O-Acetyl-Substituted Phenol Ester Synthesis via Direct Oxidative

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O-Acetyl Substituted Phenol Ester Synthesis via Direct Oxidative Esterification Utilizing Ethers as An Acylating Source with Cu2(dhtp) Metal-Organic Framework as A Recyclable Catalyst Thien N. Lieu, Ha T T Nguyen, Ngoc D M Tran, Thanh Truong, and Nam Thanh Son Phan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02733 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 3, 2016

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Industrial & Engineering Chemistry Research

O-Acetyl Substituted Phenol Ester Synthesis via Direct Oxidative Esterification Utilizing Ethers as An Acylating Source with Cu2(dhtp) Metal-Organic Framework as A Recyclable Catalyst Thien N. Lieu, Ha T. T. Nguyen, Ngoc D. M. Tran, Thanh Truong, Nam T. S. Phan* Faculty of Chemical Engineering, Ho Chi Minh City University of Technology, VNU-HCM, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam *

Email: [email protected]

Ph: (+84 8) 38647256 ext. 5681

Fx: (+84 8) 38637504

Abstract

A metal-organic framework Cu2(dhtp) was prepared, and utilized as a recyclable catalyst for the direct C-O coupling of ethers with 2-acylphenols to generate phenol esters. The Cu2(dhtp) displayed better efficiency in the production of phenol esters than a number of MOFs as well as traditional homogeneous catalysts. The oxidant played an important role for the conversion, and aqueous tert-butyl hydroperoxide was the best choice. Heterogeneous catalysis was confirmed for the transformation, and no phenol ester generated by leached species, if any, was recorded. It was possible to reuse the Cu2(dhtp) catalyst numerous times in the conversion of 2-acylphenols and ethers to phenol esters without a noticeable deterioration in catalytic efficiency. To our best understanding, the direct esterification to produce phenol esters utilizing ethers as acylating source assisted by solid catalysts was not previously reported in the literature.

Keywords: Cu2(dhtp); phenol; ester; acylating; ether.

ACS Paragon Plus Environment

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1. Introduction

Esters have arised as one of the most noteworthy functional groups in organic synthesis, found in various bioactive natural products, agrochemicals, pharmaceuticals, and functional polymers.1-4 Traditionally, phenol esters could be achieved by the reaction between phenols and carboxylic acid derivatives in the presence of a large amount of bases, exhibiting several disadvantages.5-8 The

direct oxidative esterification would express more effective synthetic procedures for the production of phenol esters.2-4,9-15 Cheng et al. previously reported a Pd(OAc)2-catalyzed esterification reaction of aldehydes with arylboronic acids under air to form phenol esters.16 Wu et al. demonstrated a Pd2(dba)3-catalyzed production of phenol esters via the decarboxylative coupling of isatoic anhydrides with arylboronic acids under an oxygen atmosphere.17 Xuan et al. mentioned a Cu(OAc)2-catalyzed direct esterification of phenols with aldehydes to produce phenol esters utilizing organic oxidants.18 Kim et al. synthesized phenol esters using a Cu(OAc)2-catalyzed esterification of benzylic alcohols with ortho-acyl phenols.19 Lately, Kim et al. described the first demonstration of an oxidative coupling reaction between numerous dialkyl/dibenzyl ethers and ortho-acyl phenols to form phenol esters using Cu(OAc)2 catalyst.20 To add to green features for the transformation, solid catalysts should be explored.21,22 Moreover, the contamination of phenol esters with hazardous metals would be reduced if it is possible to reuse the solid catalyst.23-26

Metal-organic frameworks (MOFs) have appeared as a modern type of crystalline materials being composed of metal cations coordinated to multidentate organic molecule linkers.27-31 A sequence of MOFs could be assembled by changing the nature of the linkers as well as the


