Isomerization barriers and resonance stabilization for the conrotatory and disrotatory isomerizations of nitrogen containing tricyclo moieties

Abstract

The isomerizations of 3,4-diazatricyclo[4.1.0.0^2,7]hept-3-ene and 3,4-diazatricyclo[4.1.0.0^2,7]heptane to their corresponding products were studied by ab initio calculations. Structures were determined at the multiconfiguration self-consistent field level and energies calculated at the single state second-order MRMP level. The isomerization of 3,4-diazatricyclo[4.1.0.0^2,7]hept-3-ene occurs through four unique pathways with barriers of 36.1 and 37.9 kcal mol⁻¹ for the allowed channels, while those for the forbidden channels were 44.3 and 56.5 kcal mol⁻¹. The 12.2 kcal mol⁻¹ disparity in the disrotatory barriers is explained through electron delocalization in the transition state. The 3,4-diazatricyclo[4.1.0.0^2,7]heptane structure has eight separate reaction channels for isomerization, and the allowed barriers ranged from 37.4–43.3 kcal mol⁻¹ while the forbidden barriers ranged from 49.5–57.3 kcal mol⁻¹. Resonance stabilization for two of the forbidden pathways results in a relative energy lowering. The energy difference in the four allowed barriers is due mainly to steric considerations. The isomerization of 3,4,5-triazatricyclo[4.1.0.0^2,7]hept-3-ene through the disrotatory channel was studied to help identify stabilization effects from π bond electrons and lone pair electrons: π bond electrons showed greater contribution for molecular stabilization than lone pair electrons.