Effects of γ-Cyano substituent in allylic bridgehead solvolyses: Evidence for cyano π conjugation in carbocations

Article abstract of DOI:10.1016/S0040-4020(97)00505-X

Slightly flexible 4-methylene-3-homoadamantyl mesylate solvolyzes 2 x 10⁵ times faster than rigid 2-methylene-1-adamantyl mesylate. Placement of a cyano substituent on the (E) position of the vinylic carbon of these systems hardly alter the rate ratio (6 x 10⁵). The result suggests fortuitous cancellation of the inductive destabilizing effect of the cyano substituent by its mesomeric stabilizing effect toward the partially conjugated allylic cation. PM3 calculations on simple allylic cations as models supported the experimental results.