Diverse Transformation of Vinyl Azides with 2,2,6,6-Tetramethyl-N-oxopiperidinium

Elucidating the Mechanism of Electrooxidative Allene Dioxygenation: Dual Role of Tetramethylpiperidine N-Oxyl (TEMPO)

The cumulated π system of a nonsymmetric allene contains three distinct unsaturated carbons that imbue it with unique reactivity toward radicals as compared to its alkene and alkyne counterparts. Despite the synthetic potential of these versatile building blocks, electrochemical transformations of allenes have been historically underexplored. Myriad strategies for easy access to allenes, coupled with the resurgence of interest in sustainable oxidative transformations of hydrocarbons, prompted our efforts to conduct an in-depth investigation of a rare example of an electrochemical TEMPO-mediated allene dioxygenation. The resultant vinyl-TEMPO motif is readily postfunctionalized to install a heteroatom at each allene carbon. Mechanistic investigations, including cyclic voltammetry (CV) studies, computations, and monitoring by operando NMR (ReactNMR) were performed to lay the groundwork for future electrochemical allene functionalizations that deliver unique synthetic building blocks.

Ferrier Glycosylation Mediated by the TEMPO Oxoammonium Cation

The TEMPO oxoammonium cation has been proven to be both an efficient oxidizing reagent and an electrophilic substrate frequently found in organic reactions. Here, we report that this versatile chemical reagent can also be used as an efficient promoter for C- and N-glycosylation reactions through a Ferrier rearrangement with moderate to high yields. This unprecedented reactivity is explained in terms of a Lewis acid activation of glycal by TEMPO+ forming a type of glycal-TEMPO+ mesomeric structure, which occurs through an extended vinylogous hyperconjugation toward the π*(O═N+) orbital [LP(O1) → π*(C1═C2), π*(C1═C2) → σ*(C3-O3), and LP(O6) → π*(O═N+)]. This enables the formation of the respective Ferrier glycosyl cation, which is trapped by various nucleophiles. The extended hyperconjugation (or double hyperconjugation) toward the π*(O═N+) orbital, which confers the Lewis acid character of the TEMPO cation, was supported by natural bond orbital analysis at the M06-2X/6-311+G** level of theory.

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.

Dearomative oxyphosphorylation of indoles enables facile access to 2,2-disubstituted indolin-3-ones

A highly efficient oxidative dearomatization of indoles with H-phosphorus oxides in the presence of TEMPO oxoammonium salt has been demonstrated. Through the intramolecular oxidative dearomatization of indoles and subsequent intermolecular nucleophilic addition with phosphorus nucleophile, a variety of structurally diverse arylphosphoryl and alkylphosphoryl indolin-3-ones were obtained in good yields with a broad substrate scope and high functional-group compatibility.

Amidation-Ketonization-Selenation of Terminal Alkynes Using TEMPO and Elemental Selenium

Herein, we present a novel silver- or copper-mediated direct amidation-ketonization-selenation of terminal alkynes for the synthesis of α-oxo-selenoamides. The reaction utilized easily accessible elemental selenium as the source of selenium. In addition, the ¹⁸O labeling experiment revealed that TEMPO is the oxygen source of the carbonyl group. The reaction takes advantage of an unsaturated C≡C bond to construct new C═O, C═Se, and C-N bonds in one step.

Oxygen Atom Transfer in the Oxidation of Dimethyl Sulfoxide by Oxoammonium Cations

Cyclic oxoammonium salts and DMSO are known as important reagents for their diverse and unique reactivity. In the present work, we have studied the reaction of six- and five-membered oxoammonium salts with DMSO. The reaction includes ∼100% selective transfer of the O atom from the >N⁺═O group to the S atom of DMSO and structural rearrangement of the remaining cationic framework, leading to the formation of hydrolytically unstable iminium salts. The logarithms of the bimolecular rate constants k of the reaction correlated linearly with the reduction potentials E_{>N⁺═O/>N-O^•}, a relationship known for other electrophile-nucleophile combinations. The kinetic data and results of the DFT calculations allow for the suggestion that the studied process proceeds via the prereactive charge-transfer complex >N⁺═O···S (O)Me₂ and its direct concerted rearrangement to the iminium salts. An alternative mechanism that includes intermediate steps with discrete nitrenium cations can be ruled out on the basis of product analysis and DFT computations. The obtained results allow a deeper understanding of the redox chemistry of a pair of nitroxide radicals-oxoammonium cations.

Visible light-driven fluoroalkylthiocyanation of alkenes via electron donor–acceptor complexes

The visible light-induced fluoroalkylthiocyanation of alkenes via electron donor–acceptor complexes of perfluoroalkyl iodide reagents and K₃PO₄ was disclosed.

Electrochemical Tandem Fluoroalkylation-Cyclization of Vinyl Azides: Access to Trifluoroethylated and Difluoroethylated N-Heterocycles

A transition-metal- and oxidant-free electrochemical strategy for radical fluoroalkylation of vinyl azides was developed. The reaction was carried out under mild conditions by using inexpensive and bench-stable R_fSO₂Na (R_f = CF₃, CF₂H) as fluorination reagents. Depending on the starting material, both the electrochemical radical cyclization and dearomatization products could be obtained. This method provides a green and safe approach to synthesize fluorinated nitrogen heterocycles.

The Electrophilic Fluorination of Enol Esters Using SelectFluor: A Polar Two-Electron Process

The reaction of enol esters with SelectFluor is facile and leads to the corresponding α-fluoroketones under mild conditions and, as a result, this route is commonly employed for the synthesis of medicinally important compounds such as fluorinated steroids. However, despite the use of this methodology in synthesis, the mechanism of this reaction and the influence of structure on reactivity are unclear. A rigorous mechanistic study of the fluorination of these substrates is presented, informed primarily by detailed and robust kinetic experiments. The results of this study implicate a polar two-electron process via an oxygen-stabilised carbenium species, rather than a single-electron process involving radical intermediates. The structure-reactivity relationships revealed here will assist synthetic chemists in deploying this type of methodology in the syntheses of α-fluoroketones.

Direct oxidative dearomatization of indoles: access to structurally diverse 2,2-disubstituted indolin-3-ones

Described is an efficient oxidative dearomatization of indoles with TEMPO oxoammonium salt and a broad range of nucleophiles.

Cyanofluorination of vinyl ethers enabled by electron donor–acceptor complexes

The reaction is operationally simple and conducted under ambient conditions, allowing the access to highly functionalized α-alkoxy-β-fluoronitriles bearing quaternary carbons that are difficult to access by existing methods.