Mechanisms of hydrogenation. Part VI. Configurational inversion in the hydrogenolysis of benzyl alcohol derivatives

Abstract

The hydrogenolysis: [graphic omitted], (R = H or Me) at palladium, in a non-polar solvent, occurs with inversion of configuration, and the rate of hydrogenolysis increases with the electronegativity of the displaced group X, i.e., X = OH < OAc < OCOCF₃.

The hydrogenolysis of 1-phenylcycloalkanols: [graphic omitted] n= 4–8, the change of rate with n follows a sequence closely similar to the relative ease of sp³→sp² change in co-ordination. The rates of hydrogenation of cycloalkanones: [graphic omitted] n= 4–8, follow the converse order, i.e., of ease of sp²→sp³ change in co-ordination with increasing ring size.

It is inferred that hydrogenolysis occurs via a metal benzyl complex, e.g., [graphic omitted] PdX formed with retention of configuration, which may add hydrogen with inversion or retention of configuration depending on the metal and the nature of the group X. A previously proposed catalyst model is shown to account for these phenomena.