Protonated Forms of 2-(2-Furyl)pyrroles and Their Interconversion: ¹H NNR and Quantum-Chemical (MNDO) Study

The 2-(2-furyl) pyrroles and vinylpyrroles are protonated by various acids (HSO₃F, CF₃CO₂H, HCl, HBr) at C₅ of either the pyrrole or furan ring depending on reaction conditions (temperature and acid nature). At -80 °C only the pyrrole ring protonation is observed; at -40 °C (the reaction with HCl and HBr) an equilibrium mixture of pyrrolium (A) and furanium (B) ions is formed, with the hydrogen halides being added to the vinyl group. At 0 °C the HBr molecule adds to the protonated furane ring with the formation of 2-(2-pyrrolyl)-4-bromo-4,5-dihydrofuranium cations, which upon further heating (20 °C) are transformed to 2-(2-furyl)-4-bromo-4,5-dihydropyrrolium cations. The heats of formation (AH), atomic charges, and characteristics of the frontier molecular orbitals in the 2-(2-furyl)pyrrole molecule (1) and its protonated forms (1A) and (IB) have been calculated by MNDO method. The ΔE values for 1A and IB are in close agreement with their experimental ratio. The protonation of 1 is likely to proceed under orbital control. The internal rotational potentials in 1, 1A, and 1B have been calculated and discussed.