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CHAPTER I1
BENZIL-BENZILIC ACID REARRANGEMENT IN CROSSLINKED
MACROMOLECULAR SYSTEMS
Control over reactivity, rate and specificity can be
attained in functional transformations of organic
compounds by attaching the reactive species on a polymeric
backbone. The polymer matrix is often intended as a label
for mechanistic investigations of polymer supported
organic reactions. Molecular rearrangements are governed
by a number of parameters characteristic of the substrate
and the environment. In polymer-analogous molecular
rearrangements, the polymer provides a unique
microenvironment for the rearrangeable functional. group
and thus participates in the course of the rearrangement.
The polymer can influence the mechanistic and kinetic
responses of the functional species by its characteristic
molecular property and topographical behaviour.
The base-induced transformation' of an OC -diketone
into an oC-hydroxy acid is one of the most important
molecular rearrangements in organic chemistry 137-140. The
reaction is recognized as the prototype of a general
class of rearrangements and has been the subject of
numerous synthetic and mechanistic investigations
including applications of tracer techniques with isotopes
of carbon, hydrogen and oxygen. The rearrangement has
been investigated in the case of aromatic, semi-aromatic,
alicyclic, aliphatic as well as in heterocyclic
&- diketones 141-150. In view of this generalized nature
of the benzil-benzilic acid rearrangement it was thought
of interest to examine the effects of possible
macromolecular constraints on the rearrangement. when
carried out in a polymeric environment or on a polymer-
supported system.
s his chapter deals with the investigations of the
rearrangement of PC-diketo systems attached to an
insoluble, crosslinked polymeric network through a
covalent bond and rearrangement of benzil units existing
as part of the chain of the soluble linear polymers. The
preparation of polymeric K-hydroxy compounds and the
effects of the polymeric backbone on the ext.ent of
reaction and oxidation of the benzoin analogue into the
&-diketone are subjected to detailed study. The study
centers mainly on the rearrangement of
polymeric &-diketones into g-hydroxy acids. The effects
of molecular level reaction parameters are discussed. The
role of the macromolecular backbone on the course of the
rearrangements is investigated in detail. The molecular
character, frequency of crosslinking units and the
swellability in solvents are the factors which deem a
thorough investigation in these cases. The migratory
aptitude is studied under different reaction conditions
and the possible role of the 'polymer effect' is
discussed.
Terephthalaldehyde (TPA) was subjected to self
benzoin condensation and the resulting linear polybenzoin
was converted to polybenzil 151f152. Facile benzil-
benzilic acid rearrangement was observed in these systems.
In polybenzil derived from TPA, the rearrangeable function
exists as part of the backbone itself, not as a pendant
group, (in contrast to the polymer supported systems) and
the rearrangement process demands a chain contraction
within the polymer backbone.
RESULTS AND DISCUSSION
Polymers have been designed to serve as a support
material by immobilizing the rearranging systems and to
provide a typical hydrophobic or hydrophilic environment
for the functional groups. The chemical and physical
participation of the matrix in the course of the
rearrangement is related to the so called polymeric effect
which in turn is related to the molecular character of
the monomers. Properties such as the polar nature and
hydrophilic-hydrophobic composition are dependent on the
molecular properties of the monomers and thus a proper
choice of the monomer can produce polymer supports with
different physicochemical properties. The structurally
different polymer supports can impart different
microenvironmental effect^ on the rearranging functional
species. Two different types of polymer supports were
designed to investigate the benzil-benzilic acid
rearrangement in polymer matrices.
1 (a). Divinylbenzene (DVl3)-Crosslinked Polystyrene (1)
Styrene-DVB polymers with different crosslink
densities were prepared by suspension polymerization. The
inhibitors were removed from the monomers by washing with
0.1 N sodium hydroxide and water. The monomers in the
required ratio were dissolved in toluene and the mixture
was suspended in water containing PVA (MW 72000) as the
stabilizer. Benzoyl peroxide was used to initiate the
free radical polymerization. The size of the polyrner bead
depends on the extent of dispersion in solution, the rate
of agitation and the temperature. When the polymerization
is initiated, tough, insoluble and almost co~npletely
spherj.ca1 crosslinked beads of the polymer precipitate
out. DVB is a rigid and non-polar crosslinking agent and
the polymer produced by the copolymerization of styrene
and DVB is hard, rigid and hydrophobic (Scheme 11.1).
Scheme 11.1. Preparation of DVB-crosslinked polystyrene
DVB-crosslinked polystyrene samples with crosslink
densities 2, 5, 10, 15 and 20 mole per cent were prepared.
These "crosslink density" percentages are not absolute;
the figures indicate the relative amount of the
crosslinking agent in the polymerization mixture. The
amount of built in crosslinking agent in the crosslinked
polymer cannot be exactly determined. The crosslinking
percentages were adjusted by varying the monomer-
crosslinker ratio. Commercially available DVB c:ontainS
about 55% polymerizable isomer and the rest is a mixture
of ethyl styrene and other isomers. This composition was
taken into account in the calculation of the weight
of DVB.
The polymer sample obtained by the c~polymer~zation
was purified by repeated washing with water, ethanol,
dichloromethane, acetone and the resulting beads subjected
to soxhlet extraction with benzene for 24 h. The beads
were dried at 10oOc, weighed and the IR spectra were
recorded using KBr pellets. The spectrum was compared
with that of authentic samples. The yields ofthe pxoducts
are given in Table 11.1.
