TEMPO-tert-Butyl Nitrite: An Efficient Catalytic System for Aerobic Oxidation of Alcohols

Aerobic Oxidation of PMB Ethers to Carboxylic Acids

The first aerobic protocol of direct transformation of p-methoxybenzyl (PMB) ethers to carboxylic acids efficiently with Fe(NO3)3 ⋅ 9H2O and TEMPO as catalysts at room temperature has been developed. The reaction accommodates C-Br bond, terminal/non-terminal C-C triple bond, amide, cyano, nitro, ester, and trifluoromethyl groups. Even highly selective oxidative deprotection of different benzylic PMB ethers has been realized. The reaction has been successfully applied to the total synthesis of natural product, (R)-6-hydroxy-7,9-octadecadiynoic acid, demonstrating the practicality of the method. Based on experimental studies, a possible mechanism involving oxygen-stabilized benzylic cation has been proposed.

A facile approach to phenothiazinones via catalytic aerobic oxidation: discovery of an antiproliferative agent

The production of bioactive pharmaceutical ingredients in a sustainable manner has become essential in the modern academic and industrial community. Herein, we report a chemically robust and sustainable aerobic oxidation for the synthesis of the phenothiazinone framework, using the commercially available TEMPO/HBF4/NaNO2 co-catalytic system under an ambient atmosphere. The reaction is highly efficient with broad scopes and excellent scalability. Preliminary activity screening led to the discovery of compound 3 as a potent antiproliferative agent. The green synthesis of a variety of sulfur containing heterocycles might encourage the pursuit of biologically valuable molecules in the medicinal field.

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.

Selective Oxidation of Cyclohexanone to Adipic Acid Using Molecular Oxygen in the Presence of Alkyl Nitrites and Transition Metals as Catalysts

This paper presents a not previously reported catalytic system consisting of transition metals Co2+ and Mn2+ and alkyl nitrites R-ONO for the oxidation of cyclohexanone with oxygen to adipic acid. The influence of type and amount of catalyst, temperature, time, and type of raw material on conversion and product composition were determined. In addition, the oxidation of selected cyclic ketones such as cyclopentanone, cyclohexanone, cyclooctanone, cyclododecanone, 2-methylcyclohexanone, 3-methylcyclohexanone, and 4-methylcyclohexanone in acetic acid as solvent was performed. The results showed that R-ONO systems, under established reaction conditions, form NO·radicals, which oxidize to NO2 under a strong oxidization reaction environment. The Co2+/Mn2+/NO2 system was shown to be highly active in the oxidation of cyclic ketones with oxygen.

Revisiting the Electrochemistry of TEMPOH Analogues in Acetonitrile

Hydroxylamines, represented by 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH), are widely involved as active species in various chemical and electrochemical oxidations. The electrochemical behavior of TEMPOH is crucial to understanding the mechanisms of TEMPO-mediated redox sequences. However, compared to abundant studies on TEMPOH electrochemistry in aqueous solutions, the sole value of its oxidation potential E_ox(TEMPOH) in organic solutions was reported to be 0.7 V (vs Fc in acetonitrile), seemingly conflicting with experimentally observed facile oxidation of TEMPOH. Herein, the electrochemistry of TEMPOH derivatives in acetonitrile was revisited, featuring much smaller oxidation potentials (about 0 V) than literature ones. Acid/base effects and kinetic studies lent credibility to these new values. Such a 0.7 V energy discrepancy impelled us to review the thermodynamic properties and oxidation mechanisms of TEMPOH deduced from the old value.

Enhanced Catalytic Activity of TEMPO-Mediated Aerobic Oxidation of Alcohols via Redox-Active Metal-Organic Framework Nodes

Metal-organic frameworks (MOFs) are outstanding platforms for heterogeneous catalysis due to their tunable pore size, huge surface area, large porosity, and potential active sites. The design and synthesis of MOF/organocatalyst co-catalytic systems have attracted considerable interest owing to their high catalytic activity, low toxicity, and mild reaction conditions. Herein, we reported the synthesis of a bifunctional TEMPO-IsoNTA organocatalyst featuring a pyridyl group as an anchoring site and a TEMPO radical as a catalytic active site. By using the topologically isomorphic structures of MIL-101(Fe) and MIL-101(Cr) as co-catalysts, these MOF/TEMPO-IsoNTA systems enable the efficient aerobic oxidation of various alcohols to their corresponding aldehydes or ketones under mild conditions. Notably, the MIL-101(Fe)/TEMPO-IsoNTA system exhibits superior catalytic activity, thanks to their redox-active Fe^III-oxo nodes, which facilitate the regeneration of TEMPO-IsoNTA. Our research not only solves the problem of potential heavy metal contamination in the TEMPO-based homogeneous catalytic system, but also enriches the understanding of synergism of MOFs/organocatalysts.

