Phosphino(silyl) Nickel Complexes for Alkene Hydroboration Catalysis: Steric Tuning of Pre-Catalyst Provides High Regioselectivity

Abstract

Bis(phosphino)silyl nickel complexes featuring diethylphosphino donors on the tridentate PSiP ligand are effective pre-catalysts for room temperature alkene hydroboration with HBPin. Terminal and geminal alkenes, including a wide scope of vinyl arenes, reacted to afford exclusively the linear, anti-Markovnikov boronate ester product in all cases. Internal alkene substrates undergo Ni-mediated tandem alkene isomerization-hydroboration to afford the terminal borylation products with high selectivity. Deuterium labeling studies employing DBPin support a Ni-H-based mechanism for this reactivity, whereby alkene isomerization occurs via reversible alkene insertion/β-hydride elimination steps. We propose that steric tuning of the PSiP ligand influences the observed regioselectivity.