An Expeditious Entry to 9-Azabicyclo[3.3.1]nonane <i>N</i>-Oxyl (ABNO): Another Highly Active Organocatalyst for Oxidation of Alcohols

Shibuya, Masabumi; Tomizawa, Masaki; Sasano, Yusuke; Iwabuchi, Yoshiharu

doi:10.1021/jo900486w

Cited by 120 publications

(69 citation statements)

References 41 publications

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“…A 0.003 mol% catalyst loading was sufficient to complete the reaction with Nor-AZADO. Nor- AZADO also exhibited efficient catalytic activity in the oxidation of primary alcohol compared with TEMPO, although AZADO and 1-Me-AZADO also worked well in this case 42,43) ( Table 6). The operationally facile and scalable features in synthesizing ABNO and Nor-AZADO should make their use applicable in many types of alcohol oxidation.…”

Section: Catalytic Activities Of Abno and Nor-azadomentioning

confidence: 99%

Discovery and Exploitation of AZADO: The Highly Active Catalyst for Alcohol Oxidation

Iwabuchi

2013

Chem. Pharm. Bull.

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109

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The oxidation of primary and secondary alcohols to the corresponding aldehydes (or carboxylic acids) or ketones is a fundamental transformation in organic synthesis. Stable organic nitroxyl radicals as represented by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (1) have been used extensively to catalyze the oxidation of a number of alcohol substrates employing environmentally benign co-oxidants such as bleach (NaOCl) or PhI(OAc) 2 . Although TEMPO oxidation is better known as a method for selective oxidation of primary alcohols to the corresponding aldehydes, the TEMPO-based method is not very efficient for the oxidation of structurally hindered secondary alcohols. We designed and synthesized 2-azaadamantane N-oxyl [AZADO (

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Section: Catalytic Activities Of Abno and Nor-azadomentioning

confidence: 99%

Discovery and Exploitation of AZADO: The Highly Active Catalyst for Alcohol Oxidation

Iwabuchi

2013

Chem. Pharm. Bull.

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109

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“…Their stability is believed to be derived from the inability to form a nitrone C ¼ N double bond to the bridgehead-an elegant application of Bredt's rule 22 . AZADO (5) has recently been shown to be a powerful oxidation catalyst and is now commercially available because of the efforts of Iwabuchi and co-workers [23][24][25] , who also reported a number of other nitroxides inspired by the same design concept. Unfortunately, the bridged bicyclic design is not universally applicable.…”

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confidence: 99%

Design concept for α-hydrogen-substituted nitroxides

et al. 2015

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Stable nitroxides (nitroxyl radicals) have many essential and unique applications in chemistry, biology and medicine. However, the factors influencing their stability are still under investigation, and this hinders the design and development of new nitroxides. Nitroxides with tertiary alkyl groups are generally stable but obviously highly encumbered. In contrast, a-hydrogen-substituted nitroxides are generally inherently unstable and rapidly decompose. Herein, a novel, concept for the design of stable cyclic a-hydrogen nitroxides is described, and a proof-of-concept in the form of the facile synthesis and characterization of two diverse series of stable a-hydrogen nitroxides is presented. The stability of these unique a-hydrogen nitroxides is attributed to a combination of steric and stereoelectronic effects by which disproportionation is kinetically precluded. These stabilizing effects are achieved by the use of a nitroxide co-planar substituent in the g-position of the backbone of the nitroxide. This premise is supported by a computational study, which provides insight into the disproportionation pathways of a-hydrogen nitroxides.

