Nuclear magnetic resonance. Part IV. The protonation of indolizines

Abstract

The protonation of indolizines under reversible conditions has been studied. Indolizines unsubstituted at position 3 give 3H-indolizinium cations alone in measurable amount. 3-Substituted indolizines give mixtures of 3H- and 1H-indolizinium cations in proportions depending on the nature and position of substitution. Exceptionally 3,5-dimethylindolizine gives the 3H-cation alone, owing to intramolecular overcrowding.The protonation of indolizine at position 3 agrees with Hückel M.O. predictions of reactivity in terms of charge and frontier electron densities, but not of electrophilic localisation energies.