Thermal Isomerization at a C=N Double Bond: How Does the Mechanism Vary with the Substituent?

Abstract

Thermal isomerizations at the C=N double bonds in R₁R₂C=NC₆H₄X proceed via an inversion mechanism, in which the C=NC₆H₄X bond angle is near 180° at the transition state (TS). Available experimental data on the reactions of (CF₃)₂C=NC₆H₄X and YC₆H₄CH=NC₆H₄X have suggested that the reaction mechanism changes with substituent X. In the present study, HF, B3LYP, and MP2 computations were carried out to analyze the modes of mechanistic change with the substituent for the reactions of (CF₃)₂C=NC₆H₄X, YC₆H₄CH=NC₆H₄X, and other systems. There are two possible pathways within the inversion mechanistic framework: a planar pathway with the R₁R₂C=N double-bond plane and the aromatic ring being coplanar at the TS and a perpendicular one with the two planes being perpendicular to each other at the TS. It was found that the mechanistic change is not due to a change in the relative importance of two independent and competitive reaction pathways, but arises from a change of the character of a single TS from planar to perpendicular geometry when the substituent becomes more electron-withdrawing.