Characterization of the Zwitterionic Intermediate in 1,1-Carboboration of Alkynes

Alessandro Bismuto; Gary S Nichol; Fernanda Duarte; Michael J Cowley; Stephen P Thomas

doi:10.1002/anie.202003468

Characterization of the Zwitterionic Intermediate in 1,1-Carboboration of Alkynes

Angew Chem Int Ed Engl. 2020 Jul 27;59(31):12731-12735. doi: 10.1002/anie.202003468. Epub 2020 May 29.

Authors

Alessandro Bismuto¹, Gary S Nichol¹, Fernanda Duarte², Michael J Cowley¹, Stephen P Thomas¹

Affiliations

¹ EaStCHEM, School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh, EH9 3FJ, UK.
² Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford, OX1 3TA, UK.

PMID: 32343867
DOI: 10.1002/anie.202003468

Abstract

The reaction of a Lewis acidic borane with an alkyne is a key step in a diverse range of main group transformations. Alkyne 1,1-carboboration, the Wrackmeyer reaction, is an archetypal transformation of this kind. 1,1-Carboboration has been proposed to proceed through a zwitterionic intermediate. We report the isolation and spectroscopic, structural and computational characterization of the zwitterionic intermediates generated by reaction of B(C₆ F₅ )₃ with alkynes. The stepwise reactivity of the zwitterion provides new mechanistic insight for 1,1-carboboration and wider B(C₆ F₅ )₃ catalysis. Making use of intramolecular stabilization by a ferrocene substituent, we have characterized the zwitterionic intermediate in the solid state and diverted reactivity towards alkyne cyclotrimerization.

Keywords: 1,1-carboboration; frustrated Lewis pairs; reaction mechanisms; vinyl cation.