v. conclusion and outlook -...
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V . CONCLUSION AND OUTLOOK
The chemistry of crosslinked insoluble organic
polymeric reagents continues to be one of the active
areas of research in basic and applied organic chemistry.
Polymeric oxidising and halogenating reagents are one
among the most important classes of these reagents. In
the reactions and processes using crosslinked polymeric
reagents, advantage is made use of the insolubility of
the polymer-supported reagent and of its byproduct which
permits the easy removal of any excess reagent or spent
material from the desired product. This feature also
enables one to use a large excess of either the low-
molecular weight substrate or the insoluble polymeric-
reagent in order to increase the reaction rate and
yields. These systems can also be used for coll~mn
operations and batch processes and can be regenerated
several times. The attachment to the insoluble
macromolecular matrix can also solve the problems of
lability, toxicity or odour, often experienced with low-
molecular weight reagents. rn addition, the polymer
matrix can be so selected or tailor-made by adjusting the
macromolecular structural parameters of the three-
dimensional matrix. This can provide a specific
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microenvironment that may induce certain selectivity or
specificity at the reaction site.
with a view to developing efficient and cost-
effective polymeric oxidising reagents and to analyse the
effect of the characteristic structural parameters of the
macromolecular matrix, different types of polymeric
supports which differ widely in their molecular character
and overall structure were investigated in the present
work. specific correlation between nature of the polymer
matrix and the reactivity of the attached oxidising
function forms the overall thrust of the study here.
In order to achieve these objectives a series of new
polymeric bromo derivatives based on linear and
differently crosslinked polyvinylpyrrolidones were
developed and their chemistry investigated. The
incorporation of bromo function in linear and crosslinked
polyvinylpyrrolidones, their application as solid phase
oxidising and halogenating reagents in synthetic organic
chemistry, investigation of the reactivity of the N-
halogeno function in different microenvironment arising
from the macromolecular structure and the correlation of
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the reactivity with the nature and extent of crosslinking
are investigated.
The starting materials for the preparation of the
large number of polyvinylpyrrolidone-bromine complexes
designed and developed in these studies were either
commercially available samples of poly(N-vinyl-
pyrrolidone) or its crosslinked analogues prepared easily
by the free-radical suspension polymerisation of N-vinyl-
pyrrolidone (VP) with divinylbenzene (DVB), N,N1-
methylene-bis-acrylamide (NNMBA) or tetraethyleneglycol
diacrylate (TTEGDA). AIBN was used as the radical
initiator. These polymers on treatment with bromine in
CClq afforded the solid bromine complexes.
The polyvinylpyrrolidone-bromine complexes prepared
were characterised by IR spectroscopy, U V spectral
studies, N M R , thermogravimetry, elemental
analysis and scanning electron microscopy. The
percentage of bromine attached to the polymer could be
calculated from the thermoanalytical data. The bromine
capacities of the reagents were determined by iodometric
titration and the capacity varied in the range 1.5 to 3.8
mequiv of Br/g.
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The polymeric reagents which could be obtained as
orange powders are stable under ordinary laboratory
conditions and can be stored indefinitely without
appreciable loss of capacity. The reagents are
noncorrosive and easy to handle. They can be recycled
and reused for further synthetic reactions. The
hygroscopic nature of the polyvinylpyrrolidone gets
reduced when it was subjected to bromination.
The PVP-Br resins were found to oxidise alcohols to
carbonyl compounds in 80-98% isolable yields. They were
also used for the double bond addition of unsaturated
compounds in 70-95% yields. But d - halogenation of ketones and N-halogenation of arnides were not possible
with these reagents. Thus ketones like acetophenone or
benzophenone and arnides like succinirnide or benzamide do
not undergo any change even after prolonged treatment
with the reagent. The oxidation conditions involved
stirring of the substrate with a five-fold molar excess
of the reagent in chloroform at room temperature.
Wetting of the reagent with water is necessary for the
effective reaction in the case of crosslinked PVP-Br.
Since linear PVP-Br is soluble in water, wetting with
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water is not possible. The reactions with linear PVP-Br
were therefore carried out at refluxing temperature in
the absense of water.
