Isomerization barriers for the disrotatory and conrotatory isomerizations of 3-aza-benzvalene and 3,4-diaza-benzvalene to pyridine and pyridazine

Abstract

The isomerizations of 3-aza-benzvalene to pyridine and 3,4-diaza-benzvalene to pyridazine have been studied using ab initio methods with a multiconfigurational wavefunction. Transition states for both the allowed disrotatory and forbidden conrotatory pathways were located. The forbidden pathways proceed through an intermediate consisting of pyridine or pyridazine with a trans double bond in the ring: this trans double bond destroys the aromatic stabilization of the π electrons due to poor orbital overlap between the cis and trans π bonds. Due to the C_s molecular point group, there are two separate allowed and forbidden pathways for 3-aza-benzvalene. The separation of the barrier heights was of particular interest: the difference in activation barriers for the lowest allowed and lowest forbidden pathways in 3-aza-benzvalene was only 1.3 kcal mol⁻¹, and the lowest forbidden pathway actually had a 1.5 kcal mol⁻¹ lower barrier than the highest allowed one. The 3-aza-benzvalene structure allows energy crossing of the allowed and forbidden barriers. For 3,4-diaza-benzvalene, there was only a single allowed and single forbidden pathway, due to the C_2v point group, and they were separated by 8.4 kcal mol⁻¹, more in line with the orbital symmetry rules.