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Kinetics and Mechanism of the Arase-Hoshi R2BH-Catalyzed Alkyne Hydroboration: Alkenylboronate Generation via B-H/C-B Metathesis
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 ¡Á 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, mu-B-H-B bridged, 2-electron-3-center bonded B-C-B intermediate.
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Benzodioxan,
1,4-Benzodioxane | C8H8O2 – PubChem