The polyvinylpyrrolidone-bromine complexes can be
used for the selective oxidation of alcohols. For this a
variety of diols were studied by taking equimolar
reagent-to-alcohol ratio and also with a five fold molar
excess of the reagent. Thus hydrobenzoin, a compound
containing two secondary alcoholic groups, gave benzoin
when equimolar reagent and substrate were used. But when
a five-fold m:lar excess of the reagent was used, both
the hydroxyl groups reacted simultaneously to give the
diketo compound, benzil. Thus it is possible to control
the oxidation of diols by adjusting the reagent-to-
substrate ratio. This selectivity appears to be
originated from the slow release of bromine from the PVP
support.
The influence of various reaction parameters like
the nature of the solvent, temperature, molar percentage
of crosslinks and concentration of the reagent function
on the course of the oxidation reaction was investigated.
For linear PVP-Br, as the polarity of the solvent
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increases, the reactivity was also found to be
increasing. This is due to the effective swelling of the
hydrophilic polyvinylpyrrolidone matrix in polar
solvents. Out of the different solvents studied THF was
found to be the most suitable one.
The various crosslinking agents used for preparing
crosslinked PVP were NNMBA, DVB and TTEGDA. Polymeric
systems with varying molecular cha~acter and extent of
crosslinking were prepared and investigated to draw a
correlation between the variables of macromolecular
structure and the reactivity. The reactivity of the
reagent prepared from DVB-crosslinked polymer is less
compared to the more hydrophilic NNMBA-crosslinked
polymer. TTEGDA is a highly flexible and hydrophilic
crosslinking agent. The reactivity of the reagent
prepared from TTEGDA-crosslinked PVP is very high
compared to the other two crosslinking agents. The
enhanced reactivity is due to the hydrophilic flexible
crosslinking agent which acts as a spacer grouping
between the backbone macromolecular chain.
It was observed that by the incorporation of various
crosslinking agents and also by varying the amount of
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crosslinking, the hydrophobic-hydrophilic balance of the
polymer backbone, which have a significant effect on
reactivity, is changed. Out of the various crosslinked
polymers used 10% TTEGDA-crosslinked polymer was found to
be the most reactive in terms of duration of reaction and
capacity. o r NNMBA- and Dvn-crosslinked polymers, it
was observed that as the crosslink density increases from
3% to 2 0 8 , the capacity of the reagent was decreasing.
This is due to the decreased swelling nature of the
NNMBA- and DVB-crosslinked polymers which prevents the
diffusion of the low molecular substrates into the
polymer matrix.
The mechanical stability and the hygroscopic nature
of the polymer was found to be dependent on the crosslink
density of the system. The lightly crosslinked polymers
are soft and sticky in nature and are highly hygroscopict
especially the one prepared from TTEGDA. They are
mechanically stable also. As the crosslink density
increases, the polymer becomes rigid and the hygroscopic
nature gets reduced. The highly crosslinked polymers are
somewhat fragile.
With each of the crosslinking agent, four types of
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polymers with different molar percentage (3, 10, 15 and
20%) of crosslinks were prepared. The reactivity of the
corresponding bromo resins were studied following the
oxidation of benzoin to benzil spectrophotometrically.
For NNMBA-crosslinked PVP-Br, it was observed that 10%
crosslinked polymer was the most reactive interms of
yield and duration of reaction. with DVB-crosslinked
PVP-Br, it was tound that as the crosslink density
increases from 3% to 20%, the reactivity gradually
decreases. This is due to the rigid nature of DVB and
also due to the enhanced hydrophobicity induced to the
vinylpyrrolidone matrix from the incorporation of the
hydrophobic DVB-crosslinking agent. The order of
reactivity for TTEGDA-crosslinked PVP-Br is 15) 10) 3 >
20%. This shows that the increased hydrophilicity
enhances the reactivity of the polymeric reagent. The
hydrophilicity of the PVP matrix increases when it is
crosslinked with hydrophilic TTEGDA. The decreased
reactivity with 20% crosslinked polymer is due to the
decreased diffusion of low molecular weight reagents into
the polymer matrix, as it is highly crosslinked.
Out of the three crosslinking agents used to prepare
PVP-Br, the one which is prepared from TTEGDA was found
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to be the most suitable for synthetic conversions. DVB-
and NNMBA-crosslinked PVP-Br needs long duration for the
completion of the reaction. But with TTEGDA-crosslinked
PVP-Br, the bromination and oxidation reactions were very
fast and the products were obtained in good yields.
