Palladium-Catalyzed Esterification of Carboxylic Acids with Aryl

Letter pubs.acs.org/OrgLett

Cite This: Org. Lett. 2018, 20, 2428−2432

Palladium-Catalyzed Esterification of Carboxylic Acids with Aryl Iodides Hiroyuki Kitano,# Hideto Ito,*,†,‡ and Kenichiro Itami*,#,†,‡ #

Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa, Nagoya 464-8602, Japan Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan ‡ JST-ERATO, Itami Molecular Nanocarbon Project, Nagoya University, Chikusa, Nagoya 464-8602, Japan †

S Supporting Information *

ABSTRACT: The first palladium-catalyzed esterification of carboxylic acids with aryl iodides is described. A palladium-based catalytic system consisting of IBnF (1,3-bis((pentafluorophenyl)methyl)imidazole-2-ylidene) ligand was found to significantly accelerate the aryl−O bond-forming esterification reaction. A series of aryl iodides and carboxylic acids undergoes a palladium-catalyzed coupling reaction to provide the corresponding aryl esters in moderate to good yields. In addition, sterically hindered aryl iodides and carboxylic acids were well-tolerated yielding the corresponding aryl esters.

A

Scheme 1. Synthesis of Aryl Esters via (a) Conventional Cacyl−Oaryl Bond Forming Condensation, (b) TransitionMetal-Catalyzed Caryl−Oacyl Bond Formation, and (c) PdCatalyzed Esterification of Aryl Iodides with Carboxylic Acids (This Work)a

ryl esters are fundamental subunits used in pharmaceuticals, agrochemicals, polymers, and natural products.1 Recently, aryl esters have also attracted much interest as green and low-cost arylating agents via C−O activation in metalcatalyzed coupling reactions.2 Among various synthetic routes for aryl esters, the condensation of phenols and carboxylic acids or their derivatives, i.e., the Cacyl−Oaryl bond formations, represents one of most conventional and established transformations (Scheme 1a).3 On the other hand, less attention has been paid to the synthesis of aryl esters via Caryl−Oacyl bond formations such as the cross-coupling between aryl halides and carboxylic acids (Scheme 1b),4 while many useful metalcatalyzed Caryl−O bond forming reactions using aryl halides/ metals with alcohol, such as the Ullmann reaction, have been developed for the synthesis of aryl ethers.5 To the best of our knowledge, the use of carboxylic acids as an O-nucleophile in metal-catalyzed Caryl−Oacyl bond formation has been limited to the esterifications by (i) copper-catalyzed Chan−Lam−Evanstype coupling6 and (ii) directing group-assisted C−H activation.7 McCusker and MacMillan have recently achieved the first cross-coupling between aryl bromides and carboxylic acid by (iii) energy transfer-driven organometallic catalysis.4 Herein, we report an efficient palladium-catalyzed esterification of carboxylic acids with aryl iodides (see Scheme 1c). By using this reaction, various aryl esters were synthesized from carboxylic acids and aryl iodides in moderate to high yields. We began our study by optimizing the reaction conditions using 4-iodoanisole (1a) and pivalic acid (2a) as model substrates (see Table 1). Through exhaustive screening of metal catalysts, ligands, bases, and so on, we found that the combination of Pd(OAc)2 (5 mol %) and Ag2CO3 (3.0 equiv) promoted the desired esterification in the presence of MS4A in 1,4-dioxane at 100 °C, providing aryl ester 3aa in 32% GC yield (entry 1 in Table 1). Then screening of ligands was © 2018 American Chemical Society

a

DG = directing group.

Received: March 8, 2018 Published: April 4, 2018 2428

DOI: 10.1021/acs.orglett.8b00775 Org. Lett. 2018, 20, 2428−2432

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solvent, 1,4-dioxane gave the best result, while highly polar solvents such as acetonitrile, dimethylformamide (DMF), and 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP) completely suppressed the reaction progress (see Table S1 in the Supporting Information for details). We next investigated the effect of Pd source (entries 15 and 16 in Table 1). While Pd(PPh3)4 showed lower activity than Pd(OAc)2 (entry 15 in Table 1), the use of [Pd(allyl)Cl]2 or [Pd(cinnamyl)Cl]2 gave results that were almost comparable to those of Pd(OAc)2, in terms of yield (entries 16 and 17 in Table 1). We also found that this esterification did not proceed at all in the absence of Pd catalyst, and only 6% GC yield of 3aa was obtained in the presence of 4-bromoanisole instead of 1a. Because of higher solubility and reproducibility, we determined [Pd(cinnamyl)Cl]2 and IBnF·HBr as the optimal catalyst and the conditions in entry 17 in Table 1 as optimal reaction conditions. The use of MS4A further increased the reproducibility of yield. With the optimal conditions in hand, we examined the substrate scope of the carboxylic acid by employing 4iodoanisole (1a) in the presence of catalytic amount of [Pd(cinnamyl)Cl]2 and IBnF·HBr (Scheme 2). With the