Table 11.1. preparation of DVB-crosslinked polystyrene(1)
Crosslink Wt. of the monomer (g) Yield density ..................... (9)
% Styrene DVB
(b). Tetraethyleneglycol Diacrylate (TTEGDAI-Crosslinked Polystyrene ( 2 )
The intrinsic reactivity of a functional group
attached to a polymeric backbone is identical to that of
the low molecular weight analogue. But the
microenvironment created within the macromolecular matrix
can change the reactivity of the active site. A polar
polymeric support has been designed with TTEGDA as the
crosslinking agent. The styrene-TTEGDA copolymer thus
produced provides a different local environment for the
rearranging system and facilitates a comparison of the
effect of the molecular character of the two supports on
the extent of the rearrangement. This polymer system has
a flexible network due to the extended length of the
crosslinks (Scheme 11.2).
Scheme 11.2. I'reparation of TTEGDA-crosslinked polystyrene
PS-TTEGDA resin was prepared by solution
polymerization technique using CH OH-CHC1 mixture as the 3 3
solvent and potassium persulphate as the initiator. 5,
10, 15 and 20 percent crosslinked resins were prepared by
adjusting the monomer ratio. The resin was freed from all
low molecular weight impurities by soxhlet extraction and
characterized by IR spectral analysis. The results are
given in Table 11.2.
Table 11.2. Preparation of PS-TTEGDA resin
Crosslink Wt. of the monomer ( g ) Yield density ..................... ( g )
( % ) Styrene TTEGDA
2. Chloromethylation of the Polystyrene (Resins 1 and 2)
Functionalization of styrene-based copolymers
involves electrophilic substitution on the aromatic ring.
The first step of the polymer-analogous reaction series
employed for introducing a dicarbonyl system into the
polymer backbone is the chloromethylation of the aromatic
ring. The reaction was carried out using anh:ydrous
SnC14, as the Lewis acid catalyst and chloromethyl methyl
ether. Dichloromethane was employed as the solvent
(Scheme 11.3).
Scheme 11.3. Preparation of chloromethyl polystyrene
The chloromethylated resins (3a & 3b)were purified by
repeated washing or soxhlet extraction using suitable
solvents. The degree of chloromethylation was determined
by estimating the chlorine content. In the Volhards
method of chlorine estimation, the resin was equilibrated
with pyridine and the pyridinium chloride thus formed was
treated with silver nitrate solution in excess. AyCl was
precipitated and the excess AgN03 was titrate'd with
ammonium thiocyanate using ferric alum as the indicator.
The results are presented in Table 11.3.
Table 11.3. Chlorine capacity of PS-DVB and PS-TTEGDA resins
PS-DVB resin (3a) PS-TTEGDA resin (3b)
Crosslink Chlorine capa- Crosslink Chlorine capa- density ( % ) city (meq/g) density ( % ) city (rneq/g)
3. Synthesis of Polymeric Aldehydes (4)
An aldehyde functional group can easily be introduced
into the polymer matrix via chloromethylation. Resin 3
was purified by soxhlet extraction using chloroform. The
resin was dried and treated with dimethyl sulphoxide at
138Oc to convert the CH2C1 group into - CHO group
(Scheme 11.4).
Scheme 11.4. Preparation of polymeric aldehyde
Dimethyl sulphide was removed by repeated washing
with hot water and common organic solvents. 1:t was
subjected to soxhlet extraction using benzene. The
aldehyde function was tested by the 2,4-dinitro phenyl
hydrazine reagent. A bright orange colour was developed
in the resin beads. The extent of functionalization was
calculated by estimating the residual chlorine in the
resin. IR spectrum was recorded using KBr pellets and a
strong peak was observed at 1700 cm-l, corresponding to
the C=O stretching absorption. Results of the
estimations are given in Table 11.4.
Table 11.4. Capacity of polymeric aldehyde (4)
PS-DVB resin (4a) PS-TTEGDA resin (4b)
Crosslink Residual Aldehyde Crosslink Residual Aldehyde density chlorine capacity density chlorine capacity
( % ) (meq/g) (meq/g) ( % ) (meq/g) (meq/g)
4. Preparation and Characterization of Polymeric
Analogue of &-~ydroxy Ketone ( 5 )
The cyanide ion-catalyzed benzoin condensation of
aldehydes is one of the thoroughly investigated reaction
in organic chemistry. The reaction is an excellent
example for specific catalysis and perhaps for that
reason, was one of the early organic reactions subjected
to kinetic study at the beginning of the twentieth
century. The benzoin condensation was extended here to
polymeric aldehydes.
(a). Intrapolymeric Benzoin Condensation
The preliminary investigations of polymer-analogous
benzoin condensation were carried out using 2% DVB-
crosslinked polystyrene copolymer containing aldehyde
functions in the aromatic rings (resin 4a or 4b). The
pre-swollen resin was treated with KCN at 108-120'~~ and
the unreacted cyanide was removed by washing with water
and water-miscible organic solvents. The polymeric
aldehyde undergoes benzoin condensation giving the polymer
analogue of K-hydroxy ketone (Scheme 11.5).
KCn/ EtOH
Dioxane
~ H O ~ H O HC C I II OH 0
5a or 5b
Scheme 11.5. Intrapolymeric benzoin condensation
The product was characterized by IR spectroscopy. A
broad peak was generated in the region 3380-3440 cm-I
corresponding to the 0 - H stretching vibrations of the
benzoin analogue. Peaks at 1090 and 1270 cm-I originate
from the C-0 stretching and 0 - H deformation vibrations
respectively. The strong peak at 1695 cm-I is due to the
C=O stretching vibrations (Figure 11.1).
