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Supramolecular catalysis of esterification by hemicucurbiturils under mild conditions
Journal of Molecular Catalysis A: Chemical 365 (2012) 181–185
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Supramolecular catalysis of esterification by hemicucurbiturils under mild conditions Hang Cong a,b , Takehiko Yamato a,∗ , Xing Feng a , Zhu Tao b a b
Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi 1, Saga-shi, Saga 840-8502, Japan Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Guizhou University, Guiyang 550025, PR China
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 24 August 2012 Accepted 2 September 2012 Available online 7 September 2012 Keywords: Cucurbituril Supramolecular catalysis Esterification Macrocyclic compound Kinetics
a b s t r a c t Hemicucurbit[6]uril (HemiQ[6])-induced esterification of acids with CH3 OH was investigated. Esterification of the model substrate MA in the presence of different amounts of HemiQ[6] had reaction rate constants of k0.5 = 0.18 h−1 , k1.0 = 0.36 h−1 and k2.0 = 0.52 h−1 . These results confirm that the reaction rate increases with the ratio of catalyst to substrate. Ineffective catalysis of MA esterification with a stoichiometric amount of MeOH suggests that the mechanism for HemiQ[6]-catalyzed esterification is solvolysis. Comparing the HemiQ[6]-catalytic kinetics of MA (4-methoxy-4-oxobut-2-enoic acid) with AA (acrylic acid) and BA (benzoic acid) shows that the catalytic activities should bear relation to the size of substrates. The different conversion of sorts of substrates in the presence of HemiQ[6] reveals that the supramolecular catalysis is favor in the conjugated structures. The inefficacy of HemiQ[12] demonstrates that the catalytic capability depends on the structure of the macrocyclic compound used. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Cucurbituril family (cucurbit[n]urils, Q[n], n = 5–8 or 10) [1–3] members are important synthetic hosts in the field of supramolecular chemistry and afford facilities for capture of substrates within the nanospace cavity [4], as in the supramolecular capsules of classic macrocyclic compounds such as cyclodextrin [5] and calixarene [6]. Cucurbituril chemistry started with the identification of cucurbit[6]uril in the 1980s [7] and other members of this family in 2000 [8], but it is well known that only Q[7] has acceptable solubility in aqueous solution and none of the family members can dissolve in any organic solvent. This disadvantage prompted the modification of cucurbiturils to improve their solubility in water [9–11] and led to the identification of hemicucurbiturils (Scheme 1, HemiQ[n], n = 6 or 12) [12], which can be dissolved in CHCl3 and methanol. Thus, cucurbituril applications can now be investigated in common organic solvents. The unique properties of host–guest complexes prompted investigation of potential applications of cucurbiturils. The first study reported was supramolecular catalysis of [3 + 2] cycloaddition reaction in pioneering work by Mock [13]. Q[6]-catalytic 1,3-dipole addition has contributed to the synthesis of a few pseudo-polyrotaxanes with a Q[6] block [14]. Photochemical dimerization of olefins and coumarins via stereoselective catalysis
by Q[8] has also been developed [15,16]. Other organic reactions catalyzed via encapsulation include Knoevenagel condensation of benzaldehyde with diethyl malonate in ionic liquids [17] and IBX oxidation of alcohols in aqueous solution [18,19]. Carbocations stabilized by a dipole interaction with the carbonyl groups at the edge of cucurbiturils [20] have been successfully utilized for acceleration of oxime hydrolysis [21] and solvolysis of benzoyl chlorides [22] in the presence of Q[7]. These results prompted us to investigate if reactions including carbocation intermediates could be catalyzed by HemiQs in organic solvent, such as esterification between acids and alcohols (Scheme 1). Esterification is a classic and fundamental reaction in organic chemistry and biological systems and is catalyzed by acids [23], metal coordination complexes [24] or lipases [25]. Esterification is particularly relevant to biodiesel synthesis in efforts to overcome the scarcity of traditional fossil energy resources and to mitigate greenhouse gas emissions [26]. The reaction is also used in the synthesis of various intermediates and final products for different purposes [27,28]. To gain an understanding of hemicucurbiturilinduced esterification, sorts of acids were used as substrates including aryl acids, allyl acids and alkyl acids (Scheme 1). 2. Experimental 2.1. Materials and apparatus
∗ Corresponding author. Tel.: +81 952288679; fax: +81 952288548. E-mail addresses: [email protected] (T. Yamato), [email protected] (Z. Tao). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2012.09.002
HemiQ[n] (n = 6 or 12) samples were prepared and purified according to a method in the literature [12] and were
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H. Cong et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 181–185
Scheme 1. Structures of the acids and hemicucurbit[n]uril, n = 6 or 12.
