Selective Organocatalytic Oxygenation of Hydrocarbons by Dioxygen Using Anthraquinones and N-Hydroxyphthalimide

Synthesis of Fluoren-9-ones via Pd-Catalyzed Annulation of 2-Iodobiphenyls with Vinylene Carbonate

A palladium-catalyzed reaction for intermolecular selective C-H cyclocarbonylation of 2-iodobiphenyls is described. Intriguingly, the vinylene carbonate acts as a carbon monoxide transfer agent to enable the annulation reaction. Moreover, as a versatile synthon, fluoren-9-one can be transformed into a variety of functionalized organic molecules, such as [1,1'-biphenyl]-2-carboxylic acid, 1'H,3'H-spiro[fluorene-9,2'-perimidine] and N-tosylhydrazones.

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.

Electricity-Promoted Friedel-Crafts Acylation of Biarylcarboxylic Acids

An electricity-promoted method for Friedel-Crafts acylation of biarylcarboxylic acids is described in this research. Various fluorenones can be accessed in up to 99% yields. During the acylation, electricity plays an essential role, which might motivate the chemical equilibrium by consuming the generated TFA. This study is predicted to provide an avenue to realize Friedel-Crafts acylation in a more environmentally friendly process.

Structural and Spectroscopic Characterization of a Zinc-Bound N-Oxyphthalimide Radical

Thermolysis of a 1:1:1 mixture of ^MeLH (^MeL = {(2,6-ⁱPr₂C₆H₃)NC(Me)}₂CH), N-hydroxyphthalimide (HOPth), and diethylzinc in toluene at 77 °C provided [^MeLZn(OPth)] (1) in good yield after workup. The subsequent reduction of 1 with 1.3 equiv of KC₈ and 1 equiv of 2.2.2-cryptand, in tetrahydrofuran, provided [K(2.2.2-cryptand)][^MeLZn(OPth)] (2) in 74% yield after workup. Characterization of 2 via X-ray crystallography and electron paramagnetic resonance spectroscopy reveals the presence of an S = ¹/₂ radical on the N-oxyphthalimide ligand. Importantly, these data represent the first structural and spectroscopic confirmation of the redox activity of a metal-bound N-oxyphthalimide fragment, expanding the range of structurally characterized redox-active ligands.

Aerobic Oxidations via Organocatalysis: A Mechanistic Perspective

This review focuses on recent advances and mechanistic views of aerobic C(sp3)–H oxidations catalyzed by organocatalysts, where metal catalysis and photocatalysis are not included.

1 Introduction

2 Carbanion Route: TBD-Catalyzed C(sp3)–H Oxygenation

2.1 α-Hydroxylation of Ketones

2.2 Carbonylation of Benzyl C(sp3)–H

3 Radical Route: NHPI-Catalyzed C(sp3)–H Oxidation

3.1 N-Oxyl Radicals and Mechanisms

3.2 Oxygenation of Benzyl C(sp3)–H

3.3 Solvent Effects

4 Hydride-Transfer Route: TEMPO-Catalyzed Oxidations

4.1 Oxoammonium Cation and Mechanisms

4.2 Dehydrogenation of Alcohols

4.3 Oxygenation of Benzyl C(sp3)–H

5 Conclusions and Outlook

Lewis Acid Enables Ketone Phosphorylation to Form a C-P Bond and a C-C Bond: Synthesis of 9-Phosphoryl Fluorene Derivatives

An efficient method for the Lewis acid promotion of the synthesis 9-phosphoryl fluorenes has been reported. This method focuses on ketone phosphonylation to form a C-P bond and a C-C bond between diphenylmethanone and H-phosphinate esters, H-phosphites, and H-phosphine oxides via phospha-aldol elimination, in which a series of 9-phosphoryl fluorene derivatives were selectively obtained in moderate to excellent yields.

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.

Selective Oxidation of Benzylic C-H Bonds Catalyzed by Cu(II)/{PMo12}

Precise catalytic regulation of carbon radical generation by a highly active oxygen radical to abstract the H atom in a C-H bond is an effective method for the selective activation of C-H synthetic chemistry. Herein, we report a facile catalyst system with commercially available copper(II)/{PMo₁₂} to form a tert-butanol radical intermediate for the selective oxidation of benzylic C-H bonds. The reaction shows a broad range of substrates (benzyl methylene, benzyl alcohols) with good functional group tolerance and chemical selectivity. The corresponding carbonyl compounds were synthesized with good yields under mild conditions. DFT calculations and experimental analysis further demonstrated a reasonable carbon radical mechanism for this type of organic transformation reaction.

