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

Abstract

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 >5 000 000 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.

C(sp2)–H Borylation of Fluorinated Arenes Using an Air-Stable Cobalt Precatalyst: Electronically Enhanced Site Selectivity Enables Synthetic Opportunities

“…Structural characterization (Figure 2) established a six-coordinate cobalt complex with κ 1 and κ 2 carboxylate ligands. A single paramagnetically shifted tert -butyl resonance was observed by 1 H NMR spectroscopy, suggesting rapid interconversion of κ 1 and κ 2 carboxylate ligands on the NMR time scale, similar to related pyridine diimine 19c and terpyridine cobalt complexes. 19d,e It is also possible that the κ 1 and κ 2 forms are indistinguishable by NMR spectroscopy.…”

“…A single paramagnetically shifted tert -butyl resonance was observed by 1 H NMR spectroscopy, suggesting rapid interconversion of κ 1 and κ 2 carboxylate ligands on the NMR time scale, similar to related pyridine diimine 19c and terpyridine cobalt complexes. 19d,e It is also possible that the κ 1 and κ 2 forms are indistinguishable by NMR spectroscopy. Compound 2 exhibited excellent bench stability, as no change in the 1 H NMR spectrum of the compound was observed after exposure of the solid to air for 5 days [Figure S1, Supporting Information (SI)].…”

“…Addition of HBPin as an activator 19a produced comparable activity to 1 (entry 2). Precatalyst 2 was exposed to air for 1 h without any measurable erosion of activity or selectivity (entry 3).…”

Fifty Years of Hydrosilylation in Polymer Science: A Review of Current Trends of Low-Cost Transition-Metal and Metal-Free Catalysts, Non-Thermally Triggered Hydrosilylation Reactions, and Industrial Applications

“…Hence, this catalyst is a potential candidate for the hydrosilylation of renewable resources. Pappas and coworkers focused on the hydrosilylation of alkenes using an α-diimine nickel complex [41]. Stoichiometric studies indicated the presence of the catalyst as dimer [(2,6-iso Pr2-C6H3)NiH]2, while studies with deuterium labeling suggested the dissociation of the dimeric nickel complex which gives rise to the fast and reversible alkene insertion into the complex system.…”

Benzyltriboronates: Building Blocks for Diastereoselective Carbon–Carbon Bond Formation

“…25 The solid-state structure of 1 established a distorted square planar geometry (see Supporting Information (SI), Figure S2). Because both cobalt and nickel bis(carboxylate) compounds have proven to be readily synthesized, bench-stable precursors for a host of catalytic reactions, 18e,23d,26 ( Cy ADI)Ni(OPiv) 2 ( 2 ) (Piv = pivaloyl) was also prepared by addition of the free α -diimine to a well-defined Ni(II) pivalate precursor, [(NEt 3 )Ni(OPiv) 2 ] 2 , a compound first reported by Eremenko and co-workers. 27 …”