Selective Oxidation Of Amines Research Articles

Carbon Quantum Dots: Synthesis, Structure, Properties, and Catalytic Applications for Organic Synthesis

Carbon quantum dots (CQDs), also known as carbon dots (CDs), are novel zero-dimensional fluorescent carbon-based nanomaterials. CQDs have attracted enormous attention around the world because of their excellent optical properties as well as water solubility, biocompatibility, low toxicity, eco-friendliness, and simple synthesis routes. CQDs have numerous applications in bioimaging, biosensing, chemical sensing, nanomedicine, solar cells, drug delivery, and light-emitting diodes. In this review paper, the structure of CQDs, their physical and chemical properties, their synthesis approach, and their application as a catalyst in the synthesis of multisubstituted 4H pyran, in azide-alkyne cycloadditions, in the degradation of levofloxacin, in the selective oxidation of alcohols to aldehydes, in the removal of Rhodamine B, as H-bond catalysis in Aldol condensations, in cyclohexane oxidation, in intrinsic peroxidase-mimetic enzyme activity, in the selective oxidation of amines and alcohols, and in the ring opening of epoxides are discussed. Finally, we also discuss the future challenges in this research field. We hope this review paper will open a new channel for the application of CQDs as a catalyst in organic synthesis.

Unraveling the amine oxidative coupling activity of hierarchical porous Fe–N4–O1 single-atom catalysts: oxygen atom-mediated dual reaction pathway

Fe–NC with Fe–N4–O1 active sites exhibits splendid catalytic activity toward the selective oxidation of amines. A possible mechanism combining the oxidative pathway and ROS pathway in the presence of Fe–NC is proposed.

Visible-Light-Induced Selective Oxidation of Amines into Imines over UiO-66-NH2@Au@COF Core–Shell Photocatalysts

Visible-Light-Induced Selective Oxidation of Amines into Imines over UiO-66-NH₂@Au@COF Core–Shell Photocatalysts

Exerting charge transfer to stabilize Au nanoclusters for enhanced photocatalytic performance toward selective oxidation of amines

Atomically precise gold nanoclusters have received increasing attention in photocatalysis but suffered from self-oxidation coalescence induced by photo-generated holes. Herein, we report an efficient charge engineering strategy to improve the photo-oxidation activity and stability of Au25(PPh3)10(SC3H6Si(OC2H5)3)5Cl2 nanocluster (AuNC) taking advantage of Z-scheme AuNC/ultrathin BiOCl nanosheets (2D-BiOCl) heterojunction for visible light oxidative self-coupling of amines. The turnover frequency (TOF) over AuNC/2D-BiOCl for benzylamine can reach 1916 h―1, much higher than that of most previously reported gold catalysts with a TOF usually less than 500 h―1. Moreover, the heterojunction can be stabilized for six cycles with the intact structure of AuNC remained. The enhanced photocatalytic activity of AuNC/2D-BiOCl was predominantly attributed to the formation of Z-scheme heterojunction, which effectively accelerated the transfer and separation of photo-generated carriers. Especially, the consumption of holes in AuNC by injected electrons from 2D-BiOCl can inhibit the self-oxidation of AuNC, thus improve its stability. This study provides some insights into the design of metal nanocluster catalysts with Z-scheme charge transfer for the application of photocatalysis.

Two-dimensional/two-dimensional Z-scheme photocatalyst of graphitic carbon nitride/bismuth vanadate for visible-light-driven photocatalytic synthesis of imines

As an environmentally friendly strategy, photocatalytic synthesis of imines via selective oxidation of amines has inspired numerous interests. In comparison to conventional type-II heterojunction, the photogenerated electrons and holes on a Z-scheme photocatalyst has strong redox ability. Herein, we prepared a two-dimensional/two-dimensional (2D/2D) Z-scheme photocatalyst of graphitic carbon nitride nanosheet/bismuth vanadate (CNNS/BVO) with a large two-phase interface, which is favorable for the transfer and separation of photoexcited charge carriers. For selective oxidation of benzylamine to N-benzylidenebenzylamine under oxygen atmosphere and visible light irradiation, the CNNS/BVO showed much higher activity than the single CNNS and BVO. The optimized nanocomposite photocatalyst with a 50% CNNS/BVO mass ratio presented a high conversion (~96%) and selectivity (~99%) in 6 h oxidative coupling reaction of benzylamine. More importantly, this 2D/2D CNNS/BVO photocatalyst has good cycling stability, as well as universality for different benzylamine derivatives. Based on the experimental results, a Z-scheme mechanism was determined, where photoinduced electrons were accumulated on the conduction band (CB) of CNNS that is higher than that of BVO and holes were collected on the valence band (VB) of BVO that is lower than that of CNNS. Further, superoxide radicals and photogenerated holes are examined to the mainly active species for selective oxidation of benzylamine, and a possible reaction pathway was therefore proposed. This study demonstrates that the Z-scheme 2D/2D CNNS/BVO with high redox ability is an efficient photocatalyst for the imine synthesis.

