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Brønsted Acid Conjugates Confined in Mesoporous Silica as Catalyst for the Asymmetric Aldol Reaction
CHINESE JOURNAL OF CATALYSIS Volume 30, Issue 7, July 2009 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2009, 30(7): 587–589.
SHORT COMMUNICATION
Chiral Diamine/Brønsted Acid Conjugates Confined in Mesoporous Silica as Catalyst for the Asymmetric Aldol Reaction LI Hua, LÜ Xiaobing* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
Abstract: Chiral primary-tertiary diamine/Brønsted acid conjugates were selectively immobilized on the inner surface of mesoporous silica and used as catalyst for the asymmetric aldol reaction of acetone with various aldehydes. The catalyst showed modest reactivity and enantioselectivity and can be reused 6 times without loss of activity and enantioselectivity. As compared with silica gel as the support, an increase in ee value of the reaction product was observed with the SBA-15-immobilized organic catalyst. This was believed to be due to the confinement effect of the nanopores. Key words: asymmetric catalysis; immobilization; confinement effect; bifunctional catalyst; aldol reactionǂ
The catalytic asymmetric aldol reaction is a highly versatile and powerful C–C bond forming reaction. It is widely used in constructing natural and non-natural products. Some bifunctional catalysts that mimic enzymes for the aldol reaction have attracted extensive research activities and development [1–4]. As compared with the well-explored enamine catalysis with secondary amines, Luo et al. [4] reported that a simple chiral primary-tertiary diamine in combination with a Brønsted acid can act as an efficient enamine-based primary amine catalyst. From a practical point of view, it would be desirable to have the catalyst immobilized so that its recovery, reuse, and product purification can be facilitated, especially when the catalyst is obtained by several synthetic steps [5–7]. However, activity and enantioselectivity are often worse when a homogeneous catalyst is heterogenized, which is due to the complex structure of the support, slow diffusion, and other reasons [8]. In the pores of mesoporous materials, the confinement effect may increase or decrease enantioselectivity depending on how the interaction changes the transition states of the chiral products [9]. Here, we report the synthesis of heterogeneous catalysts containing simple chiral primary-tertiary diamine/Brønsted
acid conjugates confined in pores by post-grafting chiral trans-N,N-dialkylated diaminocyclohexanes onto mesoporous materials. The bifunctional catalysts can effectively catalyze the asymmetric aldol reaction of acetone with various aldehydes. A confinement effect originating from the mesopores was observed in the SBA-15-immobilized catalyst system. The preparation of catalyst series 11 is shown in Scheme 1. X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/Max 3400 powder diffractometer using Cu KĮ radiation (40 kV and 30 mA). Nitrogen adsorption isotherms were measured at –196 oC on a Micromeritics ASAP 2000 system in the static measurement mode. Solid-state 13C and 29Si NMR spectra were obtained on a Varian INOVA-400 spectrometer. The XRD patterns of all the samples (not shown) showed peaks that can be assigned to the (100), (110), and (200) reflections of ordered SBA-15 structures. This indicates that the support materials remained intact during post-grafting. The N2 adsorption isotherm of 11d (not shown) was of type Č. The measurement of the N2 adsorption isotherm of 11d showed that the anchoring of the organic units on SBA-15 resulted in decreases in the average pore size from 9.25 to 7.56 nm, pore
Received date: 27 April 2009. *Corresponding author. LU Xiaobing. Tel: +86-411-39893864; Fax: +86-411-39893848; E-mail: [email protected] Foundation item: Supported by the Foundation for the Author of National Excellent Doctoral Dissertation of China (200759) and the National Science Found for Distinguished Young Scholars (20625414). Copyright © 2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved. DOI: 10.1016/S1872-2067(08)60116-3
LI Hua et al. / Chinese Journal of Catalysis, 2009, 30(7): 587–589
Scheme 1. Synthesis of heterogeneous catalysts 11.
