Base promoted highly efficient copper fluorapatite catalyzed coupling of phenols with arylboronic acids under mild and ligand-free conditions

Abstract

A mild, general and highly efficient protocol has been developed for the synthesis of diaryl ethers in good to excellent yield under mild and ligand-free conditions. This is the first example in which a recyclable, heterogeneous copper fluorapatite catalyst is used for the arylation of phenols with arylboronic acids at room temperature in the presence of Cs₂CO₃ as a base and methanol as a solvent. The catalyst was recovered and reused several times without loss of catalytic activity.

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Introduction

Diaryl ethers are key structural motifs (Fig. 1) found in a variety of naturally occurring biologically and medicinally active compounds such as perrottetine (1),¹ riccardin B (2),² and K-13 (3)³. The importance of diaryl ether motifs in cyclic peptide formation has been reviewed by Rama Rao et al.⁴ The diaryl ether structural motifs play a vital role not only in life science but also in agriculture to prevent various weed-killing chemicals.⁵ The classic Ullmann coupling reaction for the synthesis of diaryl ethers has been extensively reported, however, its wide application for the synthesis of biologically active molecules containing diaryl ether motifs has been restricted because of the high reaction temperature (125–300 °C) and the long reaction time at which many functional groups are unstable and/or the racemization of the amino acid moieties occurs, resulting in a lower yield of the desired product. Also, the requirement for copper complexes in excess quantities leads to the problem of waste disposal.⁶ Although significant progress has been achieved,^7–14 almost all of the reported methods are not practical approaches for the synthesis of biologically active compounds possessing sensitive functional groups.

Fig. 1 Examples of the diaryl ether motif in natural products. R₁ and R₂ in 1 are H, OH or OMe.

Arylboronic acid is used to introduce a phenyl ring to various biologically active compounds.¹⁵ Also, carbon–heteroatom bond forming reactions of arylboronic acids with amines,¹⁶N-hydroxyphthalimides,¹⁷ amides, imides,¹⁸ and N-heterocycles,¹⁹ to give the corresponding products have been reported in the literature. To the best of our knowledge, so far only two research groups, Evans et al.^20a and Chan et al.^20b have reported the arylation of phenol with arylboronic acid over a Cu(OAc)₂ catalyst in the presence of triethyl amine or pyridine as a base and dichloromethane as a solvent. However the coupling of phenol with arylboronic acid described by Evans et al. is advantageous for the synthesis of thyroxine whereas Chan et al. studied N- and O-arylation in which few examples with phenol are reported.

Being that diaryl ether motifs are key constituent of the structural backbone of many pharmaceutical compounds, the construction of their structural units under mild and ligand-free reaction conditions compared to those of the classic Ullmann and Goldberg arylation²¹ has attracted the attention of researchers world wide. Hence, this paper describes the first report of a recyclable, heterogeneous copper fluorapatite catalyzed coupling reaction of phenols with arylboronic acids in the presence of Cs₂CO₃ as a base under mild and ligand-free reaction conditions.

As part of our continuous efforts to develop green, ecofriendly, general and cost effective methods for organic transformations,²² herein, we report a highly efficient, cost effective, general and mild method for the synthesis of diaryl ethers in good to excellent yield from the cross coupling reactions of a wide range of substituted phenols with substituted arylboronic acids over an ecofriendly, heterogeneous, reusable, ligand-free copper fluorapatite (CuFAP) catalyst in the presence of Cs₂CO₃ as a base in a methanol solvent under ambient reaction conditions (Scheme 1) which may be a practical approach for the synthesis of bioactive molecules containing diaryl ether structural units.

Scheme 1 CuFAP catalysed diaryl etherification of substituted phenol with substituted arylboronic acid.

Results and discussion

To develop a protocol for C–O cross coupling reactions, the reaction of phenol and phenyl boronic acid was initially catalyzed by the CuFAP catalyst in the presence of Cs₂CO₃ as a base in a methanol solvent under ambient conditions to give a 90% yield of biphenyl ether. The results on this substrate under ambient conditions using the ligand-free CuFAP catalyst, prepared as per the literature procedure²³ allowed us to select it as a model reaction to optimize the reaction conditions. In order to optimize the reaction conditions, the C–O coupling reaction was performed in various solvents such as methanol, ethanol, dichloromethane, dichloroethane, ethyl acetate, and acetonitrile. Methanol resulted in a high yield of the cross-coupling product, which may be due to it having the most polar and protic character (Table 1, entry 6) as compared to ethanol, dichloromethane, dichloroethane, ethyl acetate, and acetonitrile as solvents, which result in a moderate yield (Table 1, entries 1–5).

