The [4 + 2] cycloaddition products between thiobenzophenone (
1) and arylalkenes (
2) were obtained in the presence of (thia)pyrylium salts(sensitizer =
3 or
4) as electron t
ransfer photosensitizers. Although both
radical cations
1+ and
2+ are gene
rated, only the former is involvedin the process.