Simplified Free‐Radical Reaction Kinetics for p‐Xylene Oxidation to Terephthalic Acid

The kinetic models based on complex free‐radical mechanisms always involve lots of parameters, which result in model overparameterization. In this work, on the basis of free‐radical reaction mechanisms, a simplified kinetics for liquid‐phase catalytic oxidation of p‐xylene (PX) to terephthalic acid (TPA) was developed. By assuming that different peroxy radicals have equivalent reactivity, all the initiation rate constants are identical, and the differences in the rates of termination between various peroxy radicals are neglected, the kinetic model is simplified to include only six parameters that are to be determined by experiment. The kinetic model established in this paper was shown to have satisfactory precision in predicting the concentration profiles. The kinetic model proposed is even simpler than the first ‐ order kinetic model because the rate constants concerning chain propagation and termination are independent of temperature within the range investigated.     

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.     

Synthesis of 2,5‐Diformylfuran and Furan‐2,5‐Dicarboxylic Acid by Catalytic Air‐Oxidation of 5‐Hydroxymethylfurfural. Unexpectedly Selective Aerobic Oxidation of Benzyl Alcohol to Benzaldehyde with Metal=Bromide Catalysts

The alcohol group of hydroxymethylfurfural (compound 1, HMF) is preferentially oxidized by dioxygen and metal/bromide catalysts [Co/Mn/Br, Co/Mn/Zr/Br; Co/Mn=Br/(Co+Mn) = 1.0 mol/mol] to form the dialdehyde, 2,5‐diformylfuran (compound 2, DFF) in 57% isolated yield. HMF can be also oxidized, via a network of identified intermediates, to the highly insoluble 2,5‐furandicarboxylic acid (compound 5, FDA) in 60% yield. For comparison, benzyl alcohol gives benzaldehyde in 80% using the same catalyst system. Over‐oxidation (to CO₂) of HMF is much higher than that of the benzyl alcohol but can be greatly reduced by increasing catalyst concentration.     

Oxidation of p‐Cresol to p‐Hydroxybenzaldehyde with Molecular Oxygen in the Presence of CuMn‐Oxide Heterogeneous Catalyst

A high‐yield synthesis of p‐hydroxybenzaldehyde from p‐cresol and molecular oxygen was achieved over a CuMn‐oxide supported carbon catalyst. The reaction parameters such as pressure, stirring speed, reaction temperature, solvent, and the amount of sodium hydroxide in the reaction media were optimized. As a result, a high conversion of p‐cresol (99%) and a high selectivity to p‐hydroxybenzaldehyde (96%) were realized at the same time. Catalyst separation and recycling tests clearly showed that the reaction proceeded on the heterogeneous catalyst but not on dissolved species.     

Preparation of Hydroperoxides by N‐Hydroxyphthalimide‐Catalyzed Aerobic Oxidation of Alkylbenzenes and Hydroaromatic Compounds and Its Application

An efficient approach to phenols and aldehydes through the formation of hydroperoxides from alkylbenzenes was successfully achieved by aerobic oxidation using N‐hydroxyphthalimide (NHPI) as a catalyst. The oxidation of various alkylbenzenes with dioxygen by NHPI followed by treatment with a Lewis acid or triphenylphosphine led to phenols or aldehydes, respectively, in good yields. For example, the aerobic oxidation of cumene in the presence of a catalytic amount of NHPI at 75 °C and subsequent treatment with H₂SO₄ gave phenol in 77% yield. 1,4‐Dihydroxybenzene (61%) and 4‐isopropylphenol (33%) were obtained from 1,4‐diisopropylbenzene. On the other hand, dibenzyl ether was converted into phenol or benzaldehyde upon treatment of the resulting hydroperoxide with InCl₃ or PPh₃, respectively.     

Adamantane Selective Hydroxylation by 2,6‐Dichloropyridine N‐Oxide and Organoruthenium(II) Polyoxometalates as Catalyst Precursors

Organoruthenium polyoxometalates with general formula [{Ru(C₆Me₆)}₃M₅O₁₈], M  Mo, W, serve as catalyst precursors, together with 2,6‐dichloropyridine N‐oxide, to effect the hydroxylation of adamantane with conversion up to 94%, and C³‐H/C²‐H selectivity >100. Under analogous conditions, hydroxylation of cis‐decalin occurred with complete stereoretention. Control experiments and kinetic evidence suggest the in‐situ formation of a high valent Ru‐oxo species as the competent oxidant.     

