EPR Studies on the Addition of Ligated Boryl Radicals to Carbonyl Compounds

• Published in: Journal of organic chemistry Vol. 85; no. 6; pp. 4248 - 4255
• Main Authors: Walton, John C, Dai, Wen, Curran, Dennis P
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 20-03-2020
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/acs.joc.9b03420

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.