Copper-Catalyzed Aerobic Oxidations of Organic Molecules: Pathways for Two-Electron Oxidation with a Four-Electron Oxidant and a One-Electron Redox-Active Catalyst

Highly practical and efficient preparation of aldehydes and ketones from aerobic oxidation of alcohols with an inorganic-ligand supported iodine catalyst

Herein, we divulge an efficient protocol for aerobic oxidation of alcohols with an inorganic-ligand supported iodine catalyst, (NH4)5[IMo6O24]. The catalyst system is compatible with a wide range of groups and exhibits high selectivity, and shows excellent stability and reusability, thus serving as a potentially greener alternative to the classical transformations.

Copper-Catalyzed Electrochemical C-H Amination of Arenes with Secondary Amines

Electrochemical oxidation represents an environmentally friendly solution to conventional methods that require caustic stoichiometric chemical oxidants. However, C-H functionalizations merging transition-metal catalysis and electrochemical techniques are, to date, largely confined to the use of precious metals and divided cells. Herein, we report the first examples of copper-catalyzed electrochemical C-H aminations of arenes at room temperature using undivided electrochemical cells, thereby providing a practical solution for the construction of arylamines. The use of n-Bu₄NI as a redox mediator is crucial for this transformation. On the basis of mechanistic studies including kinetic profiles, isotope effects, cyclic voltammetric analyses, and radical inhibition experiments, the reaction appears to proceed via a single-electron-transfer (SET) process, and a high valent Cu(III) species is likely involved. These findings provide a new avenue for transition-metal-catalyzed electrochemical C-H functionalization reactions using redox mediators.

Copper(ii)-benzotriazole coordination compounds in click chemistry: a diagnostic reactivity study

This diagnostic study aims to shed light on the catalytic activity of a library of Cu(ii) based coordination compounds with benzotriazole-based ligands. We report herein the synthesis and characterization of five new coordination compounds formulated as [Cu^II(L⁴)(MeCN)₂(CF₃SO₃)₂] (1), [Cu^II(L⁵)₂(CF₃SO₃)₂] (2), [Cu^II(L⁶)₂(MeCN)(CF₃SO₃)]·(CF₃SO₃) (3), [Cu^II(L⁶)₂(H₂O)(CF₃SO₃)]·(CF₃SO₃)·2(Me₂CO) (4), and [Cu(L¹)₂(L^1')₂(CF₃SO₃)₂]₂·4(CF₃SO₃)·8(Me₂CO) (5), derived from similar nitrogen-based ligands. The homogeneous catalytic activity of these compounds along with our previously reported coordination compounds (6-13), derived from similar ligands, is tested against the well-known Cu(i)-catalysed azide-alkyne cycloaddition reaction. The optimal catalyst [Cu^II(L¹)₂(CF₃SO₃)₂] (10) activates the reaction to afford 1,4-disubstituted 1,2,3-triazoles with yields up to 98% and without requiring a reducing agent. Various control experiments are performed to optimize the method and examine parameters such as ligand variation, metal coordination geometry and environment, in order to elucidate the behaviour of the catalytic system.

Novel total syntheses of oxoaporphine alkaloids enabled by mild Cu-catalyzed tandem oxidation/aromatization of 1-Bn-DHIQs

Novel total syntheses of oxoaporphine alkaloids such as liriodenine, dicentrinone, cassameridine, lysicamine, oxoglaucine and O-methylmoschatoline were developed. The key step of these total syntheses is Cu-catalyzed conversion of 1-benzyl-3,4-dihydro-isoquinolines (1-Bn-DHIQs) to 1-benzoyl-isoquinolines (1-Bz-IQs) via tandem oxidation/aromatization. This novel Cu-catalyzed conversion has been studied in detail, and was successfully used for constructing the 1-Bz-IQ core.

Novel total syntheses of several oxoaporphine alkaloids were developed. The Cu-catalyzed conversion of 1-benzyl-3,4-dihydro-isoquinolines (1-Bn-DHIQs) to 1-benzoyl-isoquinolines (1-Bz-IQs) as the key step of these total syntheses has also been investigated in detail.

Copper-Catalyzed Benign and Efficient Oxidation of Tetrahydroisoquinolines and Dihydroisoquinolines Using Air as a Clean Oxidant

A green chemical method for mild oxidation of 1,2,3,4-tetrahydroisoquinolines (THIQs) and 3,4-dihydroisoquinolines (DHIQs) has been developed using air (O₂) as a clean oxidant. DHIQs and THIQs could be efficiently oxidized to isoquinolines in dimethyl sulfoxide at 25 °C under an open air atmosphere with CuBr₂ (20 mol %) as the catalyst; different bases [NaOEt and/or 1,8-diazabicyclo[5,4,0]undec-7-ene] were used for the reaction according to the patterns of substituents (R¹, R²).

