Alkene Hydrosilylation Using Tertiary Silanes with α-Diimine Nickel Catalysts. Redox-Active Ligands Promote a Distinct Mechanistic Pathway from Platinum Catalysts

Combination of the readily available α-diimine ligand, ((ArN=C(Me))₂ Ar = 2,6-ⁱPr₂-C₆H₃), (^iPrDI) with air-stable nickel(II) bis(carboxylates) generated a highly active catalyst exhibiting anti-Markovnikov selectivity for the hydrosilylation of alkenes with a variety of industrially relevant tertiary alkoxy- and siloxy-substituted silanes. A combination of the method of continuous variations with stoichiometric studies identified the formally Ni(I) hydride dimer, [(^iPrDI)NiH]₂ as the nickel compound formed following reduction of the carboxylate ligands. For the hydrosilylation of 1-octene with (EtO)₃SiH, a rate law of [Ni]^1/2[1-octene][(EtO)₃SiH] in combination with deuterium-labeling studies establish dissociation of the nickel hydride dimer followed by fast and reversible alkene insertion into (^iPrDI)NiH, consistent with turnover-limiting C-Si bond formation. The hydrosilylation of 1-octene with triethoxysilane, a reaction performed commercially in the silicones industry on a scale of >5000000 kg/year, was conducted on a 10 g scale with 96% yield and >98% selectivity for the desired product. Silicone cross-linking, another major industrial application of homogeneous hydrosilylation, was also demonstrated using the air-stable nickel and ligand precursors.