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“…Outstanding research regarding this objective has recently been made by Iwabuchi's group, who reported and commercialized a sterically less-hindered nitroxyl radical, 2-azaadamantane-N-oxyl (AZADO), as one of the highly active catalysts for various reactions, including alcohol oxidations. 39,40) Thus, we subsequently focused on the use of AZADO in our strategy. The alcohols 2a-c were hence treated with the catalytic system employing the same amounts of the adamatane reagent 1 (1.1×1/4 eq relative to the alcohols, which corresponds to 110 mol% active iodine(III) sites) and AZADO (10 mol%) combination.…”

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confidence: 99%

Speedy and Clean Hypervalent Iodine/Nitroxyl Radical Mediated Oxidation of Alcohols Using Recyclable Adamantane Reagent with Highly Active 2-Azaadamantane-<i>N</i>-oxyl Organocatalyst

Dohi

Kamitanaka

Mochizuki

et al. 2012

CHEMICAL & PHARMACEUTICAL BULLETIN

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By utilizing a specific solvent and highly active 2-azaadamantane-N-oxyl (AZADO) as an organocatalyst, we have improved the recycling protocol of the adamatane reagent 1 to be faster and more operationally simple for the hypervalent iodine/nitroxyl radical mediated alcohol oxidation. This very mild system can efficiently mediate the oxidation of a range of alcohols to carbonyl compounds with a broad substrate scope, and after the reactions, the reduced adamantane tetraiodide 1′, which automatically precipitated from the reaction mixtures as the reactions progresses, could be directly recovered by filtration.Key words alcohol oxidation; speedy process; recycling; hypervalent iodine reagent; nitroxyl radicalThe development of an efficient and clean method for the oxidation of alcohol functionalities to carbonyl groups remains an important topic widely investigated in modern synthetic chemistry.1-3) Among the present methods, the combination of the hypervalent iodine reagent [4][5][6][7][8] and nitroxyl radical catalyst, 9,10) which was first suggested by Piancatelli and Margarita's research group in the literature, 11) has gained particular attention in the past decades as a mild and metal-free versatile oxidation system. Due to the salient features of the catalytic conditions as well as the exclusive chemoselectivity toward alcohols in the presence of many oxidation-sensitive functional groups, this mild organocatalytic system has been frequently used for the selective oxidation of alcohol groups in complex molecules, [4][5][6][7][8] as represented by the early examples which appeared in the total synthesis of natural products, such as discodermolide and leucascandrolide A.12,13) Moreover, significant advances have been developed to date for the reagent and catalyst recycling, making the strategy and method even more attractive. [14][15][16][17][18][19][20][21][22][23] Regarding this interest, we have recently reported a versatile and clean method based on the use of the adamantanetype recyclable hypervalent iodine reagent 1 [24][25][26] (Fig. 1) for the organocatalytic oxidation of alcohols to aldehydes and ketones.27,28) The well-defined tetrahedral structure and sufficient interatomic distances between the iodine(III) sites in the molecule of the adamantane 1 are due to its higher reactivity and good solubility in solvents compared to the conventional polymer-supported reagents. Also, it is very stable under severe oxidative conditions, and the degradation of the molecule did not occur even after many recyclings. Furthermore, the reagent could be nearly quantitatively recovered after the reactions by utilizing the insolubility of the formed tetraiodide 1′ in a polar solvent, specifically, methanol. Based on these significant advantages, the adamantane reagent 1 would serve as a recyclable and greener alternative to conventionally-used hypervalent iodine reagents, and mediate many types of oxidation reactions with efficiencies similar to those of phenyliodine diacetate (PIDA), [29][30][31][32][33][34][35][36] w...

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An Expeditious Entry to 9-Azabicyclo[3.3.1]nonane N-Oxyl (ABNO): Another Highly Active Organocatalyst for Oxidation of Alcohols

Cited by 120 publications

References 41 publications

Discovery and Exploitation of AZADO: The Highly Active Catalyst for Alcohol Oxidation

Discovery and Exploitation of AZADO: The Highly Active Catalyst for Alcohol Oxidation

Design concept for α-hydrogen-substituted nitroxides

Speedy and Clean Hypervalent Iodine/Nitroxyl Radical Mediated Oxidation of Alcohols Using Recyclable Adamantane Reagent with Highly Active 2-Azaadamantane-<i>N</i>-oxyl Organocatalyst

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