Fullerene as a Platform for Recyclable TEMPO Organocatalysts for the Oxidation of Alcohols

Synthesis of Nitronyl Nitroxide Radical-Modified Multi-Walled Carbon Nanotubes and Oxidative Desulfurization in Fuel

Novel and highly stable nitronyl nitroxide radical (NIT) derivatives were synthesized and coated on the surface of multi-walled carbon nanotubes (MWCNTs) to improve their desulfurization performance. They were characterized by FTIR, UV-vis, SEM, XRD, Raman spectroscopy and ESR. Thiophene in fuel was desulfurized by molecular O2, and the oxidation activity of these compounds was evaluated. At a normal temperature and pressure, the degradation rates of thiophene by four compounds in 4 h can reach 92.66%, 96.38%, 93.25% and 89.49%, respectively. The MWCNTs/NIT-F have a high special activity for the degradation of thiophene, and their desulfurization activity can be recycled for five times without a significant reduction. The mechanistic studies of MWCNTs/NIT composites show that the ammonium oxide ion is the key active intermediate in catalytic oxidative desulfurization, which provides a new choice for fuel oxidative desulfurization. The results show that NIT significantly improves the photocatalytic performance of MWCNTs.

Fabricating whole pine needles biomass with phenylhydrazine-4-sulphonic acid for effective removal of cationic dyes and heavy metal ions from wastewater

We applied a holistic, sustainable, and green approach to develop an effective multipurpose adsorbent from whole pine needles (PNs), a forest waste lignocellulosic biomass. The PNs were oxidized and modified with phenylhydrazine-4-sulphonic acid (ɸHSO3H) to OPN-ɸHSO3H. The latter was characterized and tested as an adsorbent for cationic dyes, malachite green (MG), methylene blue (MB), crystal violet (CV), and metal ions (Hg2⁺ and Pb2⁺). The adsorption followed different kinetic models: Elovich for MG and MB, pseudo-second-order for CV, and pseudo-first-order for Hg2⁺ and Pb2⁺. Langmuir isotherm indicated maximum adsorption capacities of 303.4 ± 8.91 mgg-1 (MG), 331.4 ± 17.50 mgg-1 (MB), 376.6 ± 22.47 mgg-1 (CV), 210.8 ± 28.86 mgg-1 (Hg2⁺), and 172.9 ± 20.93 mgg-1 (Pb2⁺) within 30 min. Maximum removal efficiencies were 99.0% (MG), 98.0% (MB), 96.04% (CV), 95.5% (Hg2⁺), and 89.8% (Pb2⁺). The adsorbent demonstrated significant regeneration and reusability over ten cycles, proving highly efficient for both cationic dyes and metal ions, with wide potential for practical applications where more than one adsorbate is present.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Silver Nanoparticles Densely Grafted with Nitroxides as a Recyclable Green Catalyst in the Selective Oxidation of Alcohols

The selective oxidation of alcohols, leading to appropriate aldehydes, is widely recognised as one of the most important reactions in organic synthesis. With ever-increasing environmental concerns, much attention has been directed toward developing catalytic protocols that use molecular oxygen as an oxidant. An ideal green oxidation process should employ a highly active, selective and recyclable catalyst that can work with oxygen under mild conditions. This paper presents a successful application of densely grafted silver nanostructures with stable nitroxide radicals (N-AgNPs) as an effective, easily-recovered and regenerable catalyst for the selective oxidation of alcohols. The fabricated ultra-small and narrow dispersive silver nanoparticles have been fully characterised using physicochemical methods (TEM, DLS, XPS, TGA). N-AgNPs have been successfully applied to oxidise several model alcohols: benzyl alcohol, 4-pyridinemethanol, furfuryl alcohol, 1-phenyl ethanol, n-heptanol and allyl alcohol under mild conditions using oxygen as a stoichiometric oxidant. Notably, the fabricated nitroxide grafted silver nanoparticles (N-AgNPs) were reused more than ten times in the oxidation of a series of primary alcohols to corresponding aldehydes under mild conditions with very high yields and a selectivity close to 100%.

Tethered Blatter Radical for Molecular Grafting: Synthesis of 6-Hydroxyhexyloxy, Hydroxymethyl, and Bis(hydroxymethyl) Derivatives and Their Functionalization

Synthetic access to 7-CF₃-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl radicals containing 4-(6-hydroxyhexyloxy)phenyl, 4-hydroxymethylphenyl or 3,5-bis(hydroxymethyl)phenyl groups at the C(3) position and their conversion to tosylates and phosphates are described. The tosylates were used to obtain disulfides and an azide with good yields. The Blatter radical containing the azido group underwent a copper(I)-catalyzed azide-alkyne cycloaddition with phenylacetylene under mild conditions, giving the [1,2,3]triazole product in 84% yield. This indicates the suitability of the azido derivative for grafting Blatter radical onto other molecular objects via the CuAAC "click" reaction. The presented derivatives are promising for accessing surfaces and macromolecules spin-labeled with the Blatter radical.

