Aerobic Oxidation of Cyclohexane using N‐Hydroxyphthalimide Bearing Fluoroalkyl Chains

The N‐hydroxyphthalimide derivatives, F₁₅‐ and F₁₇‐NHPI, bearing a long fluorinated alkyl chain, were prepared and their catalytic performances were compared with that of the parent compound, N‐hydroxyphthalimide (NHPI). The oxidation of cyclohexane under 10 atm of air in the presence of fluorinated F₁₅‐ or F₁₇‐NHPI, cobalt diacetate [Co(OAc)₂], and manganese diacetate [Mn(OAc)₂] without any solvent at 100 °C afforded a mixture of cyclohexanol and cyclohexanone (K/A oil) as major products along with a small amount of adipic acid. It was found that F₁₅‐ and F₁₇‐NHPI exhibit higher catalytic activity than NHPI for the oxidation of cyclohexane without a solvent. However, for the oxidation in acetic acid all of these catalysts afforded adipic acid as a major product in good yield and the catalytic activity of NHPI in acetic acid was almost the same as those of F₁₅‐ and F₁₇‐NHPI. The oxidation by F₁₅‐ and F₁₇‐NHPI catalysts in trifluorotoluene afforded K/A oil in high selectivity with little formation of adipic acid, while NHPI was a poor catalyst under these conditions, forming K/A oil as well as adipic acid in very low yields. The oxidation in trifluorotoluene by F₁₅‐ and F₁₇‐NHPI catalysts was considerably accelerated by the addition of a small amount of zirconium(IV) acetylacetonate [Zr(acac)₄] to the present catalytic system to afford selectively K/A oil, but no such effect was observed in the NHPI‐catalyzed oxidation in trifluorotoluene.     

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

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 M Pa) in the presence of manganese dioxide (10 mol %) and NHPI (10 mol %) in acetic acid at 110 °C for 4 h proceeded with 97 % conversion, and gave p‐nitrotoluene in 89 % isolated yield.     

Hydroperoxidation of Tertiary Alkylaromatics Catalyzed By N‐Hydroxyphthalimide and Aldehydes under Mild Conditions

A metal‐free catalytic system consisting of an aldehyde and N‐hydroxyphthalimide (NHPI) for the selective oxidation of tertiary alkylaromatics with molecular oxygen has been developed. Cumene was oxidized efficiently to the corresponding hydroperoxide under mild conditions. The molecule‐induced homolysis between peracids generated in situ and NHPI ensured the formation of the phthalimide N‐oxyl (PINO) radical even at room temperature. Investigations on aldehyde, solvent and temperature effects allowed us to achieve good conversions with high selectivity in hydroperoxide. The optimized procedure was successfully extended to phenylcyclohexane, a valuable alternative for the production of phenol. The mechanism is discussed in detail.     

Sunlight Induced Oxidative Photoactivation of N‐Hydroxyphthalimide Mediated by Naphthalene Imides

We report the aerobic photoactivation of N‐hydroxyphthlimide (NHPI) to the phthalimido‐N‐oxyl (PINO) radical mediated by naphthalene monoimides (NI) for promoting the selective oxidation of alkylaromatics and allylic compounds to the corresponding hydroperoxides. In the absence of either NI or NHPI no oxidation was observed, meaning that the two molecules operate in a synergistic way. Sunlight as well as artificial UV‐light irradiation was necessary in order to perform the process at low temperature (30–35 °C). EPR spectroscopy confirmed the role of NI and oxygen in promoting the formation of the superoxide radicals O₂^.− which, in turn, increased the concentration of PINO radicals during the UV light irradiation of NI/NHPI mixtures in MeCN. The investigation was extended to NI bearing different substituents on the naphthalene moiety. Finally, the synthesis and application of a unique photocatalyst including the NI and NHPI moieties linked by a suitable spacer was also considered. In this case the photocatalyst showed a substrate‐dependent behaviour with some peculiarities in comparison to the system where NI and NHPI are independent units in the same reacting system. This photocatalytic system paves the way to a non‐thermal, metal‐free approach for C H bond activation towards aerobic oxidation under very mild conditions.

