Experimental and Theoretical MEDT Study of the Thermal [3+2] Cycloaddition Reactions of Aryl Azides with Alkyne Derivatives

The reactivity and regioselectivity of the [3+2] cycloaddition (32CA) reactions of a series of 4-substituted-aryl azides/alkynes in toluene have been explored experimentally. These 32CA reactions have been also studied within the Molecular Electron Density Theory (MEDT) using Density Functional Theory (DFT) calculations at the B3LYP/6-31G(d) computational level. The computed activation energies, in gas phase and in toluene, show that the 1,5-regioisomeric channels are slightly favoured kinetically as found experimentally. The topological analysis of the electron localisation function (ELF) of some relevant points of the regioisomeric reaction paths shows that while the formation of the 1,4-diaryl triazoles takes place via a low asynchronous C-N bond formation process, the formation of the favoured 1,5-diaryl triazoles takes place via a two-stage one-step mechanism, as the result of the attack of the most nucleophilic center of the aryl alkyne on the most electrocyclic center of aryl azide.