A POLAR SUBSTITUENT EFFECT ON THE RING-CLEAVAGE REARRANGEMENT OF 1-ARYLCYCLOBUTYLMETHYL GRIGNARD-REAGENTS

The kinetics of the ring-cleavage rearrangements of 1-phenylcyclobutyl methylmagnesium chloride and its p-methyl and p-chloro analogs have be en determined. The first-order rate constants are correlated by the Ha mmett equation, with rho = - 0.47. The results are consistent with a c oncerted mechanism with a cyclic transition state having significant p olar character, although polar effects on the stabilities of reactant and product may also contribute. The phenyl group itself slows the rea ction by a factor of 0.031, which is interpreted principally in terms of steric destabilization of the transition state.