or Carbon

Article Cite This: Acc. Chem. Res. 2018, 51, 1092−1105

pubs.acs.org/accounts

Copper-Catalyzed Oxidative Carbon−Carbon and/or Carbon− Heteroatom Bond Formation with O2 or Internal Oxidants Xiaodong Tang, Wanqing Wu, Wei Zeng, and Huanfeng Jiang* Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China CONSPECTUS: 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. oxidant.4 Notably, the intramolecular interactions between internal oxidants and transition metal catalysts brings many advantages, such as mild reaction conditions, high selectivity, and good functional group tolerance. Although significant achievements have been made in this area of green oxidation, the synthetic scope and utility of green oxidants for more complex molecules are still limited. Over the last decades, transition metal-catalyzed oxidative reactions have gradually become one of the most versatile methods for forming C−C and C-X bonds.5 Compared with

1. INTRODUCTION Oxidative reactions are important not only for lab-scale preparation but also for chemical industries.1 However, in many cases, the utilization of strong oxidants brings extra problems, such as harsh conditions, poor selectivity, and limited substrate scope.2 In addition, the large amount of solid waste generated in these reactions is a major problem for flow chemistry in industrial synthesis. Thus, synthetic chemists continue to develop atom-economical oxidation reactions with green oxidants. Undoubtedly, molecular oxygen (O2) is an ideal oxidant due to its natural, inexpensive, and environmentally friendly characteristics.3 Another ideal oxidant is an internal oxidant that functions not only as a substrate but also as an © 2018 American Chemical Society

Received: December 6, 2017 Published: April 12, 2018 1092

DOI: 10.1021/acs.accounts.7b00611 Acc. Chem. Res. 2018, 51, 1092−1105

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Accounts of Chemical Research

Our long-term objective is to develop green and sustainable chemistry for organic synthesis. For example, we have explored several novel and sustainable reaction media, such as supercritical carbon dioxide (scCO2)7 and ionic liquids,8 to enhance the efficiency and maximize resource utilization. To achieve that long-term goal, we have also sought to develop general and easily prepared reagents for efficient C−C and C-X bond construction. We have previously summarized our work on haloalkynes as a powerful and versatile building block9a and palladium-catalyzed aerobic oxidation of unsaturated hydrocarbons.9b This Account describes our recent progress on copper-catalyzed green oxidative reactions using O2 or internal oxidants as the oxidant. The new transformations elaborated here will provide a platform for expanding the scope and utility of green oxidation.

Scheme 1. Reaction Pathways for Copper-Catalyzed Oxidation

2. COPPER-CATALYZED OXIDATION BY O2 2.1. C−C Bond Formation

Alkynes are fundamental and frequently used moieties in organic chemistry. However, the direct synthesis of alkynes precious metals, copper is inexpensive and features a low toxicity, easy utilization, and stable properties. Moreover, copper is a multifunctional catalyst that can easily access the Cu(0), Cu(I), Cu(II), and Cu(III) oxidative states. The reaction pathway of copper-catalyzed oxidations can go through single-electron transfer (Scheme 1, path a), a two-electron process (Scheme 1, path b), or cooperative one- and twoelectron processes (Scheme 1, path c).6 Furthermore, lowvalent copper species can be reoxidized to tunable oxidative states by O2 or an internal oxidant. Consequently, coppercatalyzed oxidative reactions are one of the most practical transformation methods and is applied widely in industrial syntheses, such as the oxidative polymerization of 2,6dimethylphenol, synthesis of 2,3,5-trimethyl-p-quinone, and oxidation of alcohols.

Scheme 3. Tentative Pathway for the Synthesis of Acetylenic Derivatives

Scheme 2. Copper-Catalyzed Aerobic Oxidation of Hydrazones to Alkynes

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Accounts of Chemical Research 2.2. C−C and C−Heteroatom Bond Formation