The benzoin analogue (5a & 5b) obtained by the
cyanide ion-catalyzed condensation of polymeric aldehyde
was also characterised by noise-decoupled C ~ ~ N M R
spectroscopy. The carbon atoms C7 and C8 of the benzoin
analogue show characteristic peaks (Figure 11.2).
The carbonyl carbon shows a distinct peak at 166.0
ppm. The peak at 86.2 ppm corresponds to the carbon atom
to which the hydroxyl group is bonded. The ring carbons
exhibit characteristic peak at 126 ppm.
(b). Crossed Benzoin Condensation Between Pol.ymeric Aldehyde (4a or 4b) and Substituted Benzaldehydes
A polymer containing benzoin units is obtained by the
crossed benzoin condensation between polymer-bound
aldehyde and benzaldehyde. A heterogeneous mixture of the
two aldehydes was treated with cyanide under optimum
reaction conditions. Three possible products are expected
as depicted in Scheme 11.6.
3-0 E ~ O H AH o
Scheme 11.6. Crossed benzoin condensation between polymeric aldehyde and substituted benzaldehydes
The self condensation between low molecular aldehydes
and crossed condensation between the polymeric aldehyde
and low molecular aldehyde (Step I and I1 in Scheme 11.6)
are observed to be facile. The cyanide ion readily
attacks the low molecular aldehyde resulting in the
formation of the "active aldehyde" intermediate species,
leading to the formation of 5c or 5d and low mo:lecular
benzoin. The mixture of the product was washed with
organic solvents like toluene, benzene, dichloro~nethane
and acetone. This removes the low molecular benzoin. The
polymeric benzoin analogue was collected, and dried under
vacuum. IR spectra show peaks at 3380-3440, 1680-1700,
1270, 1090 cm-l corresponding to 0-H stretching, C=O
stretching, 0-H deformation and C-0 stretching vibrations
respectively.
A series of substituted benzaldehydes were employed
for the synthesis of crossed benzoins. The advantage of
the mixed benzoins over the self condensation product is
the increased freedom of mobility of the reaction site due
to its less rigidity (Scheme 11.7).
H-C - C H-C- C I I I OH 0
I I I OH 0
I
HC-OH I
HC-OH I C=O
Scheme 11.7. Polymeric analogues of self and mixed benzoins: rigid and flexible systems
Ortho- and para- substituted benzaldehydes were
subjected to benzoin condensation and the hydroxyl
capacity was measured by acetylation. These results were
used to draw a correlation between the substituent effect
and extent of benzoin condensation (Table 11.5).
Table 11.5. Effect of substituents on the extent of P O ~ Y ~ ~ I : analogous mixed benzoin condensation
Low molecular Hydroxyl capacity Percentage aldehyde of mixed benzoin condensation
( meq/g ( % )
Benzaldehyde 2.6
0-chlorobenzal- 1.5 dehyde
P-chlorobenzal- 1.7 dehyde
Cinnamaldehyde 2.4
Anisaldehyde 2.8
0-nitrobenzal- 1.3 dehyde
P-methyl benzal- 2.6 dehyde
The yield of the benzoin units was low when ortho-
substituted benzaldehyde was used for the condensation.
Electron-withdrawing substituents were found to decrease
the extent of the condensation reaction. This could be
due to the steric and electronic destabilization of the
intermediate species and the resultant decreased
reactivity of the species to attack the carbonyl carbon of
the polymeric aldehyde.
Various substituents were introduced into the
polymeric system through the low molecular benzaldehydes
in order to study the substituent effects on the extent of
polymeric benzil-benzilic acid rearrangement. From these
observations it appears that, the polymer-analogous
benzoin condensation is also sensitive to the electronic
and steric participation of the various substituents
present in the low molecular part of the mixed benzoins.
A weight increase approximately corresponding t.o the
molecular weight of the low molecular aldehyde was
observed during the course of mixed benzoin condensation.
(c). Effect of Crosslinking on Polymeric Benzoin Condensation
DVB-crosslinked polystyrene and TTEGDA-crosslinked
polystyrene (2, 5, 10, 15 and 20 per cent crosslink
densities) with aldehyde groups in the aromatic ring were
subjected to intrapolymeric benzoin condensation. The
reactions were carried out under identical conditions and
the benzoin units were estimated by chemical methods. The
percentage condensation was calculated and the feasibility
of the reaction was observed to decrease gradually with
increasing crosslink density (Table 11.6).
Table 11.6. Effect of crosslinking on benzoin condensation in crosslinked po:lymeric systems
PS-DVB resin PS-TTEGDA resin
Resin Cross- Percent- Resin Cross- Percent- No. link age con- No. link age con-
density densation density densation ( % ) ( % ) ( % ) ( % )
These results show a regular gradation in the extent
of condensation with crosslink density in the case of the
PS-DVB resin. Comparatively high benzoin capacity was
expected for PS-TTEGDA resin due to its flexible nature
and easy accessibility of the cyanide ion for the atldehyde
function. But the results show that in many cases the
percentage condensation is less in the case of PS-TTEGDA
resin compared to PS-DVB resin with the corresponding
crosslink density. These observations are explainable on
the basis of the chain length of the TTEGDA crosslinking
units. Due to the flexible nature of the crosslinking
units, the coiling of the polymer chain is not intensive
and thereby the aldehyde functions are spaced far apart.
Under these circumstances, due to the increased distance
between the reactive sites, a state of high dilution is
attained and the aldehyde groups are less prone to the
condensation reaction.
