Novel Brϕnsted Acidic Ionic Liquid Based on a Cyclic Guanidinium Cation: a Green, Efficient, and Recyclable Dual Slovent-catalyst System for Fisher Esterification

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CHEM. RES. CHINESE U. 2007, 2 3 ( 6 ) , 665-668 Article ID lOo5-904O(2Oo7)-06-66544

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Novel Br-ted Acidic Ionic Liquid Based on a Cyclic Guanidinium Cation : a Green, Efficient, and Recyclable Dual Slovent-catalyst System for Fisher Esterification GUO Xu, DUAN Hai-feng, SUN Hai, CAO Jun-gang and LIN Ying-jie' Department of Organic Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China Received Mar. 21, 2007

A novel Br+nsted acidic ionic liquid( IL) based on the cyclic guanidinium cation has been synthesized. This IL, as a strong Br+nsted acid catalyst or solvent, shows high catalytic activity and biphsaic behavor in the esterifcations of carboxylic acids and alcohols. The produced esters as a separate phase can be conveniently decanted out from the IL and the IL is recyclable without any loss of catalytic activity. Keywords Br+nsted acid ; Ionic liquid ; Guanidinium ; Fisher esterifcation

Introduction Esterifications of alcohols with carboxylic acids are one of the most important reactions in organic synthese?. Organic esters are important products or intermediates in chemical and pharmaceutical industries" ] . Esterification reaction is traditionally catalyzed by an inorganic acid, such as concentrated hydrochloric acid or sulfuric acid[233'.The drawbacks of the preparation method are the tedious isolation of products, the use of volatile organic solvents, and the removal of large volumes of salt waste during the neutralization of homogeneous acid. Moreover, recycling of the liquid acid catalyst is very difficult[41. To overcome these disadvantages, efforts have been made to develop heterogenous catalyst systems, such as resins["61 , supported mineral acid!'] , heteropoly , and zeolites"01 . How-ever, these solid acid catalysts have shortcomings, such as, high molecular weighvactive-site ratios, high cost, and an unstable nature'"'. Recently, ionic liquids( ILS) have been considered as one of the most promising, environmentally benign alternatives , on account of their peculiar properties, such as, nonvolatility, particular solubility in organic and inorganic

compounds, and easy The design of the ILs gives one a little more opportunity to synthesize task-specific ionic liquids ( TILs ) , which include functional groups designed to confer particular properties or reactivity according to given reactions. For example, Br+nsted acidic ionic liquids combine both advantages of solid acids ( e. g . , nonvolatility and easy recycle) and liquid acids ( e . g . , greater effective surface area and potential activity of liquid phase), and have been designed and used as dual solvent-catalysts for Fisher esterification' l4-l7] , alcohol dehydrodimeri~ation"~] , pinacoVbenzopinaco1 rearrangement^"^] , and electrophilic substitutions of indoles with aldehydes"s1. The use of Br+nsted acidic TILs to catalyze organic reactions is an area of ongoing activity, however, the development and exploring of Br+nsted acidic TSILS are currently in the preliminary stage and the number of published Br+nsted acidic TILS is limited. Herein, the authors report the synthesis of a novel Br+nsted acidic ionic liquid based on a cyclic guanidinium cation (Scheme 1 ) , and its use as a d u d solvent-catalyst in the Fisher esterification reaction.

c1

Scheme 1 Synthesis of the novel BrQlstea aadic ionic liquid Tetrafhoroboric acid(40% solution in water) and 1,31 GeneralRemarks dimethylimidazolidin-2-one were purchased from Acros CO. and used without further purification. The esterifAll the acids and alcohols were obtained from the cation reactions were carried out in a round-bottom Beijing Chemical Company and were used as received.

Experimental

*

To whom correspondence should be addressed. E-mail: linyj@jlu. edu. cn

666

%I. 23

CHEM. RES. CHINESE U.