The reactivity of the polymeric reagent was found to
be dependent on the nature of crosslinking agent and also
molar percentage of crosslinks. The three types of
crosslinking agents used differ widely in their polarity.
Thus the polymers prepared from these three types of
crosslinking agents differ in the hydrophobic/hydrophilic
nature. This affects their swelling characteristics
which in turn affects the reactivity of the attached
function. Thus by the introduction of divinylbenzene as
crosslinks, the hydrophobicity and rigidity again
increases and the reactivity of the corresponding
brominated resin decreases. Introduction of the
hydrophilic TTEGDA-crosslinking agent increases the
flexibility and hydrophilicity of the crosslinked polymer
system. From the synthetic reactions carried out with
PVP-Br which contain varying amounts of TTEGDA-
crosslinking, it was observed that 15% crosslinked
polymer was most reactive. This shows that increased
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hydrophilicity enhances the reactivity of the PVP-Br
reagent. NNMBA-crosslinked PVP-Br has a reactivity
intermediate between DVB- and TTEGDA-crosslinked PVP-Br.
In order to study the solvation properties of
crosslinked PVP-Br, solvents of varying polarity were
selected and oxidation of benzoin were carried out with
each of these solvents. Benzene was found to be the most
suitable solvent for the hydrophobic DVB-crosslinked PVP-
Br. For NNMBA-crosslinked polymer, the most effective
solvent is chloroform. When the crosslinking agent is
TTEGDA, the most suitable solvent is dichloromethane.
These studies show that the nature of the polymer
backbone dictates the most suitable solvent for the
effective reaction.
The effect of concentration of the reagent during
oxidation was studied by taking the oxidation of benzoin
to benzil as the model reaction with different reagent-
to-substrate ratio (1:1, 2:lr 3 : l r 4:lr 5:l). In all the
cases a higher excess was found to result in an enhanced
rate of reaction.
Another type of reagent developed was based on t-
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butyl hypochlorite. These reagents can also be used for
the oxidation and halogenation of' organic substrates.
The two supports used for the immobilization of this
function were PS-PEG graft copolymer and TTEGDA-
crosslinked polystyrene. Polystyrene-PEG graft
copolymers were prepared from chloromethylated
polystyrene (2% DVB-crosslinked) and t-butyl hypochlorite
function was introduced into it by a series of polymer-
analogous reactions. Here the t-butyl hypochlorite
function was separated from the polymer matrix by a long
amphiphilic polyoxyethylene chain. In TTEGDA-crosslinked
polystyrene, the ethyleneoxide linkage is in the
crosslinks and not on the pendant side chain as in the
case of PS-PEG grafts. The reactivities of the reagents
prepared from these two types of supports were compared
and it was observed that TTEGDA-crosslinked polystyrene
supported hypochlorite was more reactive than PS-PEG
supported t-butyl hypochlorite. By the introduction of
PEG300 into chloromethylated polystyrene, the pore
dimension of the original polymer will change and it will
become difficult for the substrate molecules to diffuse
into the polymer matrix. This non-availability of active
function could be the reason for the slow reactivity of
PEG-bound hypochlorite function.
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Investigation of the nature of the polymer matrix on
the reactivity of the attached oxidising function depends
on a number of factors characteristics of the
macromolecular matrix. Thus the reactivity of the PVP-Br
complexes were found to be considerably increased by
increasing the hydrophilic nature of the support. By the
selection of appropriate crosslinking agent and adjusting
the crosslink density it is possible to design polymer
matrices with desired flexibility/rigidity, mechanical
integrity and optimum hydrophobic-hydrophilic balance.
Solvent effects modify the diffusion of substrates and
products inside the resin. These studies also indicated
that linear and crosslinked PVP-Br complexes could be
developed as efficient solid phase polymeric reagents for
oxidation and bromination of organic substrates. From
the studies carried out with t-butyl hypochlorite reagent
functions on different types of polymeric supports, it
was observed that the ethyleneoxide linkage in the
crosslinks facilitates the efficiency of the synthetic
reagent. The macromolecular structural parameters
operating on the synthetic efficiency could also be
delineated from these studies.
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R E F E R E N C E S
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