Copper-Catalyzed Radical/Radical C sp 3H/PH Cross-Coupling: α-Phosphorylation of Aryl KetoneO-Acetyloximes

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Radical Cation Salt-initiated Aerobic C-H Phosphorylation of N-Benzylanilines: Synthesis of α-Aminophosphonates

A radical cation salt-initiated phosphorylation of N-benzylanilines was realized through an aerobic oxidation of the sp3 C-H bond, providing a series of α-aminophosphonates in high yields. An investigation of the reaction scope revealed that this mild catalyst system is superior in good functional group tolerance and high reaction efficiency. The mechanistic study implied that the cleavage of the sp3 C-H bond was involved in the rate-determining step.

Copper-Catalyzed Oxidative Carbon-Carbon and/or Carbon-Heteroatom Bond Formation with O2 or Internal Oxidants

Selective oxidation, a fundamental organic transformation of critical importance, produces value-added products from simple organic molecules. This process is extensively used to incorporate heteroatoms into carbon-based molecules, where high-valent metal salts, hypervalent halogen reagents, and peroxides are widely used as oxidants. Oxidation reactions are extremely challenging because their selectivity is hard to control and/or they form significant quantities of unwanted waste derived from the stoichiometric oxidants. Undoubtedly, the utilization of green oxidants such as molecular oxygen (O₂) or internal oxidants provides tunable oxidation abilities and produces no environmentally hazardous byproducts. Thus, synthetic chemists have devoted increasing attention to the utilization of green oxidants to obtain valuable products. Since the first industrial application of noble metal-catalyzed oxidation, i.e., Pd/Cu/O₂-mediated Wacker oxidation, precious metal-catalyzed organic reactions have undergone significant development in both the laboratory and industry. However, the high cost and considerable toxicity of precious metals compel chemists to explore the catalytic activities of earth-abundant, first-row transition metals. Copper is abundant, easy to utilize, and relatively insensitive to water and air. Controllable access to Cu(0), Cu(I), Cu(II), and Cu(III) oxidation states ensures that copper can be applied as a tunable and multifunctional catalyst. Copper-catalyzed transformations involve single-electron transfer (SET), two-electron processes (TEPs) and even the cooperation of SET and TEPs. More importantly, in Cu/O₂ catalytic systems, ligands, additives, and solvents can tune the oxidation state of copper from Cu(I) to Cu(III). As a result, the development of copper-catalyzed aerobic oxidative reactions is possible and desirable. Progress in these synthetic methods will enable breakthroughs in natural product synthesis, materials science, and bioorganic chemistry. This Account describes our efforts over the last several years to develop copper-catalyzed C-C or C-heteroatom bond formation reactions with oxygen or internal oxidants as the oxidant. We primarily focused on reaction with simple substrates, including cross-couplings, cycloadditions, cyclizations, and condensations. These transformations provide convenient and efficient strategies for constructing multiple bonds, such as C-C/C-O bonds, C-C/C-N bonds, and C-N/C-S bonds, in one pot. Various alkynes, furans, benzofurans, lactones, sulfones, thioethers, and nitrogen-containing heterocyclic compounds were synthesized with high selectivity and atom economy from abundant, commercially available and inexpensive starting materials. These methods were successfully applied to the construction of drug molecules and skeletons of natural products. Additionally, the designed control experiments and serendipitous observations have given us mechanistic insights into copper-catalyzed green oxidation. Uncovering the activity of copper-catalyzed green oxidation involving oxygen and oxime esters has allowed us to extend the scope of those green oxidation reactions. We believe that copper-catalyzed green oxidation transformations can be made even more eco-friendly and economical in the synthesis of valuable compounds.

A redox-economical synthesis of trifluoromethylated enamides with the Langlois reagent

A redox-economical strategy for the synthesis of trifluoromethylated enamides using copper catalysis is reported. The reaction employs the inexpensive Langlois reagent (CF₃SO₂Na) and takes place without the need of an external oxidant. The trifluoromethylated enamide products can easily be converted into the corresponding ketone, saturated amide or oxazole.

Construction of Benzene Rings by Copper-Catalyzed Cycloaddition Reactions of Oximes and Maleimides: An Access to Fused Phthalimides

A useful Cu-catalyzed cycloaddition protocol for the construction of benzene rings has been achieved. The reactions, utilizing readily available oximes and maleimides as starting materials, proceed under mild reaction conditions to generate a series of structurally interesting fused-phthalimides that are difficult to be prepared by conventional methods.

t-BuONa-mediated direct C-H halogenation of electron-deficient (hetero)arenes

An efficient halogenation of electron-deficient (hetero)arenes is described. The reaction utilizes common t-BuONa as a catalyst (for iodination) or a promoter (for bromination and chlorination), and perfluorobutyl iodide, CBr₄ or CCl₄ as the readily-available halogenating agents, respectively. The protocol features broad scope, high efficiency, mild conditions and gram scalability. An ionic pathway involving halogen bond formation and halophilic attack is proposed. The utility of the resulting iodinated heteroarenes is demonstrated in visible light-mediated C_aryl-C_aryl cross-coupling reaction.

