N-Heterocyclic Carbene Boryl Radicals: A New Class of Boron-Centered Radical

• Published in: Journal of the American Chemical Society 2009-08, Vol.131 (31), p.11256-11262
• Main Authors: Ueng, Shau-Hua, Solovyev, Andrey, Yuan, Xinting, Geib, Steven J, Fensterbank, Louis, Lacôte, Emmanuel, Malacria, Max, Newcomb, Martin, Walton, John C, Curran, Dennis P
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja904103x

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.