Photodimerization of norbornenes and cyclohexenes catalyzed by Cu(<scp>i</scp>) complexes: mechanistic similarities and differences

The Cu(I)-photocatalyzed cycloaddition reaction has become an important way to construct four-membered ring products. A recent experiment reported the photocycloaddition of norbornenes (nb) and cyclohexenes (ch) catalyzed by CuOTf. However, the atomic level details remain lacking. Herein, using high-level electronic structure methods, the initial photophysics and subsequent dimerization reaction of two Cu complexes ([Cu(nb)₂]⁺ and [Cu(ch)₂]⁺) were explored. For both species, the ¹MLCT state is initially populated and there are two triplet states (³MLCT and ³LC). Notably, in [Cu(ch)₂]⁺, an additional ¹LC state appears near ¹MLCT rendering ¹MLCT short-lived and difficult to detect experimentally as a result of an ultrafast internal conversion to ¹LC. Through direct and ³MLCT-mediated intersystem crossing, the lowest ³LC state is populated. The most striking difference of the ³LC structures of [Cu(ch)₂]⁺ and [Cu(nb)₂]⁺ is that one cyclohexene of the former undergoes a half-chair-to-chair conformational change while the latter does not. From the precursor ³LC, two successive but nonadiabatic reactions form two C-C bonds in the ³LC and S₀ states, respectively. The current work provides the first efforts to understand the atomistic details of the Cu-catalyzed dimerization of norbornenes and cyclohexenes and paves a way for rationally designing superior photocatalysts for olefins dimerization.