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Updated: Feb 9, 2026

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Radical trans-Hydroboration of Alkynes with N-Heterocyclic Carbene Boranes
Masaki Shimoi1, Takashi Watanabe2, Katsuhiro Maeda1,3
1Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan.
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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