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Homochiral coordination cages assembled from dinuclear paddlewheel nodes and enantiopure ditopic ligands: syntheses, structures and catalysis† Lianfen Chen, Jian Kang, Hao Cui, Yingxia Wang, Lan Liu, Li Zhang* and Cheng-Yong Su* A series of homochiral metal–organic cages (MOCs) have been obtained from self-assembly of Cu(II) salts with chiral N,N’-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)-bis-amino acids. Single-crystal X-ray diffraction
Received 10th December 2014, Accepted 19th February 2015 DOI: 10.1039/c4dt03782k www.rsc.org/dalton
1.
analyses reveal that these compounds show a lantern-type cage structure, in which one pair of Cu2(CO2)4 paddlewheels is linked by four diacid ligands. The resulting homochiral cages have been fully characterized by EA, TOF-MS, TGA, VTPXRD, IR, UV, and CD measurements. The catalytic tests reveal that these Cu(II) cages are effective in cyclopropanation with excellent diastereoselectivity (up to 99 : 1 E/Z). In addition, the cage catalysts can promote the aziridination reaction with PhIvNNs.
Introduction
Assemblies of metal–organic containers, including so-called metal–organic polyhedra (MOPs) or metal–organic cages (MOCs), rings and tubes,1 have attracted significant research attention in the past two decades due to their applications in the stabilization or protection of sensitive species,2 drug delivery,3 selective recognition or separation of species from a mixture,4 catalysis,5,6 the formation of unusual products under common conditions,7 etc. Since the creation of a chiral environment can facilitate the synthesis and separation of enantiomerically pure compounds,8 assembly of chiral MOCs is of understandable importance. Chirality may be introduced from achiral precursors by symmetry breaking during the selfassembly process, either from the metal center9 or the twisted ligand.10 For example, Saalfrank and co-workers reported tetrahedral M4L6 (M = MnII, CoII, NiII) MOCs with the chirality arising from the configuration of the octahedral metal centers (Δ or Λ).9a Robson described a capsule M12L8 (M = CuII) in which the ligands served as the corners and the metals as the edges, and the chirality was based on the twisted ligands.10a
MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry and Chemical Engineering, Sun yat-sen University, Guangzhou, 510275, P. R. China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: TOF-MS spectra, PXRD and CD spectra of the cage compounds. CIF files giving crystallographic data. CCDC 1038148–1038151. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03782k
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Alternatively, chirality can also be inherently embedded in MOCs by using chiral organic ligands as linkers11 or an auxiliary attached to the metal center.12 However, due to difficulties in synthesizing enantiomerically pure bridging ligands or auxiliaries, homochiral MOCs based on chiral starting materials are much fewer than racemizing MOCs prepared by the first method. Among limited examples, Stack et al. reported an M4L6 tetrahedral cluster assembled from GaIII and an enantiopure bis(catecholate) ligand.11a Fujita’s group utilized PdII with (R,R)- or (S,S)-diaminocyclohexane as an auxiliary for the self-assembly of optically active M6L4 truncated tetrahedra.12a Despite the availability of many chiral MOCs synthesized by the above two methods,13,14 only a limited number of chiral MOCs have been further applied in asymmetric catalysis.15 Fujita et al. have utilized homochiral M6L4 octahedra based on chiral diamine auxiliaries for asymmetric [2 + 2] olefin cross photoaddition, in which enantiomeric excesses of up to 50% were achieved.15a Raymond and coworkers have developed enantiopure M4L6 tetrahedra either by resolution of racemate or by chiral ligand directed assembly, and used them in enantioselective catalysis, such as Aza-Cope rearrangement and carbonyl-ene cyclization, yielding up to 78% and 69% ee, respectively.15b,c Duan and Zhang et al. have reported a chiral M3L2 cylinder-like cage, which was then applied in the asymmetric cyanosilylation of imines, resulting in modest asymmetric induction.15d Meanwhile, copper complexes are commonly used catalysts due to their diverse catalytic reactivity and low cost. The catalytic asymmetric cyclopropanation of alkenes in the presence of
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Scheme 1 The enantiopure boxylic)-bis-amino acids.
