SiliaCat TEMPO: An Effective and Useful Oxidizing Catalyst

Reproducible Green Syntheses Using Hybrid Sol-Gel Catalysts

Referring to selected examples of reproducible green syntheses using hybrid sol-gel catalysts of the SiliaCat series from different doctoral theses and research works published between 2015 and early 2024, this study briefly illustrates how said catalysts have been applied in a number of green synthetic methods of significant industrial relevance. This shows evidence that the nanochemistry "bottom-up" sol-gel approach based on catalytic species entrapped in organically modified silicas as effective and versatile heterogeneous catalysts developed between the late 1990s and 2010 has succeeded. Subsequent developments will show how the use of said materials in automated syntheses, supplying data to machine learning algorithms actually leads to faster and cheaper optimization of the reaction conditions. Said progress ultimately will further accelerate industrial uptake of heterogeneous catalysis under flow in the fine chemical industry whose reluctance to change processes was due to the need to replace financially amortized (and expensive) production plants.

Recent Advancements in Continuous-Flow Suzuki-Miyaura Coupling Utilizing Immobilized Molecular Palladium Complexes

Immobilized Pd-catalyzed Suzuki-Miyaura coupling under continuous-flow conditions using a packed-bed reactor, representing an efficient, automated, practical, and safe technology compared to conventional batch-type reactions. The core objective of this study is the development of an active and durable catalyst. In contrast to supported Pd nanoparticles, the attachment of Pd complexes onto solid supports through well-defined coordination sites is considered a favorable approach for preparing highly dispersed and stabilized Pd species. These species can be directly employed in various flow reactions without the need for pre-treatment. This concept paper explores recent achievements involving the application of immobilized Pd complexes as precatalysts for continuous-flow Suzuki-Miyaura coupling. Our focus is to elucidate the significance of the designed catalyst structures in relation to their catalytic performance under flow conditions. Additionally, we highlight various reaction systems and catalyst packing methods, emphasizing their crucial roles in establishing a practical synthesis process.

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.

Efficient covalent capping of carbon and gold with TEMPO for catalysis and spin writing

Covalent immobilization of TEMPO at carbon and gold via an aliphatic -(CH₂)₆- linker was achieved via cathodic grafting of a diamagnetic precursor, tetramethylpiperidine, with subsequent >NH to >NO oxidation to give TEMPO-capped paramagnetic interfaces (Γ_TEMPO = 5.2 × 10^-10 mol cm^-2); catalytic and spin switching potential of the thus prepared systems was demonstrated.

Enhanced polysaccharide nanofibers via oxidation over SiliaCat TEMPO

Drawing on independent work carried out by academic and industrial researchers using the immobilized TEMPO catalyst SiliaCat TEMPO, in this study we show how shifting the carboxylation process mediated by TEMPO in solution to a process mediated by the above-mentioned hybrid sol-gel catalyst allows the synthesis of insoluble polysaccharide nanofibers of superior quality, eliminating waste. This will dramatically reduce the polysaccharide nanofiber production costs opening the route to large-scale production and uptake of these versatile nanofibers in a variety of functional products where their use has been limited by high cost. The results of this study will be useful for catalysis and biotechnology researchers as well as for chemistry educators teaching green chemistry, nanochemistry, and catalysis using the outcomes of recent research.

One-Pot Method of Synthesizing TEMPO-Oxidized Bacterial Cellulose Nanofibers Using Immobilized TEMPO for Skincare Applications

In the skincare field, water-dispersed bacterial cellulose nanofibers synthesized via an oxidation reaction using 2,2,6,6–tetramethyl–1–piperidine–N–oxy radical (TEMPO) as a catalyst are promising bio-based polymers for engineered green materials because of their unique properties when applied to the surface of the skin, such as a high tensile strength, high water-holding capacity, and ability to block harmful substances. However, the conventional method of synthesizing TEMPO-oxidized bacterial cellulose nanofibers (TOCNs) is difficult to scale due to limitations in the centrifuge equipment when treating large amounts of reactant. To address this, we propose a one-pot TOCN synthesis method involving TEMPO immobilized on silica beads that employs simple filtration instead of centrifugation after the oxidation reaction. A comparison of the structural and physical properties of the TOCNs obtained via the proposed and conventional methods found similar properties in each. Therefore, it is anticipated that due to its simplicity, efficiency, and ease of use, the proposed one-pot synthesis method will be employed in production scenarios to prepare production quantities of bio-based polymer nanofibers in various potential industrial applications in the fields of skincare and biomedical research.

Superparamagnetic core–shell radical polymer brush as efficient catalyst for oxidation of alcohols to aldehydes and ketones

The conversion of catalytic oxidation of alcohols to aldehydes and ketones is more than 99% using a nitroxide polymer brush grafted on superparamagnetic nanoparticles as a catalyst. The catalyst can be easily recovered and reused.

Towards waste free organic synthesis using nanostructured hybrid silicas

As catalysis and organic synthesis come together again, the need for stable, selective and truly heterogeneous solid catalysts for clean and efficient synthetic organic chemistry has increased. Hybrid silica glasses obtained by the sol-gel nanochemistry approach can be successfully used for the waste-free synthesis of valued chemicals in various applications. This success derives from the deliberate chemical design of hybrid nanostructures capable of immobilizing and stabilizing organocatalytic species and unstable metal nanoparticles. The highly selective activity along with a broad scope and ease of application of these mesoporous materials to high-throughput reactions opens the route to faster, cleaner and more convenient processes for both small and large scale manufacturing of useful molecules.

Copper(i)/ketoABNO catalysed aerobic alcohol oxidation

A copper(i)/ketoABNO aerobic catalyst system is highly effective for the oxidation of secondary alcohols, including unactivated aliphatic substrates. The effects of pressure and gas composition on catalyst performance are examined. The radical can be employed at low loadings and it is also amenable to immobilisation on to solid supports.

Nitroxides: applications in synthesis and in polymer chemistry

This Review describes the application of nitroxides to synthesis and polymer chemistry. The synthesis and physical properties of nitroxides are discussed first. The largest section focuses on their application as stoichiometric and catalytic oxidants in organic synthesis. The oxidation of alcohols and carbanions, as well as oxidative C-C bond-forming reactions are presented along with other typical oxidative transformations. A section is also dedicated to the extensive use of nitroxides as trapping reagents for C-centered radicals in radical chemistry. Alkoxyamines derived from nitroxides are shown to be highly useful precursors of C-centered radicals in synthesis and also in polymer chemistry. The last section discusses the basics of nitroxide-mediated radical polymerization (NMP) and also highlights new developments in the synthesis of complex polymer architectures.