volume from 0.96 to 0.79 ml/g, and BET surface area from 580 to 455 m2/g. These results suggest that the immobilization predominantly occurred on the inner surface of the mesoporous materials. The 13C NMR spectrum of 11d (not shown) exhibited a chemical shift at 0, which is characteristic of the carbon that is attached to the silicon atom. The chemical shifts of the carbons of the alkyl carbon and the cyclic CH were observed in the range of 20–24, whereas the carbons of NHCH2 exhibited chemical shifts in the range of 45–56 [10]. These results show that the organic functional groups were intact after immobilization. The 29Si NMR spectrum of 11d (not shown) exhibited chemical shifts at –70 to –60 characteristic of the silicon that is attached to the carbon atom, which indicates the incorporation of the organic functional groups [11]. These chiral immobilized catalysts were evaluated for the asymmetric aldol reaction of acetone with various aldehydes (Table 1). The N-dialkylated groups in the chiral primary-tertiary diamine/Brønsted acid conjugates had a strong influence on the rate and product enantioselectivity. A low yield of 45% and product ee value less than 5% were observed in the reaction of acetone and p-nitrobenzaldehyde with the use of 11a as catalyst. The long N-dialkylated group seemed to enhance the asymmetric induction and thereby significantly increase the enantioselectivity of the product (Table 1, entries 1–5). The highest enantioselectivity was observed with the system of 11d as the catalyst. These results further show that the appropriate steric hindrance in the chiral primary-tertiary diamine/Brønsted acid conjugate catalyst is crucial to the asymmetric induction in the aldol reaction.
For comparison, a heterogeneous catalyst (11f), namely, the chiral primary-tertiary diamine/Brønsted acid conjugates immobilized on silica gel, was synthesized and also used for this reaction under the same conditions. The yield and the enantioselectivity were clearly lower than those obtained with 11d as the catalyst (Table 1, entries 4 and 6). From the catalytic reTable 1 Asymmetric aldol reaction of acetone with various aldehydes
Entry
R
Catalyst
Time (h)
Yield (%)
eeb (%)
a
<5
1
NO2
11a
48
45
2
NO2
11b
48
56a
57
a
65
3
NO2
11c
48
61
4
NO2
11d
48
74a
78
5
NO2
11e
48
67a
70
6
NO2
11f
48
41a
70
7
NO2
48
92a
93
8
H
72
23c
77
c
78
N CF3COO H NH2
11d
9
F
11d
72
42
10
CF3
11d
72
40c
76
11
CN
11d
72
54c
76
Reaction conditions: aldehydes 0.25 mmol, acetone 5 ml, catalyst 10 mol%, 25 oC. aDetermined by HPLC. bDetermined by chiral HPLC. c
Determined by 1H NMR.
LI Hua et al. / Chinese Journal of Catalysis, 2009, 30(7): 587–589
Yield or ee (%)
100 80
(1)
60
(2)
CF3COO
N
H
O
40
MeO Si O O
20 0
HN
1
2
3
4
5
H HO
NO2
6
Recycle time Fig. 1. Plots of catalytic activity and product enantioselectivity versus recycle time of 11d. (1) Yield; (2) ee. Reaction conditions: p-nitrobenzaldehyde 0.25 mmol, acetone 5 ml, catalyst 10 mol%, 25 oC, 48 h.
sults, the enhancement of the product enantioselectivity for the reaction of acetone and p-nitrobenzaldehyde catalyzed by 11d was attributed to the confinement effect of the nanopores of SBA-15. Unfortunately, compared with the corresponding homogeneous catalyst (Table 1, entry 7), the catalytic performance of catalyst 11d decreased after immobilization. Catalyst 11d was shown to be effective in the asymmetric aldol reaction of acetone and various substituted benzaldehydes, and similar enantioselectivities of the corresponding products were achieved under the same conditions (Table 1, entries 8–11). It can be noted that the bifunctional heterogeneous catalyst 11d was easily recovered by simple filtration and was subjected to reuse 6 times without any loss in activity and product enantioselectivity (Fig. 1). Based on the concept of dual activation, a proposed transition state model of the aldol reaction catalyzed by the bifunctional catalyst is shown in Scheme. 2. In the aldol reaction, the primary amine group is responsible for enamine formation with acetone through hydrogen bonding. The protonated tertiary amine group can activate the carbonyl group of 4-nitrobenzaldehyde by hydrogen bonding [4,7,12]. The Re face of the activated aldehyde is attacked by the enamine, which results in a stable transition state. The structure of the acid site and the confinement effect of the nanopores determine that the Re face of the activated aldehyde is attacked more easily. Therefore, the chiral induction in the aldol reaction was critically influenced by the steric hindrance of acid site. The hydrogen bonding and steric hindrance of the base site can influence the rate of the catalytic reaction. In summary, we have prepared heterogeneous organocatalysts by covalent grafting of chiral primary-tertiary diamine/Brønsted acid conjugates on the inner surface of SBA-15 mesoporous silica whose outer surface was first modified by treating the parent material with PhSi(OMe)3. The synthesized catalysts were active in the asymmetric aldol reaction and gave modest yield and enantioselectivity. The aldol reaction cata-
Scheme. 2. Proposed transition state model of the aldol reaction catalyzed by bifunctional catalyst.