Rational Functionalization Towards Redox-Active TEMPO Stable Free-Radical-Hydrochar Composites

Although both stable free organic radicals and biomass-derived hydrochars have emerged as appealing, green, multifunctional materials, their association has not been explored. In this study, strength is found to lie in their union, which primarily leads to stable redox-active free-radical-hydrochar composites that can generate unexpected opportunities for the development of advanced metal-free sustainable materials. The composites are obtained by a straightforward green one-pot hydrothermal procedure. The loading of stable free radicals of nitroxide type and their localization is engineered by the nature of the carbohydrate and the reaction status; vigorous reaction parameters promote faster nucleation and growth kinetics of the hydrochar products, leading to a covalent immobilization of redox species on the surface of the carbonaceous microspherical aggregates. The nitroxide free-radical-hydrochar materials demonstrate enhancements in terms of both electrocatalytic activity and capacitive features.

Magnetic field controlled C60-TEMPO catalyst for the oxidation of alcohols

TEMPO functionalized C₆₀ fullerene phosphonic acid was grafted onto the surface of magnetic IONPs. The resulting catalyst was employed in oxidation of various primary and secondary alcohols to their corresponding aldehyde or ketone analogues.

Nitroxide supported on nanometric metal oxides as new hybrid catalysts for selective sugar oxidation

A new series of supported organocatalysts, prepared by a simple method, were used for selective sugar oxidation. This approach is based on the immobilization of a nitroxide derivative through a carboxylic function on nanometric metal oxides (TiO₂, Al₂O₃ and CeO₂), allowing the recovery of the catalyst. These hybrid materials were carefully characterized by Diffuse Reflectance FT-IR spectroscopy (DRIFT), ThermoGravimetric Analysis (TGA), X-Ray Diffraction (XRD), Brunauer-Emmet-Teller surface area measurements (B.E.T.), elemental and electrochemical analyses, showing different characteristics and behaviors depending on the nature of the metal oxide used. The activity of the supported nitroxide catalyst was evaluated on methyl α-d-glucoside oxidation, used as model reaction. In all cases, high catalytic activity was highlighted, with up to 25 times less nitroxyl radical required for complete conversion than under homogeneous conditions. The influence of several experimental conditions such as the use of phosphate buffer and recyclability of the catalyst were also investigated.

Surface modification using TEMPO and its derivatives

This article provides an overview of the methods for surface modification based on the use of stable radicals: 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its derivatives. Two approaches are discussed. The first relies on the immobilization of TEMPO moieties on the surface of various materials including silicon wafers, silica particles, organic polymers as well as diverse nanomaterials. Applications of such materials with spin labeled surface/interface, in (electro)catalysis, synthesis of novel hybrid nanostructures and nanocomposites as well as in designing of organic magnets and novel energy storage devices are also included in the discussion. The second approach utilizes TEMPO and its derivatives for the grafting of polymer chains and polymer brushes formation on flat and nanostructure surfaces via Nitroxide Mediated Radical Polymerization (NMRP). The influence of such polymer modification on surface/interface physicochemical properties is also presented.

TEMPO functionalized C60 fullerene deposited on gold surface for catalytic oxidation of selected alcohols

C₆₀TEMPO₁₀ catalytic system linked to a microspherical gold support through a covalent S-Au bond was developed. The C₆₀TEMPO₁₀@Au composite catalyst had a particle size of 0.5-0.8 μm and was covered with the fullerenes derivative of 2.3 nm diameter bearing ten nitroxyl groups; the organic film showed up to 50 nm thickness. The catalytic composite allowed for the oxidation under mild conditions of various primary and secondary alcohols to the corresponding aldehyde and ketone analogues with efficiencies as high as 79-98%, thus giving values typical for homogeneous catalysis, while retaining at the same time all the advantages of heterogeneous catalysis, e.g., easy separation by filtration from the reaction mixture. The catalytic activity of the resulting system was studied by means of high pressure liquid chromatography. A redox mechanism was proposed for the process. In the catalytic cycle of the oxidation process, the TEMPO moiety was continuously regenerated in situ with an applied primary oxidant, for example, O₂/Fe³⁺ system. The new intermediate composite components and the final catalyst were characterized by various spectroscopic methods and thermogravimetry. Graphical abstractᅟ.

Synthesis, characterization and the paramagnetic properties of bottle-brush copolymers with shielding TEMPO radicals

Bottle-brush copolymers PHEMA-g-[(PAA-g-TEMPO)-b-PMA] and PHEMA-g-(PAA-g-TEMPO) with shielding TEMPO radicals were synthesized. The paramagnetic properties in bulk and solution were studied, and the results confirmed that the presence of the outer segment played an important role on the EPR spectra of copolymers.