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.