594
Chem. Mater. 2004, 16, 594-599
Chiral Germanium Zeotype with Interconnected 8-, 11-, and 11-Ring Channels. Catalytic Properties M. E. Medina, M. Iglesias, N. Snejko, E. Gutie´rrez-Puebla,* and M. A. Monge* Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Cantoblanco, 28049, Madrid, Spain Received July 25, 2003. Revised Manuscript Received October 23, 2003
The synthesis, structure, thermal stability, and catalytic properties are reported for a chiral germanate, Ge9O19(OH)2(N2C2H10)2(N2C2H8)0.5H2O, ICMM6. It is orthorhombic, of space group C2221, and has unit cell dimensions a ) 12.156(3) Å, b ) 13.960(4) Å, c ) 14.258(4) Å, volume 2419.6(11) Å3, and Z ) 4. Three kinds of interconnected helical 8-, 11-, and 11-rings channels are present in the 3D framework. ICMM6 is an active chiral catalyst for the Michael addition reaction and acetalization of aldehydes. It is stable in water and organic solvents and can be recovered and reused at least four times without losing either its structure or its reactivity.
Introduction Currently there are many efforts dedicated to the design, preparation, and development of new microporous materials1 with better characteristics for use in processes of the petroleum refining and petrochemical industries.2,3 Researchers have achieved excellent selectivity and conversion results in reactions in which microporous materials are involved. Promising enantioselectivity results have been acheived in reactions catalyzed by zeolites modified with chiral coordination complexes anchored in their internal surface.4,5 The economic importance of these processes in the synthetic chemistry industry, which relies on the availability of new and improved catalysts, has led to continued attempts to extend the range of microporous compounds available.6-10 The prospect of synthesizing chiral open-framework microporous compounds is of great interest with respect to their possible applications in enantioselective separation and synthesis. Although the geometrical principles and theoretical stabilities of tetrahedral frameworks are quite well-known,11 it is not easy to synthesize new chiral framework topologies “on demand” because it involves complex reaction mechanisms. The predictable synthesis of bulk chiral solids is usually accomplished either by growing enantiopure hosts around a resolved template or by construction of a framework from ho(1) Treacy, M. M. J.; Marcus, B. K.; Bisher, M. E.; Higgins, J. B.; Eds. Proceedings of the 12th International Zeolite Conference; Materials Research Society: Warrendale, PA, 1999. (2) Corma, A. Chem. Rev. 1995, 95, 559. (3) Corma, A. Chem. Rev. 1997, 97, 2373. (4) Chiral Catalyst Inmobilization and Recycling; Vos, D. E., Vankelecom, I. F. K., Jacobs, P. A., Eds.; Wiley-VCH: Weinheim, Germany, 2000. (5) Fine Chemical through Heterogeneous Catalysis; Sheldon, R. A., van Bekkun, H., Eds.; Wiley-VCH: Weinheim, Germany, 2001. (6) Nakayama, H.; Hayashi, A.; Eguchi, T.; Nakamura, N.; Tsuhako, M. Solid State Sci. 2002, 4 (2), 1067. (7) Claudemir, A.; Borgo; Yoshitaka, G. J. Colloid Interface Sci. 2002, 246 (15), 343. (8) Aramendı´a, M. A.; Borau, V.; Jime´nez, C.; Marinas, J. M.; Romero, F. J.; Urbano, F. J. J. Mol. Catal. A: Chem. 2002, 182-183, 25. (9) Alberti, G.; Cavalaglio, S.; Marmottini, F.; Matusek, K.; Megyeri, J.; Szirtes, L. Appl. Catal. A: General 2001, 218, 219. (10) Wiebcke, M. Microporous Mesoporous Mater. 2002, 54 (3), 331. (11) Smith, J. V. Chem. Rev. 1988, 149, and references therein.
