Copper-/Cobalt-Catalyzed Highly Selective Radical Dioxygenation of Alkenes

Tosylhydrazide-Induced 1,6-Enyne Radical Cyclization under Copper Catalysis: Access to 3,4-Dihydronaphthalen-1(2H)-one Derivatives

We describe an approach to access 4-aroyl-3-aryl-3,4-dihydronaphthalen-1(2H)-one derivatives in 41-79% yields through the Cu-catalyzed radical cyclization/desulfonylation of 1,6-enynes with tosylhydrazide under air conditions. This alternative desulfonylation strategy combines mild conditions, external oxidant-free processes, and sustainability, contributing to more environmentally friendly organic synthesis. The mechanistic studies showed that the CuCl/O2 combination serves as the source of the oxygen atom needed to form the C═O bond. The existence of tosylhydrazide is crucial for this conversion.

The diacetyliminoxyl radical in oxidative functionalization of alkenes

The intermolecular oxime radical addition to CC bonds was observed and studied for the first time. The diacetyliminoxyl radical was proposed as a model radical reagent for the study of oxime radical reactivity towards unsaturated substrates, which is important in the light of the active development of synthetic applications of oxime radicals. In the present work it was found that the diacetyliminoxyl radical reacts with vinylarenes and conjugated dienes to give radical addition products, whereas unconjugated alkenes can undergo radical addition or allylic hydrogen substitution by diacetyliminoxyl depending on the substrate structure. Remarkably, substituted alkenes give high yields of C-O coupling products despite the significant steric hindrance, whereas unsubstituted alkenes give lower yields of the C-O coupling products. The observed atypical C-O coupling yield dependence on the alkene structure was explained by the discovered ability of the diacetyliminoxyl radical to attack alkenes with the formation of a C-N bond instead of a C-O bond giving side products. This side process is not expected for sterically hindered alkenes due to lower steric availability of the N-atom in diacetyliminoxyl than that of the O-atom.

Non-noble metal-catalyzed cross-dehydrogenation coupling (CDC) involving ether α-C(sp3)-H to construct C-C bonds

Ether derivatives are widespread as essential building blocks in various drugs, natural products, agrochemicals, and materials. Modern economy requires developing green strategies with improved efficiency and reduction of waste. Due to its atom and step-economy, the cross-dehydrogenative coupling (CDC) reaction has become a major strategy for ether functionalization. This review covers C-H/C-H cross-coupling reactions of ether derivatives with various C-H bond substrates via non-noble metal catalysts (Fe, Cu, Co, Mn, Ni, Zn, Y, Sc, In, Ag). We discuss advances achieved in these CDC reactions and hope to attract interest in developing novel methodologies in this field of organic chemistry.

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.

Emerging Trends in Copper-Promoted Radical-Involved C-O Bond Formations

The C-O bond is ubiquitous in biologically active molecules, pharmaceutical agents, and functional materials, thereby making it an important functional group. Consequently, the development of C-O bond-forming reactions using catalytic strategies has become an increasingly important research topic in organic synthesis because more conventional methods involving strong base and acid have many limitations. In contrast to the ionic-pathway-based methods, copper-promoted radical-mediated C-O bond formation is experiencing a surge in research interest owing to a renaissance in free-radical chemistry and photoredox catalysis. This Perspective highlights and appraises state-of-the-art techniques in this burgeoning research field. The contents are organized according to the different reaction types and working models.

Cu(II)-Catalyzed Unsymmetrical Dioxidation of gem-Difluoroalkenes to Generate α,α-Difluorinated-α-phenoxyketones

A Cu-based catalyst system convergently couples gem-difluoroalkenes with phenols under aerobic conditions to deliver α,α-difluorinated-α-phenoxyketones, an unstudied hybrid fluorinated functional group. Composed of α,α-difluorinated ketone and α,α-difluorinated ether moieties, these compounds have rarely been reported as a synthetic intermediate. Computational predictions and later experimental corroboration suggest that the phenoxy-substituted fluorinated ketone's sp3-hybridized hydrate form is energetically favored relative to the respective nonether variant and that perturbation of the electronic character of the ketone can further encourage the formation of the hydrate. The more facile conversion between ketone and hydrate forms suggests that analogues should readily covalently inhibit proteases and other enzymes. Further functionalization of the ketone group enables access to other useful fluorinated functional groups.

Oxoammonium Salt-Mediated Regioselective Vicinal Dioxidation of Alkenes: Relying on Transient and Persistent Nitroxides

A novel, easy-to-handle, and regioselective vicinal dioxidation of alkenes under transition metal and organic peroxide free conditions has been developed. This approach uses N-hydroxyphthalimide and its analogues as the transient nitroxyl-radical precursors and 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO⁺BF₄^-) as the oxidant as well as the source of persistent nitroxide. By employing this method, multifarious structurally important dioxidation products were efficiently synthesized from simple alkenes and complex bioactive molecule derivatives.

