Substrate-Rhodium cooperativity in photoinduced ortho-alkynylation of arenes

In the realm of metallaphotocatalytic C-H activation strategy, the mode of reaction which has been mostly explored is the synergistic effect between a photocatalyst (PC) and a transition metal. In these cases, the energy and redox transfer from the PC to transition metal modulates the oxidation state which brings new mechanistic paradigms in C-H activation and enables prior elusive transformations under milder conditions. Another mode of reactivity occurs via the direct excitation of the transition metal which plays the dual role of light energy harnessing alongside performing the bond breaking and forming. This mode is advantageous because it would not require any exogenous PC, however such reactivity by transition metals is rare in literature. In this context we have developed the first photo-induced Rh-catalyzed ortho-alkynylation under ambient conditions without the requirement of silver salt, PC or any engineered substrate or catalyst. The transformation functions by the specific cooperative effect of a six-membered rhodacycle which is the photo-responsive species. The catalytic system allows the conjugation of arenes with sp3-rich pharmacophoric fragments. The control experiments as well as the computational studies resolve the mechanistic intricacies for this transformation. An outer sphere electron transfer process from Rh to alkynyl radical is operative for the present photo-induced transformation over the more common oxidative addition or 1,2-migratory insertion pathways.