mochiral building blocks. Chirality in many materials is derived from the presence of helices within a noncentrosymmetric space group rather than through chirality-enriched components in the reaction mixture. In terms of our interest in the structure-properties relationship in germanium zeotypes12-16 we present here the synthesis, structure, characterization, and catalytic properties of the chiral germanate Ge9O19(OH)2(enH2)2(en)0.5H2O (ICMM6). Experimental Section General Information. GeO2 was from Avocado (99.98%), 1,4-diazabicyclo[2.2.2]octane (DABCO) and ethylendiamine (en), were from Aldrich (98% and 99%, respectively), and 1-butanol was from Merck (99.9%). Thermogravimetric and differential thermal analyses (TGA-DTA) were performed in a SEIKO TG/ DTA 320 apparatus in N2 flow (velocity ) 50 mL/min) in the temperature range between 25 and 700 °C. IR spectra were collected in a Nicolet 20SXC FTIR spectrometer. Gas chromatography analysis (GC-MS) was performed using a HewlettPackard 5890 II with a flame ionization detector in a crosslinked methyl-silicone column [mixture of methylsilicone (OV1701) and permethylcyclodextrine as stationary phase].17 Synthesis. ICMM6 was synthesized hydrothermally in a stainless steel autoclave from reaction mixtures containing GeO2, H2O, DABCO, and 1-butanol (molar ratio of reactants 1:100:2.5:10) at 170 °C during 5 days. Variations in the procedure were explored, especially to avoid the presence of ICMM5, ASU-9,18 and ICMM4 whose syntheses also involve DABCO. The ratio of 1-butanol/water is very important to get this phase pure. After verifying the break of DABCO into ethylendiamine molecules, the synthesis was also accomplished with en. The rate of purity of both reactions was confirmed by comparison (12) Cascales, C.; Gutie´rrez-Puebla, E.; Monge, M. A.; Ruiz-Valero, C. Angew. Chem., Int. Ed. 1998, 37 (1/2), 129. (13) Cascales, C.; Gutie´rrez-Puebla, E.; Iglesias, M.; Monge, M. A.; Ruiz-Valero, C. Angew. Chem., Int. Ed. 1999, 38 (16), 2436. (14) Cascales, C.; Gutie´rrez-Puebla, E.; Iglesias, M.; Monge, M. A.; Ruiz-Valero, C.; Snejko, N. Chem. Commun. 2000, 2145. (15) Cascales, C.; Gomez-Lor, B.; Gutie´rrez-Puebla, E.; Iglesias, M.; Monge, M. A.; Ruiz-Valero, C.; Snejko, N. Chem. Mater. 2002, 14 (2), 677. (16) Medina, M. E.; Iglesias, M.; Monge, M. A.; Gutierrez-Puebla, E. Chem. Commun. 2001, 2548. (17) Miranda, E.; Sa´nchez, F.; Sanz, J.; Jime´nez, M. I.; MartinezCastro, I. J. High Resolut. Chromatogr. 1998, 21, 225. (18) Li, H.; Yaghi, O. J. Am. Chem. Soc. 1998, 120, 10569.
10.1021/cm0346879 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/21/2004
Synthesis and Structure of a Chiral Germanate
Chem. Mater., Vol. 16, No. 4, 2004 595
Table 1. Crystallographic Data
Table 2. Atomic Coordinates
Ge4.5O9.5(OH)(N2C2H10)-(N2C2H8)0.25(H2O)0.5 formula weight crystal system space group a (Å) b (Å) c (Å) alpha (°) beta (°) gamma (°) Z volume (Å3) density (calculated mg/m3) absorption coefficient (mm-1) F (000) crystal size (mm) reflections collected independent reflections goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole (eÅ-3)
