H atom of the growing chain with the electron-poor metal, such that the
electron pair in the a-C-H a-bond is partly donated to the metal; this is the so-called
31
a-agostic interaction (Scheme 1.14). The stabilization of the alkyl product of insertion by an
p-agostic interaction (of a (3-C-H bond with the metal) is also suggested by Morokuma and
coworkers, although others support either a y-agostic interaction" or no agostic interaction
at all.72
+ M —CH-2 R'
CH2 2
M CHR
CH2
1 + ,,CH
2 R'
/CSR
CH2
,,H
' M•
„CHR \
H2C
transition state
p-agostic interaction a-agostic interaction
Scheme 1.14 Illustration of agostic interactions."
Several chain transfer processes have been observed in these homogeneous systems
(Scheme 1.13). In the absence of a chain transfer agent, intramolecular (3-hydrogen
elimination produces a vinylidene-terminated polymer (23). Intramolecular p-alkyl
elimination forms an allyl terminated polymer (24). 13-Alkyl elimination is commonly
observed with catalysts possessing pentamethylcyclopentadienyl (Cp*) ligands, or when the
resulting polymer chain has no [3-hydrogens and no chain transfer agents are present.
Hydrogen is an effective chain transfer agent in these systems, which produces polymers
with saturated end groups (25). Alkyl aluminium compounds are also chain transfer
agents, where transmetallation produces an aluminium-terminated polymer (26).66
32
c) Stereoregular polymers from chiral metallocene catalysts
An advantage of well-defined homogeneous catalysts over their heterogeneous equivalents
is that the ligands of soluble metal complexes can be systematically modified to study the
effect of catalyst structure on the stereoselectivity of the reaction. The symmetry of the
metallocene, as well as the nature of the cocatalyst and the polymerization temperature
determine the polymer stereostructure.
Using ethylene bridged ligands, Wild and Brintzinger 73 obtained ethylenebis(indenyl)- and
ethylenebis(tetrahydroindenyl) titanium complexes, rac-(en)(ind)2TiC12 (27) and rac-
(en)(thind)2TiC12 (28), and their zirconium analogs. The conformationally constrained
indenyl and tetrahydroindenyl give these complexes chiral structures which could be
expected to be retained even under catalysis conditions. When activated with MAO, these
ansa-metallocenes were indeed found by Ewen 67 and also Kaminsky and Ktilper, 74 to
polymerize propylene and other alpha olefins to give highly isotactic polymers.
27
28
A range of polypropylene (PP) stereostructures can be formed by using chiral
metallocene/ MAO systems. In addition to atactic PP (random orientation of methyl
groups), isotactic PP (methyl groups all lie on one side of the outstretched polymer chain)
and syndiotactic PP (alternating methyl groups), other stereoforms of PP have been
attained. In hemiisotactic PP, every second methyl group is orientated in the same
direction, whereas the methyl groups in between adopt a random orientation. Stereoblock
isotactic PP contains isotactic blocks of alternating orientation. Another form of stereoblock
PP contains isotactic blocks alternating with atactic blocks. These stereostructures and the
pro-catalysts used are illustrated in Figure 1.5.46
(C2v ) F. F
Atactic PP
33
O O
(C2)
Syndiotactic PP
Heirdisotactic PP
Isotactic PP
(:).)1\
TiPh2 (C2v)
Stereoblock isotactic PP
(Ci)
Stereoblock isotactic-atactic PP
Figure 1.5 Relationship of pro-catalyst symmetry to polypropylene stereostructure.46
)
CH3
,\ CH3
R 01 Me
M Me • H I
CH3 Isotactic polypropylene
,\ CH3
favored
H
C
R
C
34
Two mechanisms regulate stereospecific poly(alpha olefin) formation, depending on the
catalyst symmetry: enantiomorphic-site and chain-end stereochemical control. Efficient
control of the chiral catalyst over the enantiofacial orientation of the entering olefin is
contingent on the presence of a metal-alkyl segment with at least two C atoms. This
segment appears to act as a sort of lever in transmitting the effects of the P-substituents on
the orientation of the prochiral alpha olefin. Corradini et al.75 proposed that repulsive
interactions force an olefin into that .enantiofacial approach to the metal-alkyl unit which
places the olefin substituent trans to the p-C atom of the metal-bound alkyl chain. The
metal-alkyl chain, in turn, was suggested to favor an orientation in which its C(a) - C((3)
segment is in the most open sector of the chiral ansa-metallocene ligand framework (Scheme
1.15).
14 valence electron active species
ri indicates vacant coordination site
Scheme 1.15. An example of stereocontrol in metallocene/MAO polymerization.76
35
Leclerc and Brintzinger77 rationalized the stereocontrol in terms of a-agostic interactions. Of
the two alternative Zr-C(a)-C((3) orientations resulting from binding one of the a-H atoms to
the metal centre, one orientation is sterically inaccessible since it would cause the growing
chain to collide with a 13-substituent of the chiral ligand framework; the other one is
sterically unencumbered. The a-agostic interactions control the stability of alternative
transition states for inserting an olefin into a metal-alkyl bond. The a-agostic model thus
describes the re or si orientation of an inserting olefin as being controlled by the required
placement of the p-c atom of the growing chain in the more open of the two possible
positions at a rigid three-membered Zr-H(a)-C(a) ring. 78
d) Reaction control and product distribution
An important aspect in which homogeneous olefin polymerization by metallocene catalysts
differs from heterogeneous Ziegler-Natta catalysts is the narrow molecular weight
distribution of homogeneously produced polymers. Molecular weight is strongly
influenced by the metallocene catalyst and reaction conditions employed.
A number of generalizations can be made concerning metallocene/ MAO catalysts. Catalyst
activities increase with increase in the Al : Metal ratio. As mentioned previously, large
excesses of MAO (Al : Metal > 500) is usually required for acceptable activity. A decrease in
the Al : Metal ratio and an increase in temperature result in lower average molecular weight
and (for stereospecific catalysts) decreased stereoregularity. Furthermore, the order of
activity is generally Zr > Hf > Ti. In addition alternative, potentially cheaper aluminoxanes,
such as (-EtA10-),, or (-i-BuA10-)r, and other alkylaluminium cocatalysts (e.g. the mixture of
A1Me3 and A1Me2F) show inferior activity."
In short, by raising the reaction temperature, lowering the alkene concentration and
decreasing the Al : Metal ratio, oligomers may be synthesized.
36
e) Future developments
The most remarkable aspect of metallocene-based catalysts appears to be that they can
produce an unprecedented variety of polyolefins. The range of stereo- and regioregularities,
molecular weights, copolymers and cyclopolymers available with these catalysts exceeds
that accessible with classical heterogeneous Ziegler-Natta catalysts and opens new
possibilities for producing tailored poryolefin materials. 78
The extent to which metallocene catalysts will replace traditional catalysts will depend
greatly on the relative costs of making the products. The greatest challenges involved in
utilizing this new catalyst technology in an economically feasible process are reduction of
the high cost of the cocatalyst, control of polymer morphology and the adaption of the
catalysts for use in existing polymerization plants. 79 The lack of morphological control and
reactor fouling can be addressed in practical application of metallocene catalysts by their
pre-adsorption on solid supports such as alumina or silica gels. Such heterogenized
catalysts can then readily be used in existing Ziegler-Natta plants, for instance in
solvent-free slurry or gas-phase reaction systems. Supported catalysts may permit the use
of less MAO whilst still offering facile control of polymer properties by ligand variation. 46,79
iii) Other important transition metal catalysts
a) Supported reduced transition metal oxide catalysts
Besides Ziegler-Natta catalysts, one-component, that is, metal-alkyl-free transition metal
catalyst systems are also highly active in alkene polymerization. The most important of
these are the oxide-supported catalysts such as the original Phillips chromium oxide on
silica or silica-alumina. 80
Such catalysts are prepared from Ti02, V205, Nb205, Ta2O5, Cr03, Cr203, Mo03, W03, NiO
and CoO. Supports include charcoal, silica, alumina, zinc oxide or alumina-silica. An
important characteristic of these catalysts is their specificity. The Phillips catalyst, for
instance, although highly efficient for ethylene polymerization, is ineffective in the
37
polymerization of propylene. Moreover, higher temperatures and pressures are required
for catalyst activity than do the typical Ziegler-Natta catalysts. 11
Diverse chromium-based catalysts are now used in the Phillips polymerization process.
Broadly, there are two families of supported chromium catalysts: organochromium
compounds, and catalysts based on chromium oxide. Suitable organochromium
compounds include the 7c-bonded "sandwich" compounds, bis(arene)-chromium(0) and the
chromocene derivatives, allyl-chromium(II) and - chromium(III), and various alkyls of Cr(II)
and Cr(IV) such as the (trimethylsilyl)methyl or neopentyl. Compounds varying in valence
from 0 to 4 have all produced good activity once attached to a carrier. The two types of
supported chromium catalysts that are commonly employed are depicted in Figure 1.6. 81
0 0 .• Cr
* Cr —ligand I ■ ol
0 0
(a) (b)
Figure 1.6. Two types of supported chromium catalysts used for ethylene
polymerization. In one type (a), Cr is converted to the hexavalent oxide; the
other (b) employs a reactive organochromium compound. 81
b) Homogeneous nickel catalysts
The use of complex nickel catalysts for the homogeneous dimerization and oligomerization
of ethylene has by far attracted the greatest interest among the Group VIII metals.
Unfortunately only a few cases are known in which well-defined complexes have been
applied.
Tsutsui reported that dicyclopentadienylnickel was a homogeneous catalyst for the
dimerization of ethylene. 82 Barnett83 discovered that [Ni(1e-05H7)Cp] was a highly selective,
homogeneous catalyst for the conversion of ethylene to 1-butene and n-hexenes.
38
Keim84 found that complexes of type (29) were very active catalysts for the oligomerization
of ethylene. Additionally, complex 30 converted ethylene to alpha olefins possessing 99%
linearity and >95% alpha olefin content. The geometric distribution of olefins could be
tailored by pressure and addition of Ph3P. By using phosphines other than Ph3P the catalyst
could be altered towards the synthesis of linear polyethylene. 85 Complexes similar to (30)
may be formed on reacting bis(1,5-cyclooctadiene)nickel(0) with Ph2PCH2CO2H, thiolactic
acid, o-mercaptobenzoic acid or phosphorous ylides. All these systems have been patented
by Shell and they are effective in the SHOP reaction.2 9'86
H Ph
Ph ?= gNi; Ph .7 .7 Ph \3113
29 30
R1 :..._. R2 = CF3
R1 = CF3; R 2 = CH3
1.4 Conclusion
Catalytic oligomerization 5 of olefins has attracted a lot of interest because oligomers are
useful intermediates for speciality chemicals. Whereas catalytic systems for ethylene
oligomerization are well established, processes for the oligomerization of alpha olefins by
organometallic catalysts were rare before the discovery of metallocenes. Here, metallocene
catalysis is promising, because easily accessible metallocenes like Cp2ZrC12 or alkyl-
substituted derivatives can be used as very effective catalysts for oligomerization with high
regioselectivity and also high specificity in the chain-termination reaction. 87,88
However, there are limited examples of the oligomerization of higher alpha olefins with
metallocene catalysts. Furthermore, at this stage, a detailed investigation into the general
nature of the oligomerization of higher alpha olefins with these catalysts is lacking. In the
light of the mentioned advantages, a systematic study of these reactions is certainly
warranted in order to broaden the knowledge base in this field.
CHAPTER 2
DISCUSSION
2.1 Aim of research
Linear alpha olefins are versatile and reactive intermediates for the petrochemical industry.
Alpha olefin derivatives are used in surfactants, synthetic lubricants, plastics, seals in space
shuttles and for the extraction of crude oil form the ground, to name but a few. The use of
these olefins as comonomers in the polymerization of ethylene and as feedstocks for the
production of polyalphaolefins are showing the highest growth.
The oligomerization of ethylene is the primary source of linear alpha olefins today.
However, this process restricts the manufacturer to a relatively inflexible, even-numbered
distribution of alpha olefins. The Fischer-Tropsch synthesis may be tailored to produce
liquid fuel with a larger light alkene content. 89,90 The favorable surface composition of the
catalyst suppresses secondary transformations (alkene hydrogenation and isomerization),
thus ensuring selective alpha olefin formation. 91 These alpha olefins can then be recovered
from the raw synthetic fuel streams.
Sasol isolates alpha olefins from coal-derived fuel (with a value comparable to ethylene) and
can target only those alpha olefins that are the most profitable. In addition odd-numbered
alpha olefins, like 1-pentene, can be produced. These odd-numbered alpha olefins make a
host of new products accessible to the chemical industry. Hitherto, the plastics industry
could not shift from the C4 to C5 alpha olefins in polymerization or use 1-pentene as a
comonomer in the production of polyethylene due to insufficient access to 1-pentene.
Furthermore, 1-pentene is a new raw material that can be funtionalized for use in variety of
applications.
The aim of this project was to study the oligomerization of higher alpha olefins and,
specifically that of 1-pentene. A dual approach was followed, namely (i) the synthesis of a
model 1-pentene oligomer and (ii) an evaluation of the different oligomerization catalysts.
40
2.2 Synthesis of model oligomers
As mentioned previously, a large number of isomers form during the cationic
oligomerization of alpha olefins, especially if BF3/ROH is used. In order to investigate and
rationally modify this and other closely related oligomerization processes, it was necessary
to obtain information on the structures and the degree of branching of the different isomers
present in the product mixture. Unfortunately, there is no unambiguous method to predict
the structures of the major isomers in the product mixture, or to ensure that peaks
appearing in the gas chromatogram are single isomers rather than mixtures of closely
related isomers. Furthermore, there is only a limited number, if any, of the corresponding
alkane standards commercially available. Since it is impractical to synthesize all possible
isomers, some model is usually applied to predict the major isomers. The best that can be
done at present, is to determine the average "hypothetical" structure of the product. It
should be kept in mind that the properties of such a compound might not correspond to
those of the mixture of isomers. 18 One of the aims of this project was to synthesize such a
model 1-pentene oligomer. A literature search revealed that the synthesis of these model
oligomers and related compounds have almost never been attempted.
Guppies and coworkers 22 prepared tetrasubstituted methanes having three approximately
equal alkyl branches, and a methyl branch in order to determine the effect of branching on
viscosity and thermal stability. The synthesis of 11-methyl-11-octylheneicosane was
attempted and it was found that the direct route via the addition of an organometallic
reagent to a tertiary alkyl halide did not proceed in reasonable yield. A general, indirect
route to tetraalkylmethanes in high yield was established (Scheme 2.1). This procedure
involved the condensation of ethyl cyanoacetate with a ketone to afford ethyl
alkylidenecyanoacetate, followed by the 1,4-addition of a Grignard reagent to give
a-cyano-B,[3,13-trialkylproprionate. Alkaline hydrolysis and decarboxylation yields
1343,13-trialkylproprionitrile. The addition of another Grignard reagent affords a ketone and
finally the reduction of this ketone to the alcohol, dehydration and hydrogenation produce
an alkane.