Synthesis of Imines by Selective Oxidation of Amines on alloy nanoparticles of palladium and gold under visible light irradiation at ambient temperatures

Imines are important intermediates in organic synthesis. To avoid derivatives, the research on direct oxidized amines as an alternative method is imminent. The gold (Au)–palladium (Pd) alloy nanoparticles (NPs) catalyst shows a favourable activity and high selectivity on benzylamine oxidation reaction under light irradiation. The yield of the reaction on the alloy NPs with a Au:Pd ratio of 1:1.86 in the dark was 36% at 45 °C, while the yield increased to 95% under light illumination. The study demonstrates different properties from alloy NP to pure Au NP and Pd NP catalysts which may be caused by increased surface charge heterogeneity in the alloy system. The conversion rate of the reaction can also be enhanced by light irradiation and heat. The light energy which utilized by Localized Surface Plasmon Resonance effect of gold in alloy system can highly enhanced the efficiency of reaction. These findings provide useful guidelines for designing efficient alloy catalysts with a plasmonic metal and a catalytically active transition metal for various organic syntheses driven by sunlight.

One-pot selective synthesis of azoxy compounds and imines via the photoredox reaction of nitroaromatic compounds and amines in water

A facile one-pot two-stage photochemical synthesis of aromatic azoxy compounds and imines has been developed by coupling the selective reduction of nitroaromatic compounds with the selective oxidation of amines in an aqueous solution. In the first stage (light illumination, Ar atmosphere), the light excited nitroaromatic molecule abstract H from amine to form ArNO2H and amine radical, which then form nitrosoaromatic, hydroxylamine and imine compounds. Water acts as a green solvent for the dispersion of the reactants and facilitates the formation of nitrosoaromatic and hydroxylamine intermediate compounds. In the second stage (no light, air atmosphere), the condensation of nitrosoaromatic and hydroxylamine compounds yields aromatic azoxy product with the aid of molecular oxygen in air. This photochemical synthesis achieved both high conversion and high product selectivity (>99%) at room temperature.

N-hydroxyphthalimide-TiO2 complex visible light photocatalysis

TiO2 is the most established semiconductor photocatalyst. The prominence of TiO2 is becoming increasingly obvious because its interfacial redox reactions have implication on a wide range processes such as energy conversion and environmental remediation. Herein, we exploited the surface complex created by the interaction between organic molecules with binding sites and accommodating surface of TiO2 for visible light-driven selective aerobic oxidation reactions. A novel surface complex formed between N-hydroxyphthalimide (NHPI) and TiO2 was discovered. The NHPI-TiO2 complex turned out to be an outstanding visible light photocatalyst and was successfully used in the selective oxidation of amines into imines with atmosphere O2 under blue LED irradiation. The stability of the NHPI-TiO2 complex was preserved by 3 mol% of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) acting as a cooperative catalyst. Moreover, selectivities for the imine products were also promoted by TEMPO. Superoxide anion radical (O2−) were evidenced to be the primary reactive oxygen species (ROS) to execute the oxidative conversions. This work suggests that TiO2 surface complexes can be robust photocatalysts for visible light-driven selective aerobic reactions, provided that an appropriate cooperative redox catalyst exists to channel the photocatalytic electron transfer.

