1178555-32-2Relevant articles and documents
N-heterocyclic carbene boryl radicals: A new class of boron-centered radical
Ueng, Shau-Hua,Solovyev, Andrey,Yuan, Xinting,Geib, Steven J.,Fensterbank, Louis,Lacote, Emmanuel,Malacria, Max,Newcomb, Martin,Walton, John C.,Curran, Dennis P.
supporting information; scheme or table, p. 11256 - 11262 (2009/12/03)
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.