Modular 2,3-diaryl-2H-azirine synthesis from ketoxime acetates via Cs2CO3-mediated cyclization

A practical and effective Cs₂CO₃-mediated cyclization of ketoxime acetates for the synthesis of 2H-azirines has been developed.

Synthesis of an oxygen-linked germinal frustrated Lewis pair and its application in small molecule activation

An oxygen-linked germinal frustrated Lewis pair was synthesized and its reactivities against a series of small molecules were investigated.

One-pot bifunctionalization of unactivated alkenes, P(O)–H compounds, and N-methoxypyridinium salts for the construction of β-pyridyl alkylphosphonates

An efficient phosphono-heteroarylation of unactivated alkenes was developed through a one-pot bifunctionalization process to give access to β-pyridylphosphine structural motifs.

Copper-catalyzed coupling of oxime acetates and aryldiazonium salts: an azide-free strategy toward N-2-aryl-1,2,3-triazoles

An azide-free strategy for the synthesis of N-2-aryl-1,2,3-triazoles via copper-catalyzed coupling of oxime acetates and aryldiazonium tetrafluoroborates.

Phosphinoyl-functionalization of unactivated alkenes through phosphinoyl radical-triggered distal functional group migration

Reported herein is a novel, mild, and practical protocol for the radical-mediated phosphinoyl-functionalization of unactivated alkenes through distal functional group migration.

Copper-catalyzed selective radical–radical cross-coupling for C–S bond formation: an access to α-alkylthionitriles

Oxidative radical–radical cross-coupling: a new protocol for C–S bond formation was developed by selective cross-coupling between a thiyl radical and an isobutyronitrile radical.

Barium-catalyzed C–OH/P–H dehydrative cross-coupling for C–P bond construction

A barium-catalyzed C–OH/P–H dehydrative cross-coupling protocol for the construction of C–P bonds was developed in an environmentally benign manner.

The mechanism of copper-catalyzed oxytrifluoromethylation of allylamines with CO2: a computational study

We propose a novel Cu(i)–Cu(ii) catalytic cycle for the copper-catalyzed oxytrifluoromethylation of allylamines with CO₂, which proceeds through a radical–radical cross-coupling pathway.

Metal-free electrophilic phosphination of electron-rich arenes, arenols and aromatic thiols with diarylphosphine oxides

A metal-free and mild route to aromatic organophosphorus compounds from stable phosphines.

A Visible-Light-Driven Iminyl Radical-Mediated C-C Single Bond Cleavage/Radical Addition Cascade of Oxime Esters

A room-temperature, visible-light-driven N-centered iminyl radical-mediated and redox-neutral C-C single bond cleavage/radical addition cascade reaction of oxime esters and unsaturated systems has been accomplished. The strategy tolerates a wide range of O-acyl oximes and unsaturated systems, such as alkenes, silyl enol ethers, alkynes, and isonitrile, enabling highly selective formation of various chemical bonds. This method thus provides an efficient approach to various diversely substituted cyano-containing alkenes, ketones, carbocycles, and heterocycles.

Metal-Involving Synthesis and Reactions of Oximes

This review classifies and summarizes the past 10-15 years of advancements in the field of metal-involving (i.e., metal-mediated and metal-catalyzed) reactions of oximes. These reactions are diverse in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. This consideration gives a general outlook of reaction routes, mechanisms, and driving forces and underlines the potential of metal-involving conversions of oxime species for application in various fields of chemistry and draws attention to the emerging putative targets.

Crystal Structure of Bis(2,4,6-trimethylphenyl)-phosphine Oxide

The single crystal structure of bis(2,4,6-trimethylphenyl)phosphine oxide has been determined. All interatomic distances and angles can be considered normal. The aryl substituents adopt an intermediate configuration when compared to both sterically unhindered (e.g., diphenylphosphine oxide) and congested (e.g., bis(2,4,6-tri-tert-butylphenyl)phosphine oxide) secondary phosphine oxides, illustrating the influence of steric congestion on the molecular structure.

Visible-Light Mediated Oxidative C-H/N-H Cross-Coupling between Tetrahydrofuran and Azoles Using Air

Tetrahydrofuran is a privileged structural moiety in many important organic compounds. In this work, we have developed a simple and mild catalytic oxidative amination of tetrahydrofuran mediated by visible-light catalysis. The C(sp3)-H bond of tetrahydrofuran was activated using molecular oxygen as a benign oxidant. Besides, a variety of azoles could be tolerated, providing a green route for N-substituted azoles.

Facile Construction of Quinoline-2-carboxylate Esters through Aerobic Oxidation of Alkyl 4-Anilinocrotonates Induced by a Radical Cation Salt

A facile construction of quinoline-2-carboxylate esters through an aerobic oxidation of alkyl 4-anilinocrotonates is described. In the presence of dioxygen, sp³ C-H bonds in 4-anilinocrotonates can easily be oxidized by using a catalytic amount of a radical cation salt, providing a radical intermediate. After further oxidation and domino cyclization, the desired quinoline derivatives were afforded in high yields. This reaction provides a new way to construct the pharmaceutically relevant quinoline skeleton, avoiding harsh reaction conditions and tedious starting material synthesis.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).