Crystal structure of trans-2,3-bis(mesitylthio)-4-nitro-2,3-dihydrothiophene and kinetic study of its base-catalysed aromatisation to 2-mesitylthio-4-nitrothiophene in toluene

Abstract

By means of a crystal-structure determination, the trans-configuration is unequivocally assigned to the 2,3-dihydrothiophene derivative (2), isolated as an intermediate in the cine-substitution of 3,4- dinitrothiophene (1) with mesitylenethiolate. The puckering of the 2,3-dihydrothiophene ring can be interpreted in terms of a half-chair conformation. Structural parameters establish the regiospecificity of the Buⁿ₃N-induced syn-elimination of mesitylenethiol from (2) in toluene [leading to the final product (3)], for which a carbanionic, stepwise pathway through ion pairs is postulated from kinetic evidence. A brief discussion of the proposed mechanism of formation of (2) from (1) is also presented.