Cobalt(II)-Catalyzed Acyloxylation of C–H Bonds in Aromatic Amides

Letter Cite This: Org. Lett. 2018, 20, 1062−1065

pubs.acs.org/OrgLett

Cobalt(II)-Catalyzed Acyloxylation of C−H Bonds in Aromatic Amides with Carboxylic Acids Rina Ueno, Satoko Natsui, and Naoto Chatani* Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan S Supporting Information *

ABSTRACT: The cobalt(II)-catalyzed acyloxylation of C−H bonds in aromatic amides containing an 8-aminoquinoline moiety as the directing group with carboxylic acids is reported. Various carboxylic acids including aromatic and aliphatic carboxylic acids are applicable to the reaction. The reaction displays a broad substrate scope and high functional group tolerance. The reaction is carried out under air.

T

Scheme 1. Co(II)-Catalyzed C−H Acyloxylation of Aromatic Amides with Carboxylic Acids

ransition-metal-catalyzed C−H functionalization has received increasing attention in organic synthesis because of its step- and atom-economy advantages, environmental friendliness, and the fact that prefunctionalized substrates are not needed.1 The efficiency and scope of C−H functionalization reactions at the ortho-position of arenes has been substantially improved in the past decades. One of the most powerful and reliable methods for the functionalization of C− H bonds at the ortho-position involves chelation assistance by a directing group. New types of chelation systems have been developed that now permit the characteristic functionalization of C−H bonds. One such example involves the use of an N,N′-bidentate chelation system.2 Although noble metals, such as Pd, Rh, Ir, Pt, and Ru, are typically used as catalysts in chelation-assisted C−H functionalization, the use of first-row transition metals,3 such as Fe,4 Co,5 Ni,6 and Cu,7 has also received much interest because these metals are earth abundant and therefore less expensive than noble metals. It is particularly noteworthy that air-stable and inexpensive Co(II) complexes have been recognized as efficient catalysts for C−H functionalization due to their unique catalytic activity. In fact, Co(II) catalysts, such as Co(OAc)2 and Co(acac)2, have been extensively used in 8-aminoquinoline chelation-assisted C−H functionalization reactions in recent years.2g,8 While a wide variety of Co(II)-catalyzed C−H functionalization reactions exist in which the N,N′-bidentate directing group has been developed thus far, most involve C− C bond formation, such as alkylation, allylation, arylation, alkenylation, alkynylation, and carbonylation. Despite the success of C−H amination or amidation methods,9 the formation of a C−O bond with the cleavage of C−H bonds in the Co(II) catalyst/8-aminoquinoline chelation system remains undeveloped.10,11 We herein report the Co(II)catalyzed acyloxylation of C−H bonds in aromatic amides with carboxylic acids by taking advantage of an 8-aminoquinoline chelation system (Scheme 1).12,13 The reaction of amide 1a (0.15 mmol) with 2,6dimethylbenzoic acid (2a) (0.15 mmol) in the presence of Co(acac)2 (0.06 mmol) as the catalyst and Ag2CO3 (0.15 © 2018 American Chemical Society

mmol) as an oxidant in 1,2-dichloroethane (DCE) (0.5 mL) at 100 °C for 3 h under an atmosphere of air resulted in C− H acyloxylation to give the benzoxylation product 3aa in 77% NMR yield, along with 7% of 1a being recovered (entry 1 in Table 1). A trace amount of dimerization product 4a14 was also observed in the 1H NMR spectrum of the crude reaction mixture (entry 1). Increasing the reaction temperature failed to improve the product yield (entry 2). The use of Co(OAc)2·4H2O as the catalyst resulted in a dramatic decrease in the product yield (entry 3). When 0.20 mmol of Ag2CO3 was used, 1a was completely consumed, and the isolated product yield of 3aa was increased to 78% and no dimerization product 4a was detected (entry 4). The use of 1.5 equiv of 2a gave 3aa in 84% isolated yield (entry 5). When 0.3 mmol of Ag2CO3 was used, the yield was essentially the same as that obtained when 0.2 mmol was used (entry 6). Among the solvents examined, DCE gave the best results (entries 7−9). The use of toluene as a solvent also gave good yield of the product when 2,6-dimethylbenzoic acid was used as a carboxylic acid. However, lower yields were frequently observed when other carboxylic acids were used, as a coupling partner. Gratifyingly, it was found that the use of 10 mol % of Co(acac)2 gives 3aa in 80%, which is comparable to that obtained in the reaction with 10 mol % catalyst (entry 10). Finally, the use of 1a (0.15 mmol), 2a (0.225 mmol), Co(acac)2 (10 mol %), and Ag2CO3 (0.20 mmol) in DCE Received: December 26, 2017 Published: February 2, 2018 1062

DOI: 10.1021/acs.orglett.7b04020 Org. Lett. 2018, 20, 1062−1065

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Organic Letters Table 1. Optimization of Reaction Conditions

Scheme 2. Co(II)-Catalyzed C−H Acyloxylation of Aromatic Amide 1a with Various Carboxylic Acids 2a

a Yields were determined by 1H NMR. The value in parentheses is the isolated yield. a

Products were isolated by column chromatography. bThe reaction was carried out on a 1 mmol scale. cCo(acac)2 (20 mol %) was used. d NMR yield.

