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

1. Advantages of Chemical Redox Agents 878 2. Disadvantages of Chemical Redox Agents 879 C. Potentials in Nonaqueous Solvents 879 D. Reversible vs Irreversible ET Reagents 879 E. Categorization of Reagent Strength 881 II. Oxidants 881 A. Inorganic 881 1. Metal and Metal Complex Oxidants 881 2. Main Group Oxidants 887 B. Organic 891 1. Radical Cations 891 2. Carbocations 893 3. Cyanocarbons and Related Electron-Rich Compounds 894

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Chemical Redox Agents for Organometallic Chemistry

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

The first acidity scale to be established in a pure solvent other than water was the result of the pioneering work of Conant, Wheland, and McEwen in ether or ben~ene .~ During the past 20 years an ion-pair acidity scale covering an \"effective pKa rangefrom about 15 to 40 has been developed in cyclohexylamine (CHA),6 and similar studies in other low-dielectric-constant solvents including 1,2-dimethoxyethane (DME)7a and tetrahydrofuran (THF)7b*c have been carried out. A more limited ion-pair acidity scale has been developed in liquid NH,.7d Also, during this period, acidity scales have been established in the polar non-hydrogenbond-donor (NHBD) solvents dimethyl sulfoxide (Me$0)8 and N-methylpyrrolidin-2-one (NMP)? which have relatively high dielectric constants. The pK,'s measured in these solvents differ from ion-pair pK,'s in that they are absolute, in the sense that they are based on Me2S0 and NMP as the standard states, which allows direct comparisons to be made with H20 and gas-phase pK,'s. A truly absolute acidity scale has been established in the gas phase, which, for the first time, provides intrinsic measures of structural effects free of solvent effects.1° Our purpose in this Account is (a) to discuss briefly acidities in various solvent media, (b) to present a table of representative equilibrium acidity constants in M e 8 0 solution, and (c) to illustrate ways in which these pK, data can be used. In an accompanying Account we compare acidities in Me2S0 solution with intrinsic gas-phase acidities and discuss some of the insights into solvation effects provided thereby. Acidities in H 2 0 and Me2S0. It is important to recognize that pKa values are solvent dependent. The

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Equilibrium Acidities in Dimethyl Sulfoxide Solution

Publisher Summary This chapter focuses on the electron transfer reactions in organic chemistry. The simplest elementary act in homogeneous solution chemistry is the exchange of an electron between two chemical entities. No bonds need to be formed or broken in the process, and if the two species are identical, except for a difference in oxidation state, no net chemical change takes place. Such self-exchange reactions constitute the basis of theoretical treatments of electron transfer. A more complex and chemically interesting situation is the electron-transfer reaction between two species of different chemical composition. In electron-transfer theory, it is an important postulate that electrons are transferred one by one, which does not mix well with the two-electron centered electronic theory of organic molecules. Organic electron transfers rarely occur as isolated steps because of the high chemical reactivity of odd-electron species. Processes involving one or several electron-transfer steps are not likely to preserve the stereochemical integrity of the reactants. With the present upsurge of research studies frequently invoking organic electron-transfer steps in mechanistic discussions, there is a great need of a theoretical background applicable to organic species and compatible with the language of physical organic chemistry.

Electron transfer reactions in organic chemistry

Making radical cations live longer

1,1,1,3,3,3-Hexafluoropropan-2-ol (HFP) has been found to be a superior solvent for the generation of radical cations, in many cases increasing their half-lives by a factor of 102 compared with those pertaining to trifluoroacetic acid. This study has mainly used 4-tolylthallium(III) bis(trifluoroacetate) and thallium(III) tris(trifluoroacetate) as oxidants, but DDQ could also be employed in the presence of 5% trifluoroacetic acid. Photochemical oxidation by mercury(II) bis(trifluoroacetate) also worked well. Thus room temperature recording of high quality EPR spectra of radical cations previously accessible only by flow or low-temperature procedures has become feasible.

Lennart Eberson

Papers

Electron transfer reactions in organic chemistry

Making radical cations live longer

1,1,1,3,3,3-Hexafluoropropan-2-ol as a solvent for the generation of highly persistent radical cations

Studies on Electrolytic Substitution Reactions. I. Anodic Acetoxylation

Electron-transfer mechanisms in electrophilic aromatic nitration