Table 1. Copper-Catalyzed Carboesterification of Alkenes with Acetic Anhydrides

2.2.1. Cycloaddition with C−C and C−O Bond Formation. Transition metal-catalyzed olefin difunctionalization continues to attract broad interest. In 2010, our group developed a novel copper-catalyzed aerobic oxidative carboesterification of olefins with anhydrides to form γ-lactones.11 Both Cu(I) and Cu(II) catalysts were effective. The strong Lewis acidity of Cu(OTf)2 caused this catalyst to preferably activate olefin. Thus, Cu(OTf)2 exhibited the best activity and selectivity. Under the optimal conditions, various styrenes, conjugated dienes and aliphatic alkenes were suitable for this cyclization process. When propionic anhydride was used as the substrate, the syn/anti ratio of the resulting γ-lactones was approximately 1:1 (Table 1). The [Cu]/O2 system was successfully used to replace the Mn(IV)-mediated carboesterification of olefins.12 The furan unit is a common skeleton that occurs in many natural products and important drug molecules. Methods for synthesizing highly functionalized furans from available materials, especially polysubstituted furans, are rare. In 2009, we developed a copper-catalyzed aerobic oxidative reaction for the synthesis of polysubstituted furans from alkynols and dialkyl but-2-ynedioate (Scheme 4a).13a We ensured that the oxygen atom of the newly formed carbonyl group originated from O2 via using 18-labeled oxygen as the oxidant. The catalytic pathway was initiated by the nucleophilic addition of the alkynols to dialkyl but-2-ynedioate, which was followed by cyclization/oxidation with O2 to generate (2-furyl)carbene complex 5. Finally, (2-furyl)carbine complex 4 underwent carbene oxidation with O2 metathesis to produce the αcarbonyl furans.14 To facilitate the hydroxylation of the C−C triple bond, the electron-deficient alkyne was limited specifically to dialkyl but-2-ynedioate in this method. In a further study, heterogeneous Cu2O nanoparticles were also applied as suitable catalysts, and the scope of the electron-deficient alkyne was expanded to ethyl phenylpropynoate and alkynyl ketones (Scheme 4b).13b In 2013, we reported a novel strategy for the regioselective generation of trisubstituted furans from propiolaldehyde and 1,3-dicarbonyl compounds.15 These transformations exhibited excellent functional group tolerance (Table 2). Notably,

from other functional groups is still underdeveloped. In 2014, we reported the first example of the oxidation of Ntosylhydrazones to alkynes in a [Cu]/O2 system (Scheme 2).10 The base and oxidant were two key factors in those transformations. We suspected that the organic base 1,4diazabicyclo[2.2.2]octane (DABCO) not only served as a base but also played the role of a ligand. No other oxidants besides O2 led to product formation. The present protocol could be applied in the alkynylation of biologically active compounds such as steroid 1. A possible mechanism is shown in Scheme 3. First, the Ntosyl hydrazone reacted with DABCO to form diazo 2. The carbene dimer was main product in the absence of the copper catalyst, which provided strong evidence for this step. Diazo 2 combined with Cu(I) to give copper carbene species 3, which underwent aerobic oxidation and dehydrogenation to generate terminal alkynes (R2 = H) or internal alkynes (R2 ≠ H). The in situ formed terminal alkyne was subsequently transferred to the corresponding cuprous acetylide intermediate 4 followed by Glaser coupling or Sonogashira coupling with a terminal alkyne or aryl halide to afford a 1,3-diyne or internal alkyne. Scheme 4. Copper-Catalyzed One-Pot Synthesis of Furans

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Accounts of Chemical Research Table 2. Synthesis of Trisubstituted Furans from Propiolaldehyde and 1,3-Dicarbonyl Compounds

Scheme 5. Synthesis of Different Furan Derivatives under Various Reaction Conditions

Table 3. Synthesis of Pyridines via C−N Bond Cleavage

Scheme 6. Copper-Catalyzed Aerobic Oxidative Cyclization of Phenols and Alkynes

Scheme 7. Plausible Mechanism for the Synthesis of Pyridines

different furan derivatives were obtained when the reaction conditions were adjusted. 2-Vinyl furans 6 were formed in CH2Cl2 under water- and oxygen-free conditions (Scheme 5, eq 1), whereas methyl ether furans 7 were generated when methanol was used as the solvent (Scheme 5, eq 2). Furyl dimer 8 was detected by GC-MS when THF was utilized instead of methanol (Scheme 5, eq 3), which indicated that a

copper carbene intermediate was possibly formed during the process.16 1095


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Accounts of Chemical Research Scheme 8. Synthesis of Substituted 1,3,4-Oxadiazoles and Mechanistic Hypothesis

Scheme 11. Cu(I)-Catalyzed Synthesis of Imidazo[1,5a]pyridines via C(sp3)-H Amination

