Sunlight Induced Oxidative Photoactivation ofN-Hydroxyphthalimide Mediated by Naphthalene Imides

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.

Metal-Catalyzed, Photo-Assisted Selective Transformation of Tertiary Alkylbenzenes and Polystyrenes into Carbonyl Compounds

Every year, thousands of tons of polystyrene are produced and discarded, filling landfills and polluting the marine environment. Although several degradation alternatives have been proposed, the need for an effective procedure for the chemical recycling of polystyrene still remains. Here, a vanadium-catalyzed reaction, assisted by visible light, promoted the direct, selective conversion of tertiary alkylbenzenes into acetophenone and other ketone derivatives. Likewise, standard polystyrene samples as well as polystyrenes from insulation and packaging waste could be chemically recycled into acetophenone in a scalable way regardless of their molecular weight, polydispersity, or form. Preliminary mechanistic investigations revealed the participation of singlet oxygen, superoxide, and hydroxyl radical species in this homogenously catalyzed process. Acetophenone could be used as an additive to accelerate the reaction and to increase the yields in some cases.

Selective Aerobic Oxidation of P-Methoxytoluene by Co(II)-Promoted NHPI Incorporated into Cross-Linked Copolymer Structure

A wide series of copolymer materials with various contents of 4-vinyl-diisopropyl-phtalate ester (10–90 mol%), divinylbenzene (1–11 mol%) and styrene, as monomers, were obtained by radical copolymerization. In the last steps of the synthesis, diisopropyl ester functionalities were converted into the form of N-hydroxyphthalimide (NHPI) rings. The obtained materials with the NHPI groups immobilized in the copolymer structure were studied by various physicochemical techniques, including FT-IR, UV-Vis-DR, XPS, elemental analysis, and tested as catalysts in aerobic oxidation of p-methoxytoluene in the presence of Co(II) acetate co-catalyst. Conversion of the aromatic substrate was correlated with the NHPI content and cross-linking degree. The best catalytic performance (conversions higher than 23%) was achieved for the copolymer catalysts containing 60% and 30% of 4-vinyl-diisopropyl-phtalate ester. At too high concentrations of NHPI and DVB, some of the NHPI groups were transformed into inactive (C=O)-N=O species or not available due to embedding inside the copolymer structure. The mechanism of the process involving both NHPI centers, forming phthalimide N-oxyl (PINO) radicals, and Co(II) cations was discussed. Stability of the developed catalysts was also tested. The opening of imide rings took place during the catalytic process, resulting in the formation of carboxyl groups and the release of hydroxylamine molecules. The deactivated catalyst could be easily regenerated by repeating two last steps of closing imide ring.

Chemiluminescence-promoted oxidation of alkyl enol ethers by NHPI under mild conditions and in the dark

The hydroperoxidation of alkyl enol ethers using N-hydroxyphthalimide and molecular oxygen occurred in the absence of catalyst, initiator, or light. The reaction proceeds through a radical mechanism that is initiated by N-hydroxyphthalimide-promoted autoxidation of the enol ether substrate. The resulting dioxetane products decompose in a chemiluminescent reaction that allows for photochemical activation of N-hydroxyphthalimide in the absence of other light sources.

Strain-Promoted Oxidation of Methylenecyclopropane Derivatives using N-Hydroxyphthalimide and Molecular Oxygen in the Dark

The hydroperoxidation of alkylidenecyclopropanes and other strained alkenes using an N-hydroxylamine and molecular oxygen occurred in the absence of catalyst, initiator, or light. The oxidation reaction proceeds through a radical pathway that is initiated by autoxidation of the alkene substrate. The hydroperoxides were converted to their corresponding alcohols and ketones under mild conditions.

Catalytic Aerobic Oxidation of C(sp3 )-H Bonds

Oxidation reactions are a key technology to transform hydrocarbons from petroleum feedstock into chemicals of a higher oxidation state, allowing further chemical transformations. These bulk-scale oxidation processes usually employ molecular oxygen as the terminal oxidant as at this scale it is typically the only economically viable oxidant. The produced commodity chemicals possess limited functionality and usually show a high degree of symmetry thereby avoiding selectivity issues. In sharp contrast, in the production of fine chemicals preference is still given to classical oxidants. Considering the strive for greener production processes, the use of O₂ , the most abundant and greenest oxidant, is a logical choice. Given the rich functionality and complexity of fine chemicals, achieving regio/chemoselectivity is a major challenge. This review presents an overview of the most important catalytic systems recently described for aerobic oxidation, and the current insight in their reaction mechanism.

Thiocyanate radical mediated dehydration of aldoximes with visible light and air

This work describes the first example of dehydration of aldoximes by an in situ generated radical-based hydrogen abstraction reaction.

Cellulose: To depolymerize… or not to?

