SHORT COMMUNICATION Silica sulfuric acid

Chemical Papers 62 (6) 630–634 (2008) DOI: 10.2478/s11696-008-0070-7

SHORT COMMUNICATION

Silica sulfuric acid-catalyzed expeditious environment-friendly hydrolysis of carboxylic acid esters under microwave irradiation Zheng Li*, Jing Liu, Xue Gong, Xuerong Mao, Xiunan Sun, Zhouxing Zhao Gansu Key Laboratory of Polymer Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu, 730070, China Received 10 March 2008; Revised 25 April 2008; Accepted 28 April 2008

Silica sulfuric acid was found to be an efficient, recoverable, reusable and environment-friendly catalyst for the fast hydrolysis of various carboxylic acid esters in high conversions and selectivities under microwave irradiation conditions. This protocol has the advantages of no corrosion, no environmental pollution, high reaction rate, high yield, and simple work-up procedure. c 2008 Institute of Chemistry, Slovak Academy of Sciences Keywords: silica sulfuric acid; carboxylic acid ester; hydrolysis; microwave irradiation

Ester derivatives are widely present in many drugs, natural products, and synthetic compounds. Furthermore, they are frequently used for the protection of carboxylic acids and alcohols, which, as synthons, are unmasked later in the synthesis. Therefore, the hydrolysis of carboxylic acid esters is one of the most studied chemical reactions. (Bender, 1960) The reported ester hydrolysis could be catalyzed by acid, alkali (Theodorou et al., 2007), molecular iodine (Yadav et al., 2006), Zn(II) complexes (Bazzicalupi et al., 2005), and Cu(II) complexes (Kou et al., 2004). General acidic hydrolysis of the majority of common esters occurs in the presence of strong liquid protic acids such as HCl, TFA, H2 SO4 (Bentley et al., 1987; Strazzolini et al., 2005) and HNO3 (Strazzolini et al., 2000), as catalysts dissolved in organic solvents. However, many of the above-mentioned liquid acid catalysts are corrosive and often cause heavy environmental pollution because of the difficult separation from the reaction medium. Furthermore, the reactions require long reaction times and often give unsatisfactory yields. In addition, also the organic solvents used in the reactions are hazardous chemicals and often cause environmental problems. Therefore, there is still a need for a green method for carboxylic acid ester hydrolysis. In recent years, the use of solid supports under mi-

crowave irradiation has become more popular in synthetic organic chemistry (Gershonov et al. 2007; Ranu et al., 2000; Xu et al., 2007) and heterogeneous reactions facilitated by supported reagents on various solid inorganic surfaces have received more attention (Song & Lee, 2002). The advantage of these methods over conventional homogenous reactions is that they provide higher selectivity, enhanced reaction rates, allow obtaining cleaner products, and simplify handling with reaction mixtures. Silica sulfuric acid is an important solid-supported Lewis acid known as an efficient catalyst for many reactions, e.g. the Mannich reaction (Wu et al., 2007), Beckmann rearrangement (Li et al., 2006b), Friedel– Crafts acylation (Alizadeh et al., 2007), Michael addition (Li et al., 2006a), etc. The early research development in using silica sulfuric acid was also reviewed (Salehi et al., 2006). In continuation of our ongoing program to develop environment benign methods using solid supports, an expeditious and high-yielding method for the hydrolysis of carboxylic acid esters using silica sulfuric acid as a recoverable, reusable, and environment friendly catalyst under microwave irradiation conditions is presented. Conversions and selectivities of the reactions were determined using a Shimadzu GC 2010 equipped with

*Corresponding author, e-mail: [email protected]

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Z. Li et al./Chemical Papers 62 (6) 630–634 (2008)

SiO2

OH + ClSO3H

CH2Cl2 r.t.

SiO2 OSO H + HCl (g) 3

Fig. 1. Synthesis of silica sulfuric acid.

