Azo-hydrazo conversion via [1,5]-hydrogen shifts. A combined experimental and theoretical study

Azoalkenes 6e, 6g, 6h, and 8c underwent an easy azo-hydrazo conversion via a [1,5]-hydrogen shift yielding α,β-unsaturated hydrazones. The isomerization products were characterized through spectroscopic and spectrometric techniques. In order to understand the nature of the mechanism of these [1,5]-hydrogen shifts, the transition state structures of the reactions were theoretically studied at the B3LYP/6-31G(d,p) level. Substitution effects in the propenylazo system on the kinetic and thermodynamic parameters were analyzed. An electron localization function (ELF) analysis of the electronic structure of the transition state structure associated with the azo-hydrazo conversion of the simplest 1-azopropene 6a indicates that these [1,5]-hydrogen shifts have a two-stage one-step mechanism via pseudodiradical transition states, in which a formal hydrogen atom is transferred. This finding allows us to reject the pericyclic reaction model for these [1,5]-hydrogen shift reactions.