41
-
Et0OCCI1CN +R1_8_R2 CN
C= (._ R2/ 'COOEt
R3MgX I-1,30+
R3
Ri_LCH2CN
R1
2
R4MgX H30+
a) Alkaline hydrolysis
4111,) Cu, 150 °C, (-04)
R3 CN
I 'Coot
R3 0 R3 R3 KHSO4
L
R1 -0 _(::H2cH(014)R4 R1-—CH=CHR4 + isomers
R1
2 R1
2 L2
Ni, H2
R3 -- C1--120-12 R4
R2
Scheme 2.1. General route to tetraalkylmethanes. 22
2.2.1 Synthetic target and strategy
The synthesis of the following range of hypothetical 1-pentene oligorners was attempted.
31
32
33
Dimer
Trimer Tetramer
42
It can be seen that the trimer (32) will correspond to the previously mentioned ideal
structure for a lubricant molecule. 16 This states that in 32 (Scheme 2.1) R and R2 should be
nearly identical in size. R 1 is generally a hydrogen and R3 can vary from a methyl group to
larger than R and R2. Due to the low carbon number of this trimer, C15, it can be foreseen
that the physical properties of this model will not be ideal for application as an automotive
crankcase lubricant although it might be useful in other applications.
R1
R2
R3
32
R1 = H
R = C5
R2 = C5—Me
R3 = C3 ('\/)
Figure 2.1. 1-Pentene model trimer.
The study of this model oligomer is, however, valuable in order to understand and analise
higher alpha olefin oligomerization reactions. The general retrosynthetic strategy to attain
the synthetic targets is shown in Scheme 2.2. 1-Bromopentane (34), 1,2 -epoxypentane (35)
and 2-bromopentane (36) are employed as starting materials in this proposed synthesis.
2.2.2 Synthesis of 1,2-epoxypentane
Oxiranes are common intermediates in organic synthesis. The facile nucleophilic opening of
oxiranes provides a convenient route for forming C-C es-bonds and in most cases it is
possible to anticipate the stereochemistry and regiochemistry of the ring-opening. Oxiranes
play an important role as 1,2-functionalized synthons.92
29
43
28
C-C bond formation
III
O
X = leaving group e.g. halide, OTs, 0
%
FG transformation
34
FG transformation
O
III
FG transformation Metal
FG transformation
OH
C-C bond formation
0
C-C bond formation
OH
O
OH OH
0
Metal
3.5
FG transformation
B r
36
Scheme 2.2. Retrosynthetic strategy.
i) Direct epoxidation
The direct epoxidation of an olefinic double bond is a simple route to oxiranes. This reaction
is achieved most conveniently by employing ni-chloroperbenzoic acid (MCPBA). Pure
MCPBA is, however, both shock-sensitive and potentially explosive, and consequently strict
44
regulations govern transport and supply. Although it would be the oxidation agent of
choice, the demise of MCPBA necessitated the use of safer alternatives in the large-scale
epoxidation of 1-pentene.
a) Magnesium monoperphthalate
1.0
Magnesium monoperphthalate (MMPP) is by comparison a non-shock-sensitive and non-
deflagrating peroxygen product and is thus much safer to use. It can be used to carry out a
variety of oxidation reactions. The epoxidations of cylcohexene and 2-methylbut-2-ene in 85
and 98% yield, respectively, are illustrative. 93,94
Attempted oxidation of 1-pentene with a stoichiometric amount of MMPP in aqueous
i-PrOH afforded 1,2-epoxypentane in low yield (estimated by GC). A variety of conditions
were employed, including different reaction temperatures (0 °C, 25 °C, 40 °C) and extended
reaction times, but the yield could not be improved beyond 30% (Scheme 2.3).
MMPP, i-PrOH/ H 20
C3' 35 37
bp 30 °C
40 °C, 16 h, 30 %
Scheme 2.3. MMPP epoxidation of 1-pentene.
This can presumably be attributed to the low boiling point of 1-pentene (bp 30 °C). Most
MMPP epoxidations are carried out at temperatures higher than 30 °C. 93,94 This would
imply that, depending on the reactivity of the alkene substrate, most reactions require some
activation energy to proceed satisfactorily. The epoxidation of a monosubstituted alkene,
which is generally less reactive, requires the use of stronger oxidation agents and more
severe conditions. 95 Low yields can be due to the high volatility of 37 and consequent
losses, even at room temperature. 1-Pentene's low boiling point and accompanying high
volatility proved to be a challenge in all consequent reactions with this substrate.
45
Urea hydrogen peroxide
The hydrogen bonded adduct, urea hydrogen peroxide (UHP), is formed when urea is
crystallized from aqueous hydrogen peroxide. 96
NH2 ---- HO—OH
38
UHP in combination with either acetic anhydride or trifluoroacetic anhydride can be used to
effect a wide range of peroxidations. Which anhydride is used, depends on the
nucleophilicity of the alkene: terminal alkenes give good yields using the trifluoroacetic
anhydride procedure. 94,97
UHP - Na2HPO4 0 „..-%
'.. (CF3CO)20 37 35
40 °C, 18 h, 25 %
by 30 °C
Scheme 2.4. UHP epoxidation of 1-pentene.
Attempted epoxidation of 37 by peroxytrifluoroacetic acid generated from the anhydride
and 38 in the presence of a phosphate buffer, proceeded in a low yield (Scheme 2.4). This
can again be attributed to the low boiling point of 37, since standard UHP epoxidations are
carried out in refluxing methylene chloride (bp 40 °C).
Dimethyldioxirane
Murray et al.98 have shown that solutions of dimethyldioxirane (39) in acetone (0.1M) can
be obtained through low temperature distillation of 2KHSO5.KHSO4.K2SO4 and acetone
reaction mixtures. A solution of 39 in dry acetone reacts readily with disubstituted alkenes
such as cis-3-hexene and trans-3-hexene to furnish the corresponding epoxides in nearly
quantitative yield. 99
39
46
Murray's procedure for making and isolating 39 is not always practical for large-scale
synthesis. Edwards and Curci, 180 as well as Murray98 have reported a method for forming
an excess of 39 in situ under phase-transfer conditions. Both the improved in situ generated
and isolated dimethyldioxirane were used to epoxidise 1-pentene in moderate yield (Scheme
2.5).
o—o
X (2 equivalents)
acetone
RT, 2 h, 40 % 37 35
2KHSO5.KHSO4.K2SO4 (3.2 eq)
acetone n-Bu4NHSO4, CH 2C12
NaHPO4 (aq) buffer
pH– 8, 0 °C, 50 %
37
35
Scheme 2.5. Dimethyldioxirane epoxidation of 1-pentene.
The consequent isolation of 35 proved to be more problematic. Both atmospheric and
vacuum distillation was not successful in separating the epoxide from the solvent, acetone.
This epoxide 35 was found to be unstable in extended atmospheric distillation. Separation
by vacuum distillation at a lower temperature was unsuccessful because the boiling points
of the epoxide and acetone were in the same range.
ii) Indirect epoxidation
Although several other oxidation reagents are generally used for the epoxidation of
alkenes,92 it was decided to abandon the direct epoxidation route. The indirect route via a
halohydrin is a viable method for producing epoxides which formally uses the reaction with
a halogen to effect the oxidation.
1-Pentene was transformed into the iodohydrin (Scheme 2.6) or bromohydrin (Scheme 2.7)
derivatives through the in situ generation of the hypohalous acids (HOX) from H5106 or
NaBrO3 in the presence of a suitable reducing agent.101
a) Iodohydrin
47
H5106
OH
I 37
NaHS03
0 °C to rt, 4 h, 85 % 40
Scheme 2.6. Formation of iodohydrin.
Since the reaction is exothermic, the reducing agent (NaHSO3 as a 1M solution) was added
dropwise to a mixture of H5106 and 37 in CH3CN/H20 at 0 °C in order to minimize losses of
1-pentene. After the addition was completed the reaction mixture was allowed to warm to
room temperature and to proceed for another 4 hours, after which work-up afforded a
yellow-brown liquid in 85% yield. The liquid was analysed by GC and GC/MS which
indicated the presence of two products in a 1 : 2 ratio.
The MS data of all organic compounds were determined by using a quadrupole ion trap
mass spectrometer as detector for a gas chromatograph. This spectrometer employs three
electrodes - two end-cap electrodes that are normally at ground potential and between them
a ring electrode to which a radio frequency (rf) voltage, often in the megaHertz range is
applied - to generate a quadrupole electric field. 102
The two products had identical MS data. The EI spectra showed a weak ion at m/z 213
which corresponds to hydride abstraction from 40. This phenomenon (the presence of a
M+ - 1 peak) was observed in the mass spectra of all'of the organic compounds. The loss of
water to yield C5H9I+ with m/z 196, of an iodide ion (m/z 127) to furnish an ion with m/z
87 and of both to form an ion with m/z 69 were observed. The following fragments were
also present: C4H7+ (m/z 55), CH31+ (m/z 142), C3F17+ (m/z 43) and C2H401+ (m/z 171). The
1H and 13C NMR spectra also indicated the presence of two compounds. The main
compound could be identified as the desired 1-iodo-2-pentanol. The 1 H-NMR spectrum of
the mixture showed the expected resonances of one methyl group (5 0.82, t, J 6.8 Hz), 4 alkyl
protons as two overlapping multiplets at 6 1.27 - 1.44, the hydroxylic proton as a broad
singlet (6 2.33) and the proton on the hydroxylic carbon as a 1-proton multiplet (6 3.40 -
3.43). The iodohydrin moiety was recognized by a characteristic AB system indicative of the
diastereotopic protons lb-H (8 3.12, jib,2 6.6 Hz, Jas 10.2 Hz) and la-H (5 3.36, Jia,2 3.7 Hz, JAB
48
10.2 Hz). The 13C NMR of the mixture also confirmed the presence of 40 as the main
compound in the mixture. The resonances of the methyl carbon (6 13.20, C-5), two alkyl
carbons (6 17.83, C-4; 6 35.21, C-3), iodo-substituted carbon (6 20.09, C-1) and hydroxylic
carbon (5 68.11, C-2) were present.
OH
41
It can be argued that the second compound is a regioisomer (41) or conformational isomer of
40, but no firm evidence exists. Although the literature 101 states that the iodohydroxylation
of terminal alkenes occurs with high regioselectivity in a Markovnikov fashion to afford the
1-iodo-2-hydroxy derivative, we consider the alternative, i.e. the formation of
conformational isomers as unlikely.
b) Bromohydrin
OH NaBrO3, pH — 1
Br NaHSO3
35
0°C to rt, 4 h, 80 % 42
Scheme 2.7. Formation of bromohydrin.
A similar procedure to the iodohydrin formation was used to generate 42 in 80% yield.
NaBrO3 could be used in an acidic (pH = 1) medium in the presence of NaHSO3 to form the
hypobromous acid. The GC and GC/MS analyses indicated the presence of three products
with identical MS data in a 1 : 2 : 4 ratio. Both 1 H and 13C NMR also showed the presence of
additional products with closely related structures.
By following the same reasoning as before it can be postulated that these products are
regioisomers. Again this could not be proven without any doubt. The analytical data did
confirm that the major component in the mixture was 1-bromo-2-pentanol.
The molecular ion was absent in the EI-MS and CI-MS spectra of 42. The loss of water from
42 gave two prominent peaks at m/z 148 and m/z 150 corresponding to fragments
49
containing the two bromine isotopes; the loss of a bromide ion from this ion (m/z 69)
furnished an intense peak. Other weak ion peaks could be attributed to the fragmentation
of 42. The 1H NMR spectrum indicated the presence of a methyl group (6 0.90, t, J 6 Hz)
and 4 alkyl protons as two overlapping multiplets (6 1.34 - 1.57). The hydroxylic proton was
represented by a broad singlet 6 2.20 (confirmed by D20 exchange) and the proton on the
hydroxylic carbon as a multiplet 6 3.74 - 3.76 (confirmed by proton decoupling). The
diastereotopic halide protons were again characterized by an AB spectrum: lb-H (6 3.38, J1b,2
7.1 Hz, JAB 10.3 Hz) and 1a-H (6 3.53, Jia,23.3 Hz, JAB 10.3 Hz). 13C NMR clearly identified 42
as the main product in the mixture. The data confirmed the presence of the methyl carbon
(6 13.80, C-5), two alkyl carbons (6 18.73 and 6 37.07, C-4 and C-3), bromo-substituted carbon
(6 40.52, C - 1) and the hydroxylic carbon (6 70.72, C - 2).
c) 1,2-E poxypentane
Epoxides can be formed from halohydrins by treatment with a base to generate the alkoxide.
The alkoxide should be anti to the adjacent halide in order to allow the substitution of the
halide to give the epoxide via a SN2 process (Scheme 2.8).
OH
„X R
B-
Scheme 2.8. Illustration o f the preparation of an epoxide from a halohydrin.
The epoxide (35) was prepared without any solvent by adding the halohydrin dropwise to a
large excess of finely powdered NaOH (Scheme 2.9). The reaction was exothermic and 35
could be distilled under vacuum as it formed; this method circumvented the problem of
separating the epoxide from a solvent. The epoxide isolated in this manner was found to be
pure (> 95%) by GC analysis. The boiling point of this compound was determined as
67 - 69 °C. The total yield for the synthesis of the epoxide from 1-pentene is approximately
65% which is higher than the yields obtained in the direct route.
50
OH Base
40
35
OH
Br Base
42 35
0 80%
O 75%
Scheme 2.9. Preparation of 1,2-epoxypentane.
The EI-MS of 35 afforded a weak peak at m/z 85 which corresponds to hydride abstraction
from 35. Further fragments observed were the loss of H2O (m/z 68) and a CH3 fragment
(m/z 71). The 1H NMR of the epoxide in CDCb indicated the presence of two compounds
of which one could be the epoxide. The GC analysis of the product in CDC13 showed that a
new product formed: two peaks were observed instead of one. It was thought that the other
originated from the addition of residual DC1 (HC1) in the CD03 to the epoxide. Due to the
large extent of peak overlap the spectrum could not be assigned with certainty and the
NMR analyses were repeated in deuteurated pyridine in which only one product was
observed. The 1H NMR confirmed the presence of the methyl group (6 0.94, tt, J 7.5 and 0.9
Hz), two alkyl groups as a 4-proton multiplet (6 1.42 - 1.49) and the secondary epoxide
proton (6 2.80 - 2.84, m). The diastereotopic protons lb-H (6 2.38, Jlb,2 2.77 Hz, JAB 5.4 Hz)
and la-H (6 2.94, ,la,2 4.2 Hz, JAB 5.4 Hz) on the terminal carbon atom again yielded an AB
spectrum. The 13C NMR spectrum showed the characteristic resonances of C-5 (6 14.03), C-4
and C-3 (6 19.63 and 6 34.90) and the two epoxide carbons C-1 and C-2 (6 46.21 and 6 51.54).
