Bifunctional nickel precatalysts of amido-functionalized N-heterocyclic carbenes for base-free Michael reaction under ambient conditions

A series of new bifunctional nickel precatalysts, [1-(R)-3-N-(benzylacetamido)imidazol-2-ylidene]2 Ni [R = Me (1b), i-Pr (2b), and CH2Ph (3b)], containing a Lewis acidic metal site and a Lewis basic amido-N site in a pendent ligand sidearm, have been successfully designed for base-free Michael addition reaction. Specifically, the nickel (1–3)b complexes catalyzed the highly desired base-free Michael addition reactions of representative cyclic 5-membered β-dicarbonyl and β-ketoester substrates with a variety of activated olefinic compounds in air at ambient temperature in good to excellent yield. The nickel (1–3)b complexes were synthesized from the reactions of the corresponding imidazolium chloride salts, (1–3)a, with NiCl2•6H2O in presence of K2CO3 as a base in 55–73% yield. The density functional theory (DFT) studies performed on the nickel complexes suggested the presence of a strong Ni–NHC σ-interaction in these complexes.

Graphical abstractA series of bifunctional nickel precatalysts of amido-functionalized N-heterocyclic carbenes have been successfully employed in carrying out the highly desired base-free Michael reaction in air at ambient temperature for a representative cyclic 5-membered β-dicarbonyl and β-ketoester substrates with a variety of activated olefinic compounds in good to excellent yield.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesis of chelated amido-functionalized NHC complexes of Ni. ► DFT studies on Ni–NHC complexes. ► Bifunctional Ni–NHC precatalysts for base-free Michael reaction.