Solvent- and additive-free oxidative amidation of aldehydes using a recyclable oxoammonium salt

A range of acyl azoles have been prepared from aromatic, heteroaromatic, and aliphatic aldehydes by means of an oxidative amidation reaction. The methodology employs a substoichiometric quantity of an oxoammonium salt as the oxidant. It avoids the need for additives such as a base, is run solvent-free, and the oxoammonium salt is recyclable.

Aerobic oxidation and oxidative esterification of alcohols through cooperative catalysis under metal-free conditions

The ABNO@PMO-IL-Br material obtained by anchoring 9-azabicyclo[3.3.1]nonane-3-one N-oxyl (keto-ABNO) within the mesopores of periodic mesoporous organosilica with bridged imidazolium groups is a robust bifunctional catalyst for the metal-free aerobic oxidation of numerous primary and secondary alcohols under oxygen balloon reaction conditions. The catalyst, furthermore, can be successfully employed in the first metal-free self-esterification of primary aliphatic alcohols affording valued esters.

Photocatalytic C-H Activation with Alcohol as a Hydrogen Atom Transfer Agent in a 9-Fluorenone Based Metal-Organic Framework

Hydrogen atom transfer (HAT) has become an attractive strategy for the activation of hydrocarbon feedstocks. Alcohols, as inexpensive and efficient hydrogen transfer reagents, have limited application in C-H functionalization due to the difficulty in the alkoxy radical acquisition. 9-Fluorenone moieties were incorporated into the metal-organic framework (MOF) as a photocatalyst; through the formation of hydrogen bonds between the carbonyl group of a ligand and alcohol, alkoxy radicals could be obtained by the visible-light-driven oxidation of 2,2,2-trichloroethanol via proton-coupled electron transfer (PCET). Effectively photocatalyzed intermolecular coupling reactions between phenyl vinyl sulfone and aldehyde or cyclic ether were realized through the HAT pathway. Compared to homogeneous catalysts, the heterogeneous MOF photocatalyst improved the catalytic efficiency and could be recycled at least five times. The microenvironment of the Zn-OFDC channel was beneficial for the formation of hydrogen bonds and stability of alkoxy radicals.

SBA-15 Supported 1-Methyl-2-azaadamanane N-Oxyl (1-Me-AZADO) as Recyclable Catalyst for Oxidation of Alcohol

Herein, we designed and synthesized an SBA-15 supported 1-methyl-2-azaadamanane N-oxyl (1-Me-AZADO) and investigated its catalytic performance for selective oxidation of alcohols under Anelli's conditions. The first example of immobilization of 1-Me-AZADO was very important to advance the oxgenation effectively because this supported N-oxyl has excellent catalytic activity for oxidation of alcohols to carbonyl compounds, and more importantly, it can be conveniently recovered and reused at least 6 times without significant effect on its catalytic efficiency.

Synthesis and crystal structures of nickel(ii) and palladium(ii) complexes with o-carboranyl amidine ligands

