Aerobic Oxidation Conditions Research Articles

Innovative Application of Polyether Amine as a Recyclable Catalyst in Aerobic Thiophenol Oxidation

Polyether amines are versatile compounds characterized by a flexible structure, consisting of polyoxypropylene and polyoxyethylene as the backbone, with amine groups at each end. They have widespread applications in various industrial processes and daily life. Despite their versatility, the utilization of polyether amines as base catalysts is rare. In this study, one kind of three-arm polyether amine 1 was employed as an environmentally friendly, cost-effective catalyst for the aerobic oxidation of thiophenols, leading to the synthesis of disulfides. The oxidative coupling of thiols serves as a fundamental pathway for the production of disulfides, which are vital in both chemical and biological processes. In contrast to known methods for thiol oxidation, this polyether amine-based catalytic process eliminates the need for expensive stoichiometric oxidants and minimizes the formation of over-oxidized by-products. Using a mere 0.5 mol % of the polyether amine 1 as the catalyst, a remarkable > 96% yield was achieved for all 16 tested substrates, encompassing a diverse range of functional groups, under the catalytic aerobic oxidation conditions. Furthermore, it is noteworthy that over 90% of the polyether amine catalyst can be efficiently recovered for reuse without loss of activity, making this a sustainable and cost-effective catalytic approach.

Efficient aerobic and DMSO-oxidative sulfenylation reactions catalyzed by Na2S2O3

Two efficient acid-base-free procedures are introduced for the sulfenylation of cyclic enaminone compounds under aerobic and DMSO-oxidative conditions catalyzed by Na2S2O3. According to procedures, a mixture of an aryl thiol and a cyclic enaminone are reacting together in DMF (aerobic oxidative conditions) or DMSO in the presence of a catalytic amount of Na2S2O3 (0.1 equiv in DMSO and 0.4 equiv in DMF) to produce the related α-sulfenylated cyclic enaminone compound in high yields. Both reactions are not satisfactory in the absence of Na2S2O3, so the reaction in DMF (aerobic condition) does not produce any product at all and the conversion in DMSO progresses by less than 50% within 24 h.

Application of Iodine as a Catalyst in Aerobic Oxidations: A Sustainable Approach for Thiol Oxidations.

Iodine is a well-known oxidant that is widely used in organic syntheses. Thiol oxidation by stoichiometric iodine is one of the most commonly employed strategies for the synthesis of valuable disulfides. While recent advancements in catalytic aerobic oxidation conditions have eliminated the need for stoichiometric oxidants, concerns persist regarding the use of toxic or expensive catalysts. In this study, we discovered that iodine can be used as a cheap, low-toxicity catalyst in the aerobic oxidation of thiols. In the catalytic cycle, iodine can be regenerated via HI oxidation by O2 at 70 °C in EtOAc. This protocol harnesses sustainable oxygen as the terminal oxidant, enabling the conversion of primary and secondary thiols with remarkable efficiency. Notably, all 26 tested thiols, encompassing various sensitive functional groups, were successfully converted into their corresponding disulfides with yields ranging from >66% to 98% at a catalyst loading of 5 mol%.

Construction of Alkylidene Fluorene Scaffolds Using Pd-Catalyzed Direct Arene/Alkene Coupling Strategy

A versatile synthetic method for the construction of alkylidene fluorenes and their heteroarene derivatives has been developed successfully by means of a Pd(II)-catalyzed direct C-H/C-H coupling of o-alkenyl biaryls. Use of the Pd(OAc)2 catalyst under aerobic oxidation conditions gives rise to the corresponding alkylidene fluorenes having various functional groups and diversely fused polycyclic systems. The resulting products can serve as versatile synthetic building blocks for the construction of structurally diverse polycyclic arenes and heteroarenes.

Solvent-free aerobic photocatalytic oxidation of alcohols to aldehydes over ZnO/C3N4

Benzyl alcohol can exclusively yield 99.8% benzaldehyde over a Z-scheme heterojunction photocatalyst of ZnO/C3N4 under solvent-free aerobic photocatalytic oxidation conditions.

