Light can be considered an ideal reagent for environmentally friendly, 'green' chemical synthesis; unlike many conventional reagents, light is non-toxic, generates no waste, and can be obtained from renewable sources. Nevertheless, the need for high-energy ultraviolet radiation in most organic photochemical processes has limited both the practicality and environmental benefits of photochemical synthesis on industrially relevant scales. This perspective describes recent approaches to the use of metal polypyridyl photocatalysts in synthetic organic transformations. Given the remarkable photophysical properties of these complexes, these new transformations, which use Ru(bpy)(3)(2+) and related photocatalysts, can be conducted using almost any source of visible light, including both store-bought fluorescent light bulbs and ambient sunlight. Transition metal photocatalysis thus represents a promising strategy towards the development of practical, scalable industrial processes with great environmental benefits.

We report that Ru(bipy)3Cl2 can serve as a visible light photocatalyst for [2+2] enone cycloadditions. A variety of aryl enones participate readily in the reaction, and the diastereoselectivity in the formation of the cyclobutane products is excellent. We propose a mechanism in which a photogenerated Ru(bipy)3+ complex promotes one-electron reduction of the enone substrate, which undergoes subsequent radical anion cycloaddition. The efficiency of this process is extremely high, which allows rapid, high-yielding [2+2] cyclizations to be conducted using incident sunlight as the only source of irradiation.

Ruthenium(II) polypyridyl complexes promote the efficient radical cation Diels–Alder cycloaddition of electron-rich dienophiles upon irradiation with visible light. These reactions enable facile [4+2] cycloadditions that would be electronically mismatched under thermal conditions. Key to the success of this methodology is the availability of ligand-modified ruthenium complexes that enable the rational tuning of electrochemical properties of the catalyst without significantly perturbing the overall photophysical properties of the system.

Photochemical reactions are remarkable for their ability to easily assemble cyclobutanes and other strained ring systems that are difficult to construct using other conventional synthetic methods. We have previously shown that Ru(bpy) 3 2+ is an efficient photocatalyst that promotes the [2+2] cycloadditions of electron-deficient olefins with visible light. Here, we show that Ru(bpy) 3 2+ is also an effective photocatalyst for the [2+2] cycloaddition of electron-rich olefins. This transformation is enabled by the versatile photoelectrochemical properties of Ru(bpy) 3 2+ , which enables either one-electron reduction or one-electron oxidation of interesting organic substrates under appropriate conditions. Cyclobutanes are prominent structural features of many bioactive natural products.1 Arguably the most straightforward method for the preparation of cyclobutane rings is the [2+2] photocycloaddition of olefins, and the utility of this prototypical photochemical reaction has been demonstrated in numerous synthetic applications.2 Nevertheless, the requirement for irradiation with high energy UV light is a disadvantage of this reaction in terms of the cost, scalability, and environmental impact of the methodology.3 We recently reported a new approach to [2+2] enone cycloadditions catalyzed by Ru(bpy) 3 Cl 2 upon irradiation with low-intensity visible light.4 Several other groups5 have also recently become interested in similar strategies for utilizing the well-studied photoredox properties of metal polypyridyl complexes6 in various synthetically useful transformations.Notably, each of the methods recently developed by us,4 MacMillan,5a,b Stephenson,5c-e and Akita5f has taken advantage of a reductive quenching photoredox cycle (Figure 1, Path A). In our method for [2+2] cycloaddition of enones, for example, the photoexcited state (Ru*(bpy) 3 2+ ), generated upon visible light irradiation of the photocatalyst, abstracts an electron from a relatively electron-rich tertiary amine base (i-Pr 2 NEt); the resulting Ru(bpy) 3 + complex is a strong reductant that reduces an aryl enone to the key radical anion intermediate involved in the [2+2] cycloaddition. This mechanism implies that a fundamental limitation of our strategy is the requirement for an alkene that is sufficiently electron-deficient to undergo efficient one-electron reduction by Ru(bpy) 3 + ; indeed, electron-rich olefins (e.g., styrenes) do not react under the conditions we previously reported.We therefore became interested in designing a complementary method for photooxidative electron transfer catalysis that could engage electron-rich olefins in productive [2+2] tyoon@chem.wisc.edu. Supporting Information Available: Experimental procedures and spectral data for all new compounds (PDF format) are provided. This information is available free of charge via the Internet at http://pubs.acs.org. has been extensively investigated for solar energy applications,6a and the most well-studied among these systems rely on an oxidative quenching cycle (Figure 1, ...

We report a method for the crossed [2+2] cycloaddition of styrenes using visible light photocatalysis. Few methods for the synthesis of unsymmetrically substituted cyclobutanes by photochemical [2+2] cycloaddition are known. We show that careful tuning of the electrochemical properties of a ruthenium photocatalyst enable the efficient crossed [2+2] cycloaddition of styrenes upon irradiation with visible light. We outline the logic that enables high crossed chemoselectivity, and we also demonstrate that this reaction is remarkably efficient; gram-scale reactions can be conducted with as little as 0.025 mol% of the photocatalyst.