The Influence of Boryl Substituents on the Formation and Reactivity of Adjacent and Vicinal Free Radical Centers

Radicals containing α-boronate substituents were generated by bromine abstraction from 1-bromoalkyldioxaborolanes (boronic esters), by addition to vinyl boronate, and by hydrogen abstraction from alkyldioxaborolanes and observed by EPR spectroscopy. Unsymmetrically substituted α-boronate radicals displayed selective line broadening in their low-temperature spectra from which barriers to internal rotation about ^•CH₂−B(OR‘)OR bonds were found to be 3 ± 1 kcal mol^-1. Use of an empirical relationship between barrier height and bond dissociation energy led to BDE[(RO)₂BCH₂−H] = 98.6 kcal mol^-1. Rate constants for hydrogen abstraction from 2,4,4,5,5-pentamethyl-1,3,2-dioxaborolane by tert-butoxyl radicals were determined from competitive EPR and product studies and found to be relatively small, comparable 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 extremely difficult. The structures and energetics of α-boronate radicals were computed by DFT methods (B3LYP/6-31G*). This predicted reductions in the rotation barriers of X₂B−CH₂^• radicals for increasing alkoxy substitution at B (X = Me or MeO) and corresponding increases in the X₂BCH₂−H bond dissociation energies. The B3LYP-computed BDE[(MeO)₂BCH₂−H] was in excellent agreement with the analogous value derived from the experimental rotation barrier. Radicals containing β-boronate substituents were generated from the corresponding 2-bromoalkylboronic esters and characterized by EPR spectroscopy. At higher temperatures the main product from trialkyltin and triethylsilyl radical promoted reactions of 2-(2-bromohexyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane was 1-hexene. This was best accounted for by a mechanism involving initial S_H2 attack on the borolane and subsequent bromine atom elimination from the displaced 2-bromohexyl radical.