N-heterocyclic carbene boryl radicals: a new class of boron-centered radical

S. E. Ueng, A. Solovyev, X. Yuan, S. J. Geib, L. Fensterbank, E. Lacote, M. Malacria*, M. Newcomb*, John Christopher Walton*, D. P. Curran*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

188 Citations (Scopus)

Abstract

Reduction of xanthates by N-heterocyclic carbene boranes (NHC-boranes) has been suggested to occur by a radical chain mechanism involving heretofore unknown NHC-boryl radicals. In support of this suggestion, both the expected borane dithiocarbonate product and an unexpected borane xanthate product have now been isolated. These are the first NHC-boranes with boron-sulfur bonds, and their structures have been secured by spectroscopic and crystallographic means. The first rate constants for H-atom transfer from an NHC borane complex were determined by using the ring opening of a substituted cyclobutylcarbinyl radical as a clock reaction. The rate constant for reaction of the NHC-borane with a secondary alkyl radical at ambient temperature is 4 × 104 M-1 s-1 , and the Arrhenius function displayed an entropic term (log A term) that was typical for a bimolecular reaction. The B-H bond dissociation energy of an NHC-borane complex has been estimated at 88 kcal/mol. The putative NHC-boryl radical in these transformations has been detected by EPR spectroscopy. Spectral analysis suggests that it is a π-radical, analogous to the benzyl radical.
Original languageEnglish
Pages (from-to)11256-11262
Number of pages7
JournalJournal of the American Chemical Society
Volume131
Issue number31
Early online date16 Jul 2009
DOIs
Publication statusPublished - 12 Aug 2009

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