Site‐Selective Dearomative [2+2] Photocycloaddition of Naphthalene Derivatives With Alkenes

Photocycloaddition between aromatics and alkenes is a fundamental class of reactions in organic chemistry. Conventional approaches have predominantly harnessed triplet‐state manifolds to achieve [4+2] cycloadditions. However, reversing the intrinsic site‐selectivity from [4+2] to [2+2] cycloaddition under triplet‐state conditions remains a major challenge. In this study, we report that the excited‐state behavior of naphthalene derivatives can be profoundly modulated through coordination to a chiral Lewis acid catalyst, enabling catalyst‐controlled site‐selective [2+2] photocycloadditions. The catalytic system, assembled from readily available Tb(OTf) 3 and a chiral pyridine‐2,6‐bis(oxazoline) (PyBox) ligand, accommodates a diverse array of electronically and sterically differentiated naphthalenes and alkenes, delivering cyclobutanes in 47%–97% yields with up to >20:1 site‐selectivity (ss), >20:1 diastereomeric ratio (dr), >99% enantiomeric excess (ee). Mechanistic investigations employing femtosecond‐resolved transient absorption spectroscopy reveal that ligand coordination extends the triplet excited‐state lifetime of the aromatic while enabling selective excitation of the aromatic precursors over the alkene adducts. Kinetic studies elucidate the cycloaddition manifold and demonstrate that the [2+2] cycloadduct undergoes rearrangement to the [4+2] product primarily via a retro‐[2+2]/[4+2] sequence.