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)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.