Ruthenium-Catalyzed Anti-Markovnikov Selective Hydroboration of Olefins

Ruthenium-catalyzed selective hydroboration of styrenes and aliphatic olefins with pinacolborane (HBpin) is reported. This efficient transformation provided products with exclusive anti-Markovnikov selectivity, and this hydroboration protocol is compatible with olefins having electronic and steric divergence as well as diverse functional groups. Hydroboration occurred at room temperature under solvent-free conditions with minimal catalyst load (0.05 mol %) and provided high TON (>1980; >990 per Ru). Mechanistic studies confirmed the involvement of intermediate [{(η6-p-cymene)RuCl}2(μ-H-μ-Cl)] (2). A catalytic cycle including a mononuclear ruthenium intermediate is proposed. The rationale for observed anti-Markovnikov selectivity is provided from reversible 1,3-hydride transfer leading to the regioselective 1,2-insertion on olefins.