Selectivity Switch in the Aerobic 1,2‐Propandiol Oxidation Catalyzed by Diamine‐Stabilized Palladium Nanoparticles

AbstractPalladium nanoparticles stabilized by a sterically demanding secondary diamine ligand have been synthesized by hydrogen reduction of a palladium acetate complex bearing the corresponding diimine ligand. The obtained nanoparticles were used to catalyze the aerobic oxidation of 1,2‐propandiol in n‐hexane, and after their heterogenization onto a high surface area carbon, in water. In n‐hexane (2,4‐dimethyl‐1,3‐dioxolan‐2‐yl) methanol has been obtained as major product, whereas in water acetic acid with a selectivity of >85 % has been achieved. The selectivity switch observed was a clear induced by water. The robustness of diamine‐stabilized palladium nanoparticles under real aerobic oxidation conditions has been proved by recycling experiments, TEM measurements of the recovered catalysts and by comparison of its performance with that of palladium nanoparticles generated by the metal vapor synthesis technique and supported onto the same carbon in the absence of the stabilizing diamine ligand.

Synthesis of 2, 4-Disubstituted Quinazolines via One-Pot Three-Component Assembly

Catalyst free efficient and straightforward method is accounted for the synthesis of 2,4-disubstituted quinazolines utilizing urea was seen as a modest, nontoxic, easily biodegradable, and powerful reaction medium. This productive and straight forward method furnished the corresponding 2,4-disubstituted quinazolines in significant yields employing a one-pot three-component reaction of 2-amino benzophenones, aldehyde, and urea under aerobic oxidation conditions. Additionally, 2-methyl-4-phenylquinazolines were synthesized using ammonium acetate with readily available 2-amino benzophenones.

Copper-Catalyzed Oxidative Cleavage of the C-C Bonds of β-Alkoxy Alcohols and β-1 Compounds.

Copper-catalyzed aerobic oxidation conditions were employed to promote the C–C bond cleavage of β-alkoxy alcohols and β-1 compounds (lignin model compounds). Besides these compounds, various 1,2 and 1,3-diols were successfully converted to aldehydes. We propose the Cu(I)-catalyzed mechanism explaining the C–C cleavage of these 1,2 and 1,3-dihydroxy compounds and β-alkoxy alcohols based on XPS data. Although our reaction conditions do not include large excess of bases and elaborated ligand-modified catalysts, copper salts with/without Me-TBD show good catalytic activities for C–C bond cleavage of various lignin model compounds.

Efficient Stereoselective Carbocyclization to cis-1,4-Disubstituted Heterocycles Enabled by Dual Pd/Electron Transfer Mediator (ETM) Catalysis

An efficient Pd/ETM (ETM = electron transfer mediator)-cocatalyzed stereoselective oxidative carbocyclization of dienallenes under aerobic oxidation conditions has been developed to afford six-membered heterocycles. The use of a bifunctional cobalt complex [Co(salophen)-HQ] as hybrid ETM gave a faster aerobic oxidation than the use of separated ETMs, indicating that intramolecular electron transfer between the hydroquinone unit and the oxidized metal macrocycle occurs. In this way, a class of important cis-1,4-disubstituted six-membered heterocycles, including dihydropyran and tetrahydropyridine derivatives were obtained in high diastereoselectivity with good functional group compatibility. The experimental and computational (DFT) studies reveal that the pendent olefin does not only act as an indispensable element for the initial allene attack involving allenic C(sp3)–H bond cleavage, but it also induces a face-selective reaction of the olefin of the allylic group, leading to a highly diastereoselective formation of the product. Finally, the deuterium kinetic isotope effects measured suggest that the initial allenic C(sp3)–H bond cleavage is the rate-limiting step, which was supported by DFT calculations.

