Asymmetric Base‐Free Michael Addition at Room Temperature with Nickel‐Based Bifunctional Amido‐Functionalized N‐Heterocyclic Carbene Catalysts. Issue 9 (16th February 2015)

• Record Type: Journal Article
• Authors: Rao, Mitta Nageswar; Haridas, Meera; Gangwar, Manoj Kumar; Rajakannu, Palanisamy; Kalita, Alok Ch.; Ghosh, Prasenjit
• Is Part Of: European journal of inorganic chemistry. Issue 9(2015)
• Page Start: 1604
• Page End: 1615
• Publication Date: 2015-02-16
• Subjects: Asymmetric catalysis -- Michael addition -- Bifunctional catalysts -- Carbene ligands -- Nickel; Chemistry, Inorganic -- Periodicals; Organometallic chemistry -- Periodicals; Bioinorganic chemistry -- Periodicals; Solid state chemistry -- Periodicals; 546
• DOI: 10.1002/ejic.201403224
• Languages: English
• ISSNs: 1434-1948

Abstract

Abstract: A series of nickel‐based chiral bifunctional catalysts (1d –3d ) with N‐heterocyclic carbene (NHC) ligands derived from (1 R )‐(–)‐menthol, (1 S )‐(–)pinene, and (1 R )‐(+)‐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]2 Ni [R = (1 S )‐menthyl (1d ), (1 S )‐pinane (2d ), and (1 R )‐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 derivative1d, 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 of1d –3d . Abstract : A series of nickel‐based chiral bifunctional catalysts with N‐heterocyclic carbene ligands derived from readily available and inexpensive (1 R )‐(–)‐menthol, (1 S )‐(–)pinene, and (1 R )‐(+)‐camphor synthons successfully catalyze asymmetric base‐free Michael addition reactions under ambient conditions.