Thiophenes reactivity indices

For benzo[. ]thiophene, there is a it-MO delocalization between the two aromatic rings. For benzo[r]thiophene, the orbitals of the five-membered rings are localized on the heteroatom, C-1 and C-3, and there is no it-MO delocalization on the heterocyclic five-membered rings. These results are in agreement with the theoretical aromaticity of these molecules as are theoretical results from the reactivity indexes. 

Aromatic substitution reactions are often complicated and multistep processes. A correlation, however, in many cases can be found between the charged attacking species and the electron density distribution in the molecule attacked during electrophilic and nucleoph c substitution. No such correlation is expected in radical substitution where the attacking particles are neutral, rather a correlation between the reactivities of separate bonds and a free valency index of the bond order. This allows the prediction of the most reactive bonds. Such an approach has been used by researchers who applied quantum calculations to estimate the reactivities of the isomeric thienothiophenes and to compare them with thiophene or naphthalene. " Until recently quantum methods for studying reactivities of aromatics and heteroaromatics were developed mainly in the r-electron approximation (see, for example, Streitwieser and Zahradnik ). The M orbitals of a sulfur atom were shown not to contribute substantially to calculations of dipole moments, polarographic reduction potentials, spin-density distribution, ... 

Calculations have been carried out to estimate the relative reactivities of the isomeric thienothiophenes in comparison with thiophene or naphthalene. The localization energy is expected to be the most adequate index of reactivity (76AHC(19)123). 

The DFT-derived Fukui indexes as reactivity descriptors make it possible to predict the preferential sites of electrophilic attack on pyrrole, furan and thiophene molecules. Their relative reactivities depend on the local softness of the most reactive site in each system. The results obtained by this approach are in total agreement with experiment (05MI1). 

---