Condensed heterocycles. 39. Quantum-chemical calculations of the reactivities of some condensed sulfur and selenium heterocycles
A number of isomeric thienothiophenes, selenophenothiophenes, and selenopheno-selenophenes, as well as their C-protonated forms (σ complexes), were calculated by the self-consistent-field (SCF) MO LCAO method with the CNDO/2 approximation and the spd basis. Their reactivities in electrophilic substitution reactions were investigated theoretically by means of the localization energy approach. The results were compared with the available experimental data. The dependence of the specificity of electrophilic attack on the size of the attacking reagent was investigated for compounds with various heteroatoms. It is shown that the reactivity of the α position relative to the selenium atom should increase as the size of the attacking reagent increases.