Single-electron transfer , photoredox

The qualitative reaction profile given in Fig. 11 shows how a light-induced single-electron transfer process can be coupled to suitable follow-up steps to facilitate the formation of permanent two-electron photoredox products. Larger bond and shape reorganizations of excited state molecules, which typically involve the population of CT states or the formation of Jahn-Teller distorted species, are very helpful to achieve such... 

The application of inexpensive photoactive copper complexes has given great impulse to visible light-mediated photoredox catalysis, e.g. for C-C and C-N coupling reactions. Irradiation of Cu-complexes both triggers metal centred single electron transfer processes and assists the Cu-catalysed elemental steps, in which the substrate is covalently bonded. Successful extension of cooperative copper photocatalysis to other reactions is expected for the near future. " ... 

Indeed, photoredox catalysis with Ru polypyridine complexes has emerged as a powerful tool for redox reactions including formation of carbon-carbon bonds based on oxidation of sp C-H bonds via single-electron-transfer (SET) processes. Results that are closely related to those shown in Schemes 33,34, and 35, where the carbon-carbon bond formation resulted from the benzyUc sp C-H oxidative activation in the presence of BuOOH, have been recently reported for the regioselective functionalization of tetrahydroisoquinolines with cyanide and a variety of nucleophiles arising from ketones, nitroalkanes, allyltrimethylsilane, silyl enol ethers, 1,3-dicarbonyl compounds under photocatalytic conditions [67-70] as illustrated in Scheme 62 [67]. Other applications of Ru(bipy)3Cl2 in photocatalytic cycUzation reactions involving carbon-carbon btmd formation have appeared [71, 72]. 

Abstract Photoredox catalysis by well-known nithenium(II) polypyridine complexes is a versatile tool for redox reactions in synthetic organic chemistry, because they can effectively catalyze single-electron-transfer (SET) processes by irradiation with visible light. These favorable properties of the catalysts provide a new strategy for efficient and selective radical reactions. Salts of tris(2,2 -bipyridine)mthenium (II), [Ru(bpy)3], were first reported in 1936. Since then, anumber of works related to artificial photosynthesis and photofunctional materials have been reported, but only limited efforts had been devoted to synthetic organic chemistry. Remarkably, since 2008, this photocatalytic system has gained importance in redox reactions. In this chapter, we will present a concise review of seminal works on ruthenium photoredox catalysis around 2008, which will be followed by our recent research topics on trifluoromethylation of alkenes by photoredox catalysis. 

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. 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

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