Thermal and microwave assisted reactions of 2,5-disubstituted thienosultines with [60]fullerene: non-Kekulé biradicals and self-sensitized oxygenation of the cycloadduct

Abstract

Refluxing an o-dichlorobenzene solution of 2,5-disubstituted thienosultines 10a– f with [60]fullerene for 2–24 h gave both 1:1 and 2:1 cycloadducts in 37–79% isolated yields. The reaction was highly accelerated by microwave irradiation giving comparable yields of cycloadducts. Sultines 10a– f underwent cheletropic extrusion of SO 2 to form the corresponding non-Kekulé biradical intermediates 11a– f , which were subsequently trapped by [60]fullerene to form corresponding cycloadducts. The activation energy barriers (Δ G c ≠) determined for the boat-to-boat inversion of these 4′,5′,6′,7′-tetrahydrobenzo[ c]thieno-[5′,6′:1,2][60]fullerene adducts 12a– f were found to be in the range of 13.5–14.8 kcal/mol. Unexpectedly, one of the monoadduct 12a was found to be labile when kept in air under ambient light. Two new products 15 (a sulfine-enone) and 16 (an endione) were isolated from the decomposed 12a and were found to derive from self-sensitized singlet oxygen reaction on the 2,5-dimethylthieno moiety of 12a .