characterized by 1 H NMR. HemiQ[6] (CDCl3 , ı): 3.40 (s, 24H), 4.67 ppm (s, 12H); HemiQ[12] (CDCl3 , ı): 3.36 (s, 24H), 4.67 ppm (s, 12H). The acids were obtained commercially (Tokyo Kasei Kogyo Co., Ltd.) and used without further purification. 1 H NMR spectra were recorded at 25 ◦ C on a JEOL JNM-Al00 spectrometer in a mixture of CDCl3 and CD3 OD. High performance liquid chromatography (HPLC) was performed using a JASCO with a UV detector. The acids and corresponding esters were successfully separated by a Wakosil 5SIL chromatographic column (250 mm × 4.6 mm). 2.2. Catalytic esterification experiments In the cases of MA, AA, BA, PA (propanoic acid) and ACA (1adamantanecarboxylic acid), acids (0.01 mmol) were added to a mixture of CDCl3 and CD3 OD (1:1, 0.6 ml) and the solution was transferred into an NMR tube. HemiQ[n] (n = 6 or 12) was added to the acid solution at a corresponding ratio. The NMR tube was directly heated to 60 ◦ C and monitored by 1 H NMR over time. Reactant conversion was directly confirmed by 1 H NMR spectral data. For the other substrates, acids (0.01 mmol) were added to a mixture of CHCl3 and CH3 OH (1:1, 1.0 ml) and HemiQ[6] was added to the solution in a ratio of 1:1. The solution was heated to 60 ◦ C, and then it was diluted with ethyl acetate to an appropriate concentration for the measure by HPLC. 3. Results and discussion MA is the direct product of the alcoholysis reaction of maleic anhydride in methanol, but second-order alcoholysis cannot occur spontaneously and requires a catalyst such as another organic acid. In the 1 H NMR spectrum of MA (Fig. 1b), the resonances at ı 6.29 and 6.30 ppm can be attributed to protons on the olefin of MA, and the proton resonance of the OCH3 group appears at ı 3.79 ppm. In general, there are obvious differences in chemical shift patterns between Qs-bound and free guests due to the shielding effect of the macrocyclic cavity and the deshielding effect of the carbonyl
groups on the portals. However, we observed no change in chemical shift in 1 H NMR spectra of a mixture of MA and HemiQ[6] (Fig. 1c) compared to spectra of the free guest (Fig. 1b) and the macrocyclic compound (Fig. 1a). Fig. 1d reveals broadening of the resonance signals for HemiQ[6] after the mixture was heated. This can be explained as follows. The flexible structure of the host is frozen and includes various confirmations of the 2-imidzolidinone units and methylene bridges. The host–guest interaction is also confirmed by the prominent change of IR absorption of the carbonyl groups on binding HemiQ[6] (Supporting Information, Fig. S5). MA esterification occurred spontaneously, with conversion of 34.7%, and complete conversion was achieved by heating for ∼10 h. The macrocyclic compound unit was also applied for the screening of the catalysis, but MA cannot be esterified in the presence of 2imidazolidinone in a ratio of 1:6. It could be speculated from the result that the cavity of HemiQ[6] is the catalytic residence of the esterification. Kinetic plots of the catalytic esterification of MA in the presence of different amounts of HemiQ[6] are shown in Fig. 2. As expected, the esterification rate increased with the amount of HemiQ[6]. The kinetics of HemiQ[6]-catalyzed esterification can be described by a pseudo-first-order formula [26]: Ct = C0 × [1 − exp(−kn × t)],
(1)
where C0 is the initial MA concentration and Ct is the MA concentration at time t, k is the corresponding reaction rate constant, and the subscript n represents the ratio of MA to HemiQ[6]. The kinetic constants calculated suggest that supramolecular catalysis greatly depends on the ratio of MA to HemiQ[6]. Addition of 0.005 mmol of catalyst led to esterification with a reaction rate constant of 0.18 h−1 (k0.5 ). Addition of double this amount to yield a HemiQ[6]/MA ratio of 1:1 led to a constant of k1.0 = 0.36 h−1 , which is double the value for k0.5 . A further increase in the amount of HemiQ[6] in the reaction system improved the percentage conversion of the substrate and a kinetic constant of k2.0 = 0.52 h−1 was obtained, with a curve fitting observed for HemiQ[6]-catalyzed esterification kinetics (Fig. 2).
