3-Halopropenals as β-Acylvinyl Anion Equivalents

Significance

The authors report an NHC-catalyzed reaction of 3-halopropenals with β,γ-unsaturated α-keto esters and azodicarboxylates to give butenolides and pyrazolones, respectively, in good yields. In the course of the reaction, the NHC derived from 1 and 3-halopropenals form Breslow intermediates, reacting with the corresponding electrophiles. Subsequent ring closure releases the catalyst and after elimination of HX the products are formed.