NHC-Catalyzed Reactions of Aryloxyacetaldehydes: A Domino Elimination/Conjugate Addition/Acylation Process for the Synthesis of Substituted Coumarins

N-Heterocyclic carbenes (NHCs) catalyze a domino Michael addition/acylation reaction to form 3,4 through addition of the NHC to an aryloxyaldehyde followed by elimination of a phenoxide leaving group, generating an enol intermediate. This transient nucleophile generated in situ performs a 1,4-addition onto a conjugate acceptor, and the carbene catalyst Is regenerated upon acylation of the phenoxide anion resulting in formation of 3,4-dihydrocoumarins.