Selective oxidation of amines using O2 catalyzed by cobalt thioporphyrazine under visible light

A highly efficient oxidation of amines including primary and secondary amines into imines using O2 as the oxidant was realized by cobalt tetra(2-hydroxymethyl-1,4-dithiin)porphyrazine (CoPz(hmdtn)4) under irradiation of visible light (λ ≥ 420 nm). Particularly, the photocatalytic selective oxidation of benzylamine into N-benzylidenebenzylamine was thoroughly studied in order to get more insight into the reaction kinetics and mechanism for the oxidation of amine. The photocatalytic oxidation of benzylamine was observed to follow pseudo-first-order reaction kinetics. Moreover, a reasonable linear relationship between the log(kX/kH) values and the Brown-Okamoto constant (σ+) parameters was obtained for oxidation of para- and meta-substituted benzylamines. Besides imine, the presence of other intermediate products such as NH-imine and H2O2 was further evidenced in this photocatalytic system. Additionally, the reactive oxygen species (ROSs) generated in this photocatalytic process were confirmed by electron spin resonance (ESR) technique. The possible photocatalytic transformation pathway of amine into imine was also proposed, involving the dehydrogenation of amine to form NH-imine and the nucleophilic addition of NH-imine by free amine to afford the final imine.

ChemInform Abstract: 3‐Methyl‐4‐oxa‐5‐azahomoadamantane as an Organocatalyst for the Aerobic Oxidation of Primary Amines to Oximes in Water.

AbstractThis green and selective oxidation of amines gives E‐type oximes in high yields.

Visible‐Light‐Induced Selective Photocatalytic Aerobic Oxidation of Amines into Imines on TiO2

Imines are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and agricultural chemicals. Selective oxidation of amines into their corresponding imines with dioxygen is one of the most-fundamental chemical transformations. Herein, we report the oxidation of a series of benzylic amines into their corresponding imines with atmospheric dioxygen as the oxidant on a surface of anatase TiO(2) under visible-light irradiation (λ>420 nm). The visible-light response of this system was caused by the formation of a surface complex through the adsorption of a benzylic amine onto the surface of TiO(2). From the analysis of products of specially designed benzylic amines, we demonstrated that a highly selective oxygenation reaction proceeds via an oxygen-transfer mechanism to afford the corresponding carbonyl compound, whose further condensation with an amine would generate the final imine product. We found that when primary benzylic amines (13 examples), were chosen as the substrates, moderate to excellent selectivities for the imine products were achieved (ca. 38-94%) in moderate to excellent conversion rates (ca. 44-95%). When secondary benzylic amines (15 examples) were chosen as the substrates, both the corresponding imines and aldehydes were detected as the main products with moderate to high conversion rates (ca. 18-100%) and lower selectivities for the imine products (ca. 14-69%). When tribenzylamine was chosen as the substrate, imine (27%), dibenzylamine (24%), and benzaldehyde products (39%) were obtained in a conversion of 50%. This report can be viewed as a prototypical system for the activation of C-H bonds adjacent to heteroatoms such as N, O, and S atoms, and oxofuctionalization with air or dioxygen as the terminal oxidant under visible-light irradiation using TiO(2) as the photocatalyst.

Graphite‐Supported Gold Nanoparticles as Efficient Catalyst for Aerobic Oxidation of Benzylic Amines to Imines and N‐Substituted 1,2,3,4‐Tetrahydroisoquinolines to Amides: Synthetic Applications and Mechanistic Study

Selective oxidation of amines using oxygen as terminal oxidant is an important area in green chemistry. In this work, we describe the use of graphite-supported gold nanoparticles (AuNPs/C) to catalyze aerobic oxidation of cyclic and acyclic benzylic amines to the corresponding imines with moderate-to-excellent substrate conversions (43-100%) and product yields (66-99%) (19 examples). Oxidation of N-substituted 1,2,3,4-tetrahydroisoquinolines in the presence of aqueous NaHCO3 solution gave the corresponding amides in good yields (83-93%) with high selectivity (up to amide/enamide=93:4) (6 examples). The same protocol can be applied to the synthesis of benzimidazoles from the reaction of o-phenylenediamines with benzaldehydes under aerobic conditions (8 examples). By simple centrifugation, AuNPs/C can be recovered and reused for ten consecutive runs for the oxidation of dibenzylamine to N-benzylidene(phenyl)methanamine without significant loss of catalytic activity and selectivity. This protocol "AuNPs/C+O2" can be scaled to the gram scale, and 8.9 g (84 % isolated yield) of 3,4-dihydroisoquinoline can be obtained from the oxidation of 10 g 1,2,3,4-tetrahydroisoquinoline in a one-pot reaction. Based on the results of kinetic studies, radical traps experiment, and Hammett plot, a mechanism involving the hydrogen-transfer reaction from amine to metal and oxidation of M-H is proposed.

Selective Oxidation of Amines and Sulfides

AbstractFor Abstract see ChemInform Abstract in Full Text.