lyzed by the chiral primary-tertiary diamine/Brønsted acid conjugates immobilized on SBA-15 gave a higher ee value than that catalyzed by the same organic groups anchored on silica gel. The enhanced enantioselectivity for this reaction was attributed to the confinement effect of the nanopores of SBA-15. However, the activity and enantioselectivity of immobilized catalyst were lower than those of the corresponding homogeneous catalyst.
Acknowledgments The authors are grateful to acknowledge Prof. ZHANG Weiping (Dalian Institute of Chemical Physics, Chinese Academy of Sciences) for the help in characterization of the resulted heterogeneous catalysts.
References 1 Tang Z, Jiang F, Yu L T, Cui X, Gong L Z, Mi A Q, Jiang Y Z, Wu Y D. J Am Chem Soc, 2003, 125: 5262 2 Liu Y X, Sun Y N, Tan H H, Liu W, Tao J C. Tetrahedron: Asymmetry, 2007, 18: 2649 3 List B, Lerner R A, Barbas C F. J Am Chem Soc, 2000, 122: 2395 4 Luo S Z, Xu H, Li J Y, Zhang L, Cheng J P. J Am Chem Soc, 2007, 129: 3074 5 Luo S Z, Zheng X X, Chen J P. Chem Commun, 2008: 5719 6 Margelefsky E L, Zeidan R K, Davis M E. Chem Soc Rev, 2008, 37: 1118 7 Zhong L, Xiao J L, Li C. Chin J Catal, 2007, 28: 673 8 Li C, Zhang H D, Jiang D M, Yang Q H. Chem Commun, 2007: 547 9 Li C. Catal Rev, 2004, 46: 419 10 Jiang D M, Yang Q H, Wang H, Zhu G R, Yang J, Li C. J Catal, 2006, 239: 65 11 Haake M, Pines A, Reimmer J A, Seydoux R. J Am Chem Soc, 1997, 119: 11711 12 Huh S, Chen H T, Wiench J W, Pruski M, Lin V S Y. Angew Chem, Int Ed, 2005, 44: 1826
SHORT COMMUNICATION
Chiral Diamine/Brønsted Acid Conjugates Confined in Mesoporous Silica as Catalyst for the Asymmetric Aldol Reaction LI Hua, LÜ Xiaobing* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
Abstract: Chiral primary-tertiary diamine/Brønsted acid conjugates were selectively immobilized on the inner surface of mesoporous silica and used as catalyst for the asymmetric aldol reaction of acetone with various aldehydes. The catalyst showed modest reactivity and enantioselectivity and can be reused 6 times without loss of activity and enantioselectivity. As compared with silica gel as the support, an increase in ee value of the reaction product was observed with the SBA-15-immobilized organic catalyst. This was believed to be due to the confinement effect of the nanopores. Key words: asymmetric catalysis; immobilization; confinement effect; bifunctional catalyst; aldol reactionǂ
The catalytic asymmetric aldol reaction is a highly versatile and powerful C–C bond forming reaction. It is widely used in constructing natural and non-natural products. Some bifunctional catalysts that mimic enzymes for the aldol reaction have attracted extensive research activities and development [1–4]. As compared with the well-explored enamine catalysis with secondary amines, Luo et al. [4] reported that a simple chiral primary-tertiary diamine in combination with a Brønsted acid can act as an efficient enamine-based primary amine catalyst. From a practical point of view, it would be desirable to have the catalyst immobilized so that its recovery, reuse, and product purification can be facilitated, especially when the catalyst is obtained by several synthetic steps [5–7]. However, activity and enantioselectivity are often worse when a homogeneous catalyst is heterogenized, which is due to the complex structure of the support, slow diffusion, and other reasons [8]. In the pores of mesoporous materials, the confinement effect may increase or decrease enantioselectivity depending on how the interaction changes the transition states of the chiral products [9]. Here, we report the synthesis of heterogeneous catalysts containing simple chiral primary-tertiary diamine/Brønsted