SiO2 OSO H 3

O R1 O R2

+ H2O

O +

R1 MWI, 7-12 min

R2 OH

OH

Fig. 2. Hydrolysis of carboxylic acids esters in the presence of silica sulfuric acid.

a 15 m-0.5 mm RTX-1 capillary column and a FID detector. Microwave reactions were conducted in a modified microwave oven fitted with a condenser (LGWP650, China). The esters were prepared according to standard methods and their purities were established prior to their use by measuring their melting points, boiling points, or using GC. Silica sulfuric acid was prepared according to the reported method. The silica-supporting capacity for SO3 H was ca 2.5 mmol g−1 (Zolfigol, 2001). Carboxylic acid ester (5 mmol), silica sulfuric acid (0.2 g, 0.5 mmol based on the amount of silicasupported SO3 H groups) and distilled water (0.14 g, 7.5 mmol) were added to a round-bottomed flask. Then the flask was transferred into the modified microwave oven fitted with a condenser. The mixture was subjected to microwave irradiation at the power of 455 W for the appropriate time, as indicated in Table 1. Progress of the reaction was monitored by thin layer chromatography (petroleum ether to ethyl acetate volume ratio, ϕr = 3:1). After completion of the reaction, the reaction mixture was cooled to room temperature and diluted with acetone (5 mL). Then, the resulting mixture was filtered to recover the catalyst and the filtrate was used to determine conversion and selectivity of the reaction by GC. The analytical samples were obtained either by recrystallization from aqueous ethanol (ϕr = 5:1) or by chromatography using a petroleum ether–ethyl acetate mixture (ϕr = 20:1) as eluent after the solvent removal by evaporation. All products were characterized by the comparison of their melting or boiling points, IR, and 1 H NMR spectra with those of authentic samples. Silica sulfuric acid was prepared through a reaction of silica gel (80–200 mesh) with chlorosulfonic acid in methylene chloride (Fig. 1) and it was characterized by IR: ν˜/cm−1 : 1282 (S—O), 886, 852 (S—O). Initially, phenyl acetate was selected as a substrate to examine the feasibility of the hydrolysis of esters using silica sulfuric acid as catalyst under microwave irradiation. Generally, the reaction rate and yield were increased over the amount of catalyst. It was found that phenyl acetate (5 mmol) and distilled water (7.5 mmol) catalyzed by 10 mmol of silica sulfuric acid (i.e. 0.2 g, 0.5 mmol based on the silica-supported SO3 H) was the optimized ratio for the hydrolysis reaction (Fig. 2). The smaller amount of catalyst used, the lower yield and even prolonged reaction time were found. On the other hand, using higher amount of catalyst, no increase of conversion to the desired product was observed. In order to select the approximate microwave power, the reaction was also conducted at dif-

ferent power levels ranging from 325 W to 650 W. It was observed that the power level of 455 W was the most promising for the reaction because lower power levels resulted in poor yields and higher power lead to carbonization of the substrate. To explore the hydrolysis of esters using silica sulfuric acid as a catalyst, a series of representative aromatic esters and aliphatic esters as substrates were examined under microwave irradiation conditions (Table 1). It is noteworthy that not only aromatic carboxylic acid esters but also aliphatic carboxylic acid esters were smoothly hydrolyzed under the given conditions. It was found that aryl acetates were hydrolyzed to give acetic acid and the corresponding phenols without any difficulty (Table 1, entries 1– 15). However, presence of electron-withdrawing groups (such as nitro- and chloro-) on the aryl rings favored ester hydrolysis (Table 1, entries 3–7, 13–15). Furthermore, the more potent was the electron-withdrawing group located on the aryl ring, the higher conversion of esters was observed. In contrast, the aryl acetates bearing electron-donating groups (such as methyl and tert-butyl) on the aryl rings afforded slightly lower conversions and selectivities (Table 1, entries 2, 10– 12). Alkyl acetates were also efficiently hydrolyzed to acetic acid and the corresponding alcohols in good conversions and selectivities (Table 1, entries 16–18). Benzoates could also readily hydrolyze to produce benzoic acid and the respective phenols or alcohols in high conversions and selectivities (Table 1, entries 19– 25). The different substituents on the aryl rings had no obvious effect on the hydrolysis reaction. However, the steric effect played an important role in the hydrolysis of benzoates. For example, the hydrolysis of o-hydroxybenzoate exhibited lower conversion and selectivity because of the steric hindrance caused by a substitution of the ortho position (Table 1, entry 25). Chosen reaction conditions were also suitable for the hydrolysis of diesters, such as dibutyl phthalate, to efficiently produce phthalic acid and butanol in excellent conversion and selectivity (Table 1, entry 26). The reusability of silica sulfuric acid was also tested using phenyl acetate and p-nitrophenyl acetate as substrates (Table 2). An insignificant decrease of the substrate conversions and product selectivities was observed after four runs. A comparison of hydrolysis of the selected esters using microwave irradiation with that using traditional heating was also conducted. It was observed that using the microwave irradiation method, reac-


