Photochemical Electron‐Transfer Generation of Arylthiirane Radical Cations with Tetranitromethane and Chloranil — Some Novel Observations

The radical cations from 2,2,3,3‐tetraphenylthiirane ( 1a ), 2,2‐bis(4‐methoxyphenyl)‐3,3‐diphenylthiirane ( 1b ), and trans ‐2,3‐diphenylthiirane ( 1c ) have been generated by photoinduced electron transfer (PET) reactions with tetranitromethane [C(NO 2 ) 4 ] and chloranil (CA). A charge‐transfer complexe (CTC) absorption is observed by UV/Vis spectroscopy between thiiranes ( 1 ) and C(NO 2 ) 4 . On the other hand, quenching studies with azulene suggest that the ET reaction occurs between thiiranes and the triplet CA ( 3 CA). The photochemical reaction of the CTC between thiiranes 1 and C(NO 2 ) 4 yields mainly the corresponding alkenes from the fragmentation of the radical‐cation intermediate 1 · + , together with the products derived from nitration on the phenyl rings. However, oxygen transfer to afford the sulfoxides is not found. A marked solvent effect is observed in this reaction, with cage coupling favored in CH 2 Cl 2 (nitration derivatives as primary products) and non‐cage coupling observed in CH 3 CN (the alkene as the primary product). The PET reactions between 1a – b and CA, in the presence of CH 3 OH (or another possible oxygen‐centered nucleophile), give the ketone derivatives through ring opening, followed by oxidative cleavage. Conversely, under the same experimental conditions, the thiirane 1c affords only trans ‐stilbene 2c . This different behavior is ascribed to a different spin density in the corresponding singly occupied molecular orbital (SOMO) of the radical cation. For 1c +· , the spin density is concentrated at the sulfur atom, whereas for 1a ·+ and 1b ·+ , the charge is distributed onto the aromatic rings. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)