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cations,32-34 thus optimizing the diversity in structure and properties of the materials.35-40 Even though more endeavors are demanded for the development of these materials, possible applications of MOFs have attracted notable concentration throughout the most recent years.41-45 In the field of catalysis, MOFs have gained immense worldwide interests from both academia and industry.45-52 Different from conventional solid catalysts, all metal sites or organic ligands might be valuable for catalytic conversions.53-56 Furthermore, the nature of MOFs would allow for systematically tuning the steric and electronic properties of the central metal ions, appreciably like in metal complex homogeneous catalyst systems.57-60 With all these advantages, MOFs would connect the difference between homogeneous and heterogeneous catalysis in organic synthesis.61,62 Indeed, a diversity of conversions have been performed utilizing MOFs as heterogeneous catalysts.63-70 Among numerous celebrated MOFs, materials containing copper sites have been explored as catalysts for abundant organic reactions.65,71-78 Herein, we would like to present the synthesis of phenol esters by the direct coupling of ethers with 2-acylphenols utilizing Cu2(dhtp) MOF as a recyclable catalyst. To our best understanding, the direct synthesis of phenol esters utilizing ethers as acylating source assisted by solid catalysts was not previously reported in the literature.

2. Experimental

2.1. Catalyst preparation

The Cu2(dhtp) was synthesized in compliance with a procedure previously reported by Sanz et al.79 In a representative preparation, a solution of Cu(NO3)2.3H2O (1.18 g, 4.88 mmol) in N,N’-


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dimethylformamide (47 ml) was introduced to round-bottom flask. H2dhtp (H2dhtp = 2,5dihydroxyterephthalic acid; 0.44 g, 2.22 mmol), and isopropanol (3 ml) were then added to the flask. The mixture was vigorously stirred for 15 min to gain a clear solution, and was then equally added to seven 10 ml pressurized vials. The vials were tightly covered and heated at 85 o

C without interruption for 18 h. Reddish crystals were produced on the wall of the vials

throughout the time of the experiment. Subsequent to this period, the vials were cooled down to ambient temperature. The crystals were collected by decantation, and washed thoroughly with N,N’-dimethylformamide (3 x 20 ml). The solid was then immersed in isopropanol (3 x 20 ml) at ambient temperature for solvent interchange. Afterwards, the product was dried under vacuum in a shlenkline at 150 oC for 6 h, obtaining 0.50 g of Cu2(dhtp) in the shape of reddish black crystals (59 % yield regarding 2,5-dihydroxyterephthalic acid).

2.2. Phenol ester synthesis utilizing Cu2(dhtp) catalyst

In a representative reaction, 2-acetyl phenol (0.0409g, 0.3 mmol) was introduced to a pressurized vial accommodating the catalyst (0.0015 g, 3 mol%). A mixture of diphenyl ether (0.051g, 0.3 mmol) and dimethyl sulfoxide (1 ml) was then added to the vial. The catalyst amount was determined regarding the copper/2-acetyl phenol mole proportion. Dibenzyl ether (0.0891g, 0.45 mmol) and aqueous tert-butyl hydroperoxide (tBuOOH/TBHP; 0.193g, 5 equivalents) were then introduced, and the mixture was continuously stirred at 80 oC for 6 h. Samples were taken, quenched with water (1 ml). Ethyl acetate (3 ml) was used for the extraction, and the organic phase was shaked vigorously with anhydrous Na2SO4, and then analyzed by GC regarding the diphenyl ether internal standard. The expected product, 2-acetylphenyl benzoate, was isolated


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using column chromatography. The product specification was additionally verified by GC-MS, 1

H NMR, and

13

C NMR. For the catalyst recycling experiment, the catalyst was collected by

centrifugation, and washed thoroughly with dichloromethane, activated in a shlenkline under vacuum for 6 h at 150 oC, and reused for new experiment.

3. Results and discussion

3.1. Catalyst characterization

The Cu2(dhtp) was obtained in 59% yield, using 2,5-dihydroxyterephthalic acid as the organic linker and copper nitrate trihydrate as the metal source, in compliance with a procedure previously reported by Sanz et al.79 After the solvent exchange and the activation steps, the material was characterized by standard analysis techniques (Fig. S1 – Fig. S7). X-ray powder diffraction observation demonstrated that a crystalline framework was acquired, with a remarkably sharp peak at 2θ of 7.0 being recorded (Fig. S1). Scanning electron microscopy studies disclosed that well-formed crystals were generated (Fig. S2). Transmission electron microscopy exploration suggested that a porous framework was achieved, though its pore arrangement was elaborate (Fig. S3). Nitrogen physisorption measurements revealed that the framework would have pore diameter of less than 10 Å, being in microporous range (Fig. S4). Langmuir surface areas of 1300 m2/g were recorded, which should be higher than those of ordinary microporous materials (Fig. S5). A noticeable weight-loss stage of appoximately 10% was noticed in the thermogravimetric analysis result when the temperature extended to over 275 o

C, signaling the thermal stability of the material (Fig. S6). The C=O stretching vibration noted


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for 2,5-dihydroxyterephthalic acid (1651 cm-1) was clearly shifted to lower wave number in Fourier transform infrared spectra of the framework, supported the deprotonation of carboxyl groups due to the reaction with metal cations (Fig. S7).

