Aerobic oxidation catalysis with stable radicals

A Cooperative Effect in a Novel Bimetallic Mo-V Nanocomplex Catalyzed Selective Aerobic C-H Oxidation

In this study, a new heterobimetallic Mo(VI)-V(V) organosilicon Schiff base complex has been prepared and characterized by different techniques, such as FTIR, Raman, MS, ICP-AES, TGA, and XPS. The bimetallic nanocomplex, revealed by TEM images, showed high oxidation stability and desired activity in the aerobic oxidation of a structurally diverse set of benzylic alcohols in ethanol as a safe solvent. Further, oxidation of benzylic hydrocarbons successfully occurred, producing the target compounds in high yields and excellent selectivities. Our results demonstrated a cooperative effect between Mo(VI) and V(V) as redox active sites in an organosilicon Schiff base framework. A facile and practical reusability of the solid catalyst at the end of the reaction was observed.

Oxidative addition of verdazyl halogenides to Pd(PPh3)4

A novel approach to the preparation of stable Pd-substituted verdazyls was developed through the direct oxidative addition of iodoverdazyls to Pd(PPh₃)₄.

[FeII(L•)2][TCNQF4•−]2: A Redox-Active Double Radical Salt

The reaction of [FeII(L•)2][BF4]2 with LiTCNQF4 results in the formation of [FeII(L•)2][TCNQF4•−]2·2CH3CN (1) (L• is the neutral aminoxyl radical ligand 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide; TCNQF4 is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane). Single-crystal X-ray diffraction; Raman, Fourier-transform infrared (FTIR) and ultraviolet–visible spectroscopies; and electrochemical studies are all consistent with the presence of a low-spin FeII ion, the neutral radical form (L•) of the ligand, and the radical anion TCNQF4•−. 1 is largely diamagnetic and the electrochemistry shows five well-resolved, diffusion-controlled, reversible one-electron processes.

Ruthenium carbonyl complexes supported by pyridine-alkoxide ligands: synthesis, structure and catalytic oxidation of secondary alcohols

Five novel trinuclear ruthenium carbonyl complexes were synthesized and their application in oxidation of secondary alcohols was also researched.

The Use of Molecular Oxygen for Liquid Phase Aerobic Oxidations in Continuous Flow

Molecular oxygen (O₂) is the ultimate “green” oxidant for organic synthesis. There has been recent intensive research within the synthetic community to develop new selective liquid phase aerobic oxidation methodologies as a response to the necessity to reduce the environmental impact of chemical synthesis and manufacture. Green and sustainable chemical processes rely not only on effective chemistry but also on the implementation of reactor technologies that enhance reaction performance and overall safety. Continuous flow reactors have facilitated safer and more efficient utilization of O₂, whilst enabling protocols to be scalable. In this article, we discuss recent advancements in the utilization of continuous processing for aerobic oxidations. The translation of aerobic oxidation from batch protocols to continuous flow processes, including process intensification (high T/p), is examined. The use of “synthetic air”, typically consisting of less than 10% O₂ in N₂, is compared to pure O₂ (100% O₂) as an oxidant source in terms of process efficiency and safety. Examples of homogeneous catalysis and heterogeneous (packed bed) catalysis are provided. The application of flow photoreactors for the in situ formation of singlet oxygen (¹O₂) for use in organic reactions, as well as the implementation of membrane technologies, green solvents and recent reactor solutions for handling O₂ are covered.

2-Azaadamantane N-oxyl (AZADO)/Cu Catalysis Enables Chemoselective Aerobic Oxidation of Alcohols Containing Electron-Rich Divalent Sulfur Functionalities

The chemoselective oxidation of alcohols containing electron-rich sulfur functionalities (e.g., 1,3-dithianes and sulfides) into their corresponding carbonyl compounds with the sulfur groups can sometimes be a demanding task in modern organic chemistry. A reliable method for this transformation, which features azaadamantane-type nitroxyl radical/copper catalysis using ambient air as the terminal oxidant is reported. The superiority of the developed method was demonstrated by comparing it with various conventional alcohol oxidation methods.