On the other hand, in the PS-DVB resin, the reactive
sites are rigidly held in the backbone due to the well-
defined morphology of the resin. DVB is more rigid than
TTEGDA crosslinks and the aldehyde groups are more closer
in the backbone. Therefore benzoin condensation is more
feasible in this polymer system.
(a). Effect of Hyperentropic Factor on Intrapolymeric Benzoin Condensation
The proximity of the pendant aldehyde groups bonded
to the phenyl rings in the polymer matrix is a decisive
factor which controls the extent of benzoin condensation.
The facile intrapolymeric benzoin condensation gives
evidence for the effective site-site interaction in
polymer-bound aldehydes. This was again tested by
preparing polymeric aldehydes with varying functional
group capacity. The resulting aldehydes were subjected to
benzoin condensation under identical reaction conditions
and the extent of intrapolymeric benzoin condensation was
estimated. The results show a linear relationship between
the functional group capacity of aldehyde and hydroxyl
value of benzoin (Figure 11.3). This gives evidence for
the effective site-site interaction in crosslinked
polymers. Otherwise, no intrapolymeric benzoin
condensation is possible in the case of aldehydes with
very low functional group capacity.
0 1 2 3 4 5
-CHO Capacity (meq/g)
Figure 11.3. Effect of hyperentropic f act(or on intrapolymeric benzoin condensation
5. Synthesis of Polymeric Diketones (6)
A diketo group was introduced into the polymer matrix
by oxidising the &-hydroxy ketone (resin 5a, 5b, 5c or
5d) using nitric acid as the oxidising agents. The yellow
colour of the benzoin analogue was intensified during
oxidation. In a second method resin 5 was treated. with
copper (11) acetate, ammonium nitrate and aqueous acetic
acid. The product was purified by repeated washing until
it is free from the last trace of acid. Polymeric
analogues of benzil (6a-6d) were obtained in almost
quantitative yield (Scheme 11.8).
H-C - C I OH
II 0
Scheme 11.8. Oxidation of polymeric analogue of benzoin into benzil
The IR spectrum shows a strong band at 1700 crn-I
corresponding to the C=O stretching vibration. The
spectrum appears as a single peak in this region with a
shoulder. This observation suggests a trans configuration
for the dicarbonyl groups. Therefore both the carbonyl
groups are not IR active. This configuration is
attributable to the rigidity of the polymer backbone and
the resulting spatial strain. The strong band in the
region 3440 cm-l which was present in the benzoin analogue
disappeared (or diminished) during oxidation
(Figure 11.4).
"1 3 C' CP-MAS spectrum was also used for monitoring the
benzoin-benzil conversion. The peaks at 173.6 ppm and
194.2 ppm were assigned to the carbon atoms of the
dicarbonyl group. The peak at 194.2 ppm is entire1:y a new
peak and was not present in the benzoin analogue (Figure
11.5). This shows that the carbon (C7) to which the
hydroxyl group is bonded in the benzoin analogue is
converted to a carbonyl carbon. The presence of well
separated peaks at 173.6 ppm and 194.2 ppm correspond to
the carbonyl carbons show an environmental difference .of
the carbon atoms. This appears to be imparted by the
backbone material.
The unreacted hydroxyl group in the benzil analogue
was estimated by acetylation method. The rearrangeable
diketo function was calculated from these results.
Benzil-Benzilic Acid Rearrangement in Crosslinked
Polystyrene Networks
The diketo resin was subjected to benzil-benzilic
acid rearrangement under basic conditions. The pre-
swollen resin was treated with potassium hydroxide and
absolute ethanol at 1 2 0 ~ ~ . A gentle and constant magnetic
stirring was applied throughout the course of the
reaction. The hydroxide ion attacks the carbonyl carbon
of the diketo group and the resulting species undergoes a
benzil-benzilic acid type rearrangement (Scheme 11.9).
'm C- C *-@$ C - C-OR -'@ II I I I I I / \ 0 0 0 - 0 0 - C-OR
I\ 0
'C"
HO / 'COO-
Scheme 11.9. Polymer-analogous benzil-benzilic acid rearrangement
The potassium salt of the & -hydroxy acid was
treated with HC1-dioxane mixture and stirred at room
temperature. The free acid was washed with water and
dried. The resin shows the characteristic tests for an
organic acid. The carboxyl group was estimated by
equilibrating a weighed quantity of pre-swollen sample
with standard sodium hydroxide at room temperature. The
hydroxyl group was also estimated and the results obtained
from the estimation are agreeable .with that of the
carboxyl values.
The IR band in the region 3380-3440 cm-I which was
present in the benzoin analogue and disappeared on
oxidation, reappeared during rearrangement. The resin was
analysed at different time intervals of the reaction and a
gradual reappearance of the peak was observed. This
corresponds to the 0-H stretching absorption of the
tertiary hydroxyl group. The shoulder of the C=O
stretching band which was present in the benzil analogue
in the region 1680-1700 cm-I disappeared during the course
of the rearrangement. However, the carbonyl absorption
remains strong and intense in this region with a small
shift to the higher wavenumber region. The C-0 stretching
and 0-H deformation absorptions also reappeared during the
rearrangement. A typical IR spectrurn of polymeric
analogue of benzilic acid to (7) is given in Figure 11.6.
The carbon atom bonded to the hydroxyl and carboxyl
groups shows a characterisitc peak at 89.2 ppm in the
c13 NMR spectrum (Figure 11.7) . The peak at 194.2 ppm in
the benzil analogue disappeared and. the peak at
173.6 ppm was shifted to 166.6 ppm. The low-field shift
of the carbonyl carbon clearly indicates the generation of
-COOH group from dicarbonyl group 153-156. The results are
in accordance with the results obtained from chemical
analysis and IR spectra.
