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Catalytic activitiy of chiral phosphinooxazolidine ligands immobilized on SBA-15 for the asymmetric allylic substitution
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
501
Catalytic activitiy of chiral phosphinooxazolidine ligands immobilized on SBA-15 for the asymmetric allylic substitution Pong Hyun Chong^, Yong Ku Kwon^, Chung Young Lee^ and Geon-Joong Kim^ ^Department of Chemical Engineering, Inha University Incheon 402-751, Korea. ^Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea. ^Department of Industrial Chemistry, Inha Technical College, Incheon 402-752, Korea. The asymmetric Pd-catalyzed allylic substitution using chiral phosphinooxazolidines immobilized on SBA-15 mesoporous silica could be applied with success & high enantioselectivities were attainable in the allylic substitution. 1. INTRODUCTION Asymmetric catalysis is becoming the preferred approach because of its low environmental impact and high potential productivity. In particular, asymmetric allylic alkylation has become one of the more useful reactions. During the last decade, various enantioselective catalysts have been developed for palladium-catalyzed allylic substitution reactions[l]. In particular, chiral phosphinooxazolidine ligands are one of the effective ligands in this reaction[2,3]. In this study, we developed new chiral oxazolidines and immobilized these ligands on the supports. Chiral phosphinooxazolidine ligands could be readily synthesized by condensation of alkyl aminoalcohols and (2-diphenylphosphino) benzaldehyde. So, we have synthesized new oxazolidines using aminoalcochols containing sufur atom(methionol, s-methyl cysteinol, sbenzyl cysteinol) and various aldehydes(2-thiophenecarbxaldehyde, 2-pyridinealdehyde). The sequent anchoring method of reacting a functionalized support with reactive groups of aminoalcohols, step by step, make it possible to synthesize various unsymmetrical chiral oxazolidines of different structure and to immobilize them onto solid supports. The heterogeneous catalysts offer practical advantages of the facile separation from reactants and products, as well as recovery and reuse[4-7]. But some disadvantages can be expected in heterogeneous catalysis in terms of reaction rates and enantioselectivity. Our attention was directed to the development of a method anchoring the optically selective chiral ligands on the solid supports[5]. A few studies concerning the immobilization of amino alcohol derivatives onto MCM-41 mesoporous materials and their catalytic properties in the asymmetric allylic alkylation have been published in the open literature[5-7]. The use of these mesoporous silicas with regular pore diameters has expanded the range of applications for the catalysis of large substrates and incorporating bulky complexes for enantioselective catalytic reactions.
502
Ri
Qupporp
CI OH
*^2C03
Scheme. 1 Preparation of phosphinooxazolidines immobilized on supports 2. EXPERIMENTAL 2.1. Preparation of phosphinooxazolidines immobilized on the mesoporous material Scheme. 1 shows the method to immobiUze phosphinooxazolidines on the mesoporous supports. Heterogenized phosphinooxazolidines ligands were prepared by the reaction of chloro functionalized supports with an amino alcohol in boiling toluene. 2.2. Allylic substitution reaction Similarly to the Trost's procedure[8], which is very often employed as a test allylic substitution reaction, the reactions of l,3-diphenyl-2-propenyl acetate with nucleophile produced by reacting dimethylmalonate, M(9-bis(trimethylsilyl)acetamide(BSA), and potassium acetate as a catalyst were performed at room temperature in the presence of nallylpalladium chloride dimmer and the chiral ligands. 3. RESULTS AND DISCUSSION The catalytic activities of homogeneous phosphooxazolidines were investigated in the asymmetric allylic substitution of l,3-diphenyl-2-propenyl acetate with dimethyl malonate and the results are summarized in Table 1. The high enantioselectivity was obtained when 2.5 mol% Pd-allyl dimmer and 10 mol% ligand to the substrate were employed. As can be seen in this Table, the enantioselectivity was strongly dependent on the structure of amino alcohols. The homogeneous phosphinooxazolidine ligand Al and A3 show the very high enantioselectivities. The structure of ligands used as catalysts in this work is shown in Scheme 2.
