Competition between [2+1]- and [4+1]-cycloaddition mechanisms in reactions of conjugated nitroalkenes with dichlorocarbene in the light of a DFT computational study

The competition between [2+1] and [4+1] channels regarding reactions of conjugated nitroalkenes with dichlorocarbene was explored based on B3LYP/6-31G(d) calculations. It was found that, in the case of cycloadditions involving parent nitroethene and its 1-substituted analogs, the [2+1] scheme should be treated as possible only from the kinetic process point of view. On the other hand, in similar reactions involving 2-substituted nitroethenes, both channels considered may compete. Additionally, mechanistic aspects of all cycloadditions were analyzed. It was found that the considered [2+1]-cycloadditions proceed via a non-polar mechanism with a biradicaloidal transition state (TS), whereas [4+1]-cycloadditions proceed via a polar mechanism with a zwitterionic TS.