Photoredox‐Induced Radical 6‐<i>exo</i>‐<i>trig</i> Cyclizations onto the Indole Nucleus: Aromative versus Dearomative Pathways

Alpers, Dirk; Brasholz, Malte; Rehbein, Julia

doi:10.1002/ejoc.201700150

Cited by 9 publications

(3 citation statements)

References 42 publications

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“…Bromomalonate 24 might undergo homolysis under this set of reaction conditions as an efficient radical initiator. α-Bromomalonates are known to form malonyl radicals via SET or an energy-transfer pathway. − In the energy-transfer pathway, bromine radicals are also formed; this raised doubts about the actual radical abstracting hydrogen from 1,3-dioxolane. In this regard, we performed a stoichiometric experiment between bromomalonate 24 and 1,3-dioxolane, and malonate 27 was formed in only 45% yield (unoptimized) (Scheme d).…”

supporting

confidence: 92%

Acetal Addition to Electron-Deficient Alkenes with Hydrogen Atom Transfer as a Radical Chain Propagation Step

2021

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We describe a visible-light-promoted addition of a hydrogen atom and an acetal carbon toward various electrondeficient alkenes. 1,3-Dioxolane is converted to its radical species in the presence of persulfate and an iridium catalyst upon visible light irradiation, which then reacts with electron-deficient alkenes. The reaction operates via a radical chain mechanism, a less commonly observed pathway for this class of transformation. Hydrogen atom transfer from 1,3-dioxolane to α-malonyl radicals is corroborated by experimental and density functional theory studies.

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confidence: 92%

Acetal Addition to Electron-Deficient Alkenes with Hydrogen Atom Transfer as a Radical Chain Propagation Step

2021

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“…Stern–Volmer quenching experiments revealed the excited state of the catalyst 1 + is quenched by TEA and transient absorption measurements confirmed this interaction occurrs via reductive quenching photoinitiated electron transfer (PET) to form the reduced complex 1 0 . The quenching of 1 + * by TEA was previously demonstrated in photocatalytic water reduction studies, , and included in the postulated mechanisms of numerous synthetic transformations. − Subsequent oxidation of 1 0 back to 1 + was proposed to occur via electron transfer between the transition metal complex and the imine substrate, despite a large difference between the formal potential of 1 0 ( E 0 ′( 1 + / 1 0 ) = −1.47 V vs SCE) and the peak potentials of the imines (e.g., E p red = −2.18 V vs SCE for N -(diphenylmethylene)-1-phenylmethanamine; Scheme c). Herein we reveal a heretofore unknown mechanistic cycle of the iridium photoredox catalyst 1 + , in which the 1 0 intermediate is converted into a new complex characterized by a large negative shift in reduction potential.…”

Section: Introductionmentioning

confidence: 97%

The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)₂(dtb-bpy)]⁺ Enabling Access to Energy Demanding Organic Substrates

et al. 2019

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We report the discovery of a tandem catalytic process to reduce energy demanding substrates, using the [Ir(ppy) 2 (dtb-bpy)] + (1 + ) photocatalyst. The immediate products of photoinitiated electron transfer (PET) between 1 + and triethylamine (TEA) undergo subsequent reactions to generate a previously unknown, highly reducing species (2). Formation of 2 occurs via reduction and semisaturation of the ancillary dtb-bpy ligand, where the TEA radical cation serves as an effective hydrogen atom donor, confirmed by nuclear magnetic resonance, mass spectrometry, and deuterium labeling experiments. Steady-state and time-resolved luminescence and absorption studies reveal that upon irradiation, 2 undergoes electron transfer or proton-coupled electron transfer (PCET) with a representative acceptor (N-(diphenylmethylene)-1-phenylmethanamine; S). Turnover of this new photocatalytic cycle occurs along with the reformation of 1 + . We rationalize our observations by proposing the first example of a mechanistic pathway where two distinct yet interconnected photoredox cycles provide access to an extended reduction potential window capable of engaging a wide range of energy demanding and synthetically relevant organic substrates including aryl halides.

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“…[62a] Recently, Rehbein et al discovered a visible light-induced radical 6-exotrig cyclizations to the indole nucleus for synthesizing hydropyridoindoles (Scheme 60). [95] The plausible mechanism is proposed in Scheme 60. Firstly, bromomalonate-tethered indole is reduced to alkyl radical 172 through a SET process by the reduced ruthenium species.…”

Section: Ruthenium Photocatalysismentioning

confidence: 99%

Recent Developments in Photo‐Catalyzed/Promoted Synthesis of Indoles and Their Functionalization: Reactions and Mechanisms

2020

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The use of clean and renewable light sources is increasingly common in organic synthesis due to its safety, practicality and economy. Recently, photoredox catalysis has shown great application values in organic transformations because of its advantages of environmentally friendly and abundant resources. Indoles and their derivatives (indolines, oxindoles and isatins) are the core skeletons of some important organic compounds and widely‐present in various natural products and pharmaceuticals with different biological activities. Therefore, the research on the synthesis and modification of indoles is particularly important for chemists and pharmacologists. This review summarizes the effects of photocatalysis on indole synthesis and modification in recent decades. These transformations are accomplished by using metal photocatalysts (i. e., Ir, Ru, Ni, Cu, Fe, Au, Rh, TiO2, etc.) or non‐metallic photocatalysts (i. e., Rose Bengal, Eosin Y, quinones, naphthols, N‐heterocyclic carbenes, carbazoles, pyrylium salts, etc.), or without the need of photocatalysts. The detailed mechanisms of these photo‐catalyzed/promoted organic reactions are also highlighted deeply. And we hope this review will be helpful to researchers interested in this promising field of photocatalyzed transformations.

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Photoredox‐Induced Radical 6‐exo‐trig Cyclizations onto the Indole Nucleus: Aromative versus Dearomative Pathways

Cited by 9 publications

References 42 publications

Acetal Addition to Electron-Deficient Alkenes with Hydrogen Atom Transfer as a Radical Chain Propagation Step

Acetal Addition to Electron-Deficient Alkenes with Hydrogen Atom Transfer as a Radical Chain Propagation Step

The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)₂(dtb-bpy)]⁺ Enabling Access to Energy Demanding Organic Substrates

Recent Developments in Photo‐Catalyzed/Promoted Synthesis of Indoles and Their Functionalization: Reactions and Mechanisms

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Photoredox‐Induced Radical 6‐exo‐trig Cyclizations onto the Indole Nucleus: Aromative versus Dearomative Pathways

Cited by 9 publications

References 42 publications

Acetal Addition to Electron-Deficient Alkenes with Hydrogen Atom Transfer as a Radical Chain Propagation Step

Acetal Addition to Electron-Deficient Alkenes with Hydrogen Atom Transfer as a Radical Chain Propagation Step

The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)2(dtb-bpy)]+ Enabling Access to Energy Demanding Organic Substrates

Recent Developments in Photo‐Catalyzed/Promoted Synthesis of Indoles and Their Functionalization: Reactions and Mechanisms

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The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)₂(dtb-bpy)]⁺ Enabling Access to Energy Demanding Organic Substrates