11. POLYMERIC REAGENTS: CONCEPTS A BARACTE.FA8TICS

The chemical reaction. of to

the chemistry of their preparation, were little studied

until the 1950's. In recent years, however, there has

been an upsurge of activity in the chemistry of

macromolecules occasioned partly from the need to extend

the range of application of existing polymeric materials

and partly from the development of polymers for non-

material uses. The latter include polymeric reagents

which consists of reactive functions bound to an

inscluble support 31i32. The exhausted reagent is easily

removed from the reaction medium and in favourable

circumstances 33 it can be regenerated . Such

functionalized resins have many applications as ion-

exchangers34, chelating agents 35f 36, reagents for organic

synthesis 37-40, catalysts 41-45, and media for trapping

unstable reaction intermediates 46,47 Organometallic

polymers have potentially useful semiconducting

properties4'. There is a growing interest in polymers as

vehicles for carrying biologically active molecules such

as drugs and hormones 49i50. These are all concerned with

the synthetic aspects of polymer-supported reagents.

This review attempts to give a critical account of the

effects of different structural parameters on the course

of the reactions using polymer-supported reagents.

Because of its macromolecular environment, a

functional group attached to a polymer chain may have a

quite different reactivity from the analogous group in a

small molecule. Neighbouring group effects, which may be

either activating or deactivating, are well-documented

and have no exact counterpart in small molecules.

Factors which affect the chemical reactivity of polymers

have been reviewed by many workers 51,52

11. 1. The Polymer Support

A proper choice of the polymer matrix is an important

factor for successful utilization of polymeric reagents.

It is increasingly recognized that, when polymers are

used as supports for catalysts or reagents, the

reactivity and selectivity of the supported reagents or

catalysts may be seriously changed by the so-called

polymer effects 53-56. The support interacts with the

surrounding medium. It may or may not swell, depending

on its thermodynamic affinity with the medium and its

method of synthesis. It may selectively absorb one of

the reactants or products, as a result of preferential

solvation. Many types of readily available natural and

synthetic polymers have been chemically functionalized

for use as reactive supports. The macromolecule can be a

linear species capable of forming a molecular solution in

a suitable solvent, or alternatively a crosslinked

species, or the so-called resin, which though readily

being solvated by a suitable solvent remain

macroscopically insoluble. Of the two approaches, the

use of resins has been more widespread because of the

practical advantages occuring from their insolubility.

The ease of chemical modifications of a resin, and

success of its subsequent application as a reagent or a

catalyst, can depend substantially on the physical

properties of the resin. Functionalized polymeric

supports must possess a structure which permits enough

diffusion of reagents into the reactive sites, a

phenomenon which depends on the extent of swelling or

solvation, the effective pore size and pore volume, and

the chemical and mechanical stability of the resins under

the conditions of a particular chemical reaction or a

reaction sequence. These in turn depend on the degree of

crosslinking of the resin and the conditions employed for

the preparation of the resin.

~ o t h linear and crosslinked polymers are used as

supports for various purposes. The advantage associated

with the use of linear polymers include the fact that the

reaction will be carried out in solution in a homogeneous

medium without much diffusion problems and with equal

accessibility to all the functional groups of the

polymer. This may be a significant advantage in a

reaction which is known to be sluggish or involves

substrates which have a large size and may not be able to

penetrate the pores of a crosslinked polymer. But here

separation of the polymer from low molecular weight

contaminants is difficult. Another potential problem

with the use of linear polymers is the possibility of

side reaction producing unwanted crosslinks during

reaction. In contrast, crosslinked polymers, being

soluble in all solvents, offer the greatest ease of

processing as they can be prepared in the form of

spherical beads which do not coalesce when placed in a

suspending solvent and can be separated from low

molecular weight contaminants by simple filtration and

washing with various solvents. Insolubility does not

mean lack of reactivity. In appropriate solvents,

polymer beads with low degrees of crosslinking swell

extensively, exposing their inner reactive groups to the

soluble reagents.

suspension polymerization is the usual method

employed for the preparation of crosslinked polymers.

There are various factors which determine the particle

size of the polymer beads. Smaller particles are

obtained by increasing the water/monomer ratio or

diluting the organic phase with a solvent for the polymer

to be produced. Increasing the amount of crosslinker

has the opposite effect. Temperature is also an important

factor. However the two most important factors are the

choice of the dispersing agent and the stirring process.

