Quantum-chemical investigation of the mechanism of the sulfonation of pyrrole

A detailed quantum-chemical study of the sulfonation of pyrrole with regard to the effect of the solvent (the model of overlapping spheres) on the energy characteristics of the formation of the σ-complexes produced during attack on the α-and β-positions of the heterocycle and their possible transformation paths was made by density functional theory the B3LYP/6-31G(d) method]. The possibility of mutual transformations between the isomeric σ-complexes by α/β-migration of the SO₃ is examined. The formation of pyrrolesulfonic acids was studied for the case of the intramolecular rearrangement of the complexes. Comparison of the activation energies shows that in contrast to the gas-phase reaction the formation of the β-sulfonic acid is preferred in methylene chloride: the solvation energy of the α-isomer of the σ-complex is higher than the energy for the transition state of its rearrangement and its product, α-pyrrolesulfonic acid, leading to an increase in the kinetic barrier and to a decrease of the energy gain on the path to the formation of the latter. The opposite variation of the energy characteristics on the path to the β-isomer with regard to solvation leads to agreement between the calculated data and the experimentally observed preferred formation of the β-pyrrolesulfonic acid. Dedicated to Mikhail Grigor’evich Voronkov. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1647–1654, November, 2006.