Scheme 9. Synthesis of Pyrazoles and Indazoles and Plausible Reaction Pathway

Table 4. Synthesis of 1,2,4-Triazoles and 1,3,5-Triazines from Amidines

Table 5. Oxidative Sulfonation of Alkenes

Scheme 10. Copper-Catalyzed Synthesis of 2-HaloSubstituted N-Heterocyclics

The tandem strategy, which is initiated by hydroxylation, has also been utilized to synthesize benzofurans. The nucleophilic addition/aerobic oxidative cyclization strategy for benzofurans uses phenols and inactivated internal alkynes as starting materials (Scheme 6).17 In 2013, Sahoo’s group reported the same transformation using Pd as the catalyst and a stoichiometric amount of copper salts as the oxidant.18a Meanwhile, Shi and co-workers performed those transformations with several equivalents of Cu(OTf)2.18b We found that using the polar solvent PhNO2 and adding ZnCl2

as an additive could produce the desired products in good to excellent yield with oxygen as the terminal oxidant.18b 2.2.2. Condensation with C−C and C−N Bond Formation. In the past few years, transition metal-catalyzed cleavage of C−N bonds has become a popular research topic. However, the reassembly of two bond cleavage fragments in the desired structure is challenging and of high atom economy. In 2013, we developed a copper-catalyzed aerobic oxidative fragment-assembling strategy for constructing pyridine deriva1096


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1,3,4-oxadiazoles (Scheme 8).21 The proposed catalytic pathway was initiated by substrate isomerization to give intermediate 11, which reacted with the Cu(II) catalyst to form intermediate 12. Metalation at the imine sp2 C−H bond of intermediate 12 gave metallacycle 13, which was followed by reductive elimination to generate the desired product. The copper catalyst was regenerated by the oxidation of O2. Although the substrate scope was limited, this reaction provided a novel way to synthesize 1,3,4-oxadiazoles via copper-catalyzed intramolecular C−H bond activation and C−O bond formation. 2.3.2. C−N Bond Formation. The success of coppercatalyzed oxidative C−O bond formation in the synthesis of 1,3,4-oxadiazoles inspired us to explore the construction of the corresponding N-containing heterocycles through coppercatalyzed oxidative C−N bond formation. Using this strategy, we developed a copper-catalyzed direct oxidative intramolecular C(sp2)−H amination reaction to form pyrazoles and indazoles (Scheme 9).22 Radical inhibitors (hydroquinone, (2,2,6,6tetramethylpiperidin-1-yl)oxyl (TEMPO)) did not inhibit the reaction, which indicated that a radical process was not involved in this transformation. Based on the above control experiment and a previous report,23 we proposed that the cyclization reaction started with the reaction of the substrate with Cu(OAc)2 to produce Cu−N adduct 14. Subsequent formation of metallacycle 15 by electrophilic metalation or C−H bond activation was followed by reductive elimination to give the products and a low-valent copper species, which was subsequently reoxidized by O2 to regenerate the active catalyst. Haloalkynes are highly electron-deficient alkynes, and nitrogen-containing nucleophiles can undergo nucleophilic addition with haloalkynes to form enamine intermediates.24 In 2013, we utilized o-aminopyridine as a binucleophile to react with haloalkynes, and after the enamine intermediate formed, C−N bond formation occurred under the [Cu]/O2 system (Scheme 10).25 This novel and efficient method successfully constructed various 2-halo-substituted imidazo[1,2-a]pyridines, imidazo[1,2-a]pyrazines, and imidazo[1,2-a]pyrimidines. Furthermore, the newly formed 2-halo-substituted N-heterocycles could be easily transformed to other functionalized Nheterocyclics via Pd-catalyzed coupling reactions. In addition to copper-catalyzed C(sp2)−N bond formation to form the corresponding heterocycles, we successfully developed a Cu(I)-catalyzed method for the direct C(sp3)−H amination of 2-acylpyridines with aliphatic amines to construct multifunctional imidazo[1,5-a]pyridines (Scheme 11).26 Interestingly, Cu(I) salts were suitable catalysts, while the corresponding Cu(II) salts exhibited no catalytic activity, and adding TEMPO completely inhibited the transformation. The intramolecular isotope effect (KH/KD = 1.10) of 17 indicated that C(sp3)-H bond cleavage was not the rate-limiting step. The proposed catalytic pathway was initiated by dehydrative condensation to form imine 16, which was followed by coordination of a Cu(I) ion to two N atoms to generate intermediate 17. Intermediate 17 then combined with O2 to form Cu(II)-superoxo radical 18, and a six membered Cu(III) species 19 was obtained via intramolecular hydrogen absorption/isomerization/oxidation processes. Finally, the product was formed by reductive elimination. Nitrogen-enriched heterocycles such as 1,2,4-triazoles and 1,3,5-triazines widely exist in drug molecules and exhibit excellent bioactivities. In 2015, we disclosed a novel method for the synthesis of 1,2,4-triazoles and 1,3,5-triazines from amidines