Crosslinked polymers can also be prepared by popcorn

polymerization. Here the rnixture of monovinyl compound

and small amounts of divinyl monomer are mixed together

an2 warmed gently in the absence of any initiator and

solvent, resulting in white and opaque glassy polymers.

These are insoluble and extremely porous materials which

are capable of absorbing large quantities of solvents,

due to the permanent voids present in them.

Particles with a narrow size distribution are easy to

obtain in the submicron range using emulsion

5 7 polymerization without an emulsifier . A simpler

process has been recently described involving the

dispersion pclymerization of styrene in a non-solvating

medium containing a dissolved cellulosic polymer. The

58 particle size thus obtained, was around 10-20 microns . For the larger sizes (100-500 microns) which are

preferred as supports for functional groups in organic

synthesis or catalysis, the simplest way to get beads

59 with a given particle size is to use a sieving process .

Gel-type resins are generally lightly crosslinked and

appear transluscent and have no permanent porosity, but

swell to varying degrees in many organic solvents. They

are often referred to as microporous, because the space

between the crosslinks occupied by the swelling solvent

are considered as small pores. The mobility inside the

micropores is restricted due to the high viscosity of the

solutions. The supported species are also less mobile

than the dissolved ones, but they are far from totally

i~miscible.

Supports like poly(methylmethacrylate), polyvinyl

alcohol and cellulose which were experimented earlier

were rejected later due to the synthetic inconveniences

associated with them. Majority of the polymeric reagents

prepared so far utilizes crosslinked polystyrene as the

suFport material on account of its commercial

availability and ease of functionalisation. But they

were found to be incompatible with polar solvents and

substrates due to the strong hydrophobic nature of the

6 0 polystyrene matrices. Reagents based on polyamides ,

Poly(viny1pyridine) 61>62and poly (N-acrylylpyrrolidines) 6 3

were also used to immobilize reagent function. Oxidising

reagents based on these resins were found to be superior

than polystyrene due to the better hydrophobic/

hyarophilic balance they can provide. Polyszccharides

5 4 also have been used as immobilizing media . A wide

range of inorganic supports have also been modified and

are found suitable for various catalytic and

6 5 chromatographic applications . Organic synthesis using

reagents supported on inorganic materials has been

reviewed by McKillop and Young 66,67 However, they

cannot meet the capacity demands of polymeric reagents

as a result of their low loading capabilities.

11. 2. Chemical Modification of the Supports

Studies on chemical reactions of polymers were rather

scant until the 1950's. After the introduction of the

4 solid phase peptide synthesis by ~errifield in 1963,

there has been intense activity on polymer modification

by functionalization and in their use in different areas

of chemistry and technology.

The attachement of functional groups to polymers is

frequently the first step towards the preparation of

speciality polymers, for example, for biomedical

application, or as supported reagents. Two approaches

have been used for the introduction of active functional

groups in the polymer matrix: (I) by polymerization of

the monomers containing the desired functional group or

(2) by chemical modification of the non-functionalized,

preformed polymer. Many functional linear polymers can

be prepared, by the former method, without difficulty by

free-raeical, anionic, cationic, coordination or group

transfer polymerization. However, for most purposes,

crosslinked polymers are more attractive than the linear

ones. The preparation of crosslinked polymers in good

physical form is most readily achieved by suspension

polymerization 6e-70 A difficulty with this method is

that considerable manipulation of the copolymerization

procedure may be necessary to ensure a good yield of the

required copolymer and in the case of resins, to ensure

also a satisfactory physical form. Moreover, this type

of polymerisation requires an appropriately substituted

mcnomer. If a suitable monomer is not available, it is

necessary to synthesise one which carries the required

functional group. Eut the advantage here is that the

degree of functionalization of the product is more easily

controlled, and the structure can be determined easily by

analysis of the relevant comonomer prior to

polymerization.

The chemical modification of the preformed polymer is

the more accepted and most extensively used method for

preparing polymeric reagents and protecting groups. It

is particularly attractive for the preparation of

crosslinked polymers because one can start with

commercially available microporous or macroporous polymer

beads of good physical form and size with a known

percentage of crosslinking and porosity. The product

polymer has the same physical form as the original

polymer. Here the reactivity and accessibility of the

reaction site may be limited as compared to small

molecules and in most cases it depends on the proper

choice of a swelling or suspending solvent. Thus more

drastic conditions are required to get a good yield.