2.2.3 Synthesis of the dimer analogue
The reaction of an epoxide with a carbon nucleophile generates a new carbon-carbon
6-bond, and is a versatile protocol for the preparation of a variety of alcohols provided that
the ring opening takes place regioselectively. Owing to the polarity and the strain of the
three-membered ring, epoxides undergo reaction with a number of organometallic
reagents.92
51
A summary of organocopper reagent types is given in Table 2.1. 103 Addition reactions of
these species are sometimes curtailed owing to the competing reactions arising from the
Lewis acidity or basicity of the organometallics used. 104,105 The carbon nucleophiles of choice
are organocuprates, because Grignard reagents sometimes result in concomitant formation
of side products arising from the substitution by a halide ion.104,105,106 Higher order mixed
organocuprates of the general formula R2Cu(CN)Li are superior to R2CuLi. 107 Although the
nucleophilic opening of oxiranes by Grignard reagents has been limited in scope due to side
reactions, it has been shown that the reaction proceeds smoothly in the presence of copper
catalysts. 108
Table 2.1 Types of organocopper reagents 103
Type Preparation
Copper-catalysed Grignard reagents
Mono-organocopper reagents
(organocopper reagents)
Homocuprate reagents
(cuprate or Gilman reagents)
Heterocuprate reagents
Higher order homocuprates, e.g.
Rn,+„CurnLin (m + n > 2)
Higher order heterocuprates,
e.g. higher order cyanocuprates
RMgX (or RLi) + < 25 mol% CuX
RM + CuX —> RCu
2 RM + CuX —> R2CuM
RM + CuZ --> RCu(Z)M
3 RLi + 2 CuI —> R3Cu
5 RLi + 3 CuI -3 R5Cu3Li2
2 RM + CuCN —> R2Cu(CN)M2
Species Comments
R = alkyl, aryl, vinyl, etc.
CuX is usually CuI or CuBr•SMe2
CuZ is usually CuSPh or CuO-t-Bu
RM is usually RLi or RMgX
Ether or THE used as solvent
Reactions usually carried out at - 78 ° C to 0 ° C
under N2 or Ar
Solubilizing ligands [e.g. Me2S, (EtO)3P, Bu3P] are
often added
Addition of Lewis acids (e.g. BF3, A103) or
Me3SiX can enhance activity
Copper-catalysed Grignard reagents are still of great utility owing to their ease of
preparation and the normal requirement of only a modest excess of organometallic reagent.
They are considered as the first choice of reagent for nucleophilic addition and
stoichiometric reagents is only investigated if problems are encountered.103
52
Copper-catalysed Grignard openings of epoxides are commonly used reactions. 109 Some
model C-C bond formation reactions were attempted using this methodology; the results are
summarized in Table 2.2.
Table 2.2 Summary of model C-C bond formation reactions
Grignard reagent a
Epoxide Reaction Product c Yield
conditions b
MgBr
MgBr
0 °C, 2h
/KT 10% CuId -30 °C to rt, 5.5 he
Mixture of more than 10 products
Mixture of 4 products
OH 79%
OH 10% CuCN f
99% MgBr -30 °C to rt, 5.5 he
O g 10% CuCNIf 30 °C to rt, 5.5 he
OH
95%
Grignard reagents were prepared by standard procedurello from 1-bromobutane as a 0.8
mol/dm3 solution in dry diethyl ether.
Reactions were carried out under positive N2 pressure in flame dried glassware using dry,
oxygen-free THF as solvent.
Product mixtures were analysed by GC and GC/MS and purified by Kugelrohr distillation prior
to NMR analysis. Analytical data confirmed the structure and constitution of the alcohols.
Cul was prepared from CuSO4 and NaI.
The Cu(I) salt (10 mol %) and the Grignard reagent (1 equivalent) were stirred at -30 °C for 30
minutes. An equimolar amount of epoxide in THF was added dropwise to the reaction mixture
at -30 °C and the mixture was kept at this temperature for 1 hour. The reaction mixture was then
allowed to warm to room temperature over a period of 4 hours.
Obtained commercially.
Prepared from 1-iodo-2-pentanol and determined by GC analysis to be 95% pure.
53
The results confirmed the expected side reactions for the addition of a Grignard reagent to
an epoxide. The facile addition of the Grignard reagent in the presence of catalytic Cu(I)
salts to the oxirane moiety is in accord with the literature. 108 A high quality of the Cu(I) salt
is of utmost importance to the success of the subsequent reaction - this is also reflected in the
model studies. The prepared Cul can contain impurities such as iodine and the
accompanying product of disproportionation Cu(II). 109 The higher yield with CuCN can
possibly be attributed to the higher purity of the commercially obtained salt.
Considering the results obtained with the model studies, the formation of the dimer carbon
skeleton was attempted using copper-catalysed Grignard reagent ring opening of
1,2-epoxypentane (Scheme 2.10). The Grignard reagent was easily generated by standard
procedurello from 2-bromopentane as a 0.8 mol/dm 3 solution in diethylether. Two closely
related products (with identical MS data) were detected in a 1 : 1 ratio by GC and GC/ MS
(Figure 2.2).
OH MgBr
10% CuCN
-30 °C to rt, 5.5 h, 70%
43 35 44
Scheme 2.10. Formation of Cio alcohol.
Initially it was envisaged that these products formed as a result of a loss of regioselectivity
thus forming the following byproduct:
OH
45
100z
TOT -
500 550 8.33 9.16
Figure 2.2. Gas chromatogram of two alcohol products.
It is known that the nature of the organometallic reagent can have a marked effect on the
regioselectivity of the reaction. 103,109 Different organometallic reagents were generated and
their addition to 35 studied in order to observe their influence on the product distribution.
These results are summarised in Table 2.3 and it was clear that the product distribution was
not influenced by using different nucleophilic species. Thus, the possibility that the
products were regioisomers was unlikely. It was proposed that the two products were
diastereomers resulting from the generation of two stereogenic centres in 44. It should be
noted at this stage that although the presence of four stereoisomers can be expected,
enantiomeric pairs could not be observed by the analytical techniques employed.
Consequently although enantiomers certainly form only the formation of diastereomers are
discussed.
54
44
55
Table 2.3 Influence of different organometallic nucleophiles on the following general
reaction:
OH Metal
44
35
Organometallic
reagenta
Mg Br
Reaction
conditions b
No additive
Product
distribution c
Mixture of more than 10 products
Yield
Mg Br
/\) 10% CuCN 1 : 1 70 %
30% CuCN 1 : 1 69 %
60% CuCN 1 : 1 69 %
Cu(CN)MgBr
/\) No additive 1 : 1 72%
Cu(CN)(MgBr)2
)2
No additive 1 : 1 75%
The organometallic reagents were prepared according to standard procedure.lmim
The reaction mixture was kept at -30 °C while 35 dissolved in THE was added dropwise. The
reaction mixture was kept at this temperature for 1 hour and then allowed to warm to room
temperature over 4 hours.
Product distribution was determined by GC/MS (approximate ratios are given).
These stereogenic centres form as a result of the stereospecific trans opening of the epoxide
by two possible nucleophilic approaches (Scheme 2.11)
H IMe
(c),
H Me
56
Scheme 2.11. Formation of two stereogenic centres.
The mixture was purified by Kugelrohr distillation (70 °C, 0.2 mrnHg) and an inseparable
mixture of the two diastereomers in a 1 : 1 ratio was obtained. These two products had
identical MS spectra. The EI-MS indicated that the molecular ion of 44 was very unstable
and fragmentation occured to a large extent. This is characteristic of long chain alkyl
compounds (especially branched ones) and the chemical ionization technique was
subsequently used in order to observe the molecular ion. The CI-MS of 44 afforded a very
weak molecular ion at m/z 158 which is in agreement with a molecular formula of C121 -1220.
Further fragmentation included the loss of a water molecule (m/z 140) and the subsequent
typical fragmentation of a saturated hydrocarbon, i.e. a series of m/z differing by 14 amu
(corresponding to a difference of CH2). The 1 H NMR spectrum of the mixture indicated the
presence of 9 protons as three overlapping triplets (5 0.83 - 0.94), 10 alkyl protons as a
multiplet centred at 5 1.36 overlapping with a 1-proton multiplet centred at 5 1.67, the
hydroxylic proton (5 2.5) and the hydrogen on the alcoholic carbon (5 3.65 - 3.71, m). The
13C NMR spectrum corroborated the presence of two similar compounds; almost all peaks
were duplicated and the chemical shift difference was more pronounced in proximity to the
stereogenic centres. The DEPT technique was used to assist in the assignment of the carbon
atoms. The analysis confirmed the presence of three methyl carbons [5 14.11 (2 C), 14.31 and
14.36, C-1 and C-9; 5 19.26 and 20.37, C-7'], five CH2 carbons, all duplicated (5 18.37 and
18.85, C-2; 5 19.95 and 20.03, C-8; 5 38.99 and 40.02, C-7; 5 40.22 and 40.59, C-3; 5 45.07 and
45.36, C-5) and two CH carbons also in duplicate (5 28.97 and 29.41, C-6; 5 69.42 and 69.81,
C-4).
57
It was assumed that since there were no anomalies in the analytical data two diastereomers
of the expected product 44 did form and no skeletal rearrangement took place.
2.2.4 Formation of the trimer analogue
i) Bromination of the Cio alcohol
Alkyl bromides are useful precursors for organometallic compounds and can be formed by
the substitution of a hydroxy group. HBr is generally used but for secondary alcohols a SN1
and SN2 mechanism can operate simultaneously and competitively. The intermediate in the
SN1 pathway has the tendency to isomerize to a more stable cation which gives rise to
isomeric products.
Promotion of an SN2 displacement mechanism, and hence greater regioselectivity may be
effected by using PBr3 or PBrs. The use of phosphorus reagents, however, avoids
rearrangement only to some extent in secondary alcohol substrates. Other regioselective
brominating reagents are chlorotrimethylsilane/lithium bromide and hexamethyldisilane/
pyridinium perbromide. 111 Reaction with Ph3P and Br2 or CBr4 as well as the Mitsunobu
procedure (Ph3P, diethyl azodicarboxylate, LiBr) 112 allows the formation of alkyl bromides
under mild conditions.
A model secondary alcohol, 2-heptanol (prepared in the study of C-C bond formation
reactions, Table 2.2) was brominated using PPh3/CBr 4 (Scheme 2.12). Two equivalents of
PPh3 were added portionwise to a mixture of one equivalent of the alcohol and two
equivalents of CBr4 at 0 °C in CH3CN. The reaction mixture was then refluxed for 30
minutes and the resultant product after work-up was analysed with GC/MS. The alcohol
45 was converted quantitatively to the corresponding alkyl bromide 46, but separation from
the reaction byproducts such as phosphines and bromomethanes proved difficult.
OH
58
45
Ph3P / CBr4
CH3CN
0 °C to reflux, 1 h 46
Scheme 2.12. Bromination of 2-heptanol.
The branched alcohol 44 was also brominated by using this procedure in order to ascertain if
this product could be isolated (Scheme 2.13). The purification was unsuccessful.
Alcohol 44 was subsequently brominated with PBr3 using standard procedure. 113,114,115 PBr3
(0.33 equivalents) was added dropwise to the neat alcohol (44) while the temperature was
kept between -5 °C and 5 °C. The reaction was allowed to warm to room temperature and
to proceed for eight hours, after which work-up and purification by Kugelrohr distillation
afforded the product mixture as a clear liquid in 80% yield.
OH Ph3P / CBr4
CH3CN
0 °C to reflux, 1 h 44
47
Scheme 2.13. Bromination of alcohol 44.
Both GC and GC/MS indicated that carbocation rearrangement did take place and the
product contained 10% of an isomeric alkylbromide (lowering the yield of 47 to
approximately 72%). This isomer formed in the same proportion under a variety of
controlled reaction conditions (e.g. under nitrogen, rigorous drying and purification of
reagents, in dry diethyl ether at -30 °C, the removal of excess HBr by passing a N2 stream
through the solution and shorter reaction times). Furthermore, this byproduct could not be
separated from the desired product and it was decided to use the mixture in further
reactions.
The analysis also indicated that as expected, two diastereomers of 47 were present in a 1: 1
ratio. The CI-MS (and El-MS) data of all three compounds were the same. The molecular
ion (m/z 221) was absent, but the ion corresponding to the loss of HBr (m/z 141) and the
59
typical alkane fragmentation (repetitive loss of 14 mass units) were present. 1H NMR
indicated the resonances of 47 as the main product: the three methyl groups overlapped as a
9-proton multiplet 6 0.83 - 0.93, the secondary and tertiary protons overlapped as a 11-
proton multiplet 6 1.17 - 1.92 and the bromide proton was represented by a 1-proton
multiplet centred at 6 4.09. The 13C NMR spectrum confirmed the presence of three
compounds of which two were diastereomers of 47. The DEPT technique was used to assist
in the assignment of the peaks of these diastereomers; almost all peaks were duplicated.
The resonances of three methyl carbons (6 13.47, 6 14.25 and 14.30, C-9; 6 18.67 and
19.73, C-7'), the five methylene carbons (6 19.93, 6 20.70 and 20.79, C-8; 6 38.13 and
39.66, C-7; 6 41.21 and 42.08, C-3; 6 46.51 and 46.99, C-5) and two CH carbons (6 30.85, C6; 6
56.50 and 56.89, C-4) were found. The peaks representing the third compound were in the
same range as the peaks of the diastereomers thus confirming the theory that it was a
structural isomer.
ii) The synthesis of the C15 alcohol
a) Grignard formation
Br
47
Mg
MgBr
diethyl ether
48
Scheme 2.14. Generation of the Cio Grignard reagent.
Attempted synthesis of the secondary Grignard reagent 48 by standard procedure 11° in
diethyl ether failed. No active organometallic species formed, even under tightly controlled
reaction conditions (e.g. under a nitrogen atmosphere, rigorous drying and purification of
alkyl bromide and solvent), extended reflux (up to 7 days), use of a higher boiling solvent
(THF, dlisopropyl ether) and a large excess of magnesium. The alkyl halide 47 might be
unreactive because it is a secondary alcohol and the branching and long chain nature may
cause steric hindrance.
60
It is known116 that simple organic halides are sometimes rather unreactive in the preparation
of their Grignard reagents. Modifications have been developed to enable the formation of
these difficultly accessible Grignard reagents.117 ,116 These includes the use of higher boiling
and more strongly coordinating solvents, 118 catalysis with transition metal salts / 119
physicall 2o and chemical121 methods to produce highly activated magnesium. Among these
modifications, chemically activated magnesium has been widely used.
The addition of catalytic amounts of iodine to the reaction mixture, the use of stoichiometric
iodine-activated magnesium (Mg/ I2), finely divided Mg (formed by stirring Mg turnings
under N2 for one day) 122 and magnesium activated by acidic wash did not result in the
formation of the Grignard reagent.
If the R group remains the same the reactivity of organic halides in the formation of
Grignard reagents is: RI > RBr > RC1. 1111 Consequently the bromide 47 was converted into
the alkyl iodide 49 by refluxing with 1.25 equivalents of NaI in acetone (Scheme 2.15). This
iodide was also unreactive towards magnesium.