Table 6. Oxidative Sulfenylation of Aromatic Rings

Scheme 12. Proposed Mechanism for Copper-Catalyzed Oxidative C(sp2)−H Sulfonation/Sulfenylation

tives in a process involving C−N bond cleavage.19 The optimized system was successfully applied to the synthesis of symmetrical 2,4,6-trisubstituted pyridines. Furthermore, when 1-(4-methoxyphenyl)ethanone and benzylamine were treated with other methyl ketones, the asymmetrical pyridines could be formed as major products (Table 3). The control experiments showed that benzylamine could be completely transformed to imine under the [Cu]/O2 system. According to the above control experiments and related mechanistic studies by Stahl,20 the reaction mechanism was proposed as shown in Scheme 7. First, imine 9 was formed via the single-electron oxidation of benzylamine by Cu(II) and subsequent aminolysis. Afterward, the reversible hydrolysis of imine 9 gave benzaldehyde and benzylamine. Subsequently, 1,4-dihydropyridine 10 was generated by the Lewis acidpromoted (Cu(II)) condensation of ketone, benzylamine, and benzaldehyde. Finally, copper-catalyzed aerobic oxidative C−N bond cleavage produced the desired pyridine and released one molecule of benzaldehyde. 2.3. C−Heteroatom Bond Formation

2.3.1. Cyclization with C−O Bond Formation. In 2012, we described an efficient copper-catalyzed aerobic oxidative approach via intramolecular C−O bond formation to generate 1097


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Accounts of Chemical Research Table 7. Copper-Catalyzed Oxidative [2 + 2+1] Cycloaddition To Form 1,3-Oxazoles

denitrogenation, and in the presence of a CuCl catalyst and LiBr additive, the oxidative sulfonation of alkenes was achieved with high chemo- and regioselectivity (Table 5). The sulfonyl hydrazides could also be used as a thioether source to couple with electron-rich aromatic rings or heteroaromatic rings (Table 6). Pyridine-directed C−H sulfenylation could also be achieved in 64% yield. A tentative mechanism is illustrated in Scheme 12. We proposed intermediate 20 as a key intermediate not only because it was obtained as the main byproduct but also because it could realize the sulfonation of indole. Initially, sulfonothioate 20 formed from the oxidative decomposition of sulfonyl hydrazides.29a Subsequently, the sulfonothioate combined with a Cu(I) complex to afford the active sulfenyl cuprate(III) 21 or sulfonyl cuprate(III) 22, which could activate the C(sp2)−H bond of aromatic rings or alkenes to form intermediate 23 or 24.29b Finally, reductive elimination provided aryl sulfides or vinyl sulfones, respectively. 2.3.4. Cycloaddition with C−O and C−N Bond Formation. The C−N triple bond in nitrile is quite stable. Thus, the effective activation of nitrile is difficult, especially in a mild catalytic [Cu]/O2 system. To date, few examples have been reported for the formation of N-heterocycles through copper-catalyzed aerobic oxidative cyclization involving nitrile.30 In 2012, we reported a copper-catalyzed aerobic oxidative [2 + 2+1] cycloaddition reaction for the regioselective synthesis of 1,3-oxazoles from an aryl-substituted alkyne, nitrile and water.31 Under the optimized reaction conditions, both symmetrical and unsymmetrical alkynes were suitable substrates. The biologically active molecule 25,32 which is a COX-2 inhibitor, was effectively synthesized with this method in three total steps (Table 7). An isotopic labeling study with H218O clearly demonstrated that the oxygen atoms of the products originated from water. A

