Bage, Andrew D.

Evans, Louise A

Hunt, Thomas A.

Leach, Andrew G.

Lloyd-jones, Guy C.

Nieto-sepulveda, Eduardo

Thomas, Stephen P

English

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 labelling (10B/2H), and density functional theory (DFT) calculations. For the Cy2BH catalyzed hydroboration 4-fluorophenylacetylene by pinacolborane, the resting state is the antiMarkovnikov addition product ArCH=CHBCy2. Irreversible and turnover-rate limiting reaction with pinacolborane (k ~7´10-3 M-1s1) 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 E-Ar-CH(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-centre bonded B-C-B intermediate.<br/

Edinburgh Research Explorer

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 × 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.</p

Nieto-Sepulveda, Eduardo

Bage, Andrew D

Hunt, Thomas A

Leach, Andrew G

Lloyd-Jones, Guy C

The University of Manchester - Institutional Repository

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

https://www.research.ed.ac.uk/portal/files/117778268/20191029_Lloyd_Jones_ja_2019_101143_accepted.pdf

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

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Edinburgh Research Explorer

The University of Manchester - Institutional Repository