Quantum-chemical study of the protonation of pyrrolo[2,1-b]thiazole and its selenium and tellurium analogs

The possibility of proton attack on various centers in pyrrolo[2,1-b]thiazole (1) has been evaluated. The results of semiempirical (MNDO, AM1, and PM3) andab initio (6-31G*) calculations were compared. The MNDO and 6-31G* methods give “chemically proper” and qualitatively coincident results. Analysis of the intramolecular (geometric and electronic) reorganization of molecule1, depending on the protonation center, has been carried out. The most probable attack centers, depending on the mechanism of electrophilic reaction, have been recognized. The energy parameters of intramolecular prototropic rearrangements in cation1 and the “blocking” factor value of methyl groups reducting the corresponding complex stability have been evaluated. It has been established that the relative stability of the protonated forms does not change on going to pyrrolo[2,1-b]selenium- and telluriumazoles, but the range of variations is considerably narrowed in the series S>Se>Te.