Another problem is in the purification of the modified

product. With low molecular weight reagents,

crystallisation, distillation or chromatography can be

applied. But with crosslinked polymers these methods are

not applicable. Since few reactions are either

quantitative or free from side reactions, the final

product is often the one in which a large proportion of

the functional groups have been modified; but this also

contains some unchanged functionalities and the

impurities resulting from side reactions. The amount of

such impurities will depend on the specific reaction

which are carried out on a polymer. In some cases the

presence of impurities on the polymer will have no

noticeable effect on its end-use. Many reviews on the

chemical modification o f polymers have been

published 71-73

Of the many chemical modification of polystyrenes,

chloromethylation and bronination are most generally

useful. chloromethylation was originally carried out

using chloromethyl ether and a Lewis acid such as stannic

75 chloride74 or zinc chloride . Chloromethylation of

styrene-EVE resin without chloromethyl ether has been

7 6 reported recently . Another reagent used for the

chloromethylation is dimethoxymethane with thionyl

77 chloride and Lewis acid . Although known for a long

time78, the chloromethylation reaction was applied to

polystyrene and its copolymers only towards the middle of

this century. In 1952, Cow Chemical and Rohm

and Hass companyB0 were the first to publish, almost

simultaneously, patents regarding the preparation of ion-

exchange resinsby chloromethylation of polystyrene and

its copolymers.

he chloromethylated polystyrenes play an important

role in Merrifield's synthesis of peptides61, as well as

in many other reactions carried out on the

chloromethylated resin. The study of chloromethylati.on

8 2 and of its uses have been reviewed by many workers -

11. 3. Swelling Characteristics

Despite the ever-increasing knowledge relating to

chemical reactions occuring on polymer supports, there

still exists only a primitive level of understanding of

the physical and chemical nature of immobilized

substrates and the role which the solvent plays in these

8 3 heterogeneous systems . Network polymers, like linear

ones, are diluted by solvents when the polymer-solvent

interactions lead to an increased entropy of mixing.

However, complete mixing cannot occur in polymer

networks. The support shvuld have a backbone compatible

with both solvents and reactants to favour the

equivalance of all functional groups as in homogeneous

system. AS the network is swollen by the solvent, the

network junctions and chain are forced apart to

accommodatethe increasing volume fraction of the solvent.

The resulting strained conformation results in a

retractive force which tends to bring the network chains

into a more probable conformation. As the volume

fraction of solvent increases, so do the retractive

forces. Eventually the entropy of dilution and the

retractive network forces balance and a state of

8 4 equilibrium is achieved .

Flory has proposed an analogy between swelling

8 5 equilibrium and osmotic equilibrium . In this analogy

the retractive force of the swollen network is viewed as

equivalent to an osmotic pressure when the gel is in

equilibrium with the solvent. According to Flory, the

pressure is sufficient to increase the chemical potential

of the solvent in the gel so that it equals that of the

excess solvent surrounding the swollen network polymer.

These considerations about the swelling of polymer

networks give rise to the expectation of a fundamental

relationship between swelling and the nature of the

8 6 polymer and the solvent . The extent of swelling is

inversely proportional to the density of crosslinks in

the network and is highly dependent on the solvent and

the temperature.

The factors that control the solvation of the bound

reagents and transport of the reactants in the polymer

can be modified either by chemical reaction on the

pendant groups or by alteration of the physical nature of

the polymer. The accessibility of functional groups

present in the functionalized polymer supports for

chemical modification is an important paraweter for

87 deciding its utility . Good solvents diffuse quickly

into the crosslinked polymeric networks, resulting in

swelling. Microporous resins possess porosity only in a

swollen condition because of swelling, also known as gel

8 8 porosity . As the degree of crosslinking decreases, gel

networks result which consists largely of solvent with

only a small fraction of polymer backbone. By using a

copolymerization technique, the chemical structure of the

polymer can be varied over a wide range in orderto obtain

a support with a particular combination of properties.

The hydrophobic or hydrophilic character of the polymer

can be changed by changing the nature and ratio of the

comonomer units. Thus, solvent compatibility with the

resin can also be adjusted by proper selection of the

comonomer units in the polymer. The swelling studies are

important fcr identifying the good solvents in order to

select the suitable reaction medium for performing

reactions on polymer supports.

