Mechanism of Dicyanoanthracene-Photosensitized Oxygenation of 1,1,2,2-Tetraarylcyclopropanes and 1,1,3,3-Tetraarylpropenes

Article abstract of DOI:10.1021/jo00311a011

1,1,2,2-Tetraphenylcyclopropane (2a) and electron-donor-substituted 1,1-diaryl-2,2-diphenylcyclopropanes 2b-f as well as correspondingly substituted 1,1-diaryl-3,3-diphenylpropenes 5a-e and 3,3-diaryl-1,1-diphenylpropenes 6a-e were irradiated in CCl4 and acetonitrile in the presence of oxygen and various sensitizers.The cyclopropanes as well as the propenes are inert toward singlet oxygen in both solvents.In electron-transfer-induced oxygenation reactions, photosensitized by 9,10-dicyanoanthracene in acetonitrile, cyclopropanes 2d-f, carrying efficient electron-donating 4-methoxyphenyl and 4-phenoxyphenyl groups, yield 1,2-dioxolanes 3d-f exclusively.Cyclopropanes 2b and 2c, which carry less efficient electron-donating 4-methylphenyl groups, give rise to dioxolanes 3b and 3c, respectively, as major products.In addition, allylic hydroperoxides 4b and 4c are formed, which are further oxygenated to benzophenone (10) and the corresponding diaryl ketones 7b and 7c. 1,1,2,2-Tetraphenylcyclopropane (2a) yields dioxolane 3a and allylic hydroperoxide 4a in a ratio of 3:2 as major products; in addition, 1,1,3,3-tetraphenylpropene (5a=6a) is formed as a minor product that is oxygenated under the reaction conditions to benzophenone (10) and diphenylacetaldehyde (8).By use of biphenyl (co-sensitizer), lithium perchlorate (special salt effect), and p-benzoquinone (quencher of O2^.-), it is shown that cyclopropanes 2a-f are oxygenated in chain reactions involving (1) 1,3-radical cations 2^.+ rather than 1,3-triplet biradicals and (2) triplet ground-state oxygen rather than the superoxide radical anion.Use of 1,8-dihydroxyanthraquinone as a sensitizer supports these results.Propenes 5a-e and 6a-e yield ketones and aldehydes as major products by reactions of 1,2-radical cations 5^.+ and 6^.+ with O2^.- as the oxygenating species.Dioxolanes and allylic hydroperoxides are not produced from these propenes.A mechanism is developed for the electron-transfer-induced photooxygenation of 1,1,2,2-tetraarylcyclopropanes 2 that shows that the increase of the resonance stabilization of the 1,3-radical cation 2^.+, caused by substitution of phenyl groups by electron-releasing aryl groups and demonstrated by the concomitantly decreasing oxidation potential of 2, plays the essential role in determining oxygenation rates and product formation.