Scheme. 2
503
The increase of reaction temperature led to a slight decrease of enantioselectivity. These results show that low reaction temperature is efficient for the increase of enantioselectivity. THF solvent is usually used in the Palladium-catalyzed allylic substitution reaction. The high enantioselectivity was also observed by using dichloromethane as a solvent. We have prepared new phosphinooxazolidine ligands starting with the sulfiir-containing amino alcohols and used them as catalysts in the same reaction. Chiral phosphooxazolidine (Al) prepared from methioninol showed a high ability to induce chirality in the target molecules with 88% ee. The greater enantioselectivity provided by ligand prepared from methioninol in comparison with s-methinonol may be rationalized by the longer tether between the nitrogen and sulfur as a donor atoms. The chiral phosphooxazolidine ligands immobilized on the mesoporous material were applied for the pd-catalyzed reaction to evaluate their catalytic activity against the parent homogeneous catalysts. The results obtained on a supported chiral catalyst were compared with those obtained by using the analogous homogeneous catalyst. A higher ee% was obtained under the same reaction conditions employing Merrifield's resin as a support. When the chiral phosphooxazolidine ligands anchored on polystyrene were used in the reaction, B1 and B3 gave products of 80% ee and 89% ee, respectively. In the case of SBA-15, capping of silanols with TMS improved the enantioselectivity, but this inorganic support was less efficient in the Pd-catalysed allylic substitution reaction. Heterogenized catalyst was simply separated by filtration from the product and it could be reused again. Table 1 Asymmetric Pd-catalyzed allylic substitution of l,3-diphenyl-2-propenyl acetate Q^^
3eqCH2(C02Me)2
^ ^^
3eqBSA 3mol%KOAc
CH(C0iMe)2
^
2.5mol% [Pd(allyl)CIJ2 10mol% ligand
Entry
Catalyst
Benzaldehyde'^
TempCt:)
Yield(%)
ee%'
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Al Al Bl (Polymer) B1(SBA-15) Bl (Silica gel) A2 B2(Polymer) B2(SBA.15) A3 B3(Polymer) B3(SBA-15) B3(Silica gel) A4 B4(SBA-15) CI C2
PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 thiophene pyridyl
10 50 10 10 10 10 10 10 10 10 10 10 10 10 10 10
98 100 88 74 62 88 66 40 98 75 70 60 99 55 60 58
88 61 80 65 43 55 44 35 98 89 72 58 84 52 3 20
>h2=2--diphenylposphinobenzaldehyd e, THF solvent
•etermi nded by HPLC with chiralcel OD column(25cm x 0.46cm) : 1% 2-propanol in hexane, flow rate=0.5mL/min, tR(min)=25.6(R), 27.5(S)
504
As summarized in Table 1, the enantioselective catalytic activities of the phosphinooxazolidines immobilized on solid supports are slightly lower than those of the corresponding homogeneous phosphino-oxazolidines. SBA-15-supported catalysts gave much higher reacon rates and higher asymmetric induction than silica gel-supported ones. Highly ordered mesoporous silica supports were found to be better inorganic support than amorphous silica gel. The ligands prepared from 2-pyridinealdehyde and 2-thiophenecarboxaldehyde afford 320% ee. On the basis of asymmetric allylic substitution reaction, the chiral complexes immobilized on mesoporous material by the present procedure can be applied as an effective heterogenized homogeneous catalyst for the asymetric reactions. 4. CONCLSIONS New heterogeneous catalysts employing various amino alcohols immobilized on SBA-15 have been synthesized and they were applied to the asymmetric allylic substitution. The enantioselectivity was strongly dependent on the structure of amino alcohol and the enantiomeric excess varied substantially from one amino alcohol to another. SBA-15 has served as a potential support for the heterogenized chiral catalysts in the asymmetric reduction of aromatic ketones to alcohols. ACKNOWLEDGMENT This work was supported by grant No. 2000-1-30700-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation and partially by Inha Technical College. REFERENCES 1. A. K. Ghosh, P. Mathivanan and J. Cappiello, Tetrahedron Asymmetry, 9 (1998) 1. 2. H. Steinhagen, M. Reggelin and G. Helmchen, Angew. Chem. Int. Ed. Engl. 36 (1997) 2108. 3. B. J. Nagy, P. Sutra, F. Fajula, D. Brunei, P. Lentz, G. Daelen, Colloids and Surfaces. 158 (1999)21. 4. G. Giffels, J. Beliczey, M. Felder and U. Kragel, Tetrahedron; Asymmetry, 9 (1998) 691. 5. G.-J. Kim and J.-H. Shin, Tetrahedron Lett., 40 (1999) 6827. 6. S. W. Kim, S. J. Bae, T. Hyeon and B. M. Kim, Microporous and Mesoporous Materials, 44(2001)523. 7. N. Bellocq, D. Brunei, M. Lasperas, P. Moreau, Stud. Surf Sci. Catal., 108 (1997) 485. 8. B. M. Trost and D. L. van Vranken, Chem. Rev. 96 (1996) 395.
