Quantum-Chemical Study on Positional Selectivity in the Trimethylsilylation and Sulfonation of Pyrrole and <Emphasis Type="Italic">N</Emphasis>-Alkylpyrroles

Positional selectivity (α:β ratio) of electrophilic substitution in pyrrole, N-methylpyrrole, and N-tert-butylpyrrole was analyzed by ab initio RHF/6-31G(d), MP2/6-31G(d)//RHF/6-31G(d)] and DFT B3LYP/6-31G(d)] calculations of some σ-complexes derived from the substrates. The results of calculations with the use as model electrophilic species of trimethylsilyl cation MP2/6-31G(d)//RHF/6-31G(d) and B3LYP/6-31G(d)] and SO₃ molecule B3LYP/6-31G(d)] instead of proton are fairly consistent with the experimental data, according to which trimethylsilylation of pyrrole and its N-substituted derivatives with trimethylsilyl trifluoromethanesulfonate, as well as sulfonation with pyridine-sulfur trioxide complex, gives the corresponding β-substituted products.