Switching on elusive organometallic mechanisms with photoredox catalysis

Despite advances in carbon–carbon fragment couplings, the ability to forge carbon–oxygen bonds in a general fashion via nickel catalysis has been largely unsuccessful; here, visible-light-excited photoredox catalysts are shown to provide transient access to Ni(iii) species that readily participate in reductive elimination, leading to carbon–oxygen coupling. Despite advances in carbon–carbon fragment couplings, the ability to create carbon–oxygen bonds via nickel catalysis has been largely unsuccessful. Here David MacMillan and colleagues show that visible-light-excited photoredox catalysts can provide transient access to Ni(III) species that readily participate in reductive elimination. Using this synergistic merger of photoredox and nickel catalysis, the authors develop a highly efficient and general carbon–oxygen coupling reaction using alcohols and aryl bromides. Transition-metal-catalysed cross-coupling reactions have become one of the most used carbon–carbon and carbon–heteroatom bond-forming reactions in chemical synthesis. Recently, nickel catalysis has been shown to participate in a wide variety of C−C bond-forming reactions, most notably Negishi, Suzuki–Miyaura, Stille, Kumada and Hiyama couplings1,2. Despite the tremendous advances in C−C fragment couplings, the ability to forge C−O bonds in a general fashion via nickel catalysis has been largely unsuccessful. The challenge for nickel-mediated alcohol couplings has been the mechanistic requirement for the critical C–O bond-forming step (formally known as the reductive elimination step) to occur via a Ni(iii) alkoxide intermediate. Here we demonstrate that visible-light-excited photoredox catalysts can modulate the preferred oxidation states of nickel alkoxides in an operative catalytic cycle, thereby providing transient access to Ni(iii) species that readily participate in reductive elimination. Using this synergistic merger of photoredox and nickel catalysis, we have developed a highly efficient and general carbon–oxygen coupling reaction using abundant alcohols and aryl bromides. More notably, we have developed a general strategy to ‘switch on’ important yet elusive organometallic mechanisms via oxidation state modulations using only weak light and single-electron-transfer catalysts.