Triplet state reactivity of iminium ions in organocatalytic asymmetric [2 + 2] photocycloadditions

Enantioselective iminium-ion catalysis has demonstrated significant synthetic potential for constructing chiral molecules. The photochemistry of iminium ions has experienced a recent renaissance. While singlet-state (S₁) reactivity has been widely used for diverse synthetic applications, triplet-state (T₁) reactivity remains largely reliant on external sensitizers. In this study, we explored the ability of chiral iminium ions embedded within an indole core to directly access the elusive T₁ reactivity, enabling organocatalytic enantioselective [2 + 2] photocycloadditions. Optical and transient spectroscopic techniques, complemented by DFT and TD-DFT calculations, revealed the pivotal role of the T₁ state intermediate. The broad applicability of our findings is demonstrated by the synthesis of various chiral cyclobutane-containing polycyclic scaffolds with high yields and excellent optical purity.