acid conjugates confined in pores by post-grafting chiral trans-N,N-dialkylated diaminocyclohexanes onto mesoporous materials. The bifunctional catalysts can effectively catalyze the asymmetric aldol reaction of acetone with various aldehydes. A confinement effect originating from the mesopores was observed in the SBA-15-immobilized catalyst system. The preparation of catalyst series 11 is shown in Scheme 1. X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/Max 3400 powder diffractometer using Cu KĮ radiation (40 kV and 30 mA). Nitrogen adsorption isotherms were measured at –196 oC on a Micromeritics ASAP 2000 system in the static measurement mode. Solid-state 13C and 29Si NMR spectra were obtained on a Varian INOVA-400 spectrometer. The XRD patterns of all the samples (not shown) showed peaks that can be assigned to the (100), (110), and (200) reflections of ordered SBA-15 structures. This indicates that the support materials remained intact during post-grafting. The N2 adsorption isotherm of 11d (not shown) was of type Č. The measurement of the N2 adsorption isotherm of 11d showed that the anchoring of the organic units on SBA-15 resulted in decreases in the average pore size from 9.25 to 7.56 nm, pore
Received date: 27 April 2009. *Corresponding author. LU Xiaobing. Tel: +86-411-39893864; Fax: +86-411-39893848; E-mail: [email protected] Foundation item: Supported by the Foundation for the Author of National Excellent Doctoral Dissertation of China (200759) and the National Science Found for Distinguished Young Scholars (20625414). Copyright © 2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved. DOI: 10.1016/S1872-2067(08)60116-3
LI Hua et al. / Chinese Journal of Catalysis, 2009, 30(7): 587–589
Scheme 1. Synthesis of heterogeneous catalysts 11.
volume from 0.96 to 0.79 ml/g, and BET surface area from 580 to 455 m2/g. These results suggest that the immobilization predominantly occurred on the inner surface of the mesoporous materials. The 13C NMR spectrum of 11d (not shown) exhibited a chemical shift at 0, which is characteristic of the carbon that is attached to the silicon atom. The chemical shifts of the carbons of the alkyl carbon and the cyclic CH were observed in the range of 20–24, whereas the carbons of NHCH2 exhibited chemical shifts in the range of 45–56 [10]. These results show that the organic functional groups were intact after immobilization. The 29Si NMR spectrum of 11d (not shown) exhibited chemical shifts at –70 to –60 characteristic of the silicon that is attached to the carbon atom, which indicates the incorporation of the organic functional groups [11]. These chiral immobilized catalysts were evaluated for the asymmetric aldol reaction of acetone with various aldehydes (Table 1). The N-dialkylated groups in the chiral primary-tertiary diamine/Brønsted acid conjugates had a strong influence on the rate and product enantioselectivity. A low yield of 45% and product ee value less than 5% were observed in the reaction of acetone and p-nitrobenzaldehyde with the use of 11a as catalyst. The long N-dialkylated group seemed to enhance the asymmetric induction and thereby significantly increase the enantioselectivity of the product (Table 1, entries 1–5). The highest enantioselectivity was observed with the system of 11d as the catalyst. These results further show that the appropriate steric hindrance in the chiral primary-tertiary diamine/Brønsted acid conjugate catalyst is crucial to the asymmetric induction in the aldol reaction.
For comparison, a heterogeneous catalyst (11f), namely, the chiral primary-tertiary diamine/Brønsted acid conjugates immobilized on silica gel, was synthesized and also used for this reaction under the same conditions. The yield and the enantioselectivity were clearly lower than those obtained with 11d as the catalyst (Table 1, entries 4 and 6). From the catalytic reTable 1 Asymmetric aldol reaction of acetone with various aldehydes
Entry
R
Catalyst
Time (h)
Yield (%)
eeb (%)
a
<5
1
NO2
11a
48
45
2
NO2
11b
48
56a
57
a
65
3
NO2
11c
48
61
4
NO2
11d
48
74a
78
5
NO2
11e
48
67a
70
6
NO2
11f
48
41a
70
7
NO2
48
92a
93
8
H
72
23c
77
c
78
N CF3COO H NH2
11d
9
F
11d
72
42
10
CF3
11d
72
40c
76
11
CN
11d
72
54c
76
Reaction conditions: aldehydes 0.25 mmol, acetone 5 ml, catalyst 10 mol%, 25 oC. aDetermined by HPLC. bDetermined by chiral HPLC. c
Determined by 1H NMR.
