Red'ko A. Physicochemical characteristics of N-acyloxypyridinium salts and their catalitic properties in the inverse phase-transfer catalysis

The reactions of N-acetyloxypyridinium and N-benzoyloxypyridinium salts formation and acyl groups transfer reactions between pyridine N-oxides 77 in number have been studied. Thermodinamic characteristics for reactions and their activation parameters have been obtained and quantitative relationships between kinetic and equilibrium parameters have been found out. The reactions proceed in a single step via SN2 mechanism with a low degree of bond rupture in the transition state. Equilibrium constants of different acyl group transfer reactions 28 in number between N-acyloxypyridinium salts and pyridine N-oxides in acetonitrile solutions have been obtained. The equilibrium constants are independent of transferring acyl group nature. Rate constants for benzoyl group transfer reactions 19 in number between N-oxides, N-oxides and 4-dimethylaminopyridine have been measured. Reactions proceed via enforced concerted mechanism through early associative transition state. Benzoylation reaction of 4-nitrobenzoate ion by benzoyl chloride in two-phase system (water-dichloromethane) in the presence of pyridine N-oxides has been investigated. It has been found that catalytic efficiency decrease for the salts which include bad leaving groups (high basisity N-oxides). The kinetic scheme and mechanism of catalytic carboxylate benzoylation has been proposed.