Oxidation Potential Tunable Organic Molecules and Their Catalytic Application to Aerobic Dehydrogenation of Tetrahydroquinolines

In this work, oxidation potential tunable organic molecules, alkyl 2-phenyl hydrazocarboxylates, were disclosed. The exquisite tuning of their oxidation potentials facilitated a catalytic dehydrogenation of 1,2,3,4-tetrahydroquinolines in the presence of Mn(Pc) and O₂.

The Hydrazine-O2 Redox Couple as a Platform for Organocatalytic Oxidation: Benzo[c]cinnoline-Catalyzed Oxidation of Alkyl Halides to Aldehydes

An organocatalytic oxidation platform that capitalizes on the capacity of hydrazines to undergo rapid autoxidation to diazenes is described. Commercially available benzo[c]cinnoline is shown to catalyze the oxidation of alkyl halides to aldehydes in a novel mechanistic paradigm involving nucleophilic attack, prototropic shift, and hydrolysis. The hydrolysis and reoxidation events occur readily with only adventitious oxygen and water. A survey of the scope of viable substrates is shown along with mechanistic and computational studies that give insight into this mode of catalysis.

CuCl/TMEDA/nor-AZADO-catalyzed aerobic oxidative acylation of amides with alcohols to produce imides

Although aerobic oxidative acylation of amides with alcohols would be a good complement to classical synthetic methods for imides (e.g., acylation of amides with activated forms of carboxylic acids), to date, there have been no reports on oxidative acylation to produce imides. In this study, we successfully developed, for the first time, an efficient method for the synthesis of imides through aerobic oxidative acylation of amides with alcohols by employing a CuCl/TMEDA/nor-AZADO catalyst system (TMEDA = teramethylethylendiamine; nor-AZADO = 9-azanoradamantane N-oxyl). The proposed acylation proceeds through the following sequential reactions: aerobic oxidation of alcohols to aldehydes, nucleophilic addition of amides to the aldehydes to form hemiamidal intermediates, and aerobic oxidation of the hemiamidal intermediates to give the corresponding imides. This catalytic system utilizes O2 as the terminal oxidant and produces water as the sole by-product. An important point for realizing this efficient acylation system is the utilization of a TMEDA ligand, which, to the best of our knowledge, has not been employed in previously reported Cu/ligand/N-oxyl systems. Based on experimental evidence, we consider that plausible roles of TMEDA involve the promotion of both hemiamidal oxidation and regeneration of an active CuII-OH species from a CuI species. Here promotion of hemiamidal oxidation is particularly important. Employing the proposed system, various types of structurally diverse imides could be synthesized from various combinations of alcohols and amides, and gram-scale acylation was also successful. In addition, the proposed system was further applicable to the synthesis of α-ketocarbonyl compounds (i.e., α-ketoimides, α-ketoamides, and α-ketoesters) from 1,2-diols and nucleophiles (i.e., amides, amines, and alcohols).

The Catalytic Reaction of Nitroxides with Peroxyl Radicals and Its Relevance to Their Cytoprotective Properties

Sterically-hindered nitroxides such as 2,2,6,6-tetramethylpiperidin- N-oxyl (TEMPO) have long been ascribed antioxidant activity that is thought to underlie their chemopreventive and anti-aging properties. However, the most commonly invoked reactions in this context-combination with an alkyl radical to give a redox inactive alkoxyamine or catalysis of superoxide dismutation-are unlikely to be relevant under (most) physiological conditions. Herein, we characterize the kinetics and mechanisms of the reactions of TEMPO, as well as an N-arylnitroxide and an N, N-diarylnitroxide, with alkylperoxyl radicals, the propagating species in lipid peroxidation. In each of aqueous solution and lipid bilayers, they are found to be significantly more reactive than Vitamin E, Nature's premier radical-trapping antioxidant (RTA). Inhibited autoxidations of THF in aqueous buffers reveal that nitroxides reduce peroxyl radicals by electron transfer with rate constants ( k ≈ 10⁶ to >10⁷ M^-1 s^-1) that correlate with the standard potentials of the nitroxides ( E° ≈ 0.75-0.95 V vs NHE) and that this activity is catalytic in nitroxide. Regeneration of the nitroxide occurs by a two-step process involving hydride transfer from the substrate to the nitroxide-derived oxoammonium ion followed by H-atom transfer from the resultant hydroxylamine to a peroxyl radical. This reactivity extends from aqueous solution to phosphatidylcholine liposomes, where added NADPH can be used as a hydride donor to promote nitroxide recycling, as well as to cell culture, where the nitroxides are shown to be potent inhibitors of lipid peroxidation-associated cell death (ferroptosis). These insights have enabled the identification of the most potent nitroxide RTA and anti-ferroptotic agent yet described: phenoxazine- N-oxyl.