A number of new nido-carboranyl amidines 10-R(CH₂)_nNHC(Et)[double bond, length as m-dash]HN-7,8-C₂B₉H₁₁ (n = 2, R = OH, OMe, and NMe₂; n = 3, R = OH) were synthesized by the nucleophilic addition of amino alcohols and N,N-dimethylethylenediamine to the highly activated -C[triple bond, length as m-dash]N⁺- triple bond of the 10-propionitrilium derivative of nido-carborane. A similar reaction of 10-EtC[triple bond, length as m-dash]N-7,8-C₂B₉H₁₁ with ethylenediamine unexpectedly resulted in the cleavage of the C[triple bond, length as m-dash]N bond to form the ammonium derivative 10-H₃N-7,8-C₂B₉H₁₁. The complexation of the synthesized carboranyl amidines 10-MeO(CH₂)₂NHC(Et)[double bond, length as m-dash]HN-7,8-C₂B₉H₁₁ and 10-Me₂N(CH₂)₂NHC(Et)[double bond, length as m-dash]HN-7,8-C₂B₉H₁₁ with nickel and palladium phosphine complexes [(Ph₃P)₂MCl₂] (M = Ni, Pd) was studied. The reactions with 10-MeO(CH₂)₂NHC(Et)[double bond, length as m-dash]HN-7,8-C₂B₉H₁₁ result in half-sandwiched metallacarborane complexes with the retention of one triphenylphosphine ligand [3-Ph₃P-3-(8-MeOCH₂CH₂N[double bond, length as m-dash]C(Et)NH)-3,1,2-MC₂B₉H₁₀], while the reactions with 10-Me₂N(CH₂)₂NHC(Et)[double bond, length as m-dash]HN-7,8-C₂B₉H₁₁ proceed with the complete loss of the phosphine ligands and lead to the formation of complexes with the η⁵:κ²(N,N')-coordinated carboranyl amidine ligand [3,3-(8-Me₂NCH₂CH₂N[double bond, length as m-dash]C(Et)NH)-3,1,2-MC₂B₉H₁₀]. The crystal molecular structures of the synthesized complexes were determined by single crystal X-ray diffraction.

Synthesis of TEMPO radical decorated hollow porous aromatic frameworks for selective oxidation of alcohols

A bottom-up approach was developed to prepare TEMPO radical decorated hollow aromatic frameworks (HPAF-TEMPO) by using TEMPO radical functionalized monomers and SiO2 nanospheres as templates. The accessible inner layer, high density of TEMPO sites, and hybrid micro-/mesopores of the HPAF-TEMPO enable the aerobic oxidation of a broad range of alcohols with high efficiency and excellent selectivity.

Metal–organic framework/carbon nitride nanosheets composites (TMU-49/CNNSs): efficient photocatalyst for aerobic oxidation of alcohols under visible light

The TMU-49/CNNSs composite was used as a photocatalyst for oxidation of alcohols.

Sequential Connection of Mutually Exclusive Catalytic Reactions by a Method Controlling the Presence of an MOF Catalyst: One-Pot Oxidation of Alcohols to Carboxylic Acids

A functionalized metal-organic framework (MOF) catalyst applied to the sequential one-pot oxidation of alcohols to carboxylic acids controls the presence of a heterogeneous catalyst. The conversion of alcohols to aldehydes was acquired through aerobic oxidation using a well-known amino-oxy radical-functionalized MOF. In the same flask, a simple filtration of the radical MOF with mild heating of the solution completely altered the reaction media, providing radical scavenger-free conditions suitable for the autoxidation of the aldehydes formed in the first step to carboxylic acids. The mutually exclusive radical-catalyzed aerobic oxidation (the first step with MOF) and radical-inhibited autoxidation (the second step without MOF) are sequentially achieved in a one-pot manner. Overall, we demonstrate a powerful and efficient method for the sequential oxidation of alcohols to carboxylic acids by employing a readily functionalizable heterogeneous MOF. In addition, our MOF in-and-out method can be utilized in an environmentally friendly way for the oxidation of alcohols to carboxylic acids of industrial and economic value with broad functional group tolerance, including 2,5-furandicarboxylic acid and 1,4-benzenedicarboxylic acid, with good yield and reusability. Furthermore, MOF-TEMPO, as an antioxidative stabilizer, prevents the undesired oxidation of aldehydes, and the perfect "recoverability" of such a reactive MOF requires a re-evaluation of the advantages of MOFs from heterogeneity in catalytic and related applications.

Transition-Metal-Free Oxidative Cross-Coupling of Tetraarylborates to Biaryls Using Organic Oxidants

Readily prepared tetraarylborates undergo selective (cross)-coupling through oxidation with Bobbitt's salt to give symmetric and unsymmetric biaryls. The organic oxoammonium salt can be used either as a stoichiometric oxidant or as a catalyst in combination with in situ generated NO2 and molecular oxygen as the terminal oxidant. For selected cases, oxidative coupling is also possible with NO2 /O2 without any additional nitroxide-based cocatalyst. Transition-metal-free catalytic oxidative ligand cross-coupling of tetraarylborates is unprecedented and the introduced method provides access to various biaryl and heterobiaryl systems.

