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Jacobsen asymmetric epoxidation of olefins
R = aryl, alkenyl, alkynylR’ = bulky group
Metal complexes of porphyrins, salens, phthlocyanins catalyze the reaction with O2 or O–donors (H2O2, ROOH, PhIO, NaOCl, RCOOOH, py-O, etc).
Typically, the O-donor generates a high-valent metal-oxo complex and the electrophilic oxo-atom is transferred to the hydrocarbon substrate.
Although porphyrin complexes are not so readily available as salen complexes, they can be used in combination with O2.
Reference: Katsuki, T. Synlett. 2003, 3, 281
For the Jacobsen epoxidation, a “lock-and-key” mechanism operates: the transition statecomplex with the lowest energy is the one leading to the major product.
R R'
(S,S)-cat (4 mol%)NaOCl (aq), pH 11
CH2Cl2, 4 oC R R'
H HO
O
Mn
N
tBu
tBu O
N
tBu
tBuCl
(S,S)-catJacobsen's pre-catalyst
Mechanistic considerations
Reference: Katsuki, T. Adv. Synth. Catal. 2002, 344, 131
cis-disubstituted olefins are better substrates than trans-disubstituted olefins.Trisubstituted olefins are very good substrates.
The observed selectivity is based on a side-on approach of the olefin.
O
MnN
tBu
tBuO
N
tBu
tBuO
L
s
Mn
O
L
s
shallow stepped conformation
Mn
O
L
s
deeply stepped conformation
The olefin approaches such that its bulkier substituent (L) is away from the 3’-substituent to minimize repulsion. The substituents on the benzylic carbons in the 3 or 3’ positions are directed away from the incoming olefin and strong repulsion cannot be expected. In order to improve enantioselectivities, a binaphtyl unit as the chiral element was used.
Trans-olefins are not good substrates because the desired orientation of the incoming olefin is destabilized by the interaction of the downward substituent (S) with the salen ligand. Deeply-folded Mn(salen)s are expected to be the catalyst suitable for trans-olefins.
Sharpless asymmetric dihydroxylation of olefins
O
OsO
OO
O
OsO
OO
O
OsO
OO
NR3
R3N: +
fast
slow
H2Ooxidation
OsO4 + NR3 catalytic
HO OH
"Os" stoichiometric
+
+
Ligands used for AD
AD-mix- contains:K3[Fe(CN)6]
K2CO3
(DHQD)2-PHALK2OsO2(OH)4
Conditions: t-BuOH, H2O (1:1)0 oC, 6-24 h
N
OMe
N
H Os
O
O O
ONN
O O
N
MeO
H
N
N
MeO
N
Et
H OH
R
N N
Ph
Ph
DHQD-R
N
N N
O
R = PYR R = PHAL R = IND
Oligomerization and Polymerization of
Olefins
Textbook H: Chapter 22.1 – 22.4, 22.9.1.1
Textbook A: Chapter 15.1, 15.3
Outline
Oligomerization SHOP: Shell Higher Olefin Process
Polymerization Polymers: definitions, structural/property relationships Historical aspects of Ziegler/Natta polymerization Heterogenous catalysts Metallocene catalysts: co-catalysts, mechanism Living polymerization Late transition metal catalysts
Ni-catalyzed oligomerization
SHOP: commercialized in 1977; in 1993 global annual production capacity was 106 tons.
C6 – C18 olefins: commercial valueThe rest: isomerization and metathesis
Ph2P
NiOO
-
Ph2P
NiO
H
O CH2=CH2
Ph2P
NiOO
Ph2P
NiOO
CH2=CH2
Ph2P
NiOO
CH2=CH2
Ph2P
NiOO Rn
Rn
Polymers: Definitions• Monomer: Any substance that can be converted to polymers.
• Polymer: Macromolecule built up by linking together large numbers of smaller molecules.
• Copolymer: Macromolecule built up by linking together two different monomers.
Naturally occurring polymers:
Synthetic polymers:
R
-olefin polyolefin
R
n
• Polymer structure greatly affects polymer properties.
H2NOH
O
R
HN
O
R n
aminoacid peptide
Synthesis of polymers Condensation reactions: all molecules are involved in the steady growth of
species of higher molecular weight. Addition reactions: reaction of the initiating species with monomers; a
limited number of growing polymer molecules exists in excess of monomers. Radical (and living radical) polymerization
Anionic polymerization Cationic polymerization Coordination polymerization
ethylene LDPE
radical
initiatorn
n
ethylene HDPE
n
Polymer molecular weight
Mn = njMj
nj
Mw = Mj2nj
Mjnj
PDI = Mw
Mn Molecular WeightW e i g h t f r a c t i o n
narrow molecular weight distribution
broad molecular weight distribution
Weight fraction
• Many important mechanical properties of a polymer depend on and vary with molecular weight: melting point, Tm (crystalline part); glass transition temp., Tg; crystallinity; strength; modulus (the relation between stress and deformation); viscosity; morphology of the polymer particles.
Mn: number average molecular weight; Mw: weight average molecular weight
•Gel Permeation Chromatography (GPC) is a tool for polymer molecular weight determination.
