Cobalt-Catalyzed Regiodivergent Hydrosilylation of Vinylarenes and Aliphatic Alkenes: Ligand- and Silane-Dependent Regioselectivities

Abstract

We report a regiodivergent hydrosilylation of alkenes catalyzed by catalysts generated in situ from bench-stable Co­(acac)2 and phosphine- or nitrogen-based ligands. A wide range of vinylarenes and aliphatic alkenes reacted to afford either branched (45 examples) or linear (37 examples) organosilanes in high isolated yields (average: 84%) and high regioselectivities (from 91:9 to >99:1). This transformation tolerates a variety of functional groups including ether, silyloxy, thioether, epoxide, halogen, amine, ester, boronic ester, acetal, cyano, and ketone moieties. Mechanistic studies suggested that the hydrosilylation of alkenes catalyzed by the cobalt/bisphosphine system follows the Chalk–Harrod mechanism (with a Co–H intermediate), and the hydrosilylation of alkenes catalyzed by the cobalt/pyridine-2,6-diimine system follows the modified Chalk–Harrod mechanism (with a Co–Si intermediate). Systematic studies with sterically varied silanes revealed that the steric properties of silanes play a pivotal role in controlling the regioselectivity of vinylarene hydrosilylation and the chemoselectivity of the reactions of aliphatic alkenes and silanes catalyzed by the cobalt/pyridine-2,6-diimine system.

Bench‐Stable Cobalt Pre‐Catalysts for Mild Hydrosilative Reduction of Tertiary Amides to Amines and Beyond

“…The alkylcobalt species produced from the Co hydride by 2,1‐ and 1,2‐insertion of olefins can be interconverted via β ‐H elimination and reinsertion of the alkene into the Co−H bond. For styrene, keeping in mind the preferential formation of the secondary benzylcobalt intermediate,, lower b/l ratio of hydrosilylation products for dpephos (bite angle of 102.2°) vs xantphos (bite angle of 111.7°) can be rationalized by the competing rates of β ‐H elimination and silane addition upon reducing the bite angle of the ligand. In contrast, for unhindered aliphatic alkenes, undergoing 1,2‐insertion to form primary alkylcobalt species,, addition of PhSiH 3 appears preferential over β ‐H elimination and results in exclusive formation of anti ‐Markovnikov products ( 2 b – 2 d , Scheme ).…”

“…Similar to (dpephos)CoCl 2 /LiBHEt 3 , regioselective Markovnikov hydrosilylation of aryl alkenes and anti ‐Markovnikov hydrosilylation of aliphatic alkenes with PhSiH 3 was recently reported by Ge et al . for Co(acac) 2 /xantphos (xantphos=4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene) . Using dpephos in place of xantphos, addition of PhSiH 3 to styrene at 25 °C resulted in reduced branched‐to‐linear ( b/l ) product ratio (76 : 24 vs 97 : 3 for xantphos) and lower styrene conversion (15 % vs >98 % for xantphos) .…”

“…for Co(acac) 2 /xantphos (xantphos=4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene) . Using dpephos in place of xantphos, addition of PhSiH 3 to styrene at 25 °C resulted in reduced branched‐to‐linear ( b/l ) product ratio (76 : 24 vs 97 : 3 for xantphos) and lower styrene conversion (15 % vs >98 % for xantphos) . Compared to Co(acac) 2 /dpephos, (dpephos)CoCl 2 /LiBHEt 3 ‐catalyzed reactions showed very similar b/l ratios (Table , entries 6–8); however, significantly higher conversion of styrene (71 %) was observed at room temperature (Table , entry 8) …”

“…The mechanism of Co/diphosphine‐catalyzed hydrosilylation of alkenes with PhSiH 3 was studied by Ge et al . and the reactions were suggested to proceed via Chalk‐Harrod mechanism (with a Co−H intermediate), and the regioselectivity of the silane addition was rationalized by the preferential 2,1‐migratory insertion of the vinylarenes into the Co−H bond (electronic preference) vs 1,2‐insertion for aliphatic alkenes (driven by steric preference) ,.…”

“…Due to the skyrocketing prices of these metals, the development of more economical non‐precious metal based catalysts has been an intensive area of research in the last decade . In this regard, earth abundant base transition metal catalysts (Mn, Fe, Co, Ni) have become very attractive alternatives to conventional precious metal systems, Whereas, a series of base metal catalysts have been developed for hydrosilylation of alkenes, alkynes, aldehydes, ketones– and even esters, the examples of application of simple commercially available bench‐stable systems are still scarce ,,. Base‐metal catalyzed hydrosilative reduction of amides is even more challenging, and there is only a handful of reported examples, most of which require rather high temperatures (≥100 °C) and long reaction times,…”