The chemical and spectral analyses give conclusive
evidence for the intrapolymeric rearrangement. The
reaction was observed to be facile in the crosslinked
networks inspite of the environmental constraints imposed
by the rigid, crosslinked, high molecular weight polymeric
backbone. However, the possibility for the complete
detachment of the migrating group from the migration
origin during the rearrangement can be ruled out. If
complete bond breaking occurs before the bond formation
with the migration terminus, it would be difficult for the
bulky polymeric moiety to migrate to the carbon atom.
Therefore, it appears that the rearrangement involves a
cyclic transition state, with bond breaking and bond
formation taking place in a single step (Scheme 11.10).
C - C
0 0 II i l 0 0
'@+\q$i 5 +q C C / \
SO-?' KO-C OH HO- C I I ' 'OH II
0 0 0
Scheme 11-10. Mechanism of polymeric benzil-benzilic acid rearrangement
(a). Effect of Substituents
A series of polymeric 1,2-diketones with different
substituents in the aromatic ring were prepared by the
oxidation of mixed benzoins. Merrifield resin was used as
the matrix. These resins were subjected to benzil-
benzilic acid rearrangement under identical conditions.
The hydroxyl and carboxyl capacity of the resins were
estimated and compared. Results of the preliminary
investigations are given in Table 11.7.
Table 11.7. Hydroxyl values in polymers 5c, 6c and 7c and carboxyl capacity of resin 7c
Structure Reaction Temp. Hydroxyl value (meq/g) Carboxyl of time ....................... value of
diketone (h) (OC) Benzoin ~enzil Benzilic benzilic acid acid
5c 6c 7c (meq/g )
Facile rearrangement was observed in these mixed
diketones also. It was also observed that the reaction
conditions like the extent of reaction and temperature are
less rigorous in the case of mixed benzils than those
required in the case of benzil obtained by self-
condensation. The system is less rigid and the reactive
sites are more exposed to the reagent. Hence the
reactions are more feasible in such cases.
The rearrangement of mixed benzils also follow the
same mechanism as shown in Scheme 11.11.
Scheme 11.11. Mechanism of the rearrangement of mixed benzil into benzilic acid
Mixed diketones with o-chloro, p-chloro, p-methyl, p-
methoxy, o-nitro groups and some other groups as the
substituents were subjected to rearrangement. The effects
of the substituents were investigatd based on the
percentage migration of the diketone. The percentage
migration, in each case, was calculated from the
rearrangeable diketo groups and the hydroxyl and carboxyl
capacity of the rearranged product. The results are
given in Table 11.8.
Table 11.8. Effect of substituents on polymer-analogous benzil-benzilic acid rearrangement
Structure of the diketone
Hydroxyl Diketo Hydroxyl Percentage capacity capacity capacity migration of benzoin of benzil of benzi-
lic acid 5c 6c 7 c
(meq/g ) (meq/g ) (meq/g ) ( % )
These results indicate that the' ortho effect is
prominent in polymer analogous molecular rearrangements.
Polymeric benzil with o-chloro substituent gives only
37.8% migration whereas the corresponding para-compound
gives 63.7% migration. Ortho-nitro compound shows only
41.6% migration. This can be explained on the basis of
the steric participation of the ortho substituent which is
not favourable for the rearrangement since the formation
of a new covalent bond between the migrant and the
migration terminus is hindered.
Diketones with electron donating substituents were
observed to give comparatively high yields of the product.
The +I effect of the methyl and methoxy groups favours
the attack of migrant to the cabonyl carbon which is the
migration terminus. Thus, diketones with p-methyl and
p-methoxy groups showed 94 .0 and 88.5 per cent migration
respectively. Diketone without any substituent in the
phenyl ring gives 90.4 per cent migration.
However, the studies of the substituent effect on the
extent of polymer-analogous benzil-benzilic acid
rearrangement do not indicate any regualr trend in the
electronic effects of substituents. For example, the +I
effect of the -OCH3 group is larger than that of the -CH3
group. But diketone with methyl substituent gives 94.0
per cent migration whereas the corresponding methoxy
systems gives only 88.5 per cent migration. These results
suggest that the molecular level reaction parameters are
subject to complications by the inestimable polymer
effecl..~ arising from the complexity of the crosslinked
systems.
(b). Effect of the Nature of Crosslinking
The molecular characteristics of the crosslinking
agents like its polarity, hydrophilicity and rigidity
were found to affect the migratory aptitude of the
rearrangeable functions. For a comparative study, two
different types of polymer supports were employed. PS-DVB
resin is a typical hydrophobic polymer with rigid
crosslinking units. On the other hand, PS-TTEGDA resin is
a hydrophilic and polar polymer support with flexible
crosslinking. The diketo group was attached to both the
polymers and subjected to rearrangement under identical
reaction conditions. Dioxane was used as the solvent for
PS-TTEGDA resin whereas toluene was used as the solvent
for PS-DVB resin. The results are presented in
Table 11.9.
