Phenazine-Based Donor Acceptor Systems as Organic Photocatalysts for "Metal-free" C-N/C-C Cross-Coupling

With an aim to achieve a balance between ground-state and excited-state reduction potential of donor acceptor systems for efficient C-N/C-C cross-coupling, a series of donor acceptor systems DA1-DA4 have been synthesized by varying the donor strength and connecting positions. With an increase in donor strength, systematic elevation in the ground-state reduction potential and decrease in the HOMO-LUMO gap was observed. Interestingly, all the derivatives DA1-DA4 could catalyze the C-N bond formation reaction between activated aryl halides and amines at low catalytic loading under metal-free conditions without the need of any external base upon irradiation with white LED. A balance was realized in the case of derivative DA2, which exhibits high efficiency in C-N couplings. Different control experiments support the validity of the energy as well as electron transfer pathways in the visible light-mediated C-N bond formation. This study further reveals the potential of derivative DA1 in "metal-free" Sonogashira coupling involving activated aryl halides which is attributed to its high excited-state reduction potential.