NHC-Catalyzed, Microwave-Assisted and Base-Dependent Reactions of Substituted Cinnamaldehydes Towards γ-Lactones or Saturated Esters

Abstract

N-Heterocyclic carbenes (NHCs) generated from imidazolium, imidazopyridinium and imidazoquinolinium hydrochlorides in situ using base and microwave irradiation were employed as organocatalysts to yield homoenolates from cinnamaldehyde and its aryl-substituted analogs. Depending on the catalytic base either γ-lactones were prepared when the strong base tBuOK was used or the corresponding saturated ethyl 3-aryl propionates were synthesized with the milder DIPEA base. Novel imidazopyridinium and imidazoquinolinium hydrochlorides were synthesized and were compared to commercially available NHCs IMes and IDip for their catalytic activity. The optimum reaction conditions were established using IMes and the broad scope of both reactions was investigated using the 4 most efficient NHCs.