The stereochemistry of the reaction of Grignard and lithium reagents with phenyl- and methyl-propiolic acids

Abstract

Derivatives of phenyl-(Ia—c) and methyl-propiolic acid (IIa—b) undergo 1,2-addition with Grignard reagents. The amides of phenylpropiolic acid (ld—f) also undergo 1,4-addition, particularly at high temperatures. Products of 1,4-addition are also obtained in the presence of copper(I) or cobalt(II) salts. The acids (Ia) and (IIa) with methyl- and phenyl-magnesium bromide, respectively, yield predominantly trans-β-methylcinnamic acid (V), but the esters (Ib) and (IIb) give more of the cis-isomer (VI) in similar reactions. The cis-isomer (VI) is obtained preferentially on addition of methylcopper to phenylpropiolic acid or its methyl ester, but propiolate salt yields the trans-acid (V). It is assumed that an enolate with an sp² hybridised α-carbon atom is an intermediate. Methyl-lithium can add in a 1,4-manner to some of the derivatives of these acetylenic acids.