3.2. Catalytic studies

OH

O +

O

Cu-MOF-74 tBuOOH, DMSO, 80oC

O

O

O

Scheme 1. The reaction between dibenzyl ether and 2-acetyl phenol utilizing Cu2(dhtp) catalyst.

The Cu2(dhtp) was prepared with 59% yield by solvothermal approach in compliance with a procedure previously reported by Sanz et al.,79 and was characterized using traditional approaches (Fig. S1 – Fig. S7). The Cu2(dhtp) was utilized as a solid catalyst for the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate in the role of the major product (Scheme 1). First, the influence of temperature to the direct esterification was explored. The reaction was conducted in dimethyl sulfoxide for 20 h, at 10 mol% catalyst, with 2-acetyl phenol:benzyl ether mole fraction of 1:2, utilizing 3 equivalents of tBuOOH in decane, at ambient temperature, 40 oC, 60 oC, 80 oC, 100 oC, and 120 oC, respectively. No evidence of 2acetylphenyl benzoate was recorded for the reaction performed at ambient temperature or at 40 o

C after 20 h. Elevating the temperature to 60 oC generated 37% yield of 2-acetylphenyl benzoate

after 20 h. The phenol ester yield could be upgraded to 52% when the temperature was increased to 80 oC. Nevertheless, performing the reaction at higher than 80 oC eventuated a considerable


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reduction in the yield of 2-acetylphenyl benzoate due to the oxidation of the starting materials (Fig. 1). Repeated experiments gave similar results. Undoubtedly, in the first report of the reaction between dibenzyl ethers and ortho-acyl phenols to produce phenol esters using Cu(OAc)2 catalyst, Kim et al. performed the experiment at 80 oC.20

100 80 Yield (%)

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60 40 20 0 RT

40

60

80

100

120

o

Temperature ( C)

Fig. 1. Yields of 2-acetylphenyl benzoate at varied temperatures.

Another item that must be discussed for the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate is the catalyst amount. The reaction was performed in dimethyl sulfoxide for 20 h, at 80 oC, with 2-acetyl phenol:benzyl ether mole fraction of 1:2, utilizing 3 equivalents of tBuOOH in decane, at 3 mol%, 5 mol%, 7 mol%, 10 mol%, and 15 mol% catalyst, respectively. The reaction was not able to continue in the absence of the framework catalyst, and no evidence of 2-acetylphenyl benzoate was recorded after 20 h. This observation confirmed the requirement of employing the solid catalyst for the reaction. As


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previously noted, the reaction utilizing 10 mol% catalyst could afford 52% yield after 20 h. Interesting, it was observed that 51% yield was still obtained after 20 h for the reaction conducted at 3 mol% catalyst. Under this condition, the yield was not significantly enhanced by extending the catalyst amount. Indeed, similar yields of 2-acetylphenyl benzoate were recorded for the reaction utilizing more than 5 mol% catalyst (Fig. 2). In the first report of the coppercatalyzed reaction between dibenzyl ethers and ortho-acyl phenols to produce phenol esters, Kim et al. utilized a number of copper salts, and pointed out that 5 mol% Cu(OAc)2 should be used.20 We then tried to improve the yield of the desired product by using 3 mol% catalyst, and changing the reaction condition. 100 80 Yield (%)

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60 40 20 0 0

3

5

7

10

15

Catalyst amount (mol%)

Fig. 2. Yields of 2-acetylphenyl benzoate at varied catalyst amounts. Like other reactions via direct C-H bond activation, an oxidant must be imperative for the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate utilizing the framework catalyst. It was subsequently determined to explore the impact of the oxidant amount. The reaction was performed in dimethyl sulfoxide for 20 h, at 80 oC, with 2-acetyl