Very few measurements of the pore size and pore

size distribution have been carried out directly in

swollen resin~~'-~l. one method is based on small angle

X-ray scattering from which a distribution of the

electron density in a material can be deduced. A more

powerful method ,thermoporometry9~ has also been used. It

I s based on freezing point measurements of solvents

inside the pores.

11.4. Monitoring of Polymer-supported Reactions

The increased difficulty of analysis of insoluble

materials compared to low nolecular weight reagents has

discouraged many organic chemists from using supported

reagents and catalysts and has resulted in inadequate

characterization of the repcrted materials. ~lemental

analysis of nitrogen, phosphorous, hydrogen or sulphur is

used commonly and allows for the quantification of the

reaction. However there are some difficulties, since

undesired functionalisation may result by side reaction,

or the polymer may contain trapped impurities which

invalidate the analytical data.

The most reliable quantitative methods are specific

analysis of a typical functional group. A quaternary

ammonium or phosphonium ion catalyst should be analysed

for the chloride or bromide ion released during its

synthesis. Chloromethylated polystyrene can be analysed

sinilarly by modified Volhard's method.

IR spe~troscopy is another useful tool available for

the direct study of supported species. This is a good

method for following the course of the reaction of

polymers. Chloromethyl groups in polystyrene can be

identified by their 1260cm-1 IR band. The sensitivity of

the method has been enhanced by the use of Fourier

Transform infrared spectrometers.

Solid state high resolution nmr technique is very

useful for the analysis of crosslinked polymers. The

03-95, multiple-phase sequences (MP) , technique comprises

dipolar decoupling (ED), and magic angle sample rotation

( A ) . In addition, cross-polarization (cP) may be used

to enhance weak 13c resonances 93194. These experiments

provide both dynamic and structural informations as do

conventional broadline measurements, but the resolution

of individual resonances provides a detailed description

of molecular behaviour at the monomeric level.

13c-NMR spectra of polystyrene gels highly swollen in

CDC13 may have linewidths of 4 l O H z for peaks of the

96 pendant functional groups . Functional groups bound to

silica gel surfaces gave high resolution 13c-NMR spectra 9 7

when the functional groups are well solvated - PEG

chains bound to highly swellable polystyrenes may be

analyzed by quantitative 1 3 ~ - ~ ~ ~ peak area comparisons

with the aromatic peak of the polystyrene if the spin-

lattice relaxation lines and Nuclear Overhauser

96 Enhancement factors are known . The number of

chloromethyl groups produced by free radical chlorination

of poly(p-methylstyrene) has been analysed by

9 8 quantitative 13c-~MR spectroscopy

1 Most polymer structural studies involve either H or

13 C-NMR, but other nucleiiare also valuable. 15N-N~R

spectroscopy is potentially valuable in studies of

polyamides, though its low sensitivity in natural

abundance (ca. ten-fold less than 13c) is a serious

restriction. Nevertheless Kricheldorf and Hull have

carried out extensive investigations "-lo2 using this

technique. The 15N chemical shift is strongly sensitive

to the location of the amide groups and in many cases it

is more informative than the carbonyl 13c signal also

used for this purpose. In contrast to 15N - NNR the

problem posed by "F-KMR is not one of sensitivity but

one of obtaining fluid samples of 19F containing

polymers, which are generally high melting and insoluble

materials at normal temperatures. The principal

advantage of 13~-EyiR is the wide chemical shift

dispersion. Because of this, useful spectra in the

presence of significant line broadening due to restricted

mobility. In particular, the structure of crosslinked

polymers can often be studied if the system is swollen.

Manatt et.allo3 have used this approach to investigate the

chloromethylation of crosslinked polystyrene in the

phenyl 4 - position; a dispersion of reactivities was,

however, not reflected in a dispersion of chemical

shifts.

11. 5. Advantages and Limitations of Polymer-Supported

React ions

The main advantages offered by the use of polymeric

carriers in organic synthesis are insolubilization and

immobilization of the attached species and the

possibility of creating special microenvironmental and

steric effects. AS the reagent is attached to the

insoluble polymer the product work-up will be easy. In

the case of soluble p~lymers~ultrafiltration or selective

104 precipitation removes soluble polymers . In this

approach usually an excess of polymeric reagent is taken,

so that a high yield of product is obtained in solution.

