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

• Published in: Journal of the American Chemical Society 2019-11, Vol.141 (46), p.18600-18611
• Main Authors: Nieto-Sepulveda, Eduardo, Bage, Andrew D, Evans, Louise A, Hunt, Thomas A, Leach, Andrew G, Thomas, Stephen P, Lloyd-Jones, Guy C
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.9b10114

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.