Stable Tetravalent Phosphonium Enolate Zwitterions

Three-component coupling reactions between trialkylphosphines, methyl propiolates, and aldehydes produced 1:1:1 dipolar adducts in moderate to excellent yields. The product phosphonium enolate zwitterions were isolated as crystalline solids. X-ray crystallographic analyses of these single crystals established unequivocally the dipolar structures of these tetravalent phosphonium enolate zwitterions. Because phosphonium enolates are the first key intermediates in the nucleophilic phosphine-mediated catalysis of α,β-unsaturated carbonyl compounds, this study provides crucial insight into the mechanisms of Morita−Baylis−Hillman-type reactions.