Photoredox cycling

Schematic representation of the photoredox cycling of iron in the photic zone of surface waters. The important features are the following ...

It is now realized that copper as metal next to iron and chromium participates in photoredox cycles and its role cannot be ignored. The most important part of the cycle is photoreduction of Cu(II) to Cu(I) induced by solar light and oxidation of ligands to their environmentally benign forms. Then Cu(I) is easily re-oxidized to Cu(II), which can coordinate the next ligand molecule, and thereby the Cu photocatalytic cycles contribute to continuous environmental cleaning. Besides oxida-tion/reduction, other critical processes relevant to the copper cycles are adsorption/desorption and precipitation/dissolution... 

Figure 1. Scheme for photochemical production of co-reductant to drive the oxidation of hydrocarbons by mimicking the cytochrome-P450 cycle. The SnP sensitized photoredox cycle is on the left the P 50 catalytic cycle is shown on the right. 

SCHEME 2.22. Photoredox cycle of Ru(bipy)3Cl2 (reduction potentials at 293 K). 

Figure 16 Photoredox cycles coupled with hydrogen evolution in the reducdon terminal end.

Another target reaction in the reduction terminal end of photoredox cycles has been the reduction of carbon dioxide. Pioneering works have been reported [393-403]. Interesting examples of photochemical fixation of carbon dioxide have also been reported for A1 porphyrins [404-406]. The insertion reaction of CO2 into Al-X bond (X = R, OR, SR, NR2) of Al porphyrin (3) was observed... 

Scheme 4.10 gives an insight into this reaction mechanism. It is supposed that the electron-rich iridium-complex Ir(ppy)2(dtb-bpy) generates the electrophilic tri-fluoromethyl radical via a single-electron transfer. This trifluoromethyl radical reacts with the enamine of the organocatalyst 7 and enoUzable aldehydes highly enantioselectively. The second catalytic cycle, the photoredox cycle, was reaUzed by oxidation/reduction processes of transition metal complexes with the aid of light, as depicted in Scheme 4.10 for the trifluoromethylation of aldehydes. 

Substrates with a heteroatom in proximity to the carboxylic acid, benzylic and homobenzylic carboxylic acids were transformed more rapidly due to faster CO2 extrusion, and it was found that only two equivalents of Select-fluorwas needed for these substrates. Fluoride elimination was not observed under the basic conditions, even for substrates which might be expected to have a propensity to display this type of reactivity. Mechanistic studies support an oxidative quenching pathway in which initial reduction of the N-F bond of Selectfluor initiates the photoredox cycle with oxidation/dec-arbo)q lation of the carboxylates leading to the intermediate allq l radicals. 

The photoredox reaction of chromimn(III) complexes with ligands such as oxalate, citrate, or edta can proceed under environmental conditions. In consequence, these pollutants imdergo oxidative degradation, but simultaneously the harmful and toxic chromate(VI) is generated. To close the photocatal5rtic cycle, Cr(VI) had to vmdergo successive photoredox processes. 

Although photoelectrochemical systems are able to offer respectable conversion efficiencies, the refinement of other solution-based processes continues. Gratzel has reviewed photoredox processes, paying particular attention to the use of organized assemblies such as micelles and vesicles. He emphasizes the central role of efficient colloidal metal catalysts in these schemes and also describes the recent development of bifunctional redox catalysts that allow the combination of cycles for the generation of hydrogen and oxygen. 

Consequently, the Rovis group identified a productive dual-catalysis mode that enables the asymmetric a-acylation of tertiary amines with aldehydes facilitated by the powerful combination of chiral NHC catalysis and photoredox catalysis. m-DNB (dinitrobenzene) is likely to induce an oxidative quenching cycle of [Ru(bpy)3] under these conditions, with adventitious oxygen likely being the terminal oxidant (Scheme 7.14). 

Recently, Zeitler and co-workers [107] have demonstrated the viability of an asymmetric organocatalytic cycle using the organic dye eosin Y as photoredox... 

Figure 2 summarizes four features of polyoxometalate (Pox) photoredox catalysis (i) the two general classes of reactions, equations (2) and (3) (top), (ii) the classes of substrates, SubH2, that have been photochemically oxidized or otherwise transformed by polyoxometalates in the presence of light (top), (iii) the basic processes that add to equations (2) and (3) in the form of a catalytic cycle (middle), and (iv) definitions of three classes of polyoxometalate complexes based on their reactivity (bottom). Note that equations (4), (5) and (6) in the cycle sum to equation (2) and equations (4), (5), and (7) sum to equation (3). As is apparent in Figure 2 and will be elaborated below, a major feature of the photochemistry of polyoxometalate systems is the rich thermal chemistry that is induced by the photoredox processes. The fact that coupled and subsequent thermal processes can be extensively modulated by altering reaction conditions is a principal reason why polyoxometalate photochemistry is so versatile and promising. 

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