Br Nal
acetone
44 reflux, 6h, 80% 46
Scheme 2.15. Preparation of Clo iodide.
The entrainment procedurel 23 was successful in generating the Grignard reagent 48 in
approximately 60% yield. In this procedure all the "inert" halide 47 was added to
magnesium (> 2 equivalents) in dry diethyl ether and the resulting suspension refluxed.
The entrainment reagent ethylene dibromide (stoichiometric amount) dissolved in dry
diethyl ether was then added dropwise. The auxiliary halide keeps the magnesium clean
and forms ethylene and magnesium dibromide, thus activating the magnesium for reaction
with the inert halide without introducing a second Grignard rPagent.
49 35
MgBr
10% CuCN
-30 °C to rt, 5.5 h, 70%
61
b) Carbon-carbon bond formation
The nucleophilic ring opening of 35, using the same methodology as before proceeded
smoothly to afford 50 as a clear oil in 70% yield (Scheme 2.16). Three stereogenic centres are
present in alcohol 50 and thus a total of eight stereoisomers can be expected of which the
four diastereomers were observed.
OH
Scheme 2.16. Formation of C15 alcohol.
The product was purified by Kugelrohr distillation (130 °C, 0.15 mm Hg) and analysed by
GC/MS. The chromatogram confirmed the presence of the diastereomers as well as some
structural isomers of 50. The constitutional isomer of 47 could also have formed a Grignard
reagent and reacted with 35 to yield another isomer of 50. These isomers represented less
than 5% of the total reaction mixture. The different isomers could not be separated and thus
alcohol 50 was not purified any further.
The molecular ion of 50 (m/z 228) was absent in the CI-MS and the loss of water (m/z 210)
gave rise to a weak ion peak. This can be attributed to the highly branched nature of 50
which causes the molecular ion to fragment easily. The expected fragmentation to form
clusters of peaks 14 mass units apart was observed. Other fragments within each cluster
corresponding to additional losses of one or two hydrogen atoms were observed. Cleavage
to yield secondary carbocations is responsible for the relatively high intensities of a C101 -119+
(m/z 139) and a C5F111+ (m/z 71) fragment. The 1 H NMR spectrum verified the presence of
4 methyl groups resonating as overlapping triplets 8 0.78 - 0.89, 8 methylene groups
resonating as a multiplet centred at 8 1.23 overlapping with 2 CH protons (5 1.63, m). The
resonances of a hydroxylic proton (6 2.33, s) and the proton on the alcoholic carbon (6 4.52,
m) were also observed. The 13C NMR spectrum established the presence of the four
62
diastereomers of 50, some peaks were quadrupled. The DEPT technique was used to assist
in the assignment of the carbon atoms. The resonances of 3 methyl carbons as exceptionally
broad peaks were found (6 14.13, 14.40 and 14.48; C-1, C-9' and C-11). The broadening of
the peaks are due to the presence of diasteromeric carbons resonating at nearly identical
chemical shifts. The four diastereomer peaks of the methyl carbon on the stereogenic centre
(8 19.81, 19.91, 20.00, 20.35; C-9"), the eight groups of methylene carbons C-2 (6 20.07, 20.13
and 20.21), C-10 (8 21.09, 21.17 and 21,24), C-8' (6 32.56, 32.71, 32.97 and 33.00), C-7' (8 33.21,
33.39, 33.42 and 33.66), C-9 (6 37.88, 38.36, 38.40 and 38.62), C-7 (6 38.64, 38.87 and 38.98),
C-5 (6 39.66, 39.78 and 39.87), C-3 (6 40.11, 40.15, 40.27 and 40.41) and the three CH carbons
C-8 (6 30.14, 30.22, 30.27 and 30.34), C-6 (5 36.10, 36.22, 36.29 and 36.34) and C-4 (6 67.60,
broad) were observed.
iii) Synthesis of a model trimer
Although the C15 model trimer could be formed from alcohol 50 by the elimination of the
alcohol group followed by reduction, this was viewed as a cumbersome route. It was
envisaged that alkane 32 could be formed by the displacement of a secondary halide by an
organometallic species (Scheme 2.17).
Metal
47
32
Scheme 2.17. Preparation of model trimer.
This C-C bond formation was attempted by using a Grignard reagent as the organometallic
species. This Grignard reagent was prepared by standard procedureno from
1-bromopentane as a 0.8 mol.dm- 3 solution in diethyl ether. The copper(I) catalysed
nucleophilic displacement of the halide did not proceed at all. Neither did reaction with the
higher order organocuprate nor catalysis of this reaction with a Lewis acid (BF3.Eb0).
63
It is known that most higher order cyanocuprates are very effective in the displacement of
secondary bromides. 124 The role of the gegenion in the displacement reaction has been
studied. 125 The replacement of the organolithium with an organomagnesium reagent
usually dramatically decreases the rate of the reaction.
An organolithium reagent was formed by standard procedure 126 from the primary alkyl
halide and lithium pellets. From this organolithium a higher order cyanocuprate was
formed and reacted with 47 at -50 °C. The reaction was allowed to warm to 0 °C and
proceed for 6 hours after which work-up afforded a clear oil in 50% yield.
The GC/ MS confirmed the formation of two diastereomers of 32 in a 1: 1: ratio as the main
products. A byproduct representing 5% of the reaction mixture was observed. The CI-MS
furnished an extremely weak ion peak at m/z 211 which corresponds to hydride abstraction
from 32. Furthermore, the characteristic alkane fragmentation i.e. a series of m/z differing
by 14 mass units was observed. The 1H NMR spectrum confirmed that no functional groups
were present, only alkane peaks were detected. The resonances of the main classes of
protons were found, namely, the four methyl groups (6 0.83 - 0.89), nine methylene groups
(centred at 8 1.22) overlapping with the two CH groups centred at 5 1.29. The 13C NMR data
corroborated to some extent that two diastereomers were present; only some of the peaks
were duplicated. The presence of the following alkane carbons were established using the
DEPT technique: 4 methyl carbons [6 14.06 (3 C) and 14.49; C-11, C-1, C-9 and C-5']; 9
methylene carbons (6 22.67 - 36.08, some peaks overlapped); and two CH carbons [6 32.45, (2
C), C-4; 6 37.15 and 37.36, C-6].
2.2.5 Attempted synthesis of model tetramer
In order to synthesise the model tetramer (33) by the same protocol as before, the alcohol
group in 50 had to be converted into a good leaving group. This C15 alcolhol proved to be
extremely unreactive and prone to elimination as exemplified in Table 2.4.
The unreactivity of this secondary alcohol lies in part in the long chain branched nature of
the alkyl substituents. This functional group is sterically hindered and hence a low
64
reactivity was observed. Conversely, the 13-hydrogen is more accessible and elimination to
form an alkene was favored. In other words, the reaction proceeding via a SN1 pathway
yields an unstable carbocation while the reaction via a SN2 pathway is sterically hindered.
The functionalization of the substrate and the attempted synthesis of the tetramer using this
route was abandoned.
Table 2.4: Attempted functional group transformation
General reaction :
OH
50
X
Reaction conditions
Result (leaving group)
Br PBr3 (0.33 eq) 0 °C to rt, 16 h No reaction 80 °C, 2 h No reaction 80 °C, 16 h No reaction
PBr3 (3 eq) 140 °C, 12 h Elimination
I
Mitsunobu conditions PPh3, imdazole, 12 100 °C,16 h Elimination rt, 1 h Elimination
OTs TsC1 (1.5 eq), pyridine 0 °C, 1h
TsC1 (1.5 eq), NaH (1.2 eq)
No reaction
Tosylate formed, product unstable and eliminated
65
2.3 Catalyst survey
Although the oligomerization of other alpha olefins has been studied, there is limited
knowledge concerning the oligomerization of 1-pentene with common catalysts.
Furthermore, there are almost no systematic studies on the oligomerization of higher alpha
olefins that enables the comparison of different catalysts and alpha olefin activity. In this
project, some of the main oligomerization catalysts were evaluated for the dimerization and
trimerization of higher alpha olefins and especially of 1-pentene.
2.3.1 Cationic oligomerization
The first and most common of these catalysts is the BF 3/ROH cationic oligomerization
system. BF3/n-BuOH was employed to produce 1-pentene oligomers and this reaction was
used as standard to compare the different catalysts.
The reaction was carried out in a reproducible fashion by bubbling BF 3(gas) rapidly through
a mixture of 1-pentene (2.7 equivalents), heptane (3 equivalents) and n-BuOH (0.05
equivalents) for 5 minutes, the gas flow was then reduced to such a rate that the pressure in
the system remained approximately constant. The temperature of the reaction mixture rised
initially to ± 40 °C after which the reaction was allowed to proceed for another 30 minutes
and then quenched with aqueous NaOH. Work-up afforded a light brown oil in high yield.
This oil was hydrogenated over 1% Pd/C (10% m/m) with H2 (40 - 50 bar) for 2 hours. The
degree of unsaturation was determined by standard procedure 127 and it was found that the
product was completely hydrogenated. Both this product and the unsaturated oil were
analyzed by GC and GC/ MS. Dimers through to pentamers were formed and the general
distribution is summarized in Table 2.5. The product distribution peaked at the trimer
which is in accord with results obtained for other feedstocks. At higher temperatures the
distribution shifted to the higher oligomers but still peaked at the trimer.
66
Table 2.5. Product distribution of BF3/n-BuOH oligomerization of 1-pentene.
Oligomer Cu C15 C20 C25
Oh 15 47 23 15
The GC/MS analyses indicated that, as expected, individual fractions consisted of a large
number of isomers (Figure 2.3) - even more than can be explained in terms of the classic
cationic mechanism. It has been found that product quality not only depends on oligorner
distribution, but also on the specific structures present in the various oligomers. 26 Due to
the formation of stable secondary ions, the mass spectra of branched hydrocarbons should
include intense peaks that deviate from that expected for straight-chain hydrocarbons. This
phenomenon provides a convenient method to determine the branching pattern of the
isomeric mixtures formed in the BF3/n-BuOH oligomerization of 1-pentene. It was,
however, found that although each oligomer had a unique fragmentation pattern, it was
impossible to assign specific structures unambigUously to the isomers. The GC/MS
analyses did indicate that, as expected, branched oligomers were formed, even though the
different isomers could not successfully be distinguished, because of the complexity of the
mixture.
100v.
TOT-
600 000 1000 1200 1400 10.00 13.33 16.66 19.99 23.33
Figure 2.3. Gas chrornatrogram of 1-pentene trimer.
67
The trimer (C15) was isolated by high vacuum distillation (0.20 mmHg): dimer 80 - 85 °C,
trimer 85 - 115°C, and tetramer 125 - 145 °C. The isolated, hydrogenated trimer fraction was
used in viscosity studies of 1-pentene oligomers, which will be discussed later.
2.3.2 Radical oligomerization
Almost all organic peroxides are photo- and thermally sensitive because of the facile
cleavage of the weak oxygen-oxygen bond; AH = -125.6 to -184.2 kJ/mo1. 128 The initially
generated species, the free radicals, are reactive intermediates with very short lifetimes, e.g.
t112 = 10-3 S.129 As a consequence organic peroxides are used as initiators for many different
free-radical reactions, especially in commercial polymerizations.
The activity of different organic peroxide radical initiators in the oligomerization of 1-
pentene was investigated. These include: dibenzoyl peroxide, one of the first peroxides
used in the industry; 26 di-tert-butyl peroxide, patented by Mobil for the oligomerization of
lower oligomers from other processes; 27 and dicumyl peroxide, reported in a French patent
application for the oligomerization of C9 - Cu alpha olefins. 13°
Unless otherwise stated, the reactions were performed under pressure in an autoclave with
the catalyst, monomer and a small amount of heptane (as internal standard); initiation
occurring by thermal homolysis. The results are summarized in Table 2.6. GC and GC/ MS
analyses of the reaction mixtures were complicated by the presence of catalyst
decomposition products. Most of these decomposition products had retention times and
peak intensities similar to those of the oligomers. The estimation of the product distribution
and yields are thus only rough approximations.
68
Table 2.6. Radical oligomerization of 1-pentene with organic peroxide initiators.
Initiator Reaction conditions. Resultsb•
Conversion % C10 C15 C20
Dibenzoyl peroxide
37 °C; Reflux, UV light 24 h; 5 mol% catalyst
- - - -
100 °C; 40 psi 24 h; 2 mol% catalyst
8 5 3 -
150 °C; 80 psi 24 h; 5 mol% catalyst
10 4 5 1
150 °C; 140 psi 24 h; 10 mol% catalyst
10 4 5 1
150 °C; 170 psi 24 h; 15 mol% catalyst
10 4 5 1
Dicumyl peroxide
100 °C; 40 psi 4 h; 2 mol% catalyst
- - -
100 °C; 40 psi 24 h; 2 mol% catalyst
5 1 4 -
150 °C; 80 psi 24 h; 2 mol% catalyst
5 1 4 -
150 °C; 80 psi 24 h; 5 mol% catalyst
15 5 10 -
150 °C; 86 psi 24 h; 10 mol% catalyst
25 10 15 -
150 °C; 96 psi 24 h; 15 mol% catalyst
25 10 15 -
Di-t-butyl peroxide
100 °C; 50 psi 24 h; 2 mol% catalyst
- - -
150 °C; 90 psi 24 h; 5 mol% catalyst
30 5 20 5
150 °C; 90 psi 24 h; 10 mol% catalyst
40 10 30 -
150 °C; 90 psi 24 h; 15 mol% catalyst
50 15 35
1-Pentene : Heptane in a 3 : 1 ratio.
Maximum pressures are quoted (after temperature equilibration, about 1 hour).
Reaction mixtures were allowed to cool down to room temperature and analyzed with
GC/MS.
Conversions and product composition were calculated by comparison with the peak area of
the internal standard and unconverted monomer.
Primary and secondary dialkyl peroxides undergo competing electrocyclic reactions upon
thermal decomposition (Scheme 2.18) and are more susceptible to radical-induced
decomposition. 131,132 Consequently di-tert-alkyl peroxides are used as initiators in most
applications. In the absence of reactive substrates, tertiary dialkyl peroxides yield primarily
69
t-alcohols, ketones, hydrocarbons and minor amounts of ethers, epoxides and carbon
monoxide. Decomposition products from primary and secondary dialkyl peroxides include
aldehydes, ketones, alcohols, hydrogen, hydrocarbons, carbon monoxide and carbon
dioxide.
H H 0 , 1 I .. • II
R— C-0-0=CR,
R"CR' + H2 I R" R
I "
solvent
Scheme 2.18. Decomposition of a secondary organic peroxide.
Conversions with each of these catalysts were low (< 30%), but it seemed that di-t-butyl
peroxide was the better initiator. Although each oligomer consisted of a number of isomers,
fewer isomers formed than in the cationic oligomerization. An increase in catalyst
concentration increased the conversion. A certain temperature was necessary in order to
initiate radical formation, for these catalysts 150 °C was sufficient.
