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EPR studies of the generation, structure, and reactivity of n-heterocyclic carbene borane radicals

Walton, JC and Brahmi, MM and Fensterbank, L and Lacôte, E and Malacria, M and Chu, Q and Ueng, SH and Solovyev, A and Curran, DP (2010) EPR studies of the generation, structure, and reactivity of n-heterocyclic carbene borane radicals. Journal of the American Chemical Society, 132 (7). 2350 - 2358. ISSN 0002-7863

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Abstract

N-Heterocyclic carbene boranes (NHC-boranes) are a new "clean" class of reagents suitable for reductive radical chain transformations. Their structures are well suited for their reactivity to be tuned by inclusion of different NHC ring units and by appropriate placement of diverse substituents. EPR spectra were obtained for the boron-centered radicals generated on removal of one of the BH3 · hydrogen atoms. This spectroscopic data, coupled with DFT computations, demonstrated that the NHC-BH2radicals are planar Π-delocalized species. tert-Butoxyl radicals abstracted hydrogen atoms from NHC-boranes more than 3 orders of magnitude faster than did C-centered radicals, although the rate decreased markedly for sterically shielded NHC-BH3 centers. Combinations of two NHC-boryl radicals afforded 1,2-bisNHC-diboranes at rates which also depended strongly on steric shielding. The termination rate increased to the diffusion-controlled limit for sterically unhindered NHC-boryls. Bromine atoms were rapidly transferred to imidazole-based NHC-boryl radicals from alkyl, allyl, and benzyl bromides. Chlorine-atom abstraction was, however, much less efficient and only observed for sterically unhindered NHC-boryls reacting with allylic and benzylic chlorides. For an NHC-borane containing a bulky thexyl substituent at boron, the tertiary H atom of the thexyl group was selectively removed. The resulting ß-boron- containing alkyl radical rapidly underwent ß scission of the B-C bond with production of an NHC-boryl radical and an alkene. © 2010 American Chemical Society.

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Details

Item Type: Article
Status: Published
Creators/Authors:
CreatorsEmailPitt UsernameORCID
Walton, JC
Brahmi, MM
Fensterbank, L
Lacôte, E
Malacria, M
Chu, Q
Ueng, SH
Solovyev, A
Curran, DPcurran@pitt.eduCURRAN
Date: 3 March 2010
Date Type: Publication
Journal or Publication Title: Journal of the American Chemical Society
Volume: 132
Number: 7
Page Range: 2350 - 2358
DOI or Unique Handle: 10.1021/ja909502q
Schools and Programs: Dietrich School of Arts and Sciences > Chemistry
Refereed: Yes
ISSN: 0002-7863
PubMed ID: 20104896
Date Deposited: 12 Feb 2013 15:10
Last Modified: 13 Jun 2021 01:55
URI: http://d-scholarship-dev.library.pitt.edu/id/eprint/17311

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