Scheme 13. Proposed Mechanism for the Copper-Catalyzed Synthesis of Oxazole Derivatives

via copper-catalyzed aerobic oxidative C(sp3)-H functionalization.27 When three different reaction partners, trialkylamines, DMSO and DMF, were used, 2,4,6-trisubstituted and 2,6disubstituted 1,3,5-triazines (trialkylamine) and 1,3-disubstituted 1,2,4-triazoles (DMSO/DMF) were obtained. This transformation had good functional group tolerance and produced various three-nitrogen-containing heterocycles in good to excellent yields (Table 4). 2.3.3. Cross-Coupling with C−S Bond Formation. Copper-catalyzed C−H bond activation/intramolecular C−X bond formation was applied to the construction of various heterocycles, and we extended this strategy to the intermolecular version to construct linear products via C−S bond formation. The regio- and stereoselective synthesis of sulfones and thioethers was achieved via the copper-catalyzed oxidative coupling of terminal olefins with sulfonyl hydrazides.28 The sulfonyl hydrazide could undergo dehydrogenation and 1098


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Accounts of Chemical Research Scheme 14. Proposed Pathways for Copper-Catalyzed Oxidation with Oxime Ester

preliminary reaction pathway was proposed based on the experimental results, as shown in Scheme 13. Initially, the nucleophilic attack of alkynes by nitrile produced intermediate 26, which was attacked by H2O to give intermediate 27. Next, the coordination of a hydroxy group and copper center formed intermediate 28. Finally, the reductive elimination of intermediate 28 afforded the 1,3-oxazole product, and the reduced copper species could be oxidized by O2 to complete the catalytic cycle.33

Table 8. Synthesis of Sulfone Derivatives

3. COPPER-CATALYZED OXIDATION BY INTERNAL OXIDANTS Copper-catalyzed direct transformations involving internal oxidants are attractive since they have many advantages, such Scheme 15. Synthesis of Enaminones and Plausible Reaction Pathway Scheme 16. Studies on the Reaction Mechanism for the Synthesis of Sulfone Derivatives

as the lack of a requirement for external oxidants, mild conditions and good selectivities. The N−O bond in oxime ester can be easily cleaved by a copper catalyst to act as an oxidant. According to studies on copper-catalyzed transformations of oxime ester,34 two major pathways are generally proposed. The first process involves organocopper(III) intermediates, which are formed via oxidative insertion of Cu(I) into the N−O bond of oxime ester (Scheme 14, eq 1). The other pathway is a single-electron transfer (SET) process, in which an imine radical intermediate is generated via homolytic cleavage of the N−O bond by Cu(I). Subsequently, the imine radical isomerizes to an α-carbon radical and then

forms the target product (Scheme 14, eq 2, path a). Alternatively, the imine radical might also associate with Cu(I) to form a Cu(II) enamide intermediate, which would undergo oxidative cyclization to provide the N-heterocycles (Scheme 14, eq 2, path 1b). 1099


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Accounts of Chemical Research Table 9. Conversion of Pyridine and Isoquinolines to Imidazo[1,2-a]pyridines and Imidazo[1,2-a]isoquinolines

Scheme 17. Plausible Reaction Mechanism

Scheme 18. Synthesis of 1H-Indazoles and Proposed Pathway

3.2. C-Heteroatom Bond Formation

3.2.1. Cross-Coupling with C−S Bond Formation. Transition metal-catalyzed oxidative C−S bond formation through C−H bond activation is an attractive process. In 2014, our group successfully developed a novel method for producing sulfone derivatives by the oxidative coupling of sodium sulfinates and oxime esters via C(sp3)-S bond formation.36 β-Sulfonylvinylamines were formed in situ, and β-ketosulfones were obtained upon hydrolysis. Very few methods were previously available for constructing βsulfonylvinylamines.37 This strategy showed a broad substrate scope under mild conditions (Table 8). Free radical scavengers completely inhibited the reaction, and a low yield of coupling product was obtained when 1,1diphenylethylene was added to trap the sulfonyl radical. According to the above results, we suggested a radical pathway,

3.1. C−C Bond Formation

Selective radical/radical cross-coupling represents a powerful approach for achieving C−C bond formation. Although considerable achievements have been made in this area, little attention has been paid to realizing this process under redoxneutral conditions.34c In 2017, we discovered a coppercatalyzed reaction of α-oxocarboxylic acids with oxime esters to form enaminones via radical/radical cross-coupling under redox-neutral conditions (Scheme 15).35 Cu(II) salts showed no activity in those transformations, and free radical inhibitors completely suppressed the reaction. We thus proposed a radical−radical coupling mechanism.34c The transformation was limited to (het)aryl-substituted substrates due to the stability of the corresponding radical intermediates. 1100