577.78 orthorhombic C2221 12.156(3) 13.960(4) 14.258(4) 90 90 90 8 2419.6(11) 3.172 11.126 2196 0.20 × 0.20 × 0.20 9549 3648 (Rint ) 0.0626) 1.069 R1 ) 0.0392, wR2 ) 0.0973 R1 ) 0.0416, wR2 ) 0.0987 1.462 and -1.385
between XRD powder experimental diagrams and that calculated with the single-crystal structure. X-ray Structure Determinations. To carefully study the absolute structure of ICMM6, four crystals of ICMM6 were selected and mounted on a Siemens Smart-CCD diffractometer equipped with a normal focus, 3-kW sealed tube. A summary of the fundamental crystal and refinement data for one of them is given in Table 1. In three of the crystals, data were collected at room temperature over the full sphere of the reciprocal space by a combination of four frame sets. The θ range was 3.62 to 31.15°. The crystal-to-detector distance was 5.02 cm. Cell parameters were determined and refined by least-squares fit of all reflections collected. Each frame exposure time was of 20 s covering 0.3° ω. The first 100 frames were re-collected at the end of the data collection to monitor crystal decay. Structure was solved by direct methods. The hydrogen atoms of the OH group and those of the water molecule were located in difference Fourier maps. The final cycles of least-squares refinements for the four crystals included atomic coordinates and anisotropic thermal parameters for all nonhydrogen atoms except for those involved in the amine molecules, which were isotropically refined. The Flack x parameter values were 0.01(3), 0.01(1), 0.03(2), and 0.04(3) for the same coordinates in the four cases, and are indicative of their correct absolute structure. The values of goodness-of-fit on F2 and final R indices [I > 2σ(I)] corresponding to the other three crystals collected are as follows: II, 0.938 and 0.0292; III, 1.042 and 0.0382; and IV, 0.937 and 0.0513. Neutral-atom scattering factors for all atoms were used. Anomalous dispersion and secondary extinction corrections were applied. All calculations were performed using the SHELXTL program.19 Acetalization of Aldehydes with Trimethylorthoformate (TMOF). Activation of the catalyst was performed by heating the solid at 373 K for 12 h to remove water. After this time, a solution of the carbonyl compound (2.7 mmol) (benzaldehyde or 2-(R,S)phenyl propionaldehyde (C6H5)CH(CH3)COH), and TMOF (12.7 mmol) in tetrachloromethane (3 mL) was added to a suspension of the activated catalyst. The mixture was magnetically stirred at reflux temperature for 2.5 h. Samples were taken at intervals, and the reaction products were analyzed by gas chromatography. General Procedure for the Michael Addition. In a typical experiment, a mixture of enone (1 mmol) [methylvinyl ketone (MVK), acroleine (AC), or 5-methoxy-2,5(H)-furanone (Ψ)], and benzenethiol (1 mmol), 2-methyl,2-propanethiol (1 mmol), or nitroethane (1 mmol) in each case was added to a suspension of preactivated ICMM6 (10 wt %). The mixture was stirred at 323-333 K. The evolution of reaction results was monitored by gas chromatography. (19) SHELXTL Version 6.10 software package.
Ge4.5O9.5(OH)(N2C2H10)-(N2C2H8)0.25(H2O)0.5 atom
x
y
z
U (eq)
