Post-spin crossing dynamics determine the regioselectivity in open-shell singlet biradical recombination

Radical recombination is among the fastest reactions in organic chemistry. Achieving high levels of selectivity in this type of reaction is rather challenging. In a recent report on visible-light-induced dearomative cycloaddition of pyrrole-tethered vinylcyclopropanes, extraordinary regioselectivity was observed during the intramolecular open-shell singlet biradical recombination step. In order to address the origin of this unusual selectivity, comprehensive computational studies have been performed, which suggests that it is not the result of any thermodynamic or kinetic factors but dominated by novel dynamic effects that are operational after the spin crossing from triplet to open-shell singlet state due to the unique shape of multi-spin-state potential energy surfaces. The results presented in this study provide new insights into how the dynamic effects can work in a multi-spin-state radical process and advance the understanding of a fundamental organic reaction.