MECHANISM OF THE GRIGNARD ADDITION-REACTION .16. HOMOLYTIC AND CONCERTED MECHANISMS IN THE REACTION OF ALPHA,BETA-UNSATURATED CARBONYL-COMPOUNDS WITH GRIGNARD-REAGENTS

Kinetic measurements have shown that the addition of Grignard reagents to alpha,beta-unsaturated carbonyl compounds takes place either by a concerted mechanism or by a homolytic mechanism. Phenylmagnesium bromide, which is incapable of homolysis, reacts rapidly in a 1,4-fashion if an s-cis conformation exists between the C = C and the C = O bonds, but only 1,2-addition takes place if the conformation is s-trans. tert-Butylmagnesium bromide is unsuited to the concerted reaction, but 1,4-addition takes place via homolysis. Primary and secondary Grignard reagents, like phenyl, react rapidly in a concerted manner with s-cis substrates, but unlike phenyl, these Grignard reagents may, with s-trans substrates, produce some 1,4-adduct via the homolytic mechanism.