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.