Metallaphotoredox catalysis refers to the combination of a photocatalyst and a transition metal catalyst to promote an organic transformation. The excited state of the photocatalyst reduces or oxidizes an organic substrate through single-electron transfer; this step typically results in the formation of an organic radical that subsequently enters a separate catalytic cycle involving the transition metal catalyst. The photocatalyst is regenerated through a second electron transfer step at some point in the mechanism. Action of the photocatalyst can accelerate transition metal-catalyzed reactions that would be slower otherwise, such as cross-coupling reactions. Common photocatalysts include 2,2’-bipyridine complexes of ruthenium(II) and 2-phenylpyridine complexes of iridium(III). Synthetic modification of the structures of these ligands modulate the redox properties of the photocatalyst, enabling oxidation or reduction of a wide variety of organic substrates. Nickel, palladium, gold, and other metals have been employed in the transition metal catalyst, although nickel complexes are most commonly used.
Photo-excitation of a molecule promotes an electron to a higher energy level, leaving a hole in the energy level where the electron originally resided. Thus, the excited state contains both a high-energy electron and a low-energy hole and it is both a stronger reducing agent and stronger oxidizing agent than the ground state. Photo-excitation can therefore enable the oxidation or reduction of electron-rich or electron-poor organic substrates respectively; the result is typically formation of an organic radical. In metallaphotoredox catalysis, a separate catalytic cycle involving a transition metal catalyst happens concurrently with this photoredox process. The organic radical enters this cycle most commonly via coordination to the metal catalyst, which results in its formal oxidation. Reductive elimination of two organic fragments turns over the transition metal-catalyzed process.
Acceleration through photocatalysis has been observed for a number of different types of transition metal-catalyzed cross-coupling reactions that establish carbon–carbon bonds. Examples include decarboxylative cross-coupling reactions of sp3-hybridized carboxylic acids with organohalides, trifluoromethylation reactions of aryl diazonium salts and arylboronic acids, and aldehyde C(sp2)–H cross-couplings with aryl halides and with C–H arenes bearing directing groups.