In some of these reactions side products remain attached

to the polymer, thus facilitating the product

purification. This allows the polymeric reagents to be

used in column or in batch process. They can be

regenerated and reused.

A soluble, low molecular weight compound, when

attached to a crosslinked polymer, acquires the latter's

property of complete insolubility in all common solvents.

Moreover, if the polymer is of high porosity, the bound

molecules will remain freely accessible to solvent and

solute molecules and therefore do not lose much of the

reactivity they exhibit in solution.

The polymer matrix contributes a special environment

for carrying out reaction. It will generally impose on

molecules diffusing into it certain defined steric

requirements determined by pore or channel structure, by

substituents on the polymer backbone and by thedista~ce

between attached molecules and the polymer backbone.

This may induce some specificity at the reaction site.

The reactivity of an unstable reagent or catalyst may

be attenuated when supported on a resin and the corrosive

action of certain reagents can be minimized and also

toxic and malodorous materials can be rendered

environmentally more acceptable. Results obtained by the

e of polymeric reagents are often difficult or

impossible to duplicate in reactions under conventional

homogeneous conditions.

However there are some disadvantages such as high

costs in preparing a reagent, reduction of the degree of

functionalisation during regeneration, slow reactions and

low reactivities, low product yields, difficulties with

separation of impurities and additional synthesis time.

Some of these can be overcome by a proper choice of the

support.

11. 6 , Nature of t h e Polymer Backbone

The support, in addition to holding the reactive

groups, plays an important role in determining the

reactivity of the reagent moieties 105,106 It was

orlglnallythought that the polymer support acts as an

inert medium whichdoes not have any significant effect on

the reactions of the attached reactive function. But

later studies showed that the macromolecular matrix does

influence the reactions of the bound species. A

polymeric reagent has a different reactivity from the

conventional low molecular weight reagents due to the

effects of several factors. These factors could be the

incompatibility of the reactants and the polymer, the

steric hindrance by the polymer cr the adsorption of the

products on the resin. Thus more drastic condition may

be required to reach satisfactory conversion. The

polymer properties can be modified either by chemical

reactions on pendant groups or by changing the

physical nature of the polymers, such as their ~hysical

form, solvation behaviour and porosity. Such properties

have a great importance for the functionalization

reaction and for the eventual application of the reactive

polymers, and must be considered during the design of a

polymeric reagent. Appropriate choice of support

structure and properties as well as rea

can overcome the problems of slow

yields of products than in homogeneou

As crosslinked polymers are insolub

and since more than 90% of the reactive groups are within

the beads, it is clear that for reactions to take place

the low molecular weight reactants must diffuse into the

polymer beads. With microporous polymers diffusion will

be very slow unless the reaction solvent swells the

beads. With polymer supported reagents where the loading

of reactive groups will generally be high, the swelling

properties change considerably as the original functional

groups are transformed into others. with macroporous

polymers access to the functional groups within the

macropores generally present no problems; but when

functional groups are within the dense and more highly

crosslinked regions of the matrix the choice of reaction

solvent will again be important if the functional groups

are to be readily accessible.

With crosslinked polymers the mobility of the polymer

chains and hence immobilised reactive functional groups

is restricted. with lightly crosslinked resins such as

1% or 2% crosslinked polystyrenes the mobility is not

reduced significantlyl but as the crosslink density

increases a point is eventually reached where a small but

significant fraction of the functional groups cannot

reach others. Then a degree of permanent site isolation

is achieved. The state at which it occurs depends on the

degree of functionalization, the extent of crosslinkingl

the distribution of functional groups, the extent of

swelling of the polymer matrix and the type of polymer

used.

11. 7. Molecular Character and Extent of crosslinking

The degree of crosslinking is an important factor

which determines the reactivity of the attached reagent

function. The polymeric reagent should h ~ v e a porous

structure to allow diffusion of reactants and solvents to

the reactive sites. This depends on total surface area,

total pore volume, and the average pore diameter. These

physical parameters are closely interrelated. These

factors depend on the degree of crosslinking and the

condition employed for the preparation of the resin. A

high porosity allows good flow of solvent and substrate

particles into the interior of the polymer matrix and

does not hinder the penetration of substrate leading to

high activity of the polymeric reagent.

As the degree of crosslinking decreases, gel networks

result which ccnsists largely of solvent with only a

small fraction of the polymer backbone. The crosslinker

ratio controls in an inversely proportional sense the

degree of swelling. As the degree of crosslinking

increases, the ability of the network to expand in a good

solvent becomes reduced and penetration of reagents into

the interior decreases.