Flavin Nitroalkane Oxidase Mimics Compatibility with NOx/TEMPO Catalysis: Aerobic Oxidization of Alcohols, Diols, and Ethers

Biomimetic flavin organocatalysts oxidize nitromethane to formaldehyde and NO_x-providing a relatively nontoxic, noncaustic, and inexpensive source for catalytic NO₂ for aerobic TEMPO oxidations of alcohols, diols, and ethers. Alcohols were oxidized to aldehydes or ketones, cyclic ethers to esters, and terminal diols to lactones. In situ trapping of NO_x and formaldehyde suggest an oxidative Nef process reminiscent of flavoprotein nitroalkane oxidase reactivity, which is achieved by relatively stable 1,10-bridged flavins. The metal-free flavin/NO_x/TEMPO catalytic cycles are uniquely compatible, especially compared to other Nef and NO_x-generating processes, and reveal selectivity over flavin-catalyzed sulfoxide formation. Aliphatic ethers were oxidized by this method, as demonstrated by the conversion of (-)-ambroxide to (+)-sclareolide.

Dual-fixations of europium cations and TEMPO species on metal-organic frameworks for the aerobic oxidation of alcohols

The efficient and selective aerobic oxidation of alcohols has been investigated with judicious combinations of europium-incorporated and/or TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl)-functionalized zirconium-based porous metal-organic frameworks (MOFs). Although MOFs are well-known catalytic platforms for the aerobic oxidation with radical-functionalities and metal nanoparticles, these systematic approaches involving metal cations and/or radical species introduce numerous interesting aspects for cooperation between metals and TEMPO for the aerobic oxidation of alcohols. The role of TEMPO as the oxidant in the heterogeneous catalytic aerobic oxidation of alcohols was revealed through a series of comparisons between metal-anchored, TEMPO-anchored, and metal and TEMPO-anchored MOF catalysis. The fine tunability of the MOF allowed the homogeneously and doubly functionalized catalysts to undergo organic reactions in the heterogeneous media. In addition, the well-defined and carefully designed heterogeneous molecular catalysts displayed reusability along with better catalytic performance than the homogeneous systems using identical coordinating ligands. The role of metal-cation fixation should be carefully revised to control their coordination and maximize their catalytic activity. Lastly, the metal cation-fixed MOF displayed better substrate tolerance and reaction efficiencies than the TEMPO-anchored MOF or mixture MOF systems.

Selective construction of alkaloid scaffolds by alcohol-based direct and mild aerobic oxidative Pictet–Spengler reactions

Tetrahydro-β-carboline and β-carboline alkaloid scaffolds can be selectively obtained by direct aerobic oxidative Pictet–Spengler reactions of tryptamines with alcohols using TBN/TEMPO as the catalysts and oxygen as the oxidant under mild conditions.

UV light promoted 'Metal'/'Additive'-free oxidation of alcohols: investigating the role of alcohols as electron donors

UV light promoted selective oxidation of primary and secondary alcohols has been demonstrated under ‘metal-free’ and ‘additive-free’ conditions. Under the optimized conditions, a variety of aromatic, heteroaromatic, and alicyclic alcohols have been examined for their transformations to the corresponding carbonyl compounds. The mechanistic studies emphasize the important role of substrate (alcohol) and solvent (DMSO) in the generation of superoxide radical which is a vital intermediate for the transformation. This study also highlights the role of air as the oxidant in the oxidation process. Further, the practical application of the strategy has also been demonstrated for the oxidation of the alcoholic moiety in cholesterol.

The present study demonstrates the important role of alcohols themselves as electron donors for their oxidative transformations to the corresponding carbonyl compounds in the absence of any metal/oxidant and external photosensitizer.

Zr-Metal-Organic Frameworks Featuring TEMPO Radicals: Synergistic Effect between TEMPO and Hydrophilic Zr-Node Defects Boosting Aerobic Oxidation of Alcohols

Metal-organic frameworks (MOFs) featuring multiple catalytic units are excellent platforms for heterogeneous catalysis. However, the synergism between multiple catalytic units for catalysis is far from being well understood. Herein, we reported the synthesis of a robust 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) radical-functionalized Zr-MOF (UiO-68-TEMPO) in the form of single-crystalline and microsized crystals with varied missing linker defects. Detailed catalytic studies and theoretical calculations reveal that the synergistic effect between the TEMPO radicals and hydrophilic and defective Zr-nodes endows UiO-68-TEMPO with superior catalytic activity toward aerobic oxidation of alcohols. Our work not only offers a new route to design and synthesize highly effective MOF catalysts but also provides insights into the synergism between multiple catalytic sites.