CoCl2-promoted TEMPO oxidative homocoupling of indoles: access to tryptanthrin derivatives

The first example of one-step synthesis of tryptanthrin derivatives using indoles as the only substratesviadirect C–H transformation.

Effect of heteroatom and functionality substitution on the oxidation potential of cyclic nitroxide radicals: role of electrostatics in electrochemistry

Electrostatic effects on electrochemical oxidation potentials of heteroatomic and functional substituted nitroxides were studied both experimentally and computationally.

Structural Effects on the pH-Dependent Redox Properties of Organic Nitroxyls: Pourbaix Diagrams for TEMPO, ABNO, and Three TEMPO Analogs

Electrochemical studies of the reduction and oxidation reactions of five different organic nitroxyls have been performed across a wide pH range (0-13). The resulting Pourbaix diagrams illustrate structural effects on their various redox potentials and on the p K_a values of the corresponding hydroxylamine and hydroxylammonium ions. Evidence is also given for the reversible formation of a hydroxylamine N-oxide when nitroxyls are oxidized in alkaline media. Structural effects on the thermodynamics of this reaction are assessed.

Unprecedented Concomitant Formation of Cu2O-CD Nano-Superstructures During the Aerobic Oxidation of Alcohols and Their Catalytic Use in the Propargylamination Reaction: A Simultaneous Catalysis and Metal Waste Valorization (SCMWV) Method

Copper-cyclodextrins (CDs)-catalyzed aerobic oxidation of alcohols under aqueous conditions and a concomitant formation of Cu₂O-cyclodextrin nano-superstructures (Cu₂O-CD nps) during the reaction are reported. The use of affordable copper and cyclodextrin combination for aerobic oxidation precluding organic solvents makes it a benign methodology. Intriguingly, a diverse array of Cu₂O-CD nps with unique morphologies was obtained by varying copper salts, cyclodextrins, and bases. The nano-superstructures were characterized by different techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, differential scanning calorimetry-thermogravimetric analysis, scanning electron microscopy, time of flight secondary-ion mass spectrometry, and transmission electron microscopy to confer their authenticity. Interestingly, the nano-superstructures showed promising catalytic efficiency for a one-pot three-component propargylamination reaction. The used particles were found to be recoverable and recyclable for propargylamination for up to three cycles, with no loss of catalytic activity. Moreover, the concomitant formation of Cu₂O-CD nanostructures and their self-segregation during an aerobic oxidation reaction under homogenous conditions is a first-of-its-kind method depicting simultaneous catalysis and metal waste valorization (SCMWV). Overall, this new approach of reaping the benefits of homogenous metal catalysis and simultaneously sequestrating the metal into a high-value product might pave the way to develop many such SCMWV protocols in future.

Mechanically induced oxidation of alcohols to aldehydes and ketones in ambient air: Revisiting TEMPO-assisted oxidations

The present work addresses the development of an eco-friendly and cost-efficient protocol for the oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones by mechanical processing under air. Ball milling was shown to promote the quantitative conversion of a broad set of alcohols into carbonyl compounds with no trace of an over-oxidation to carboxylic acids. The mechanochemical reaction exhibited higher yields and rates than the classical, homogeneous, TEMPO-based oxidation.