H. Cong et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 181–185
183
Table 1 HemiQ[6]-catalytic esterification of acids with methanol. Time (h)
Conversiona (%)
1
12
84.0
2
12
90.7
3
24
13.1
4
24
58.8
5
12
86.9
6
18
70.8
7
24
–
8
24
–
Entries
Fig. 1. 1 H NMR spectra of the esterification of MA in the presence of HemiQ[6]. Spectra of (a) HemiQ[6], (b) MA, (c) a mixture of MA and HemiQ[6] in a ratio of 1:1 in CDCl3 /CD3 OD (1:1) and (d) after the mixture was heated at 60 ◦ C. The conversion was confirmed by comparing the integrated proton resonances for the olefin in MA (ı 6.29, 6.30 ppm) with those for the product, dimethyl maleate (ı 6.31 ppm).
Substrates
For the smaller substrate acrylic acid (AA), the HemiQ[6]catalytic esterification with CD3 OD is also observed. There is no any change of the proton chemical shift of AA in the presence of HemiQ[6] in a 1:1 ratio to be observed, and only the broadened resonance peak of HemiQ[6] (Fig. 3) represents the host–guest interaction as same as the case of MA. The conversion of AA at the
a The conversion of entries 1–6 was measured by HPLC, and the esterification of entries 7 and 8 were traced by 1 H NMR.
Fig. 2. Kinetic plots of MA esterification catalyzed by different ratios of HemiQ[6].
different time could be non-linear fitted to give a kinetic constant of kAA = 1.22 h−1 with the formula (1) (Fig. 3, insert), which is almost 4 folds of the k1.0 in the MA system, so the HemiQ[6]-mediated esterification of AA is obviously faster than the MA. BA was also used to investigate the catalytic activity of HemiQ[6] in the esterification system. 1 H NMR spectra for BA (Fig. 4a) and a mixture of BA and HemiQ[6] (Fig. 4b) reveals no obvious changes in resonance signal for protons on the aryl ring. However, heating of the BA and HemiQ[6] solution induced broadening of the resonance peaks (Fig. 4c), reflecting interaction between BA and HemiQ[6]. 1 H NMR monitoring of conversion revealed that supramolecular catalysis was still effective for BA esterification at a HemiQ[6]/BA ratio of 1:1, but the reaction was much slower than for HemiQ[6]catalyzed MA esterification. The conversion of only 45.2% after 5 h is less than two-thirds of that in the MA system. Over the next 5 h, conversion improved by ∼13%, but the further 4% improvement
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Fig. 3. 1 H NMR spectra of the esterification of AA in the presence of HemiQ[6]. Spectra of (a) AA, (b) AA and HemiQ[6] in a ratio of 1:1 in CDCl3 /CD3 OD (1:1) and (c) after the mixture was heated at 60 ◦ C. The conversion was confirmed by comparing the integrated proton resonances for the acrylic group in AA (ı 6.43 ppm) with those for the product, methyl benzoate (ı 6.41 ppm). Insert: kinetic plots of AA esterification in the presence of HemiQ[6] in the ratio of 1:1.