501
Catalytic activitiy of chiral phosphinooxazolidine ligands immobilized on SBA-15 for the asymmetric allylic substitution Pong Hyun Chong^, Yong Ku Kwon^, Chung Young Lee^ and Geon-Joong Kim^ ^Department of Chemical Engineering, Inha University Incheon 402-751, Korea. ^Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea. ^Department of Industrial Chemistry, Inha Technical College, Incheon 402-752, Korea. The asymmetric Pd-catalyzed allylic substitution using chiral phosphinooxazolidines immobilized on SBA-15 mesoporous silica could be applied with success & high enantioselectivities were attainable in the allylic substitution. 1. INTRODUCTION Asymmetric catalysis is becoming the preferred approach because of its low environmental impact and high potential productivity. In particular, asymmetric allylic alkylation has become one of the more useful reactions. During the last decade, various enantioselective catalysts have been developed for palladium-catalyzed allylic substitution reactions[l]. In particular, chiral phosphinooxazolidine ligands are one of the effective ligands in this reaction[2,3]. In this study, we developed new chiral oxazolidines and immobilized these ligands on the supports. Chiral phosphinooxazolidine ligands could be readily synthesized by condensation of alkyl aminoalcohols and (2-diphenylphosphino) benzaldehyde. So, we have synthesized new oxazolidines using aminoalcochols containing sufur atom(methionol, s-methyl cysteinol, sbenzyl cysteinol) and various aldehydes(2-thiophenecarbxaldehyde, 2-pyridinealdehyde). The sequent anchoring method of reacting a functionalized support with reactive groups of aminoalcohols, step by step, make it possible to synthesize various unsymmetrical chiral oxazolidines of different structure and to immobilize them onto solid supports. The heterogeneous catalysts offer practical advantages of the facile separation from reactants and products, as well as recovery and reuse[4-7]. But some disadvantages can be expected in heterogeneous catalysis in terms of reaction rates and enantioselectivity. Our attention was directed to the development of a method anchoring the optically selective chiral ligands on the solid supports[5]. A few studies concerning the immobilization of amino alcohol derivatives onto MCM-41 mesoporous materials and their catalytic properties in the asymmetric allylic alkylation have been published in the open literature[5-7]. The use of these mesoporous silicas with regular pore diameters has expanded the range of applications for the catalysis of large substrates and incorporating bulky complexes for enantioselective catalytic reactions.
502
Ri
Qupporp
CI OH
*^2C03
Scheme. 1 Preparation of phosphinooxazolidines immobilized on supports 2. EXPERIMENTAL 2.1. Preparation of phosphinooxazolidines immobilized on the mesoporous material Scheme. 1 shows the method to immobiUze phosphinooxazolidines on the mesoporous supports. Heterogenized phosphinooxazolidines ligands were prepared by the reaction of chloro functionalized supports with an amino alcohol in boiling toluene. 2.2. Allylic substitution reaction Similarly to the Trost's procedure[8], which is very often employed as a test allylic substitution reaction, the reactions of l,3-diphenyl-2-propenyl acetate with nucleophile produced by reacting dimethylmalonate, M(9-bis(trimethylsilyl)acetamide(BSA), and potassium acetate as a catalyst were performed at room temperature in the presence of nallylpalladium chloride dimmer and the chiral ligands. 3. RESULTS AND DISCUSSION The catalytic activities of homogeneous phosphooxazolidines were investigated in the asymmetric allylic substitution of l,3-diphenyl-2-propenyl acetate with dimethyl malonate and the results are summarized in Table 1. The high enantioselectivity was obtained when 2.5 mol% Pd-allyl dimmer and 10 mol% ligand to the substrate were employed. As can be seen in this Table, the enantioselectivity was strongly dependent on the structure of amino alcohols. The homogeneous phosphinooxazolidine ligand Al and A3 show the very high enantioselectivities. The structure of ligands used as catalysts in this work is shown in Scheme 2.
