Kinetics and Mechanism of the Arase-Hoshi R2BH-Catalyzed Alkyne Hydroboration: Alkenylboronate Generation via B-H/C-B Metathesis

Abstract

The mechanism of R₂BH-catalyzed hydroboration of alkynes by 1,3,2-dioxaborolanes has been investigated by in situ ¹⁹F NMR spectroscopy, kinetic simulation, isotope entrainment, single-turnover labeling (¹⁰B/²H), and density functional theory (DFT) calculations. For the Cy₂BH-catalyzed hydroboration 4-fluorophenylacetylene by pinacolborane, the resting state is the anti-Markovnikov addition product ArCH = CHBCy₂. Irreversible and turnover-rate limiting reaction with pinacolborane (k ≈ 7 × 10^-3 M^-1 s^-1) regenerates Cy₂BH and releases E-Ar-CH═CHBpin. Two irreversible events proceed in concert with turnover. The first is a Markovnikov hydroboration leading to regioisomeric Ar-C(Bpin)═CH₂. This is unreactive to pinacolborane at ambient temperature, resulting in catalyst inhibition every ∼10² turnovers. The second is hydroboration of the alkenylboronate to give ArCH₂CH(BCy₂)Bpin, again leading to catalyst inhibition. 9-BBN behaves analogously to Cy₂BH, but with higher anti-Markovnikov selectivity, a lower barrier to secondary hydroboration, and overall lower efficiency. The key process for turnover is B-H/C-B metathesis, proceeding by stereospecific transfer of the E-alkenyl group within a transient, μ-B-H-B bridged, 2-electron-3-center bonded B-C-B intermediate.