N-hydroxyphthalimide as an effective mediator for the oxidation of alcohols by electrolysis

Masui, Masaichiro; Ueshima, Takahiro; Ozaki, Shigeko

doi:10.1039/c39830000479

Cited by 93 publications

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“…[22] Subsequently, Masui and coworkers reported that NHPI acts as an efficient electron carrier in the electrochemical oxidation of secondary alcohols. [23] The use of NHPI in an autoxidation reaction was first reported by Foricher and coworkers of Hoffmann La Roche in 1986. [24] Various terpenes and steroids were oxidized to the corresponding allylic hydroperoxides in the presence of a stoichiometric amount of NHPI.…”

Section: Reviewsmentioning

confidence: 99%

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

2004

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The use of nitroxyl radicals, alone or in combination with transition metals, as catalysts in oxidation processes is reviewed from both a synthetic and a mechanistic viewpoint. Two extremes of reactivity can be distinguished: stable (persistent) dialkylnitroxyls, such as the archetypal TEMPO, and reactive diacylnitroxyls, derived from N-hydroxy imides, such as N-hydroxyphthalimide (NHPI). The different types of reactivity observed are rationalized by considering the bond dissociation energies (BDEs) of the respective N-hydroxy precursors, substrates and reaction intermediates. Reactive diacylnitroxyl radicals are generated in situ from the corresponding N-hydroxy compound. The protagonist, NHPI, catalyzes a wide variety of free radical autoxidations, improving both activities and selectivities by increasing the rate of chain propagation and/or decreasing the rate of chain termination. In the absence of metal co-catalysts improved conversions and selectivities are obtained in the autoxidation of hydrocarbons to the corresponding alkyl hydroperoxides. For example, cyclohexylbenzene afforded the 1-hydroperoxide in 97.6% selectivity at 32% conversion when the autoxidation was performed in the presence of 0.5 mol % NHPI, and the product hydroperoxide as initiator, at 100 8C. This forms the basis for a potential coproduct-free route from benzene to phenol. In combination with transition metal co-catalysts, notably cobalt, NHPI and related compounds, such as N-hydroxysaccharin NHS, afford effective catalytic systems for the effective autoxidation of hydrocarbons, e.g., toluenes to carboxylic acids, under mild conditions. In the case of the less reactive cycloalkanes, NHS proved to be a more active catalyst than NHPI which is attributed to the higher reactivity of the intermediate nitroxyl radical, resulting from the replacement of a carbonyl group in NHPI by the more strongly electron-attracting sulfonyl group. Stable dialkylnitroxyl radicals, exemplified by TEMPO, catalyze oxidations of, e.g., alcohols, with single oxygen donors such as hypochlorite and organic peracids. The reactions involve the intermediate formation of the corresponding oxoammonium cation as the active oxidant. Alternatively, in conjunction with transition metals, notably ruthenium and copper, they catalyze aerobic oxidations of alcohols. These reactions involve metal-centered dehydrogenations and the role of the TEMPO is to facilitate regeneration of the catalyst (Ru and Cu) and oxidation of the alcohol (Cu) via hydrogen abstraction or one-electron oxidation processes. Detailed mechanistic investigations, including kinetic isotope effects, revealed that oxoammonium cations are not involved as intermediates in these reactions. In contrast, oxoammonium cations are involved in the aerobic oxidation of alcohols catalyzed by the copper-dependent oxidase, laccase, in combination with TEMPO. This different mechanistic pathway is attributed to the much higher redox potential of the copper(II) in the enzyme. Similarly, N-hydroxy compounds such as ...

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Section: Reviewsmentioning

confidence: 99%

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

2004

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“…No oxidation occurred in the presence of hydroquinone, which confirmed the free radical pathway of the reaction. In addition, after Phen, NHPI and Br 2 were mixed without any substrate in acetonitrile, the solutions color gradually changed from brown-red (color of Br 2 ) to yellow (color of PINO), [9] which proved visually the production of PINO radicals. Moreover, PINO signals could be found by means of in situ FT-IR spectroscopy.…”

Section: Communicationsmentioning

confidence: 99%

Highly Efficient and Metal‐Free Aerobic Hydrocarbons Oxidation Process by an o‐Phenanthroline‐Mediated Organocatalytic System

