UNEXPECTED LACK OF STEREOSELECTIVITY IN THE HORNER-WADSWORTH-EMMONS REACTION WHEN APPLIED TO THE SYNTHESIS OF (E)- AND (Z)-METHYL A-ARYLCINNAMATES

A facile synthetic route to chloroaromatic derivatives of (E)- and (Z)-methyl alpha-arylcinnamates (methyl 2,3-diarylpropenoates) is described. The reaction proceeded with good to excellent yields in all cases and separation of the E and Z isomers was easily achieved. The Horner-Wadsworth-Emmons reaction of phosphonates 2 with aromatic aldehydes afforded mixtures of E and Z isomers in approximately 1:1 ratios, except for the more E-biased product mixtures of 3:1, obtained when 2 was reacted with 2,6-dichlorobenzaldehyde. The high proportion of the Z isomers in the former systems was unexpected considering results reported for similar systems. An explanation based on the combined contributions of both steric and electronic factors is suggested for this lack of stereoselectivity.