Manifestations of Bridgehead-Bridgehead Interactions in the Bicyclo[1.1.1]pentane Ring System

A series of 3-halo-substituted bicyclo[1.1.1]pentane-1-carboxylic acids 1 (Y = COOH; X = F, Cl, Br, I, and CF(3)) as well as the parent compound 1 (Y = COOH, X = H) have been prepared, and a study of some of their properties have been made. It was found that their reactions with xenon difluoride cover a wide range of reactivities. On one hand, the fluoro acid 1 (Y = COOH, X = F) displayed no apparent reaction at all while, on the other, the bromo acid 1 (Y = COOH, X = Br) and parent compound 1 (Y = COOH, X = H) underwent ready reaction with complete disintegration of the ring system. A possible explanation is advanced based on polar kinetic and thermodynamic effects governing the lifetime of an intermediate acyloxy radical species. The relative ease of oxidation of the carboxylates 1 (Y = COO(-); X = H, F, Cl, Br, I, CF(3), and COOCH(3)), as mirrored by their peak oxidation potential values (E(p)) determined by cyclic voltammetry, also covers a wide range. These data coupled with the dissociation constants (pK(a)) of some of the acids 1 (Y = COOH; X = H, F, Cl, and CF(3)) reflect significantly on the modes of transmission of electronic effects acting through the bicyclo[1.1.1]pentane ring system.