Photodimerization of norbornenes and cyclohexenes catalyzed by Cu(I) 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)] and [Cu(ch)]) were explored. For both species, the MLCT state is initially populated and there are two triplet states (MLCT and LC). Notably, in [Cu(ch)], an additional LC state appears near MLCT rendering MLCT short-lived and difficult to detect experimentally as a result of an ultrafast internal conversion to LC. Through direct and MLCT-mediated intersystem crossing, the lowest LC state is populated. The most striking difference of the LC structures of [Cu(ch)] and [Cu(nb)] is that one cyclohexene of the former undergoes a half-chair-to-chair conformational change while the latter does not. From the precursor LC, two successive but nonadiabatic reactions form two C-C bonds in the LC and S 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.