Quantum chemical studies of sulfonation of pyrrole with pyridine sulfur trioxide

Abstract

In the framework of the density functional theory (B3LYP/6-31G(d) method), a detailed quantum chemical study of sulfonation of pyrrole with pyridine sulfur trioxide in a dichloroethane medium was performed with allowance for its influence on the energy characteristics of the reaction at the IEFPCM level. A direct transfer of the electrophile SO₃ from pyridine to pyrrole was studied as an example of the formation of the intermediate σ-complexes from the starting compounds. A possibility of preliminary dissociation of pyridine sulfur trioxide was considered. The similarity of energy barriers on the way to isomeric σ-complexes was due to the closeness of early transition states of the SO₃ molecule addition to the α- or β-C atom of the pyrrole molecule to the starting compounds on the reaction coordinate. At the first step of the reaction, the latter should form with approximately equal rates and then be subjected to rapid deprotonation involving pyridine, which leads to the reaction products. Comparatively slow interconversions are also possible. Under these conditions, the formation of the pyridinium salts of α- and β-pyrrolesulfonic acids can be reversible. The α-isomer of the product is kinetically less stable, which is favorable for the predominance of the β-isomer in the reaction mixture.