Comparative behaviour of σ-complexes in phenylation of 4-methylpyridine by benzoyl peroxide in refluxing benzene

Abstract

Isotope effects have been determined for the phenylation of [2-²H]- and [3-²H]-4-methylpyridine by benzoyl peroxide in refluxing benzene. The primary deuterium isotope effect k_H/k_D for hydrogen abstraction from the σ-complex at the 3-position in 4-methylpyridine is 3.7 whereas at the 2-position k_H/k_D= 1. In the presence of nitrobenzene there is no isotope scrambling and it is therefore suggested that isomer distributions obtained from product analyses resulting from benzoyl peroxide decomposition in the presence of nitrobenzene reflect the true measure of relative rates at which the isomeric σ-complexes are formed initially. We have tried to understand the different behaviours of σ-complexes at position 2 and 3 in 4-methylpyridine.