A new mechanistic insight into the molecular mechanisms of the addition reactions of 2-aryl-3-nitro-2H-chromenes to pyrazoles and cyclopentadienes

Abstract

The molecular mechanism of the addition reactions of 2-aryl-3-nitro-2H-chromenes to pyrazoles was evaluated in the framework of molecular electron density theory (MEDT) based on the results of the wb97xd/6-311+G(d,p) (IEFPCM-integral equation formalism polarizable continuum model) quantum chemical calculations. It was found that the title reaction was realized under a controlled local interaction between the electrophilically activated β-carbon atom of the nitrovinyl moiety and the nucleophilically activated nitrogen sp²-atom of the pyrazole. The first reaction stage was the formation of a zwitterionic intermediate, which was further converted to the final product via a [1,4]-proton sigmatropic shift. The reorganization of the electron density was evaluated using the BET (bond evolution theory) technique. The additional calculations in a solvent other than THF for other nitrochromene systems showed that the proposed mechanism can be treated as a general route for a wide range of similar transformations. The obtained results were compared with an analogous study performed for the addition of 2-aryl-3-nitro-2H-chromenes to pyrazoles according to the Diels–Alder scheme and for the reaction between the same nitrochromene and cyclopentadienes.