     

Synthesis of N‐Alkoxy Amines via Catalytic Oxidation of Hydrocarbons

Sterically hindered N‐alkoxy amines 3 are synthesized in good yields by coupling nitroxides 2 with hydrocarbyl radicals generated in situ by t‐BuOOH hydrogen abstraction from hydrocarbons. The reaction is catalyzed by copper halides as well as by onium iodides.     

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

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.     

Hydroxylamine as a Source for Nitric Oxide in Metal‐Free 2,2,6,6‐ Tetramethylpiperidine N‐Oxyl Radical (TEMPO) Catalyzed Aerobic Oxidation of Alcohols

The communication reports on the metal‐free 2,2,6,6‐tetramethylpiperidine N‐oxyl radical (TEMPO) catalyzed aerobic oxidation of various alcohols to aldehydes and ketones. A novel catalyst system that uses 1–4 mol% of TEMPO in combination with 4–6 mol% of aqueous hydroxylamine is introduced. No other additives are necessary and corrosive by‐products are not formed during oxidation. Nitric oxide which is important for the catalytic cycle is generated in situ by reaction of the hydroxylamine with TEMPO. A catalytic cycle for the overall oxidation process is suggested.     

Innovation of Hydrocarbon Oxidation with Molecular Oxygen and Related Reactions

An innovation of the aerobic oxidation of hydrocarbons through catalytic carbon radical generation under mild conditions was achieved by using N‐hydroxyphthalimide (NHPI) as a key compound. Alkanes were successfully oxidized with O₂ or air to valuable oxygen‐containing compounds such as alcohols, ketones, and dicarboxylic acids by the combined catalytic system of NHPI and a transition metal such as Co or Mn. The NHPI‐catalyzed oxidation of alkylbenzenes with dioxygen could be performed even under normal temperature and pressure of dioxygen. Xylenes and methylpyridines were also converted into phthalic acids and pyridinecarboxylic acids, respectively, in good yields. The present oxidation method was extended to the selective transformations of alcohols to carbonyl compounds and of alkynes to ynones. The epoxidation of alkenes using hydroperoxides or H₂O₂ generated in situ from hydrocarbons or alcohols and O ₂ under the influence of the NHPI was demonstrated and seems to be a useful strategy for industrial applications. The NHPI method is applicable to a wide variety of organic syntheses via carbon radical intermediates. The catalytic carboxylation of alkanes was accomplished by the use of CO and O₂ in the presence of NHPI. In addition, the reactions of alkanes with NO₂ and SO₂ catalyzed by NHPI provided efficient methods for the synthesis of nitroalkanes and sulfonic acids, respectively. A catalytic carbon‐carbon bond forming reaction was achieved by allowing carbon radicals generated in situ from alkanes or alcohols to react with alkenes under mild conditions. 1 Introduction 2 Discovery of NHPI as Carbon Radical Producing Catalyst from Alkanes 2.1 Historical Background 2.2 Catalysis of NHPI in Aerobic Oxidation 3 NHPI‐Catalyzed Aerobic Oxidation 3.1 Oxidation of Benzylic Compounds 3.2 Alkane Oxidations with Molecular Oxygen 3.3 Oxidation of Alkylbenzenes 3.4 Practical Oxidation of Methylpyridines 3.5 Preparation of Acetylenic Ketones via Alkyne Oxidation 3.6 Oxidation of Alcohols 3.7 Selective Oxidation of Sulfides to Sulfoxides 3.8 Production of Hydrogen Peroxide by Aerobic Oxidation of Alcohols 3.9 Epoxidation of Alkenes using Molecular Oxygen as Terminal Oxidant 4 Carboxylation of Alkanes with CO and O₂ 5 Utilization of NO_x in Organic Synthesis 5.1 First Catalytic Nitration of Alkanes using NO₂ 5.2 Reaction of NO with Organic Compounds 6 Sulfoxidation of Alkanes Catalyzed by Vanadium 7 Carbon‐Carbon Bond Forming Reaction via Catalytic Carbon Radicals Generated from Various Organic Compounds Assisted by NHPI 7.1 Oxyalkylation of Alkenes with Alkanes and Dioxygen 7.2 Synthesis of α‐Hydroxy‐γ‐lactones by Addition of α‐Hydroxy Carbon Radicals to Unsaturated Esters 7.3 Hydroxyacylation of Alkenes using 1,3‐Dioxolanes and Dioxygen 8 Conclusions     