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N,N’-(bicyclo[2,2,2]oct-7-ene-tetracar-
chiral copper complexes shows significant potential as a general method for the synthesis of optically pure cyclopropanes, which is of great importance in organic and biological chemistry. Investigations of enantioselectivity and diastereoselectivity in intermolecular cyclopropanation reactions have principally focused on diazoacetate derivatives. Exceptional enantiocontrol (up to 99% ee) has been achieved in reactions of diazoacetates with alkenes using chiral salicylaldimine-, semicorrinato-, and bis-oxazolidine-liganded Cu(II) catalysts.16 Nevertheless, high diastereoselectivity was obtained in few cases and the control of E-/Z-diastereoselectivity still remains a challenge.16f,g In addition, transition metal-catalyzed aziridination reactions have also been studied widely, considering that aziridine rings have drawn special attention as important organic intermediates and bioactive natural compounds.17 Considering the fact that a large number of copper-based MOCs have been synthesized,18,19 and few of them have been tested in carbene/nitrene transfer reactions,5c in this work, a series of homochiral Cu-containing coordination cages have been synthesized from the self-assembly of Cu(II) salts with enantiopure N,N′-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)-bisamino acids (Scheme 1). The catalytic capabilities of the obtained chiral cages in carbene/nitrene transfer reactions have been studied. To the best of our knowledge, no coordination cages have been examined in such asymmetrical cyclopropanation/aziridination reactions.
2. Experimental 2.1
General materials and methods
All the reagents in the present work were obtained from commercial sources and were used directly without further purification. The elemental analyses were performed with a PerkinElmer 240 elemental analyzer. HRESI-MS was performed using a Bruker Daltonics ESI-Q-TOF maXis4G, the data analyses of ESI-TOF mass spectra were processed using Bruker Data Analysis software and the simulations were performed using Bruker Isotope Pattern software. GC-MS data were obtained on an Agilent 7890A GC instrument coupled with a 5975 mass detector. Infrared spectra on KBr pellets were collected with a Nicolet/Nexus-670 FT-IR spectrometer in the region of 4000–400 cm−1. UV-Vis spectra were tested on a Shimadzu/ UV-3600 spectrophotometer. 1H NMR spectra were recorded
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using a Bruker AVANCE III 400 (400 MHz). Chemical shifts were quoted in parts per million ( ppm) referenced to the appropriate solvent peak or 0 ppm for TMS. HPLC spectra were obtained on Agilent 1200 series. PXRD patterns were recorded using a Bruker D8 Advance diffractometer at 40 kV and 40 mA with a Cu target tube and a graphite monochromator. Thermogravimetric (TG) analyses were performed under an N2 atmosphere at a heating rate of 10 °C min−1 using a NETZSCH TG 209 system. CD spectra were recorded on a JASCO J-810 circular dichroism spectrometer. Cautions! Although we have not experienced any problem in the handling of these compounds (e.g. methyl phenyldiazoacetate and copper(II) perchlorate), extreme care should be taken when manipulating them due to their explosive nature. 2.2
Syntheses of ligands