The main differences in reactivity of mono- and diunsaturated fatty acid esters during aerobic oxidation

During the aerobic oxidation of esters of both mono- and diunsaturated fatty acids and their mixtures, epoxide compounds are first forming, as well as the peroxide compounds with different structures and reactivities depending on the number of double bonds in the original molecule. The mild aerobic oxidation conditions in the column-type apparatus led to a number of features of the process. It was shown that both types of esters are consumed in independent ways, despite the fact that the whole process is radical. Therefore, this means the absence of processes of transfer of oxidation chains, as well as the chains of radical reactions themselves. Thus, we can conclude that the process under study is radical without chain transfer from one type of ester to another. Peroxides forming from monounsaturated fatty acid esters are able to participate in catalytic epoxidation by hydroperoxides, which proceeds with high selectivity. From diunsaturated fatty acid esters, at least two types of peroxides with different reactivities are formed. One of them can enter into catalytic epoxidation reaction, the other forms secondary oxidation products, which are mainly represented by the products of the Baeyer–Villiger reaction. In this contribution upon using different mono- and diunsaturated fatty acid esters, the mechanism of products formation has been investigated and discussed.

Efficient base-free oxidation of monosaccharide into sugar acid under mild conditions using hierarchical porous carbon supported gold catalysts

The highly selective synthesis of d-xylonic acid and d-gluconic acid from monosaccharide catalyzed by Au/NC-3 under base-free aerobic oxidation conditions.

TEMPO‐Mediated Synthesis of N‐(Fluoroalkyl)imidazolones via Reaction of Imidazoles with Iodofluoroacetate

AbstractWe report a TEMPO‐mediated oxidative copper‐catalyzed synthesis of N‐(fluoroalkyl)imidazolones via the radical addition of imidazoles with iodofluoroacetate. A possible key intermediate involving TEMPO was observed by ESI‐MS. We also found that aerobic oxidation conditions were effective for the transformation in the presence of a copper catalyst, enabling access to a number of N‐(fluoroalkyl)imidazolones in moderate to good yields.magnified image

Metal‐Free Synthesis of 5‐Trifluoromethyl‐1,2,4‐Triazoles from Iodine‐Mediated Annulation of Trifluoroacetimidoyl Chlorides and Hydrazones

AbstractA metal‐free approach for the synthesis of 5‐trifluoromethyl‐1,2,4‐triazoles from trifluoroacetimidoyl chlorides and hydrazones has been achieved under aerobic oxidative conditions. The reaction proceeds through a cascade base‐promoted intermolecular C−N bond formation and iodine‐mediated intramolecular C−N bond oxidative coupling sequence. The protocol features broad substrate scope and can be scaled up to gram scale.magnified image

Air‐Induced One‐Pot Synthesis of N‐Sulfonylformamidines from Sulfonyl Chlorides, NaN3, and Tertiary/Secondary Amines

An efficient and practical one‐pot strategy for the synthesis of N‐sulfonylformamidines was developed by directly using easily available sulfonyl chlorides, sodium azide and tertiary/secondary amines in the presence of 5 mol‐% CuBr2 under aerobic oxidative conditions.

Age Boundaries and Stratigraphic Importance of Microbiota of the Lower Riphean Kaltasy Formation of the Volga–Uralia Area

The association of organic-walled microfossils of the Kaltasy Formation, which includes morphologically complex large forms, is considered on the basis of previous and new materials. In general, the Kaltasy microbiota is similar to many associations of the Neoproterozoic (Late Riphean and Vendian) microfossils and, originally, was erroneously interpreted as Late Riphean, in spite of missing Late Riphean index taxa and geological evidence of its Early Riphean age. The Early Riphean age of the formation was supported by results of Re–Os, U–Pb, and other modern methods of isotope geochronology, whereas the low 87Sr/86Sr ratios and almost invariant δ13C values of its rocks are comparable with isotopic characteristics of the Mesoproterozoic sequences of >1300 Ma. The application of geochemical methods using high-reagent Fe (FeHR) and C and S isotopes has showed that relatively deep rocks of the Arlan Subformation of the Kaltasy Formation accumulated under aerobic oxidation conditions (ratio FeHR/FeT < 0.38). This contradicts a common concept of the presence of an anoxic ocean in the Mezoproterozoic, but explains the presence of morphologically complex large eukaryotic forms of microbiota, which were abundant in Neoproterozoic oxygenic basins.

Aerobic Oxidation Approaches to Indole-3-carboxylates: A Tandem Cross Coupling of Amines-Intramolecular Mannich-Oxidation Sequence.

