[2+2] Cycloaddition

Abstract

Any process involving the breakage of two π bonds and the formation of two s bonds to form a four‐membered ring structure in general belong to [2+2] cycloaddition. There are various chemical reactions based on [2+2] cycloaddition. The cycloaddition of olefins under photochemical conditions is known as photochemical [2+2] cycloaddition and the reversion of [2+2] cycloaddition under UV irradiation referred to as [2+2] photocycloreversion. The molecules with olefinic structure are found to undergo photochemical [2+2] cycloaddition even in solid states. It has been found that in singlet [2+2] photocycloaddition, the regioselectivity and stereoselectivity are determined by the pericyclic minimum and exciple (or excimer) minimum, and the energy barrier between these two minima. The allenes and ketenes have also been found to undergo [2+2] cycloadditions. Furthermore the study finds that for cyclic olefins, as the size of carbocyclic ring decreases, double bonds are more bent and twisted, therefore, the cyclic olefins undergo the [2+2] cycloaddition thermally.