RESUMO
Nickel carbenes are key reactive intermediates in the catalytic cyclopropanation of olefins and other reactions, but isolated examples are scarce and generally rely on low coordination numbers (≤ 3) to stabilize the metal-ligand multiple bond. Here we report the isolation and characterization of a stable tetracoordinated nickel carbene bearing a triphosphine pincer ligand. Its nucleophilic character is evidenced by reaction with acids, and it can transfer the carbene fragment to CO to form a ketene. A computational study of the Ni=C chemical bond sheds light on the role of the third phosphine in the pincer framework to the stabilization of the nickel carbene fragment.
RESUMO
Oxidative addition of aryl halides to Ni(0) is a ubiquitous elementary step in cross-coupling and related reactions, usually producing a square-planar Ni(II)-aryl intermediate. Here we show that a triphosphine ligand supports oxidative addition at a tris-ligated Ni(0) center to cleanly form stable five-coordinate Ni(II)-aryl compounds. Kinetic and computational studies support a concerted, two-electron mechanism rather than radical halogen abstraction. These results support the idea that oxidative addition to triphosphine Ni(0) species may be more generally involved in Ni/phosphine catalytic systems.
RESUMO
A nickel pincer complex is found to catalyze alkyl-alkyl Kumada coupling reactions of 1,3- and 1,4-substituted cyclohexyl halides and tetrahydropyrans with an excellent diastereoselectivity. The mechanistic investigation of the coupling reactions provides evidence that the activation of alkyl halides is reversible.