Theoretical Studies on the Mechanisms of [2+2] Cycloaddition Reactions

[2 + 2] cycloaddition reaction is one type of the most important reactions in the field of organic chemistry, and the mechanisms' of [2 + 2] cycloaddition reactions are always hot topic both for experimental and theoretical researchers. In this paper, three types of [2 + 2] cycloaddition reactions, including that of simple olefins or alkynes, cumulenes, Th compounds, have been classified and reviewed. The obtained results indicate that the cycloaddition reactions involving simple olefins or alkynes are proceeding in diradical mechanism generally, and the others are proceeding in either concerted or zwitterion mechanism. All of which have been elucidated with frontier molecular orbital interactions.