Oxidation of the primary OH groups in cellulose is a pivotal reaction both at lab and industrial scale, leading to the value-added products, i.e. oxidized cellulose which have tremendous applications in medicine, pharmacy and hi-tech industry. Moreover, the introduction of carboxyl moieties creates prerequisites for further cellulose functionalization through covalent attachment or electrostatic interactions, being an essential achievement designed to boost the area of cellulose-based nanomaterials fabrication. Various methods for the cellulose oxidation have been developed in the course of time, aiming the selective conversion of the OH groups. These methods use: nitrogen dioxide in chloroform, alkali metal nitrites and nitrates, strong acids alone or in combination with permanganates or sodium nitrite, ozone, and sodium periodate or lead (IV) tetraacetate. In the case of the last two reagents, cellulose dialdehydes derivatives are formed, which are further oxidized by sodium chlorite or hydrogen peroxide to form dicarboxyl groups. A major improvement in the cellulose oxidation was represented by the introduction of the stable nitroxyl radicals, such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). However, a major impediment for the researchers working in this area is related with the severe depolymerisation occurred during the TEMPO-mediated conversion of CH₂OH into COOH groups. On the other hand, the cellulose depolymerisation represent the key step, in the general effort of searching for alternative strategies to develop new renewable, carbon-neutral energy sources. In this connection, exploiting the biomass feed stocks to produce biofuel and other low molecular organic compounds, involves a high amount of research to improve the overall reaction conditions, limit the energy consumption, and to use benign reagents. This work is therefore focused on the parallelism between these two apparently antagonist processes involving cellulose, building a necessary bridge between them, thinking how the reported drawbacks of the TEMPO-mediated oxidation of cellulose are heading towards to the biomass valorisation, presenting why the apparently undesired side reactions could be turned into beneficial processes if they are correlated with the existing achievements of particular significance in the field of cellulose conversion into small organic compounds, aiming the general goal of pursuing for alternatives to replace the petroleum-based products in human life.

N-Hydroxyphthalimide catalysts as bioactive pro-oxidants

N-Hydroxyphthalimide organocatalysts bearing lipophilic moieties exhibit a cytotoxic action by promoting oxidative stress in cells.

Functionalization of Cyclodextrins with N-Hydroxyphthalimide Moiety: A New Class of Supramolecular Pro-Oxidant Organocatalysts

N-hydroxyphthalimide (NHPI) is an organocatalyst for free-radical processes able to promote the aerobic oxidation of a wide range of organic substrates. In particular, NHPI can catalyze the hydroperoxidation of polyunsaturated fatty acids (PUFA). This property could be of interest for biological applications. This work reports the synthesis of two β-cyclodextrin derivatives (CD5 and CD6) having a different degree of methylation and bearing a NHPI moiety. These compounds, having different solubility in water, have been successfully tested for the hydroperoxidation of methyl linoleate, chosen as the PUFA model molecule.

Aerobic oxidation of alkylaromatics using a lipophilic N-hydroxyphthalimide: overcoming the industrial limit of catalyst solubility

4,4'-(4,4'-Isopropylidenediphenoxy)bis(N-hydroxyphthalimide), which is a new lipophilic analogue of N-hydroxyphthalimide, can act as an effective catalyst in the aerobic oxidation of alkylaromatics under reduced amounts of polar cosolvent. The catalyst was selected on the basis of an in-depth study of the influence that substituents on the aromatic ring of N-hydroxyphthalimide exert on determining the NO-H bond dissociation energy (BDE). BDE values for a range of model molecules are calculated by DFT and measured by EPR spectroscopy. The new catalyst can be successfully employed in the aerobic oxidation of cumene, ethylbenzene, and cyclohexylbenzene, affording, in all cases, good conversions and high selectivity for the corresponding hydroperoxide. The effect of solvent, catalyst, and temperature has also been investigated.

Theoretical design of multi-nitroxyl organocatalysts with enhanced reactivity for aerobic oxidation

Higher catalytic performances of N,N',N''-trihydroxyisocyanuric acid (THICA), N,N-dihydroxypyromellitimide (NDHPI), and N-hydroxynaphthalimide (NHNI) than that of N-hydroxyphthalimide (NHPI) have been demonstrated recently in aerobic oxidation. Herein, the rational design of reactive multi-nitroxyl organocatalysts has been addressed theoretically by using systematic analysis of some important properties and catalytic activities of yet-to-be-synthesized catalysts. Our results show that 1) NHNI and its analogue, similar to THICA, unlike NHPI and others, are unsuitable for solvent- or mediator-free catalysis due to their strong intramolecular hydrogen-bonding interactions; 2) increasing the reactive hydroxyimide groups on the same aromatic ring, or doped N atoms or ionic-pair groups onto the aromatic ring, can improve catalytic reactivity, whereas appropriate enlargement of conjugated aromatic systems results in unchanged activity; 3) the newly designed catalysts are more active than NHPI and NHNI and have catalytic activities comparable to NDHPI and THICA; 4) the ionic-pair supported case is suggested to be a very active catalyst, even towards inert propane, and can be used as a novel model catalyst for further improvements. The present work will be helpful in designing reactive hydroxyimide organocatalysts.