LI Hua et al. / Chinese Journal of Catalysis, 2009, 30(7): 587–589
Yield or ee (%)
100 80
(1)
60
(2)
CF3COO
N
H
O
40
MeO Si O O
20 0
HN
1
2
3
4
5
H HO
NO2
6
Recycle time Fig. 1. Plots of catalytic activity and product enantioselectivity versus recycle time of 11d. (1) Yield; (2) ee. Reaction conditions: p-nitrobenzaldehyde 0.25 mmol, acetone 5 ml, catalyst 10 mol%, 25 oC, 48 h.
sults, the enhancement of the product enantioselectivity for the reaction of acetone and p-nitrobenzaldehyde catalyzed by 11d was attributed to the confinement effect of the nanopores of SBA-15. Unfortunately, compared with the corresponding homogeneous catalyst (Table 1, entry 7), the catalytic performance of catalyst 11d decreased after immobilization. Catalyst 11d was shown to be effective in the asymmetric aldol reaction of acetone and various substituted benzaldehydes, and similar enantioselectivities of the corresponding products were achieved under the same conditions (Table 1, entries 8–11). It can be noted that the bifunctional heterogeneous catalyst 11d was easily recovered by simple filtration and was subjected to reuse 6 times without any loss in activity and product enantioselectivity (Fig. 1). Based on the concept of dual activation, a proposed transition state model of the aldol reaction catalyzed by the bifunctional catalyst is shown in Scheme. 2. In the aldol reaction, the primary amine group is responsible for enamine formation with acetone through hydrogen bonding. The protonated tertiary amine group can activate the carbonyl group of 4-nitrobenzaldehyde by hydrogen bonding [4,7,12]. The Re face of the activated aldehyde is attacked by the enamine, which results in a stable transition state. The structure of the acid site and the confinement effect of the nanopores determine that the Re face of the activated aldehyde is attacked more easily. Therefore, the chiral induction in the aldol reaction was critically influenced by the steric hindrance of acid site. The hydrogen bonding and steric hindrance of the base site can influence the rate of the catalytic reaction. In summary, we have prepared heterogeneous organocatalysts by covalent grafting of chiral primary-tertiary diamine/Brønsted acid conjugates on the inner surface of SBA-15 mesoporous silica whose outer surface was first modified by treating the parent material with PhSi(OMe)3. The synthesized catalysts were active in the asymmetric aldol reaction and gave modest yield and enantioselectivity. The aldol reaction cata-
Scheme. 2. Proposed transition state model of the aldol reaction catalyzed by bifunctional catalyst.
lyzed by the chiral primary-tertiary diamine/Brønsted acid conjugates immobilized on SBA-15 gave a higher ee value than that catalyzed by the same organic groups anchored on silica gel. The enhanced enantioselectivity for this reaction was attributed to the confinement effect of the nanopores of SBA-15. However, the activity and enantioselectivity of immobilized catalyst were lower than those of the corresponding homogeneous catalyst.
Acknowledgments The authors are grateful to acknowledge Prof. ZHANG Weiping (Dalian Institute of Chemical Physics, Chinese Academy of Sciences) for the help in characterization of the resulted heterogeneous catalysts.
References 1 Tang Z, Jiang F, Yu L T, Cui X, Gong L Z, Mi A Q, Jiang Y Z, Wu Y D. J Am Chem Soc, 2003, 125: 5262 2 Liu Y X, Sun Y N, Tan H H, Liu W, Tao J C. Tetrahedron: Asymmetry, 2007, 18: 2649 3 List B, Lerner R A, Barbas C F. J Am Chem Soc, 2000, 122: 2395 4 Luo S Z, Xu H, Li J Y, Zhang L, Cheng J P. J Am Chem Soc, 2007, 129: 3074 5 Luo S Z, Zheng X X, Chen J P. Chem Commun, 2008: 5719 6 Margelefsky E L, Zeidan R K, Davis M E. Chem Soc Rev, 2008, 37: 1118 7 Zhong L, Xiao J L, Li C. Chin J Catal, 2007, 28: 673 8 Li C, Zhang H D, Jiang D M, Yang Q H. Chem Commun, 2007: 547 9 Li C. Catal Rev, 2004, 46: 419 10 Jiang D M, Yang Q H, Wang H, Zhu G R, Yang J, Li C. J Catal, 2006, 239: 65 11 Haake M, Pines A, Reimmer J A, Seydoux R. J Am Chem Soc, 1997, 119: 11711 12 Huh S, Chen H T, Wiench J W, Pruski M, Lin V S Y. Angew Chem, Int Ed, 2005, 44: 1826