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Accounts of Chemical Research Table 10. Synthesis of Indolo[1,2-a]quinazoline Derivatives

Scheme 19. Synthesis of 2-Aminothiazoles and Thiazoles in a Copper/Oxime Ester System

Scheme 20. Synthesis of N-Heterocycles via Cu(II) Enamide Intermediates

Scheme 21. Transformations of Pyrroles

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Accounts of Chemical Research Scheme 22. Construction of Nature-Based Pyridine Skeletons

synthesizing 2-aminothiazoles is the Hantzsch reaction, which involves the coupling of α-halocarbonyl compounds with thioureas. In 2016, we developed a new strategy for synthesizing 2-aminothiazoles from oxime esters and isothiocyanates via a copper-catalyzed oxidative cyclization reaction (Scheme 19, eq 1).41 Encouraged by these results, we also developed a copper-catalyzed [3 + 1 + 1]-type condensation reaction of oxime, anhydrides and potassium thiocyanate (KSCN) to achieve the synthesis of thiazoles (Scheme 19, eq 2).42

but an organometallic pathway could not be overlooked (Scheme 16). In the radical pathway (path a), the C−S bond was formed through the addition of the sulfonyl radical into the C−C double bond, while in the organometallic pathway (path b),34b the C−S bond was constructed via the reductive elimination of an organocopper(III) intermediate. 3.2.2. C−N Bond Formation. The direct dearomatization of pyridine is very difficult owing to the low energy of its π system. In 2013, we reported the transformation of pyridine to imidazo[1,2-a]pyridines via copper-catalyzed aerobic cyclization of oxime esters.38 Various aryl and alkyl oxime esters were good coupling partners. In addition to pyridines, isoquinolines were also suitable substrates for those reactions (Table 9). A control experiment showed that free radical scavengers (TEMPO or ethene-1,1-diyldibenzene) did not prohibit the transformation. We further conducted an intermolecular competition reaction with equal amounts of pyridine and d5pyridine and measured a kinetic isotope effect (KIE) of 1, which suggested that H atom abstraction from pyridine was not the rate-determining step. According to the above results, a preliminary reaction pathway was proposed, as shown in Scheme 17. Initially, the oxidative addition of Cu(I) into the oxime ester formed Cu(III) intermediate 29, in which the Cu− N bond of the Cu(III) intermediate inserted into pyridine to afford the intermediate, 30. Next, isomerization and intramolecular H atom abstraction produced copper ring intermediate 31. Finally, reductive elimination and oxidative aromatization led to the final product. In 2013, we developed an efficient method for the synthesis of 1H-indazoles via a copper-catalyzed cascade cyclization reaction from 2-bromoaryl oxime esters and amines (Scheme 18).39 In the presence of the copper catalyst, intermediate 32 was first generated via an Ullmann-type reaction from 2bromophenyl oxime esters and aniline. Then, intermediate 32 could go through two possible pathways to give the desired product. The amino group could directly attack the oxime ester group to produce the desired product (path a). In path b, Cu(III) intermediate 33 was formed by the oxidative addition of Cu(I) into the N−O bond of the oxime ester followed by coordination of the nitrogen atom to the copper center to form intermediate 34. Finally, reductive elimination of intermediate 34 afforded the product.30 In addition to the amines, we also attempted to react indole and pyrrole with 2-haloaryl oxime esters. Thus, a convenient and practical copper-catalyzed cascade annulation for the synthesis of indolo[1,2-a]-quinazolines via N-1 and C-2 difunctionalization of indoles with 2-iodoketoxime ester was developed (Table 10).40 3.2.3. C−S and C−N Bond Formation. 2-Aminothiazole is a common unit in drug molecules. The classical method for