Tong

Miao

2005

Adv Synth Catal

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A highly efficient o-phenanthroline-mediated, metal-free catalytic system has been developed for oxidation of hydrocarbons with dioxygen in the presence of N-hydroxyphthalimide; various hydrocarbons were efficiently and high selectively oxidized, e.g., ethylbenzene to acetophenone in 97% selectivity and 76% conversion, under mild conditions. Keywords: C À H activation; hydrocarbons; molecular oxygen; organic catalysis; oxidation; o-phenanthrolineThe aerobic selective oxidation of hydrocarbons is a major goal of todays research in catalysis as selectively oxidized hydrocarbons can be used as feedstock for the preparation of fine chemicals.[1] In catalysis research field, catalytic efficiency and product selectivity remain a challenge when molecular oxygen is employed as terminal oxidant.[2] Traditional approaches for oxidation are based on metal-catalyzed activation of hydrocarbons.[3] Aiming at improving oxidation efficiency, Ishiis group and others developed an efficient catalytic system of N-hydroxyphthalimide (NHPI) combined with metal mediators (cobalt, manganese, etc.), which exhibited considerable activity in oxidation processes.[4] Recently, due to concerns of metal toxicity and metal-sensitive products, extensive research has been carried out on metal-free C À H activations, [5] thus combining organic mediators with NHPI becomes an ideal catalytic system. Our group has reported a biomimetic non-metal system composed of anthraquinones, HY zeolite and NHPI.[6] Excellent selectivity for hydrocarbon oxidation was obtained. In the above autoxidation processes, the active catalytic species of phthalimide N-oxyl radical (PINO) originated from NHPI via proton and electron transfer with the assistance of catalyst mediators. O-Phenanthroline (Phen) or bipyridine (or analogues) in combination with a metal has been employed to the electron-transfer process or to some oxidation processes, [7] but has rarely employed for hydrocarbon oxidation in the absence of metal complexes. Herein, focusing on more efficient organic mediators, we have developed a new o-phenanthroline (or analogues)-mediated, metal-free catalytic system for the oxidation of hydrocarbons in the presence of NHPI and a cocatalyst. In this communication, molecular bromine (Br 2 ), which has shown a particular promotion effect in some oxidation process, [8] was chosen as a cocatalyst. To test the efficiency of our catalytic system, ethylbenzene was used as a model substrate (Scheme 1). The reaction was carried out under 0.3 MPa of O 2 at 353 K for 2 h. Table 1 shows the results of oxidation using Phen or different analogues as catalyst mediators in the presence of NHPI and Br 2 . When Phen was employed, a 76% conversion and 97% selectivity for acetophenone (1) was obtained (entry 1). The conversion decreased (67 or 65%) and the selectivity for 1 was almost unchanged when 2, 2'-bipyridine or bathophenanthroline replaced Phen as catalyst mediators (entries 2 and 3). However, when neocuproine, bathocuproin, or 2,2'-bichinolin was employed, the conv...

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“…[3] The formed PINO abstracts the hydrogen of a C À H bond in organic substrates to produce carbon-centered radicals, and then propagates the autoxidation. In 1983, Masui et al [4] had documented that the oxidation of NHPI to PINO occurs on the anode when they employed NHPI as mediator in the electrochemical oxidation of secondary alcohols to ketones. Subsequently, there are many methods reported of NHPI-catalyzed oxygenations, however, PINO generation always relies on the assistance of various auxiliaries (Scheme 1).…”

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confidence: 99%

Manganese Dioxide and N‐Hydroxyphthalimide. An Effective Catalytic System for Oxidation of Nitrotoluenes with Molecular Oxygen

Yang

Zheng

et al. 2007

Adv Synth Catal

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The inexpensive manganese dioxide has been proven to be an efficient auxiliary for oxidizing N-hydroxyphthalimide (NHPI) to form the phthalimide N-oxyl radical via reduction and reoxidation. The combination of manganese dioxide and NHPI could catalyze effectively the oxidation of nitrotulenes by molecular oxygen. Thus, the oxidation of p-nitrotoluene with molecular oxygen (0.4 MPa) in the presence of manganese dioxide (10 mol %) and NHPI (10 mol %) in acetic acid at 110 8C for 4 h proceeded with 97 % conversion, and gave p-nitrotoluene in 89 % isolated yield.Keywords: aerobic oxidation; catalysis; N-hydroxyphthalimide; manganese dioxide; nitrotolueneThe oxyfunctionalization of alkylarenes with molecular oxygen as an ultimate oxidant is a key technology for converting petroleum-based feedstocks to useful primary and special chemicals. The success of these oxygenations always depends largely on the development of catalysts to promote productivity, the rate of reaction, as well as the selectivity of products.[1] Due to its very highly catalytic efficiencies even under mild conditions in the aerobic oxidation of a wide variety of organic substrates, N-hydroxyphthalimide (NHPI) is attracting continuous attention.[2] The general consensus for the NHPI-catalysis mechanism is that the one-electron oxidation of NHPI to the phthalimide N-oxyl radical (PINO) must take place first. [3] The formed PINO abstracts the hydrogen of a C À H bond in organic substrates to produce carbon-centered radicals, and then propagates the autoxidation. In 1983, Masui et al. [4] had documented that the oxidation of NHPI to PINO occurs on the anode when they employed NHPI as mediator in the electrochemical oxidation of secondary alcohols to ketones. Subsequently, there are many methods reported of NHPI-catalyzed oxygenations, however, PINO generation always relies on the assistance of various auxiliaries (Scheme 1). The reported auxiliaries could include generally two sorts: the cocatalyst and the radical initiator. The transition metal ions, [5] including Co, [2a] Mn, [6] V, [7] Ce, [8] etc., are well-documented cocatalysts. Notably, the combination of NHPI with Co or with Co and Mn together affords an effective catalytic system, known as the Ishii system, for the autooxidation of a broad range of organic substrates. For example, the oxidation of toluene in the presence of NHPI (10 mol %) and CoA C H T U N G T R E N N U N G (OAc) 2 (0.5 mol %) in acetic acid afforded benzoic acid with 96 % selectivity in 84 % conversion even at ambient temperature and pressure. [2a,b] Based on the assumption that redoxactive organic compounds could oxidize NHPI to PINO, we found that anthraquinones can serve as purely organic cocatalysts for NHPI-catalyzed oxygenations. [9a,b] On the other hand, some radical initiators, such as alkyl hydroperoxide, [10] AIBN, [11] and aldehyde, [12] can also mediate the formation of PINO in the NHPI-based oxidation, although they are consumed during the reactions and are not recoverable.The investigations revealed...

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N-hydroxyphthalimide as an effective mediator for the oxidation of alcohols by electrolysis

Cited by 93 publications

References 0 publications

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

Highly Efficient and Metal‐Free Aerobic Hydrocarbons Oxidation Process by an o‐Phenanthroline‐Mediated Organocatalytic System

Manganese Dioxide and N‐Hydroxyphthalimide. An Effective Catalytic System for Oxidation of Nitrotoluenes with Molecular Oxygen

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