# Preparation of Mesoporous SBA-16 Silica-Supported Biscinchona Alkaloid Ligand for the Asymmetric Dihydroxylation of Olefins

Authors: Shaheen M. Sarkar; Md. Eaqub Ali; Md. Lutfor Rahman; Mashitah Mohd Yusoff
Journal: Journal of Nanomaterials
 (2014)

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## Abstract

Optically active cinchona alkaloid was anchored onto mesoporous SBA-16 silica and the as-prepared complex was used as a heterogeneous chiral ligand of osmium tetraoxide for the asymmetric dihydroxylation of olefins. The prepared catalytic system provided 90–93% yield of vicinal diol with 92–99% enantioselectivity. The ordered mesoporous SBA-16 silica was found to be a valuable support for the cinchona alkaloid liganded osmium catalyst system which is frequently used in chemical industries and research laboratories for olefin functionalization.


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## Body

## 1. Introduction

The discovery of highly ordered mesoporous materials has opened up new fields of research in advanced chemistry, modern electronics, and nanotechnology [1–3]. Ordered mesoporous SBA-16 is a nanostructured porous material with a 3D cubic arrangement of mesopores that corresponds to theIm3m space group [4–9]. The surface properties of such materials could be significantly modified by adding organic groups and various functionalities onto them [10]. Our interest in the field led us to prepare SBA-16 silica-supported biscinchona alkaloid for osmium-catalyzed asymmetric dihydroxylation (AD) of olefins. Osmium-catalyzed asymmetric dihydroxylation of olefins is an attractive method for the synthesis of optically active diols [11–14]. Cinchona alkaloid-based osmium complexes are harmless and known to be the most effective chiral catalysts for AD reactions in terms of both reactivity and enantioselectivity [15–18].However, the high cost and toxicity of osmium are a serious concern and many efforts have been devoted to overcome the issue including the development of heterogeneous catalyst ligand to trap the osmium [19, 20]. Immobilization of homogeneous catalysts onto various supports has emerged as a major route to prepare heterogeneous catalysts [21, 22]. Such a heterogeneous catalyst system offers practical advantages in catalyst separation and potential recycling over its homogeneous counterpart [23, 24]. Silica gel such as mesoporous silica MCM-41 and SBA-15 has been successfully used as inorganic supports for the immobilization of homogeneous catalysts [25–27]. However, despite having highly ordered mesopores, SBA-16 silica has been scarcely explored in this area. Herein, we described the synthesis of SBA-16 supported cinchona alkaloid ligand and tested it for the osmium catalyzed AD reaction of olefins to diols, a key reaction in organic synthesis.


## 2. Experimental Details

### 2.1. Preparation of the SBA-16 Silica

SBA-16 silica was synthesized at room temperature under acidic condition using Pluronic F127 (EO106PO70EO106, M
w
=
12.6
K) as a structure-directing agent (SDA) [28]. The acidic solution was made by adding 1.5 g of deionized (DI) water to 120 g of 2 M HCl solution at room temperature. Subsequently, 8.5 g of tetraethoxysilane (TEOS) was added onto the solution and stirring continued for 20 h. The reaction mixture was kept at 100°C for 48 h. During this time the solid SBA-16 was produced under static conditions in a Teflon-lined vessel. The solid product was recovered and washed with DI water. Calcination was carried out slowly by increasing temperature from room temperature to 500°C in 8 h and heating at 500°C for another 6 h.


### 2.2. Preparation of the 1,4-Bis(9-O-quininyl)phthalazine 3

NaH (2.24 mmol) at 0°C was slowly added to a stirred solution of 1,4-dichlorophthalazine (0.5 mmol) and quinine4 (1.15 mmol) in THF (8 mL). The solution was stirred at 60°C for 2 h and then it was quenched at 0°C by careful addition of water. The mixture was extracted in ethyl acetate (EtOAc) and the solvent was removed under reduced pressure. The residue was purified by short column chromatography to separate 1,4-bis(9-O-quininyl)phthalazine3 and 82% yield was obtained.


### 2.3. Preparation of the Triethoxysilanized 1,4-Bis(9-O-quininyl)phthalazine 2

1,4-Bis(9-O-quininyl)phthalazine3 (0.5 mmol) was added to a solution of (3-mercapto-propyl)triethoxysilane (1.25 mmol) and α,α′-azoisobutyronitrile (AIBN) (0.10 mmol) in degassed chloroform (10 mL) under N2 atmosphere. The reaction mixture was refluxed for 30 h and concentrated under reduced pressure. The residue was purified by flash short column on silica gel to give compound2 with 80% yield.