in conversion at 15 h indicates that esterification was close to the point of chemical equilibrium. The above kinetic plots have been collected in the insert of Fig. 4 to be curve fitted with the kinetic constant kBA = 0.19 h−1 , which is almost half of that in the esterification of MA and the one tenth of kAA . These results demonstrate that effective esterification largely depends on the substrate structure. The aryl ring of BA is larger than the substituent of MA and AA, and therefore it make against the catalytic esterification by this macrocyclic compound. However, the esterification of the smallest substrate AA can be preformed more easily than the others. Further studies for the purpose of the universality and selectivity of this supramolecular catalysis have been carried out in the
presence of equivalent HemiQ[6] in the mixture of CHCl3 and CH3 OH. As shown in Table 1, the results suggest that the catalytic activities should prefer the conjugated structure. Two alkyl acids, PA and ACA, were subjected to the same catalytic esterification, but no reaction occurred. These negative results confirm that catalysis depends on the substrate structure, and is favored for aryl and allyl substrates. However, there are some different catalytic activities of HemiQ[6] on the esterification of substituted benzoic acid, which tends to prove that the electronic effect and steric effect of the substrates seem to exist for these aryl acid systems. Almost all of the aryl acids (Entries 1, 2, and 4–6) can be converted to the corresponding esters with very satisfying conversion in the presence of HemiQ[6], but only 13.1% conversion in the case of SA (Entry 3)
Fig. 4. 1 H NMR spectra of the esterification of BA in the presence of HemiQ[6]. Spectra of (a) BA, (b) BA and HemiQ[6] in a ratio of 1:1 in CDCl3 /CD3 OD (1:1) and (c) after the mixture was heated at 60 ◦ C. The conversion was confirmed by comparing the integrated proton resonances for the benzyl group in BA (ı 7.45 ppm) with those for the product, methyl benzoate (ı 7.38 ppm). Insert: kinetic plots of BA esterification in the presence of HemiQ[6] in the ratio of 1:1.
H. Cong et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 181–185
can be found. The result indicates that the inductive effect of oOH group should be disadvantage of the esterification, because it is not able to stabilize the carbon cation intermediate in the course of the formation of ester. On the other hand, the conversions of both o-ABA (o-aminobenzoic acid) (Entry 1) and SA (salicylic acid) (Entry 3) are less than that of the p-substituted substrates, p-ABA (paminobenzoic acid) (Entry 2) and p-HBA (p-hydroxybenzoic acid) (Entry 4), which denotes the existence of the steric effect in this supramolecular catalysis. For a random selection, four substrates, MA, BA, ACA and PA, were subjected to esterification with a stoichiometric amount of MeOH in the presence of HemiQ[6] in CDCl3 , but no ester product was synthesized in more than 24 h. These results indicate that the catalytic activity of HemiQ[6] greatly depends on the substrate concentration and suggest that the mechanism for esterification is a solvolytic reaction between the acids and the CH3 OH solvent. To understand the dependence of supramolecular catalysis on the structure of the macrocyclic catalyst, HemiQ[12] was used as an alternative to catalyze acid esterification in CDCl3 /CD3 OD (1:1) solution. Only resonance broadening was observed for HemiQ[12] singlets in 1 H NMR spectra of a heated mixture of BA and HemiQ[12] (Supporting Information, Fig. S4), and supramolecular catalysis was ineffective for esterification of the four acids. Thus, a more flexible structure and a larger cavity, as in HemiQ[12], are negative factors in this catalytic esterification.
