CHAPTER I1 BENZIL-BENZILIC ACID REARRANGEMENT IN CROSSLINKED MACROMOLECULAR SYSTEMS

Transcription

1 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 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

2 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 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 polymersupported 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

3 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 benzilbenzilic 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

4 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).

5 Scheme 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 monomercrosslinker 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.

6 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 Table 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

7 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 I'reparation of TTEGDA-crosslinked polystyrene

8 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 Table 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

9 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 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.

10 Table Chlorine capacity of PS-DVB and PS-TTEGDA resins PS-DVB resin (3a) PS-TTEGDA resin (3b) Crosslink Chlorine capa- Crosslink Chlorine capadensity (%) 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 Preparation of polymeric aldehyde

11 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 Table 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)

12 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 functions in the aromatic rings (resin 4a or 4b). aldehyde The pre-swollen resin was treated with KCN at '~~ 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).

13 KCn/ EtOH Dioxane ~ H O ~ H O HC C I II OH 0 5a or 5b Scheme Intrapolymeric benzoin condensation The product was characterized by IR spectroscopy. A broad peak was generated in the region 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~~NMR 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 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.

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16 (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 E~OH AH o Scheme Crossed benzoin condensation between polymeric aldehyde and substituted benzaldehydes

17 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 , , 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).

18 H-C - C H-C- C I I I I II OH 0 OH 0 I HC-OH I HC-OH I C=O Scheme 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).

19 Table Effect of substituents on the extent of PO~Y~~I: analogous mixed benzoin condensation Low molecular aldehyde Hydroxyl capacity of mixed benzoin Percentage condensation ( meq/g (%) Benzaldehyde chlorobenzal- 1.5 dehyde P-chlorobenzal- 1.7 dehyde Cinnamaldehyde 2.4 Anisaldehyde nitrobenzal- 1.3 dehyde P-methyl benzal- 2.6 dehyde The yield of the benzoin units was low when orthosubstituted 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.

20 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).

21 Table 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 condensity 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

22 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 welldefined 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

23 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 CHO Capacity (meq/g) Figure Effect of hyperentropic f act(or on intrapolymeric benzoin condensation

24 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 II OH 0 Scheme Oxidation of polymeric analogue of benzoin into benzil

25 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 ppm and ppm were assigned to the carbon atoms of the dicarbonyl group. The peak at 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 ppm and 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.

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28 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 120~~. 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 'm *-@$ C- C benzil-benzilic acid type rearrangement (Scheme 11.9). -'@ C - C-OR II II I I I / \ C-OR I\ 0 'C" HO / 'COO- Scheme Polymer-analogous benzil-benzilic acid rearrangement

29 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 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 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.

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31 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 ppm in the benzil analogue disappeared and. the peak at ppm was shifted to ppm. The low-field shift of the carbonyl carbon clearly indicates the generation of -COOH group from dicarbonyl group 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).

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33 C-C 0 0 II il 0 0 '@+\q$i 5 +q C / \ C SO-?' KO-C OH HO- C I I ' 'OH II Scheme 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 benzilbenzilic 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.

34 Table 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 selfcondensation. 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

35 Scheme 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.

36 Table 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 benzilic 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

37 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.

38 (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 Table Effect of the nature of crosslinking on the rearrangement Crosslink Crosslinking density agent Duration Solvent of reaction Percentage migration (%) (h) (%) 5 10 DVB 75 Toluene 82.3 TTEGDA 60 Dioxane 85.2 DVB 75 Toluene 74.0 TTEGDA 6 0 Dioxane 75.0 DVB 75 Toluene 54.5 'TTEGDA 60 Dioxane 60.0

39 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

40 identical conditions using toluene as the solvent. The percentage migration was calculated by chemical methods. Typical results are given in Table Table 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 benzilic 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 Hydroxyl groups obtained by the partial cleavage of the ester functions of a few crosslinking units were exempted from the calculations.

41 Table 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 benzilic 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-

42 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 benzilbenzilic 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 and

43 Table Effect of solvation on benzil-benzilic acid rearrangement in PS-DVB matrix Crosslink Percentage migration (%) density... (%) Dioxane THF Methanol Water Benzene Toluene Table Effect of solvation on benzil-benzilic acid rearrangement in PS-TTEGDA matrices Crosslink Percentage migration (%) density... (%) Dioxane THF Methanol Water Benzene Toluene

44 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 nonpolar solvents like benzene and toluene. In a goodswollen 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

45 Crosslink Density (8) Figure Effect of solvation on the extent of benzil-benzilic acid rearrangement in DVBcrosslinked 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 solvents (Fiyure I1.9 ). in these

46 Crosslink Density (%) Figure 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

47 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 TTEGDAcrosslinked 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 characteristics of the solvents which influence the 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

48 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% TTEGDAcrosslinked polystyrene matrix indicate that, benzilbenzilic 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. rate constant values are given in Table The 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 0.23 x (rnole/litre)-l rnin - 1. The reactions were

49 Table 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 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 benzilbenzilic 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 and

50 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 BaC1-1 (meq/g) (N) (N) 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

51 the rate. chloride The effect is more pronounced with barium (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, = KC1 = BaC * I \ Y Concentration of the salt [N) Figure Salt effect on benzil-benzilic acid rearrangement in polymeric matrices

52 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

53 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 0 OH 0 I OH Scheme Synthesis of polybenzoin from terephthalaldehyde 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

54

55 different chemical and magnetic environment. This is due to the presence of carbonyl and hydroxyl groups in the molecule (m) and functional correspond to the ring protons (s) is due to the -&H proton and (s) corresponds to the hydroxyl I proton in the molecule. (m) (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 Scheme Oxidation of polybenzoin into polybenzil The precipitated polybenzil was washed repeatedly with water to remove nitric acid. The products show

56 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 (rn) corresponding to the ring protons. This indicates the presence of only one type of ring protons. The benzoin analogue showed two peaks ( ) 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 (s) present in the benzoin analogue disappeared during the formation of benzil. The -?H disappeared during oxidation. I proton and hydroxyl proton (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

57

58 was facile in these systems just like the rearrangement of pendant gc-diketo systems. The reaction is depicted in Scheme Scheme 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 (s) corresponds to the carboxyl proton (in) and (s) are due to the aromatic ring protons and hydroxyl proton respectively.

59

60 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.

61 MOLECULAR REARRANGEMENT IN MACROMOLECULAR CAVITIES