### 2.4. Immobilization of Biscinchona Alkaloid2 onto SBA-16 Silica 1

SBA-16 silica (1.0 g) was suspended in toluene and refluxed with compound2 (145 mg, 0.124 mmol). After 12 h, the powder was collected by filtration and washed with methanol and methylene chloride. After drying under vacuum at 70°C, SBA-16-supported alkaloid1 (1.077 g) was obtained. Elemental analysis and weight gain showed that 0.073 mmol of 1,4-bis(9-O-quininyl)phthalazine was anchored on 1.0 g of SBA-16-supported chiral Ligand1.


### 2.5. Characterization of SBA-16-Supported Chiral Ligand 1

Powder X-ray diffractometry (Philips PW 1729) was used for the determination of crystalline structure using Cu Ka radiation over 0.5
°
≤
2
θ
≤
3. The XRD sample of SBA-16 was analyzed at 30°C. The diffractograms showed 3 peaks at 2
θ
≈
0.74
°, 1.1°, and 1.4° that corresponded to (110), (200), and (211) planes, respectively, in the cubicIm3m structure. The transmission electron microscopy (TEM) was performed with a FEI Tecnai G2 microscope operated at 200 kV. The TEM sample was prepared by placing a few drops of SBA-16 powder dispersed in acetone on a carbon grid and allowing it to dry for 5 min before TEM analysis. Large particles were crushed by submerging them in liquid nitrogen followed by mechanical grinding in a mortar prior to acetone dispersion. The nitrogen adsorption-desorption measurements were performed at −196°C on a Micromeritics ASAP 2020 surface area and porosity analyzer. Approximately 0.5 g of SBA-16 was degassed at 300°C for 9 h before taking the measurement. The surface area determination was performed by the Brunauer-Emmett-Teller (BET) method [29] over the relative pressure (P
/
P
0) range of 0.05–0.2. The pore-size distribution was determined using the Broekhoff-de Boer (BdB) method [30] applied to the adsorption branch. Finally, the total pore volume was calculated from the amount of adsorbed N2 at P
/
P
0
=
0.99, and the microporous volume was determined using the t-plot method.


### 2.6. Asymmetric Dihydroxylation of Olefin Using SBA-16-Supported Chiral Ligand 1

A mixture of SBA-16-supported biscinchona alkaloid1 (1 mol%), potassium ferricyanide (1.5 mmol), potassium carbonate (1.5 mmol), and OsO4 (1 mol%, 0.5 M in water) intert-butyl alcohol-water (6 mL, 1 : 1, v/v) was stirred at room temperature for 30 min. Olefin (0.5 mmol) was added at once and stirred for 7~15 h. The reaction mixture was diluted with water and CH2Cl2 and the immobilized Ligand1 was separated by filtration. The crude product was purified by flash column chromatography, and the enantiomeric excess of the diol was determined by chiral gas chromatography (GC) analysis (Agilent HP Chiral-20B 30MX0.25MMX0.25UM GC Column).


## 2.1. Preparation of the SBA-16 Silica

SBA-16 silica was synthesized at room temperature under acidic condition using Pluronic F127 (EO106PO70EO106, M
w
=
12.6
K) as a structure-directing agent (SDA) [28]. The acidic solution was made by adding 1.5 g of deionized (DI) water to 120 g of 2 M HCl solution at room temperature. Subsequently, 8.5 g of tetraethoxysilane (TEOS) was added onto the solution and stirring continued for 20 h. The reaction mixture was kept at 100°C for 48 h. During this time the solid SBA-16 was produced under static conditions in a Teflon-lined vessel. The solid product was recovered and washed with DI water. Calcination was carried out slowly by increasing temperature from room temperature to 500°C in 8 h and heating at 500°C for another 6 h.