Table 11.9. Effect of the nature of crosslinking on the rearrangement
Crosslink Crosslinking Duration Solvent Percentage density agent of reaction migration
( % ) (h) ( % )
DVB 75 Toluene 82.3 5
T T E G D A 6 0 Dioxane 85.2
DVB 7 5 Toluene 74.0 10
T T E G D A 6 0 Dioxane 75.0
DVB 7 5 Toluene 54.5
'TTEGDA 6 0 Dioxane 60.0
85.2 per cent migration was observed in 60 h for 5%
PS-TTEGDA resin. But for 5% PS-DVB resin only 82.3 per
cent migration was observed in 75 h duration. This was
the case with resins of 10 and 15 per cent crosslink
density. The results are explainable on the basis of the
crosslinking pattern of the two polymer networks. The
reactive sites are buried deep in the rigid polymer
network in the case of PS-DVB resin and the diffusion
controlled movement of the hydroxide ion into the interior
of the polymer is difficult. But the reactive sites are
more available to the reagent in the case of PS-TTEGDA
resin due to its less rigid nature.
(c). Effect of the Degree of Crosslinking
The microenvironmental effect of the polymeric
backbone on the extent of migration of the rearrangeable
functional group attached to it is determined by the
frequency of crosslinking units within the matrix. A
correlation between the percentage migration and extent of
crosslinking was obtained using two 'different polymer
supports with varying crosslink densities.
PS-DVB resin with 2, 5, 10, 15 and 20 per cent
crosslink densities were prepared and converted to the
corresponding benzil analogues by a series of polymer
analogous reactions. Benzaldehyde was used for preparing
mixed benzoins. Rearrangement was carried out under
identical conditions using toluene as the solvent. The
percentage migration was calculated by chemical methods.
Typical results are given in Table 11.10.
Table 11-10. Effect of the divinylbenzene content on the extent of rearrangement
Crosslink Hydroxyl Diketo Hydroxyl Percentage density capacity capacity capacity migration
( % ) of benzoin of benzil of benzi- lic acid
5c 6 c 7 c (meq/g) (meq/g ) (meq/g) ( % )
Diketo function was introduced into the hydrophilic
PS-TTEGDA resins with 5, 10, 15 and 20 per cent crosslink
densities. Polar solvents like dioxane is more compatible
to the polar polymer matrix and the reaction is carried
out in dioxane under identical conditions. The &-hydroxy
acid formed by the rearrangement was estimated. The
results are given in Table 11.11. Hydroxyl groups
obtained by the partial cleavage of the ester functions of
a few crosslinking units were exempted from the
calculations.
Table 11.11. Effect of the tetraethyleneglycol diacrylate content on the benzil-benzilic acid rearrangement
Crosslink Hydroxyl Diketo Hydroxyl Percentage density capacity capacity capacity migration
( % ) of benzoin of benzil of benzi- lic acid
5d 66 7d (meq/g 1 (meq/g (meq/g 1 (%)
Diketo systems attached to PS-DVB and PS-TTEGDA
resins show regular decrease in the extent of
rearrangement with increase in crosslink density. 2% PS-
DVB resin gives 94.4% migration whereas 20 per cent PS-DVB
resin gives only 25% migration. The trend is similar in
the case of PS-TTEGDA resin also. Here the 5% resin gives
85.2% migration whereas the 20% matrix gives only 38.4%
migration.
These results can be explained as arising from the
increased rigidity of the polymer matrix and hence the
poor accessibility of the reagent to the reactive sites
rather than the steric participation of the polymer matrix
on the course of the rearrangement. The diffusion-
controlled penetration of the reagent into the interior of
the matrix is prevented by the high frequency crosslinks.
The decrease in the extent of rearrangement with the
increased degree of crosslinking is more prominent in the
case of PS-DVB resin due to the high rigidity of the
matrix. Hydrolysis of ester crosslinking units was
observed to some extent in the case of PS-TTEGDA resin.
Due to this hydrolytic reaction, unexpectedly higher
functional group capacities were recorded in the
estimation processes. Hence, a control experiment was
conducted in the case of PS-TTEGDA resin and this
functional group value was subtracted from the final
values (Table 11.11).
(a). Effect of Solvation
The effect of solvent on the course of benzil-
benzilic acid rearrangement was studied by using
1,2-diketo systems attached to a hydrophobic PS-DVB matrix
and a hydrophilic PS-TTEGDA matrix (resins 6c and 6d) . A
series of solvents with varying polarity were used for the
investigations. The rearrangement was carried out in
basic medium under identical conditions. The solvents
used for the studies are toluene, benzene, methanol,
water, THF and dioxane. The extent of rearrangement was
calculated in all the cases. Typical results are given in
Tables 11.12 and 11.13.
Table 11-12. Effect of solvation on benzil-benzilic acid rearrangement in PS-DVB matrix
Crosslink Percentage migration ( % ) density ...............................................
( % ) Dioxane THF Methanol Water Benzene Toluene
Table 11.13. Effect of solvation on benzil-benzilic acid rearrangement in PS-TTEGDA matrices
- - --
Crosslink Percentage migration ( % ) density ...............................................
( % ) Dioxane THF Methanol Water Benzene Toluene
Favourable interaction between the polymeric matrix
and the solvent is an essential factor for the effective
functionalization and functional group transformation in
polymer matrices. In polymer supported strategy, the
functional groups are anchored or immobilized on the
polymer support. These reactive sites are distributed on
the surface of the polymer beads or it may be buried in
between the crosslinks. If the polymer and solvent are
compatible, the movement of the reagent is facilitated by
the good swelling behaviour of the backbone and hence the
reaction rate increases. If the solvent and the polymer
are totally incompatible, the reaction is almost
inhibited. Benzene and toluene are found to be the best
solvents for the benzil-benzilic acid rearrangement in
hydrophobic PS-DVB networks (Figure 11.8).