• Molecular weight distribution gives information about the distribution of different molecular weight chains within a polymer sample.
Ziegler/Natta polymerization: introduction• Karl Ziegler: German chemist, Nobel prize 1963• 1953/54: oligomerization of ethylene by trialkylaluminum compounds (high pressure and temperature).• In the presence of trace transition metals, it was found that the reaction took place at a much lower temperature and pressure.• Polymer had a linear, unbranched structure with high molecular weight, i.e. HDPE
http://www.nobel.se/chemistry/laureates/1963/
R2Al-R +n
Transition Metal
Catalyst• Giulio Natta: Italian chemist, Nobel prize 1963• Learned of Ziegler’s research, and applied findings to other -olefins such as propylene and styrene.• Resulting polypropylene was made up of two fractions: amorphous (atactic) and crystalline (tactic). Polypropylene is not produced in radical initiated reactions.
propylene polypropylene
n
Metallocene catalysts• 1955: Natta reported that Cp2TiCl2 activated by AlEt3 could polymerize ethylene with low activities.
• 1981: Sinn and Kaminsky discovered a high-activity catalyst:
x AlMe3 + x H2O Al
Me
O x+ 2x CH4
• The key to reactivity was the co-catalyst generated from the adventitious water, “methylalumoxane(s)” or MAO:
ZrCH3
CH3
+ AlMe3 + "H2O"ethylene
Very active catalyst!
Role of MAO• Ziegler/Natta catalysts are water sensitive; excess MAO serves to dry the solvent and monomers.
• MAO can abstract alkyl groups from a complex and generate cations. In this case, MAO is transformed into a weakly-coordinating anion.
• MAO can alkylate metal halides.
Al
Me
O x+ H2O Al
OH
O x+ CH4
Al
Me
O x+ [M]-Cl Al
Cl
O x+ [M]-Me
Al
Me
O x+ [M]-Me Al
Me
O x+ [M]
Me
Reference: Eilertsen, J. L. et al. Inorg. Chem. 2005, 44, 4843
Activation of metallocenes and olefin insertion
• The alkyl resides in one of the equatorial sites and the olefin binds to the other.
• The active metallocene catalyst is a cationic alkyl.
ZrMe
ZrMe
ZrMe
ZrMe
ZrMe
Zr
Me
ZrMe
Me+ AgBPh4
- Ag- CH3CH3
Zr(IV) d0
16 e
ZrMe
[BPh4]
Zr(IV) d0
14 e
-Olefin insertion
• -olefins can insert from two positions:
1,2-insertionR
1-position
2-position
ZrMe
ZrMe
ZrMe
R
RR
Zr
RMe
ZrMe
ZrMe
ZrMe
R
Zr
MeRR
R
2,1-insertion
• 1,2-addition is the major mode of insertion; 2,1-insertion usually leads to chain termination.
Chain termination
• -Hydrogen elimination
• -Alkyl elimination
• Chain transfer to co-catalyst
(L) MR
+P
R
(L) MP
RHR
(L) MP
RHR
(L) M (L) MH
+R
P
R
P
RRH
(L) MP
RRH
(L) MR'
+ R'2Al
R
P(L) M
R'Al
R'2(L) M
P
RH
+ AlR'3
R
P
Living polymerization: A special case
(L) M
R'
R
n+1
(L) MR
R
activator
activation
initiation (ki)
R'
propagation (kp)
n(L) M
R'
RR'(L) M
R
ki = rate of initiationkp = rate of propagation
• Initiator and intermediates are stable under reaction conditions.• There is no chain termination.• ki ≥ kp
This means that the rate of initiation is greater than rate of propagation and that all the metal centers are initiated before propagation takes place.
• Polymers with narrow molecular weight distributions are obtained.
Ti-based heterogeneous Ziegler/Natta catalystsFirst generation (Solid solution)• Different crystalline modifications of Ti(III) chloride, (TiCl3), and Al(C2H5)2Cl
Second generation (Donor modified)• TiCl3/AlR3/Lewis Base (e.g. ethers, esters, ketones, amines and phosphines)• Certain Lewis bases increase the stereospecificity of polymerization and increase the activity of the catalyst.
Third generation (Supported)• TiCl4 + Al/MgCl2/Lewis Base/AlR3
• Increase in catalyst surface area greatly increases polymerization activity.
Ti
Cl
CH2
Cl
Cl
Cl
P
Al
EtEt
H
H
H
HParticle
Core
Cossée-Arlman mechanism
M-P +1
23
MP
12
3
1,2-insertion
M P
3
2
13,1-insertion
MP
12
3
2,1-insertion
Late transition metal catalysts
N N
M
Pol
+
ArAr N
N NM Ar
Cl Cl
MAO
EthenePropene
Polymers
M = Ni, Pd
N N
M
S Pol
+
ArAr
M = Ni, Pd
COOMe
COOMe
MeOOC
Brookhart M = Fe, Co
Ethylene polymerization catalysts
Ethylene-acrylate copolymerization catalysts