Mechanism of the reaction of trialkyl phosphites with α-halogenoacetophenones in alcoholic solvents

Abstract

Second-order rate constants and activation parameters have been determined for the reactions of trialkyl phosphites in alcoholic media with α-chloro- or α-bromo-acetophenones having various substituents in the benzene ring (3,4-Me₂, 4-Me, H, 4-Cl, and 4-I). Linear Hammett plots are consistent with the involvement of a common first intermediate in the rate-determining stage, leading to the formation of vinyl phosphate, α-hydroxyphosphonate, and acetophenone. The results provide the first kinetic evidence in support of the initial formation of the previously suggested betaine as a common intermediate in alcoholic solution. Further reaction then involves rearrangement to give the vinyloxyphosphonium species (and hence the Perkow product) or protonation followed by dealkylation to give the α-hydroxyphosphonate. Evidence for the possible formation of dehalogenated ketone via solvolysis of the α-hydroxyphosphonium intermediate is also presented.