Understanding the Fate of the Banert Cascade of Propargylic Azides. Sigmatropic versus Prototropic Pathway

The Banert cascade is an efficient synthetic strategy for obtaining 4,5-disubstituted 1,2,3-triazoles. The reaction can proceed via a sigmatropic or prototropic mechanism depending on the substrate and the conditions. In this work, the mechanism of both pathways from propargylic azides with different electronic features were investigated using DFT, QTAIM and NBO approaches. The calculated energies barriers were consistent with the experimental data. Three patterns of electron density distribution on the transition structures were observed, which reflected the behaviours of the reactants in the Banert cascade. The stronger conjugative effects were associated with lower/higher free activation energies of sigmatropic/prototropic reactions, respectively. A clear relationship between the accumulation of the charge at the C3 atom of propargylic azides with the energy barriers for prototropic reactions was found. Thus, the obtained results would allow the prediction of the reactions course evaluating reactants.