3.3. C−C and C-Heteroatom Bond Formation

The combination of oxime ester with a copper catalyst can form a Cu(II) enamide intermediate, which is a weakly nucleophilic reagent. Guan, Yoshikai and their co-workers used electrondeficient compounds to capture a Cu(II) enamide intermediate and form N-heterocycles.34d,e Our recent work in this area included the synthesis of a variety of pyrroles, pyrazoles and pyridine. In 2013, we reported a copper-catalyzed [3 + 2]-type cyclization reaction for the synthesis of pyrroles from oxime esters and dialkyl but-2-ynedioates (Scheme 20, eq 1).43 To further demonstrate the synthetic applications of this protocol, we transformed the pyrrole products to pyrrolo-[2,1-a]isoquinolines via a ruthenium-catalyzed aerobic oxidative cyclization reaction (Scheme 21),44 where pyrrolo[2,1-a]isoquinoline is the skeleton structure of lamellarine alkaloids, which exhibit potentially useful biological activities, such as antitumor and anti-HIV activities.45 Later, we attempted to cyclize a Cu(II) enamide intermediate with an imine formed in situ from amine and paraformaldehyde to synthesize pyrazoles in the presence of a copper catalyst (Scheme 20, eq 2).46 There are few examples of the synthesis of pyrazoles via N−N bond formation.30a In 2015, we further cyclized a Cu(II) enamide intermediate with α,β-unsaturated ketones formed in situ from aldehydes and activated methylene compounds to construct functionalized pyridines (Scheme 20, eq 3).47 To further demonstrate the application value of this method, we successfully transformed oxime esters derived from natural products (L-(−)-carvone and β-ionone) to the corresponding pyridine skeletons in moderate yields (Scheme 22).

4. CONCLUSIONS AND OUTLOOK This Account has highlighted our recent studies on the development of green oxidation processes through copper catalysis with O2 or internal oxidants. We developed a variety of practical and efficient methods for the synthesis of useful and fundamental moieties from readily available materials. Furthermore, those strategies have been applied to the elaboration of pharmaceutical molecules and skeletons of natural products. 1102



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Meanwhile, we are also interested in elucidating the mechanistic principles that underlie copper-catalyzed green oxidative reactions, which will provide a foundation for the development of even greener and more atom-economical methodologies in copper-catalyzed oxidation. We envision that this Account will help to promote continued interest in the field of copper-catalyzed green oxidation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaodong Tang: 0000-0002-0051-105X Wei Zeng: 0000-0002-6113-2459 Huanfeng Jiang: 0000-0002-4355-0294 Notes

The authors declare no competing financial interest. Biographies Xiaodong Tang received his PhD from South China University of Technology with Professor Huanfeng Jiang in 2016. He then joined Southern Medical University. His current research is focused on the development of green organic synthetic methods and drug synthesis. Wanqing Wu received her PhD from Peking University with Professor Zhen Yang and Chi-Sing Lee in 2010. She then joined Professor Huanfeng Jiang’s group as a postdoctoral researcher at South China University of Technology (SCUT). In 2014, she was promoted to Professor, and her research interests include the development and applications of new synthetic methods. Wei Zeng received his Bachelor’s (1994), Master’s (2000) and Ph.D. (2003) degrees at Sichuan University with Professor Shenying Qin. In 2004 he joined Professor V. W. W. Yam’s lab at The University of Hong Kong as a Research Assistant. Then he joined Professor C. Melander’s group (The North Carolina State University, 2005-2006) and Professor S. R. Chemler’s lab (The State University of New York at Buffalo, 2006-2008) as a postdoctorate, respectively. He joined South China University of Technology as a full Professor in 2008, and his lab’s research interests involve the development of novel organic synthetic methodologies including asymmetric catalysis. He was a recipient of New Century Excellent Talents in University, Ministry of Education, China (2009). Huanfeng Jiang received his PhD from the Shanghai Institute of Organic Chemistry (SIOC) with Professor Xiyan Lu in 1993. He then joined the Guangzhou Institute of Chemistry as a research fellow. In 2003, he moved to South China University of Technology (SCUT) as the Leading Professor of Chemistry. He received the Chinese Chemical Society-BASF Young Investigator’s Award in 2002 and the National Natural Science Funds for Distinguished Young Scholar in 2006. His research interests focus on synthetic methodology and green and sustainable chemistry.

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ACKNOWLEDGMENTS We are deeply indebted to all of the co-workers, whose names can be found in the references, for their significant contributions to the projects described herein. We gratefully acknowledge financial support from the National Key Research and Development Program of China (2016YFA0602900) and the National Natural Science Foundation of China (21420102003 and 21672072). 1103


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