Efficient metal‐free aerobic oxidation of aromatic hydrocarbons utilizing aryl‐tetrahalogenated N‐hydroxyphthalimides and 1,4‐diamino‐2,3‐dichloroanthraquinone

BACKGROUND: Hydrocarbon oxidation reactions are central to numerous processes that convert bulk chemicals into useful and higher‐value products. In this investigation, an efficient metal‐free catalytic system for aerobic oxidation of aromatic hydrocarbons was successfully established by synthesizing a series of aryl‐tetrahalogenated N‐hydroxyphthalimides and applying these compounds with 1,4‐diamino‐2,3‐dichloroanthraquinone (DADCAQ). RESULTS: Ethylbenzene was oxidized with 82.3% conversion and 86.9% selectivity to acetophenone catalyzed by the system of TCNHPI/DADCAQ under 0.3 MPa of molecular oxygen at 100 °C for 5 h. Other hydrocarbons were oxidized with high efficiency using this catalytic system. For example, indane can be converted completely to indan‐1‐one with 98.0% selectivity. CONCLUSION: A highly efficient metal‐free catalytic system consisting of TCNHPI and DADCAQ was developed for the oxidation of aromatic hydrocarbons with molecular oxygen. Aryl‐halogen substituents served to significantly increase the activities of the catalytic system. The results in this study can form the basis for the design of an efficient hydrocarbon oxidation process. Copyright © 2008 Society of Chemical Industry     

TEMPO and Carboxylic Acid Functionalized Imidazolium Salts/Sodium Nitrite: An Efficient,Reusable, Transition Metal‐Free Catalytic System for Aerobic Oxidation of Alcohols

An effective catalytic system comprising a 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) functionalized imidazolium salt ([Imim‐TEMPO]⁺ X⁻), a carboxylic acid substituted imidazolium salt ([Imim‐COOH]⁺ X⁻), and sodium nitrite (NaNO₂) was developed for the aerobic oxidation of aliphatic, allylic, heterocyclic and benzylic alcohols to the respective carbonyl compounds with excellent selectivity up to >99%, even at ambient conditions. Notably, the catalyst system could preferentially oxidize a primary alcohol to the aldehyde rather than a secondary alcohol to the ketone. Moreover, the reaction rate is greatly enhanced when a proper amount of water is present. And a high turnover number (TON 5000) is achieved in the present transition metal‐free aerobic catalytic system. Additionally, the functionalized imidazolium salts are successfully reused at least four times. This process thus represents a greener pathway for the aerobic oxidation of alcohols into carbonyl compounds by using the present task‐specific ionic liquids in place of the toxic and volatile additive, such as hydrogen bromide, bromine, or hydrogen chloride (HBr, Br₂ or HCl), which is commonly required for the transition metal‐free aerobic oxidation of alcohols.     

Visible‐Light‐Induced Metal‐Free Allylic Oxidation Utilizing a Coupled Photocatalytic System of g‐C3N4 and N‐Hydroxy Compounds

Graphitic carbon nitride (g‐C₃N₄) and N‐hydroxy compounds can function as a non‐metal photocatalytic system to activate O₂ for the selective allylic oxidation under mild conditions, avoiding the employment of any metal derivative or organic oxidizing agents. Interestingly, the novel photocatalytic system affords a remarkably high selectivity towards the formation of aldehydes, especially in the oxidation of toluene. By combining the unique nature of g‐C₃N₄ (surface basicity, semiconductor features, high stability) and the remarkable catalytic oxidation reactivity of nitroxyl radicals, this photocatalytic system opens up a mild and efficent access for C H bond activation.     