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phenol:benzyl ether mole fraction of 1:2, utilizing 3 mol% catalyst, with 1, 2, 3, 4, 5, and 6 equivalents of tBuOOH in decane, respectively. The conversion was not able to continue when tBuOOH was not used, and no evidence of 2-acetylphenyl benzoate was recorded after 20 h. In the presence of 1 equivalent of tBuOOH, only 14% yield of the phenol ester was observed after 20 h. Extending the amount of the oxidant caused a marked enhancement in 2-acetylphenyl benzoate yield. The reaction applying 2 equivalents of the oxidant produced the product in 33% yield after 20 h, while 52% yield was observed for that employing 3 equivalents of the oxidant. It was possible to upgrade the yield to 74% after 20 h by employing 4 equivalents of the oxidant. With 5 equivalents of the oxidant, the reaction could offer 86% yield after 20 h. However, utilizing more oxidant did not enhance the conversion (Fig. 3). Moreover, experimental result proved that aqueous tBuOOH, a cheaper oxidant, exhibited similar efficiency as compared to the corresponding oxidant in decane. Other oxidants such as potassium persulfate, tert-butyl peroxybenzoate, di-tert-butyl peroxide, hydrogen peroxide were inappropriate for the reaction (Fig. 4).


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100 80 Yield (%)

60 40 20

H

iu m

pe rs yd ul fa ro te ge n pe ro xi de

TB P D

TB PB

Po ta ss

tB uO

O H

(in

de Aq ca ue ne ou ) s tB uO O H

0

Oxidant

Fig. 3. Yields of 2-acetylphenyl benzoate at varied oxidants. 100 80 Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20 0 0

1

2

3

4

5

6

Oxidant amount (equiv.)

Fig. 4. Yields of 2-acetylphenyl benzoate with varied oxidant amounts.


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The solvent might display an impressive impact, being subject to the character of the catalyst. Accordingly, the influence of varied solvents to the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate using the framework catalyst was studied. The reaction was performed at 80 oC for 20 h, with 2-acetyl phenol:benzyl ether mole fraction of 1:2, utilizing 4 equivalents of aqueous tBuOOH, at 3 mol% catalyst, in N,N’-dimethylformamide, N,N’-dimethylacetamide, toluene, diglyme, dichlorobenzene, n-butanol, and dimethyl sulfoxide as solvent, respectively. It was noted that N,N’-dimethylformamide was totally not suitable as solvent for the reaction, and no 2-acetylphenyl benzoate was recorded after 20 h. Less than 20% yields were observed for the transformation conducted in N,N’-dimethylacetamide, toluene, diglyme, or dichlorobenzene, while 23% yield was observed for the situation of n-butanol. Dimethyl sulfoxide expressed the best efficiency, producing the phenol ester in 74% yield after 20 h (Fig. 5). Furthermore, the amount of solvent (e.g. the concentration of the starting materials) displayed a noteworthy influence to the generation of 2-acetylphenyl benzoate, having performed the reaction in dimethyl sulfoxide for 20 h at 80 oC, with 2-acetyl phenol:benzyl ether mole fraction of 1:2, utilizing 4 equivalents of aqueous tBuOOH, with 3 mol% catalyst, at 2acetyl phenol concentration of 0.075 M, 0.1 M, 0.15 M, 0.3 M, and 0.6 M, respectively. Diminishing the reactant concentration to less than 0.3 M considerably reduced the yield of the product (Fig. 6).


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100 80 Yield (%)

60 40 20

,N

N

N

,N

’-d im

et hy lfo ’-d rm im am et hy id la e ce ta m id e To lu en e D D ig ic ly hl m or e ob en ze ne nD bu im ta et no hy l ls ul fo xi de

0

Solvent

Fig. 5. Yields of 2-acetylphenyl benzoate in varied solvents. 100 80 Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20 0 0.6

0.3

0.15

0.1

0.075

2-Acetyl phenol concentration (M)

Fig. 6. Yields of 2-acetylphenyl benzoate at varied 2-acetyl phenol concentrations.