Scheme. 2
503
The increase of reaction temperature led to a slight decrease of enantioselectivity. These results show that low reaction temperature is efficient for the increase of enantioselectivity. THF solvent is usually used in the Palladium-catalyzed allylic substitution reaction. The high enantioselectivity was also observed by using dichloromethane as a solvent. We have prepared new phosphinooxazolidine ligands starting with the sulfiir-containing amino alcohols and used them as catalysts in the same reaction. Chiral phosphooxazolidine (Al) prepared from methioninol showed a high ability to induce chirality in the target molecules with 88% ee. The greater enantioselectivity provided by ligand prepared from methioninol in comparison with s-methinonol may be rationalized by the longer tether between the nitrogen and sulfur as a donor atoms. The chiral phosphooxazolidine ligands immobilized on the mesoporous material were applied for the pd-catalyzed reaction to evaluate their catalytic activity against the parent homogeneous catalysts. The results obtained on a supported chiral catalyst were compared with those obtained by using the analogous homogeneous catalyst. A higher ee% was obtained under the same reaction conditions employing Merrifield's resin as a support. When the chiral phosphooxazolidine ligands anchored on polystyrene were used in the reaction, B1 and B3 gave products of 80% ee and 89% ee, respectively. In the case of SBA-15, capping of silanols with TMS improved the enantioselectivity, but this inorganic support was less efficient in the Pd-catalysed allylic substitution reaction. Heterogenized catalyst was simply separated by filtration from the product and it could be reused again. Table 1 Asymmetric Pd-catalyzed allylic substitution of l,3-diphenyl-2-propenyl acetate Q^^
3eqCH2(C02Me)2
^ ^^
3eqBSA 3mol%KOAc
CH(C0iMe)2
^
2.5mol% [Pd(allyl)CIJ2 10mol% ligand
Entry
Catalyst
Benzaldehyde'^
TempCt:)
Yield(%)
ee%'
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Al Al Bl (Polymer) B1(SBA-15) Bl (Silica gel) A2 B2(Polymer) B2(SBA.15) A3 B3(Polymer) B3(SBA-15) B3(Silica gel) A4 B4(SBA-15) CI C2
PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 PPh2 thiophene pyridyl
10 50 10 10 10 10 10 10 10 10 10 10 10 10 10 10
98 100 88 74 62 88 66 40 98 75 70 60 99 55 60 58
88 61 80 65 43 55 44 35 98 89 72 58 84 52 3 20
>h2=2--diphenylposphinobenzaldehyd e, THF solvent
•etermi nded by HPLC with chiralcel OD column(25cm x 0.46cm) : 1% 2-propanol in hexane, flow rate=0.5mL/min, tR(min)=25.6(R), 27.5(S)
504
As summarized in Table 1, the enantioselective catalytic activities of the phosphinooxazolidines immobilized on solid supports are slightly lower than those of the corresponding homogeneous phosphino-oxazolidines. SBA-15-supported catalysts gave much higher reacon rates and higher asymmetric induction than silica gel-supported ones. Highly ordered mesoporous silica supports were found to be better inorganic support than amorphous silica gel. The ligands prepared from 2-pyridinealdehyde and 2-thiophenecarboxaldehyde afford 320% ee. On the basis of asymmetric allylic substitution reaction, the chiral complexes immobilized on mesoporous material by the present procedure can be applied as an effective heterogenized homogeneous catalyst for the asymetric reactions. 4. CONCLSIONS New heterogeneous catalysts employing various amino alcohols immobilized on SBA-15 have been synthesized and they were applied to the asymmetric allylic substitution. The enantioselectivity was strongly dependent on the structure of amino alcohol and the enantiomeric excess varied substantially from one amino alcohol to another. SBA-15 has served as a potential support for the heterogenized chiral catalysts in the asymmetric reduction of aromatic ketones to alcohols. ACKNOWLEDGMENT This work was supported by grant No. 2000-1-30700-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation and partially by Inha Technical College. REFERENCES 1. A. K. Ghosh, P. Mathivanan and J. Cappiello, Tetrahedron Asymmetry, 9 (1998) 1. 2. H. Steinhagen, M. Reggelin and G. Helmchen, Angew. Chem. Int. Ed. Engl. 36 (1997) 2108. 3. B. J. Nagy, P. Sutra, F. Fajula, D. Brunei, P. Lentz, G. Daelen, Colloids and Surfaces. 158 (1999)21. 4. G. Giffels, J. Beliczey, M. Felder and U. Kragel, Tetrahedron; Asymmetry, 9 (1998) 691. 5. G.-J. Kim and J.-H. Shin, Tetrahedron Lett., 40 (1999) 6827. 6. S. W. Kim, S. J. Bae, T. Hyeon and B. M. Kim, Microporous and Mesoporous Materials, 44(2001)523. 7. N. Bellocq, D. Brunei, M. Lasperas, P. Moreau, Stud. Surf Sci. Catal., 108 (1997) 485. 8. B. M. Trost and D. L. van Vranken, Chem. Rev. 96 (1996) 395.