International Collaboration Project of Guizhou Province (Grant No. [2011]7003), and the Natural Science Foundation of Guizhou Province (Grant No. [2008]75). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata. 2012.09.002. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
4. Conclusion We developed a method for effective supramolecular hemicucurbit[6]uril catalysis of esterification of carboxylic compounds. HemiQ[6]-induced esterification depends on the amount of HemiQ[6], and a greater amount of catalyst should increase the reaction rate. The electronic and steric structures of the substrates affect the supramolecular catalysis; only the conjugated acids could be catalyzed using this method. In the screening of the macrocyclic compound, HemiQ[12] was ineffective in catalyzing the esterification, so the structure of the catalyst should be a crucial factor. The reaction results for the cases of four organic acids selected randomly with a stoichiometric amount of MeOH suggest that the mechanism for esterification is alcoholysis in CH3 OH solvent. Acknowledgements We acknowledge the support of the National Natural Science Foundation of China (Grant Nos. 20972034 and 21162003), the
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Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Supramolecular catalysis of esterification by hemicucurbiturils under mild conditions Hang Cong a,b , Takehiko Yamato a,∗ , Xing Feng a , Zhu Tao b a b
Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi 1, Saga-shi, Saga 840-8502, Japan Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Guizhou University, Guiyang 550025, PR China
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 24 August 2012 Accepted 2 September 2012 Available online 7 September 2012 Keywords: Cucurbituril Supramolecular catalysis Esterification Macrocyclic compound Kinetics
a b s t r a c t Hemicucurbit[6]uril (HemiQ[6])-induced esterification of acids with CH3 OH was investigated. Esterification of the model substrate MA in the presence of different amounts of HemiQ[6] had reaction rate constants of k0.5 = 0.18 h−1 , k1.0 = 0.36 h−1 and k2.0 = 0.52 h−1 . These results confirm that the reaction rate increases with the ratio of catalyst to substrate. Ineffective catalysis of MA esterification with a stoichiometric amount of MeOH suggests that the mechanism for HemiQ[6]-catalyzed esterification is solvolysis. Comparing the HemiQ[6]-catalytic kinetics of MA (4-methoxy-4-oxobut-2-enoic acid) with AA (acrylic acid) and BA (benzoic acid) shows that the catalytic activities should bear relation to the size of substrates. The different conversion of sorts of substrates in the presence of HemiQ[6] reveals that the supramolecular catalysis is favor in the conjugated structures. The inefficacy of HemiQ[12] demonstrates that the catalytic capability depends on the structure of the macrocyclic compound used. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Cucurbituril family (cucurbit[n]urils, Q[n], n = 5–8 or 10) [1–3] members are important synthetic hosts in the field of supramolecular chemistry and afford facilities for capture of substrates within the nanospace cavity [4], as in the supramolecular capsules of classic macrocyclic compounds such as cyclodextrin [5] and calixarene [6]. Cucurbituril chemistry started with the identification of cucurbit[6]uril in the 1980s [7] and other members of this family in 2000 [8], but it is well known that only Q[7] has acceptable solubility in aqueous solution and none of the family members can dissolve in any organic solvent. This disadvantage prompted the modification of cucurbiturils to improve their solubility in water [9–11] and led to the identification of hemicucurbiturils (Scheme 1, HemiQ[n], n = 6 or 12) [12], which can be dissolved in CHCl3 and methanol. Thus, cucurbituril applications can now be investigated in common organic solvents. The unique properties of host–guest complexes prompted investigation of potential applications of cucurbiturils. The first study reported was supramolecular catalysis of [3 + 2] cycloaddition reaction in pioneering work by Mock [13]. Q[6]-catalytic 1,3-dipole addition has contributed to the synthesis of a few pseudo-polyrotaxanes with a Q[6] block [14]. Photochemical dimerization of olefins and coumarins via stereoselective catalysis
by Q[8] has also been developed [15,16]. Other organic reactions catalyzed via encapsulation include Knoevenagel condensation of benzaldehyde with diethyl malonate in ionic liquids [17] and IBX oxidation of alcohols in aqueous solution [18,19]. Carbocations stabilized by a dipole interaction with the carbonyl groups at the edge of cucurbiturils [20] have been successfully utilized for acceleration of oxime hydrolysis [21] and solvolysis of benzoyl chlorides [22] in the presence of Q[7]. These results prompted us to investigate if reactions including carbocation intermediates could be catalyzed by HemiQs in organic solvent, such as esterification between acids and alcohols (Scheme 1). Esterification is a classic and fundamental reaction in organic chemistry and biological systems and is catalyzed by acids [23], metal coordination complexes [24] or lipases [25]. Esterification is particularly relevant to biodiesel synthesis in efforts to overcome the scarcity of traditional fossil energy resources and to mitigate greenhouse gas emissions [26]. The reaction is also used in the synthesis of various intermediates and final products for different purposes [27,28]. To gain an understanding of hemicucurbiturilinduced esterification, sorts of acids were used as substrates including aryl acids, allyl acids and alkyl acids (Scheme 1). 2. Experimental 2.1. Materials and apparatus
∗ Corresponding author. Tel.: +81 952288679; fax: +81 952288548. E-mail addresses: [email protected] (T. Yamato), [email protected] (Z. Tao). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2012.09.002
HemiQ[n] (n = 6 or 12) samples were prepared and purified according to a method in the literature [12] and were
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H. Cong et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 181–185
Scheme 1. Structures of the acids and hemicucurbit[n]uril, n = 6 or 12.
