Protonated forms of N-vinylpyrroles

The reaction of N-vinylpyrroles with hydrogen chloride at low temperatures has been studied. At -80‡C the α-position of the heterocycle is protonated while at higher temperatures products of HCl addition to the double bond and protonation of the ring are formed. The C-Cl bond of the chloroethyl radical of the product of HCl addition to the vinyl group is readily dissociated resulting in reversible interconversion of the enantiomers of the racemic mixture which is formed. By introducing a diastereotopic marker (i-Pr) this interconversion can be monitored by dynamic NMR. It has been found that the energy of this process varies with HCl concentration and with the substituents on the ring.