Ni(0) catalysts

Previous reports described broad usage of nickel(0) complexes of N-heterocyclic carbene (NHC) ligands that are prepared either in situ from air-sensitive precursors or that are prepared and handled in an inert atmosphere glovebox. Nickel(0) NHC complexes where dimethyl fumarate stabilizes the complex and provides air stability are known, but the stabilizing dimethyl fumarate renders the catalysts too stable and inactive for most applications. Many examples of air-stable Ni(II) catalysts are known, but they typically do not involve NHC ligands, and they require reduction to the active Ni(0) form. To date, there are no examples of moderately air-stable Ni(0) complexes of NHC ligands that display good catalytic reactivity in transformations of broad interest to the pharmaceutical or polymer industries.

The synthesis and catalytic activity of several classes of NHC–Ni(0) precatalysts stabilized by electron-withdrawing alkenes are described. Variations in the structure of fumarate and acrylate ligands modulate the reactivity and stability of the NHC–Ni(0) precatalysts and lead to practical and versatile catalysts for a variety of transformations. The catalytic activity and efficiency of representative members of this class of catalysts have been evaluated in reductive couplings of aldehydes and alkynes and in N-arylations of amines.