Aryl migration reactions in the alkaline hydrolysis of vinylphosphonium ions. The role of the electron sink and the effects of substitution at the migration terminus

Abstract

The vinylphosphonium ions Ar₂[graphic omitted](Ar′)C(R)CHZ (1; Ar = Ph; Ar′=o-chlorophenyl, m-trifluoromethylphenyl, 2-furyl, 2-thienyl, and 1-methylpyrrol-2-yl; R = H; and Z = CO₂Et)(encountered as intermediates in the reactions of the phosphines Ar₂PAr′ with ethyl propiolate in the presence of water) undergo hydrolysis with predominant migration of Ar′ from phosphorus to adjacent carbon to form Ar₂P(O)C(R)(Ar′)CH₂Z (4) in contrast to the predominant loss of Ar′ which occurs in the hydrolysis of related ions in which Z = Ph. The vinylphosphonium ions (1; Ar = Ph, Ar′=o-chlorophenyl or 2-thienyl, R = Ph, and Z = CO₂Et)(from the reaction of Ar₂PAr′ with ethyl phenylpropiolate in the presence of water) undergo hydrolysis with loss of either the vinylic group or the group Ar′. It is suggested that when R = Ph, the migration reaction is inhibited as a result of steric crowding in the intermediate phosphorane. When this is reduced, as in the hydrolysis of the phosphonium ion derived from 5-methyldibenzophosphole and ethyl phenylpropiolate, predominant migration occurs even when R = Ph. The implications of this result for a recently suggested alternative mechanism of the aryl migration reaction are considered.