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The merger of transition metal catalysis and photocatalysis, termed metallaphotocatalysis, has recently emerged as a versatile platform for the development of new, highly enabling synthetic methodologies. Photoredox catalysis provides access to reactive radical species under mild conditions from abundant, native functional groups, and, when combined with transition metal catalysis, this feature allows direct coupling of non-traditional nucleophile partners. In addition, photocatalysis can aid fundamental organometallic steps through modulation of the oxidation state of transition metal complexes or through energy-transfer-mediated excitation of intermediate catalytic species. Metallaphotocatalysis provides access to distinct activation modes, which are complementary to those traditionally used in the field of transition metal catalysis, thereby enabling reaction development through entirely new mechanistic paradigms. This Review discusses key advances in the field of metallaphotocatalysis over the past decade and demonstrates how the unique mechanistic features permit challenging, or previously elusive, transformations to be accomplished. Transition metal catalysis is well established as an enabling tool in synthetic organic chemistry. Photoredox catalysis has recently emerged as a method to effect reactions that occur through single-electron-transfer pathways. Here we review the combination of the two to show how this provides access to highly reactive oxidation states of transition metals and distinct activation modes that further enable the synthetic chemist.

Electrochemical synthesis can provide more sustainable routes to industrial chemicals 1 – 3 . Electrosynthetic oxidations may often be performed ‘reagent-free’, generating hydrogen (H 2 ) derived from the substrate as the sole by-product at the counter electrode. Electrosynthetic reductions, however, require an external source of electrons. Sacrificial metal anodes are commonly used for small-scale applications 4 , but more sustainable options are needed at larger scale. Anodic water oxidation is an especially appealing option 1 , 5 , 6 , but many reductions require anhydrous, air-free reaction conditions. In such cases, H 2 represents an ideal alternative, motivating the growing interest in the electrochemical hydrogen oxidation reaction (HOR) under non-aqueous conditions 7 – 12 . Here we report a mediated H 2 anode that achieves indirect electrochemical oxidation of H 2 by pairing thermal catalytic hydrogenation of an anthraquinone mediator with electrochemical oxidation of the anthrahydroquinone. This quinone-mediated H 2 anode is used to support nickel-catalysed cross-electrophile coupling (XEC), a reaction class gaining widespread adoption in the pharmaceutical industry 13 – 15 . Initial validation of this method in small-scale batch reactions is followed by adaptation to a recirculating flow reactor that enables hectogram-scale synthesis of a pharmaceutical intermediate. The mediated H 2 anode technology disclosed here offers a general strategy to support H 2 -driven electrosynthetic reductions. A quinone-mediated hydrogen anode design shows that hydrogen can be used as the electron source in non-aqueous reductive electrosynthesis, for a more sustainable way to make molecules at larger scale.

Transition metal catalyzed cross-coupling is an incredibly powerful platform for the construction of organic architectures, since the advent of these technologies their impact has been felt throughout the chemical sciences and society more broadly. Despite the advances that have been made in this field over the last half century, limitations with respect to the ability to construct Csp3 rich frameworks, and carbon–heteroatom bonds with particularly electron deficient heteroatom coupling partners remains unsolved challenges. The moieties which would result from the realization of these transformations are of long-standing interest in the pharmaceutical industry, and academic medicinal chemistry discovery programs, due to unique biological properties they may possess.Over the last decade photoredox catalysis has arisen as highly versatile platform for forging carbon–carbon and carbon–heteroatom bonds in a variety of molecular settings. In particular the merger of photoredox catalysis and transition metal catalysis has facilitated the development of a litany of new transformations which have the potential to vastly expedite the synthesis of organic frameworks of both academic and industrial interest, many of which have traditionally been difficult or impossible to access with prior art.The design and development of a new paradigm for selective functionalization of C–H bonds adjacent to alcohols (an incredibly prevalent functional handle) is discussed. This strategy utilized a combination of three catalytic cycles in conjunction with a Lewis acid activation mode to access radical species from simple alcohols, which can then be cross coupled with aryl halides to deliver benzylic alcohol products in a highly modular fashion.In addition, a novel cobalt photocatalyzed coupling of N-aryl amides and boronic acids is discussed. The development of a novel cobalt photoexcitation pathway facilitates the coupling of these partners via an open-shell homolytic substitution pathway. This methodology allows access to highly sterically encumbered diaryl amides in excellent yield, and also highlights the potential utility of base-metal chromophores in photocatalysis.

Lignin-derived aromatic chemicals offer a compelling alternative to petrochemical feedstocks, and new applications are the focus of extensive interest. 4-Hydroxybenzoic acid (H), vanillic acid (G), and syringic acid (S) are readily obtained via oxidative depolymerization of hardwood lignin substrates. Here, we explore the use of these compounds to access biaryl dicarboxylate esters that represent biobased, less toxic alternatives to phthalate plasticizers. Chemical and electrochemical methods are developed for catalytic reductive coupling of sulfonate derivatives of H, G, and S to access all possible homo- and cross-coupling products. A conventional NiCl2/bipyridine catalyst is able to access the H–H and G–G products, but new catalysts are identified to afford the more challenging coupling products, including a NiCl2/bisphosphine catalyst for S–S and a NiCl2/phenanthroline/PdCl2/phosphine cocatalyst system for H–G, H–S, and G–S. High-throughput experimentation methods with a chemical reductant (Zn powder) are shown to provide an efficient screening platform for identification of new catalysts, while electrochemical methods can access improved yields and/or facilitate implementation on larger scale. Plasticizer tests are performed with poly­(vinyl chloride), using esters of the 4,4′-biaryl dicarboxylate products. The H–G and G–G derivatives, in particular, exhibit performance advantages relative to an established petroleum-based phthalate ester plasticizer.