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

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

Asymmetric enamine catalysis.

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

/pdf/introduction-of-fluorine-and-fluorine-containing-functional-1i8kwiad20.pdf

Introduction of Fluorine and Fluorine-Containing Functional Groups

In recent years, photoredox catalysis has come to the forefront in organic chemistry as a powerful strategy for the activation of small molecules. In a general sense, these approaches rely on the ability of metal complexes and organic dyes to convert visible light into chemical energy by engaging in single-electron transfer with organic substrates, thereby generating reactive intermediates. In this Perspective, we highlight the unique ability of photoredox catalysis to expedite the development of completely new reaction mechanisms, with particular emphasis placed on multicatalytic strategies that enable the construction of challenging carbon–carbon and carbon–heteroatom bonds.

http://pubs.acs.org/doi/pdf/10.1021/acs.joc.6b01449

Photoredox Catalysis in Organic Chemistry

The cyclization of nitrogen-centered radicals onto silyl enol ethers is an efficient method for the synthesis of polyhydroxylated alkaloids as the 2-hydroxymethylpyrrolidine core can be readily accessed from a linear precursor. During our studies on the synthesis of polyhydroxylated alkaloid CYB-3, we found that the diastereoselectivity of the cyclization was dependent on a complex combination of sterics and olefin geometry. A more thorough understanding of the factors that lead to high diastereoselectivites would greatly expand the utility of this methodology in complex natural product synthesis. We have found that cyclization diastereoselectivities of substrates with alkyl or aryl substitution were excellent regardless of olefin geometry or substitution pattern. When electronegative substituents were introduced adjacent to the silyl enol ether, only Z-silyl enol ethers provide high diastereoselectivites. Temperature, steric size of the silyl group, and sterics and electronics of the metal hydride affect...

Unique diastereoselectivity trends in aminyl radical cyclizations onto silyl enol ethers.

The photoelectrochemical decomposition of lignin model compounds at a BiVO4 photoanode is demonstrated with simulated sunlight and an applied bias of 2.0 V. These prototypical lignin model compounds are photoelectrochemically converted into the corresponding aryl aldehyde and phenol derivatives in a single step with conversion of up to ≈64 % over 20 h. Control experiments suggest that vanadium sites are electrocatalytically active, which precludes the need for a redox mediator in solution. This feature of the system is corroborated by a layer of V2 O5 deposited on BiVO4 serving to boost the conversion by 10 %. Our methodology capitalizes on the reactive power of sunlight to drive reactions that have only been studied previously by electrochemical or catalytic methods. The use of a BiVO4 photoanode to drive lignin model decomposition therefore provides a new platform to extract valuable aromatic chemical feedstocks using solar energy, electricity and biomass as the only inputs.

Photoelectrochemical Decomposition of Lignin Model Compound on a BiVO4 Photoanode.

The Mitsunobu reaction is a powerful transformation for the one-pot activation and substitution of aliphatic alcohols. Significant efforts have focused on modifying the classic conditions to overcome problems associated with purification from phosphine-based byproducts. Herein, we report a phosphine free method for alcohol activation and substitution that is mediated by sulfuryl fluoride. This new method is effective for a wide range of primary alcohols using phthalimide, di-tert-butyl-iminodicarboxylate, and aromatic thiol nucleophiles in 74 % average yield. Activated carbon nucleophiles and a deactivated phenol were also effective for this reaction in good yields. Secondary alcohols were also successful substrates using aryl thiols, affording the corresponding sulfides in 56 % average yield with enantiomeric ratios up to 99:1. This new protocol has a distinct synthetic advantage over many existing phosphine-based methods as the byproducts are readily separable. This feature was exploited in several examples that did not require chromatography for purification. Furthermore, the mild reaction conditions enabled further in situ derivatization for the one-pot conversion of alcohols to amines or sulfones. This method also provides a boarder nucleophile scope compared to existing phosphine-free methods.

One-Pot Substitution of Aliphatic Alcohols Mediated by Sulfuryl Fluoride.

Cooperative heterobimetallic catalysis was used as a design principle to achieve a highly reactive system for the enantioselective conjugate addition of cyanide to α,β-unsaturated imides. A dual-catalyst pathway involving chiral (salen)Al complex 1b and chiral (pybox)Er complex 4b provides measurable improvements in rates and enantioselectivities relative to single-catalyst systems. Mechanistic studies point to a cooperative bimetallic mechanism involving activation of the imide by the Al complex and activation of cyanide by the Er complex.

Cooperative Dual Catalysis: Application to the Highly Enantioselective Conjugate Cyanation of Unsaturated Imides.

The potential-controlled incorporation of DOPC liposomes (100 nm diameter) into an adsorbed octadecanol layer on Au(111) was studied using electrochemical and in situ fluorescence microscopy. The adsorbed layer of octadecanol included a small amount of a lipophilic fluorophore—octadecanol modified with BODIPY—to enable fluorescence imaging. The deposited octadecanol layer was found not to allow liposomes to interact unless the potential was less than −0.4 V/SCE, which introduces defects into the adsorbed layer. Small increases in the capacitance of the adsorbed layer were measured after introducing the defects, allowing the liposomes to interact with the defects and then annealing the defects at 0 V/SCE. A change in the adsorbed layer was also signified by a more positive desorption potential for the liposome-modified adsorbed layer as compared to that for an adsorbed layer that was porated in a similar fashion but without liposomes present in the electrolyte. These subtle changes in capacitance are diffi...

Potential-Dependent Interaction of DOPC Liposomes with an Octadecanol-Covered Au(111) Surface Investigated Using Electrochemical Methods Coupled with in Situ Fluorescence Microscopy

Glenn M. Sammis

Papers

Unique diastereoselectivity trends in aminyl radical cyclizations onto silyl enol ethers.

Photoelectrochemical Decomposition of Lignin Model Compound on a BiVO4 Photoanode.

One-Pot Substitution of Aliphatic Alcohols Mediated by Sulfuryl Fluoride.

Cooperative Dual Catalysis: Application to the Highly Enantioselective Conjugate Cyanation of Unsaturated Imides.

Potential-Dependent Interaction of DOPC Liposomes with an Octadecanol-Covered Au(111) Surface Investigated Using Electrochemical Methods Coupled with in Situ Fluorescence Microscopy