Cobalt-Catalyzed Decarboxylative C–H (Hetero)Arylation for the

Letter Cite This: Org. Lett. 2017, 19, 5589-5592

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Cobalt-Catalyzed Decarboxylative C−H (Hetero)Arylation for the Synthesis of Arylheteroarenes and Unsymmetrical Biheteroaryls Yanrong Li,† Fen Qian,† Mengshi Wang,† Hongjian Lu,*,† and Guigen Li*,†,‡ †

Institute of Chemistry and BioMedical Sciences, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China ‡ Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States S Supporting Information *

ABSTRACT: A cobalt-catalyzed decarboxylative cross-coupling reaction of (hetero)aryl carboxylic acids with benzothiazoles or benzoxazoles is reported. This represents a first example of metal-catalyzed decarboxylative C−H heteroarylation of benzo-fused heterocycles. The transformation provides a convenient route, with good yields and functional group tolerance, to various important arylheteroaryl and unsymmetrical biheteroaryl structural motifs.

B

Scheme 1. Transition-Metal-Catalyzed Decarboxylative C−H Heteroarylation of Heteroarenes

iheteroaryl compounds, such as those in Figure 1, are a class of privileged scaffolds with interesting biological and

Figure 1. Selected biheteroaryl structures.

Pd(OAc)2/CuCO3 system.8a In 2012, Su et al. reported a Pdcatalyzed decarboxylative C−H heteroarylation reaction of thiophenes, providing various 2-heteroarylthiophenes with Ag2CO3 as the oxidant.8b Currently, catalytic decarboxylative C−H heteroarylation systems for the synthesis of unsymmetrical biheteroaryls employ noble metal−palladium catalysts and have a narrow substrate scope with nonbenzo-fused heterocycles and limited heteroaryl carboxylic acids. In view of the importance of biheteroaryls,1 the development of environmentally friendly decarboxylative C−H heteroarylation of benzo-fused heterocycles will be of prime synthetic value. Recently, due to its inexpensiveness, low toxicity, and unique catalytic modes, Co has been extensively explored in the catalysis of C−H bond functionalization.10 Inspired by our recent work on Co-catalyzed C−H bond functionalization,5c,11,12 we report here the first example of Co-catalyzed decarboxylative C−H heteroarylation of benzo-fused heterocycles, providing various types of biheteroaryl structural motifs (Scheme 1B). This catalytic system could be used in arylation reactions for the synthesis of arylheteroarenes.

physical properties and are often found in organic functional materials, pharmaceuticals, ligands, and natural products.1 Transition-metal-catalyzed cross-coupling reactions of two functionalized heteroaromatic units are the traditional methods with which to access biheteroaryl moieties.2 Recently, transitionmetal-catalyzed C−H heteroarylation has emerged as an efficient and straightforward approach to the synthesis of biheteroaryls.3,4 Decarboxylative C−H functionalization,5 which merges the modern C−H functionalization processes with state-of-the-art decarboxylation protocols, uses widely available carboxylic acids as attractive alternatives to traditional aryl (pseudo)halides and organometallic counterparts to minimize waste products by loss of CO2 and opens novel perspectives regarding functional and space diversity.6,7 In the carboxylate substrate, the position of the C−C bond formation is predefined, and in the hydrocarbon reactant, dimerization is minimized. In this way, it becomes an ideal strategy for C−C bond formation. In fact, construction of biheteroaryl compounds by transition-metal-catalyzed decarboxylative C−H heteroarylation by a few systems has been reported (Scheme 1A).8,9 In 2010, Greaney et al. reported a decarboxylative C−H heteroarylation reaction of oxazoles catalyzed by a © 2017 American Chemical Society

Received: September 1, 2017 Published: October 9, 2017 5589

DOI: 10.1021/acs.orglett.7b02730 Org. Lett. 2017, 19, 5589−5592

Letter

Organic Letters Table 1. Optimization of Reaction Conditionsa

Scheme 2. Synthesis of Unsymmetric Biheteroarylsa

entry

[Co]

ligand

solvent

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22d 23e 24f 25g 26h

CoBr2 CoBr2 CoBr2 CoBr2 CoBr2 CoBr2 CoCl2 Co(acac)2 Co(OAc)2 CoCO3 Co(NO3)2·6H2O CoClO4·6H2O Co(acac)3 CoBr2 CoBr2 CoBr2 CoBr2 CoBr2 CoBr2 CoBr2

− PCy3 PPh3 TPO IMe·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl IPr·HCl

C6H5F C6H5F C6H5F C6H5F C6H5F C6H5F C6H5F C6H5F C6H5F C6H5F C6H5F C6H5F C6H5F C6H5CF3 C6H5Cl C6H5Br C6H5CH3 2-F-C6H4CF3 3-F-C6H4CF3 4-F-C6H4CF3 C6H5F 2-F-C6H4CF3 2-F-C6H4CF3 2-F-C6H4CF3 2-F-C6H4CF3 2-F-C6H4CF3