characterized by 1 H NMR. HemiQ[6] (CDCl3 , ı): 3.40 (s, 24H), 4.67 ppm (s, 12H); HemiQ[12] (CDCl3 , ı): 3.36 (s, 24H), 4.67 ppm (s, 12H). The acids were obtained commercially (Tokyo Kasei Kogyo Co., Ltd.) and used without further purification. 1 H NMR spectra were recorded at 25 ◦ C on a JEOL JNM-Al00 spectrometer in a mixture of CDCl3 and CD3 OD. High performance liquid chromatography (HPLC) was performed using a JASCO with a UV detector. The acids and corresponding esters were successfully separated by a Wakosil 5SIL chromatographic column (250 mm × 4.6 mm). 2.2. Catalytic esterification experiments In the cases of MA, AA, BA, PA (propanoic acid) and ACA (1adamantanecarboxylic acid), acids (0.01 mmol) were added to a mixture of CDCl3 and CD3 OD (1:1, 0.6 ml) and the solution was transferred into an NMR tube. HemiQ[n] (n = 6 or 12) was added to the acid solution at a corresponding ratio. The NMR tube was directly heated to 60 ◦ C and monitored by 1 H NMR over time. Reactant conversion was directly confirmed by 1 H NMR spectral data. For the other substrates, acids (0.01 mmol) were added to a mixture of CHCl3 and CH3 OH (1:1, 1.0 ml) and HemiQ[6] was added to the solution in a ratio of 1:1. The solution was heated to 60 ◦ C, and then it was diluted with ethyl acetate to an appropriate concentration for the measure by HPLC. 3. Results and discussion MA is the direct product of the alcoholysis reaction of maleic anhydride in methanol, but second-order alcoholysis cannot occur spontaneously and requires a catalyst such as another organic acid. In the 1 H NMR spectrum of MA (Fig. 1b), the resonances at ı 6.29 and 6.30 ppm can be attributed to protons on the olefin of MA, and the proton resonance of the OCH3 group appears at ı 3.79 ppm. In general, there are obvious differences in chemical shift patterns between Qs-bound and free guests due to the shielding effect of the macrocyclic cavity and the deshielding effect of the carbonyl
groups on the portals. However, we observed no change in chemical shift in 1 H NMR spectra of a mixture of MA and HemiQ[6] (Fig. 1c) compared to spectra of the free guest (Fig. 1b) and the macrocyclic compound (Fig. 1a). Fig. 1d reveals broadening of the resonance signals for HemiQ[6] after the mixture was heated. This can be explained as follows. The flexible structure of the host is frozen and includes various confirmations of the 2-imidzolidinone units and methylene bridges. The host–guest interaction is also confirmed by the prominent change of IR absorption of the carbonyl groups on binding HemiQ[6] (Supporting Information, Fig. S5). MA esterification occurred spontaneously, with conversion of 34.7%, and complete conversion was achieved by heating for ∼10 h. The macrocyclic compound unit was also applied for the screening of the catalysis, but MA cannot be esterified in the presence of 2imidazolidinone in a ratio of 1:6. It could be speculated from the result that the cavity of HemiQ[6] is the catalytic residence of the esterification. Kinetic plots of the catalytic esterification of MA in the presence of different amounts of HemiQ[6] are shown in Fig. 2. As expected, the esterification rate increased with the amount of HemiQ[6]. The kinetics of HemiQ[6]-catalyzed esterification can be described by a pseudo-first-order formula [26]: Ct = C0 × [1 − exp(−kn × t)],
(1)
where C0 is the initial MA concentration and Ct is the MA concentration at time t, k is the corresponding reaction rate constant, and the subscript n represents the ratio of MA to HemiQ[6]. The kinetic constants calculated suggest that supramolecular catalysis greatly depends on the ratio of MA to HemiQ[6]. Addition of 0.005 mmol of catalyst led to esterification with a reaction rate constant of 0.18 h−1 (k0.5 ). Addition of double this amount to yield a HemiQ[6]/MA ratio of 1:1 led to a constant of k1.0 = 0.36 h−1 , which is double the value for k0.5 . A further increase in the amount of HemiQ[6] in the reaction system improved the percentage conversion of the substrate and a kinetic constant of k2.0 = 0.52 h−1 was obtained, with a curve fitting observed for HemiQ[6]-catalyzed esterification kinetics (Fig. 2).
