Visible Light Photocatalytic Aerobic Oxygenation of Indoles and pH as a Chemoselective Switch

Direct and selective oxygenation of C−H bonds to C−O bonds is regarded as an effective tool to generate high‐value products. However, these reactions are still subject to challenges such as harsh reaction conditions, use of expensive transition metal catalysts, and involvement of stoichiometric oxidants. To avoid these, molecular oxygen would be ideal as oxidant, as the byproduct is water or hydrogen peroxide. Additionally, achieving these reactions by using metal‐free catalysts would contribute to green and sustainable chemical synthesis. This Minireview summarizes recent reports on C−H oxygenation reactions with metal‐free catalysts and molecular oxygen under visible‐light conditions.

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“…[46] This radicala nion further reacted with the correspondingr adical cation from the substrate and formed the monooxygenated species.…”

Section: Visible-light-driven Aerobic Oxygenation Of Indolesmentioning

confidence: 99%

Metal‐Free Photocatalysts for C−H Bond Oxygenation Reactions with Oxygen as the Oxidant

2019

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“…Pleasingly, only the desired mono-alkylated compound 8 a was obtained without any trace of bis-indole compound 7 a. [19] It is worth mentioning that in all cases, 8 was obtained as a single product without concurrent formation of bis-indoles 7. The use of a smaller amount of indole 6 a (1.5 equivalents), led to a reduced yield and a partial racemization of one of the two diastereomers (entry 2).…”

mentioning

confidence: 89%

Combining Organocatalysis and Photoredox Catalysis: An Asymmetric Synthesis of Chiral β‐Amino α‐Substituted Tryptamines

et al. 2019

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A stereoselective synthesis of functionalized β-amino α-substituted tryptamines has been achieved by a sequential asymmetric organocatalytic three-component electrophilic amination and photocatalyzed Friedel-Crafts reaction of indoles with moderate to high diastereoselectivities (up to > 99 : 1 dr) and excellent enantioselectivities (up to > 99 % ee). We also demonstrated that the β-amino α-substituted tryptamines can be successfully engaged in a further catalytic step, furnishing various α,β-substituted tryptamines without erosion of enantioselectivity and with complete diastereoselectivity.

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“…[87] Intermediate IV is then converted into the carbonyl compounds after deprotonation. [88] Twom echanisms for visiblelight-driven aerobic photooxygenationo fi ndoles were proposed (Scheme13). [87] Then, H 2 is produced by the reactiono f the Co III ÀHs pecies with H + to regenerate [Co III (dmgH) 2 pyCl].…”

Section: Photocatalytic Hydroxylation Of Substrates With Organophotocmentioning

confidence: 99%

“…AcrCÀMes reduces the Co II species to regenerate Acr + ÀMes and the Co I speciest hat reacts with H + to produce the Co III ÀH species. [88] Electron transferf rom indoles to the triplet excited state ( 3 DPZ*) occurs to produce indole radical cation because the one-elec-tron reduction potentialo f 3 DPZ* (0.91 Vv s. SCE) is more positive than the one-electron oxidation potentialo fi ndole (À0.04 Vv s. SCE). [87] The oxygen atom in the carbonyl compound comes from water and ap rotoni s used as the oxidant for the oxidation of alkenes.…”

Section: Photocatalytic Hydroxylation Of Substrates With Organophotocmentioning

confidence: 99%

Photocatalytic Oxygenation Reactions Using Water and Dioxygen

2019

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Water (H2O) is the most environmentally benign reductant and is oxidized to evolve dioxygen (O2)—the greenest oxidant—in photosystem II. This Minireview focuses on photocatalytic oxygenation of substrates with H2O as an oxygen source and O2 as an oxidant. Metal complexes can be oxidized by two molecules of one‐electron oxidants with H2O to produce high‐valent metal–oxo complexes, which act as active oxidants for oxygenating organic substrates. When an appropriate oxidant is employed for the substrate oxidation, the reduced oxidant can be oxidized by dioxygen to regenerate the oxidant when water and dioxygen are used as an oxygen source and an oxidant, respectively. Photoinduced electron transfer from a substrate (S) to the excited state of complex [(L)MIII]+ produces a substrate radical cation (S.+), accompanied by the regeneration of [(L)MII]. S.+ then reacts with H2O to produce an OH adduct radical that is oxidized by [(L)MIII]+ to yield an oxygenated product (SO), in which the oxygen atom originates from H2O, accompanied by regeneration of [(L)MII]. Photocatalytic oxidation of H2O by O2 to produce H2O2 is combined with the catalytic oxygenation of substrates with H2O2 to produce the oxygenated products, in which the oxygen atom originates from O2 at the beginning but later from water. This Minireview provides a promising strategy for oxygenation of substrates by using H2O as an oxygen source and O2 as the greenest oxidant.

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Visible Light Photocatalytic Aerobic Oxygenation of Indoles and pH as a Chemoselective Switch

Cited by 163 publications

References 66 publications

Metal‐Free Photocatalysts for C−H Bond Oxygenation Reactions with Oxygen as the Oxidant

Metal‐Free Photocatalysts for C−H Bond Oxygenation Reactions with Oxygen as the Oxidant

Combining Organocatalysis and Photoredox Catalysis: An Asymmetric Synthesis of Chiral β‐Amino α‐Substituted Tryptamines

Photocatalytic Oxygenation Reactions Using Water and Dioxygen

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