CYCLOADDITION REACTIONS OF 2,3-DIDEHYDROTHIOPHENE GENERATED BY FLOW VACUUM THERMOLYSIS OF THIOPHENE-2,3-DICARBOXYLIC ANHYDRIDE

Article abstract of DOI:10.1016/0040-4020(81)85005-3

Flow vacuum thermolysis (FVT) of thiophene-2,3-di-carboxylic anhydride (2) in the presence of 2,3-dimethylbutadiene (6) gives, in addition to 5,6-dimethylthianaphthene (9), small quantities of a dihydrodimethylthianaphthene (12) and another dimethylthianaphthene (13) which is probably also formed by dehydrogenation of 12 with chloranil.The partial structures of these minor products are consistent with their being formed by a <2+2>-cycloaddition between 6 and an intermediate aryne, 2,3-didehydrothiophene (1), followed by a rearrangement of the resulting adduct 11 and dehydrogenation.FVT of 2 in the presence of 2,5- (17b) or 3,4-dimethylthiophene (17c) also gave a mixture of the dimethylthianaphthenes (18, 22, 23) which can be rationalized as arising by a <4+2>- and two <2+2>-cycloadditions of the aryne 1 to the thiophenes 17 with subsequent desulfurization.The lack of equilibration of the products 18, 22, and 23 was demonstrated and their origin as a function of the structure and reactivity of the aryne 1 discussed.