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Table 1 HemiQ[6]-catalytic esterification of acids with methanol. Time (h)
Conversiona (%)
1
12
84.0
2
12
90.7
3
24
13.1
4
24
58.8
5
12
86.9
6
18
70.8
7
24
–
8
24
–
Entries
Fig. 1. 1 H NMR spectra of the esterification of MA in the presence of HemiQ[6]. Spectra of (a) HemiQ[6], (b) MA, (c) a mixture of MA and HemiQ[6] in a ratio of 1:1 in CDCl3 /CD3 OD (1:1) and (d) after the mixture was heated at 60 ◦ C. The conversion was confirmed by comparing the integrated proton resonances for the olefin in MA (ı 6.29, 6.30 ppm) with those for the product, dimethyl maleate (ı 6.31 ppm).
Substrates
For the smaller substrate acrylic acid (AA), the HemiQ[6]catalytic esterification with CD3 OD is also observed. There is no any change of the proton chemical shift of AA in the presence of HemiQ[6] in a 1:1 ratio to be observed, and only the broadened resonance peak of HemiQ[6] (Fig. 3) represents the host–guest interaction as same as the case of MA. The conversion of AA at the
a The conversion of entries 1–6 was measured by HPLC, and the esterification of entries 7 and 8 were traced by 1 H NMR.
Fig. 2. Kinetic plots of MA esterification catalyzed by different ratios of HemiQ[6].
different time could be non-linear fitted to give a kinetic constant of kAA = 1.22 h−1 with the formula (1) (Fig. 3, insert), which is almost 4 folds of the k1.0 in the MA system, so the HemiQ[6]-mediated esterification of AA is obviously faster than the MA. BA was also used to investigate the catalytic activity of HemiQ[6] in the esterification system. 1 H NMR spectra for BA (Fig. 4a) and a mixture of BA and HemiQ[6] (Fig. 4b) reveals no obvious changes in resonance signal for protons on the aryl ring. However, heating of the BA and HemiQ[6] solution induced broadening of the resonance peaks (Fig. 4c), reflecting interaction between BA and HemiQ[6]. 1 H NMR monitoring of conversion revealed that supramolecular catalysis was still effective for BA esterification at a HemiQ[6]/BA ratio of 1:1, but the reaction was much slower than for HemiQ[6]catalyzed MA esterification. The conversion of only 45.2% after 5 h is less than two-thirds of that in the MA system. Over the next 5 h, conversion improved by ∼13%, but the further 4% improvement
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Fig. 3. 1 H NMR spectra of the esterification of AA in the presence of HemiQ[6]. Spectra of (a) AA, (b) AA and HemiQ[6] in a ratio of 1:1 in CDCl3 /CD3 OD (1:1) and (c) after the mixture was heated at 60 ◦ C. The conversion was confirmed by comparing the integrated proton resonances for the acrylic group in AA (ı 6.43 ppm) with those for the product, methyl benzoate (ı 6.41 ppm). Insert: kinetic plots of AA esterification in the presence of HemiQ[6] in the ratio of 1:1.
