Competitive cyclisation of singlet and triplet nitrenes. Part 6. The cyclisation of 2-azidophenyl thienyl sulphides

Abstract

A number of derivatives of the title azides [namely 2-azidophenyl 2-thienyl, 5-methyl-2-thienyl, 3-methyl-2-thienyl, and 3,5-dimethyl-2-thienyl sulphides (5) and 2-azidophenyl 3-thienyl sulphide (6)] have been decomposed thermally and photochemically. The influence of differing solvents and temperatures, of the presence of various dienophiles, of added triethyl phosphite, and of photosensitisation with acetophenone was also studied. Pyrrolo-[2,1-b]benzothiazoles were produced in most cases, often in good yield. 2-Azidophenyl 3,5-dimethyl-2-thienyl sulphide gave 2-(3-thienyl)benzothiazole in addition to the pyrrolobenzothiazole. The formation of the products has been rationalised by consideration of a ring-opening–ring-closure mechanism of an intermediate spiro-thienobenzothiazole, and analogous literature reactions have been reinterpreted in this light.