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 describ ed. 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 af forded 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 re acted with 2,6-dichlorobenzaldehyde. The high proportion of the Z isom ers in the former systems was unexpected considering results reported for similar systems. An explanation based on the combined contribution s of both steric and electronic factors is suggested for this lack of stereoselectivity.