PS-DVB resin undergoes effective swelling in non-
polar solvents like benzene and toluene. In a good-
swollen benzil analogue, the movement of the hydroxide ion
into the interior of the polymer is facilitated. This
favours the attack of the hydroxide ion at the carbonyl
carbon of the diketo system and hence the percentage
migration. PS-DVB resin does not show an effective
swelling in solvents like water and methanol and the
movement of the attacking species is hindered and thereby
the percentage migration is reduced. PS-TTEGDA resin, on
the other hand, shows poor swelling property in
0 5 10 15 20 25
Crosslink Density ( 8 )
Figure 11.8. Effect of solvation on the extent of benzil-benzilic acid rearrangement in DVB- crosslinked polystyrene matrix
hydrot'arbon solvents. However, it shows good swelling
behaviour in polar solvents like dioxane, THF and
methanol. PS-TTEGDA resin immobilized diketo systems
therefore undergo higher extents of migration in these
solvents (Fiyure I1 . 9 ) .
Crosslink Density ( % )
Figure 11.9- Effect of solvation on the extent of benzil-benzilic acid rearrangement in TTEGDA-crosslinked polystyrene matrix
Crosslinked polymers are macroscopically insoluble in
almost all the solvents. They can be solvated only to a
limited extent. This limited solvation also depends on
the molecular character of the polymer backbone. However,
by absorbing considerable amount of solvent, the
crosslinked polymeric network can expand largely and
become extremely porous forming a pseudo-gel. With
increased crosslinking, the polymer becomes more and more
rigid and free space in between the crosslinks available
for penetration of solvent is reduced. Thus the ability
for uptake of solvents is reduced. DVB and TTEGDA-
crosslinked polystyrenes represent two different types of
polymer supports with entirely different molecular
properties. The compatibility of the polymer with
different solvents is thus different depending on the
nature of the solvent. The migratory aptitude in polymer-
analogous molecular rearrangement is determined by the
characteristics of the solvents which influence the
swelling pattern of the matrix. In solvents which cannot
effectively swell the polymer network, movement or
diffusion of the reagent within the network to the
migration origin is difficult and hence the rate and
overall extent of the rearrangement are considerably
decreased.
7. Kinetics of Benzil-Benzilic Acid Rearrangement in
Crosslinked Polymeric Matrices
Standard kinetic analysis of polymer supported
reactions continues to be a challenging problem. Due to
the true heterogeneity of polymer supported reaction
systems, attempt to quantify the differences in reaction
rates between polymer supported reactions and its low
molecular analogues can be misleading. Moreover it may
not be accurate to speculate on possible mechanistic
difference between the homogenous and supported reactions.
Moreover diffusional limitation is imposed on reactions
occuring in crosslinked polymeric networks. All these
factors limit the utility and interpretation of kinetic
observations.
However, rate constants calculated based on the
probable rate equation can be used as a probe to
differentiate between reactivity under various reaction
conditions. Studies using easily swellable 5% TTEGDA-
crosslinked polystyrene matrix indicate that, benzil-
benzilic acid rearrangement in polymeric matrices follows
the second order reaction kinetics13'. Kinetics of the
rearrangement was followed in different solvents by
titrimetric method and the rate constants were calculated.
The k values are a measure of the solvation effect. The
rate constant values are given in Table 11.14.
From the results it is clear that dioxane is the best
solvent for benzil-benzilic acid rearrangement. In
dioxane medium, the observed k value was 3.08 x
-1 -1 (mole/litre) min . A gradual decrease in rate constant
values was observed with change in the polarity of the
solvent. The k value for the reaction in water is only
- 1 0.23 x (rnole/litre)-l rnin . The reactions were
Table 11-14. Rate constants of benzil-benzilic acid rearrangement in polymeric matrices
Solvent Solvent/water Rate constant ratio k
(mole/litre)-'min-'
Dioxane 2:l 3.08 x
THF 2:l 2.36 x
Methanol 2:l 0.81 x
Water - 0.23 x
carried out at the boiling point of the solvents and all
other reaction conditions were kept constant. Rate
constants were calculated by following the concentration
of potassium hydroxide present in the bulk phase
titrimeterically. Pre-swollen resin, rather than dry
resin is more suitable for carring out these studies for
getting consistent results.
8. Investigations of Salt Effect
Kinetic studies were carried out in presence of added
salts with different ionic strengths. It was observed
that the rate constants of polymer-analogous benzil-
benzilic acid rearrangement are sensitive to the presence
of these salts. KC1 and BaC12 with different
concentrations were employed for the studies. The kinetic
observations are presented in Tables 11.15 and 11.16.
Table 11-15: Salt effect on benzil-benzilic acid rearrangement in polymer matrices: Effect of KC1
Concent- Concent- Concent- k ration of ration of ration of (mole/litre)-' benzil KOH KC1 (meq/g ) (N) (N) mi n -1
Table 11.16: Salt effect on benzil-benzilic acid rearrangement in polymer matrices14 Effect of BaC12
Concent- Concent- Concent- k ration of ration of ration of (mole/litre)-l benzil KOIl BaC1
(N) (N) -1
(meq/g ) min
The kinetic picture of the benzil-benzilic acid
rearrangement in polymer matrices was complicated by the
addition of outside ions. KC1 or BaC12 increases the
ionic strength of the reaction medium and this increases
the rate. The effect is more pronounced with barium
chloride (Figure 11-10). The results are in accordance
with the positive salt effect observed in the
case of low-molecular weight benzil-benzilic acid
rearrangement 138,139
Figure 11.10- Salt effect on benzil-benzilic acid rearrangement in polymeric matrices
1.0 - 0 = KC1
= BaC12
0.8 -
* I 0 3 0.6 - \ Y
0.4 6
0.2- .
