Specific catalytic action of large molecules containing imidazole groups
INTRODUCTION In recent years, studies of the reaction rates of small molecules in the presence of synthetic polymers containing catalytically active functions have contributed significantly to the understanding of the mechanism of enzyme behaviour1. Of particular interest in the field of polymeric catalysis have been the studies of the solvolyses of esters catalyzed by polymers containing pendent imidazole groups because the imidazole moiety has been shown to play a prolific role in enzymatic reactions2—7. In reactions catalyzed by poly-4(5)-vinylimidazole and by poly-5(6)vinylbenzimidazole, three types of cooperative effects have been indicated. (i) Cooperative interaction between two imidazole groups and substrate where neutral imidazole acts as a nucleophile on the substrate and anionic imidazole acts as a general base on the intermediate. This type of bifunctional catalysis was indicated in the poly-4(5)-vinylimidazole catalyzed solvolysis of PNPA (p-nitrophenyl acetate) and in the poly-5(G)vinylbenzimidazole catalyzed solvolyses of PNPA, NABA (4-acetoxy-3nitrobenzoic acid) and NABS (sodium 4-acetoxy-3-nitrobenzenesulphonate), all solvolyses occurring at high pH values3' '. (ii) Cooperative interaction between two neutral, pendent imidazole groups and substrate where at intermediate pH the nucleophilic imidazole reaction is catalyzed by another neutral imidazole function acting as a general base6. (iii) Cooperative interaction between neutral and protonated imidazole groups in the solvolyses of NABA and NABS. The protonated sites on the polymer chain, which do not catalyze the solvolysis of PNPA, apparently serve as binding sites for the negatively charged esters, thereby facilitating a solvolytic attack from a neighbouring imidazole group. Because the solvolytic reactions are dependent on the concentrations of both protonated and neutral sites on the polymer chain, bell-shaped pH-rate profiles were obtained3' 4. A cooperative effect analogous to that described in (iii) (i.e., utilization of electrostatic attraction of substrate to the polymer chain) was observed in the solvolysis of the positively charged ester 3-acetoxy-N-trimethylanilinium iodide (ANTI) catalyzed by a copolymer of 4(5)-vinylimidazole and acrylic acid5. Although this copolymer was a less efficient catalyst than imidazole in solvolyzing PNPA and NABA, it exhibited marked selectivity towards ANTI at high pH values. The higher reactivity of the copolymer was rationalized