Synthesis and transformation of sulfides of the thiophene series. 34. Quantum-chemical studies of the specificity of electrophilic attack and isomerization under C-protonation conditions in the alkylmercaptothiophene series

Abstract

2-Methoxy-5-methylthiothiophene, 2,5-bis(methylthio)thiophene, and all of their possible C-protonated forms (σ complexes) were subjected to calculations by the self-consistent-field (SCF) MO LCAO method within the CNDO/2 (complete neglect of differential overlap/2) valence approximation. The results obtained were used to explain the peculiarities of the behavior of these compounds in electrophilic-substitution reactions (Vilsmeier formylation and acetylation). It is shown that the most preferred sites of electrophilic attack in both compounds are the 3 and 5 positions, while the least active site is the 2 position in 2-methoxy-5-methylthiophene, as a result of which one should not observe either migration or splitting out of an OCH₃ group, in agreement with the available experimental data. The energetics of the secondary transformations of the σ complexes formed in the reaction, viz., isomerization and elimination of the SCH₃ ⁺ group, were estimated.