Table 1 Effect of solvents on the diaryl etherification of phenol with phenyl boronic acid^a

Entry	Solvent	Yield^b(%)
a Reaction conditions: phenyl boronic acid (0.50 mmol), phenol (0.55 mmol), Cs2CO3 (0.75 mmol), solvent (3 ml), catalyst (50 mg), room temperature, 8 h. b Isolated yields.
1	Acetonitrile	67
2	Ethyl acetate	54
3	Dichloromethane	31
4	Dichloroethane	28
5	Ethanol	79
6	Methanol	90

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After achieving a high yield in the methanol solvent, studies focused on the investigation of the influence of various bases on the C–O coupling reaction, with Cs₂CO₃ found to be the most effective base to achieve a high yield (Table 2 entry 7) as compared to NaOH, KOH, Na₂CO₃, and K₂CO₃ (Table 2, entries 3–6). The high product yield when using Cs₂CO₃ as the base may be due to the increasing electropositivity of group IA cations in the order Cs > K > Na, which facilitates the deprotonation of the phenol through the increasing electronegativity of the carbonate salt in the solvent as well as the high solubility of Cs₂CO₃ in the methanol solvent (Table 2). However, no reaction (N. R.) was observed when using organic bases such as Et₃N and pyridine (Table 2, entries 1,2).

Table 2 Effect of the base on the diaryl etherification of phenol with phenyl boronic acid^a and the solubility of inorganic bases in methanol.

Entry	Solvent	Solubility in MeOH (g per 100 mL) at RT	Yield^b (%)
a Reaction conditions: phenyl boronic acid (0.50 mmol), phenol (0.55 mmol), base (0.75 mmol), methanol (3 ml), CuFAP (50 mg), room temperature, 8 h. b Isolated yields. c Converted from g per 100 g to g per 100 mL.
1	Triethyl amine	—	N. R.
2	Pyridine	—	N. R.
3	NaOH	23.8^26a	24
4	KOH	28.17^26b ^c	31
5	Na2CO3	0.17^26c ^c	53
6	K2CO3	4.84^26d ^c	69
7	Cs2CO3	44.52^26c ^c	90

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The promising results on the optimized reaction conditions using Cs₂CO₃ as a base in methanol solvent over the ligand-free CuFAP catalyst encouraged us to investigate the feasibility of this methodology for a wide range of substituted phenols and substituted arylboronic acids for diaryl etherification. As shown in Table 3, substituted phenols possessing a variety of functional groups reacted with phenyl boronic acid with Cs₂CO₃ as the base in methanol solvent to obtain diaryl ethers as products in moderate to excellent yields. No desired cross-coupling product was obtained under the same reaction conditions in the absence of the CuFAP catalyst (Table 3, entry 1).

Table 3 Diaryl etherification of substituted phenols with phenyl boronic acid^a

Entry	Substituted Phenol	Product	Time (h)	Yield^b(%)
a Reaction conditions: phenyl boronic acid (0.50 mmol), substituted phenol (0.55 mmol), Cs2CO3 (0.75 mmol), methanol (3 ml), catalyst (50 mg), room temperature, 8 h. b Isolated yields. c No reaction with base (Cs2CO3) without CuFAP catalyst and vice versa.
1			24	N. R.^c
2			8	90
3			6	96
4			6	94
5			7	92
6			8	87
7			24	49
8			12	80
9			12	84
10			10	86
11			10	84
12			10	82

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The phenols with electron donating groups such as 4-methoxy phenol, 4-methyl phenol, and 4-tert-butyl phenol, (Table 3, entries 3–5) provided excellent yields as compared to phenol and 3-methylphenol (Table 3, entry 2 and 6) while phenols with electron withdrawing groups such as 4-chlorophenol and 4-iodophenol provided moderate yields after longer reactions times (Table 3, entries 8 and 9) except in the case of 4-nitrophenol (Table 3, entry 7), whereas 4-phenyl phenol, α-naphthol and β-naphthol also provided moderate yields (Table 3, entries 10–12).

To widen the scope of the methodology using the ligand-free CuFAP catalyst for O-arylation, the coupling of various substituted arylboronic acids with phenol have been investigated; the results are summarized in Table 4. The substituted phenyl boronic acids with electron withdrawing groups such as 4-fluorophenyl boronic acid, 4-chlorophenyl boronic acid, 4-iodophenyl boronic acid and 4-nitrophenyl boronic acid (Table 4, entries 2–5) provided excellent yields as compared to phenyl boronic acid (Table 4, entry 1). However, substituted phenyl boronic acids with electron donating groups such as 3-methylphenyl boronic acid, 4-methylphenyl boronic acid, 2-methoxyphenyl boronic acid, 3,4,5-trimethoxyphenyl boronic acid and 4-methoxyphenyl boronic acid were successfully coupled with phenol to give the corresponding diaryl ethers in moderate yields (Table 4 entries 6–10).