Application of dialkyl azodicarboxylate frameworks featuring multi-functional properties

The application of dialkyl azodicarboxylates as versatile reagents in Mitsunobu, oxidation, electrophilic, amination and carbonylation reactions is reviewed.

Solvent-controlled chemoselective N-dealkylation-N-nitrosation or C-nitration of N-alkyl anilines with tert-butyl nitrite

A metal-free, acid-free and chemoselective N-dealkylation-N-nitrosation or C-nitration of N-alkyl anilines has been developed.

Electrochemical Performance of ABNO for Oxidation of Secondary Alcohols in Acetonitrile Solution

The ketones was successfully prepared from secondary alcohols using 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) as the catalyst and 2,6-lutidine as the base in acetonitrile solution. The electrochemical activity of ABNO for oxidation of 1-phenylethanol was investigated by cyclic voltammetry, in situ Fourier transform infrared spectroscopy (FTIR) and constant current electrolysis experiments. The resulting cyclic voltammetry indicated that ABNO exhibited much higher electrochemical activity when compared with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) under the similar conditions. A reasonable reaction mechanism of the electrocatalytic oxidation of 1-phenylethanol to acetophenone was proposed. In addition, a series of secondary alcohols could be converted to the corresponding ketones at room temperature in 80⁻95% isolated yields.

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.

Molecular design of stable diarylnitroxides

The review is devoted to diarylnitroxides, which constitute an important type of organic radicals. These compounds are much less investigated than their alkyl counterparts. Meanwhile, they are of great interest, since they provide extensive opportunities for targeted structural modification and control of electronic properties of a molecule for a particular practical application. The existing trends of molecular design of stable diarylnitroxides and general approaches to their synthesis are discussed. Structural details and spin density distribution in diarylnitroxides are considered. Factors determining the stability of both the diarylnitroxide radicals and their oxidized and reduced forms and the redox properties of this class of organic radicals are addressed.

The bibliography includes 128 references.

Tetramethylpiperidine N-Oxyl (TEMPO), Phthalimide N-Oxyl (PINO), and Related N-Oxyl Species: Electrochemical Properties and Their Use in Electrocatalytic Reactions

N-Oxyl compounds represent a diverse group of reagents that find widespread use as catalysts for the selective oxidation of organic molecules in both laboratory and industrial applications. While turnover of N-oxyl catalysts in oxidation reactions may be accomplished with a variety of stoichiometric oxidants, N-oxyl reagents have also been extensively used as catalysts under electrochemical conditions in the absence of chemical oxidants. Several classes of N-oxyl compounds undergo facile redox reactions at electrode surfaces, enabling them to mediate a wide range of electrosynthetic reactions. Electrochemical studies also provide insights into the structural properties and mechanisms of chemical and electrochemical catalysis by N-oxyl compounds. This review provides a comprehensive survey of the electrochemical properties and electrocatalytic applications of aminoxyls, imidoxyls, and related reagents, of which the two prototypical and widely used examples are 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) and phthalimide N-oxyl (PINO).

TEMPO-Appended Metal-Organic Frameworks as Highly Active, Selective, and Reusable Catalysts for Mild Aerobic Oxidation of Alcohols

Metal-organic frameworks (MOFs) decorated with stable organic radicals are highly promising materials for redox catalysis. Unfortunately however, the synthesis of chemically robust MOFs typically requires harsh solvothermal conditions, which are not compatible with organic radicals. Here, we describe the synthesis of two isoreticular families of stable, mixed component, zirconium MOFs with UiO-66 and UiO-67 structures and controlled amounts of covalently attached TEMPO radicals. The materials were obtained using a relatively low-temperature, HCl-modulated de novo method developed by Hupp and Farha and shown to contain large amounts of missing cluster defects, forming nanodomains of the reo phase with 8-connected clusters. In the extreme case of homoleptic UiO-67-TEMPO(100%), the material exists as an almost pure reo phase. Large voids due to missing clusters and linkers allowed these materials to accommodate up to 2 times more of bulky TEMPO substituents than theoretically predicted for the idealized structures and proved to be beneficial for catalytic activity. The TEMPO-appended MOFs were shown to be highly active and recyclable catalysts for selective aerobic oxidation of a broad range of primary and secondary alcohols under exceptionally mild conditions (room temperature, atmospheric pressure of air). The influence of various parameters, including the pore size and TEMPO content, on the catalytic activity was also comprehensively investigated.