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

Organic transformations mediated by the transient formation of iminium ions have shown remarkable synthetic potential for the construction of enantioenriched molecules. The possibility to access their first singlet excited state (S1) under light irradiation has led to the development of previously inaccessible transformations. However, the triplet state (T1) reactivity remains limited and typically requires external photosensitizers. Here we show that structurally modified chiral iminium ions, integrated into extended π-systems, directly engage in T1 reactivity. This modified conjugated architecture was designed to overcome the intrinsic photophysical limitations of conventional iminium ion chemistry, enabling access to previously inaccessible excited-state reaction manifolds. The resulting system allows organocatalytic enantioselective [2 + 2] photocycloadditions without the need for external sensitizers. Mechanistic studies, involving spectroscopic techniques and computational methods, elucidate the role of the T1 intermediate as the key reactive intermediate. Harnessing the triplet state reactivity of iminium ions has remained a long-standing challenge in photocatalysis. Now it has been shown that structural modification allows direct access to this manifold, overcoming the need for external photosensitizers. This strategy allows the development of organocatalytic enantioselective [2 + 2] photocycloadditions to generate complex cyclobutanes with high stereocontrol.