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Subsequently, we tried to improve the yield of 2-acetylphenyl benzoate by investigating the influence of the reactant mole fraction to the direct esterification. The reaction was performed in dimethyl sulfoxide for 20 h, at 80 oC, utilizing 4 equivalents of aqueous tBuOOH, at 3 mol% catalyst, with 2-acetyl phenol:benzyl ether mole fraction of 1:1, 1:1.5, 1:2, 1:2.5, and 1:3, respectively. In the first report of the reactions between dibenzyl ethers and ortho-carbonyl phenols to produce phenol esters using Cu(OAc)2 as catalyst, Kim et al. conducted the transformation with 2 equivalents of dibenzyl ethers.20 In this work, it was monitored that utilizing 2 equivalents of dibenzyl ether could afford 74% yield of 2-acetylphenyl benzoate after 20 h. Expanding the amount of dibenzyl ether was not the best option as the yield was not upgraded to a greater extent. Undoubtedly, the reaction utilizing 3 equivalents of dibenzyl ether produced the phenol ester in 71% yield after 20 h. Reducing 2-acetyl phenol:benzyl ether mole fraction to 1:1.5, the yield of 2-acetylphenyl benzoate could be slightly enhanced to 76% after 20 h. However, the reaction utilizing 1 equivalent of dibenzyl ether provided 69% yield after 20 h (Fig. 7). These observations suggested that the yield of the phenol ester could not be upgraded remarkably by changing the reactant mole fraction. As mentioned earlier, the oxidant amount expressed a considerable influence to the reaction. It was noticed that the reaction utilizing 1.5 equivalents of dibenzyl ether and 5 equivalents of aqueous tBuOOH could continue to 88% yield after 6 h. This reaction condition was accordingly employed for next experiments.


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100 80 Yield (%)

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60 40 20 0 1:1

1:1.5

1:2

1:2.5

1:3

Reactant mole fraction

Fig. 7. Yields of 2-acetylphenyl benzoate at varied 2-acetyl phenol:benzyl ether mole fractions.

As the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate using the framework catalyst was conducted in solution phase, the leaching assessment must be addressed. In some situations, the reaction might not progress under true heterogeneous catalysis assignable to the leaching matter. The leaching assessment was accordingly conducted to test if any 2-acetylphenyl benzoate was produced via homogeneous catalysis. The reaction was performed in dimethyl sulfoxide for 6 h, at 80 oC, with 2-acetyl phenol:benzyl ether mole fraction of 1:1.5, utilizing 5 equivalents of aqueous tBuOOH, at 3 mol% catalyst. Subsequent to the first 2 h, the solid framework catalyst was isolated. The reaction solution was afterwards transported to a new pressurized vial, and heated at 80 oC with magnetic stirring for further 4 h. Reaction progression, if any, was evaluated by GC as previously explained. It was noticed that no phenol esters generated by leached species, if any, was recorded (Fig. 8). These results would


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confirm that the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate was able to continue only if the solid catalyst was present in the reaction mixture, and the donation of leached species to the generation of the desired phenol ester, if any, was inconsiderable.

100 80 Yield (%)

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60 40 3 mol% catalyst

20

Leaching test

0 0

1

2

3

4

5

6

7

Time (h)

Fig. 8. No donation from homogeneous catalysis to the generation of 2-acetylphenyl benzoate was recorded.

To understand the mechanism of the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate, more mechanistic studies were conducted. The reaction was performed in dimethyl sulfoxide for 6 h, at 80 oC, with 2-acetyl phenol:benzyl ether mole fraction of 1:1.5, utilizing 5 equivalents of aqueous tBuOOH, at 3 mol% catalyst. To verify the requisite of the oxidant in the conversion, ascorbic acid or 2,2,6,6-tetramethylpiperidin-1yl)oxidanyl (usually known as TEMPO) as the antioxidant was introduced to the reactor after the


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first 1 h reaction time. The mixture was heated at 80 oC with magnetic stirring for further 5 h. The conversion was tremendously influenced by the radical scavenger (Fig. 9). It could be proposed that ascorbic acid or TEMPO trapped the radicals generated in the catalytic cycle. In an other experiment, pyridine was used as a catalyst poison. Subsequent to the first 2 h reaction time, pyridine was introduced to the reactor, and the resulting mixture was heated at 80 oC with magnetic stirring for further 4 h. No extra 2-acetylphenyl benzoate was recorded when the catalyst poison was present (Fig. 10). It must be emphasized that the reaction was not be able to progress in the absence of the framework catalyst.