Electrochemical Oxidative Difunctionalization of Alkenes to Access α-Oxygenated Ketones

Dioxygenation of alkenes was developed by the combination of electrochemical synthesis and aerobic oxidation, leading to easy accessibility of α-oxygenated ketones in an eco-friendly fashion. Using air as the oxygen source and the absence of transition metals were the critical features of this protocol. A wide range of alkenes and N-hydroxyimides were found to be compatible and provided α-oxygenated ketones in moderate to high yields.

Visible light-induced aerobic dioxygenation of α,β-unsaturated amides/alkenes toward selective synthesis of β-oxy alcohols using rose bengal as a photosensitizer

The first visible light-induced aerobic dioxygenation of alkenes for the selective synthesis of β-oxy alcohols was developed using non-toxic rose bengal as a photosensitizer.

A copper-catalyzed asymmetric oxime propargylation enables the synthesis of the gliovirin tetrahydro-1,2-oxazine core

The bicyclic tetrahydro-1,2-oxazine subunit of gliovirin is synthesized through a diastereoselective copper-catalyzed cyclization of an N-hydroxyamino ester. Oxidative elaboration to the fully functionalized bicycle was achieved through a series of mild transformations. Central to this approach was the development of the first catalytic, enantioselective propargylation of an oxime to furnish a key N-hydroyxamino ester intermediate.

Sulfated tungstate/dioxygen: a new catalytic system for oxysulfonylation of styrenes to form β-keto sulfones

A new green cat./O₂ oxidation system is mild, safe, heterogeneous, simple to make, stable, noncorrosive, produces N₂ as by product, and has an easy to recover–reuse catalyst and wide substrate scope.

Aerobic Metal-Free Dioxygenation of Alkenes with tert-Butyl Nitrite and N-Hydroxylamines

Metal-free dioxygenation of alkenes with tert-butyl nitrite and N-hydroxylamines (N-hydroxyphthalimide, N-hydroxybenzotriazole, and N-hydroxysuccinimide) is described to produce β-aminoxy nitrate esters using air as the oxidant. These organic nitrates can be readily converted into 1,2-diols and 1,2-diketone with broad substrate scope and functional group diversity.

Cobalt salt-catalyzed carbocyclization reactions of α-bromo-N-phenylacetamide derivatives

A Co(ii)-catalyzed carbocyclization reaction for the synthesis of 4-substituted quinolin-2-(1H)-ones is described.

PhI(OAc)2-mediated dialkoxylation of 4-aminostyrenes through a dearomatization process under metal-free conditions

A PhI(OAc)₂-mediated dialkoxylation of 4-aminostyrenes has been developed to afford 2,3-disubstituted 1,4-dioxane or 1,2-dimethoxyethane derivatives.

Novel and efficient transformation of enamides into α-acyloxy ketones via an acyl intramolecular migration process

Hydrogen peroxide and anhydride mediated transformation of enamides to afford substituted α-acyloxy ketones is described.

Iron(iii)-catalyzed aerobic dioxygenation of styrenes using N-hydroxyphthalimide and N-hydroxybenzotriazole

The dioxygenation of alkenes with NHPI/HOBt in air is described using commercial iron salt as the catalyst under ambient conditions with a broad substrate scope and functional group tolerance. The products can be readily converted into the corresponding 1,2-diols in high yields.

Manganese-catalysed hydroperoxidation of carbon–carbon double bonds using molecular oxygen present in air and hydroxylamine under ambient conditions

A highly efficient manganese-catalysed hydroperoxidation of carbon–carbon double bonds of enynes as well as styrene derivatives using N-hydroxyphthalimide, N-hydroxybenzotriazole or N-hydroxysuccinimide was developed.

Selectfluor-mediated highly selective radical dioxygenation of alkenes

Selectfluor mediated highly selective radical dioxygenation of alkenes using hydroxylamines and O₂ was achieved under mild conditions.

A Catalyst-Free Synthesis of Phosphinic Amides Using O-Benzoylhydroxylamines

A practical approach for the synthesis of phosphinic amides via the coupling of secondary phosphine oxides (SPOs) with O-benzoylhydroxylamines has been reported. Simply heating the mixture of SPOs and O-benzoylhydroxylamines in the presence of K(2)CO(3) gave the phosphinic amides in moderate to excellent yields under an open air system. This method provides a practical and catalyst-free method for the synthesis of various synthetically valuable phosphinic amides.