Solvent‐Free Aerobic Oxidation of Alcohols Catalyzed by an Efficient and Recyclable Palladium Heterogeneous Catalyst

A simple alumina‐supported palladium catalyst prepared by an adsorption method is highly efficient and recyclable in the solvent‐free oxidation of alcohols with molecular oxygen. The adsorption method results in high dispersion of palladium probably as mononuclear or oligonuclear species on alumina surface. These palladium species are transformed to small Pd nanoparticles (ca. 5 nm), which are probably the true active species, during the course of alcohol oxidation.     

Nanocrystalline Magnesium Oxide Stabilized Palladium(0): An Efficient Reusable Catalyst for Room Temperature Selective Aerobic Oxidation of Alcohols

Nanocrystalline magnesium oxide‐stabilized palladium(0) [NAP‐Mg‐Pd(0)], as an efficient catalytic system has been employed for the selective oxidation of alcohols using atmospheric oxygen as a green oxidant at room temperature. Various alcohols could be transformed into their corresponding aldehydes or ketones in good to excellent yields using a set of optimal conditions. NanoActive™ Magnesium Oxide Plus, [NAP‐MgO] with its three‐dimensional structure and well‐defined shape acts as an excellent support for well dispersed palladium(0) nanoparticles. This catalyst can be recovered and reused for several cycles without any significant loss of catalytic activity.     

烃类氧化新的N-羟基邻苯二甲酰亚胺基催化氧化体系

烃类氧化反应是石油化工中一类重要反应。空气和氧气是容易得到且环境友好的氧化剂,所以以氧为氧化剂的具有高选择性和转化率的催化体系的开发在烃类氧化中具有极其重要意义。本文在对传统的烃类分子氧氧化催化剂进行讨论的基础上,对近年来出现的以N-羟基邻苯二甲酰亚胺为基础的新催化剂体系进行了综述。该催化体系催化的烃类分子氧氧化反应具有反应条件温和、选择性高等优点。     

Highly efficient and organic nitrogen‐containing cation‐promoted aerobic oxidation of alkylaromatics in the presence of N‐hydroxyphthalimide

BACKGROUD: Direct oxidation of alkylaromatics to the corresponding aromatic ketone is an important process in the manufacture of perfumes, pharmaceuticals, flavors, dyes and agrochemicals. For example, tetralin is oxidized to produce 1‐tetralinone, which is a key intermediate in the commercial production of 1‐naphthol, 2‐hydroxy‐1‐tetralone, aureolic acid antibiotics and other pharmaceuticals. RESULTS: In this investigation, it was found that alkylaromatics could be efficiently and selectively oxidized to the corresponding aromatic ketones when methyl violet was employed as a promotor in the presence of N‐hydroxyphthalimide (NHPI); tetralin was oxidized with 89% conversion and 76% selectivity to 1‐tetralinone under 0.3 MPa of O₂ at 75 °C for 2.5 h. The effects of temperature, oxygen pressure, reaction time and additive inclusion were studied in detail. A possible reaction mechanism for tetralin oxidation has been proposed. CONCLUSION: It was demonstrated that methyl violet could efficiently promote the aerobic oxidation of alkylaromatics in the presence of NHPI under mild conditions. Further investigations indicated that the nitrogen cation had a crucial promotion effect in the oxidation process. Copyright © 2009 Society of Chemical Industry     

Organocatalytic Radical Involved Oxidative Cross‐Coupling of N‐Hydroxyphthalimide with Benzylic and Allylic Hydrocarbons

The cross‐coupling reaction between N‐hydroxyphthalimide and various benzylic and allylic hydrocarbons was realized through an organocatalytic radical‐mediated process involving C(sp³) O bond formation using tert‐butyl hydroperoxide (t‐BuOOH) as an oxidant and tetra‐n‐butylammonium iodide [(n‐Bu]₄NI] as a catalyst, during which the phthalimide N‐oxyl (PINO) radical and benzylic and allylic radicals were generated in situ and underwent the selective radical/radical cross‐coupling reaction. This novel method provides a convenient metal‐free approach to the synthesis of O‐alkylated hydroxy imides under mild reaction conditions.