2.3.3 Metallocene catalysts
Metallocenes in the presence of methylaluminoxane are receiving a lot of attention, both
academically and industrially as a new generation of homogeneous, single-site Ziegler-
Natta catalysts. The focus of this investigation was the oligomerization of alpha olefins,
because higher C5 - C20 alpha olefins are of increasing importance as a source for many
industrially important chemicals. Although various catalysts have been described for the
oligomerization of alpha olefins, little attention has been paid to the application of
metallocene/ MAO systems in this process. These catalysts should advantageously combine
high catalyst activity with the potential to control properties such as molar mass, molar
mass distribution and stereoregularity through rational ligand design at the transition metal
centre. Depending on the chain termination, the type of end group may also be controlled.
In this preliminary study, the potential of metallocene/MAO catalysts to fine-tune the
oligomer characteristics, was investigated.
51
Ti
70
As a starting point in our study, the oligomerization of 1-pentene, 1-hexene and 1-octene by
titanocene dichloride (51) in the presence of MAO was examined. As mentioned previously
a decreased Al / Metal ratio, increased temperature and lowered alkene concentration
should result in the formation of oligomers. Oligomerizations were carried out in 100 ml
flamed-out Schlenk-flasks under argon at an oil bath temperature of 40 °C, unless stated
otherwise. The MAO and metallocene (Al : Metal in a ratio of 150: 1 as opposed to a ratio of
greater than 500 required for polymerization) were preactivated for a short period before the
monomer was added. After the specified reaction time, the reaction mixtures were allowed
to cool to room temperature and the reactions were quenched with a 10% HC1/ Me0H
solution. After work-up, the isolated oligomers were analyzed with GC/MS. The results
are compiled in Table 2.7.
Initially the yields of the reaction were not reproducible, especially when 1-pentene was
used as starting material. As before, this might be attributed to the high volatility and low
boiling point of 1-pentene. Losses of monomer at the reaction temperature of 40 °C could
influence conversion and product distribution. To ensure reproducibility, the
oligomerization reactions were repeated until at least two runs were consistent.
The data indicated maximum conversions of 37% and 48% for 1-pentene and 1-hexene
respectively after 24 - 48 hours. Longer reaction times increased the selectivity for higher
oligomers. 1-Octene showed decreasing conversion with longer reaction times and this
could be due to the formation of higher oligomers or polymers which could not be observed
by GC analyses. In strong contrast to the cationic oligomerization where a multitude of
isomers formed, each oligomer fraction consisted of one compound only (>95%). A small
amount of products was formed with 15 mass units more than the expected oligomer, e.g. a
C11 compound where a C10 was expected.
71
Table 2.7. Oligomerization of alpha olefins with titanocene dichloride/MAO.'
Alpha
olefin"
Reaction Conditions
Results`
Conver-
siond TONe Yields % f
(Selectivity % g) Dimer Dimer Trimer Trimer Tetra-
+ Me + Me mer 1-pentene 50 °C, 2h 0 - - - - - - - (800 eq.) 10 ml toluene
1-pentene 50 °C, 24h 0.30% 2 - - 0.08 - 0.22 (800 eq.) 10 ml toluene (25) (75)
1-pentene 40 °C, 24h 33% 124 14 1 18 - - (375 eq.) (43) (2) (55)
1-pentene 40 °C, 48h 37% 139 16 1 20 - - (375 eq.) (44) (2) (54)
1-pentene 40 °C, 72h 37% 139 9 1.5 20 1.5 5 (375 eq.) (25) (4) (54) (4) (13) 1-hexene 50 °C, 24h 39% 146 36.6 2 0.4 (375 eq.) (94) (5) (1) 1-hexene 40 °C, 24h 48% 180 46.6 1 - 0.4 - (375 eq.) (97) (2) (1) 1-hexene 40 °C, 48h 48% 180 43 2 3 - - (375 eq.) (89) (4) (7) 1-hexene 40 °C, 72h 64% 240 53 1.3 7.7 12 - (375 eq.) (83) (2) (12) (3) 1-octene 40 °C, 24h 50% 250 50 - - - (375 eq.) (100) 1-octene 40 °C, 48h 35% 131 34 - 1 - - (375 eq.) (97) (3) 1-octene 40 °C, 72h 16% 60 16 - - - (375 eq.) (100)
Cp2TiC12 (1 eq.); MAO (150 eq.).
Alpha olefins were dried over activated molecular sieves prior to use. •
Yields and seletivities were calculated from the GC/MS peak areas of each oligomer by
comparison with an internal standard.
Conversion —
mass of productx 100%
mass of monomer
TON = turnover number — mol of converted monomer
mol of catalyst
1) mol of compound X
Yield of compound X = x 100% mol of monomer
yield of compound g) Selectivity for compound X — x 100%
total yield
72
The reaction mechanism for the formation of these methylated products with the titanocene
dichloride/ MAO catalyst is depicted in Scheme 2.19. Firstly an excess of MAO generates
the active methylated metallocene complex 52. The alkene coordinates with the transition
metal followed by the insertion of the olefin into the metal-alkyl bond to form complex 53.
As noted in the literature study, 1,2-cis olefin insertion is mainly observed for metallocene
catalysts. The second olefin also coordinates with the transition metal and inserts into the
growing metal-alkyl chain to give complex 54. This complex (54) can then either form the
dimer 56 and a hydridotitaniumcomplex 55 by 13-hydride elimination or react with a third
olefin to yield trimer 57. This termination reaction is the most commonly observed for the
metallocene catalysts and may occur via an intermediate hydride complex or by direct
hydride transfer to the monomer. 46
Thus this dimer (and trimer) contains one more methyl group than expected due to methyl
insertion from a Ti-Me starting group. This was also observed by Janiak and coworkers in
the dimerization of propylene and 1-hexene by Cp2ZrC12 and MA0. 133 This effect is,
however, only important in the dimerization step; in subsequent catalytic cycles a Cp2Ti+-H
species formed by 13-H elimination is the only discerning chain transfer process.
The complex 55 is the main catalyst present in the reaction mixture and forms the normal
dimers and trimers as shown in Scheme 2.20. As before, propagation occurs by insertion of
the monomer double bond into the growing chain and termination occurs by 13-hydrogen
transfer.
The 1-pentene dimer (C1oH20) and the trimer (C15F130), 1-hexene dimer (C12H24), and 1-octene
dimer (C16H32) was isolated and purified by Kugelrohr distillation. The structures of these
products (Figure 2.4) were determined by 1 H and 13C NMR. GC analyses of the product
indicated that only one compound was present in each fraction.
73
Me
Ti
+ [—A1-0—] n Me
CI
52
+Ti? \ Me
MAO
51
+Tf Me
54
0
+Ti
1,2-cis-insertion
13-hydride elimination
52
1,2-cis-insertion
1,2-cis-insertion
O +Ti
(3-hydride elimination R R R 55 +
Me
57
Scheme 2.19. Formation of methylated oligorners.
0
+Tr"
H
74
R\ 55 +Tf
H
O •■•.,..R
1,2-cis-insertion
1,2-cis-insertion
O +Ti
n-hydride elimination of)
59
60 R = C3H7
61 R = C4H9
62 R = C6H13
1,2-cis-insertion i e •- • ■,
O +Ti
3-hydride elimination 55 +
63 R = C3H7
Scheme 2.20. Formation of normal dimers and trimers.
75
60
63
1 -pentene dimer
1 -pentene trimer
61
62
1 -hexene dimer
1-octene dimer
Figure 2.4. Structure of oligomers.
The EI/MS of the 1-pentene dimer 60 (C1oH20) gave a molecular ion at m/z 140 and an ion at
m/z 139 corresponding to hydride abstraction. Further fragmentation was the typical
clusters of peaks spaced 14 mass units apart. The 1H NMR spectrum indicated the
resonances of two methyl groups as triplets (6 0.87, J 6 Hz; 6 0.88, J 7.5 Hz) and 8 methylene
protons as overlapping multiplets 6 1.26 - 6 1.47. The resonances of the four methylene
protons on the carbons alpha to the quaternary carbon overlapped in a broadened triplet
(6 1.98, J 7.8 Hz) and the vinylidene protons resonated as a singlet at 6 4.68. Ten carbon
peaks were distinguished in the 13C NMR spectrum of 60. The multiplicities of the peaks
were confirmed by the proton-coupled 13C NMR spectrum: two methyl carbons as quartets
(6 13.8 and 14.0), six methylene carbons as triplets (6 20.8 - 38.1), the terminal vinylidene
carbon (6 108.5, t) and the quaternary alkene carbon (6150.2, s) were observed.
The structures of the other dimers were elucidated in analogous manner, the resonances of
the expected number of methyl and methylene groups were found and each oligomer
exhibited the characteristic vinylidene end group resonances. The diagnostic resonances of
the CH group, the three methyl groups, the nine methylene groups and the vinylidene
group of the trimer 63 were observed.
The 1-pentene tetramer was also isolated but NMR analyses proved to be complicated due
to the large extent of peak overlap of the resonances of the methylene carbons in the 13C
NMR spectrum. The presence of the vynilidene end group and the four methyl groups
76
were, however, established. A molecular ion with m/z 280 corresponding to a constitution
of C2oH40, was observed in the CI-MS data.
The metallocene component of the catalyst and the reaction conditions were subsequently
modified to study the influence on the oligomerization reactions.
i) Metal
It was noted in Chapter 1 that in general zirconocene derivatives are more active
polymerization catalyst than their titanocene counterparts. The activity of zirconocene
dichloride 64 as a catalyst for the oligomerization of alpha olefins was investigated and the
results are given in Table 2.8. The general reaction conditions are similar to those quoted in
the previous table.
64
Oligomerization reactions using Cp2ZrC12/MAO as catalyst was more reproducible than
reactions using Cp2TiC12/ MAO. In general, conversions were higher, only marginally so for
1-pentene, but this catalyst was clearly superior for 1-hexene and 1-octene. Longer reaction
times increased conversions slightly, except for 1-octene, as was observed previously.
Dimers were predominantly formed without significant portions of the higher oligomers.
These dimers consisted of single compounds that formed with >95% selectivity.
77
Table 2.8. Oligomerization of alpha olefins with Cp2ZrC12/ MAO.
Alpha olefin
Reaction Conditions
Results
Conver- sion
TON Yields % (Selectivity %)
Dimer Dimer + Me
Trimer Trimer + Me
Tetra- mer
1-pentene 40 °C, 24h 37 139 35.5 - 1.5 - - (375 eq.) .. (96) (4)
1-pentene 40 °C, 48h 36 135 34.6 - 0.4 - - (375 eq.) (96) (4)
1-pentene 40 °C, 72h 42 158 40.7 - 1.3 - - (375 eq.) (97) (3) 1-hexene 40 °C, 24h 76 285 76 - - - (375 eq.) (100) 1-hexene 40 °C, 48h 78 293 77.2 - 0.8 - - (375 eq.) (99) (1) 1-hexene 40 °C, 72h 83 311 82.2 - 0.8 - - (375 eq.) (99) (1) 1-octene 40 °C, 24h 89 334 89 - - - (375 eq.) (100) 1-octene 40 °C, 48h 56 210 54 - 2 - - (375 eq.) (96) (4) 1-octene 40 °C, 72h 41 154 40 - 1 - - (375 eq.) (98) (2)
ii) Metallocene trichloride
The relationship of the electron density at the metal centre and the oligomerization activity
was investigated. The substitution of a cyclopentadienyl ligand with a chloride should
increase the positive charge on the metal due to the greater electron-withdrawing properties
of the halide. This in turn should strengthen the bonding between the metal and other
ligands, particularly the bond to the olefin which will then become less reactive. Titanocene
trichloride (65) was used as catalyst precursor. The results of the oligomerizations with
CpTiCl3/MAO are compiled in Table 2.9.
\T,
/Cl
/ a a
65
78
Table 2.9. Oligomerization of alpha olefins with CpTiC13/ MAO. a
Alpha olefin
Reaction Conditions
Results
Conver- sion
TON Yields % ' (Selectivity %)
Diener Dimer + Me
Trimer Trimer + Me
Tetra- mer
1-pentene (375 eq.)
40 °C, 24h 5 19 ..
- - 5 (100)
- -
1-pentene (375 eq.)
40 °C, 48h 8 30 0.41 (5.1)
0.07 (0.9)
7.52 (94)
- -
1-pentene (375 eq.)
40 °C, 72h 9 34 - - 7.83 (87)
0.63 (7)
0.54 (6)
1-hexene (375 eq.)
40 °C, 24h 14 53 11.6 (83)
- 2.4 (17)
-
1-hexene (375 eq.)
40 °C, 48h 14 53 13.9 (99)
0.1 (1)
- -
1-hexene (375 eq.)
40 °C, 72h 6 23 3 (50)
0.6 (10)
2.4 (40)
- -
1-octene (375 eq.)
40 °C, 24h 5 19 4.8 (96)
- 0.2 (4)
- -
1-octene (375 eq.)
40 °C, 48h 8 30 8 (100)
- - -
1-octene (375 eq.)
40 °C, 72h 6 23 5.9 (99)
- 0.1 (1)
- -
a) All reaction conditions and calculations are analogous to those quoted in the previous tables.
As expected, the conversions were considerably lower than for the dichloride catalyst
species. A higher selectivity was observed for the higher oligomers. Furthermore, the
individual oligomer fractions and in particular the dimer fraction of 1-pentene and 1-hexene
consisted of more than five products. Steric effects may decrease the selectivity: a number of
insertion or termination reactions may take place, because there is less steric hindrance
around the metal centre.
iii) Substituted cyclopentadienyl rings
Tetramethyl substituted cyclopentadienyl ligands were used to correlate substitution on the
ligands at the transition metal centre with alpha olefin oligornerization activity. The
metallocene 66 was prepared from TiC14 and [CpMe4]1.1+. The EI-MS of this compound
furnished a molecular ion at m/z 360 corresponding to a constitution of (C9H13)2TiC12 and
titanium isotope peaks were observed. Both the 1H and 13C NMR spectra revealed the
79
expected equivalence of the ring ligands. The 1H NMR resonances of two different sets of
methyl groups as singlets (6 1.89 and 6 2.03) and the two equivalent ring protons (6 5.97)
were found. The 13C spectrum was in accord with this: two sets of methyl carbons (8 13.02
and 6 14.81) and three different sets of ring carbons (8 113.09, 8 125.16 and 6 135.91) were
observed.
Attempted oligomerization reactions with 66 in the presence of MAO (similar ratios of
catalyst, cocatalyst and monomer than before) were unsuccessful. Neither 1-pentene, 1-
hexene or 1-octene were oligomerized, even after a 100 hours at 40 °C. This can be ascribed
to the steric bulk of the ligand around the metal centre. Although polymerization of
ethylene and co-oligomerization with alpha olefins have been observed with substituted
cyclopentadienyl ligands, 1M it seems as if the steric hindrance is detrimental to the
oligomerization of higher alpha olefins. In several studies 135'136 it has been established that
highly substituted cyclopentadienyl ligands for example Cr (Cp*
pentamethylcyclopentadienyl) exhibit the lowest activities in polymerization reactions.
iv) Temperature
Ambient reaction temperatures would be economically favored in industrial applications.