100 3 mol% catalyst Adding TEMPO

80 Yield (%)

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Adding ascorbic acid

60 40 20 0 0

1

2

3

4

5

6

7

Time (h)

Fig. 9. Influence of ascorbic acid and TEMPO on 2-acetylphenyl benzoate yield.


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100 80 60

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 3 mol% catalyst

20

Pyridine test

0 0

1

2

3

4

5

6

7

Time (h)

Fig. 10. Impact of pyridine on 2-acetylphenyl benzoate yield. O

OH OH

Cu(II)

O

Cu(II) O

O

Cu(I)

O C Cu(III) O O

O

O

O

O C

O

OH O

Scheme 2. Suggested reaction mechanism.


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To verify the importance of the acetyl group in ortho-acetyl substituted phenols, the reaction between phenol and dibenzyl ether was carried out in dimethyl sulfoxide for 6 h, at 80 oC, with phenol:benzyl ether mole fraction of 1:1.5, utilizing 5 equivalents of aqueous tBuOOH, at 3 mol% catalyst. However, no phenol ester was detected in this case. This observation would suggest that the acetyl group at ortho position in phenol should be necessary for the bidentate coordination with copper sites in the catalytic cycle. Indeed, Kim et al. reported the same result for the reaction using Cu(OAc)2 catalyst.20 Kappe,80 Reddy81 et al. previously indicated that the carbonyl group adjacent to the hydroxy moiety in phenol acted as a directing group for several catalytic crossdehydrogenative-coupling reactions. In an other experiment, the oxidation of dibenzyl ether with aqueous tBuOOH in the presence of Cu2(dhtp) was investigated. GC-MS analysis indicated that benzyl alcohol, benzaldehyde, benzoic acid, and benzyl benzoate were detected in the mixture. We also carried out the reaction of 2-acetyl phenol with benzyl alcohol, benzaldehyde, benzoic acid, and benzyl benzoate, respectively, utilizing the copper-based framework catalyst. It was found that the expected product, 2acetylphenyl benzoate, was generated for the case of benzyl alcohol and benzaldehyde. However, no evidence of phenol ester was observed for the case of benzoic acid, and benzyl benzoate. Although further studies are needed to elucidate the reaction mechanism, the formation of benzoyl radical from dibenzyl ether, benzyl alcohol, and benzaldehyde in the catalytic cycle should be possible. Based on these observations and the literature,20 the pathway for the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate was recommended (Scheme 2).


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100 80 Yield (%)

60 40 20

C

)2 (D

AB C

O ) u3 (B TC )2 C C u2 u( BD (P BD C ) C )2 (B PY ) C u2 (d ht p)

N

i2 (B D

C

Fe (B P

D

C

C

)

)

0 Fe (B D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Catalyst

Fig. 11. Yields of 2-acetylphenyl benzoate with varied MOFs as catalyst.

To underline the noteworthy benefit of employing the Cu2(dhtp) catalyst for the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate, its catalytic efficiency was differentiated with other MOFs. The reaction was performed in dimethyl sulfoxide for 6 h, at 80 oC, with 2-acetyl phenol:benzyl ether mole fraction of 1:1.5, utilizing 5 equivalents of aqueous tBuOOH, at 3 mol% catalyst. Ni2(BDC)2(DABCO), Fe(BDC), and Fe(BPDC) were not catalytically active for the reaction, and no evidence of 2-acetylphenyl benzoate was recorded after 6 h. Although Cu3(BTC)2 previously expressed high productivity in plentiful reactions, this Cu-MOF provided low activity in the synthesis of phenol esters, affording only 8% yield after 6 h. The Cu(BDC)-catalyzed coupling of dibenzyl ether with 2-acetyl phenol was able to progress