0 0.1 0.2 0.3 0.4 0.5
Concentration of the salt [N)
9. Molecular Rearrangements in Macromolecular
Solutions
The course of benzil-benzilic acid rearrangement in
insoluble crosslinked polymeric networks was discussed
earlier in this section. The effects of polymeric
backbone - its molecular character, frequency of
crosslinking and swellability were mentioned in this
connection. The preparation of a soluble polybenzoin from
a dialdehyde, its oxidation to the polybenzil and
rearrangement of the polybenzil into soluble &-hydroxy
acid by benzil-benzilic acid rearrangement are
investigated in this section.
The studies of molecular rearrangements in insoluble,
crosslinked polymeric matrices have some limitations due
to the heterogeneity of the system. The purification and
characterization of the product is difficult in these
systems. The interpretation of the results become
difficult in many cases. Such difficulties are largely
not there in the case of linear soluble, polymers. The
products are isolable with sufficient purity and
charactt:i~:ization is less difficult in these polymers.
(a). Synthesis of Polybenzoin ( 8 )
A polymeric analogue of benzoin was obtained by the
self-benzoin condensation of terephthalaldehyde. A
solution of the dialdehyde in dimethylformamide (DMF) was
subjected to benzoin condensation by using potassium
cyanide as the catalyst. A light yellow coloured
polymeria product was precipitated on acidification of the
reaction mixture. The last traces of the potassium
cyanide was removed by repeated washing with water. It
was expected that the dialdehyde undergoes a self-benzoin
condensation in presence of the cyanide ions
(Scheme 11.12).
OHC -Q CHO KCN Et OH/DHF II I
0 OH 0 OH
Scheme 11.12- Synthesis of polybenzoin from tere- phthalaldehyde
The product was characterized by spectral analysis.
IR spectrum was recorded using KBr pellets. The spectrum
shows characteristic peaks at 3440, 1680 and 1080 cm-I
corresponding to the 0-H str., C=O str., and C-0 str.
vibrations respectively.
NMR spectrum (Figure 11.11) was recorded in DMSO.
The polymer contains two types of ring protons with
different chemical and magnetic environment. This is due
to the presence of carbonyl and hydroxyl functional
groups in the molecule. 6 7.5-7.2 (m) and 6 8.1-7.9 (m)
correspond to the ring protons. 6 6.0 (s) is due to the -&H proton and 54.5-4.3 (s) corresponds to the hydroxyl
I
proton in the molecule.
(b). Synthesis of Polybenzil (9)
The polybenzoin obtained by the self-benzoin
condensation of TPA was oxidised to the correspondiny
polybenzil. The benzoin was heated with concentrated
nitril' acid. The secondary hydroxyl group was oxidised
into the carbonyl group. The reaction is depicted in
Scheme 11.13.
Scheme 11.13. Oxidation of polybenzoin into polybenzil
The precipitated polybenzil was washed repeatedly
with water to remove nitric acid. The products show
strong IR absorptions at 1700 cm-I corresponding to the
C=O stretching vibration. The band at 3440 cm-' which was
present in the benzoin analoguc disappeared during
oxidation reaction.
NMR spectral analysis gave some interesting results.
The spectrum (Figure 11.12) shows a peak at
68.18-8.01 (rn) corresponding to the ring protons. This
indicates the presence of only one type of ring protons.
The benzoin analogue showed two peaks ( 6 8.1-7.2) in this
region. This is due to the transformation of the
secondary alcoholic group into a carbonyl group resulting
the formation of a dicarbonyl system during oxidation.
Furthermore, the peaks at 66.0 (s) and 54.5-4.3 (s)
present in the benzoin analogue disappeared during the
formation of benzil. The -?H proton and hydroxyl proton I
disappeared during oxidation.
(c). Synthesis of Benzilic Acid (10) from Polybenzil: Benzil-Benzilic Acid Rearrangement
The benzil analogue was subjected to benzil-benzilic
acid rearrangement by applyiny the rearrangement
conditions. The resulting solution was acidified using
dilute HC1 and free benzilic acid was precipitated. From
the product analysis it is evident that, the rearrangement
was facile in these systems just like the rearrangement of
pendant gC-diketo systems. The reaction is depicted in
Scheme 11.14.
Scheme 11.14. Conversion of polybenzil from TPA into OC -hydroxy acid
The product was characterized by chemical and
spectroscopic methods. IR spectrum shows absorptions at
3450 cm-I corresponding to the 0 - H vibrations, which was
not present in polybenzil.
NMR spectrum (Figure 11.13) was recorded in DMSO.
10.1 (s) corresponds to the carboxyl proton. 8.1-
7.9 (in) and 6 4.4 (s) are due to the aromatic ring
protons and hydroxyl proton respectively.
The foregoing investigations indicate that a
rearrangeable functional group attached to a linear or
crosslinked polymeric support can undergo intramolecular
migrations under suitable conditions. The molecular
properties , and morphological characteristics of the
backbone are decisive factors in controlling the migratory
aptitudes. However, the rearrangement can be effected in
these macromolecular networks like any other solution
phase low molecular weight reactions. The effect of
molecular level reaction parameters and physical and
chemical nature of the polymeric matrix on the migratory
aptitude can be investigated. A systematic analysis of
the results offers a better understanding of the
mechanistic aspects of supported reactions and the
polymer effects .
MOLECULAR REARRANGEMENT IN MACROMOLECULAR CAVITIES