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.






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Figure 1


Ruthenium polypyridyl complexes: versatile visible light photocatalysts.

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Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

Ionic liquids are salts that are liquid at low temperature (<100 degrees C) which represent a new class of solvents with nonmolecular, ionic character. Even though the first representative has been known since 1914, ionic liquids have only been investigated as solvents for transition metal catalysis in the past ten years. Publications to date show that replacing an organic solvent by an ionic liquid can lead to remarkable improvements in well-known processes. Ionic liquids form biphasic systems with many organic product mixtures. This gives rise to the possibility of a multiphase reaction procedure with easy isolation and recovery of homogeneous catalysts. In addition, ionic liquids have practically no vapor pressure which facilitates product separation by distillation. There are also indications that switching from a normal organic solvent to an ionic liquid can lead to novel and unusual chemical reactivity. This opens up a wide field for future investigations into this new class of solvents in catalytic applications.

Ionic Liquids-New "Solutions" for Transition Metal Catalysis.

In this review, we highlight the use of organic photoredox catalysts in a myriad of synthetic transformations with a range of applications. This overview is arranged by catalyst class where the photophysics and electrochemical characteristics of each is discussed to underscore the differences and advantages to each type of single electron redox agent. We highlight both net reductive and oxidative as well as redox neutral transformations that can be accomplished using purely organic photoredox-active catalysts. An overview of the basic photophysics and electron transfer theory is presented in order to provide a comprehensive guide for employing this class of catalysts in photoredox manifolds.

Organic Photoredox Catalysis

Ionic liquids as electrolytes

Selected patented cross-coupling reaction technologies.

Stereochemical control of microbial reduction. 17. A method for controlling the enantioselectivity of reductions with bakers' yeast

L'oxoetanoate ethyl-2 est reduit par 3 methodes: la levure Saccharomyces cerevisiae libre en milieu aqueux, la levure immobilisee dans l'eau, la levure immobilisee dans l'hexane. La stereochimie de la reduction des alpha ceto esters par Saccharomyces cerevisiae est controlee par des conditions de reaction specifiques

Stereochemical control on yeast reduction of .alpha.-keto esters. Reduction by immobilized bakers' yeast in hexane

The displacement reaction of various non-activated aryl halides with cyanide ions was investigated in the presence of palladium salts. Various aryl iodides and bromides were converted into the corresponding aryl cyanides in good yields under mild conditions. The addition of certain substances, such as potassium hydroxide and potassium carbonate, enhanced the catalytic activity. Hexamethylphosphoric triamide, in which potassium cyanide was scarcely soluble, was an outstanding solvent for the reaction. The reduced palladium species was supposed to be the active catalyst.

Atsuyoshi Ohno

Papers

Stereochemical control of microbial reduction. 17. A method for controlling the enantioselectivity of reductions with bakers' yeast

Stereochemical control on yeast reduction of .alpha.-keto esters. Reduction by immobilized bakers' yeast in hexane

Nucleophilic Displacement Catalyzed by Transition Metal. I. General Consideration of the Cyanation of Aryl Halides Catalyzed by Palladium(II)

Effect of solvent structure on enantioselectivity of lipase-catalyzed transesterification

Stereochemical control in yeast reduction.