The oligomerization activity of zirconocene dichloride at room temperature was studied
since it was the most active metallocene catalyst at 40 °C (Table 2.10). Monomer conversions
comparable to the higher temperature oligomerizations were observed after 24 hours at
room temperature. After this period the yields decreased, probably due to polymerization
80
and the formation of higher oligomers. Dimers consisting of only one product were isolated
and insignificant amounts of higher oligomers formed.
Table 2.10. Room temperature oligomerization of alpha olefins with Cp2ZrC12/ MAO.
Alpha olefin
Reaction Conditions
Results ' Conver-
sion - TON Yields %
(Selectivity %) Dimer Dimer
+ Me Trimer Trimer
+ Me Tetra- mer
1-pentene rt, 24h 36 135 35 - 1 - - (375 eq.) (96) (4)
1-pentene rt, 48h 9 34 7.7 - 1.3 - - (375 eq.) (86) (14)
1-pentene rt, 72h 6 23 4.5 - 1.5 - - (375 eq.) (75) (25) 1-hexene rt, 24h 90 338 90 - - - (375 eq.) (100) 1-hexene rt, 48h 33 124 33 - - - - (375 eq.) (100) 1-hexene rt, 72h 39 146 32 - 7 - - (375 eq.) (83) (17) 1-octene rt, 24h 86 323 86 - - - (375 eq.) (100) 1-octene rt, 48h 12 45 12 - - - - (375 eq.) (100) 1-octene rt, 72h 12 45 12 - - - - (375 eq.) (100)
While this study was in progress Bergman and Christoffers 137 also published that
Cp2ZrC12/ MAO catalysed the oligomerization of C4 - C7 alpha olefins at 25 °C to form
dimers in a 80 - 90% yield.
v) Purity of the alpha-olefin monomer
1-Hexene containing 2 - 3% oxygenate impurities were used as monomer to examine the
sensitivity of metallocene/ MAO catalysts to impurities. Oligomerizations with both
Cp2TiC12/ MAO and Cp2ZrC12/ MAO yielded conversions and product distributions almost
identical to those obtained with pure 1-hexene. This illustrated one of the proposed
functions of methylaiuminoxane as cocatalyst, namely as a scavenger of impurities.
81
2.3.4 Comparison of different catalysts
Comparison of viscosity data
The viscosity of the hydrogenated trimer fractions from the BF3/n-BuOH and
Cp2TiC12/MAO oligomerization of 1-pentene was determined at different temperatures by
using an Ostwald viscometer. 138,139 These results are tabulated below:
Table 2.11. Viscosity/temperature relationship of 1-pentene trimer fractions.
Temperature Viscosity
(°C) •(cStokes)
BF3/n-BuOH Cp2TiC12/MAO
30 22 63.1
35 19.1 44.6
40 17.3 38.3
The viscosity of the single component trimer was higher than the viscosity of the
corresponding mixture of isomers formed with the BF3/n-BuOH system. This may be due
to the higher degree of regularity that can prevent the molecules from sliding over each
other with ease.
General comparison
The BF3/n-BuOH system yielded a 1-pentene oligomer distribution that peaked at the trimer
and consisted of a large number of isomers. Although the monomer conversion was high,
the trimer was not formed with a very high selectivity. The radical oligomerizations of 1-
pentene gave low conversions and a smaller number of isomers under the selected reaction
„ditto k_viLns.
82
Of the three classes of catalysts surveyed, metallocenes clearly offers new avenues to explore
in oligomerization. By employing the appropriate metallocene in the presence of
methylaluminoxane, higher alpha olefin dimers and trimers can be formed in a high yield at
ambient temperatures. Furthermore, a single oligomer can be formed with high selectivity
as opposed to a number of isomers. Of the metallocene catalysts investigated Cp2ZrC12 was
the most active. It should further be possible to tailor reaction conditions in order to form
dimers or trimers exclusively.
The choice of which catalyst to use and develop will depend on the intended application of
the oligomer. This will determine whether a single component oligomer is required or
whether a mixture of isomers is advantageous. At this stage the BF3/ROH catalyst system
is more economical and forms trimers in a higher yield than the metallocene catalysts. On
the other hand, the metallocene can form dimers with a 100% selectivity and in addition
oligomers consisting of only one component.
In the production of synthetic lubricants the formation of highly branched longer alpha
olefin (Cs to C12) oligomers by the BF 3/ROH catalyst is favorable. The oligomers of shorter
alpha olefins (C4 to C7), however, exhibits too much branching and thus the metallocene-
based catalysts present the opportunity to form less branched oligomers for application in
this field.
2.4 Conclusion
The synthesis of model oligomers represented a unique opportunity to relate specific
molecular structures with certain properties. Firstly, the analytical data of branched alkanes
are somewhat ambiguous. The mass spectra of these compounds give an indication of the
branching in the molecule, but molecules with more than one branch fragment easily,
therefore complicating structural determination. Comparison with the GC/MS data of
standard branched alkanes, particularly those structurally similar to alpha olefin oligomers,
should be a valuable tool in the structure determination of these compounds.
Furthermore, it was found possible to differentiate between diastereomers in the GC/MS
analyses of these types of compounds. The implication is that the composition of
83
oligomerization reaction mixtures may be much less complex than appears at first. The
presence of these diastereomeric compounds with identical mass spectra should always be
considered in the analyses of complex mixtures.
A route to a model 1-pentene dimer and trimer, and possibly a model tetramer, was
developed. By using this methodology model 1-pentene oligomers can be prepared if
certain factors are kept in mind: the unreactivity of substrates, ease of cationic isomerization
and the separation of reaction products. The synthesis of other model oligomers with
different branching patterns would certainly be of value.
Metallocene-based oligomerization of higher alpha olefins presents opportunities for further
research. The reduction of the MAO/metallocene ratio is of great industrial importance and
so is the use of cationic metallocene catalysts. In addition, the use of bridged ligands will
facilitate steric control and thus the syntheses of optically active oligomers.
Metallocene catalysts have started a revolution in the polymer industry and also promise a
new era in the oligomerization of higher alpha olefins.
84
CHAPTER 3
EXPERIMENTAL
3.1 General
Apparatus and standard procedure
All air- and moisture sensitive reactions were performed under an inert gas (N2 or Ar)
atmosphere with dry solvents in flamed-out glassware. Room temperature (rt) refers to ca.
20 - 25 °C
Solvents and reagents
Solvents were purified and distilled prior to use. Ether, tetrahydrofuran, pentane, hexane,
benzene and toluene were dried over sodium wire, with benzophenone as indicator.
Chloroform and dichloromethane were dried over phosphorous pentoxide. Alpha olefins
(1-pentene, 1-hexene and 1-octene) were left over activated molecular sieves for 24 hours,
distilled and stored over freshly activated molecular sieves. 1-Pentene was obtained from
Sasol with >99% purity. 1-Hexene with 97 - 98% purity (2 - 3% oxygenates) was also
obtained from Sasol and higher purity 1-hexene was purchased commercially. All other
reagents were purchased from commercial suppliers and used without further purification
unless stated otherwise.
Instrumentation
1) Gas chromatography
A Varian 3400 gas chromatograph equipped with a hydrogen flame ionization detector and
a variable-split glass capillary injector was used in this study. The carrier gas was nitrogen
with a linear flow rate of 2 ml.min -1 . The data system used in conjunction with the Varian
3400 was a Microsep Datamodule M741. A DB1 fused-silica capillary column
85
(30m x 0.25 mm i.d.) coated with 100% methyl groups, was used. Samples were injected
into the gas chromatograph (20/1 split, 250 °C), the oven was kept at 50 °C for 5 minutes,
programmed to 250 °C at 10 °C.min- 1 and then held isothermally for 10 minutes. The
detector was operated at 300 °C. Retention times (R e) are given in minutes.
Gas chromatography/mass spectrometry
A Varian Saturn 3 GC/MS was used to collect GC, El-MS and CI-MS data of all organic
compounds (mass values m/z 40 •- 600). A DB5-MS fused-silicon capillary column
(30m x 0.25 mm i.d.) coated with 5% phenyl and 95% methyl groups was used. Helium
was the carrier gas with a flow rate of 1 ml.min -1 . Samples were injected into the gas
chromatograph (10/1 split, 260 °C) with an appropriate oven program; retention times (Re)
are given in minutes. The oven was held at 40 °C for 5 minutes and programmed to 250 °C
at 10 °C.min-1, 10 minutes isotherm, unless stated otherwise. The GC/MS transfer line was
maintained at 260 °C with a filament emission of 12 1.t.A for both EI and CI, an ionizing
voltage of 50 eV and a source temperature of 170 °C. • Chemical ionization was accomplished
using methane as reagent gas.
NMR spectroscopy
1H- and 13C NMR spectra were recorded by means of a Varian VXR 200 (200 MHz) or Varian
Gemini 2000 (300 MHz) spectrometer. Tetramethylsilane and deuterochloroform were used
as internal standards for 1H- and 13C-spectra, respectively. Unless otherwise stated, the
spectra were recorded in CDCb. Chemical shifts are given in parts per million (5) downfield
from the signal of tetramethylsilane.
The following abbreviations are used throughout:
s = singlet ct = quartet
d = doublet
m = multiplet
t = triplet
br = broadened
86
4) Mass spectroscopy
Mass spectra of organometallic catalysts were recorded on a Finnigan-MAT 8200 mass
spectrometer (70 eV).
3.2 Standard chemical methods
Standard Grignard solutions
Magnesium (1.05 eq.) was placed in a flamed-out flask under N2, together with dry diethyl
ether (5 eq.), a crystal of iodine and a third of the alkyl halide (0.33 eq.). The reaction started
refluxing at room temperature and the remainder of the alkyl halide (0.66 eq.) in diethyl
ether was added at such a rate that the ether refluxed smoothly. The reaction was refluxed
for 30 minutes, cooled to room temperature and then standardized.
Titration of organometallic reagents
The organometallic reagent was added dropwise to a solution of 2,2'-bipyridyl (10 mg) in
dry THE (1 ml) under N2 until a color change was observed (dark red). Another 1.00 ml
aliquot portion of the organometallic reagent was added to the solution after which the
resulting solution was titrated with dry s-BuOH until the color of the charge transfer
complex was discharged (dark red —> milky yellow). This method was then repeated by
adding 1.00 ml of the organometallic followed by titration with s-BuOH. The molarity was
determined from the following equations :
col (RM) Molarity (RM) =
ml (RM) used
mmol (RM) = mmol (s - BuOH)
density (s - BuOH) x ml (s - BuOH)
MW (s - BuOH)
where RM = RMgX or RLi
At least three consistent measurements were made to ensure accuracy.
87
Gilman test for free RLi or RMgX
An equal volume of a 1% benzene solution of Michler's ketone
[4,4'-bis(dimethylamino)benzophenone] was added to an ethereal solution of the
organometallic (e.g. 2.5 ml sample) and the mixture was allowed to warm to room
temperature. Water (1.5 ml) was then added and the mixture was stirred vigorously for 5
minutes. A 0.2% solution of 12 in glacial acetic acid was added dropwise and the
development of a characteristic greenish-blue aqueous layer confirmed the presence of
RMgX or RLi in the original solution.
Addition of a Grignard reagent to an epoxide in the presence of catalytic Cu(I) salts
The ethereal solution of the Grignard reagent (1 eq.) was added dropwise to a stirred slurry
of CuCN (0.1 eq.) in THF (1 m1/0.1 g CuCN) under N2 at -30 °C and the mixture was stirred
at this temperature for 30 minutes. The reaction mixture was then cooled to - 50 °C, the
epoxide (1 eq.) in THF was added dropwise and the reaction mixture was allowed to warm
to room temperature over a period of 5 hours. The reaction was quenched with a
NH4C1/ NH3 solution (pH 8 - 9) and filtered through Celite ® . The filtrate was extracted with
ether (3x) and the combined organic layers washed with a saturated solution of NaCl (aq)
and dried over MgSO4. After filtration the solvent was removed in vacuo and the crude
product usually purified by distillation.
Hydrogenation of oligomers
The oligomers were hydrogenated over 1% Pd/C (10% m/m) with H2 (40 - 50 bar) for 2
hours at room temperature. The resulting liquid and catalyst were separated by filtration
through a sintered glass funnel. The degree of unsaturation was determined by halogen
addition.
Determination of the degree of unsaturation
A portion of a 0.3 N bromine solution (10 - 15 ml) was placed in a 500 ml flask fitted with a
ground glass opening. The flask was closed immediately and cooled in ice-water. The
sample (0.2 - 0.3 g = VV) was added, the closed flask shaken vigorously and placed in a cold
88
bath for 2 minutes. 20 ml of a 10% KI solution and 75 ml of H2O was added and the iodine
titrated with a 0.1 N thiosulfate solution (standardized) in the presence of a starch indicator.
A blank control was titrated at the same time. After the first titration, 5 ml of a 2% K103
solution was added and the iodine liberated was again titrated with thiosulfate.
b — a — 2c Bromine number = x C x 7.99
a = nil of thiosulfate needed for titrating the sample;
b = ml of thiosulfate needed in the control;
c = ml of thiosulfate needed after adding iodate to the titrated
solution;
W= sample weight in g;
C = concentration of thiosulfate solution.
3.3 Preparation of model oligomers
Epoxidations of 1-pentene with MMPP, 93m UHP94,97 and dirriethyldioxirane 98,100 were carried
out according to standard procedure.
1-Iodo-2-pentanol (40)
1 M NaHSO3 (96 ml) was added dropwise over a period of about 4 hours to a solution of 1-
pentene (40 mmol, 4.4 ml) and HI04.2H20 (48 mmol, 10.94 g) in CH3CN (80 ml) and H2O (24
ml) at 0 °C and the mixture was stirred at room temperature for another 8 hours. • The
resulting solution was extracted with diethyl ether (150 ml x 3) and the combined organic
layers washed with aqueous Na2SO3 and dried over MgSO 4. After filtration, the solvent was
evaporated in vacuo to leave the crude product as a yellow-brown liquid consisting of two
compounds (by GC) which was used without further purification (7.28 g, 85%). These
products exhibited identical EI-MS data as determined by GC-MS.
Rt (GC) 15.06 (50%) and 16.39 (50%)
EI-MS:
m/z 213 (M+ - 1, 0.4%), 196 (41), 142 (39), 127 (16), 87 (100), 69 (38), 55 (47), 45 (68),
43 (74), 41 (84)
89
NMR (CDC13, 200 MHz):
814 0.82 (3 H, t, J4,5 6.8 Hz, 5-Me), 1.27 - 1.44 (4 H, m, 3-CH2, 4-CH2),
2.33 (1 H, br s, OH), 3.12 (1 H, dd, J1,2 6.6 Hz, JAB 10.2 Hz, lb-H),
3.36 (1 H, dd, J1,2 3.7 Hz, JAS 10.2 Hz, la-H), 3.40 - 3.43 (1 H, m, 2-H);
8c 13.20 (C-5), 7.83 (C-4), 20.09 (C-1), 35.21 (C-3), 68.11 (C-2).