## 2.2. Preparation of the 1,4-Bis(9-O-quininyl)phthalazine 3

NaH (2.24 mmol) at 0°C was slowly added to a stirred solution of 1,4-dichlorophthalazine (0.5 mmol) and quinine4 (1.15 mmol) in THF (8 mL). The solution was stirred at 60°C for 2 h and then it was quenched at 0°C by careful addition of water. The mixture was extracted in ethyl acetate (EtOAc) and the solvent was removed under reduced pressure. The residue was purified by short column chromatography to separate 1,4-bis(9-O-quininyl)phthalazine3 and 82% yield was obtained.


## 2.3. Preparation of the Triethoxysilanized 1,4-Bis(9-O-quininyl)phthalazine 2

1,4-Bis(9-O-quininyl)phthalazine3 (0.5 mmol) was added to a solution of (3-mercapto-propyl)triethoxysilane (1.25 mmol) and α,α′-azoisobutyronitrile (AIBN) (0.10 mmol) in degassed chloroform (10 mL) under N2 atmosphere. The reaction mixture was refluxed for 30 h and concentrated under reduced pressure. The residue was purified by flash short column on silica gel to give compound2 with 80% yield.


## 2.4. Immobilization of Biscinchona Alkaloid2 onto SBA-16 Silica 1

SBA-16 silica (1.0 g) was suspended in toluene and refluxed with compound2 (145 mg, 0.124 mmol). After 12 h, the powder was collected by filtration and washed with methanol and methylene chloride. After drying under vacuum at 70°C, SBA-16-supported alkaloid1 (1.077 g) was obtained. Elemental analysis and weight gain showed that 0.073 mmol of 1,4-bis(9-O-quininyl)phthalazine was anchored on 1.0 g of SBA-16-supported chiral Ligand1.


## 2.5. Characterization of SBA-16-Supported Chiral Ligand 1

Powder X-ray diffractometry (Philips PW 1729) was used for the determination of crystalline structure using Cu Ka radiation over 0.5
°
≤
2
θ
≤
3. The XRD sample of SBA-16 was analyzed at 30°C. The diffractograms showed 3 peaks at 2
θ
≈
0.74
°, 1.1°, and 1.4° that corresponded to (110), (200), and (211) planes, respectively, in the cubicIm3m structure. The transmission electron microscopy (TEM) was performed with a FEI Tecnai G2 microscope operated at 200 kV. The TEM sample was prepared by placing a few drops of SBA-16 powder dispersed in acetone on a carbon grid and allowing it to dry for 5 min before TEM analysis. Large particles were crushed by submerging them in liquid nitrogen followed by mechanical grinding in a mortar prior to acetone dispersion. The nitrogen adsorption-desorption measurements were performed at −196°C on a Micromeritics ASAP 2020 surface area and porosity analyzer. Approximately 0.5 g of SBA-16 was degassed at 300°C for 9 h before taking the measurement. The surface area determination was performed by the Brunauer-Emmett-Teller (BET) method [29] over the relative pressure (P
/
P
0) range of 0.05–0.2. The pore-size distribution was determined using the Broekhoff-de Boer (BdB) method [30] applied to the adsorption branch. Finally, the total pore volume was calculated from the amount of adsorbed N2 at P
/
P
0
=
0.99, and the microporous volume was determined using the t-plot method.


## 2.6. Asymmetric Dihydroxylation of Olefin Using SBA-16-Supported Chiral Ligand 1

A mixture of SBA-16-supported biscinchona alkaloid1 (1 mol%), potassium ferricyanide (1.5 mmol), potassium carbonate (1.5 mmol), and OsO4 (1 mol%, 0.5 M in water) intert-butyl alcohol-water (6 mL, 1 : 1, v/v) was stirred at room temperature for 30 min. Olefin (0.5 mmol) was added at once and stirred for 7~15 h. The reaction mixture was diluted with water and CH2Cl2 and the immobilized Ligand1 was separated by filtration. The crude product was purified by flash column chromatography, and the enantiomeric excess of the diol was determined by chiral gas chromatography (GC) analysis (Agilent HP Chiral-20B 30MX0.25MMX0.25UM GC Column).