Cu-Catalyzed Aerobic Oxidative N-N Coupling of Carbazoles and Diarylamines Including Selective Cross-Coupling

A Cu-catalyzed method has been identified for aerobic oxidative dimerization of carbazoles and diarylamines to the corresponding N-N coupled bicarbazoles and tetraarylhydrazines. The reactions proceed under mild conditions (1 atm O2, 60-80 °C) with a catalyst composed of CuBr·dimethylsulfide and N, N-dimethylaminopyridine. Reactions between carbazole and diarylamines show unusually selective cross-coupling, even with a 1:1 ratio of the two substrates. This behavior was found to arise from reversible formation of the tetraarylhydrazine. Formation of this species is kinetically favored, but cleavage of the N-N bond under the reaction conditions leads to selective formation of the thermodynamically favored cross-coupling product.

Intramolecular Hydrogen Bonding Enhances Stability and Reactivity of Mononuclear Cupric Superoxide Complexes

[(L)CuII(O2•-)]+ (i.e., cupric-superoxo) complexes, as the first and/or key reactive intermediates in (bio)chemical Cu-oxidative processes, including in the monooxygenases PHM and DβM, have been systematically stabilized by intramolecular hydrogen bonding within a TMPA ligand-based framework. Also, gradual strengthening of ligand-derived H-bonding dramatically enhances the [(L)CuII(O2•-)]+ reactivity toward hydrogen-atom abstraction (HAA) of phenolic O-H bonds. Spectroscopic properties of the superoxo complexes and their azido analogues, [(L)CuII(N3-)]+, also systematically change as a function of ligand H-bonding capability.

Oxidation Catalysis by an Aerobically Generated Dess-Martin Periodinane Analogue

Hypervalent iodine(V) reagents, such as Dess-Martin periodinane (DMP) and 2-iodoxybenzoic acid (IBX), are broadly useful oxidants in chemical synthesis. Development of strategies to generate these reagents from dioxygen (O₂ ) would immediately enable use of O₂ as a terminal oxidant in a broad array of substrate oxidation reactions. Recently we disclosed the aerobic synthesis of I(III) reagents by intercepting reactive oxidants generated during aldehyde autoxidation. In this work, aerobic oxidation of iodobenzenes is coupled with disproportionation of the initially generated I(III) compounds to generate I(V) reagents. The aerobically generated I(V) reagents exhibit substrate oxidation chemistry analogous to that of DMP. The developed aerobic generation of I(V) has enabled the first application of I(V) intermediates in aerobic oxidation catalysis.

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.

Perfluorocyclopentadienyl Radical Derivative as an Organocatalyst for Oxidative Coupling of Aryl- and Thienylmagnesium Compounds under Atmospheric Oxygen

The oxidative homocoupling reaction of Grignard reagents in the presence of atmospheric oxygen molecules proceeded in the presence of a heptafluorotolyl-substituted perfluorocyclopentadienyl radical. The turnover number (TON) was over 30 for the coupling reactions of PhMgBr to give biphenyl. The organocatalyst could couple thienylmagnesium compounds to give bithiophene derivatives in up to 94% yield. Furthermore, a gram-scale synthesis of 6,6'-dimethoxybiphenyl-2,2'-diyl-bis(phosphonic acid diethyl ester) was demonstrated. Stabilization of the phenyl radical for the inhibition of the side reaction was also considered using DFT calculations.

Protein-Derived Cofactors Revisited: Empowering Amino Acid Residues with New Functions

A protein-derived cofactor is a catalytic or redox-active site in a protein that is formed by post-translational modification of one or more amino acid residues. These post-translational modifications are irreversible and endow the modified amino acid residues with new functional properties. This Perspective focuses on the following advances in this area that have occurred during recent years. The biosynthesis of the tryptophan tryptophylquinone cofactor is catalyzed by a diheme enzyme, MauG. A bis-Fe^IV redox state of the hemes performs three two-electron oxidations of specific Trp residues via long-range electron transfer. In contrast, a flavoenzyme catalyzes the biosynthesis of the cysteine tryptophylquinone (CTQ) cofactor present in a newly discovered family of CTQ-dependent oxidases. Another carbonyl cofactor, the pyruvoyl cofactor found in classes of decarboxylases and reductases, is formed during an apparently autocatalytic cleavage of a precursor protein at the N-terminus of the cleavage product. It has been shown that in at least some cases, the cleavage is facilitated by binding to an accessory protein. Tyrosylquinonine cofactors, topaquinone and lysine tyrosylquinone, are found in copper-containing amine oxidases and lysyl oxidases, respectively. The physiological roles of different families of these enzymes in humans have been more clearly defined and shown to have significant implications with respect to human health. There has also been continued characterization of the roles of covalently cross-linked amino acid side chains that influence the reactivity of redox-active metal centers in proteins. These include Cys-Tyr species in galactose oxidase and cysteine dioxygenase and the Met-Tyr-Trp species in the catalase-peroxidase KatG.