flask fitted with a reflux condenser. The acids and alcohols were added to the ionic liquid. The reaction proceeded for a period of time that ranged from 2 to 10 h with vigorous stirring at 25-120 C. The reaction was monitored by GC-MS( Agilent GC: 6890 MS: 5973N). After the reaction was completed, the ester could be simply decanted out from the ionic liquid. The ionic liquid was reused after removal of water under vacuum( 1.33 Pa) at 100 “c. ‘H and l3 C NMR spectra were recorded on a Brucker 300 instrument. 2 Synthesis of Ionic Liquid 2. I Preparation of Vihneier Salt 1 To a solution of 1 , 3-dimethylimidazolidin-2-one (33.5 g, 0.294 mol) in 50 mL of toluene, oxalyl chloride ( 50.53 g , 0.330 mol) was added dropwise, within 20 min, at room temperature, in nitrogen atmosphere. The reaction mixture was stirred at 60 “c for 5 h. The resulting precipitate was filtrated, washed with toluene, and dried in vacuum. Next it was recrystallized from the mixture of acetonitrile and ethyl acetate ( 1 : 5 , volume ratio ) to yield colorless prisms 1 (40.2 g, 81% yield), m. p. 95-97 T . ‘H NMR (300 MHz, CDCl,, “MS) , 6: 3.34( 9 , 6 H ) , 4.37 ( s , 4H). IR( KBr) , ij/cm-’ : 163. 2.2 Preparation of Gmnidine 2 To a stirred solution of vilsmeier salt 1 in dry dichloromethane, n-butylamine ( 52.56 g, 0.72 mol ) was added dropwise, at 0 C for 1 h. Then the reaction mixture was heated to 50 C and stirred at that temperature for 18 h. After cooling to mom temperature, 120 mL of 35% NaOH aqueous solution was added dropwise into the mixture carefully and the mixture was extracted with dichloromethane(50 mL x 4 ) . The organic phases were combined, washed with saturated aqueous solution of NaC1( 30 mL x 3 ) , and evaporated under vacuum to afford a brown liquid as the crude product. The resulting residue was distilled under vacuum to afford a colorless liquid 2 ( 29.41 g, 87% yield). H NMR (300 MHz, CDCl,, TMS) , 6: 3.34( t , 2 H ) , 3.14 ( 9 , 4 H ) , 2 . 7 9 ( s , 6 H ) , 1.48-1.58(m, 2H), 1.31-1.43(m, 2 H ) , 0.91(t, 3H). 2.3 Preparation of Gmnidininium Tetraflumoborate 3 Guanidine 2( 16.9 g , 0.10 mol) was placed in a three-necked flask with a magnetic stirrer and was cooled to 0 C . Then tetrafluoroboric acid (0. 10 mol, 40% solution in water) was slowly added to the above precooled solution. The mixture was maintained at that temperature and stirred for 3 h. The resulting colorless ionic liquid was dried in vacuum at 60 “c to remove water and used for esterification. NMR spectroscopic

data of the ionic liquid 3 : ‘H NMR( 300 MHz , CDC1, , TMS), 6: 0 . 8 6 ( t , 3 H ) , 1.23-1.35(m, 2H), 1.55-1.65( m, 2 H ) , 2.99( s, 9 H ) , 3 . 3 2 ( 2 H ) , 3.61(s, 4 H ) , 6.20(1H). 2 . 4 Typical Estenfiation Procedure Alcohol(0. 10 mol) , acid(0. 10 mol) , and 5 mL of IL 3 were added in a round-bottom flask with a reflux condenser. The reaction mixture was stirred at 80 “c and the reaction was monitored by GC-MS( Shimadzu GC-MS QP 5000). After the reaction was completed, the ester was simply decanted out from the IL 3, and the IL 3 was reused after removal of water under vacuum( 1.33 Pa) at 100 t for 1 h.

Results and Discussion The synthesis of IL 3 included three steps. The vilsmerier salt 1 was prepared in a high yield with high purity, according to the procedure described in the literature [ 191 , from the reaction of 1,3-dimethylimidazolidin-2-one with oxayl chloride. When the reaction of the vilsmerier salt 1 with n-butylamine proceeded to completion, pentaalkylguanidine 2 was obtained after simple distillation in vacuum. The novel Br4nsted IL 3, based on the guanidinium cation, was conveniently obtained by simply mixing pentaalkylguanidine 2 with tetrafluoroboric acid( 40% aqueous solution) at 0 C . The fresh guanidinium ionic liquid 3 is a pale yellow viscous liquid, which is miscible in water, methanol, and dichloromethane. IL 3 is partially immiscible in esters, alkanes, and aromatic hydrocarbons. On account of these properties, IL 3 can form biphasic systems with these solvents. The treatment of IL 3 under vacuum at 120 T for 24-75 h results in no loss of mass and proves that IL 3 is stable at a high temperature, unlike strong acids dissolved in conventional IL, which frequently continue to emit noxious vapors and harm the environment. Esterification is one type of classical acid-promoted organic reaction and is usually used to measure catalytic activity of an acid. In this study, the authors chose the esterifcation of caproic acid with 1-hexanol as the model reaction( Scheme 2) to observe the effects of reaction time, reaction temperature, and the amount of IL on the esterification. The esterification was carried out with IL 3 as the dual solvent-catalyst system, and the results are listed in Table 1. CH, ( CH,),COOH

+ CH, ( CH,),CH,OH

CH, (CH,),COOCH,( CH,),CH,

IL 3

Scheme 2 EsteMcation of caproic acid with l-hemanol in IL 3

GUO Xu et d.

No. 6

Table 1 Results of e~terificatio~ of caproic acid with 1-hexanolin IL 3 under dinerent conditions Reaction V( IL 3)/ Conversion

Reaction Enhytemperature/gC 1 r. t.

50 80

2 3 4

100

5 6 7 8 9 10

120 80 80 80 80 80 80 80 80 80 80

11 12 13 14 15

*

,

timdh 2 2 2 2 ' 2 1 1.5 2 10 24 2 2 2 2 2

mL

5 5 5 5 5 5 5 5 5 5 1 3 5 10 15

Selectivity

(96 1 63 75 98 98 98 73 82 98 98 98 72 , 78 98 98 98

Conversion and selectivity was determined by GC-MS.