Ge(1) Ge(2) Ge(3) Ge(4) Ge(5) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) N(1) N(2) N(3) N(4) C(1) C(2) C(3) H(1A) H(1B) H(1D) H(1E) H(2A) H(2B) H(31) H(32) H(4A) H(4B) H(10) H(120)
1.0000 0.8647(1) 0.8231(1) 0.7577(1) 0.7919(1) 0.8181(4) 0.7746(4) 0.7133(4) 1.0000 0.8254(4) 0.8917(3) 1.0483(4) 0.8059(4) 0.7224(4) 0.8991(5) 0.8973(3) 0.6110(30) 0.0287(10) 0.0098(11) -0.1410(20) 0.1660(20) -0.0229(17) 0.0541(15) -0.0258(17) -0.1003 -0.0078 0.1019 0.0151 0.0199 -0.0031 -0.1794 -0.1794 0.2054 0.2054 0.8891 0.6560(60)
0.8450(1) 0.6249(1) 0.7962(1) 0.9106(1) 0.7296(1) 0.6727(3) 0.6207(3) 0.8303(3) 0.5831(5) 0.5043(3) 0.7522(3) 0.8466(3) 0.8172(3) 0.8620(4) 0.7528(5) 0.9413(3) 0.5000 0.0881(9) 0.4179(10) 0.5000 0.5000 0.1674(14) 0.4729(12) 0.4579(12) 0.1556 0.2270 0.0981 0.0311 0.3541 0.4479 0.4749 0.5251 0.5369 0.4631 0.7224 0.5360(50)
0.7500 0.7660(1) 0.8977(1) 0.7227(1) 0.6025(1) 0.8754(2) 0.6660(3) 0.6346(4) 0.7500 0.8062(3) 0.7177(3) 0.6254(3) 1.0164(3) 0.8339(3) 0.5237(4) 0.7197(3) 1.0000 0.6396(7) 0.8985(9) 1.0000 1.0000 0.7006(11) 0.9744(12) 0.9820(13) 0.7020 0.6709 0.6374 0.6666 0.8978 0.8434 1.0480 0.9520 0.9594 1.0405 0.4753 0.9780(50)
11(1) 9(1) 11(1) 13(1) 14(1) 16(1) 19(1) 29(1) 43(3) 18(1) 11(1) 17(1) 21(1) 22(1) 35(1) 18(1) 163(12) 71(3) 31(3) 124(9) 93(6) 93(5) 57(4) 66(4) 139 139 107 107 46 46 187 187 139 139 53 5
Results and Discussion According to the crystal structure determination, the composition of the obtained material is Ge9O19(OH)2(enH2)2(en)0.5H2O. Final atomic coordinates, bond distances, and bond angles are given in Tables 2 and 3. In this compound there are five Ge atoms per asymmetric unit: two of them are tetrahedrically coordinated [range of bond lengths Ge-O 1.704(4)-1.781(4) Å; O-Ge-O bond angles 97.7(2)-115.7(2)°; Ge-O-Ge 119.2(2)°]; two form trigonal bypiramids [range of bond lengths Ge-O 1.750(5)-2.066(4) Å, O-Ge-O bond angles 81.4(2)-174.3(2)°, Ge-O-Ge 92.5(2)-141.3(3)°]; and the fifth, which is located in a special position, forms an octahedron [range of bond lengths Ge-O, 1.872(4)1.904(4) Å; O-Ge-O bond angles 88.4(2)-178.7(3)°, and Ge-O-Ge 117.1(2)°]. These three kinds of Ge polyhedra (GeO4, GeO5, and GeO6) are linked among them by sharing corners; besides, every two GeO5 bypiramids share also the O2-O6 edge-forming dimmers. These connections together with the existence of a µ3-oxo (O6) that links Ge3-Ge2-Ge5 give rise to the noncentrosymmetric nine germanium clusters SBUs shown in Figure 1. Looking at the Ge positions, the 9SBU can be depicted as formed by two (related by a 2-fold axis) 4SBUs linked by an octahedral Ge atom situated on the axis. Ge-Ge distances are in the ranges 3.086-3.663 (involving the central Ge1), 2.891-3.082 (for Ge in the same 4SBU), and 3.322-5.942 Å (among Ge of different 4SBUs). The structure can be thought of as formed by 11R- pseudo
596
Chem. Mater., Vol. 16, No. 4, 2004
Medina et al.
Table 3. Bond Lengths and Angles for Ge9O19(OH)22- Framework bond lengths (Å) Ge(1)-O(7)#1 Ge(1)-O(7) Ge(1)-O(11)#1 Ge(1)-O(11) Ge(1)-O(6) Ge(1)-O(6)#1 Ge(2)-O(4) Ge(2)-O(1) Ge(2)-O(2) Ge(2)-O(5) Ge(2)-O(6) Ge(3)-O(8) Ge(3)-O(7)#1 Ge(3)-O(1) Ge(3)-O(9) Ge(4)-O(5)#2 Ge(4)-O(11) Ge(4)-O(9) Ge(4)-O(3) Ge(5)-O(10) Ge(5)-O(3) Ge(5)-O(2) Ge(5)-O(8)#3 Ge(5)-O(6) O(4)-Ge(2)#1 O(5)-Ge(4)#4 O(7)-Ge(3)#1 O(8)-Ge(5)#5