Table 1. Screening of Reaction Conditions

entry

Pd catalyst

ligand

Ag salt

yielda (%)

1 2 3 4 5 6 7 8 9 10 11e 12f 13 14g 15 16 17i

Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(PPh3)4 [Pd(ally)Cl]2 [Pd(cinnamyl)Cl]2

none PCy3·HBF4b PPh3b SPhos BINAP bipy IPr·HCl IBnF·HBr IBn·HBr IBnF·HBr IBnF·HBr IBnF·HBr IBnF·HBr IBnF·HBr IBnF·HBr IBnF·HBr IBnF·HBr

Ag2CO3 Ag2CO3 Ag2CO3 Ag2CO3 Ag2CO3 Ag2CO3 Ag2CO3 Ag2CO3 Ag2CO3 Ag2O none none none AgOPivh Ag2CO3 Ag2CO3 Ag2CO3

32 37 25 32 32 3 66 78 (75)c 51 34 0 0 0 55 49 67 77d (77)c

Scheme 2. Substrate Scope of Carboxylic Acids 2 in Esterification with 4-Iodoanisole (1a)

a

Determined by gas chromatography (GC), using dodecane as an internal standard. b10 mol %. cIsolated yield. dDetermined by 1H NMR using CH2Br2 as an internal standard. eK2CO3 (3.0 equiv) was used. fCs2CO3 (3.0 equiv) was used. gReaction without pivalic acid (2a). h1.5 equiv. iReaction time = 42 h. bipy = 2,2′-bipyridyl. IPr = 1,3-bis(2,6- diisopropylphenyl)imidazole-2-ylidene.

conducted using phosphine ligands such as PCy3, PPh3, SPhos, and BINAP, but the yield of 3aa was not improved (entries 2− 5 in Table 1). The use of nitrogen-based ligand such as 2,2′bipyridyl (bipy) dramatically lowered the yield (entry 6 in Table 1, 3% yield). To our delight, N-heterocyclic carbene (NHC) ligands such as 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr) effectively facilitated the esterification reaction (entry 7 in Table 1). Especially, hydrogen bromide salt of (1,3-bis((perfluorophenyl)methyl)imidazole-2-ylidene) (IBnF)8 showed further increase of yield (entry 8 in Table 1, 78% yield). The reaction with more electron-rich dibenzyl NHC ligand (IBn) was also tested for comparison, but the yield of 3aa decreased to 55% (entry 9 in Table 1).9 Setting IBnF· HBr as an optimal ligand precursor, we further investigated the effect of silver salt (entries 10−14 in Table 1). The use of Ag2O resulted in lower yield (entry 10 in Table 1, 34% yield). Silver salts are essential for this reaction; the use of K2CO3 and Cs2CO3, instead of Ag2CO3, or the absence of silver salt completely shuts down the reaction (entries 11−13 in Table 1). Interestingly, the employment of silver pivalate instead of the combination of pivalic acid (2a) and Ag2CO3 also gave the product 3aa in 51% yield (entry 14 in Table 1). This result implies that mixing 2a and Ag2CO3 would also give AgOPiv in situ, and it works as effective transmetalation or ligand exchanging agents in the present catalytic reaction. As the

exception of pivalic acid (2a), tertiary alkyl carboxylic acids such as tert-amyl carboxylic acid (2b) and 1-methyl-cyclohexane-1-carboxylic acid (2c) gave the corresponding products 3ab and 3ac in 78% and 69% yields, respectively. The reactions with other tertiary carboxylic acids such as 1-phenycyclopentane-1-carboxylic acid (2d) and 1-methylcyclopropropane-1carboxylic acid (2e) afforded esters 3ad and 3ae in moderate yields. The bulkiness around carboxylic acid seems to be 2429

