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
Abstract2-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.