A tandem aerobic oxidation protocol has been developed for the facile synthesis of indole-3-carboxylates. Two readily available starting materials, anilines and benzylamines, were efficiently cross-coupled under the o-naphthoquinone-catalyzed aerobic oxidation conditions to the corresponding 2-arylmethyleneaminophenylacetates that in turn smoothly underwent the Cu(II)-catalyzed intramolecular Mannich reaction. The resulting indoline derivatives were aerobically oxidized to indole-3-carboxylates, providing a ready access to indole derivatives from two simple amine derivatives.

Aerobic α,β-C(sp3)-H Bond Difunctionalization and C-N Bond Cleavage of Triethylamine: Difunctional Ammonium Iodide Enabling the Regioselective Synthesis of 4-Arylpyrimido[1,2-b]indazoles.

A novel method for the regioselective synthesis of 4-arylpyrimido[1,2-b]indazoles has been developed via the dual C(sp3)-H bond functionalization and C-N bond cleavage of triethylamine. The elusive acyclic enamine intermediates are effectively in situ generated and captured by aromatic aldehydes to form a wide array of tricyclic products from 3-aminoindazoles under the NH4I-mediated aerobic oxidative conditions. This reaction features easily available feedstock, green and economic conditions, and valuable products.

Copper/TEMPO Redox Redux: Analysis of PCET Oxidation of TEMPOH by Copper(II) and the Reaction of TEMPO with Copper(I).

Copper salts and organic aminoxyls, such as TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), are versatile catalysts for aerobic alcohol oxidation. Previous reports in the literature contain conflicting proposals concerning the redox interactions that take place between copper(I) and copper(II) salts with the aminoxyl and hydroxylamine species, TEMPO and TEMPOH, respectively. Here, we reinvestigate these reactions in an effort to resolve the conflicting claims in the literature. Under anaerobic conditions, CuIIX2 salts [X = acetate (OAc), trifluoroacetate (TFA), and triflate (OTf)] are shown to promote the rapid proton-coupled oxidation of TEMPOH to TEMPO: CuIIX2 + TEMPOH → CuIX + TEMPO + HX. In the reaction with acetate, however, slow reoxidation of CuIOAc occurs. This process requires both TEMPO and HOAc and coincides with the reduction of TEMPO to 2,2,6,6-tetramethylpiperidine. Analogous reactivity is not observed with trifluoroacetate and triflate species. Overall, the facility of the proton-coupled oxidation of TEMPOH by CuII salts suggests that this process could contribute to catalyst regeneration under aerobic oxidation conditions.

Synthesis of Multi‐substituted Dihydropyrazoles by Copper‐Mediated [4+1] Cycloaddition Reaction of N‐Sulfonylhydrazones and Sulfoxonium Ylides

AbstractA general and expeditious approach for the copper mediated synthesis of multi‐functionalized dihydropyrazoles from N‐sulfonylhydrazones and sulfoxonium ylides has been achieved under aerobic oxidative conditions. The formal [4+1] cycloaddition reaction exhibits many notable features and can be easily scaled up to gram scale.magnified image

The kinetics, catalysis and inhibition of the Baeyer—Villiger reaction in the processes of liquid-phase oxidation of organic compounds

Literary data on kinetics, catalysis and inhibition of the oxidation reaction of carbonyl compounds with peroxy acids according to the Baeyer—Villiger reaction under aerobic liquidphase oxidation conditions have been considered and discussed. The main reaction channel involves a reversible formation of α-hydroxyperoxy ester and its rearrangement to an ester or a lactone. In the case of homolytic decomposition of α-hydroxyperoxy ester no esters are formed. At all steps the formation and transformation of α-hydroxyperoxy ester are catalyzed by carboxylic acids. The possibility of formation of the second intermediate, presumably dioxirane, is shown. Catalysts of the oxidation processes such as variable-valency metal salts influence the kinetics at all steps in the Baeyer—Villiger reaction. Inhibition of ester formation in the presence of cobalt and manganese salts is associated with catalysis of homolytic decomposition of peroxy acid and α-hydroxyperoxy ester.