The influence of boryl substituents on the formation and reactivity of adjacent and vicinal free radical centers

Radicals containing alpha-boronate substituents were generated by bromine a bstraction from 1-bromoalkyldioxaborolanes (boronic esters), by addition to vinyl boronate, and by hydrogen abstraction from alkyldioxaborolanes and o bserved by EPR spectroscopy. Unsymmetrically substituted alpha-boronate rad icals displayed selective line broadening in their low-temperature spectra from which barriers to internal rotation about (CH2)-C-.-B(OR')OR bonds wer e found to be 3 +/- 1 kcal mol(-1). Use of an empirical relationship betwee n barrier height and bond dissociation energy led to BDE[(RO)(2)BCH2-H] = 9 8.6 kcal mol(-1). Rate constants for hydrogen abstraction from 2,4,4,5,5-pe ntamethyl-1,3,2-dioxaborola by tert-butoxyl radicals were determined from c ompetitive EPR and product studies and found to be relatively small, compar able to those of unactivated methyl groups. Hydrogen abstraction from bis(4 ,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)methane was found to be extremel y difficult. The structures and energetics of cr-boronate radicals were com puted by DFT methods (B3LYP/6-31G*). This predicted reductions in the rotat ion barriers of X2B-CH2. radicals for increasing alkoxy substitution at B ( X = Me or MeO) and corresponding increases in the X2BCH2-H bond dissociatio n energies. The B3LYP-comIjuted BDE[(MeC))2BCH2-H] was in excellent agreeme nt with the analogous value derived from the experimental rotation barrier. Radicals containing P-boronate substituents were generated from the corres ponding 2-bromoalkylboronic esters and characterized by EPR spectroscopy. A t higher temperatures the main product from trialkyltin and triethylsilyl r adical promoted reactions of 2-(2-bromohexyl)-4,4,5,5-tetramethyl-1,3,2-dio xaborolane was l-hexene. This was best accounted for by a mechanism involvi ng initial S(H)2 attack on the borolane and subsequent bromine atom elimina tion from the displaced 2-bromohexyl radical.