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methoxyl group. The reactions with other para-substituted iodobenzenes such as 1-iodo-4-methylbenzene (1d), 1-n-butyl4-iodobenzene (1e), 1-tert-butyl-4-iodobenzene (1f), and 4iodo-1,1′-biphenyl (1g) afforded the corresponding esters 3da, 3ea, 3fa, and 3ga in moderate yields. The reaction with 4-(4iodophenyl)morpholine (1h) resulted in low yield of the product 3ha (27%). However, iodobenzenes having electron deficient para-substituents such as chloro, methoxycarbonyl, and acetoamido groups (1i, 1j, and 1k) did not provide the products. In these reactions, 14%, 70%, and 77% of 1i, 1j, and 1k remained unreacted after reactions, respectively. Multisubstituted and electron-rich aryl iodides such as 1-iodo-4methoxy-2-methybenzene (1l), 1-iodo-2,4-dimethoxybenzene (1m), 1-iodo-2-methoxyl-4-methylbenzene (1n), 1-iodo-3,4dimethyliodobenzene (1o), and mesityl iodide (1p) worked well to afford the corresponding products 3la, 3ma, 3na, 3oa, and 3pa (60%−86%). These results suggest that both electron density of the benzene ring and the steric hindrance around an iodine atom are the key factors for the reaction efficacy. Indeed, the reaction with 1-iodonaphthalene (1q), whose iodine atom is hindered by a hydrogen atom on the peri-position, afforded the ester 3qa in good yield (62%). Furthermore, double esterification with 4,4′-diiodobiphenyl (1r) gave the corresponding ester 3ra in 25%, whose value is comparable to the yield found in the single esterification forming 3ga. A gram-scale synthesis was examined using 2.0 g (8.6 mmol) of 4-iodoanisole (1a) and 1.3 g (13 mmol) of pivalic acid (2a) under optimal reaction conditions (Scheme 4). As a result, the esterification reaction successfully worked to afford 4methoxyphenyl pivalate (3aa) in 81% yield (1.5 g of product).

important for the reaction progress. However, the reaction with 1-adamantanecarboxylic acid (2f) somehow decreased the yield of 3af (10%). The reaction efficiency also decreased when secondary alkyl carboxylic acids were used. Cyclohexanecarboxylic acid (2g), tetrahydro-2H-pyran-4-carboxylic acid (2h) and cyclobutanecarboxylic acid (2i) provided the aryl esters 3ag, 3ah, and 3ai in low to moderate yields. Less bulkier primary alkyl carboxylic acids such as propionic acid (2j), isopentanoic acid (2k), and nonanoic acid (2l) carboxylic acids resulted in low yields of product 3aj, 3ak, and 3al (8%−26%). While benzoic acid (2m) did not furnish the ester 3am, the use of 2,4,6-trimethyl benzoic acid gave the corresponding ester 3an in moderate yield. These results also suggest that the steric hindrance around the carboxylic acid facilitated the esterification reaction.10 Notably, over 71% conversions of 1a were observed in the reaction using 2a−2l and 2n, while the reaction with 2m resulted in the only 14% conversion of 1a. We also investigated the substrate scope of aryl iodides (Scheme 3). p-Methoxy (1a) and o-methoxy (1c) substituted phenyl iodides can participate in the present coupling reaction, but m-methoxyphenyl iodide (1b) gave the corresponding product 3ba in quite low yield, albeit almost all of the 1b was consumed, likely because of the inductive effect of metaScheme 3. Substrate Scope of Aryl Iodides 1 in the Esterification with Pivalic Acid (2a)a

Scheme 4. Gram-Scale Synthesis of Aryl Ester 3aa

a

We proposed the reaction mechanism as follows: [PdCl(cinnamyl)]2 and IBnF·HBr would easily form [PdIICl(cinnamyl)(IBnF)] by Ag2CO3 or in-situ-generated AgOPiv, and then Pd0−IBnF complex 4 can form through the formal reductive elimination of cinnamyl chloride (Figure 1).11 Then, the oxidative addition of 4-iodoanisole (1a) to 4 can occur to form a aryl−Pd(II) complex (5), which undergoes the anion exchanging with in-situ-generated AgOPiv to form aryl palladium(II) pivalate (6). Finally, the reductive elimination of aryl pivalate 3aa can occur along with the regeneration of Pd0−IBnF complex 4.12 In summary, we have developed the first palladium-catalyzed esterification reaction of carboxylic acids with aryl iodides. Electron-rich aryl iodides showed good reactivity to this reaction, even in the gram-scale synthesis. Sterically hindered aryl iodides and carboxylic acids were well-tolerated to furnish the corresponding aryl esters in good yields. Further investigation of the ligand effect and mechanistic studies are currently ongoing in our laboratories.

Reaction time: 42 h. 2430

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Figure 1. Proposed catalytic mechanism on esterification of 4iodoanisole (1a) with pivalic acid (2a).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00775. Detailed experimental procedures and spectral data for all compounds, including scanned image of 1H, 13C NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H. Ito). *E-mail: [email protected] (K. Itami). ORCID

Hideto Ito: 0000-0002-4034-6247 Kenichiro Itami: 0000-0001-5227-7894 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the ERATO program from JST (No. JPMJER1302 to K.I.), JSPS KAKENHI (Grant Nos. JP26810057, JP16H00907, and JP17K19155 (H.I.)), the SUMITOMO Foundation (No. 141495 to H.I.) and DAIKO Foundation (H.I.). The computations were performed using the Research Center for Computational Science, Okazaki, Japan. ITbM is supported by the World Premier International Research Center Initiative (WPI), Japan.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b00775 Org. Lett. 2018, 20, 2428−2432