Solar Synthesis: Prospects in Visible Light Photocatalysis

Background Interest in photochemical synthesis has been motivated in part by the realization that sunlight is effectively an inexhaustible energy source.Chemists have also long recognized distinctive patterns of reactivity that are uniquely accessible via photochemical activation. However, most simple organic molecules absorb only ultraviolet (UV) light and cannot be activated by the visible wavelengths that comprise most of the solar energy that reaches Earth’s surface. Consequently, organic photochemistry has generally required the use of UV light sources. Visible light photocatalysis. (A) Transition metal photocatalysts, such as Ru(bpy)32+, readily absorb visible light to access reactive excited states. (B) Photoexcited Ru*(bpy)32+ can act as an electron shuttle, interacting with sacrificial electron donors D (path i) or acceptors A (path ii) to yield either a strongly reducing or oxidizing catalyst toward organic substrates S. Ru*(bpy)32+ can also directly transfer energy to an organic substrate to yield electronically excited species (path iii). bpy, 2,2'-bipyridine; MLCT, metal-to-ligand charge transfer. Advances Over the past several years, there has been a resurgence of interest in synthetic photochemistry, based on the recognition that the transition metal chromophores that have been so productively exploited in the design of technologies for solar energy conversion can also convert visible light energy into useful chemical potential for synthetic purposes. Visible light enables productive photoreactions of compounds possessing weak bonds that are sensitive toward UV photodegradation. Furthermore, visible light photoreactions can be conducted by using essentially any source of white light, including sunlight, which obviates the need for specialized UV photoreactors. This feature has expanded the accessibility of photochemical reactions to a broader range of synthetic organic chemists. A variety of reaction types have now been shown to be amenable to visible light photocatalysis via photoinduced electron transfer to or from the transition metal chromophore, as well as energy-transfer processes. The predictable reactivity of the intermediates generated and the tolerance of the reaction conditions to a wide range of functional groups have enabled the application of these reactions to the synthesis of increasingly complex target molecules. Outlook This general strategy for the use of visible light in organic synthesis is already being adopted by a growing community of synthetic chemists. Much of the current research in this emerging area is geared toward the discovery of photochemical solutions for increasingly ambitious synthetic goals. Visible light photocatalysis is also attracting the attention of researchers in chemical biology, materials science, and drug discovery, who recognize that these reactions offer opportunities for innovation in areas beyond traditional organic synthesis. The long-term goals of this emerging area are to continue to improve efficiency and synthetic utility and to realize the long-standing goal of performing chemical synthesis using the sun. Synthesis in the Spotlight Most organic molecules absorb little or no visible light. Consequently, conventional organic photochemistry has relied on excitation in the ultraviolet regime, with the drawback that the high energy involved can lead to undesirable by-products. Over the past several years, an alternative strategy has emerged involving the visible excitation of metal complexes (primarily ruthenium and iridium) that can then engage in electron or energy transfer with organic compounds. The ensuing reactivity patterns complement thermally accessible outcomes without introducing detrimental quantities of excess energy. Schultz and Yoon (10.1126/science.1239176) review developments in this rapidly advancing area of photoredox catalysis. Chemists have long aspired to synthesize molecules the way that plants do—using sunlight to facilitate the construction of complex molecular architectures. Nevertheless, the use of visible light in photochemical synthesis is fundamentally challenging because organic molecules tend not to interact with the wavelengths of visible light that are most strongly emitted in the solar spectrum. Recent research has begun to leverage the ability of visible light–absorbing transition metal complexes to catalyze a broad range of synthetically valuable reactions. In this review, we highlight how an understanding of the mechanisms of photocatalytic activation available to these transition metal complexes, and of the general reactivity patterns of the intermediates accessible via visible light photocatalysis, has accelerated the development of this diverse suite of reactions.