in conversion at 15 h indicates that esterification was close to the point of chemical equilibrium. The above kinetic plots have been collected in the insert of Fig. 4 to be curve fitted with the kinetic constant kBA = 0.19 h−1 , which is almost half of that in the esterification of MA and the one tenth of kAA . These results demonstrate that effective esterification largely depends on the substrate structure. The aryl ring of BA is larger than the substituent of MA and AA, and therefore it make against the catalytic esterification by this macrocyclic compound. However, the esterification of the smallest substrate AA can be preformed more easily than the others. Further studies for the purpose of the universality and selectivity of this supramolecular catalysis have been carried out in the
presence of equivalent HemiQ[6] in the mixture of CHCl3 and CH3 OH. As shown in Table 1, the results suggest that the catalytic activities should prefer the conjugated structure. Two alkyl acids, PA and ACA, were subjected to the same catalytic esterification, but no reaction occurred. These negative results confirm that catalysis depends on the substrate structure, and is favored for aryl and allyl substrates. However, there are some different catalytic activities of HemiQ[6] on the esterification of substituted benzoic acid, which tends to prove that the electronic effect and steric effect of the substrates seem to exist for these aryl acid systems. Almost all of the aryl acids (Entries 1, 2, and 4–6) can be converted to the corresponding esters with very satisfying conversion in the presence of HemiQ[6], but only 13.1% conversion in the case of SA (Entry 3)
Fig. 4. 1 H NMR spectra of the esterification of BA in the presence of HemiQ[6]. Spectra of (a) BA, (b) BA and HemiQ[6] in a ratio of 1:1 in CDCl3 /CD3 OD (1:1) and (c) after the mixture was heated at 60 ◦ C. The conversion was confirmed by comparing the integrated proton resonances for the benzyl group in BA (ı 7.45 ppm) with those for the product, methyl benzoate (ı 7.38 ppm). Insert: kinetic plots of BA esterification in the presence of HemiQ[6] in the ratio of 1:1.
H. Cong et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 181–185
can be found. The result indicates that the inductive effect of oOH group should be disadvantage of the esterification, because it is not able to stabilize the carbon cation intermediate in the course of the formation of ester. On the other hand, the conversions of both o-ABA (o-aminobenzoic acid) (Entry 1) and SA (salicylic acid) (Entry 3) are less than that of the p-substituted substrates, p-ABA (paminobenzoic acid) (Entry 2) and p-HBA (p-hydroxybenzoic acid) (Entry 4), which denotes the existence of the steric effect in this supramolecular catalysis. For a random selection, four substrates, MA, BA, ACA and PA, were subjected to esterification with a stoichiometric amount of MeOH in the presence of HemiQ[6] in CDCl3 , but no ester product was synthesized in more than 24 h. These results indicate that the catalytic activity of HemiQ[6] greatly depends on the substrate concentration and suggest that the mechanism for esterification is a solvolytic reaction between the acids and the CH3 OH solvent. To understand the dependence of supramolecular catalysis on the structure of the macrocyclic catalyst, HemiQ[12] was used as an alternative to catalyze acid esterification in CDCl3 /CD3 OD (1:1) solution. Only resonance broadening was observed for HemiQ[12] singlets in 1 H NMR spectra of a heated mixture of BA and HemiQ[12] (Supporting Information, Fig. S4), and supramolecular catalysis was ineffective for esterification of the four acids. Thus, a more flexible structure and a larger cavity, as in HemiQ[12], are negative factors in this catalytic esterification.
International Collaboration Project of Guizhou Province (Grant No. [2011]7003), and the Natural Science Foundation of Guizhou Province (Grant No. [2008]75). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata. 2012.09.002. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
4. Conclusion We developed a method for effective supramolecular hemicucurbit[6]uril catalysis of esterification of carboxylic compounds. HemiQ[6]-induced esterification depends on the amount of HemiQ[6], and a greater amount of catalyst should increase the reaction rate. The electronic and steric structures of the substrates affect the supramolecular catalysis; only the conjugated acids could be catalyzed using this method. In the screening of the macrocyclic compound, HemiQ[12] was ineffective in catalyzing the esterification, so the structure of the catalyst should be a crucial factor. The reaction results for the cases of four organic acids selected randomly with a stoichiometric amount of MeOH suggest that the mechanism for esterification is alcoholysis in CH3 OH solvent. Acknowledgements We acknowledge the support of the National Natural Science Foundation of China (Grant Nos. 20972034 and 21162003), the
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