1.872(4) 1.872(4) 1.885(4) 1.885(4) 1.904(4) 1.904(4) 1.761(2) 1.788(4) 1.799(4) 1.840(4) 1.935(4) 1.729(4) 1.745(4) 1.755(4) 1.781(5) 1.704(4) 1.751(4) 1.776(5) 1.767(5) 1.750(5) 1.761(5) 1.781(4) 1.830(5) 2.066(4) 1.761(2) 1.704(4) 1.745(5) 1.830(5)
angles (°) O(7)#1-Ge(1)-O(7) O(7)#1-Ge(1)-O(11)#1 O(7)-Ge(1)-O(11)#1 O(7)#1-Ge(1)-O(11) O(7)-Ge(1)-O(11) O(11)#1-Ge(1)-O(11) O(7)#1-Ge(1)-O(6) O(7)-Ge(1)-O(6) O(11)#1-Ge(1)-O(6) O(11)-Ge(1)-O(6) O(7)#1-Ge(1)-O(6)#1 O(7)-Ge(1)-O(6)#1 O(11)#1-Ge(1)-O(6)#1 O(11)-Ge(1)-O(6)#1 O(6)-Ge(1)-O(6)#1 O(4)-Ge(2)-O(1) O(4)-Ge(2)-O(2) O(1)-Ge(2)-O(2) O(4)-Ge(2)-O(5) O(1)-Ge(2)-O(5) O(2)-Ge(2)-O(5) O(4)-Ge(2)-O(6) O(1)-Ge(2)-O(6) O(2)-Ge(2)-O(6) O(5)-Ge(2)-O(6) O(8)-Ge(3)-O(7)#1 O(8)-Ge(3)-O(1) O(7)#1-Ge(3)-O(1) O(8)-Ge(3)-O(9) O(7)#1-Ge(3)-O(9)
178.7(3) 89.0(2) 90.1(2) 90.1(2) 89.0(2) 89.0(3) 91.2(2) 89.7(2) 177.4(2) 88.4(2) 89.7(2) 91.2(2) 88.4(2) 177.4(2) 94.2(2) 122.2(2) 117.0(2) 120.7(2) 88.8(3) 89.3(2) 93.5(2) 95.7(2) 91.2(2) 81.4(2) 174.3(2) 103.0(2) 109.9(2) 113.1(2) 109.3(2) 108.1(2)
O(1)-Ge(3)-O(9) O(5)#2-Ge(4)-O(11) O(5)#2-Ge(4)-O(9) O(11)-Ge(4)-O(9) O(5)#2-Ge(4)-O(3) O(11)-Ge(4)-O(3) O(3)-Ge(4)-O(9) O(10)-Ge(5)-O(3) O(10)-Ge(5)-O(2) O(3)-Ge(5)-O(2) O(10)-Ge(5)-O(8)#3 O(3)-Ge(5)-O(8)#3 O(2)-Ge(5)-O(8)#3 O(10)-Ge(5)-O(6) O(3)-Ge(5)-O(6) O(2)-Ge(5)-O(6) O(8)#3-Ge(5)-O(6) Ge(3)-O(1)-Ge(2) Ge(5)-O(2)-Ge(2) Ge(5)-O(3)-Ge(4) Ge(2)-O(4)-Ge(2)#1 Ge(4)#4-O(5)-Ge(2) Ge(1)-O(6)-Ge(2) Ge(1)-O(6)-Ge(5) Ge(2)-O(6)-Ge(5) Ge(3)#1-O(7)-Ge(1) Ge(3)-O(8)-Ge(5)#5 Ge(4)-O(9)-Ge(3) Ge(4)-O(11)-Ge(1)
113.0(2) 112.3(2) 111.5(2) 110.4(2) 97.7(2) 115.7(2) 108.6(2) 115.1(3) 124.9(3) 119.0(3) 96.9(2) 96.1(2) 87.7(2) 92.5(2) 89.4(2) 78.2(2) 165.8(2) 121.0(2) 107.7(2) 121.7(3) 141.3(3) 141.3(2) 131.0(2) 134.6(2) 92.5(2) 117.1(2) 132.4(3) 119.2(2) 117.5(2)
Figure 1. Representation of the 9SBUs in ICMM6 vs that of ZnGe-1A.20 Black atoms and bonds represent connections through µ3-oxo.
layers of composition [Ge9O19(OH)2]∞4- perpendicular to the [011] connected among them to give the 3D structure. Three kinds of tunnels form the ICMM6 framework: those (8R) along the a direction, that can be thought as formed by two helix kept together through one oxygen atom, O5, shared by a GeO4 and a GeO5 of two different 9SBUs of different helixes, Figure 2. Along the [011] and [01 h 1] directions run 11R helical channels, Figure 3. The result is a structure with a threedimensional net of interconnected channels, in which the amine is housed. Bu et al.20 reported centrosymmetric 9SBUs (Figure 1) giving rise to centrosymmetric frameworks and thus not chiral structures. Noncentrosymmetric 9SBUs appeared also in the framework reported by R. Xu21 in 1992. In that paper the unit cell is twice the size of ours and all the amine molecules are perfectly ordered. In our smaller and more symmetric cell we have two kinds of en molecules: one situated along the 8R channels, for which a model of disorder had to be established, and (20) Bu, X.; Feng, P.; Stucky, G. D. Chem. Mater. 2000, 12, 1811.
