N-heterocyclic carbene catalysis via the α,β-unsaturated acyl azolium

Over the past decade, the α,β‐unsaturated acyl azolium(1) has been extensively examined in modern N‐heterocyclic carbene (NHC) organocatalysis. There are five general methods currently known to access the α,β‐unsaturated acyl azolium intermediates. In four of these methods, a redox approach is exploited that involves initial formation of an acyl anion intermediates. The simplest reactions of the α,β‐unsaturated acyl azolium involve bond formation at the acyl carbon to provide ester products. The α,β‐unsaturated acyl azolium could be derived from either enal or enal equivalent and was reacted with a variety of enamines to give dihydropyridinones containing various functional groups including the ester, nitro, and nitrile. Annulation of the α,β‐unsaturated acyl azoliums can also produce cyclopropanes via (2+1) annulation. In 2012, Studer and coworkers reported the NHC‐catalyzed synthesis of cyclopropyl carboxylic acid esters from enals, sulfur ylides, and alcohols.