EPR studies on the addition of ligated boryl radicals to carbonyl compounds
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The boron-centered radicals derived from alkenyl N-heterocyclic carbene (NHC)-boranes bearing ester substituents were recently found to ring close in 5-endo mode by addition to the oxygen atoms of the ester substituents. The inference from this was that NHC-boryl radicals might add intermolecularly to carbonyl-containing substrates. Several different NHC-boryl radicals were generated by H-atom abstraction from NHC-ligated trihydroborates. Electron paramagnetic resonance (EPR) spectroscopy proved that these did indeed add to the oxygen atoms of diaryl ketones with production of the corresponding bora-ketyl radicals. The same unusual regioselectivity of addition was observed with monoaryl ketones, but no bora-ketyls were observed with dialkyl ketones. Similarly, no bora-ketyl adduct radicals were observed with esters, even esters of benzoic acid. EPR spectroscopic evidence suggested that NHC-boryl radicals were also added to the O-atoms of aromatic aldehydes. Amine-boryl and phosphine-boryl radicals were also observed to add to the O-atom of benzophenone with production of the corresponding ketyl radicals.
Walton , J C , Dai , W & Curran , D P 2020 , ' EPR studies on the addition of ligated boryl radicals to carbonyl compounds ' , The Journal of Organic Chemistry , vol. 85 , no. 6 , pp. 4248-4255 . https://doi.org/10.1021/acs.joc.9b03420
The Journal of Organic Chemistry
Copyright © 2020 American Chemical Society. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the author created accepted 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.9b03420
DescriptionJ.C.W. thanks EaStCHEM for financial support and D.P.C. thanks the US National Science Foundation. Computational support was provided through the EaStCHEM Research Computing Facility.
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