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  • Title: Radical trans-Hydroboration of Alkynes with N-Heterocyclic Carbene Boranes.
    Author: Shimoi M, Watanabe T, Maeda K, Curran DP, Taniguchi T.
    Journal: Angew Chem Int Ed Engl; 2018 Jul 20; 57(30):9485-9490. PubMed ID: 29845716.
    Abstract:
    Hydroboration of internal alkynes with N-heterocyclic carbene boranes (NHC-boranes) occurs to provide stable NHC (E)-alkenylboranes upon thermolysis in the presence of di-tert-butyl peroxide. The E isomer results from an unusual trans-hydroboration, and the E/Z selectivity is typically high (90:10 or greater). Evidence suggests that this hydroboration occurs by a radical-chain reaction involving addition of an NHC-boryl radical to an alkyne to give a β-NHC-borylalkenyl radical. Ensuing hydrogen abstraction from the starting NHC-borane provides the product and returns the starting NHC-boryl radical. Experiments suggest that the observed trans-selectivity results from kinetic control in the hydrogen-transfer reaction.
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