%A JC Walton
%A MM Brahmi
%A J Monot
%A L Fensterbank
%A M Malacria
%A DP Curran
%A E Lac?te
%J Journal of the American Chemical Society
%T Electron paramagnetic resonance and computational studies of radicals derived from boron-substituted N-heterocyclic carbene boranes
%X Fifteen second-generation NHC-ligated boranes with aryl and alkyl substituents on boron were prepared, and their radical chemistry was explored by electron paramagnetic resonance (EPR) spectroscopy and calculations. Hydrogen atom abstraction from NHC - BH2Ar groups produced boryl radicals akin to diphenylmethyl with spin extensively delocalized across the NHC, BH, and aryl units. All of the NHC - B?HAr radicals studied abstracted Br-atoms from alkyl bromides. Radicals with bulky N,N?-dipp substituents underwent dimerization about 2 orders of magnitude more slowly than first-generation NHC-ligated trihydroborates. The evidence favored head-to-head coupling yielding ligated diboranes. The first ligated diboranyl radical, with a structure intermediate between that of ligated diboranes and diborenes, was spectroscopically characterized during photolysis of di-t-butyl peroxide with N,N?-di-t-butyl-imidazol-2-ylidene phenylborane. The reactive site of B-alkyl-substituted NHC - boranes switched from the boron center to the alkyl substituent for both linear and branched alkyl groups. The ?-borylalkyl radicals obtained from N,N?-dipp-substituted boranes underwent exothermic ?-scissions with production of dipp-Imd - BH2? radicals and alkenes. The reverse additions of NHC - boryl radicals to alkenes are probably endothermic for alkyl-substituted alkenes, but exothermic for conjugated alkenes (addition of an NHC - boryl radical to 1,1-diphenylethene was observed). A cyclopropylboryl radical was observed, but, unlike other ?-cyclopropyl-substituted radicals, this showed no propensity for ring-opening. ? 2011 American Chemical Society.
%N 26
%P 10312 - 10321
%V 133
%D 2011
%R 10.1021/ja2038485
%L pittir16322