## 3. Results and Discussion

To synthesize a SBA-16-supported biscinchona alkaloid1, we started with quinine and 1,4-dichlorophthalazine following a route shown in Scheme 1.Scheme 1
Preparation of SBA-16-supported biscinchona alkaloid1.Treatment of optically active quinine4 with 1,4-dichlorophthalazine in the presence of excess NaH synthesized 4-bis(9-O-quininyl)phthalazine3 with high yield (82%). Radical reaction of dimeric quinine3 with (3-mercaptopropyl)triethoxysilane in the presence of AIBN radical initiator provided compound2 having a pendant triethoxysilane functional group. The desired immobilized biscinchona alkaloid SBA-161 (loading ratio: 0.073 mmol/g) was readily obtained by condensation of2 with surface silanols of SBA-16 support in refluxing toluene. The degree of functionalization was determined by elemental analysis and weight gain. As shown in Table 1, the surface area and pore diameter were decreased following the modification. The high resolution transmission electron microscopy (HRTEM) image of SBA-16-supported Ligand1 is shown in Figure 1. The 3D cubic structure and the pore arrays were conserved after the immobilization of 1,4-bis(9-O-quininyl)phthalazine onto SBA-16 silica and it was also confirmed by XRD (Figure 2).Table 1
Structural characteristics of SBA-16-supported Ligand1.
Sample
Surface area
Pore diameter
Pore volume
Functional group
SBA-16
820 m2/g
5.13 nm
0.73 cm3/g
—
1
490 m2/g
3.68 nm
0.45 cm3/g
0.073 mmol/gFigure 1
HRTEM image of SBA-16-supported Ligand1.Figure 2
XRD pattern of SBA-16-supported Ligand1.The AD reaction of stilbene was performed in the presence of immobilized cinchona alkaloid1 (1 mol%) and OsO4 (1 mol%) at room temperature. Potassium carbonate and potassium ferricyanide were used as a secondary oxidant intert-butyl alcohol-water mixture (1 : 1). The results are summarized in Table 2. Surprisingly, catalytic AD reactions of stilbene provided excellent enantioselectivities and high yields (entries 1 and 2). Osmium catalyst loading of 0.5 mol% was sufficient to obtain outstanding enantioseletivity as well as high reactivity. Moreover, the SBA-16-supported alkaloid-OsO4 complex could be reused for the AD reaction of stilbene without a significant loss of reactivity and enantioselectivity (entry 3). The catalyst was also highly effective to AD of methylcinnamate, 1-phenyl-1-cyclohexene, and styrene (entries 4–6).Table 2
Heterogeneous AD of olefins using SBA-16-supported Ligand1
a.
Entry
Olefin
Time (h)
Yield (%)
(%) Eeb
Config.b
1
7
93
>99
S, S
2c
7
91
98
S, S
3d
8
90
97
S, S
4
9
91
96
2R, 3S
5
10
92
94
2S, 3S
6
15
92
92
S
aThe reactions were carried out at RT; molar ratio of olefin/OsO4/supported ligand = 1/0.01/0.01. bEe and absolute configuration were determined by chiral GC analysis. cMolar ratio of olefin/OsO4/supported ligand = 1/0.005/0.005. dReaction was carried out with regenerated 1 without further addition of OsO4.The SBA-16-supported Ligand1 reported here showed somewhat higher reactivity and better asymmetric induction over amorphous silica-supported biscinchona alkaloid [25]. The improved outcome of the reaction seems to be attributed to the ordered array of chiral catalytic site on the nanopore surface of SBA-16 support. The ordered array leads to elegant site isolation which may result in enhanced enantioselectivity.Romero et al. [31] reported the asymmetric dihydroxylation reaction of olefin using ionic liquid, which involves high cost and toxicity. The yield and enantioselectivity of the styrene were also very poor (87% yield, 62% ee) [32]. On the other hand, Junttila and Hormi [33] used methanesulfonamide as an accelerator of the asymmetric dihydroxylation reaction using potassium osmate (vi) and obtained 97% ee with low yield (70%) of the diol product. It is noteworthy here that the alkaloid ligand complexes synthesized in this report produced excellent results in terms of both yield (93%) and enantioselectivity (92–99%).


## 4. Conclusion

We successfully synthesized air and moisture stable SBA-16-supported biscinchona alkaloid chiral ligand. Osmium tetraoxide readily formed a chiral complex with the SBA-16-supported biscinchona alkaloid at room temperature. The synthesized SBA-16 supported Os-complex efficiently promoted the asymmetric dihydroxylation of olefin to corresponding diols with 92–99% enantioselectivity and 93% yield, demonstrating SBA-16 silica as an excellent support material for the heterogeneous chiral ligand.



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*Source: 123680-2014-06-15.xml*