Room temperature ring expansion of N-heterocyclic carbenes and B-B bond cleavage of diboron(4) compounds

Sabrina Pietsch; Ursula Paul; Ian A Cade; Michael J Ingleson; Udo Radius; Todd B Marder

doi:10.1002/chem.201501498

Room temperature ring expansion of N-heterocyclic carbenes and B-B bond cleavage of diboron(4) compounds

Chemistry. 2015 Jun 15;21(25):9018-21. doi: 10.1002/chem.201501498. Epub 2015 May 8.

Authors

Sabrina Pietsch¹, Ursula Paul¹, Ian A Cade², Michael J Ingleson³, Udo Radius⁴, Todd B Marder⁵

Affiliations

¹ Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg (Germany).
² School of Chemistry, The University of Manchester, Manchester M13 9PL (UK).
³ School of Chemistry, The University of Manchester, Manchester M13 9PL (UK). [email protected].
⁴ Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg (Germany). [email protected].
⁵ Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg (Germany). [email protected].

PMID: 25960171
DOI: 10.1002/chem.201501498

Abstract

We report the isolation and detailed structural characterization, by solid-state and solution NMR spectroscopy, of the neutral mono- and bis-NHC adducts of bis(catecholato)diboron (B2 cat2 ). The bis-NHC adduct undergoes thermally induced rearrangement, forming a six-membered -B-C=N-C=C-N-heterocyclic ring via C-N bond cleavage and ring expansion of the NHC, whereas the mono-NHC adduct is stable. Bis(neopentylglycolato)diboron (B2 neop2 ) is much more reactive than B2 cat2 giving a ring expanded product at room temperature, demonstrating that ring expansion of NHCs can be a very facile process with significant implications for their use in catalysis.

Keywords: BB bond activation; CN bond cleavage; N-heterocyclic carbene; boron compounds; ring expansion reaction.