Borane-Catalyzed, Chemoselective Reduction and Hydrofunctionalization of Enones Enabled by B-O Transborylation

Kieran Nicholson, Thomas Langer and Stephen P. Thomas*

*EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh EH9 3FJ, United Kingdom, Email: stephen.thomased.ac.uk

K. Nicholson, T. Langer, S. P. Thomas, Org. Lett., 2021, 23, 2498-2504.

DOI: 10.1021/acs.orglett.1c00446

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Abstract

Organoborane reductants have been rendered catalytic through an isodesmic B-O/B-H transborylation applied in the borane-catalyzed, chemoselective alkene reduction and formal hydrofunctionalization of enones. The reaction proceeds via a 1,4-hydroboration of the enone and B-O/B-H transborylation with HBpin, enabling catalyst turnover.

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proposed mechanism

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General Procedure

α,β-Unsaturated ketone (1.00 eq.), 9-borabicyclo(3.3.1)nonane [H-B-9-BBN]₂(0.04 eq.) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin) (1.20 eq.) were stirred in ethyl acetate (0.5 M) at 40 °C for 16 hours. The reactions were quenched with SiO₂ (0.25g / mmol chalcone). Once effervescence had stopped the mixture was filtered and concentrated. The crude product was purified by flash column chromatography.

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Key Words

1,4-reduction, pinacolborane, 9-BBN

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ID: J54-Y2021