Visible light photoredox catalysis with transition metal complexes: applications in organic synthesis.

A fundamental aim in the field of catalysis is the development of new modes of small molecule activation. One approach toward the catalytic activation of organic molecules that has received much attention recently is visible light photoredox catalysis. In a general sense, this approach relies on the ability of metal complexes and organic dyes to engage in single-electron-transfer (SET) processes with organic substrates upon photoexcitation with visible light. Many of the most commonly employed visible light photocatalysts are polypyridyl complexes of ruthenium and iridium, and are typified by the complex tris(2,2′-bipyridine) ruthenium(II), or Ru(bpy)32+ (Figure 1). These complexes absorb light in the visible region of the electromagnetic spectrum to give stable, long-lived photoexcited states.1,2 The lifetime of the excited species is sufficiently long (1100 ns for Ru(bpy)32+) that it may engage in bimolecular electron-transfer reactions in competition with deactivation pathways.3 Although these species are poor single-electron oxidants and reductants in the ground state, excitation of an electron affords excited states that are very potent single-electron-transfer reagents. Importantly, the conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes. Figure 1 Ruthenium polypyridyl complexes: versatile visible light photocatalysts. The ability of Ru(bpy)32+ and related complexes to function as visible light photocatalysts has been recognized and extensively investigated for applications in inorganic and materials chemistry. In particular, photoredox catalysts have been utilized to accomplish the splitting of water into hydrogen and oxygen4 and the reduction of carbon dioxide to methane.5 Ru(bpy)32+ and its analogues have been used (i) as components of dye-sensitized solar cells6 and organic light-emitting diodes,7 (ii) to initiate polymerization reactions,8 and (iii) in photo-dynamic therapy.9 Until recently, however, these complexes had been only sporadically employed as photocatalysts in the area of organic synthesis. The limited exploration of this area is perhaps surprising, as single-electron, radical processes have long been employed in C–C bond construction and often provide access to reactivity that is complementary to that of closed-shell, two-electron pathways.10 In 2008, concurrent reports from the Yoon group and our own lab detailed the use of Ru(bpy)32+ as a visible light photoredox catalyst to perform a [2 + 2] cycloaddition11 and an α-alkylation of aldehydes,12 respectively. Shortly thereafter, Stephenson and co-workers disclosed a photoredox reductive dehalogenation of activated alkyl halides mediated by the same catalyst.13 The combined efforts of these three research groups have helped to initiate a renewed interest in this field, prompting a diversity of studies into the utility of photoredox catalysis as a conceptually novel approach to synthetic organic reaction development. Much of the promise of visible light photoredox catalysis hinges on its ability to achieve unique, if not exotic bond constructions that are not possible using established protocols. For instance, photoredox catalysis may be employed to perform overall redox neutral reactions. As both oxidants and reductants may be transiently generated in the same reaction vessel, photoredox approaches may be used to develop reactions requiring both the donation and the reception of electrons at disparate points in the reaction mechanism. This approach stands in contrast to methods requiring stoichiometric chemical oxidants and reductants, which are often incompatible with each other, as well as to electrochemical approaches, which are not amenable to redox neutral transformations. Furthermore, single-electron-transfer events often provide access to radical ion intermediates having reactivity patterns fundamentally different from those of their ground electronic or excited states.14 Access to these intermediates using other means of activation is often challenging or requires conditions under which their unique reactivity cannot be productively harnessed. At the same time, photoredox catalysts such as Ru(bpy)32+ may also be employed to generate radicals for use in a diverse range of established radical chemistries. Photoredox reactions occur under extremely mild conditions, with most reactions proceeding at room temperature without the need for highly reactive radical initiators. The irradiation source is typically a commercial household light bulb, a significant advantage over the specialized equipment required for processes employing high-energy ultraviolet (UV) light. Additionally, because organic molecules generally do not absorb visible light, there is little potential for deleterious side reactions that might arise from photoexcitation of the substrate itself. Finally, photoredox catalysts may be employed at very low loadings, with 1 mole % or less being typical. This Review will highlight the early work on the use of transition metal complexes as photoredox catalysts to promote reactions of organic compounds (prior to 2008), as well as cover the surge of work that has appeared since 2008. We have for the most part grouped reactions according to whether the organic substrate undergoes reduction, oxidation, or a redox neutral reaction and throughout have sought to highlight the variety of reactive intermediates that may be accessed via this general reaction manifold.15 Studies on the use of transition metal complexes as visible light photocatalysts for organic synthesis have benefited tremendously from advances in the related fields of organic and semiconductor photocatalysis. Many organic molecules may function as visible light photocatalysts; analogous to metal complexes such as Ru(bpy)32+, organic dyes such as eosin Y, 9,10-dicyanoanthracene, and triphenylpyrylium salts absorb light in the visible region to give excited states capable of single-electron transfer. These catalysts have been employed to achieve a vast range of bond-forming reactions of broad utility in organic synthesis.16 Visible light photocatalysis has also been carried out with heterogeneous semiconductors such as mesoporous carbon nitride17 and various metal oxides and sulfides.18 These approaches are often complementary to photoredox catalysis with transition metal-polypyridyl complexes, and we have referred to work in these areas when it is similar to the chemistry under discussion. However, an in-depth discussion of the extensive literature in these fields is outside the scope of this Review, and readers are directed to existing reviews on these topics.16–18