Pyrylium-mediated transformations of natural products. Part 8. Kinetics of nucleophilic displacements with pyridines as leaving groups in aqueous solution.

Abstract

Good second-order kinetics were found with k, values which were ca. 50 times less for piperidine displacements in H,O than in chlorobenzene solutions, as expected from the polarity increase. Rates for thioglycolate dianion displacements were about five times faster than for piperidine. The rate dependence on pyridine leaving group structure paralleled that previously found for non-aqueous solutions except that an additional SO3substituent group showed a small rate-decreasing effect.Earlier Parts have described the preparation of water-soluble pyrylium salts containing one or more sulphonic or carboxylic