Nickel Complexes of N/O‐Functionalized N‐Heterocyclic Carbenes as Precatalysts for Michael Reactions in Air at Room Temperature Under the Much Preferred Base‐Free Conditions

Abstract

AbstractA series of several new nickel precatalysts supported overN/O‐functionalized N‐heterocyclic carbenes (NHC) for the Michael reactions of β‐dicarbonyl, β‐keto ester, β‐diester, and α‐cyano ester compounds with α,β‐unsaturated carbonyl compounds in air at ambient temperature under the much preferred base‐free conditions are reported. Specifically, the nickel complexes, [1‐(R1‐aminocarbonylmethyl)‐3‐R2‐imidazol‐2‐ylidene]2Ni [R1 = 2‐C6H4(OMe); R2 = Me (1b), iPr (2b), CH2Ph (3b) and R1 = 2‐CH2C4H3O; R2 = Me (4b), CH2Ph (5b)] carried out the highly convenient base‐free Michael addition of the activated C–H compounds across α,β‐unsaturated carbonyl compounds in air at room temperature. The complexes 1b–5b were synthesized by the direct reaction of the respective imidazolium chloride salt with NiCl2·6H2O in CH3CN in the presence of K2CO3 as a base. The exceptional stability of 1b–5b has been attributed to the deeply buried nickel–NHC σ‐bonding molecular orbitals as evidenced from the density functional theory (DFT) studies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)