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

Catalytic dehydrogenative cross-coupling: forming carbon-carbon bonds by oxidizing two carbon-hydrogen bonds

The use of visible light sensitization as a means to initiate organic reactions is attractive due to the lack of visible light absorbance by organic compounds, reducing side reactions often associated with photochemical reactions conducted with high energy UV light. This tutorial review provides a historical overview of visible light photoredox catalysis in organic synthesis along with recent examples which underscore its vast potential to initiate organic transformations.

Visible light photoredox catalysis: applications in organic synthesis

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Principles Of Fluorescence Spectroscopy

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[674783-97-2] C23H22ClNO4 (MW 411.88)

InChI = 1S/C23H22N.ClHO4/c1-15-13-16(2)22(17(3)14-15)23-18-9-5-7-11-20(18)24(4)21-12-8-6-10-19(21)23;2-1(3,4)5/h5-14H,1-4H3;(H,2,3,4,5)/q+1;/p-1

InChIKey = LFMBERYWDLWXNO-UHFFFAOYSA-M





()



9-Mesityl-10-ethylacridinium tetrafluoroborate



[C23H22NBF4] (MW 399.24)



(photocatalyst, particularly useful for oxygenation and bromination of aromatic compounds)



Alternate Names:  10-Methyl-9-(2,4,6-trimethylphenyl)acridinium perchlorate; [Mes-Acr]+.



Physical Data: solid at 25 °C.; m.p. (BF4 Salt): 267–274 °C (dec.); λmax = 360, 430 nm; E1/2red = −0.49 V vs. SCE (MeCN); *E1/2red = +2.18 V vs. SCE (MeCN; singlet); *E1/2red = +1.45 V vs. SCE (MeCN; triplet).12



Solubility:  insol ( hexane, ether, benzene; sol acetonitrile, dichloromethane, chloroform; slightly sol H2O; X = BF4− : soluble in acetone, MeCN, DMF, DMSO, nitromethane, MeOH, EtOH, chlorinated hydrocarbon solvents such as 1,2-dichloroethane, CH2Cl2, and CHCl3; sparingly soluble in toluene and tetrahydrofuran; insoluble in nonpolar aprotic solvents, Et2O and H2O.



Form Supplied in: yellow powder. The perchlorate salt is available from both TCI and Sigma-Aldrich. The tetrafluoroborate salt is available from Sigma-Aldrich.



Analysis of Regent Purity: UV-Vis (λmax 360, 423 [log ɛ 4.20, 3.75 acetonitrile]).1H NMR.2



Purification: recrystallization from ethanol; recrystallization from MeOH/Et2O.2



Preparative Methods: for X = ClO4− : the acridinium salt is made from N-methylacridone and mesityl magnesium bromide, followed by the addition of perchloric acid.2 X = BF4− : same procedure as ClO4− salt preparation, but with HBF4·Et2O as the acid.13



Handling, Storage, and Precautions: potential skin irritant; this reagent should be handled in a fume hood. It may decompose violently or explode by mixing with combustible/flammable, heat, shock, or friction; keep away from strong reducing agents. General perchlorate safety should be exercised when handling [Mes-Acr]+ClO4−, though an incident has not been reported.

9‐Mesityl‐10‐methylacridinium Perchlorate

Photoredox catalysis and organocatalysis represent two powerful fields of molecule activation that have found widespread application in the areas of inorganic and organic chemistry, respectively. We merged these two catalysis fields to solve problems in asymmetric chemical synthesis. Specifically, the enantioselective intermolecular α-alkylation of aldehydes has been accomplished using an interwoven activation pathway that combines both the photoredox catalyst Ru(bpy)3Cl2 (where bpy is 2,2′-bipyridine) and an imidazolidinone organocatalyst. This broadly applicable, yet previously elusive, alkylation reaction is now highly enantioselective and operationally trivial.

Merging Photoredox Catalysis with Organocatalysis: The Direct Asymmetric Alkylation of Aldehydes.

A single-pot three-component coupling reaction of silylglyoxylates (1), terminal alkynes, and aldehydes in the presence of ZnI2 and Et3N is presented. The products of the reaction, densely functionalized silyl-protected glycolate aldols (2), can be converted to the corresponding acetonides (3) in a one-pot deprotection/ketalization sequence. A variety of terminal alkynes and aldehydes can be successfully employed to give a range of highly functionalized, fully protected 1,2-diols in good yields and moderate diastereoselectivities. Mechanistic experiments suggest that the zinc acetylide reacts with the silylgyloxylate (1) in a chemoselective manner. Using an unoptimized (+)-N-methylephedrine and Zn(OTf)2 system, silyl-deprotected adduct 2 was formed in 64% ee and 89:11 dr.

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Three‐Component Coupling Reactions of Silylglyoxylates, Alkynes, and Aldehydes: A Chemoselective One‐Step Glycolate Aldol Construction.

A method of making an anti-Markovnikov addition product is carried out by reacting an acid with an alkene or alkyne in a dual catalyst reaction system to the exclusion of oxygen to produce said anti-Markovnikov addition product; the dual catalyst reaction system comprising a single electron oxidation catalyst in combination with a hydrogen atom donor catalyst. Compositions useful for carrying out such methods are also described.

Direct anti-markovnikov addition of acids to alkenes

The products are isolated as inseparable mixtures of diastereomers, some also containing traces of additional diastereomers not shown in the scheme.

David A. Nicewicz

Papers

9‐Mesityl‐10‐methylacridinium Perchlorate

Merging Photoredox Catalysis with Organocatalysis: The Direct Asymmetric Alkylation of Aldehydes.

Three‐Component Coupling Reactions of Silylglyoxylates, Alkynes, and Aldehydes: A Chemoselective One‐Step Glycolate Aldol Construction.

Direct anti-markovnikov addition of acids to alkenes

Synthesis of Highly Substituted Tetrahydrofurans by Catalytic Polar‐Radical‐Crossover Cycloadditions of Alkenes and Alkenols.