1-Bromo-2-pentanol (42)
NaBrO3 (48 mmol, 7.24 g) was dissolved in water (24 ml) and the solution was adjusted to a
pH of 1 with 2 M H2SO4. The resulting solution was cooled to 0 °C and 1-pentene (40 mmol,
4.4 ml) and CH3CN (80 ml) were added; 1 M NaHSO 3 (96 ml) was added dropwise over a
period of 4 hours and the reaction was stirred at room temperature for a further 8 hours.
The reaction mixture was extracted with diethyl ether (150 ml x 3), the combined organic
layers washed with aqueous NaSO3 and dried over MgSO4. After filtration the solvent was
evaporated in vacuo to leave the crude product as a dark brown liquid consisting of 3
products (by GC) which was used without further purification (5.35 g, 80%). These
products exhibited identical EI-MS and CI-MS data as determined by GC/MS.
Rt (GC) 13.9 (14%), 14.3 (29%) and 15.6 (57%).
CI-MS:
m/z 167 (M+ + 1, absent), 166 (M+ - 1, absent), 150 (18%) 148 (20), 81 (1.4) 79 (1.6),
69 (100)
NMR (CDC13, 200 MHz):
8H 0.90 (3 H, t, J1,2 6 Hz, 5-Me), 1.34 - 1.57 (4 H,,m, 3-CH2, 4-CH2),
2.20 (1 H, s, disappears on deuteration, OH),
3.38 (1 H, dd, J1,2 7.1 Hz, JAB 10.3 Hz, lb-H),
3.53 (1 H, dd, J1,2 3.3 Hz, JAB 10.3 Hz, la-H),
3.74- 3.76 (1 H, m, verified by spin-spin decoupling, 2-H);
8c 13.30 (C-5), 18.73 (C-4), 37.07 (C-3), 40.52 (C-1), 70.72 (C-2).
1,2-Epoxypentane (35)
The reaction was carried out in vacuum distillation apparatus fitted with a liquid nitrogen
trap to facilitate collection of the product. The neat halohydrin 40 (0.1 mol, 24.40 g) or 42
90
(0.1 mol, 16.71 g) was added dropwise to stirred, finely powdered NaOH (1 mol, 40 g) under
a vacuum of ± 30 mm Hg. The reaction was exothermic (temperatures of up to 60 °C was
noted) and the epoxide 35 was distilled and trapped as it formed. The resultant clear liquid
(6.46 - 6.89 g, 75 - 80%) was dried over MgSO4, filtered and stored under N2 over activated
molecular sieves. GC analysis indicated that 35 was isolated with > 97% purity and no
further purification was undertaken.
Rt (GC) 6.9
CI-MS:
m/z 85 (M+ - 1, 2%), 71 (100), 68 (16)
NMR (pyridine-ds, 300 MHz):
5H 0.94 (3 H, tt, J3,5 0.9 Hz, 14,5 7.5 Hz, 5-Me),
1.42 - 1.49 (4 H, m, 3-CH2, 4-CH 2),
2.38 (1 H, dd, J1,2 2.7 Hz, JAB 5.4 Hz, lb-H),
2.64 (1 H, dd, J1,2 4.2 Hz, JAB 5.4 Hz, la-H), 2.80 - 2.84 (1 H, m, 2-H);
Sc 14.03 (C-5), 19.63 (C-4), 34.90 (C-3), 46.21 (C-1), 51.54 (C-2).
Butylmagnesium bromide
Butylmagnesium bromide was prepared by standard procedure from Mg (0.105 mol, 2.55 g),
a crystal of iodine, 1-bromobutane (0.1 mol, 10.5 ml) and dry diethyl ether (100m1).
[BuMgBr] = 0.80 mol.dm-3
2-Heptanol (45)
2-Heptanol (1.15 g, 99%) was prepared as a dark yellow liquid by standard addition of
ethereal butylmagnesium bromide (0.01 mol, 12.5 ml) to propylene oxide (0.01 mol, 0.70 ml)
in the presence of CuCN (0.001 mol, 0.09 g) with THE (2 ml) as solvent.
Rt (GC): 12.25
EI-MS:
m/z 116 (M+ - 1, 14%), 98 (95), 97 (100), 83 (76), 70 (43), 69 (34)
NMR (CDC13, 200 MHz):
SH 0.83 (3 H, br t,16,7 7.3 Hz, 7-Me), 1.11 (3 H, d, 11,2 6.18 Hz, 1-Me),
91
1.23 - 1.36 (8 H, m, 4 x CH2), 2.17 (1 H, br s, OH), 3.71 (1 H, m, 2-H).
4-Nonanol
4-Nonanol (1.37 g, 95%) was prepared as a light yellow liquid by standard addition of
butylmagnesium bromide (0.01 mol, 12.5 ml) to 1,2-epoxypentane (0.01 mol, 0.86 g) in the
presence of CuCN (0.001 mol, 0.09 g) with THF (2 ml) as solvent.
Rt (GC): 16.5
EI-MS:
m/z 143 (M+ - 1, 7%), 126 (9), 125 (15), 111 (8), 101 (22), 97 (14), 85 (55), 83 (100), 71 (51),
69 (26)
NMR (CDC13, 200 MHz):
51-1 0.86 and 0.90 (6 H, 2 x t, 1-Me, 9-Me), 1.20 - 1.42 (12 H, m, 6 x CH2),
2.82 (1 H, s, OH), 3.72 (1 H, m, 4-H);
5c 13.99 (C-9), 14.06 (C-1), 18.78 (C-2), 22.61 (C-8), 25.28 (C-6), 31.89 (C-7), 37.29 (C-5),
39.49 (C-3), 6 72.12 (C-4).
2-Pentylmagnesium bromide (43)
Grignard reagent 43 was prepared by standard procedure from Mg (0.525 mol, 12.76 g), a
crystal of iodine, 2-bromopentane (0.5 mol, 63 ml) and dry diethyl ether (500 ml).
[2-Pentylmagnesium bromide] = 0.75 mol.dm- 3
6-Methyl-4-nonanol (44)
a) By copper(I) catalysed Grignard addition to epoxide 35
Alcohol 44 (22.38 g, 70%) was prepared by standard addition of Grignard reagent 43
(0.20 mol, 270 ml) to epoxide 35 (0.20 mol, 17.23 g) in the presence of CuCN (0.02 mo1,1.79 g)
in THF (40 ml). The product was purified by Kugelxohr distillation (70 °C, 0.2 mm Hg) and
a mixture of 2 diastereomers (1 : 1) was isolated. The EI-MS and CI-MS data of these two
compounds as determined by GC/ MS were identical.
92
By addition of a lower order cuprate of 43 to epoxide 35
CuCN (0.01 mol, 0.9 g) was suspended in 50 ml of dry THF, cooled to -78 °C and Grignard
reagent 43 (0.02 mol, 26.65 ml) was added. The mixture was stirred at this temperature until
a negative Gilman test resulted (± 15 minutes) and then epoxide 35 (0.01 mol, 0.86 g) in THF
(5 ml) was added dropwise over 30 minutes. Stirring was continued at -78 °C for 1 hour and
then aqueous NH4C1/NH3 (pH 8 - .9) was added. Similar work-up as before afforded
alcohol 44 as a mixture of two diastereomers (1 : 1) in a yield of 72% (1.14 g).
By addition of a higher order cyanocuprate of 43 to epoxide 35
CuCN (0.01 mol, 0.9 g) was suspended in 50 ml of dry THF, cooled to -78 °C and Grignard
reagent 43 (0.02 mol, 26.65 ml) was added. The reaction was stirred at this temperature until
a negative Gilman test resulted (± 15 minutes). The mixture was then allowed to warm until
all of the CuCN was dissolved (ca. -10 °C), thereby forming the higher order cuprate. The
reaction was again cooled to -78 °C and the epoxide'35 (0.01 mol, 0.86 g) in THF (5 ml) was
added dropwise over 30 minutes. Stirring was continued at -78 °C for 1 hour and the same
work-up as before gave the 1 : 1 mixture of diastereomers of 44 (1.2 g, 75%).
Rt (GC): 17.48 and 17.66
EI-MS:
m/z 158 (M+, 0.8%), 157 (4), 156 (9), 140 (11), 139 (17), 138 (15), 115 (18), 114 (14), 98 (24),
97 (99), 96 (36), 85 (100), 84 (31), 83 (34), 70 (75), 69 (24), 68 (26)
NMR (CDC13, 200 MHz):
OFT 0.83 - 0.94 (9 H, m, 3 x Me), 1.36 (10 H, m, 5 x CH2), 1.67 (1 H, m, 6-H),
2.5 (1 H, br s, OH), 3.65 - 3.71 (1 H, m, 4-H);
8c 14.11 (C-1), 14.31 and 14.36 (C-9), 19.26 and 20.37 (C-7'), 18.37 and 18.85 (C-2),
19.95 and 20.03 (C-8), 28.97 and 29.41 (C-6), 38.99 and 40.02 (C-7),
40.22 and 40.59 (C-3), 45.07 and 45.36 (C-5), 69.42 and 69.81 (C-4).
Multiplicities were confirmed using a DEPT experiment.
93
4-Bromo-6-methylnonane (47)
PBr3 (0.033 mol, 0.32 ml) was added dropwise to the neat alcohol 44 (0.01 mol, 1.60 g) kept
between -5 °C and +5 °C. After the addition was completed, the reaction mixture was
allowed to warm to room temperature and to proceed for eight hours. The reaction was
shaken with one-third its volume of concentrated sulfuric acid and the acid layer was drawn
off and discarded. The residual organic phase was mixed with an equal volume of 50%
Me0H and aqueous ammonia was added with intermittent shaking until the solution was
alkaline to phenolphtalein. The organic layer was separated and washed once with an equal
volume of 50% Me0H, then dissolved in diethyl ether, dried over MgSO4, filtered and the
solvent removed in vacuo. The crude product was purified by Kugelrohr distillation (120 °C,
0.2 mm Hg) to yield a mixture of three compounds as a clear oil (1.77 g, 80%). These
compounds exhibited identical EI-MS data as determined by GC/ MS.
Rt (GC/ MS): 14.64 (45%), 14.71 (45%) and 15.16 (10%)
EI-MS:
m/z 222 (M+ + 1, absent), 220 (M+ - 1, absent), 140 (8%), 98 (18), 97 (25), 96 (9), 85 (100),
84 (22), 83 (18), 71 (79), 70 (19), 69 (24)
NMR (CDC13, 200 MHz):
8H 0.83 - 0.93 (9 H, m, 3 x Me), 1.17 - 1.92 (11 H,. m, 5 x CH2, 6-H), 4.09 (1 H, m, 4-H);
8c 13.47 (C-1), 14.25 and 14.30 (C-9), 18.67 and 19.73 (C-7'), 19.93 (C-2),
20.70 and 20.79 (C-8), 30.85 (C-6), 38.13 and 39.66 (C-7), 41.21 and 42.08 (C-3),
46.51 and 46.99 (C-5), 56.50 and 56.89 (C-4). Multiplicities were confirmed using a
DEPT experiment.
6-Methylnonyl-4-magnesium bromide (48)
Alkyl bromide 47 (0.03 mol, 6.63 g) dissolved in dry ether (30 ml) was added to magnesium
turnings (0.07 mol, 1.69 g) under N2 and maintained at a gentle reflux. 1,2-Dibromoethane
(0.03 mol; 2.16 ml) in dry ether (30 ml) was added dropwise over a period of 10 hours. The
mixture was then refluxed for another 2 hours, allowed to cool to room temperature and
standardized.
[6-Methylnonyl-4-magnesium bromide] = 0.28 mol.dm-3
94
8-Methyl-6-propyl-4-undecanol (50)
Alcohol 50 (2.40 g, 70%) was prepared as a viscous yellow oil by standard addition of
Grignard reagent 49 (0.015 mol, 53.6 ml) to epoxide 35 (0.015 mol, 1.30 g) in the presence of
CuCN (0.0015 mol, 0.13 g) in THE (3 ml). The product was purified by vacuum distillation
(130 °C, 0.15 mm Hg) and a mixture of 5 products was isolated. These products exhibited
identical EI-MS data as determined by. GC/MS.
Rt (GC/MS): 11.63 (24%), 11.65 (24%), 11.71 (24%), 11.74 (24%)and 12.01 (4%)
EI-MS:
m/z 228 (M+, absent), 209 (0.8%), 140 (5), 139 (16), 138 (6), 111 (7), 110 (8), 98 (15), 97 (34),
96 (54), 85 (48), 84 (30), 83 (17), 71 (90), 70 (46), 69 (25), 57 (94), 56 (27), 55 (39),
43 (100), 42 (19), 41 (74)
NMR (CDC13, 200 MHz):
8H 0.78 - 0.89 (12 H, m, 4 x Me), 1.23 (16 H, m, 8 x CH 2), 1.63 (2 H, m, 6-H, 8-H),
2.33 (1 H, s, OH), 4.52 (1 H, m, 4-H);
Sc 14.13, 14.40 and 14.48 (br, C-1, C-9' and C-11), 19.81, 19.91, 20.00 and 20.35 (C-9"),
20.07, 20.13 and 20.21 (C-2), 21.09, 21.17 and 21.24 (C-10),
30.14, 30.22, 30.27 and 30.34 32.56, 32.71, 32.97 and 33.00 (C-8'),
33.21, 33.39, 33.42 and 33.66 (C-7'), 36.10, 36.22, 36.29 and 36.34 (C-6),
37.88, 38.36, 38.40 and 38.62 38.64, 38.87 and 38.98 (C-7),
39.66, 39.78 and 39.87 (C-5), 40.11, 40.15, 40.27 and 40.41 (C-3), 67.60 (br, C-4).
Multiplicities confirmed using a DEPT experiment.
Pentyllithium
Lithium shot (0.25 mol, 1.74 g) was placed in a flame-dried flask under an Ar atmosphere
and washed with dry hexane (5 x 8 ml) followed by addition of 6.5 ml hexane. The slurry
was cooled to -10 °C and 1-bromopentane (0.10 mol, 12.40 ml) dissolved in 12.5 ml hexane
was added slowly over a period of 40 minutes while the temperature was maintained at
-10 °C. After the addition was completed the reaction was warmed to room temperature
and stirring was continued for 2 hours. The slurry became light purple in color and
afforded a clear solution after filtration under. Ar via a cannula through a sintered glass
funnel.
95
[C5H11Li] = 3.20 mol.dm-3
4-Methyl-6-propylundecane (32)
CuCN (0.01 ml, 0.90g) was stirred in dry THF (25 ml) and cooled to -78 °C. Pentyllithium
(0.02 mol, 3.2 ml) was added and the reaction was stirred until a negative Gilman test
resulted (± 15 minutes). The reaction mixture was allowed to warm to -10 °C to effect
complete solution of the CuCN. The reaction mixture was cooled to -78 °C and the alkyl
bromide 47 (0.01 mol, 2.2 g) in THF (5 ml) was added dropwise, after which the reaction was
allowed to proceed for 1 hour and then quenched with aqueous NH4C1/ NH3 (pH 8 - 9). The
resulting suspension was filtered through Celite ®, the filtrate extracted with ether (20 ml x 3)
and washed with Na2SO3. The solution was dried over MgSO4, filtered and the solvent
removed in vacuo to yield alkane 32 as a clear oil (1.06 g, 50%). Three products with
identical EI-MS data were distinguished by GC/MS.
