Michael addition of cyclic β-oxo ester and α-methyl cyano ester substrates with activated olefins by iron complexes of benzimidazole derived N-heterocyclic carbene ligands

Michael addition reactions of cyclic β-oxo ester and α-methyl cyano ester substrates with acceptor activated olefins were catalyzed by iron N−heterocyclic carbene (NHC) complexes of the type, {Cp[1-benzyl-3-alkyl-benzimidazol-2-ylidene]Fe(CO)₂}I, where [alkyl = i-Pr (1) and Et (2)] in the presence of L-proline as chiral auxiliary under ambient conditions. In case of the cyclic β-oxo ester substrates good yields of up to 68% and enantioselectivities of up to 18% were observed while for the α-methyl cyano ester substrates similar yields of up to 82% and enantioselectivities of up to 8% were observed. Notably, for the cyclic β-oxo ester substrates, the five membered ethyl-2-oxocyclopentane-1-carboxylate gave superior yields and enantioselectivities over the six membered ethyl-2-oxocyclohexane-1-carboxylate. In case of α-methyl cyano ester substrates, methyl vinyl ketone gave higher yields as compared to the other acceptor activated olefins namely, ethyl vinyl ketone and acrylonitrile. Based on the mass spectrometry studies, that detected the presence of L-proline adduct species (1–2)A, L-proline and substrate adduct species (1–2)B and L-proline and product adduct species (1–2)C in the catalysis reaction mixture, a mechanistic cycle has been proposed for the Fe−NHC mediated Michael addition reaction.