25 15 5 29 41 65 18 15 trace 41 10 trace trace 66 46 34 41 82 (81c) 72 62 0 36 18 29 11 59

CoBr2 CoBr2 CoBr2 CoBr2 CoBr2

a

Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), and 2fluorobenzotrifluoride (0.8 mL), isolated yield. bWith 1.0 mmol scale.

and 2-fluorobenzotrifluoride proved to be an optimal solvent providing 3a in 81% isolated yield (entry 18). Control experiments showed that no desired product was formed in the absence of a cobalt salt (entry 21). When the reaction was carried out under the reduced catalyst loading, ligand or 2a, the decreased yield of 3a was observed (entries 22−26). At lower reaction temperatures, the yield of 3a was greatly decreased (Table S1 in SI). With the optimum reaction conditions, a wide range of benzofused heterocycle substrates were screened (Scheme 2). The reaction of benzothiazoles bearing Cl or Br atoms in different positions of the phenyl ring proceeded smoothly, providing the corresponding halogenated biheteroaryl products in good to excellent yields (3b, 3g−j). This is useful because the halogen atoms in the products could provide more options for further functional transformations. Aromatic rings substituted with electron-donating groups such as methoxyl (3c−d, 3k, 3q) and electron-withdrawing groups (such as Cl and Br) were tolerated. The scope of benzo-fused heterocycles is not limited to benzothiazoles. Benzoxazoles are also efficient substrates and provided the desired products in good yields (3n−o). Without further optimizing reaction conditions, nonfused heterocycles such as 5-phenyloxazole and methyl 5-phenyloxazole-4carboxylate could react with 2a to give the cross-coupling products, despite in low yields (3s (37%), 3t (33%) in SI). No desired product was observed when 4,5-dimethylthiazole was used as a coupling component. The scope of heteroaryl carboxylic acids was then examined under the optimal reaction conditions. Substituted and unsubstituted benzothiophene-2-carboxylic acids at the 3-position reacted with various benzothiazoles or benzoxazoles to produce the corresponding decarboxylative cross-coupling products 3a−e, 3n in good yields. In addition, benzofuran-2-carboxylic acids with substituents in different positons of the aromatic ring also reacted smoothly (3f−m, 3o). More interestingly, other heteroaryl carboxylic acids, such as 1-methyl-indole-2-carboxylic acid, benzothiazole-2-carboxylic acid, and thiazole-5-carboxylic acid, were tolerated in this catalytic

a Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), [Co] (0.04 mmol), ligand (0.04 mmol), Ag2CO3 (0.5 mmol), and solvent (0.8 mL), 160 °C, 24 h. bCrude 1H NMR yields determined by using dibromomethane as an internal standard. cIsolated yield. d10 mol % CoBr2 was used. e5 mol % CoBr2 was used. f10 mol % IPr·HCl was used. g5 mol % IPr·HCl was used. h2 equiv of 2a was used. PCy3 = Tricyclohexyl phosphine, TPO = Triortho-tolylphosphine, IMes = 1,3Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, IPr = 1,3-Bis(2,6-diisopropylphenyl)-imidazol-2-ylidene.

Initially, we examined the decarboxylative cross-coupling of benzothiazole (1a) with 3-methylbenzothiophene-2-carboxylic acid (2a) in the presence of catalytic amounts of CoBr2 with Ag2CO3 as an oxidant (Table 1, entry 1). The desired product, 2(3-methylbenzothiophen-2-yl)benzothiazole (3a), was obtained in 25% yield, with the symmetric biheteroaryl 2,2′-bibenzothiazole (3a′),13 generated in 19% yield from the oxidative homocoupling reaction of benzothiazole (1a) (Scheme S1 in Supporting Information (SI)). To minimize the formation of such homocoupling products, an extra ligand was added to the reaction system to control both the reaction rate of the Ag-mediated decarboxylation and rate of the Co-promoted process (entries 2− 6). While phosphine ligands give lower yields, N-heterocyclic carbene ligands promote the decarboxylative C−H functionalization and inhabit the formation of bibenzothiazole 3a′. When 20 mol % of IPr·HCl was added, 3a was formed in 65% yield (entry 6). Several different types of commercially available cobalt salts were screened (entries 6−13), and it was found that Co complexes, including Co(acac)2, CoCO3, and Co(NO3)2· 6H2O, catalyzed the reaction, producing low yields of 3a. Subsequently, different solvents were examined (entries 14−20) 5590