The mechanical stability of the matrix is also found

to be dependent on crosslink density. Lightly

crosslinked resins appear to be fragile towards

mechanical stirring. Better physical stability can be

achieved by increasing the crosslink density. Eut the

access of reactive groups in highly crosslinked networks

is considerably diminished as they are flanked by large

frequencies of crosslinks leading to a decreased

reactivity 307,108

when divinylbenzene-crosslinked poly(N- '

bromoacrylamide) prepared from differently crosslinked

(3, 10, 15 and 20%) polyacrylamides were used for the

oxidation of alcohols to carbonyl compounds, it was

observed that the reagent prepared from 3% crosslinked

polyacrylamide was most efficient in terms of reaction

period and yield of the products10g. c his shows that the

reactivity of the attached function is dependent on the

molar percentage of crosslinks in the polymer. A similar

trend was observed in the case of polymer supported t-

110 butyl hypochlorite resin also . With polystyrene

Supp~rted peroxy acids, a 1% crosslinked resin gave

better results than those prepared from 2% crosslinked

111 polystyrene during the oxidation of olefins .

When compared with linear polymers, gel polymers are

usually found to be slightly less reactive as reaction

will be limited by diffusion of the reagent within the

resin pores. The reaction yield can be affected by the

degree of crosslinking, with lower yields observed for

highly crosslinked resins112. These facts suggest that

resins with very low degrees of crosslinking would be

most suitable, as increased swelling would result in

higher accessibility through enhanced diffusion

properties. Sometimes it is possible to increase the

accessibility of the functional group by using a more'

hydrophilic crosslinking agent. Thus when poly(N-

bromoacrylamide) was prepared using a highly hydrophilic

crosslinking agent such as TTEGDA, it was observed that

15% crosslinked resin was the most reactive than the 3,

113 10 and 20% crosslinked systems .

11. 8 . Separation between the Polymer Matrix and

Reactive Functions

The mobility of functional groups is much lower when

they are directly attached to the polymer matrix. The

accessibility and the reactivity of such groups should

also be restricted under such conditions. The mobility

and reactivity of the attached function can be increased

when the reactive functions are anchored onto the support

through spacer arms. spacers have also been employed in

other fields of polymer utilization, such as drug

delivery systerr., latex supported immunoassays and comb-

like polymer liquid crystals. The commonly used spacers

are hexamethylene diamine114, polyethyleneglycol and

B K ( C H ~ ) ~ Er 11 6 ~f the functional groups initially

present in the polymer are not sufficiently isolated, the

spacer may become doubly coupled with the polymer and is

effectively lost. Such reactions have been used to study

site isolation117. For this reason, the spacer is often

chosen carrying two different chain ends.

~ h s procedure for using a spacer has recently been

extended by Tomoi 'Ie via the preparation of a monomer

carrying a spacer. The monomer was prepared by attaching

hexamethylene dibromide into the p-position of styrene.

The same idea is currently exploited by Guyot et-a1 119

using a styrene derivative with a polyoxyethylene

spacer.

A detailed investigation of the effect of spacer

methylene groups in the oxidation of alcohols was carried

120 out using a polymer supported t-butyl hypochlorite . when the hypochlorite function was separated from the

polystyrene matrix by spacer methylene groups, a drastic

increase in the rate of oxidation was observed. For this

hypochlorite functions with five, four, three, one and no

spacer methylene groups separating the hypochlorite

function from the polymer backbone were prepared and

employed for the oxidation of alcohols. The reagent with

five methylene groups as spacer groups exhibited the

greatest reactivity in terms of reaction time and product

yield and the reactivity gradually decreased with least

reactivity in the case of the reagent with no spacer. In

the assymmetric Robinson cyclisation reaction using

polymer bound L-proline as catalyst, incorporation of

spacer was found to improve the catalytic efficiency

121 considerably . In phase transfer catalysts also the

introduction of long spacer chains enhanced the activity

identical to the non-supported system 122,123

11. 9. Polymer-Supported Oxidising Reagents

An increasing number of polymeric reagents have been

developed for use in organic synthesis 124,125 AS

methods for the functionalisation of polymers have

improved parallel tc the development of different types

of polymeric reagents synthesis and uses of polymeric

oxidising reagents have become the natural goals.