N-Hydroxyphthalimide/benzoquinone-catalyzed chlorination of hydrocarbon C-H bond using N-chlorosuccinimide

The direct chlorination of C-H bonds has received considerable attention in recent years. In this work, a metal-free protocol for hydrocarbon C-H bond chlorination with commercially available N-chlorosuccinimide (NCS) catalyzed by N-hydroxyphthalimide (NHPI) with 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ) functioning as an external radical initiator is presented. Aliphatic and benzylic substituents and also heteroaromatic ones were found to be well tolerated. Both the experiments and theoretical analysis indicate that the reaction goes through a process wherein NHPI functions as a catalyst rather than as an initiator. On the other hand, the hydrogen abstraction of the C-H bond conducted by a PINO species rather than the highly reactive N-centered radicals rationalizes the high chemoselectivity of the monochlorination obtained by this protocol as the latter is reactive towards the C(sp3)-H bonds of the monochlorides. The present results could hold promise for further development of a nitroxy-radical system for the highly selective functionalization of the aliphatic and benzylic hydrocarbon C-H.

Efficient catalytic oxidation of methyl aromatic hydrocarbon with N-alkyl pyridinium salts

A series of N-alkyl pyridinium salts were synthesized and employed as metal-free catalyst for the selective oxidation of methyl aromatic hydrocarbon with molecular oxygen. The electronic effect of the substitutes was found to be an important factor for the catalytic performance. With the introduction of electron-donating substitute –N(CH₃)₂, the conversion of p-xylene and selectivity of p-toluic acid could be simultaneously increased. 1-Benzyl-4-N,N-dimethylaminopyridinium salt showed the highest catalytic activity, and 95% conversion with 84% of selectivity to p-toluic acid could be obtained for the selective oxidation of p-xylene. Several methyl aromatic hydrocarbons could all be efficiently oxidized with the reported catalyst at the absence of any metal species.

A series of N-alkyl pyridinium salts were synthesized and employed as metal-free catalyst for the selective oxidation of methyl aromatic hydrocarbon with molecular oxygen.

Catalytic Synthesis of 2,5-Furandicarboxylic Acid from Concentrated 2,5-Diformylfuran Mediated by N-hydroxyimides under Mild Conditions

Producing polyester monomer 2,5-furandicarboxylic acid (FDCA) from biomass as an alternative to fossil-derived terephthalic acid has drawn much attention from both academy and industry. In this work, an efficient FDCA synthesis was proposed from 10.6 wt % 2,5-diformylfuran (DFF) in acetic acid using a combined catalytic system of Co/Mn acetate and N-hydroxyimides. The intermediate product of 5-formyl-2-furandicarboxylic acid (FFCA) possesses the least reactive formyl group. N-hydroxysuccinimide was found to be superior to N-hydroxyphthalimide in catalyzing the oxidation of the formyl group in FFCA intermediate, affording a near 95 % yield of FDCA under mild conditions of 100 °C. Trace maleic anhydride was detected as by-product, which mainly came from the oxidative cleavage of DFF via furfural, furoic acid and 5-acetoxyl-2(5H)-furanone as intermediates.

Mild and Practical Dirhodium(II)/NHPI-Mediated Allylic and Benzylic Oxidations with Air as the Oxidant

Aerobic allylic and benzylic oxidations catalyzed by dirhodium(II) complexes with N-hydroxyphthalimide (NHPI) are described. The open flask reaction occurs at mild temperature, using air as the oxidant. Mechanistic studies suggest that dirhodium(II) complexes axially coordinate with NHPI to activate the O-H bond in NHPI and decrease the bond-dissociation energy (BDE).

Metastable 9-Fluorenone: Blueshifted Fluorescence, Single-Crystal-to-Single-Crystal Reactivity, and Evaluation as a Multimodal Fingermark Visualization Treatment

A metastable form of 9-fluorenone (MS9F) has been characterized using Raman spectroscopy, fluorimetry, and X-ray diffraction techniques. MS9F emits blue fluorescence (λ_max =495 nm) upon 365 nm irradiation and undergoes a single-crystal-to-single-crystal (SCSC) transformation to reach the ground state form (GS9F) over approximately 30 minutes, whereupon it emits the expected green fluorescence. A structure-property relationship for this fluorescent behavior has been posited. MS9F and GS9F were applied as a means of visualizing latent fingermarks on a nonporous surface. This approach identified three different modes of fluorescent fingermark visualization using 9-fluorenone.

Aerobic conversion of benzylic sp3 C–H in diphenylmethanes and benzyl ethers to CO bonds under catalyst-, additive- and light-free conditions

Catalyst-, additive- and light-free aerobic conversion of benzylic C–H to CO bonds is, for the first time, reported.