     

Efficient and Selective Aerobic Alcohol Oxidation Catalyzed by Copper(II)/2,2,6,6,‐Tetramethylpiperidine‐1‐oxyl at Room Temperature

A simple and efficient copper(II)/2,2,6,6,‐tetramethylpiperidine‐1‐oxyl (TEMPO)‐catalyzed aerobic oxidation of both primary and secondary benzylic, allylic, and aliphatic alcohols to their corresponding aldehydes and ketones at room temperature using the copper(II) complex [Cu(μ‐Cl)(Cl)(phen)]₂ as the Cu(II) source is reported. The conversion of both electron‐rich and electron‐neutral benzyl alcohols is smooth and faster than those of electron‐deficient ones. The chemoselectivity of a primary benzyl alcohol over the secondary alcohol is also observed. Alcohols regarded as difficult substrates for oxidation due to their coordinating ability with transition metal catalyst such as 4‐(methylthio)benzyl alcohol and 3‐pyridinemethanol are also oxidized easily. In addition, a lignin model alcohol is oxidized to the corresponding aldehyde in excellent yield. Conversions of benzylic and allylic alcohols are faster as compared to those of aliphatic alcohols in accordance with their C_α H bond strengths. A plausible mechanism of the TEMPO‐based catalytic cycle is proposed.     

Activation of Dioxygen by Cobaloxime and Nitric Oxide for Efficient TEMPO‐Catalyzed Oxidation of Alcohols

The aerobic oxidation of alcohols to their corresponding carbonyl compounds could be efficiently accomplished by using the combination of cobalt nitrate, dimethylglyoxime and 2,2,6,6‐tetramethylpiperidine 1‐oxyl (TEMPO) as a novel catalytic system, and various alcohols including primary and secondary benzylic, allylic and aliphatic alcohols could be quantitatively converted to the corresponding aldehydes or ketones at 70 °C under 0.4 MPa dioxygen pressure in dichloromethane. During the oxidation, the in situ generated cobaloxime and nitric oxide (NO) were responsible for the activation of dioxygen, respectively, thereby, two concerted catalytic routes exist: cobaloxime‐activating‐dioxygen TEMPO‐catalyzed and NO‐activating‐dioxygen TEMPO‐catalyzed aerobic oxidation of alcohols.     

A Continuous Flow Solution to Achieving Efficient Aerobic Anti‐Markovnikov Wacker Oxidation

An aerobic anti‐Markovnikov Wacker oxidation for the flow‐synthesis of arylacetaldehydes is reported. In the process, flow chemistry techniques have provided a means to control and minimise the over‐oxidation of sensitive products. The reaction showed general applicability to various functionalised styrenes and provided a process capable of a multi‐gram scale.     

Size‐Dependent Catalytic Activity of Supported Palladium Nanoparticles for Aerobic Oxidation of Alcohols

Silica‐alumina (SiO₂‐Al₂O₃)‐supported palladium catalysts prepared by adsorption of the tetrachloropalladate anion (PdCl₄²⁻) followed by calcination and reduction with either hexanol or hydrogen were studied for the aerobic oxidation of alcohols. The mean size of the Pd particles over the SiO₂‐Al₂O₃ support was found to depend on the Si/Al ratio, and a decrease in the Si/Al ratio resulted in a decrease in the mean size of the Pd nanoparticles. By changing the Si/Al ratio, we obtained supported Pd nanoparticles with mean sizes ranging from 2.2 to 10 nm. The interaction between the Pd precursor and the support was proposed to play a key role in tuning the mean size of the Pd nanoparticles. The Pd/SiO₂‐Al₂O₃ catalyst with an appropriate mean size of Pd particles could catalyze the aerobic oxidation of various alcohols to the corresponding carbonyl compounds, and this catalyst was particularly efficient for the solvent‐free conversion of benzyl alcohol. The intrinsic turnover frequency per surface Pd atom depended significantly on the mean size of Pd particles and showed a maximum at a medium mean size (3.6–4.3 nm), revealing that the aerobic oxidation of benzyl alcohol catalyzed by the supported Pd nanoparticles was structure‐sensitive.