P(RNCH2CH2)3N: an efficient promoter for the direct synthesis of E-α,β-unsaturated esters

Upon reacting ethyl acetate or methyl propionate with a variety of aromatic aldehydes in the presence of 1.06–1.2 equiv. of the pro-azaphosphatranes, P(MeNCH2CH2)3N, P(i-PrNCH2CH2)3N or [P(i-PrNCH2CH2)2(NHCH2CH2)N at 40–50°C for 2–6 h in isobutyronitrile, the corresponding α,β-unsaturated esters were formed as the only products. Ethyl acetate reacts with aldehydes to form exclusively E-isomers while the higher homologue methyl propionate gives rise to a mixture of E and Z isomers with the former as the major product. When used as the solvent, methyl propionate selectively forms the E-α,β-unsaturated ester. The reaction is not as successful for the preparation of α,β-unsaturated ketones.