DOI: 10.1021/acs.orglett.7b02730 Org. Lett. 2017, 19, 5589−5592

Letter

Organic Letters system and provided the desired products (3p−r) with different biheteroaryl skeleton structures. When indole-2-carboxylic acid was used to couple with benzothiazole (1a), no desired product was obtained. Although Pd14 and first row transition metals such as Ni15 and Cu16 can be utilized in the synthesis of arylheteroarenes by decarboxylative C−H arylation reactions, the carboxylic acid substrate scope of these reactions is limited to a few aryl carboxylic acids bearing strong electron-withdrawing groups in the ortho position of the phenyl ring. To expand the utility of the reaction system, the scope of aryl carboxylic acids was explored (Scheme 3). The reaction of 2-nitrobenzoic acid derivatives with electron-

Scheme 4. Mechanistic Studies

Scheme 3. Synthesis of Arylheteroarenesa

H/D exchange in the reaction of deuterated substrate D-1n and 3methylbenzothiophene-2-carboxylic acid (2a) was essentially absent (Scheme 4B). Primary kinetic isotope effects were observed in parallel experiments (Scheme 4C), suggesting that the C−H bond cleavage of benzoxazoles may be the turnoverlimiting step in the reaction. Two intermolecular competition experiments were carried out (Scheme 4D), suggesting that electron density in aromatic rings of benzoxazoles does not affect the reaction rate. On the basis of these results and previous reports,10,11 we propose a possible Co(III/IV/II) catalytic cycle (Path 1, Scheme 5). First, the Co(II) catalyst is oxidized by Ag2CO3 to afford the Scheme 5. Proposed Catalytic Cycle a Reaction conditions: 1 (0.2 mmol), 4 (0.6 mmol), and 2fluorobenzotrifluoride (0.8 mL), isolated yield.

donating or -withdrawing groups in the aromatic ring proceeded smoothly providing the expected products in good to high yields (5a−f). Various halogen substituents on the aromatic ring of carboxylic acids were tolerated. The substrate scope of the reaction was not limited to 2-nitrobenzoic acid derivatives. Benzoic acids with a fluorine atom in ortho position of the phenyl ring were also efficient substrates, affording the fluorinated arylheteroarene products (5g−i) in moderate yields. Finally, aryl carboxylic acid was used to couple with different benzo-fused heterocycles, providing arylheteroarenes (5j−r) in good yields. However, benzoic acid derivatives without a fluorine atom or NO2 group in the ortho position of the phenyl ring, such as benzoic acid, 5-bromo-2-chlorobenzoic acid, 2-chloro-5-nitrobenzoic acid, and 2,6-dimethoxybenzoic acid, could not react with benzothiazole (1a) under standard conditions. In order to further understand the mechanism of this catalytic decarboxylative cross-coupling reaction, the control experiments described in Scheme 4 were conducted. When the reaction was performed under standard conditions in the presence of 1.0 equiv of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), a free radical scavenger, the yield of 3a was greatly reduced (Scheme 4A), suggesting that a radical mechanism may be involved in the reaction. The H/D experiment showed that the intermolecular

Co(III) species CoIIIX3. With the assistance of Ag2CO3, this reacts with benzothiazole (1) to generate the Co(III) intermediate A through a cooperative deprotonation and metallization process. The aryl radical species, which is formed from an aryl carboxylic acid in the presence of Ag2CO3,17 reacts with intermediate A to afford a Co(IV) species B. Subsequent elimination of cobalt from B leads to the desired product 3 and regenerates the Co(II) catalyst. Since silver salts are also known to promote the decarboxylation of aromatic acids to organometallic silver-aryl intermediates,18 a catalytic cycle via Co(II/III/I) (Path 2, Scheme 5) cannot be excluded for this reaction. In summary, we have developed a first decarboxylative C−H (hetero)arylation reaction of benzo-fused heterocycles with a CoBr2/IPr·HCl based system. The key to this reaction is the addition of extra N-heterocyclic carbene ligands which establishes a balance between the Ag-mediated decarboxylation and the Copromoted C−H functionalization process. Compared to the 5591

DOI: 10.1021/acs.orglett.7b02730 Org. Lett. 2017, 19, 5589−5592

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Organic Letters

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established catalytic systems, this method is advantageous because of its broad tolerance of benzo-fused heterocycles and (hetero)aryl carboxylic acids, catalysis by inexpensive metals, and flexible construction of various biheteroaryl and arylheteroarene structural motifs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02730. Experimental details and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongjian Lu: 0000-0001-7132-3905 Guigen Li: 0000-0002-9312-412X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge financial support from the National Natural Science Foundation of China (Nos. 21332005, 21472085, 21672100), the Fundamental Research Funds for the Central Universities (No. 020514380114), and the Robert A. Welch Foundation (D-1361, USA).

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REFERENCES

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DOI: 10.1021/acs.orglett.7b02730 Org. Lett. 2017, 19, 5589−5592