Asymmetric Base‐Free Michael Addition at Room Temperature with Nickel‐Based Bifunctional Amido‐Functionalized N‐Heterocyclic Carbene Catalysts

Abstract

AbstractA series of nickel‐based chiral bifunctional catalysts (1d–3d) with N‐heterocyclic carbene (NHC) ligands derived from (1R)‐(–)‐menthol, (1S)‐(–)pinene, and (1R)‐(+)‐camphor have been successfully designed for asymmetric Michael addition reactions under base‐free conditions. The NHC complexes, namely, [1‐R‐3‐{N‐(phenylacetamido)}imidazol‐2‐ylidene]2Ni [R = (1S)‐menthyl (1d), (1S)‐pinane (2d), and (1R)‐isobornyl (3d)], bearing chiral ancillaries on the amido‐functionalized side arms of the NHC ligands, performed the bifunctional catalysis of the asymmetric base‐free Michael addition reaction of the α‐methyl cyano ester substrates ethyl 2‐cyanopropanoate (4), isopropyl 2‐cyanopropanoate (5), and tert‐butyl 2‐cyanopropanoate (6) with the activated olefinic substrates methyl vinyl ketone (7) and acrylonitrile (8) in 63–98 % yields with enantiomeric excess (ee) values of 2–75 % at room temperature in 8 h. More interestingly, only the longest of the three catalysts, the menthol derivative 1d, showed significant chiral induction of up to 75 % ee; this has been attributed to the reduction of the steric influence owing to the relatively distant dispositions of the chiral ancillaries from the catalytically active metal center that arise as a consequence of the cis geometries of 1d–3d.