19


to 50% yield after 6 h, while the reaction utilizing Cu2(PBDC)2(BPY) catalyst produced the phenol ester in 82% yield. Among these MOFs, Cu2(dhtp) displayed the best exhibition, with 88% yield of the product being recorded after 6 h (Fig. 11). Furthermore, the catalytic activity of Cu2(dhtp) in the reaction between dibenzyl ether and 2-acetyl phenol was also differentiated to a number of homogeneous catalysts such as Cu(NO3)2, Cu(OAc)2, Ni(NO3)2, Fe(NO3)3, 2,5dihydroxyterephthalic acid, the mixture of 2,5-dihydroxyterephthalic acid and Cu(OAc)2. Interestingly, this framework could display better catalytic efficiency for this conversion than these homogeneous catalysts. Certainly, 79% yield was noted for the reaction using Cu(OAc)2 catalyst, while 88% yield was obtained for the case of the framework catalyst (Fig. 12). It must be mentioned that Cu(OAc)2 was soluble in dimethyl sulfoxide, and therefore this homogeneous catalyst could not be recycled and reused for the reaction. 100 80 Yield (%)

60 40 20

2d ht p C u( O Ac )2 C u( O Ac )2 C u2 (d ht p)

H

2d ht p

+

H

O 3) 3 N i(N O 3) 2

Fe (N

u( N

O

3) 2

0

C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 41

Catalyst

Fig. 12. Yields of 2-acetylphenyl benzoate with varied homogeneous catalysts.


20

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As mentioned earlier, the Cu2(dhtp) displayed better efficiency in the production of phenol esters than a number of MOFs as well as traditional homogeneous catalysts. To underline the remarkable feature of this catalyst in the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate, one point that must be explored is the catalyst reusability. In perfect situation, it should be possible to reuse the solid catalyst for abundant times. The Cu2(dhtp) was accordingly studied for reusability in the reaction over 6 consecutive runs. The reaction was performed in dimethyl sulfoxide for 6 h, at 80 oC, with 2-acetyl phenol:benzyl ether mole fraction of

1:1.5, utilizing 5 equivalents of aqueous tBuOOH, at 3 mol% catalyst.

Subsequent to the first run, the catalyst was removed, and washed thoroughly with dichloromethane, heated at 150 oC in a shlenkline under vacuum for 6 h, and reused in new experiments. It was possible to reuse the catalyst plentiful times in the reaction between dibenzyl ether and 2-acetyl phenol to produce 2-acetylphenyl benzoate without a noticeable deterioration in catalytic efficiency. Undoubtedly, the 6th use of the catalyst still provided 86% yield of 2acetylphenyl benzoate (Fig. 13). Furthermore, XRD (Fig. 14) and FT-IR (Fig. 15) analysis determination of the reused catalyst revealed that the framework structure could be preserved after the experiments.


21


100

Yield (%)

80 60 40 20 0 1

2

3

4

5

6

Run

Fig. 13. Catalyst recycling investigation.

1200

Relative Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 41

900

600 (a) 300 (b) 0 5

10

15

20

25

30

2-Theta scale

Fig. 14. XRD determination of the new (a) and recovered (b) catalyst.


22

Page 23 of 41

100 (a) 80

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

(b)

40

20

0 4000

3600

3200

2800

2400

2000

1600

1200

800

400

-1

Wave number (cm )

Fig. 15. FT-IR analyses of the new (a) and recovered (b) catalyst.

Table 1. The synthesis of phenol esters utilizing Cu2(dhtp) catalyst.

Entry

2-Acylphenols

Ethers

Esters

Isolated yields (%)

1

OH

O

O

O


O

O

70

23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2

Page 24 of 41

OH O O O

3

OH

O

O

O

O

70a O

4

OH

O

O

65a

O

O

OH O

O

O

O O

5

68a

O

O

O

52a O

6

OH O

O

O

O

60 O

O

O

Br

Br


24

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7

OH

O

O

50 O

8

OH

O

O

O

O

55 O

a

O

O

The reaction was conducted in 20 h.