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Table 1. Hydrolysis of various carboxylic acid esters catalyzed by silica sulfuric acid under microwave irradiationa Reaction ConSelectime versionb tivityb Entry

Table 1. (continued) Reaction ConSelectime versionb tivityb Entry

Ester

Ester min

%

min

%

%

12

94

92

12

98

93

9

89

95

10

80

96

12

97

84

12

93

78

12

90

80

11

98

95

11

98

98

11

73

95

12

68

75

11

90

94

% OCOCH3

OCOCH3

1

10

98

15

Cl

16

CH3

93 O

OCOCH3

10

2

93

O C2H5

90 O

C(CH3)3

3

17

OCOCH3

O2N

O

8

97

96

OCOCH3

O2N

NO2

CH3 O

7

98

OCOCH3

20 8

98

(CH2)2CH(CH3)2

95 19

5

COO

COO

21

COO

OCOCH3

CH3

Cl

9

98

95

7

97

96

22

NO2

OCOCH3

23

O2N

7

C(CH3)3

97

NO2

6

(CH2)3CH3

O

18 4

CH3

COO

Cl

COOCH2

COOCH3 NO2

24

OCOCH3

10

8

83

96

OH

25 COO(CH2)4CH3

OCOCH3

10

9

93

92

OCOCH3

10

H3C

COO(CH2)3CH3

12

94

91 a) All products were characterized by comparison of their melting or boiling points, IR, and 1 H NMR spectra with those of authentic samples; b) Deduced from the GC analysis.

OCOCH3

10

11

95

92

CH3

OCOCH3

12

11

95

83

12

98

93

12

90

84

CH3 OCOCH3

13 Cl OCOCH3 Cl

14 Cl

COO(CH2)3CH3

26

tions could be completed within a much shorter time (8–12 min) compared to the traditional method (16–20 h). The conversions and selectivities for the microwave irradiation method were also slightly higher than those obtained using traditional heating (Table 3). Silica sulfuric acid was found to be an efficient, recyclable and reusable catalyst for hydrolysis of various carboxylic acid esters under microwave irradiation conditions. The catalyst is cheap and stable. Compared to the homogeneously and also heterogeneously catalyzed hydrolysis of esters carried out under traditional heating, the presented protocol showed advantages of higher conversion and selectivity, shorter re-


633


Table 2. Reusability of silica sulfuric acid deduced from the GC analysis Conversion/%

Selectivity/%

Substrate 1st run

2nd run

3rd run

4th run

1st run

2nd run

3rd run

4th run

98

97

95

94

93

93

92

91

98

97

96

94

97

96

94

93

OCOCH3

OCOCH3

NO2

Table 3. Comparison of the microwave irradiation and traditional heating influence on the hydrolysis of carboxylic acid estersa Microwave irradiation Substrate

Traditional heating

Time min

Conversion %

Selectivity %

Time h

Conversion %

Selectivity %

12

98

93

20

88

91

10

93

92

20

85

90

10

97

92

16

87

88

8

98

97

20

96

96

OCOCH3

Cl OCOCH3

OCOCH3

CH3 OCOCH3

NO2

a) Deduced from the GC analysis.

action times, no environmental pollution, and simpler manipulation with the reactants. Acknowledgements. The authors thank the National Natural Science Foundation of China (20772096) and the Gansu Natural Science Foundation (3ZS061-A25-033) for financial support of this work.

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