EPR and preparative studies of 5-endo cyclizations of radicals derived from alkenyl NHC-boranes bearing tert-butyl ester substituents
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Radical H atom abstraction from a set of N-heterocyclic carbene (NHC) complexes of alkenylboranes bearing two tert-butyl ester substituents was studied by EPR spectroscopy. The initial boraallyl radical intermediates rapidly ring closed onto the O atoms of their distal ester groups in 5-endo mode to yield 1,2-oxaborole radicals. Unexpectedly, two structural varieties of these radicals were identified from their EPR spectra. These proved to be two stable rotamers, in which the carbonyl group of the tert-butyl ester was oriented toward and away from the NHC ring. These rotamers were akin to the s-trans and s-cis rotamers of α,β-unsaturated carbonyl compounds. Their stability was attributed to the quasi-allylic interaction of their unpaired electrons with the carbonyl units of their adjacent ester groups. EPR spectroscopic evidence for two rotamers of the analogous methyl ester containing NHC-oxaborole radicals was also obtained. An improved synthetic procedure for preparing rare NHC-boralactones was developed involving treatment of the alkenyl NHC-boranes with AIBN and tert-dodecanethiol.
Dai , W , Curran , D P & Walton , J C 2019 , ' EPR and preparative studies of 5- endo cyclizations of radicals derived from alkenyl NHC-boranes bearing tert -butyl ester substituents ' , Journal of Organic Chemistry , vol. 84 , no. 4 , pp. 2102-2111 . https://doi.org/10.1021/acs.joc.8b03112
Journal of Organic Chemistry
Copyright © 2019 American Chemical Society. This work has been made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1021/acs.joc.8b03112