Diradical Character of Cyclic Allenes Enables Their Thermal (2 + 2) Cycloadditions.

(2 + 2) Cycloadditions are a versatile transformation often employed in the synthesis of cyclobutanes. Such cycloadditions usually require the use of light or another synthetic workaround, given that thermal (2 + 2) cycloadditions are orbital symmetry-forbidden for unstrained all-suprafacial transition states. A unique type of (2 + 2) cycloaddition involves transient strained cyclic allenes, which proceed readily at ambient temperature without external stimuli. In the present study, computations and experiments show that the feasibility of these cycloadditions is tied to the inherent diradical character of the cyclic allenes. Moreover, comparative stereochemical transfer experiments for Diels-Alder and (2 + 2) cycloadditions are consistent with strained cyclic allenes having both closed- and open-shell character, thus rendering them diradicaloids. This study is expected to prompt the further strategic use of diradicaloids for the assembly of complex scaffolds.