Intermolecular 2 + 2 Carbonyl-Olefin Photocycloadditions Enabled by Cu(I)-Norbornene MLCT.

Photocycloadditions are often typified by the oxetane-forming Paternò-Büchi reaction. However, the mechanistic constraints of carbonyl excitation and olefin interception have limited this attractive oxetane-forming pathway. Here we describe the use of a Cu(I) precatalyst that achieves selective olefin activation via coordination to the metal center. Significantly, this intermolecular 2 + 2 carbonyl-olefin photocycloaddition engages alkyl ketones, which are more challenging to accommodate via direct irradiation pathways. Mechanistic investigations support the in situ formation of a Cu-norbornene resting state that undergoes a MLCT leading to oxetane formation.