Aerobic Oxidation of 4-Alkyl-N,N-dimethylbenzylamines Catalyzed by N-Hydroxyphthalimide: Protonation-Driven Control over Regioselectivity

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.

Chemoselective Oxyfunctionalization of Functionalized Benzylic Compounds with a Manganese Catalyst

Whilst allowing for easy access to synthetically versatile motifs and for modification of bioactive molecules, the chemoselective benzylic oxidation reactions of functionalized alkyl arenes remain challenging. Reported in this study is a new non‐heme Mn catalyst stabilized by a bipiperidine‐based tetradentate ligand, which enables methylene oxidation of benzylic compounds by H₂O₂, showing high activity and excellent chemoselectivity under mild conditions. The protocol tolerates an unprecedentedly wide range of functional groups, including carboxylic acid and derivatives, ketone, cyano, azide, acetate, sulfonate, alkyne, amino acid, and amine units, thus providing a low‐cost, more sustainable and robust pathway for the facile synthesis of ketones, increase of complexity of organic molecules, and late‐stage modification of drugs.

Selective functionalization of benzylic C-H bonds of two different benzylic ethers by bowl-shaped N-hydroxyimide derivatives as efficient organoradical catalysts

A highly efficient, site-selective benzylic C-H bond amination of two different benzylic ether substrates was described by using bowl-shaped N-hydroxyimide organoradical catalysts with diethyl azodicarboxylate. The synthetic utility of this approach is demonstrated by the subsequent transformation of the amination products into the corresponding aldehydes and alkylhydrazines.

Change of Selectivity in C-H Functionalization Promoted by Nonheme Iron(IV)-oxo Complexes by the Effect of the N-hydroxyphthalimide HAT Mediator

A kinetic analysis of the hydrogen atom transfer (HAT) reactions from a series of organic compounds to the iron(IV)-oxo complex [(N4Py)Fe^IV(O)]²⁺ and to the phthalimide N-oxyl radical (PINO) has been carried out. The results indicate that a higher activating effect of α-heteroatoms toward the HAT from C-H bonds is observed with the more electrophilic PINO radical. When the N-hydroxy precursor of PINO, N-hydroxyphthalimide (NHPI), is used as a HAT mediator in the oxidation promoted by [(N4Py)Fe^IV(O)]²⁺, significant differences in terms of selectivity have been found. Product studies of the competitive oxidations of primary and secondary aliphatic alcohols (1-decanol, cyclopentanol, and cyclohexanol) with alkylaromatics (ethylbenzene and diphenylmethane) demonstrated that it is possible to modify the selectivity of the oxidations promoted by [(N4Py)Fe^IV(O)]²⁺ in the presence of NHPI. In fact, alkylaromatic substrates are more reactive in the absence of the mediator while alcohols are preferably oxidized in the presence of NHPI.

Photo-induced oxidative cleavage of C-C double bonds for the synthesis of biaryl methanone via CeCl3 catalysis

A Ce-catalyzed strategy is developed to produce biaryl methanones via photooxidative cleavage of C-C double bonds at room temperature. This reaction is performed under air and demonstrates high activity as well as functional group tolerance. A synergistic Ce/ROH catalytic mechanism is also proposed based on the experimental observations. This protocol should be the first successful Ce-catalyzed photooxidation reaction of olefins with air as the oxidant, which would provide inspiration for the development of novel Ce-catalyzed photochemical synthesis processes.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

N-Hydroxyphthalimide: A Hydrogen Atom Transfer Mediator in Hydrocarbon Oxidations Promoted by Nonheme Iron(IV)-Oxo Complexes

The oxidation of a series of hydrocarbons by the nonheme iron(IV)-oxo complex [(N4Py)Fe^IV═O]²⁺ is efficiently mediated by N-hydroxyphthalimide. The increase of reactivity is associated to the oxidation of the mediator to the phthalimide N-oxyl radical, which efficiently abstracts a hydrogen atom from the substrates, regenerating the mediator in its reduced form.

Activation and Deactivation Strategies Promoted by Medium Effects for Selective Aliphatic C-H Bond Functionalization

Selective functionalization of unactivated aliphatic C-H bonds represents an important goal of modern synthetic chemistry. Differentiating between such bonds in organic molecules with high levels of selectivity remains a crucial issue, and a profound understanding of even the subtlest reactivity trends is needed. Among the methods that have been developed, those based on hydrogen atom transfer (HAT) have attracted considerable interest. Within this framework, medium effects have proved effective in altering the reactivity and site selectivity in synthetically useful C-H functionalization procedures. In this Review, the mechanistic features behind the available strategies are discussed. It is shown that hydrogen bonding and acid-base interactions can promote C-H bond activation or deactivation toward HAT reagents, thereby providing fine-control over the site selectivity and product chemoselectivity as well as useful guidelines for future development and applications.