Reaction of aromatic N-oxides with dipolarophiles. IX. Formation of 1:2 cycloadduct of 2-alkylpyridine N-oxides and phenyl isocyanates and a frontier molecular orbital study.

Abstract

A series of 1 : 2 cycloadducts of a new type was obtained from the reaction of 2-alkylpyridine N-oxides with phenyl isocyanates. The reaction mechanisms are discussed in terms of frontier molecular orbital theory based on CNDO/2 and MINDO/2 calculations and kinetic data. The small solvent effect and activation parameters (Ea=14 kcal/mol, ΔS=-46 e.u.) observed for the reaction of 2-alkylpyridine N-oxides with phenyl isocyanates indicate that the cycloaddition proceeds via concerted pathways involving little charge separation in the transition state.