Oxidation-induced C-H amination leads to a new avenue to build C-N bonds

In this work, we develop an oxidation-induced C-H functionalization strategy, which not only leads to a new avenue to build C-N bonds, but also leads to different site-selectivity compared with "classic directing-groups". The high selectivity of our new catalytic system originates from the heterogeneous electron-density distribution of the radical cation species which are induced by single electron transfer between the aromatics and oxidant-Cu(ii) species.

CuBr2-Catalyzed Mild Oxidation of 3,4-Dihydro-β-Carbolines and Application in Total Synthesis of 6-Hydroxymetatacarboline D

A green chemical method for the conversion of 3,4-dihydro-β-carbolines to β-carbolines has been developed using air as the oxidant. With 15 mol % CuBr₂ as the catalyst, 3,4-dihydro-β-carbolines could be efficiently oxidized to β-carbolines in dimethyl sulfoxide at room temperature in the presence of 1,8-diazabicyclo[5,4,0]undec-7-ene (or Et₃N). By applying this method, the first total synthesis of 6-hydroxymetatacarboline D was performed through 12 steps in 22% overall yield starting from l-5-hydroxy-tryptophan.

Effect of Redox "Non-Innocent" Linker on the Catalytic Activity of Copper-Catecholate-Decorated Metal-Organic Frameworks

Two new UiO-68 type of Zr-MOFs featuring redox non-innocent catechol-based linkers of different redox activities have been synthesized through a de novo mixed-linker strategy. Metalation of the MOFs with Cu(II) precursors triggers the reduction of Cu(II) by the phenyl-catechol groups to Cu(I) with the concomitant formation of semiquinone radicals as evidenced by EPR and XPS characterization. The MOF-supported catalysts are selective toward the allylic oxidation of cyclohexene and it is found that the presence of in situ-generated Cu(I) species exhibits enhanced catalytic activity as compared to a similar MOF with Cu(II) metalated naphthalenyl-dihydroxy groups. This work unveils the importance of metal-support redox interactions in the catalytic activity of MOF-supported catalysts which are not easily accessible in traditional metal oxide supports.

Distinguishing ionic and radical mechanisms of hydroxylamine mediated electrocatalytic alcohol oxidation using NO–H bond dissociation energies

An idea is proposed to sort N-oxyl radicals with respect to their mechanisms of electrocatalytic alcohol oxidation by knowing the NO–H bond dissociation energies of their precursors.

Cu-catalyzed mild and efficient oxidation of THβCs using air: application in practical total syntheses of perlolyrine and flazin

A mild, efficient and environmentally benign method for synthesis of aromatic β-carbolines via Cu(ii)-catalyzed oxidation of 1,2,3,4-tetrahydro-β-carbolines (THβCs) was developed, in which air (O₂) was used as the clean oxidant. This method has advantages such as environmentally friendliness, mildness, very good tolerance of functional groups, high yielding and easy experiment operation. In addition, this new methodology was successfully applied in the efficient and practical total syntheses of β-carboline alkaloids perlolyrine and flazin.

A mild, efficient and ecofriendly method for synthesis of β-carbolines via Cu-catalyzed aerobic oxidation of 1,2,3,4-tetrahydro-β-carbolines (THβCs) was developed. In addition, this method was successfully applied in the practical total syntheses of perlolyrine and flazin.

Bioinspired design of a hybrid bifunctional enzymatic/organic electrocatalyst for site selective alcohol oxidation

TEMPO and AdhD were chemically conjugated into a bifunctional catalyst that selectively oxidizes primary and secondary alcohols.

Asymmetric hydrofunctionalization of minimally functionalized alkenes via earth abundant transition metal catalysis

The development of earth-abundant transition metal-catalysed highly enantioselective hydrofunctionalization of minimally functionalized alkenes was summarized.

Copper-catalyzed C–S direct cross-coupling of thiols with 5-arylpenta-2,4-dienoic acid ethyl ester

A selective copper (Cu)-catalyzed C–S bond direct cross-coupling of thiols with 5-arylpenta-2,4-dienoic acid ethyl ester was developed. Notably, various biologically active 5-phenyl-3-phenylsulfanylpenta-2,4-dienoic acid ethyl ester derivatives were efficiently synthesized under moderate conditions. Finally, a plausible Cu(i)/Cu(iii) reaction mechanism was proposed.

A selective copper (Cu)-catalyzed C–S bond direct cross-coupling of thiols with 5-arylpenta-2,4-dienoic acid ethyl ester was developed.

Copper-mediated domino C–H iodination and nitration of indoles

An efficient and cost-effective copper-mediated aerobic oxidative C–H iodination and nitration of indoles via double C–H functionalization is reported. The domino process proceeds smoothly under mild aerobic conditions to give 3-iodo-2-nitroindoles in one step with high regioselectivity and a broad substrate scope.