The work was subsequently developed to the synthesis of several phenol esters by the direct esterification reaction utilizing ethers as acylating source. In the first experiment sequence, we conducted the reaction between 2-acetyl phenol and different ethers, including dibenzyl ether, 2,2’-dimethyl dibenzyl ether, 3,3’-dimethyl dibenzyl ether, 4,4’-dimethoxy dibenzyl ether, di-nbutyl ether, respectively. The reaction was performed in dimethyl sulfoxide for 6 h, at 80 oC, with 2-acetyl phenol:ether mole fraction of 1:1.5, utilizing 5 equivalents of aqueous tBuOOH, at 3 mol% catalyst. Under this condition, 2-acetylphenyl benzoate was achieved in an isolated yield of 70% by the coupling of dibenzyl ether with 2-acetyl phenol. We also tried to detect byproducts of the reaction, and GC-MS analysis revealed that trace amounts of benzyl alcohol, benzaldehyde, benzoic acid, and benzyl benzoate were present in the mixture. This could be rationalized based on the fact that the excess dibenzyl ether could be oxidized in the presence of the oxidant and the copper-based framework catalyst. Ethers containing a substitutent decreased


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

the yield of the desired product, and the reaction period had to be extended to 20 h. The reaction between 2,2’-dimethyl dibenzyl ether with 2-acetyl phenol could continue to 68% yield of 2acetylphenyl 2-methylbenzoate after 20 h. Similarly, 70% yield of 2-acetylphenyl 3methylbenzoate was obtained after 20 h for the reaction between 3,3’-dimethyl dibenzyl ether and 2-acetyl phenol, while 65% yield of 2-acetylphenyl 4-methoxybenzoate was recorded for that using 4,4’-dimethoxy dibenzyl ether as acylating source. Di-n-butyl ether was less reactive than dibenzyl ether, though the transformation still provided 52% yield of 2-acetylphenyl butyrate after 20 h. In the second experiment sequence, we performed the reaction between dibenzyl ether and different 2-carbonyl-substituted phenols, including 2-acetyl-4-bromo phenol, 2-acetyl naphthol, and 2-benzoyl phenol, respectively. Under this condition, the transformation could provide 60% yield of 2-acetyl-4-bromophenyl benzoate after 6 h. It was also noticed that 50% yield of 2-acetylnaphthalen-1-yl benzoate and 55% yield of 2-benzoylphenyl benzoate were recorded after 6 h (Table 1).

3. Conclusions

The Cu2(dhtp) MOF was prepared by a solvothermal protocol, and was characterized by a number of approaches. The framework could be utilized as a recyclable catalyst for the reaction between ethers and 2-acylphenols to produce phenol esters. The solvent displayed a remarkable impact to the generation of the phenol esters, and dimethyl sulfoxide should be the best choice for the conversion. The Cu2(dhtp) were more catalytically active than a number of MOFs as well as traditional homogeneous. Heterogeneous catalysis was confirmed for the conversion, and no phenol ester generated by leached species, if any, was recorded. It was possible to reuse the


26

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


framework catalyst plentiful times in the conversion of 2-acylphenols and ethers to phenol esters without a noticeable deterioration in catalytic efficiency. The feature that phenol esters could be produced utilizing ethers as acylating source assisted by solid catalysts would be fascinated to the chemical industry.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. X-ray powder diffractograms, SEM micrograph, TEM micrograph, pore size distribution, nitrogen adsorption/desorption isotherm, TGA analysis, FT-IR spectra of the catalyst; 1H and

13

C NMR

spectra of all products.

AUTHOR INFORMATION Corresponding Author Nam T. S. Phan* Ph: (+84 8) 38647256 ext. 5681

Fx: (+84 8) 38637504


Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS


27


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Page 28 of 41

We would like to thank the Vietnam National University - Ho Chi Minh City (VNU-HCM) for financial funding under grant number TX2016-20-05 via annual research program.

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Synthesis of Enol Carbamates and 2-Carbonyl-Substituted Phenol Carbamates. Angew. Chem. Int. Ed. 2011, 50, 11748-11751.


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O-Acetyl Substituted Phenol Ester Synthesis via Direct Oxidative Esterification Utilizing Ethers as An Acylating Source with Cu2(dhtp) Metal-Organic Framework as A Recyclable Catalyst Thien N. Lieu, Ha T. T. Nguyen, Ngoc D. M. Tran, Thanh Truong, Nam T. S. Phan* Faculty of Chemical Engineering, Ho Chi Minh City University of Technology, VNU-HCM, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam *


Ph: (+84 8) 38647256 ext. 5681

Fx: (+84 8) 38637504

Graphical Abstract OH

O +

Cu2(dhtp) TBHP, DMSO, 80oC

O

O

O

O


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254x190mm (96 x 96 DPI)


O-Acetyl-Substituted Phenol Ester Synthesis via Direct Oxidative

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