Figure 2. View of the 11R layers perpendicular to the [011] direction (top); and the complete structure formed by the connection of these layers (bottom), where channels along the a direction are shown.
the other housed along the 11R tunnels (distinction among them is made only on the basis of TGA results and their amount and charge needed to maintain
Synthesis and Structure of a Chiral Germanate
Chem. Mater., Vol. 16, No. 4, 2004 597
Figure 3. View of the 3D system of channels in ICMM6: (a) Helix and peripheral view of the channels formed by connections of two of them; O5 is shown in black; (b) and (c) along [011] and [01 h 1] directions, respectively.
electrical neutrality, as elemental analysis is not conclusive for this small amount). The water molecule connects both kinds of en molecules by hydrogen bond (range of N-O lengths 2.90-2.95 Å). Given the similarity of both frameworks, attempts to determine the ICMM6 structure with data collected and reduced with the supercell reflections were carried out. Although an enantiomeric structure was found, the refinement resulted in a set of negative determinants and some isotropic temperature factors due to correlations in the matrix. As happens in many cases of zeotype refinements where the framework and template crystallographic cells are not coincident, a compromise has to be reached to obtain a reliable refinement. In our case the model of disorder established for the en molecules allows a very good refinement. Thus, the differences between the framework reported by R. Xu and that of the current compound are those coming from a more accurate refinement (Table 1). TGA-DTA analyses of ICMM6 showed that the compound is stable up to 250 °C. A progressive weight loss of 3.9% is observed up to 250 °C, which corresponds to the loss of one water molecule and 0.5 molecule of en per formula. Between 250 and 600 °C, the structure progressively collapses by losing the other two molecules of en and the OH groups, (13.9%). These results are in excellent agreement with the calculated values of 4% and 13.8% for the two steps, respectively. The presence of both OH- anions, amine and protonated amine, is clearly observed in the IR spectrum of ICMM6. Besides the characteristic C-N, C-C, and C-H bands, it exhibits a peak at 3580 cm-1 and a band at 2700 cm-1, assigned to the stretching vibrations of the OH groups and to the characteristic N-H vibrations of the amine in its protonated form, respectively. Catalytic Activity. The performance of ICMM6 as catalyst has been tested in different reactions. It has to be said that some of the en molecules are protonated compensating the framework negative charge. Thus, it’s not possible to remove them by heating without the structure collapsing. These molecules could, to some extent, have some influence in the basic catalysis, but, as en is not a chiral amine, it is quite improbable that they act in the enantioselectivity. (21) Jones, R. H.; Chen, J.; Thomas, J. M.; George, A.; Hursthouse, M.; Xu, R.; Li, S.; Lu, Y.; Yang, G. Chem. Mater. 1992, 4, 808.
Scheme 1. Michael Addition to 5-Methoxy-2,5(H)-furanone (Ψ).
Michael Addition to Enones. The Michael addition of nucleophiles to 5-methoxy-2,5(H)-furanone catalyzed by amines or alkaline alkoxides had been extensively studied22 and applied for preparation of several chiral valuable intermediates in organic chemistry.23 On the basis of these facts we selected one of this family of reactions as a model for testing our solid base catalyst. Thus, Ge9O19(OH)2(enH2)2(en)0.5H2O has been tested in the addition of benzenethiol (reaction I), 2-methyl-2propanethiol (reaction II), and nitroethane (reaction III) to racemic 5-methoxy-2,5(H)-furanone (Scheme 1). The reaction affords 4-tert-butyl or 4-phenylthio-3,4-dihydro5-methoxy-2,5(H)-furanone, where methoxy and sulfur groups present exclusively trans stereochemistry. Yields and enantiomeric excesses (ee ) R - S/R + S) of the Michael addition obtained for our catalyst are displayed in Figure 4a. As can be seen, the Michael additions promoted by ICMM6 took place in 1.2 h (I), 23 h (II), and 24 h (III). After these reaction times the conversions (22) de Jong, J. G.; Berg, K. J.; Lensen, A. M.; Feringa, B. L. Tettrahedron Lett. 1988, 44, 7212. Farin˜a, F.; Maestro, M. C.; Martı´n, M. R.; Martı´n, M. V.; Sa´nchez, F. Heterocycles 1983, 20, 1761. (23) Feringa, B. L.; de Jong, J. C. Bull. Soc. Chim. Belg. 1992, 101, 627. d’Angelo, J.; Cave, C.; Dermade, D.; Dumas, F. Trends Chem. 1993, 4, 555.
