N-heterocyclic carbene boryl radicals: a new class of boron-centered radical.
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 x 10(4) 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 pi-radical, analogous to the benzyl radical.
Ueng
Shau-Hua
SH
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
Solovyev
Andrey
A
Yuan
Xinting
X
Geib
Steven J
SJ
Fensterbank
Louis
L
Lacôte
Emmanuel
E
Malacria
Max
M
Newcomb
Martin
M
Walton
John C
JC
Curran
Dennis P
DP