(0.5 mL) under air was chosen as the standard reaction conditions (entry 10). With the optimized reaction conditions in hand, the scope of the reaction with respect to carboxylic acids was examined (Scheme 2). A wide variety of carboxylic acids, including aromatic and aliphatic acids, was found to be applicable to the present C−H acyloxylation. The reaction was affected by the steric nature of the carboxylic acid used. The product yields increased with increasing bulkiness of the carboxylic acid (3aa > 3ab > 3ac). In contrast, the electronic nature of a substituent on a benzene ring had only a marginal effect on the efficiency of the reaction (3ae ≈ 3af ≈ 3ag). A bromide was tolerated in the reaction. Aliphatic carboxylic acids, such as pivalic acid, cyclohexancarboxylic acid, and acetic acid, also react to give the corresponding products 3aj−al. However, acetic acid was less reactive and 20 mol % of Co(acac)2 was required to obtain a reasonably good product yield. To examine the scope of the aromatic amides, we chose 2a as a standard carboxylic acid (Scheme 3). Moderate to excellent yields were obtained, and aromatic amides containing a substituent at the ortho-position gave higher yields. In the reaction of the m-methoxy-substituted substrate 1i, the reaction proceeded only at the less hindered C−H bonds to give the corresponding product 3ia in 43% yield. In the cases of o-methyl-m-bromobenzoic amide 1k, the corresponding product 3ka was formed, while the C−H bond is sterically congested. Although the reaction of thiophenecarboxamide 1o afforded the corresponding product 3oa in good yield, furancarboxamide failed to react. The reaction was not applicable to the acyloxylation of C(sp3)−H bond in 1p, and the substrate was recovered in 91% yield.

Some mechanistic studies were conducted to gain insight into the reaction mechanism. The reaction was not inhibited when (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO) was added to the reaction mixture, indicating that a free radical is not involved in this reaction (Scheme 4a). The deuterated amide 1a-d7 was reacted with 2a under the standard conditions for 30 min. No proton atoms were incorporated into unreacted starting amide 1a-d7 (Scheme 4b). Notably, no H/D exchange was observed, even in the absence of 2a (reaction scheme not shown). These experimental results indicate that C−H bond cleavage is an irreversible step in the present system, as is often observed in the Co(II)-catalyzed, aminoquinoline-directed C−H functionalization reactions.2g We next conducted competitive experiments to detect a possible kinetic isotopic effect; in this regard, two parallel experiments were carried out using 1a and 1a-d7 under standard reaction conditions for 30 min. The result revealed a KIE of 1.84, indicating that C−H bond cleavage is not the rate-determining step (reaction scheme not shown). A proposed mechanism for the reaction is shown in Scheme 5. The reaction of amide 1 with Co(II) gives the amide-coordinated Co(II) species I, which is oxidized by Ag(I) to generate the Co(III) species II.15 Cleavage of the ortho C−H bonds in II through a CMD process results in the formation of the cobaltacycle III.16 The reaction of III with the carboxylic acid 2 gives the Co(III)species IV, which undergoes reductive elimination, protonation, and oxidation by Ag(I) to afford 3 with Co(II) being regenerated. 1063

DOI: 10.1021/acs.orglett.7b04020 Org. Lett. 2018, 20, 1062−1065

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Organic Letters Scheme 3. Co(II)-Catalyzed C−H Acyloxylation of Aromatic Amides 1 with Carboxylic Acid 2aa

Scheme 5. Proposed Mechanism

use of stable and inexpensive Co(acac)2 as the catalyst and carried out under air.

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ASSOCIATED CONTENT

* Supporting Information S

a

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b04020. Experimental procedures and characterizations of new compounds (PDF)

b

Products were isolated by column chromatography. Co(acac)2 (40 mol %) was used. cCo(acac)2 (20 mol %) was used. dAg2(Co)3 (0.30 mol %) was used. eDiacycloxylation products were formed in 32%.

Scheme 4. Mechanic Studies

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Naoto Chatani: 0000-0001-8330-7478 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant in Aid for Specially Promoted Research by MEXT (17H06091). REFERENCES

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In summary, we report on a new method for introducing an oxygen functionality at the ortho-position in aromatic amides. Various carboxylic acids, including aromatic and aliphatic carboxylic acids, were found to be applicable to the reaction. The reaction displays a broad substrate scope and high functional group tolerance. The reaction is achieved by the 1064

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DOI: 10.1021/acs.orglett.7b04020 Org. Lett. 2018, 20, 1062−1065