Electron paramagnetic resonance and computational studies of radicals derived from boron-substituted N-heterocyclic carbene boranes

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 β-borylalkyl radicals obtained from N,N′-dipp-substituted boranes underwent exothermic β-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 α-cyclopropyl-substituted radicals, this showed no propensity for ring-opening.