The Puzzle of the New Type of Intermediate in the Course of [2 + 2] Cycloaddition with the Participation of Conjugated Nitroalkenes: MEDT Computational Study

The phenomena of regio- and stereoselectivity and the molecular mechanism of the [2 + 2] cycloaddition reaction between (E)-2-arylnitroethenes and the ynamine molecular system were analyzed using wb97xd/6-311 + G(d) (PCM) quantumchemical calculations. It was found that, independently of the stepwise nature of the cycloaddition, the full retention of the stereoconfiguration of the nitroalkene can be interpreted and explained. Next, the analysis of the electronic properties of the localized reaction intermediate suggests its possible zwitterionic nature. Additionally, the solvent and the substituent effect on the reaction course were also evaluated. In consequence, the proposed mechanism can be treated as general for some groups of [2 + 2] cycloaddition processes. Lastly, for the model process, the full Bonding Evolution Theory (BET) analysis along the reaction coordinate was performed. It was found that the [2 + 2] cycloaddition reaction between (E)-2-phenylonitroethene and ynamine begins with the formation of two <i>pseudoradical</i> centers at the C2 and C3 atoms. First, a C2-C3 single bond is formed in phase V by combining two <i>pseudoradical</i> centers, while the formation of a second C4-C1 single bond begins at the last, eleventh phase of the reaction path. A BET analysis of intermediate (<b>I</b>) allows it to be classified as a compound with a <i>pseudoradical</i> structure. Next to zwitterions and biradicals, it is evidently new type of intermediate on the path of the [2 + 2] cycloaddition reaction.