Thermolysis of Thiophenedicarboxylic Acid Anhydrides as a Route to Five-Membered Hetarynes

Article abstract of DOI:10.1021/ja00400a046

Flow vacuum thermolysis (FVT) of the anhydrides of thiophene-2,3- (5) and -3,4-dicarboxylic acid (6) and of thianaphthene-2,3-dicarboxylic acid (7) in the presence of thiophene, 1,3-cyclohexadiene, or benzene gave thianthrene (15a), presumably by aromatization of an intermadiate Diels-Alder adduct of the diene traps and the aryne 2,3-didehydrothiophene (8).A similar rationale explains the formation of dibenzothiophene from 7 and thiophene, the four monomethylthianaphthenes 22-25 from 5 and cyclopentadiene, 5,6-dimethylthianaphthene (15f) from 5 and 2,3-dimethylbutadiene, and a mixture of hydroxythianaphthenes (15d) from 5 and furan.The latter reaction also produces a mixture of isomeric cyclopentanothiophenes (19) which probably arise by decarbonylation of 15d.An FVT reaction of 5 with propyne as the trap gave a mixture of allenyl-(28) and propynylthiophenes 29 which probably arise by an ene reaction of the trap and the aryne 8.Decomposition of the anhydrides 5 and 6 in molten anthracene led only to Friedel-Crafts products such as thienylanthracenes 33 and quinones 34 from 5 and 39 and 40 from 6.The diene reactivity of thiophene 13a in the gas phase was demonstrated by the formation of naphthalene from its reaction with benzyne generated from the thermolysis of indanetrione and by the formation of indene from 13a plus cyclopentadiene.Possible, but improbable, interpretations of the FVT results without invoking a five-membered hetaryne intermediate are considered.