An investigation of molecular mechanism and the role of Te-bridged-atom in the formation of polysubstituted pyridines via Hetero-Diels–Alder reaction of isotellurazole with acetylenic dienophile: a molecular electron density study

An electron localization function (ELF) analysis and a detailed computational study of the hetero-Diels-Alder (HDA) reaction between isotellurazole and acetylenic dienophile have been performed. Then, B3LYP/Lanl2dz + 6-31+g(d) basis set level has been used to characterize the molecular mechanism and reactivity. The conceptual and computational DFT show that the most favorable regioisomeric product is the ortho/endo adduct, for which the energetic cost is 39.4 kcal/mol. Furthermore, ELF topological analysis envisages that the electron density of the pseudo-radical centers of the most electrophilic and nucleophilic atoms of the molecules come mainly from the charge transfer which takes place along the reaction pathway. The high asynchronicity of the bond formation means that two-stage one-step is the most appropriate mechanism for this reaction. However, the weak electronic contribution of the heavy chalcogen-bridged Tellurium atom in the C1-Te-N2 sequence has increased the activation barrier of the reaction. Otherwise, it has assisted the easy removal of the atom in the intermediary cycloadduct in the end of the reaction leading to the polysubstituted pyridine. So, despite the fact that Te atom is electronically rich, it plays a marginal role for improving reactivity of this [4+2] cycloaddition process. Graphical Abstract: Synopsis. The mechanism of the Hetero-Diels-Alder (HDA) reaction of isotellurazole with acetylenic dienophile has been studied carefully. The quantum chemical calculations and an electron localization function analysis show that the weak electronic contribution of the heavy chalcogen-bridged Tellurium atom in the C1-Te-N2 sequence has decreased the activation barrier of the reaction. Besides, it has assisted the easy removal of this atom in the intermediary cycloadduct at the end of the reaction to make possible the synthesis of pyridine ring system (high polarized bonds). Equally, the electronic density analysis of several formed radicals permits us to conclude that two-stage one-step is the mechanism of the HDA reaction