PROTONATED FORMS OF 2-(2-FURYL)PYRROLES AND THEIR INTERCONVERSION - H-1-NMR 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-degrees-C only the pyrrole ring protonation is observed; at -40-degrees-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-degrees-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-degrees-C) are transformed to 2-(2-furyl)-4-bromo-4,5-dihydropyrrolium cations. The heats of formation (DELTA-H), atomic charges, and characteristics of the frontier molecular orbitals in the 2-(2-furyl)pyrrole molecule (1) and its protonated forms (1A) and (1B) have been calculated by MNDO method. The DELTA-E values for 1A and 1B 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.