Abstract
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.
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Dedicated to E. J. Lukevics on his 70th birthday
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Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 34–40, January, 2007.
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Belen’kii, L.I., Nesterov, I.D. & Chuvylkin, N.D. Quantum-chemical investigation of the effect of solvent polarity on the direction of sulfonation of pyrrole. Chem Heterocycl Compd 43, 28–33 (2007). https://doi.org/10.1007/s10593-007-0004-2
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DOI: https://doi.org/10.1007/s10593-007-0004-2