Comparison of (5 + 2) Cycloadditions Involving Oxidopyrylium and Oxidopyridinium Ions: Relative Reactivities.

A variety of (5 + 2) cycloaddition reactions involving oxidopyridinium and oxidopyrylium zwitterions are compared to investigate the effects of nitrogen-for-oxygen substitution on reactivity. Activation barriers for nitrogen-containing systems are predicted to be larger than those for analogous oxygen-containing systems. Correlations between barrier heights and synchronicity of C-C bond formation, changes to aromaticity, reactant distortion, and interaction energies between zwitterions and alkenes were assessed, leading to the conclusion that reactivity depends more on distortion effects (including aromaticity loss) than on interaction effects (such as those associated with highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interactions).