One of the earliest reports on the use of insoluble

126 polymeric reagents relates to peracids . The most

useful supported peroxyacids to-date are various

crosslinked polystyrene containing aromatic peroxyacid

residues 127-129. Most of the polymeric aliphatic type

p e r o ~ ~ a c i d s ~ ~ ~ - ~ ~ ~ which have reasonable activity are

quite explosive in the dry or nearly dry state.

The periodate form of Amberlyst A-26 and Amberlite-

IRA 904, two commercial macroporous anion-exchange resins

and iodate form of Amberlyst A-26 prepared by standard

ion-exchange procedures can be used to oxidise various

quinols, catechols and glycols 134-136

polymer-supported chromium ( v I ) oxidising reagents,

such as polymer-bound chromate based on commercial

Amber1 yst A-26 resin I poly(viny1pyridinium 137

c h l o r ~ c h r o m a t e ) ~ ~ ~ , poly(viny1pyridinium dichromate) 139

and, polymer-supported quaternary ammonium complex

c h r ~ m a t e s l ~ ~ , have been developed and reported in the

literature for the oxidation of different types of

alcohols. The insoluble support provides a specific

environment capable of enhancing and modifying the

reactivity of the bound reagent. Therefore it is

possible to carry out oxidation under mild conditions

using solvents which may be rather unusual for Cr(V1)

oxidations.

Recently poly(vinylbenzyltripheny1 phosphonium

dichromate) has been developed and used as oxidising

reagent and the reactivity of this reagent was compared

with that of low molecular weight model reagent in

selective oxidation of alcohols to the corresponding

carbonyl compounds141. Tammani eta1 142 developed

poly(viny1pyridine)-supported silver dichromates as

versatile, mild and efficient oxidants for alcohols,

oximes, amines, thiols and aromatic hydrocarbons.

Synthesis of aldehydes and ketones from allylic and

benzylic halides using polymer-supported chromic acid was

143 reported by Cardillo eta1 . Silica gel-supported

chromic acid reagent was also used for the oxidation of

alcohols to carbonyl compounds144. Pyridinium chromate

on silica gel oxidises alcohols containing acid labile

functions 145-147

various halogen-containing polymers have also been

used in the oxidation and halogenation of organic

substrates. Polymer-supported N-halo amides and imides

148-150- The have been used for the oxidation of alcohols

imide-type of reagents were found to be more reactive.

synthesis of bromoderivatives of linear and crosslinked

polyamides were reported by Okawara etal151. Recently

Poly(N-haloacrylamides) were developed as efficient

oxidising and halogenating reagentB5'. polymeric

analogues of t-butylhypohalites lS3 and polystyrene-

supported N-chloro-N-sodiosulfonamide 154 were also

introduced as selective oxidation reagents.

Foly(4-vinylpyridine) is another important support

which is used to attach halogenating function.

christensen et-a1 reported the oxidation of thiols to

155 disulphides using polyvinylpyridine-bromine complex . Recently polyvinylpyridine complexes of bromine chloride

as reactive polymers were reported in addition reactions

and compared the reactivity with free halogen under

156 similar conditions of solvent and temperature . ~romine adsorbed on a molecular sieve was reported to be

a reagent for selective bromination of terminal double

bond157. Insoluble poly (vinylbenzyltriphenyl phosphonium

perbroside) prepared by the chemical modification of 2%

crosslinked poly(p-bromomethylstyrene) with triphenyl

phosphine followed by treatment of the obtained ~clymeric

salt with bromine, have been used in the direct

158 bromination of alkenes and carbonyl compounds . Poly(methylmethacry1ate) resin-bound triphenyl

phosphonium bromide was also used for the oxidation and

bromination of different organic substrates under mild

conditions 159

The bound permanganate ion has also been widely used

for the oxidation of various organic substrates 160-163

various oxidations using reagents on celite or alumina-

supported oxidising agentshave been reported 164-175

Although many totally new chenical reactions and

reagents are still being developed, there is an ever-

increasing effort aimed at improving well-characterized

processes, with the intention of enhancing selectivity,

simplyfying work up and for introducing regenerable

character. In this context the introduction of a wide

variety of polymer-supported reagents has made some

impact, especially in the latter two areas. However, the

attractive feature of improving the selectivity of

reagents for particular substrates has still to meet the

significant success in reactions other than simple

176 esterolysis .