The photoredox activation of organic substrates with visible light is a powerful methodology that generates reactive radical species under very mild conditions. When combined with another catalytic process in a dual catalytic system, novel, visible-light-promoted transformations have been realized that do not proceed using either catalyst in isolation. In this minireview, the state of the art in organic reactions mediated by dual catalytic systems merging photoredox activation with organo-, acid or metal catalysis is discussed.

A room-temperature intramolecular oxy- and aminoarylation of alkenes with aryldiazonium salts has been developed using a novel gold and photoredox dual-catalytic system. The compatibility of these two catalytic modes has been established for the first time and demonstrates the potential of this system as a method to expand the scope of nucleophilic addition reactions to carbon-carbon multiple bonds.

The arylation of alkyl and aromatic terminal alkynes by a dual gold/photoredox catalytic system is described.

Nitrogen modified cobalt nanoparticles are easily prepared from melamine or melamine resins. The resulting catalysts show excellent selectivity for transfer hydrogenation of N-heteroarenes.

An integrated catalytic decarboxylation/carboxylation for accessing isotopically labeled carboxylic acids with 13CO2 or 14CO2 is described. The method shows a wide scope under mild conditions, even in the context of late-stage functionalization, and does not require stoichiometric organometallics, thus complementing existing carbon-labeling techniques en route to carboxylic acids.

Intermolecular three-component oxyarylation reactions of simple alkenes have been developed using a dual gold and photoredox catalytic system. Inexpensive organic dyes could be employed as the photocatalyst using aryldiazonium salts, while the combination of gold and iridium catalysts allowed for diaryliodonium compounds to be employed as the source of the arene coupling partner. In both cases, a-arylated ether products were generated under remarkably mild conditions using readily accessible visible light sources.

A visible-light-mediated photoredox-catalyzed semipinacol-type rearrangement proceeding via 1,2 alkyl migration was developed. In this transformation, trifluoromethylation of the C=C bond of α-(1-hydroxycycloalkyl)-substituted styrene derivatives is followed by ring expansion of the 1-hydroxycycloalkyl group to deliver novel cycloalkanones with all-carbon quaternary centers. The reaction proceeds via a radical-polar mechanism, with trifluoromethylation (radical) and ring expansion (ionic) occurring in the same transformation.

Catalytic hydrogenation using molecular hydrogen represents a green and practical approach for reductions of all kinds of organic chemicals. Traditionally, in the majority of these processes the presence of transition metal catalysts is required. In this regard, noble-metal-based catalysts have largely been implemented, such as the application of iridium, palladium, rhodium, ruthenium, and others. Recently, the employment of earth-abundant 3d metals has emerged to replace the utilization of scarce noble metals because of their availability, lower cost, and often reduced toxicity. In this respect, several cobalt complexes, in the form of either molecularly well-defined or in situ-formed complexes, are receiving increasing attention from the scientific community. Importantly, the stability and reactivity of the complexes have greatly been supported by multidentate ligands under steric and/or electronic influences. For instance, tridentate or tetradentate phosphine ligands indirectly tune the reactivity of the metal center to accelerate the overall process, whereas direct participation of the ligand in pincer-type complexes through ligand-metal cooperation regulates the elementary steps in the catalytic cycle. In this Account, we emphasize specifically the advancements in cobalt-catalyzed hydrogenations using molecular hydrogen accomplished in our group. A variety of substrate classes ranging from simple molecules (e.g., carbon dioxide) to complex compounds were explored under the mild and efficient catalytic conditions. Notable examples include the reduction of carbon dioxide to afford either formates using a Co(BF)·6HO/Tetraphos catalyst system or methanol employing a Co(acac)/Triphos complex in the presence of HNTf. As interesting examples of the synthesis of fine chemicals, cobalt-promoted hydrogenations of nitriles to primary amines and reductive alkylations of indoles using carboxylic acids as alkylating agents are highlighted. Moreover, highly selective hydrogenations of N-heteroarenes under additive-free conditions were possible by the application of specific cobalt complexes. More recently, a set of carboxylic esters could be hydrogenated to the corresponding alcohols with high efficiency by the use of a well-defined cobalt-PNP pincer catalyst. In particular, the decent reactivity of cobalt catalysts enabled high selectivity and functional group tolerance to be achieved. Throughout our studies, it was found that the pairing of a suitable cobalt precursor and an appropriate tridentate or tetradentate phosphine ligand plays a crucial role harnessing the desired reactivity, while other monodentate and bidentate phosphine ligands showed no reactivity in these investigations. Our developments could provide supervisory information for the future exploration of cobalt-catalyzed hydrogenation reactions and other types of reactions involving cobalt catalysis. Furthermore, relevant contributions from other groups, remaining challenges, and future perspectives in this research area are also presented.