Direct Amination of Homoenolates Catalyzed by N-Heterocyclic Carbenes

N-Heterocyclic carbenes derived from N-mesityl-N-methyltriazolium salts are effective catalysts for generating homoenolate species from α,β-unsaturated aldehydes. The unique intermediate adds to the electrophilic nitrogen of 1-acyl-2-aryldiazenes, and the resulting activated carbonyl unit undergoes an intramolecular acylation event. This formal [3+2] cycloaddition between α,β-unsaturated aldehydes and acylaryldiazenes, catalyzed by an N-heterocyclic carbene, produces substituted pyrazolidinones in good yields. This new NHC-catalyzed reaction accommodates aromatic and alkyl α,β-unsaturated aldehydes and various aromatic diazenes. A chiral triazolium salt catalyzes the formation of the pyrazolidinone product in moderate yield and good enantioselectivity. The pyrazolidinones can undergo reductive N−N bond cleavage to give β-amino acid derivatives.