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

Walton, JC and Brahmi, MM and Monot, J and Fensterbank, L and Malacria, M and Curran, DP and Lacǒte, E (2011) Electron paramagnetic resonance and computational studies of radicals derived from boron-substituted N-heterocyclic carbene boranes. Journal of the American Chemical Society, 133 (26). 10312 - 10321. ISSN 0002-7863

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Abstract

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 - BH Ar 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 - BH · 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. © 2011 American Chemical Society. 2 2


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Details

Item Type: Article
Status: Published
Creators/Authors:
CreatorsEmailPitt UsernameORCID
Walton, JC
Brahmi, MM
Monot, J
Fensterbank, L
Malacria, M
Curran, DPcurran@pitt.eduCURRAN
Lacǒte, E
Date: 6 July 2011
Date Type: Publication
Journal or Publication Title: Journal of the American Chemical Society
Volume: 133
Number: 26
Page Range: 10312 - 10321
DOI or Unique Handle: 10.1021/ja2038485
Schools and Programs: Dietrich School of Arts and Sciences > Chemistry
Refereed: Yes
ISSN: 0002-7863
PubMed ID: 21619055
Date Deposited: 15 Nov 2012 21:09
Last Modified: 30 Mar 2021 14:55
URI: http://d-scholarship.pitt.edu/id/eprint/16322

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