Quantum-chemical investigation of the effect of solvent polarity on the direction of sulfonation of pyrrole

The relative stability of the isomeric σ-complexes formed in the sulfonation of pyrrole at the α-or β-position (the α-isomer is energetically more favorable) does not agree with the experimentally established positional selectivity of substitution (the formation of β-pyrrolesulfonic acid). However, quantum-chemical calculations of the energy parameters for the reaction of pyrrole and SO₃ with due regard to the solvation effect in the model solvent methylene chloride (ε = 8.93) lead to the conclusion that the calculated activation energy of the rearrangement to the more favorable β-pyrrolesulfonic acid for the less favorable β-isomer of the σ-complex is lower than on the path to the formation of the α-pyrrolesulfonic acid. It was shown that the significant increase in the polarity of the model medium in the transition to DMSO (ε = 46.7) does not lead to substantial change in the energy parameters of the reaction. The explanation for the positional selectivity during the sulfonation of pyrrole using Py·SO₃, according to previous data, involves the participation of the pyridine in the transformation of the σ-complexes into the products. The calculations were made by the B3LYP/6-31G(d) and HF/3-21+G methods using the model of overlapping spheres to take account of solvation. Dedicated to E. J. Lukevics on his 70th birthday __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 34–40, January, 2007.