Signaling Recognition Events with Fluorescent Sensors and Switches.

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

Graphitic carbon nitride, g-C3N4, can be made by polymerization of cyanamide, dicyandiamide or melamine. Depending on reaction conditions, different materials with different degrees of condensation, properties and reactivities are obtained. The firstly formed polymeric C3N4 structure, melon, with pendant amino groups, is a highly ordered polymer. Further reaction leads to more condensed and less defective C3N4 species, based on tri-s-triazine (C6N7) units as elementary building blocks. High resolution transmission electron microscopy proves the extended two-dimensional character of the condensation motif. Due to the polymerization-type synthesis from a liquid precursor, a variety of material nanostructures such as nanoparticles or mesoporous powders can be accessed. Those nanostructures also allow fine tuning of properties, the ability for intercalation, as well as the possibility to give surface-rich materials for heterogeneous reactions. Due to the special semiconductor properties of carbon nitrides, they show unexpected catalytic activity for a variety of reactions, such as for the activation of benzene, trimerization reactions, and also the activation of carbon dioxide. Model calculations are presented to explain this unusual case of heterogeneous, metal-free catalysis. Carbon nitride can also act as a heterogeneous reactant, and a new family of metal nitride nanostructures can be accessed from the corresponding oxides.

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Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Bronsted acids, organocatalysts, enzymes, and transition metal complexes.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.6b00018

Dual Catalysis Strategies in Photochemical Synthesis

4.2. Solid-Supported Electrolytes 2280 4.3. Solid-Supported Mediators 2282 4.4. Supported Substrate-Product Capture 2283 4.5. A Unique Electrolyte/Solvent System 2285 5. Reaction Conditions 2285 5.1. Supercritical Fluids 2285 5.1.1. Electrochemical Properties of scCO2 2285 5.1.2. Electroreductive Carboxylation in scCO2 2286 5.1.3. Electrochemical Polymerization in scCO2 2286 5.2. The Cation-Pool Method 2286 5.2.1. Generation of N-Acyliminium Ion Pools 2286 5.2.2. Generation of Alkoxycarbenium Ion Pools 2287 5.2.3. Generation of Diarylcarbenium Ion Pools 2288 5.2.4. Generation of Other Cation Pools 2289 6. Electrochemical Devices 2289 6.1. Electrode Materials 2289 6.2. Ultrasound and Centrifugal Fields 2290 6.3. Electrochemical Microflow Systems 2290 7. Combinatorial Electrochemical Synthesis 2292 7.1. Parallel Electrolysis Using a Macrosystem 2292 7.2. Parallel Electrolysis Using a Microsystem 2293 7.3. Serial Electrolysis Using a Microsystem 2294 8. Conclusions 2294 9. Acknowledgments 2294 10. References 2294

Modern Strategies in Electroorganic Synthesis

Diastereomeric (η 4 -N-substituted-2,4-hexadienamide)Fe(CO) 3 complexes having chiral amide groups are separated and characterized. Among the complexes, the amide complexes derived from (3R)- and (3S)-3-amino-hexahydro-2H-azepin-2-one are the most easily separated by fractional crystallization and column chromatography.

Separation and characterization of diastereomeric (η4-diene)Fe(CO)3 complexes containing chiral amide groups

Effects of reaction media on photoinduced electron-transfer reactions are reviewed from synthetic and mechanistic viewpoints. This review deals with the following subjects : (1) The characteristics and the problems of photoinduced electron-transfer reactions in organic photochemistry. (2) The salt effects on organic photoreactions which proceed via exciplexes and radical ion species. In this section, examples are given that the efficiencies of the photoreactions are enhanced and suppressed by the addition of salts into the reaction systems. Examples are also given that the selectivities in the photoreactions are affected by the addition of salts. (3) The effects of other additives such as oxygen and aromatic hydrocarbons on photoreactions. These additives are shown to often act as redox sensitizers or co-sensitizers in electron-transfer mediated photoreactions. (4) The solvent effects on photoreactions. The change of solvents are shown to often affect the product distributions.

Effects of Reaction Media on Photoinduced Electron-Transfer Reactions

The photochemical cis-trans isomerization of cis- and trans-1,2-bis(4-methoxyphenyl)cyclopropanes sensitized by 9,10-dicyanoanthracene in aerated acetonitrile solutions proceeds via a chain process in which a cation radical produced from the cyclopropanes is involved as a chain carrier. The quantum yield for this photoisomerization is remarkably enhanced by the addition of air and phenanthrene.

9,10-dicyanoanthracene-sensitized photoisomerization of 1,2-bis(4-methoxyphenyl)cyclopropanes involving a chain process

The reagent system generated from Cp2TiCl2(Cp=cyclopentadienyl) and LiAlH4 promoted the catalytic hydrometalation and isomerization of olefins, the catalytic ability of the reagent system for both reactions strongly depending on the composition of two reagents in the reagent system. The hydrometalation occurred in the anti-Markownikoff fashion. By these two catalytic reactions, 1-octene was effectively converted to 1-bromooctane and 2,3-dibromooctane.

Hydrometalation and isomerization of olefins by the titanium complex

The reaction of aromatic aldehydes with Fe(CO)5 or Fe3(Co)12 in pyridine gives the corresponding 1,2-diaryl-1,2-ethanediols as major products in good yields. A reactive species of this reaction is octacarbonyl diferrate [Fe2(CO)8]2−.

Yoshio Otsuji

Papers

Separation and characterization of diastereomeric (η4-diene)Fe(CO)3 complexes containing chiral amide groups

Effects of Reaction Media on Photoinduced Electron-Transfer Reactions

9,10-dicyanoanthracene-sensitized photoisomerization of 1,2-bis(4-methoxyphenyl)cyclopropanes involving a chain process

Hydrometalation and isomerization of olefins by the titanium complex

Reductive coupling of aromatic aldehydes by octacarbonyl diferrate.