Selective functionalization of benzylic C–H bonds of two different benzylic ethers by bowl-shaped <i>N</i>-hydroxyimide derivatives as efficient organoradical catalysts

Kato, Terumasa; Maruoka, Keiji

doi:10.1039/d1cc06425h

Cited by 8 publications

(9 citation statements)

References 58 publications

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“…The α-oxy-alkyl radical thus generated then adds to the NN double bond of DEAD, and the adduct hydrazinyl radical forms the next α-oxy-alkyl radical via HAT with concomitant formation of product 157 . For the same reaction with more reactive benzylic ethers, the ether can be used as a limiting reagent . In another example that involves the PINO radical, an electrocatalytic deprotection of an acetal was realized.…”

Section: Oxidation Reactionsmentioning

confidence: 99%

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Organic Synthesis Using Nitroxides

Leifert

Studer

2023

Chem. Rev.

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Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R 1 R 2 N−O • . The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R 1 and R 2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R 1 R 2 N�O + ) or reduced to anions (R 1 R 2 N−O − ), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C−O bonds, allowing for the thermal generation of C radicals through reversible C−O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

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Section: Oxidation Reactionsmentioning

confidence: 99%

“…For the same reaction with more reactive benzylic ethers, the ether can be used as a limiting reagent. 1014 In another example that involves the PINO radical, an electrocatalytic deprotection of an acetal was realized. 4-Phenyl-1,3-dioxolane-protected ketones were selected as substrates in these deprotections.…”

Section: Oxidations Of C Nucleophilesmentioning

confidence: 99%

Organic Synthesis Using Nitroxides

Leifert

Studer

2023

Chem. Rev.

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“…The regioselective amination of benzylic positions in alkylarenes [82] and ethers [83] directed by steric effects was achieved by the development of sterically hindered "bowlshaped" imide-N-oxyl radical precursors (Scheme 7). The presented example with a sterically hindered N-hydroxyimide catalyst shows the increase in selectivity compared to the reaction catalyzed by NHPI.…”

Section: N-oxyl Radical Catalysismentioning

confidence: 99%

Redox-active molecules as organocatalysts for selective oxidative transformations – an unperceived organocatalysis field

Lopat’eva¹,

Krylov²,

Lapshin³

et al. 2022

Beilstein J. Org. Chem.

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Organocatalysis is widely recognized as a key synthetic methodology in organic chemistry. It allows chemists to avoid the use of precious and (or) toxic metals by taking advantage of the catalytic activity of small and synthetically available molecules. Today, the term organocatalysis is mainly associated with redox-neutral asymmetric catalysis of C–C bond-forming processes, such as aldol reactions, Michael reactions, cycloaddition reactions, etc. Organophotoredox catalysis has emerged recently as another important catalysis type which has gained much attention and has been quite well-reviewed. At the same time, there are a significant number of other processes, especially oxidative, catalyzed by redox-active organic molecules in the ground state (without light excitation). Unfortunately, many of such processes are not associated in the literature with the organocatalysis field and thus many achievements are not fully consolidated and systematized. The present article is aimed at overviewing the current state-of-art and perspectives of oxidative organocatalysis by redox-active molecules with the emphasis on challenging chemo-, regio- and stereoselective CH-functionalization processes. The catalytic systems based on N-oxyl radicals, amines, thiols, oxaziridines, ketone/peroxide, quinones, and iodine(I/III) compounds are the most developed catalyst types which are covered here.

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“…Benzylic motifs with a C(sp 3 )–H bond (ArCHR 2 ) are present in many bioactive compounds, 1 and ∼25% of the 200 top-selling pharmaceuticals contain these motifs. 2 Therefore, the functionalization of such benzylic C–H bonds to new C–C, 3 C–N, 4 and C–O 5 bonds is the logical next step for the further modification of drug candidates. In particular, triarylmethanes (ArCHAr 2 ) and diarylalkanes (Ar 2 CHR) are some of the most attractive frameworks targeted for benzylic C–H functionalization, as they widely exist in pharmaceuticals, 6 functional materials, 7 and sensing systems.…”

Section: Introductionmentioning

confidence: 99%

Synthesis of triarylmethanes by silyl radical-mediated cross-coupling of aryl fluorides and arylmethanes

et al. 2023

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Selective functionalization of benzylic C–H bonds of two different benzylic ethers by bowl-shaped N-hydroxyimide derivatives as efficient organoradical catalysts

Abstract: A highly efficient, site-selective benzylic C-H bond amination of two different benzylic ether substrates was described by using bowl-shaped N-hydroxyimide organoradical catalysts with diethyl azodicarboxylate. The synthetic utility of this...

Cited by 8 publications

References 58 publications

Organic Synthesis Using Nitroxides

Organic Synthesis Using Nitroxides

Redox-active molecules as organocatalysts for selective oxidative transformations – an unperceived organocatalysis field

Synthesis of triarylmethanes by silyl radical-mediated cross-coupling of aryl fluorides and arylmethanes

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