Table 4 Diaryl etherification of phenol with substituted arylboronic acids^a

Entry	Aryl boronic acids	Product	Time (h)	Yield^b(%)
a Reaction conditions: substituted phenyl boronic acid (0.50 mmol), phenol (0.55 mmol), Cs2CO3 (0.75 mmol), methanol (3 ml), CuFAP (50 mg), room temperature. b Isolated yields.
1			8	90
2			6	94
3			7	92
4			8	91
5			8	91
6			9	88
7			10	84
8			10	82
9			12	80
10			10	80

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The results in Tables 3 and 4 indicate that the O-arylation cross coupling reaction is applicable to a large number of substrates having sensitive functional groups; however, the reaction time and the yield obtained are dependent on the nature of the substituents on the phenol as well on the arylboronic acids.

According to previous research work using the CuFAP catalyst for O-arylation,^22a N-arylation of heterocycles with chloro- and fluoroarenes²³ and N-arylation of heterocycles with bromo- and iodoarenes in the presence of base (K₂CO₃),^24,25 the possible mechanism proposed in Scheme 2 for the C–O cross coupling reaction may involve base promoted CuFAP catalyzed nucleophilic substitution that proceeds via the formation of the complex (A) and then the subsequent oxidative addition of phenyl boronic acid via the formation of another complex (B) followed by instant in situ reductive elimination to release the diaryl ether product (C) as well as the CuFAP catalyst in its original form to be recycled.

Scheme 2 Possible mechanism of the reaction over the ligand-free CuFAP catalyst for diaryl etherification.

Recyclability of copper fluorapatite catalyst (CuFAP)

The recyclability of the ligand-free CuFAP catalyst in the C–O cross coupling reaction was assessed using phenol and phenyl boronic acid with Cs₂CO₃ as the base in methanol solvent at room temperature; the results are summarized in Table 5. After the completion of the reaction, the catalyst was recovered quantitatively by filtration and recycled several times, however, no loss of catalytic activity was observed even after the fourth cycle (Table 5, entry 5). The catalytic activity of the reused catalyst is very much comparable with that of the fresh catalyst (Table 5, entry 1), which clearly shows that no loss and/or leaching of copper occurred during the course of the reaction and the reused catalyst exhibits an excellent performance.

Table 5 Recyclability studies of the ligand-free CuFAP catalyst for diaryl etherification of phenol with phenyl boronic acid^a

Entry	Yield (%)^b
a Reaction conditions phenyl boronic acid (0.50 mmol), phenol (0.55 mmol), Cs2CO3 (0.75 mmol), methanol (3 ml), CuFAP (50 mg), room temperature, 8 h. b Isolated yields.
1	90
2	90
3	88
4	90
5	89

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Conclusions

In conclusion, a highly efficient method for the synthesis of diaryl ethers in good to excellent yields from the cross coupling reaction of substituted phenols with substituted aryl boronic acids has been developed over an inexpensive, ligand-free, recyclable, ecofriendly, heterogeneous copper fluorapatite (CuFAP) catalyst in the presence of Cs₂CO₃ as a base in methanol solvent under ambient reaction conditions. The developed method is mild, general, simple, clean and applicable to a large number of substrates with sensitive functional groups. The CuFAP catalyst was recovered by filtration and recycled several times without loss of catalytic activity.

Experimental section

All chemicals and reagents were procured from commercial suppliers and used without further purification. The products were characterized using ¹H NMR and ¹³C NMR spectra. NMR spectra of the products were obtained using a Bruker AC-200 MHz spectrometer with TMS as the internal standard. Column chromatography was performed using silica gel, Merck grade 60–120 mesh size. TLC was performed on 0.25 mm E. Merck precoated silica gel plates (60 F₂₅₄).

General experimental procedure for diaryl etherification

Arylboronic acid (0.50 mmol), phenol (0.55 mmol), Cs₂CO₃ (0.75 mmol), methanol (3 ml) and CuFAP catalyst (50 mg) were put into a 10 ml round bottomed flask and stirred under a nitrogen atmosphere at room temperature for 6–24 h (Table 3 and 4) and the progress of the reaction was monitored by TLC. After the completion of the reaction, the reaction mixture was diluted with 10 ml of methanol followed by filtration to recover the catalyst. The filtrate was concentrated in vacuo to get the crude product, which was further purified by column chromatography on silica gel using a hexane/ethyl acetate 90 : 10 mixture to obtain the diaryl ether product.

