Electron‐Transfer Cycloreversion of 2,3‐Diaryloxetanes: Influence of the Substitution and the Photosensitizer on the Regioselectivity
The regioselectivity in the oxidative electron-transfer cycloreversion (CR) of 2,3-diaryloxetanes depends on the substitution of the aryl groups and on the nature of the electron-transfer photosensitizer. The reaction occurs with fragmentation of the C2−C3 and C4−O bonds either in the presence of electron-releasing substituents in the 3-aryl groups, or when chloranil is used as photosensitizer. Thus, CR of the methoxy-substituted derivative trans,trans-3-(4-methoxyphenyl)-4-methyl-2-phenyloxetane (1b) results in the production of trans-anethole and benzaldehyde. In this case, the trans-anethole radical cation has been detected as transient intermediate by means of laser flash photolysis. Likewise, CR of 1a, photosensitized by chloranil, results in the formation of products arising from the trapping of the trans-β-methylstyrene radical cation by chloranil-derived intermediates. The reverse regioselectivity has been previously found in the ET cycloreversion of 4-methyl-2,3-diphenyloxetane (1a) using (thio)pyrylium salts as photosensitizers. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)