(% 1

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

667

decreasing the reaction temperature and reaction time resulted in a lower yield( Entries 1 , 6 and 7). Increasing the amount of IL 3 had no impact on the conversion and selectivity( Entries 12, 13 and 15) , however, reducing the amount of IL 3 resulted in a lower conversion rate( Entries 11, 12 and 13). By comparison, the authon chose 5 mL of IL 3 and 80 't as the optimum reaction conditions. To investigate the scope and limitation of IL 3 as a dual solvent-catalyst for esterification , other acids and alcohols( Scheme 3 ) as substrates were also tested and the results are summarized in Table 2. The results show that the esterifications of the long alkyl chains aliphatic acids and primary alcohols were very satisfactory compared with those of the short alkyl chain acids and primary alcohols ( Entries 1-17 ) It is possible that the better immiscibility of the resulting esters from the long alkyl chains acids and primary alcohols facilitates the shift of the esterification equilibrium to the product's side. Esterifications of aromatic acids were more difficult compared to those of aliphatic acids. However, satisfactory results could also be obtained( Entry 1 8 ) . In the reaction of oxalic acid, a lower selectivity for the diester was observed because the monoesterifcation product was detected in the esterification reaction( Entries 1 9 , 20).

.

As seen from the results of Table 1 , IL 3 is of high catalytic activity for the esterifcation. High conversion rate and selectivity are obtained in all the cases. No byproducts such as olefins were detected. Because IL 3 is partially immiscible with esters, the esterification proceeded smoothly to completion even without simultaneous removal of the produced water, even though esterification is a reversible reaction. A maximum 98% conversion and 100% selectivity were observed with 5 mL of IL 3 as dual solvent-catalyst, in IL 3 R'COOH + ROH R'COOR a period of 2 h , at 80 't ( Entry 8 ) . Increasing the reaction temperature and reaction time did not influScheme 3 Esterification of acids with alcohols in IL 3 ence the yield of ester( Entries 5 and 14) , nevertheless Table 2 Results of esterifications of acids with alcohols in IL 3 at 80 SC for 2 h Entry 1 2b 3 4 5' 6 7 8' 9' 10 11 12 13' 14' 15 16 17 18 19b.e 20'

R'COOH HCOOH CH, COOH CH, COOH CH, COOH CH, ( CH, ) COOH CH,( CH,),COOH CH, ( CH, ) COOH CH, ( CH, ) ,COOH CH,( CH,),COOH CH, ( CH, ) ,COOH CH, (CH,),COOH CH, ( CH, ) ,COOH CH, (CH,) ,,COOH CH, (CH,) 1,COOH CH, ( CH, ) ,,COOH CH, ( CH, ) 16 COOH CH, ( CH, ) COOH PhCOOH (COOH), (COOH),

,

,

ROH CH, ( CH, ) ,CH,OH CH,CH, OH CH3 ( CH, ) 2 CHZ OH CH3( CHZ),CH,OH CH, CH, OH CH, (CH,),CH,OH CH, ( CH, ) CH, OH CH,OH CH, CH, OH CH, ( CH, ) CH,OH CH, ( CH, ) 4 CH, OH CH, ( CHz 1I ~ C H Z O H CH, OH CH, CH, OH CH, ( CH, ) CH, OH CH3( CH,),CH,OH CH3 ( CHz ) 18 CH, (CH,),CH,OH CH, CH, OH CH3 (CH,),CH,OH

,

,

,

Conversion" ( % )

Selectivity"( 96 )

70 72 80 95 78 94 95 80 94 95 98 98 93 94 94 95 98 75 83 87

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 85 88

a. Conversion and selectivity were determined by GC-MS; b. at reflwr temperature; c. volume ratio of acohol to acid is 2: 1 .

CHEM. RES. CHINESE U.

668

VOl. 23

caproic acid and 1-hexanol were chosen as the subThe water produced in the esterification reactions strates. After the reaction, with the pmcudced ester did not need to be removed because IL 3 was miscible decanted, IL 3 was separated and reused after drying with water, whereas, the esters were not. Hence the in vacuum. It was shown that IL 3 remained with high esterification could proceed smoothy to completion. catalytic activity after at least five NIIS, without any Once the reaction was completed, liquid esters could loss of catalytic activity, and the results are given in be separated from IL 3 by simple decantation. Table 3. Ultimately, the reuse of IL 3 was also studied, Table 3 Reeyelins of IL 3 in the synthesis of 1-hexylcaproicate Run

Reaction t e m p t u d c

Reaction timdh

V( IL) /mL

80 80 80 80 80

Conversion ( % )

selectivity ( % )

98 97 98 96

100

%

100

100

100 100

~~

*

Conversion and selectivity were determined by CC-MS.

Conclusions A novel Br+nsted acidic IL 3 based on the cyclic guanidinium cation has been synthesized. IL 3, as a strong Br+nsted acid catalyst and solvent shows a high catalytic activity and biphsaic behavior in the esterifications of c&boxylic acids and alcohols. The produced esters, as a separate phase, can be conveniently decanted out from IL 3, and IL 3 is recyclable without any loss of activity. Further application of IL 3 is underway in the laboratory.

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