Oxidative photoredox catalysis

Fig. 35 Oxidative photoredox catalysis using RuibipyhCPFsh (anions omitted for clarity)...

Oxidative or Reductive Photoredox Catalysis Using Ru(II) Complexes. 225... 

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). 

Zhou utilized photoredox catalysis to convert aniline derivative 95 to tri-substituted indole 96 under mild conditions using air as the oxidant.The reaction is presumed to proceed through benzylic radical 97, which cyclizes to give 98.Vinyl radical 98 is trapped by oxygen to give 99, and after loss of H2O, ultimately yields 2-acylindole 96 (140L3264). 

Guan (1,3-disubstituted indoles) [48], Rueping (photoredox catalysis) [49], and Jiao (/V-alkynylaniline hydroamination) [50]. Before Glorius, Lee and colleagues employed Pd-catalyzed oxidative cyclization in a synthesis of duocarmy-cin SA analogues [51]. Some additional examples of this oxidative cyclization to give indoles are in later chapters. 

In analogy to the addition of a trifluoromethyl group to a double bond, Akita and co-workers reported the formation of quaternary a-trifluoromethyl ketones by oxidation of the intermediate using DMSO under photoredox catalysis (Scheme 14) [46]. 

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. 

The Stephenson s work undoubtedly stimulates a rapid development of visible-light-driven C(sp )-H bond functionalization of tertiary amines. Soon afterward, numerous nucleophiles were employed under the principle of iminium ions chemistry by visible-light photoredox catalysis (Scheme 3.4). The oxidative Mannich reaction of Af-Aryl tetrahydroisoquinoUnes with ketones [17] and enol silanes [18] were developed by Rueping and Xia, respectively. As a follow-up work. Rueping et al. [19] reported an efiicient alkynylation of a-C(sp )-H bond... 

Single-electron oxidation of tertiary amines is controllable by photoredox catalysis delivering nucleophilic a-aminoalkyl radicals (Scheme 3.11). The recent findings in this area illuminate a nice addition to the above iminium ion chemistry. 

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]. 

Transition metal-catalyzed C-H activation has proven to be a reliable method for the construction of complex products. However, the requirement of stoichiometric quantities of external oxidants has limited the sustainability of this method. In 2014, Rueping and coworkers [56] developed an oxidative Heck reaction that included recycling of the metal catalyst through a photoredox process. As a continuation of their work on photoredox catalysis, they recently disclosed a new combined photoredox- and palladium-catalyzed system useful for indole... 

The proposed catal) ic cycle includes the formation of a chiral Breslow intermediate from NHC and a reactive iminium ion via photoredox catalysis (Scheme 7.9). In this reaction, oxidation of the Breslow intermediate or NHC by [Ru(bpy)3] " and terminal oxidants could be problematic, because of the similar redox potentials of this catalyst scaffold and tertiaiy amines. To suppress such unproductive pathways, careful choice of NHC with proper steric and electronic properties and the use of a weak oxidant are important. 

Scheme 7.8 Oxidative a-acylation of N-aryl THIQs under NHC and photoredox catalysis.

The encouraging results with photoredox catalysis sparked an explosion of additional investigations of visible-light-promoted oxidation of sp carbons. Many studies explored use of related polypyridyl transition metal based... 

Many metal complexes are known to be able to carry out the initial SET of typical CDC reactions, and the catalysts may also be heterogeneous. Clearly also oxygen itself can be responsible for re-generation of the higher oxidation state of the metal complex.Processes with similar mechanisms may operate that are not overall oxidative, such as allgrlations with allg l halides alpha to the carbonyl functionality, but these processes are often also genetically referred to as photoredox catalysis... 

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