Recent Advances in Homogeneous Metal-Catalyzed Aerobic C–H Oxidation of Benzylic Compounds

Csp3–H oxidation of benzylic methylene compounds is an established strategy for the synthesis of aromatic ketones, esters, and amides. The need for more sustainable oxidizers has encouraged researchers to explore the use of molecular oxygen. In particular, homogeneous metal-catalyzed aerobic oxidation of benzylic methylenes has attracted much attention. This account summarizes the development of this oxidative strategy in the last two decades, examining key factors such as reaction yields, substrate:catalyst ratio, substrate scope, selectivity over other oxidation byproducts, and reaction conditions including solvents and temperature. Finally, several mechanistic proposals to explain the observed results will be discussed.

Tandem Oxidative Ring-Opening/Cyclization Reaction in Seconds in Open Atmosphere for the Synthesis of 1-Tetralones in Water-Acetonitrile

A mild and practical tandem oxidative ring-opening/cyclization reaction mediated by Ce⁴⁺ for the synthesis of 1-tetralones is presented. This rapid transformation was completed within 30 s and conducted in an open reactor at 0 °C in a water-acetonitrile mixture. Various cyclobutanol derivatives are transformed into desired products in good to high yields, and this reaction can be easily scaled up to the gram scale.

Silica gel mediated oxidative C–O coupling of β-dicarbonyl compounds with malonyl peroxides in solvent-free conditions

For the first time silica gel was observed to activate peroxides in oxidative coupling reactions. Here we report silica gel mediated oxidative C–O coupling of β-dicarbonyl compounds with cyclic diacyl peroxides affording α-acyloxy derivatives with 100% atom efficiency. The highest yields of coupling products were achieved in solvent free conditions. C–O coupling products were prepared in yields up to 86%.

A novel radical polymerization system initiated by a redox reaction with NHPI and xanthone

A novel redox reaction system consisting of xanthone (XT) andN-hydroxyphthalimide (NHPI) for radical polymerization is developed where NHPI and XT experience a one-electron-transfer reaction, which produces two kinds of radicals, PINO radicals and cycloketyl (CK) radicals.

An Exceptionally Facile Two-Step Structural Isomerization and Detoxication via a Water-Assisted Double Lossen Rearrangement

N-hydroxyphthalimide (NHPI), which is best known as an organocatalyst for efficient C-H activation, has been found to be oxidized by quinoid compounds to its corresponding catalytically active nitroxide-radical. Here, we found that NHPI can be isomerized into isatoic anhydride by an unusually facile two-step method using tetrachloro-1,4-benzoquinone (TCBQ, p-chloranil), accompanied by a two-step hydrolytic dechlorination of highly toxic TCBQ into the much less toxic dihydroxylation product, 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic acid). Interestingly, through the complementary application of oxygen-18 isotope-labeling, HPLC combined with electrospray ionization quadrupole time-of-flight and high resolution Fourier transform ion cyclotron resonance mass spectrometric studies, we determined that water was the source and origin of oxygen for isatoic anhydride. Based on these data, we proposed that nucleophilic attack with a subsequent water-assisted Lossen rearrangement coupled with rapid intramolecular addition and cyclization in two consecutive steps was responsible for this unusual structural isomerization of NHPI and concurrent hydroxylation/detoxication of TCBQ. This is the first report of an exceptionally facile double-isomerization of NHPI via an unprecedented water-assisted double-Lossen rearrangement under normal physiological conditions. Our findings may have broad implications for future research on hydroxamic acids and polyhalogenated quinoid carcinogens, two important classes of compounds of major chemical and biological interest.

Benzylic C(sp(3))-H Functionalization for C-N and C-O Bond Formation via Visible Light Photoredox Catalysis

A visible light mediated highly selective benzylic C-H bond functionalization for intermolecular C-N and C-O bond formation is reported. This cross-dehydrogenative coupling reaction demonstrates a straightforward protocol for incorporating the heteroaromatics to the benzylic position. Benzylic oxidation of various alkyl aryls to corresponding carbonyl compounds has also been reported.

Visible light mediated efficient oxidative benzylic sp(3) C-H to ketone derivatives obtained under mild conditions using O2

A photooxygenation of benzylic sp(3) C-H reaction has been demonstrated using O2 mediated by visible light. This protocol provides a simple and mild route to obtain ketones from benzylic sp(3) C-H bonds. Various benzylic sp(3) C-H bonds can be transformed into the desired ketone derivatives in moderate to good yields. The (18)O2 labelling experiments demonstrated that the oxygen introduced into ketone originated from dioxygen. A plausible mechanism has been proposed accordingly.