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-free C-3 selective C(sp2)-C(sp3) heteroarylation of anilines with imidazo[1,2-a]pyridine derivatives via cross-dehydrogenative coupling

A general and straightforward method for the regioselective construction of C-3 heteroaryl-containing imidazo[1,2-a]pyridines via cross-dehydrogenative coupling under transition-metal-free conditions has been reported, utilizing N,N-dimethylaniline as the methylenation source and furnishing the C(sp²)-C(sp³) functionalized products in good to excellent yields. Mechanism studies indicate that a radical pathway is responsible for this transformation.

Hidden Reactivity of Barbituric and Meldrum’s Acids: Atom-Efficient Free-Radical C–O Coupling with N-Hydroxy Compounds

The reactivity of CH-acidic and structurally related enol-containing heterocycles towards N-oxyl radicals is disclosed. Traditionally, these substrates have been considered as reactants for ionic transformations. Highly selective and efficient N-oxyl radical mediated C–O coupling of substituted barbituric or Meldrum’s acids with N-hydroxy compounds (N-hydroxyimides, hydroxamic acids, oximes, and N-hydroxybenzotriazole) was achieved using inexpensive manganese-containing salts as oxidants. Metal-free C–O coupling was demonstrated using diacetyliminoxyl as both the oxidant (hydrogen-atom acceptor) and the coupling partner.

Electrifying Phthalimide-N-Oxyl (PINO) Radical Chemistry: Anodically Induced Dioxygenation of Vinyl Arenes with N-Hydroxyphthalimide

An electrochemical process of free-radical difunctionalization of vinyl arenes with N-hydroxyphthalimide resulting in vicinal dioxyphthalimides was discovered. The reaction proceeds with the use of pyridinium perchlorate and pyridine as a supporting electrolyte and a base, respectively. The present approach involves the anodic generation of stabilized phthalimide-N-oxyl (PINO) radical, which adds to the carbon-carbon double bond of vinyl arenes and recombines with the subsequently formed benzylic radical. A wide range of dioxyphthalimides were obtained in yields up to 81%.

Iodine(III) promotes cross-dehydrogenative coupling of N-hydroxyphthalimide and unactivated C(sp3)-H bonds

Cross-dehydrogenative coupling reactions provide a method to construct new chemical bonds by direct C-H activation without any pre-functionalization. Compared to functionalization of a C-H bond α- to ether oxygen, α- to carbonyl, or at a benzylic position, functionalization of unactivated hydrocarbons is difficult and often requires high temperatures, a transition-metal catalyst, or a superstoichiometric quantity of volatile, toxic, and explosive tert-butylhydroperoxide. Here, a cross-dehydrogenative C-O coupling reaction of N-hydroxyphthalimide with unactivated alkanes, nitriles, ethers, and thioethers has been realized by using iodobenzene diacetate as the radical initiator. The current protocol enables efficient functionalization of unactivated hydrocarbons and nitriles through inert C(sp³)-H bond activation under mild reaction conditions. O-substituted NHPI derivatives are generated in good yields under metal-free conditions.

Metal-Free Cross-Dehydrogenative C-O Coupling of Carbonyl Compounds with N-Hydroxyimides: Unexpected Selective Behavior of Highly Reactive Free Radicals at an Elevated Temperature

Cross-dehydrogenative C-O coupling of N-hydroxyimides with ketones, esters, and carboxylic acids was achieved employing the di-tert-butyl peroxide as a source of free radicals and a dehydrogenating agent. The proposed method is experimentally simple and demonstrates the outstanding efficiency for the challenging CH substrates, such as unactivated esters and carboxylic acids. It was shown that N-hydroxyphthalimide drastically affects the oxidative properties of t-BuOOt-Bu by intercepting the t-BuO^• radicals with the formation of phthalimide-N-oxyl radicals, a species responsible for both hydrogen atom abstraction from the CH reagent and the selective formation of the C-O coupling product by selective radical cross-recombination. The practical applicability of the developed method was exemplified by the single-stage synthesis of commercial reagent (known as Baran aminating reagent precursor) from isobutyric acid and N-hydroxysuccinimide, whereas in the standard synthetic approach, four stages are necessary.

Ultrasound accelerated synthesis of O-alkylated hydroximides under solvent- and metal-free conditions

A novel, sustainable, environmentally friendly, high substrate scope, efficient, solvent-free and metal catalyst-free method for the cross-dehydrogenative coupling (CDC) reaction between N-hydroxyphthalimide (NHPI) and benzyl/ether compounds is described. This coupling reaction proceeds through ultrasound acceleration. Compared to conventional heating conditions, the use of ultrasound techniques not only improves the reaction efficiency and enhances the reaction rate but also minimizes the side reactions.

Copper nitrate: a privileged reagent for organic synthesis

Copper has been explored as an ideal candidate for replacing noble metals in organic synthesis, especially for practical large scale preparation. Recent decades have witnessed the renaissance and improvement of copper-catalyzed and copper-mediated organic reactions. Copper nitrate is a common inorganic copper salt which has been proved to be a ubiquitous reactant in organic synthesis due to its commercial availability, stability, inexpensiveness and environmentally benign nature. Copper nitrate could be used as a nitration reagent, oxidant, catalyst or promoter, and Lewis acid as well. Remarkably, great attention has been devoted to the efficient transformation of copper nitrate into functionalized or complicated compounds through various reaction types including cyclization, C-H activation, difunctionalization, nitration, rearrangement and asymmetric synthesis with chiral ligands. Further modification of copper nitrate, such as solid-supported copper nitrate or copper nitrate complexes, extends its applications in organic synthesis. The present review highlights recent advances of copper nitrate in organic synthesis, along with the mechanisms.

Hypervalent iodine compounds for anti-Markovnikov-type iodo-oxyimidation of vinylarenes

The iodo-oxyimidation of styrenes with the N-hydroxyimide/I₂/hypervalent iodine oxidant system was proposed. Among the examined hypervalent iodine oxidants (PIDA, PIFA, IBX, DMP) PhI(OAc)₂ proved to be the most effective; yields of iodo-oxyimides are 34-91%. A plausible reaction pathway includes the addition of an imide-N-oxyl radical to the double C=C bond and trapping of the resultant benzylic radical by iodine. It was shown that the iodine atom in the prepared iodo-oxyimides can be substituted by various nucleophiles.

Selectfluor-induced C(sp2)–O coupling reaction of N-substituted anilines with hydroxylamine derivatives

We developed a novel metal-free method for the construction of C(sp²)–O bonds via oxidative cross-coupling reactions between various N-substituted anilines and hydroxylamine derivatives just using commercially available Selectfluor as an oxidant.