John C. Walton, A. McCarroll, Qiao-lan Chen
May 23, 2000
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0
Influential Citations
47
Citations
Journal
Journal of the American Chemical Society
Abstract
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 radical...