Abstract
Thienosultine is a versatile building block in organic synthesis, for example, as a precursor for the non-Kekule structure 3,4-dimethylene thiophene. Thienosultine reacts with a dieneophile in a cycloaddition at 393 to 453 K forming the slightly more stable thienosulfolene as a by-product. The reverse reaction of thienosulfolene to thienosultine is not observed in experiments and there is no cycloaddition with dienophiles under the same reaction conditions. In order to analyze experimental data, the reaction between ethene and thienosultine or thienosulfolene to form 4,5,6,7-tetrahydro- benzo[c]thiophene was investigated by quantum chemical computations at on the B3LYP/def2-TZVP level. Two reaction paths were examined in detail. The first pathway started with the Diels-Alder addition of ethene to the thiophene ring of the reactant followed by SO2 elimination. The addition of a second ethene molecule in a retro-Diels-Alder reaction yielded the thiophene product. The second pathway contained a single step in which the ethene molecule was added to the SO2 side of the reactant with the simultaneous elimination of a SO2 molecule. The calculation results show that the second pathway is more likely than the first one as the barrier in the rate determining step is about 10 kcal/mol lower. Moreover, the second pathway can be used to explain the observed selectivity of the reaction which is not possible with the first one.