598
Chem. Mater., Vol. 16, No. 4, 2004
Medina et al.
Figure 5. Acetalization of benzaldehyde (VII) and 2-(R,S)phenyl propionaldehyde (VIII).
Figure 4. (a) Conversion and enantiomeric excesses (ee) for all the Michael addition to 5-methoxy-2,5(H)-furanone tested using ICMM6 as catalyst. (b) Conversion at 24 h for all the Michael addition tested using ICMM6 as catalyst. It is worth noting that reaction I (with benzenethiol) after 1.2 h yields 100% of reaction product.
Scheme 2. Michael Addition to Acroleine and Methylvinyl Ketone.
Figure 6. Profile of powder XRD of ICMM6 I before performing the acetalization of 2-(R,S)phenyl propionaldehyde and II when recovering the catalyst after the reaction.
Scheme 3. Acetalization of 2(R,S)-phenyl Propionaldehyde.
of 5-methoxy-2,5(H)-furanone were 100% (I), 16% (II), and 10% (III), with enantiomeric excesses of 30% (I), 32% (II), and 40% (III). ICMM6 has been tested also in the Michael addition of different nucleophiles to acroleine and methylvinyl ketone (Scheme 2). Nitroethane was added to the enones (reactions IV and V) and the addition of 2-methyl,2propanethiol to MVK (reaction VI) was also performed. Reaction (IV) took place in 24 h, yielding up to 10% of the reaction product; after 96 h reaction (V) the conversion of methylvinyl ketone was 31%; and reaction (VI) gave the corresponding product (29%) after 68 h. Figure
4b resumes the conversions at 24 h of all the Michael additions to enones performed. We recovered ICMM6 after all the reactions tested, and reused it in at least four cycles more. Yields and enantioselectivity are not modified in the successively cycles. Acetalization of Aldehydes. Formation of acetals is one of the most useful protecting methods for carbonyl compounds, and a large amount of synthetic work has been done on the protection and masking of the carbonyl groups. Acetals are important in fine chemistry as intermediates and as final products. When using Ge9O19(OH)2(enH2)2(en)0.5H2O (Scheme 3), the reaction of benzaldehyde or 2-(R,S)phenyl propionaldehyde (reactions VII and VIII), with TMOF proceeds with high conversion, and the corresponding dimethyl acetal is obtained in 45% yield after 2 or 72 h, respectively, under mild conditions (60-70 °C), when using our materials (8 mol %) as catalyst (Figure 5). The ee found for 2-(R,S)phenyl propionaldehyde dimethylacetal was 57%. As well as in Michael additions, ICMM6 was recovered after all the acetalization reactions tested, and reused in at least four cycles, maintaining its reactivity,
Synthesis and Structure of a Chiral Germanate
structure (confirmed by the powder XRD [Figure 6]), and enantioselectivity in all the cases. Although the sample was planned to be a 50% mixture of crystals with Flack parameters of 0 and 1, the observed enantiomeric excesses made us refine the crystal structure of four randomly chosen crystals of the catalytically tested sample. The four of them, being the same enantiomer, indicate that in the case of having a mixture of crystals, statistically speaking, the proportion of enantiomers is far from being 50%. This is a surprising preliminary result, which is the basis for some of our continued work. In summary we have confirmed that ICMM6 is a chiral bifunctional catalyst (acid-base) for different
Chem. Mater., Vol. 16, No. 4, 2004 599
organic reactions. It is a stable material, and it can be recovered and reused without loss of activity or enantioselectivity. Acknowledgment. This work has been supported by the Spanish CICYT MAT1999-0892. M.E.M. thanks the Ministerio de Ciencia y Tecnologı´a for a predoctoral fellowship. Supporting Information Available: X-ray crystallographic file (CIF) for ICMM6. This material is available free of charge via the Internet at http://pubs.acs.org. CM0346879