Rt (GC/MS): 17.50 (48%), 17.58 (48%) and 17.98 (4%)
EI-MS:
m/z 211 (M+ - 1, 0.4%), 210 (0.6), 209 (8), 167 (6), 153 (2), 139 (9), 125 (10), 111 (16), 97 (29),
85 (80), 71 (100)
NMR (CDC13, 300 MHz):
514 0.83 - 0.89 (12 H, m, 4 x Me), 1.22 (18 H, m, 9 x CH2), 1.29 (2 H, m, 4-H, 8-H);
5c 14.06 (3 C, C-1, C-9; and C-11), 14.49 (C-5'), 19.77 (C-2), 22.68 (2 C, C-10, C-8'),
26.31 and 26.62 (C-8), 29.66 and 29.80 (C-9), 31.93 (C-7), 32.36 (C-7'), 32.45 (br, C-4),
33.61 and 33.66 (C-3), 36.08 (C-5), 37.15 and 37.36 (C-6). Multiplicities confirmed
using a DEPT experiment.
3.4 Oligomerization reactions
BF3/n-BuOH oligomerization of 1-pentene
1-Pentene (0.34 mol, 37 ml), heptane (45 ml) and n-BuOH (0.027 mol, 0.25 ml) were placed in
a three-necked round-bottomed flask fitted with a thermometer, gas inlet, reflux condenser
and two-way stopcock to a gas outlet or to a manometer. BF3 (g) was rapidly bubbled
96
through the solution for 5 minutes. The gas flow was adjusted to maintain a constant
pressure in the closed system. The temperature rised to 37 °C, after 30 minutes the gas flow
was stopped and the reaction quenched by addition of 25% NaOH (aq). The organic layer
was diluted with diethyl ether, washed with H2O (30 ml x 3), dried over MgSO4 and filtered.
The solvents and unreacted 1-pentene were removed in vacuo to yield the oligomeric
products as a yellow-brown oil (total conversion > 90%).
Bp (0.2 mm Hg): Dimer (80 - 85 °C), trimer (85 - 115 °C), tetramer (125 - 145 °C)
Rt (GC): Dimer (11.7 - 13.1, 15%), trimer (23.2 - 28.0, 47%), tetramer (32.8 - 37.4, 23%),
pentamer (40.6 - 46.8, 15%). The GC oven was programmed from 50 °C to 250 °C at
a rate of 5 °C.min- 1, 20 minutes isotherm.
EI-MS:
The complete resolution of any single product was impossible due to the large number of
isomers in each oligomer fraction. The MS data at the maximum of the broad peaks
representing the different oligomers are quoted.
Dimer
m/z 140 (M+, 1.6%), 139 (2.4), 124 (1.6), 123 (2.4), 112 (11), 111 (5), 110 (2.4), 98 (16),
97 (100), 96 (12), 85 (38), 84 (41), 83 (74), 71 (68), 70 (91), 69 (68)
Trimer
m/z 210 (M+, 1.6%), 209 (15), 153 (12), 140 (15), 139 (25), 126 (6), 125 (19), 112 (13),
111 (35), 110 (10), 98 (17), 97 (56), 96 (17), 85 (16), 84 (23), 83 (81), 82 (18), 81 (20),
71 (21), 70 (34), 69 (100), 68 (16), 67 (28)
Tetramer
m/z 280 (M+, 10%), 266 (4), 252 (4), 237 (10), 224 (8), 210 (8), 196 (10), 181 (17), 168 (4),
154 (26), 140 (35), 125 (43), 111 (72), 96 (44), 83 (68), 69 (100)
Hydrogenation of the oligomers obtained from 1 -pentene with BF3/n -BuOH as catalyst
The 1-pentene oligomers (10 g) obtained with BF3/ri-BuOH as catalyst were hydrogenated
over Pd/C (0.1 g) according to standard procedure. The bromine number of the product
was determined to be ca. 0 and thus complete hydrogenation took place.
Rt (GC): Dimer (11.6 - 13.96), trimer (23.8 - 28.2), tetramer (31.6 - 39.2),
97
pentamer (39.9 - 48.0). The GC oven was programmed from 50 °C to 250 °C at a
rate of 5 °C.min- 1, 20 minutes isotherm.
Hydrogenation the 1 -pentene trimer obtained with of BF3/n -BuOH as catalyst
The 1-pentene trimer (20 g) obtained with BF3/n-BuOH as catalysts was hydrogenated over
Pd/C (0.2 g) according to standard procedure. The bromine number of the product was
determined to be ca. 0 and thus complete hydrogenation took place.
Rt (GC/MS): 15.1 - 23.5
Viscosity (Ostwald viscometer, cStokes): 30 °C (22), 35 °C (19.1), 40 °C (17.3)
Radical oligomerization of 1 -pentene with organic peroxide initiators
The autoclave was charged with 1-pentene (3 eq.), heptane (1 eq.) and the specified mol
percentage of the organic peroxide. The system was closed, set to the required temperature
and reaction time and pressure was measured from temperature equilibration. Afterwards
the reaction vessel was allowed to cool to room temperature and analyzed without further
work-up.
Benzoyl peroxide initiated oligomerization of 1-pentene
Reagents :
1-Pentene (0.14 mol, 15 ml)
Heptane (10 ml)
Benzoyl peroxide 2% (0.003 mol, 0.94 g); 5% (0.007 mol, 2.35 g), 10% (0.014 mol,
4.69 g), 15% (0.020 mol, 7.04 g).
Dicumyl peroxide initiated oligomerization of 1-pentene
Reagents :
1-Pentene (0.14 mol, 15 ml)
Heptane (10 ml)
98
Dicumyl peroxide 2% (0.003 mol, 0.74 g); 5% (0.007 mol, 1.85 g), 10% (0.014 mol,
3.7 g), 15% (0.020 mol, 5.55 g).
c) tert-Butyl peroxide initiated oligomerization of 1-pentene
Reagents :
1-Pentene (0.14 mol, 15 ml)
Heptane (10 ml)
Di-tert-butyl peroxide 2% (0.003 mol, 0.6 ml); 5% (0.007 mol, 1.45 ml), 10% (0.014 mol,
2.9 ml), 15% (0.020 mol, 4.35 ml).
Bis(tetramethylcyclopentadienyl)titanium(IV) dichloride (66)
Tetramethylcyclopentadienyl (0.01 mol, 1.22 g) was dissolved in dry THF (25 ml), cooled to
-78 °C and n-BuLi (1.47 mol.dm-3, 0.01 mol, 6.80 ml) was added dropwise over a period of 30
minutes. The solution was stirred for another 30 minutes at this temperature and then
allowed to warm to room temperature to form a light yellow suspension of the anion. Dry
THF (20 ml) was carefully added to TiC14 (0.005 mol, 0.55 ml) under an Ar atmosphere in
order to generate the THF adduct. The anion was slowly added to this mixture via a
cannula and the resulting solution stirred for 1 hour. The solvent was evaporated and the
metallocene recrystallized from chloroform/pentane as deep red crystals (1.80 g, 50 %).
EI-MS
m/z 362 (M+, 10 %) , 360 (15), 326 325 (5), 324 (13), 241 (16), 239 (24), 205 (5), 204 (7),
203 (12), 202 (5), 201 199 121 (100), 105 (25), 93 (12), 91 (16), 79 (11), 77 (9)
NMR (CDCb, 300 MHz):
SH 1.89 (12 H, s, 4 x Me), 2.03 (12 H, s, 4 x Me), 5.97 (2 H, s, 2 x CH);
Sc 13.02 (4 C, 2-Me, 5-Me), 14.81 (4 C, 3-Me, 4-Me), 113.09 (2 C, C-1),
125.16 (4 C, C-2, C-5), 135.91 (4 C, C-3, C-4).
99
Alpha olefin oligomerization with the metallocene/MAO catalyst system
Methylaluminoxane was obtained as a 10% solution in toluene. Although it was thought
that the MAO could decompose with time, the concentration of the MAO was not estimated
and the molar mass of the [Al-Me-0] monomer was used in calculations.
The metallocene (0.05 mmol) catalyst was preactivated with the methylaluminoxane
solution (0.0075 mol, 0.5 ml) for 15 minutes at room temperature in a 100 ml flamed-out
Schlenk flask under an Ar atmosphere. The alpha olefin was added (0.019 mol), the flask
tightly sealed and the reaction mixture stirred in an oil bath at 40 °C. After the specified
reaction time, the reaction mixture was allowed to cool to room temperature and quenched
carefully with a 10% HC1/ Me0H solution. The resulting solution was extracted with
toluene (15 ml x 3), washed with water and dried over MgSO4. After filtration, the solvent
was removed in vacuo to yield the oligomeric products as a colored oil (depending on the
color of the metallocene).
Metallocenes
Cp2TiC12 (12.5 mg)
Cp2ZrC12 (13.5 mg)
CpTiC13 (11 mg)
(C9H13)2TiC12 (17.9 mg)
Alpha olefins
1-Pentene (1.33g, 2.1 ml)
1-Hexene (1.60 g, 2.4 ml)
1-0c tene (2.13 g, 3 ml)
2-Propyl-1-heptene (60)
The 1-pentene dimer 60 was prepared by standard oligomerization of 1-pentene with
Cp2TiC12/ MAO.
100
Itt (GC): 10.22
EI-MS:
m/z 140 (M+, 8%), 139 (32), 137 (3), 111 (15), 97 (41), 96 (100), 95 (42), 85 (42), 84 (28),
83 (44), 69 (40)
NMR (CDC13, 300 MHz):
51-1 0.87 (3 H, t, 16,7 6 Hz, 7-Me), 0.88 (3 H, t, J4•5 , 7.5 Hz, 5'-Me),
1.26 - 1.47 (8 H, m, 4 x CH2), 1.98 (4 H, br t, J3',4• 13,4 7.8 Hz, 3-CH2, 3'-CH2),
4.68 (2 H, s, 1-CH2)
5c 13.8 (q, C-7), 14.0 (q, C-5'), 20.84 (t, C-4'), 22.55 (t, C-6), 27.45 (t, C-4), 31.64 (t, C-5),
35.96 (t, C-3), 38.18 (t, C-3'), 108.5 (t, C-1), 150.2 (s, C-2).
2,4-Propy1-1-nonene (63)
The 1-pentene trimer 63 was prepared by standard oligomerization of 1-pentene with
Cp2TiC12/ MAO.
Rt (GC): 16.83
EI-MS:
m/z 210 (M+, 0.4%), 209 (2.4), 208 (2.8), 165 (14), 138 (13), 123 (26), 110 (36), 97 (70),
96 (61), 84 (81), 83 (100), 82 (63), 70 (44), 69 (62), 68 (42)
NMR (CDC13, 300 MHz):
OH 0.80 - 0.91 (9 H, m, 3 x Me), 1.14 - 1.30 (14 H, m, 7 x CH2), 1.39 - 1.47 (1 H, m, 441),
1.91 - 1.96 (4 H, m, 3-CH2, 3'-CH2), 4.65 - 4.71 (2 H, 2 x d, Jcis 15 Hz, 1-CH2);
8c 13.84 (q, C-9), 14.06 (q, C-7"), 14.43 (q, C-5'), 19.64 (t, C-4'), 20.77 (t, C-6"),
22.67 (t, C-8), 26.15 (t, C-6), 32.30 (t, C-7), 33.35 (t, C-5), 34.98 (d, C-4), 35.80 (t, C-5"),
37.81 (t ,C-3'), 40.27 (t, C-3), 110.20 (t, C-1), 148.91 (s, C-2).
Viscosity (Ostwald viscometer, cStokes): 30 °C (63.1), 35 °C (44.6), 40 °C (38.3)
2-Butyl-1-octene (61)
The 1-hexene dimer 61 was prepared by standard oligomerization of 1-hexene with
Cp2ZrC12/ MAO.
Rt (GC): 14.16
101
EI-MS:
m/z 168 (M+, 12%), 167 (28), 166 (86), 124 (24), 110 (100), 93 (57), 81 (71), 68 (44), 67 (45)
NMR (CDC13, 300 MHz):
5H 0.90 (3 H, t, 17,86.6 Hz, 8-Me), 0.92 (3 H, t, J5',6' 6.6 Hz, 6'-Me),
1.20 - 1.46 (12 H, m, 6 x CH2), 2.01 (4 H, br t, '74 J3,4 7.6 Hz, 3-CH2, 3'-CH2),
4.70 (2 H, 2 x d, 1 Hz, 1-CH2);
5c 13.98 (q, C-8), 14.08 (q, C -6'),-22.55 (t, C-7), 22.70 (t, C-5'), 27.88 (t, C-4),
29.19 (t, C-4'), 30.12 (t, C-5), 31.86 (t ,C-6), 35.84 (t, C-3), 36.16 (t, C-3'),
108.38 (t, C-1), 150.31 (s, C-2).
2-Hexyl-1-decene (62)
The 1-octene dimer 62 was prepared by standard oligomerization of 1-octene with
Cp2ZrC12/ MAO.
Rt (GC): 19.54
EI-MS:
m/z 224 (M+, 14%), 223 (12), 222 190, 193 (2), 169 (4), 154 (6), 138 (48), 125 (16), 111 (100),
97 (42), 83 (59), 69 (66)
NMR (CDC13, 300 MHz):
8H 0.85 - 0.89 (6 H, m, 2 x Me), 1.26 - 1.42 (20 H, m, 10 x CH2),
1.98 (4 H, br t, J3,4 7.8 Hz, 3-CH2, 3'-CH2), 4.67 (2 H, 2 x d, J ets 1.2 Hz, 1-CH2);
5c 14.03 (q, 2 C, C-10, C-8'), 22.61 (t, C-9), 22.64 (t, C-7'), 27.77 (t, C-4), 27.80 (t, C-4'),
29.10 (t, C-7), 29.28 (t, C-6), 29.44 (t, C-5'), 29.50 (t, C-5), 31.77 (t, C-8), 31.88 (t, C-6'),
36.07 (t, 2 C, C-3, C-3'), 108.35 (t, C-1), 150.52 (s, C-2).
102
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109
ACKNOWLEDGMENTS
A work of this magnitude could not have been accomplished alone. I would like to extend
my heartfelt gratitude to :
My supervisor, Prof. Fanie van Heerden, for her guidance and counsel over the past two
years.
Hennie Swanepoel, for his understanding and encouragement as well as for helping with
the typesetting of this thesis.
My parents, for their unconditional love and support through all the years.
My colleagues at RAU for practical suggestions, interesting discussions and providing a
congenial atmosphere to work in.
Dr. Hannes Malan at Sastech R&D for expert advise.
SASOL and RAU for financial assistance.
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