Characterization of selected compounds.
1-Methoxy-4-phenoxybenzene (Table 3, entry 4 and Table 4, entry 10). ¹H NMR (200 MHz, CDCl₃): δ = 8.54–8.38 (m, 3H), 8.26–8.10 (m, 4H), 7.96 (m, 2H), 3.49 (s, 3H).

¹³C NMR (200 MHz, CDCl₃): δ = 158.35, 139.90, 129.67, 124.71, 123.06, 118.54, 118.22, 115.17, 55.38.

FTIR (CHCl₃): 676, 779, 1220, 1492, 1631, 2954 cm⁻¹.

GC-MS: 200, 168, 123, 94, 73, 57.

1-(tert-Butyl)-4-phenoxybenzene (Table 3, entry 6). ¹H NMR (200 MHz, CDCl₃): δ = 7.25–7.36 (m, 4H), 6.96–7.01 (m, 4H), 6.90 (m, 1H), 2.34 (s, 9H).

¹³C NMR (200 MHz, CDCl₃): δ = 157.56, 154.67, 146.10, 129.63, 126.52, 122.89, 118.43, 34.29, 31.48.

FTIR (CHCl₃): 700, 748, 1245, 1501, 1605, 2979 cm⁻¹.

1-Methyl-3-phenoxybenzene (Table 3, entry 7 and Table 4, entry 6). ¹H NMR (200 MHz, CDCl₃): δ = 7.60–7.49 (m, 4H), 7.36–7.14 (m, 5H), 2.58 (s, 3H).

¹³C NMR (200 MHz, CDCl₃): δ = 157.14, 139.90, 129.67, 124.02, 123.06, 119.54, 118.82, 115.87, 21.38.

FTIR (CHCl₃): 698, 738, 1260, 1487, 1607, 3049 cm⁻¹.

GC-MS: 184, 107, 93, 92, 76, 51.

1-Nitro-4-phenoxybenzene (Table 3, entry 8 and Table 4, entry 5). ¹H NMR (200 MHz, CDCl₃): δ = 8.22 (d, 2H), 7.47–7.25 (m, 3H), 7.10–6.98 (m, 4H).

¹³C NMR (200 MHz, CDCl₃): δ = 163.36, 154.66, 142.59, 130.30, 125.92, 125.40, 120.53, 117.05.

FTIR (CHCl₃): 750, 858, 1250, 1370, 1495, 1604, 3058 cm⁻¹.

GC-MS: 215, 138, 122, 76, 51.

1-Chloro-4-phenoxybenzene (Table 3, entry 9 and Table 4, entry 3). ¹H NMR (200 MHz, CDCl₃): δ = 7.3–7.21 (m, 4H), 7.06 (m, 1H), 6.96–6.86 (m, 4H).

¹³C NMR (200 MHz, CDCl₃): δ = 156.84, 155.92, 129.69, 128.16, 123.61, 120.01, 118.91.

FTIR (CHCl₃): 693, 784, 1235, 1496, 1581, 2932 cm⁻¹.

GC-MS: 204, 171, 94, 73, 50.

4-Phenoxy-1,1′-biphenyl (Table 3, entry 11). ¹H NMR (200 MHz, CDCl₃): δ = 7.03–7.09 (m, 5H), 7.31–7.42 (m, 5H), 7.52–7.58 (m, 4H).

¹³C NMR (200 MHz, CDCl₃): δ = 119, 119.1, 123.35, 126.94, 128.45, 128.80, 129.79, 136.35, 140.64, 156.89.

FTIR (CHCl₃): 660, 725, 1240, 1498, 1610, 3021 cm⁻¹.

1-Phenoxynaphthalene (Table 3, entry 12). ¹H NMR (200 MHz, CDCl₃): δ = 8.16–8.11 (d, 1H, J = 5 Hz), 7.82–7.78 (d, 1H, J = 4 Hz), 7.57 (m, 1H), 7.43 (m, 2H), 7.17 (m, 1H), 7.03–6.86 (m, 6H).

¹³C NMR (200 MHz, CDCl₃): δ = 157.21, 156.57, 152.36, 134.27, 129.14, 127.10, 125.93, 122.48, 121.44, 117.89, 112.86.

FTIR (CHCl₃): 670, 790, 1370, 1506, 1619, 3058 cm⁻¹.

Acknowledgements

SMI, MYP and SSC thank CSIR New Delhi for an SRF and JRF, respectively. The authors also thank Dr V. V. Ranade, Chair, CE-PD for encouragement and support.

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