Catalysis photoredox

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

Cano-Yalo and Deronzier reported the first application photoredox catalysis to organic reactions. Diazonium salts derived from 2,3-diphenylacrylate undergo a Pschorr reaction giving phenanthrenecarboxylates [178]. 

Nicewicz and MacMillan merged later photoredox catalysis and asymmetric organocatalysis to an efficient approach to the otherwise difficult asymmetric a-alkylation of aldehydes 118 by activated alkyl bromides 117 (Fig. 30) [183]. The concept of face differentiation at the a-position of aldehydes via chiral enamines 121 provides the basis for the method. This allows the formation of functionalized... 

Fig. 30 Asymmetric aldehyde alkylation using organocatalyst 119 and reductive photoredox catalysis...

Stephenson and coworkers applied reductive photoredox catalysis to trigger radical 6-exo cyclizations of co-pyrrole or co-indole-substituted a-bromocarbonyl compounds 124 [186] as well as radical 5-exo cyclizations of 2-bromo-2-(4-pentenyl)malonates 126 (Fig. 32) [187]. These cyclization processes provide bi- or tricyclic products 125 or cyclopentanecarboxylates 127 in moderate to excellent yields. The initial radical was formed with reduced ruthenium catalyst HOB generated similarly as above from 110 and a sacrificial amine... 

Gagne and coworkers showed that Giese additions of glycosyl bromides 128 to a,p-unsaturated carbonyl compounds 129 can be successfully performed using reductive photoredox catalysis (Fig. 33) [191]. Double addition of 129 competed to a considerable extent under standard conditions, indicating that reduction of the... 

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

Zheng, Y. Lin, L. Wang, B. Wang, X. Graphitic Carbon Nitride Polymers toward Sustainable Photoredox Catalysis. Angew. Chemielnt. Ed., 2015, 54, 12868-12884. 

Tucker JW, Narayanam JMR, Krabbe SW, Stephenson CRJ (2010) Electron transfer photoredox catalysis Intramolecular radical addition to indoles and pyrroles. Org Lett 12 368-371... 

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). 

Scheme 7.14 Asymmetric a-acylation of tertiary amines mediated by an NHC and photoredox catalysis dual mode reported by Rovis.

Fig. 2 Proposed reaction mechanism for the synthesis of trifluoromethylated alkenes from potassium vinyltrifluoroborates by photoredox catalysis. Reproduced from ref. 87 with permission from The Royal Society of Chemistry.

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. " ... 

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). 

Fust L, Narayanam JMR, Stephenson CRJ (2011) Total Synthesis of (-t)-Gliocladin C Enabled by Visible-Light Photoredox Catalysis. Angew Chem Int Ed 50 9655... 

Visible-light-induced photoredox catalysis An easy access to green radical chemistry 13SL2492. 

The chemistry and apphcations of amine radical cations produced by visible hght photoredox catalysis 13BJ01977. 

Applications of visible Hght photoredox catalysis in organic synthesis 13CJO2046. 

Synthetic applications of photoredox catalysis with visible light 130BC2387. 

Visible-light-driven photoredox catalysis in the construction of carbocy-clic and heterocyclic ring systems 13EJ06755. 

Visible light photoredox catalysis with transition metal complexes and its synthetic appUcations 13CRV5322. 

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. 

Photoredox Organocatalysis. Photoredox catalysis is emerging as an extremely powerful tool in organic synthesis [103]. Photoredox catalysis relies on the ability of some compounds, such as ruthenium(II) tris(bipyridyl) complex (Ru(bipy)3 ), to absorb visible light affording a relatively kinetically stable excited species (usually in the triplet state) with enhanced redox properties (Scheme 2.22). 

The merging of photoredox catalysis with asymmetric organocatalysis was reported by MacMillan and Nicewicz in 2008 [104]. Initial studies in the MacMillan group were devoted to the imidazolidinone-catalyzed asymmetric a-functionaliza-tion of aldehydes with a-bromo ketones and esters [104a] and were subsequently expanded to enantioselective perfluoroalkylations [105] and benzylations [106]. The... 

Asymmetric a-Alkylation of Aldehydes and Organo-photoredox Catalysis... 

One year later, an extension of organo-photoredox catalysis into enantioselective a-trifluoromethylation and a-perfluoroalkylation of aldehydes was reported by MacMillan and co-workers [150, 151] that was accomplished by using a readily available iridium photocatalyst [Ir(ppy)2(dtb-bpy)]" and a commercial imidazolidinone catalyst (Scheme 8.38). A broad range of perfluoroalkyl iodides and bromides as well as a variety of aldehydes bearing various functional or bulky groups afforded corresponding a-fluoroalkylated products in high yields with excellent diastereo-and enantioselectivity (up to 99% ee). 

Although the results of the last two approaches below previously reported results, this new extension of organo-photoredox catalysis to classic organic dyes would find broadly utility across many applications. 

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