Copper‐Catalyzed Aerobic Oxidation of Amines to Imines under Neat Conditions with Low Catalyst Loading

The copper (I)‐catalyzed direct synthesis of imines from amines under mild aerobic conditions is described. The method is applicable for the synthesis of various imines from corresponding amines such as benzylic, aliphatic, cyclic secondary, heteroaromatic species and the oxidative condensation of benzylamines with anilines extends the scope of the CuCl catalytic system. Noteworthy, solvent‐free procedure, air as a benign oxidant, and the cheap and easy availability of the catalyst are the vital advantages of the method.     

Metal‐Free: An Efficient and Selective Catalytic Aerobic Oxidation of Hydrocarbons with Oxime and N‐Hydroxyphthalimide

A non‐metal catalytic system consisting of dimethylglyoxime (DMG) and N‐hydroxyphthalimide (NHPI) for the selective oxidation of hydrocarbons with dioxygen is described. The synergistic effect of DMG and NHPI ensures its efficient catalytic ability: 82.1% conversion of ethylbenzene with 94.9% selectivity for acetophenone could be obtained at 80 °C under 0.3 MPa of dioxygen in 10 h. Several hydrocarbons were efficiently oxidized to their corresponding oxygenated products under mild conditions.     

Cyanide as a Powerful Catalyst for Facile Preparation of 2‐Substituted Benzoxazoles via Aerobic Oxidation

A cyanide‐catalyzed synthesis of 2‐substituted benzoxazoles from Schiff bases via aerobic oxidation has been developed. The products from various Schiff bases were obtained in high yields in an open flask under ambient conditions without other external oxidants. We have also developed a simple one‐step protocol for the synthesis of benzoxazoles from aminophenol and the corresponding aldehydes in the presence of cyanide without isolation of imine intermediates.     

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.

     

Manganese‐Catalyzed ortho‐C−H Alkenylation of Aromatic N−H Imidates with Alkynes: Versatile Access to Mono‐Alkenylated Aromatic Nitriles

So far, the direct C−H alkenylation of aromatic nitriles with alkynes has not been achieved. Herein, we discribe the first manganese‐catalyzed C−H alkenylation of aromatic N−H imidates to access mono‐alkenylated aromatic nitriles. The reaction is accelerated by the presence of a catalytic amount of sodium pivalate. This protocol is also highlighted by the simple catalytic system, good compatibility of functional groups, and excellent mono‐/dialkenylation selectivity as well as E/Z stereoselectivity.

     

The Unusual Characteristics of the Aerobic Oxidation of 3,4‐Dimethoxytoluene with Metal/Bromide Catalysts

DMtoluene (3,4‐dimethoxytoluene; DM=3,4‐dimethoxy), a model compound for lignin oxidation, can be autoxidized in acetic acid using a Co/Mn/Br catalyst to its benzaldehyde in 51 mol % yield, and to its acid in 85–92 mol % yield. This synthesis is unusual because a number of unprecedented phenomena occur. 1) The rate of molecular oxygen reacting with the substrate is bi‐phasic, i.e., two maximum in the rate of reaction are observed. 2) In the first phase, all of the 3,4‐dimethoxytoluene is converted to intermediates, but very little to the carboxylic acid. 3) During the second maximum of activity, virtually all the intermediates are converted to the carboxylic acid with 95–100% mass accountability. 4) The rate of carbon monoxide and carbon dioxide formation is considerably higher during the second phase during which 5) 9–12% of methyl 3,4‐dimethoxybenzoate (the methyl ester of the benzoic acid) is formed. Mechanisms are suggested for these unusual phenomena.     

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.     

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.     

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 Molybdenum: An Efficient Heterogeneous Catalyst for the Aerobic Oxidation of Alcohols to Carbonyl Compounds

A nanocrystalline magnesium oxide‐stabilized molybdenum(VI) complex catalyzed the oxidation of primary and secondary alcohols to carbonyl compounds in excellent yields using molecular oxygen as stoichiometric oxidant. The nanomaterials with their three‐dimensional structure and defined size and shape act as suitable supports for metal complexes. The catalyst can be reused for four runs without any significant loss of activity.     

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.     

Oxidation of Primary Amines to N‐Monoalkylhydroxylamines using Sodium Tungstate and Hydrogen Peroxide‐Urea Complex

The sodium tungstate‐catalyzed (10 mol %) oxidation of primary amines with a urea‐hydrogen peroxide complex (UHP) gives the corresponding N‐monoalkylhydroxylamines, which are important biologically active compounds, in good to excellent yields. The method is applicable for a wide range of primary amines, including chiral benzylic amines, α‐1,2‐hydroxylamine and α‐amino esters.