Photochemical cycloaddition reactions of N-methyl(thiophthalimide) and N-methyl(dithiophthalimide) with alkenes

Abstract

Irradiation of N-methyl(thiophthalimide) with alkenes leads to reaction at the thiocarbonyl, rather than at the carbonyl, group of the thioimide. Products isolated include (2 + 2) cycloadducts (isoindole-1-spiro-2′-thietanes), a cleavage product (a 3-alkylideneisoindol-1-one) derived from such cycloadducts, or (2 + 2 + 2) cycloadducts incorporating two molecules of thioimide (1,4-dithiane-2,3-bis-spiroiso-indoles). The products are consistent with a mechanism that involves an intermediate 1,4-biradical, and they contrast markedly with those obtained from N-methylphthalimide with alkenes. N-Methyl-(dithiophthalimide) is less effective in photoreactions with alkenes, and in only one instance was a cycloadduct formed at a reasonable rate and isolated; possible reasons for this difference are discussed.