Quantum Chemical Analysis of Heteroarylnitrenium Ions and Mechanisms for Their Self-Destruction

Singlet−triplet state-energy splittings in arylnitrenium ions are substantially reduced when the aromatic system is heterocyclic with nitrogen atoms incorporated at ring positions that would otherwise better stabilize the formally exocyclic positive charge through resonance. Ring contraction rearrangements of singlet arylnitrenium ions with a nitrogen atom at an <i>ortho</i> position are predicted to proceed with barrier heights that become lower with increasing substitution of nitrogen atoms at other positions. Predilection to delocalize positive charge also controls the relative facility with which (i) <i>N</i>-acetylarylnitrenium ions undergo ring closure to form 2-oxazolium cations and (ii) arylnitrenium ions are subject to nucleophilic aromatic substitution by water.