The mechanistic aspects of the hetero Diels–Alder reactions between strongly electrophilic (E)-2-aryl-1-cyano-1-nitroethenes and ethyl vinyl ether have been analyzed on the basis of kinetic experimental results as well as quantum chemical simulations of reaction paths. These reactions may theoretically proceed via one step or stepwise, zwitterionic mechanism. It was found that the conversion of addends into adducts carried out via polar, but not zwitterionic mechanism. This is confirmed by kinetic substituent and solvent effects. Subsequently, a detailed analysis of internal reaction coordinate trajectories shows that the title reactions should be considered as examples of “two-stage one-step” cycloadditions according to Domingo terminology.

Reactions between (E)-2-aryl-1-cyano-1-nitroethenes and diazafluorene lead to acyclic 2,3-diazabuta-1,3-diene derivatives, instead of the expected pyrazoli

The present paper is a continuation of comprehensive study regarding to synthesis and properties of pyrazoles and their derivatives. In its framework an experimental and theoretical studies of thermal decomposition of the 3,3-diphenyl-4-(trichloromethyl)-5-nitropyrazoline were performed. It was found, that the decompositions of the mentioned pyrazoline system in the solution and at the melted state proceed via completely different molecular mechanisms. These mechanisms have been explained in the framework of the Molecular Electron Density Theory (MEDT) with the computational level of B3LYP/6-31G(d). A Bonding Evolution Theory (BET) examination of dehydrochlorination of the 3,3-diphenyl-4-(trichloromethyl)-5-nitropyrazoline permits elucidation of the molecular mechanism. It was found, that on the contrary for most known HCl extrusion processes in solution, this reaction is realised via single-step mechanism.