Four enantiopure N,N′-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)bis-amino acids were synthesized by a similar procedure as exampled by S,SL120: A solution of bicyclo[2,2,2]oct-7-ene2,3,5,6-tetracarboxylic dianhydride (1.000 g, 4.03 mmol) and L-alanine (0.718 g, 8.06 mmol) in acetic acid (50 mL) was stirred at room temperature overnight and then refluxed for 4 h. After the solution was cooled down at 0 °C overnight, a large amount of white solid was precipitated from the solution, and then washed with diethyl ether and dried under vacuum. Yield, 1.190 g (76%). 1H NMR (400 MHz, d6-DMSO) δ 12.82 (br, 2H), 6.06–5.82 (m, 2H), 4.54 (q, J = 7.1 Hz, 2H), 3.38 (s, 2H), 3.22 (m, 4H), 1.25 (d, J = 7.2 Hz, 6H). FTIR (KBr) v 2947–3413 (m, br), 1774–1753 (w), 1698 (s), 1398 (s), 1219 (m), 1126 (m), 1077 (w), 971 (w), 787 (w), 673 (w), 616 (m) cm−1. HRESI-MS ([M − H]−) Calcd for C18H17N2O8: 389.0999, found: 389.0970. R,R L1. The procedure is similar to that of S,SL1, and 1 D-alanine is used instead. Yield, 1.324 g (84%). H NMR (400 MHz, d6-DMSO) δ 12.82 (br, 2H), 6.06–5.82 (m, 2H), 4.54 (q, J = 7.1 Hz, 2H), 3.38 (s, 2H), 3.22 (m, 4H), 1.25 (d, J = 7.2 Hz, 6H). FTIR (KBr, cm−1): 2947–3413 (m, br), 1752 (w), 1697 (s), 1454 (w), 1398 (s), 1308 (w), 1221 (m), 1127 (m), 971 (w), 843 (w), 788 (w), 674 (w), 617 (m). HRESI-MS ([M − H]−) Calcd for C18H17N2O8: 389.0990, found: 389.0991. S,S L2. The procedure is similar to that of S,SL1, and L-valine is used instead. Yield, 1.450 g (81%). 1H NMR (400 MHz, d6DMSO) δ 12.81 (br, 2H), 6.07–6.01 (m, 2H), 4.14 (d, J = 7.8 Hz, 2H), 3.45–3.36 (m, 2H), 3.26 (m, 2H), 2.41–2.30 (m, 2H), 0.94 (d, J = 6.7 Hz, 6H), 0.68 (d, J = 6.8 Hz, 6H). FTIR (KBr, cm−1): 3432 (m, br), 2971 (m), 1702 (s), 1389 (s), 1200 (m), 1121 (m), 1061 (w), 874 (w), 697 (w), 637 (w), 556 (m). HRESI-MS ([M − H]−) Calcd for C22H25N2O8: 445.1616, found: 445.1620. S,S L3. The procedure is similar to that of S,SL1, and L-tertleucine is used instead. Yield, 1.383 g (72%). 1H NMR (400 MHz, d6-DMSO) δ 12.67 (br, 2H), 6.04 (s, 2H), 4.16 (s, 2H), 3.43 (s, 2H), 3.29 (m, 4H), 0.95 (s, 18H). FTIR (KBr, cm−1): 3415 (m, br), 2965 (m), 1784 (m), 1745 (m), 1705 (s), 1685 (s), 1375 (s), 1239 (s), 1152 (s), 982 (w), 878 (w), 706 (w), 634 (w), 593 (m). HRESI-MS ([M − H]−) Calcd for C24H29N2O8: 473.1929, found: 473.1933.
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2.3
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Syntheses of cage compounds
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S,S
R,R
Enantiomeric cages [Cu4( L1)4(H2O)3] (1) and [Cu4( L1)4(H2O)3] (2) were synthesized by a similar procedure as exampled by 1: Cu(ClO4)2·6H2O (0.037 g, 0.1 mmol) and S,SL1 (0.020 g, 0.05 mmol) were dissolved in 5 mL of mixed solvent of ethanol and DMF (50 : 1, v/v). Blue block crystals were obtained by vapour diffusion of diethyl ether into the solution after a few days (0.013 g, 55% yield, calculated based on the ligand). The obtained cages were found to crystallize as adducts of solvents and perchloric acid. Anal. Calcd for C74H92Cl2N8O51Cu4 (1·2HClO4·7H2O·C2H5OH): C, 39.77; H, 4.15, N, 5.01%; found: C, 39.89; H, 4.54; N, 5.49%. FTIR (KBr) v 3434 (s, br), 1766 (w), 1696 (s), 1637 (s), 1461 (w), 1398 (s), 1353 (m), 1303 (w), 1208 (m), 1122 (m), 974 (w), 805 (w), 679 (w), 647 (w) cm−1. HRMS (ESI+) Calcd for Cu4C72H65N8O32 [Cu4(R,RL1)4 + H]+: 1807.0880; found: 1807.0887. Cage [Cu4(R,RL1)4(H2O)3] (2) was prepared by the reaction with R,RL1 instead (0.011 g, 49% yield, calculated based on the ligand). Anal. Calcd for C80H94N10Cl2O47Cu4 (2·2HClO4·H2O· C2H5OH·2DMF): C, 42.28; H, 4.17, N, 6.16%; found: C, 42.18; H, 4.55; N, 6.27%. FTIR (KBr) v 3435 (m, br), 1764 (w), 1693 (s), 1638 (s), 1461 (w), 1400 (s), 1354 (m), 1304 (w), 1208 (m), 1121 (w), 973 (w), 804 (w), 778 (w), 682 (w), 645 (w), 612 (w) cm−1. HRMS (ESI+) Calcd for Cu4C72H65N8O32 [Cu4(R,RL1)4 + H]+: 1807.0880; found: 1807.0947. Cage [Cu4(S,SL2)4(CH3CN)2(H2O)] (3). Cu(NO3)2·3H2O (0.025 g, 0.1 mmol) and S,SL2 (0.022 g, 0.05 mmol) were dissolved in 5 mL acetonitrile. Blue block crystals were obtained by slow evaporation after two days (0.009 g, 35% yield, calculated based on the ligand). Anal. Calcd for C100H124N14O37Cu4 (3·4H2O·4CH3CN): C, 50.71; H, 5.28; N, 8.28%; found: C, 50.80; H, 5.20; N, 8.30%. FTIR (KBr) v 3448 (m, br), 2965 (w), 1770 (w), 1701 (s), 1639 (s), 1474 (w), 1388 (m), 1199 (m), 1068 (w), 821 (w), 792 (w), 756 (w), 691 (w), 658 (w), 605 (w) cm−1. HRMS (ESI+) Calcd for Cu4C88H97N8O32 [Cu4(S,SL2)4 + H]+: 2031.3389; found: 2031.3324. Cage [Cu4(S,SL3)4(EtOH)2(H2O)] (4). Cu(CF3SO3)2·4H2O (0.043 g, 0.1 mmol) and S,SL3 (0.024 g, 0.05 mmol) were dissolved in 5 mL ethanol. Blue block crystals were obtained by slow evaporation after one week (0.004 g, 16% yield, calculated based on the ligand). Anal. Calcd for C100H136N8O40Cu4 (4·5H2O): C, 51.23; H, 5.85, N, 4.78%; found: C, 51.12; H, 5.85; N, 5.21%. FTIR (KBr) v 3432 (m, br), 2962 (w), 1705 (s), 1639 (s), 1483 (w), 1382 (m), 1351 (m), 1305 (w), 1181 (m), 1088 (w), 883 (w), 825 (w), 801 (w), 764 (w), 691 (w), 658 (w), 596 (w), 473 (w) cm−1. HRMS (ESI+) Calcd for Cu4C96H113N8O32 [Cu4(S,SL3)4 + H]+: 2145.4611; found: 2145.4648. 2.4
General procedure for catalytic cyclopropanation
Styrene (87 μL, 0.75 mmol), Cu cage (0.015 mmol, 10 mol% relative to the diazo substrate) and solvent (0.1 mL) were added to a Schlenk tube and the mixture was heated to 40 °C (or 80 °C). A solution of methyl phenyldiazoacetate (0.026 g, 0.15 mmol) in a solvent (0.4 mL) was added to the reaction mixture over a period of 6 h through a syringe pump, and then
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the whole reaction mixture was stirred for an additional 12 h. The reaction mixture was subjected to GC-MS analysis directly, collecting information about the diastereoselectivities. To obtain the NMR yield of the cage-catalyzed reactions, the reaction solution was filtered through a pad of silica gel, and the filtrate was evaporated to dryness. Then the NMR yield was obtained by the addition of CH2Br2 (10.4 μL, 0.15 mmol) as the internal standard. The enantiomeric excesses of the isolated E-cyclopropanes were determined by HPLC with a Daicel Chiralcel OJ column (hexane–iPr = 90 : 10, flowing speed: 0.8 ml min−1, detected at 254 nm). 2.5
General procedure for catalytic aziridination
A solution of [N-(p-nitrophenylsulfonyl) imino]phenyliodinane (PhIvNNs, 0.040 g, 0.1 mmol) and Cu cage (0.01 mmol, 10 mol%) in styrene (1 mL) was heated at 80 °C under a nitrogen atmosphere. After 24 h, the reaction mixture was filtered through a pad of silica gel, and the filtrate was evaporated to dryness. The NMR yield was obtained through the addition of CH2Br2 (6.9 μL, 0.10 mmol) as the internal standard. The enantiomeric excesses of the isolated aziridines were determined by HPLC with a Daicel Chiralcel OD-H column (hexane–iPrOH = 90 : 10, flowing speed: 0.8 ml min−1, detected at 254 nm.) 2.6
X-ray crystallography
The X-ray diffraction data were collected with an Oxford Gemini S Ultra diffractometer for cages 1 and 3 and an Agilent Technologies SuperNova X-ray diffractometer system for cages 2 and 4 equipped with Cu Kα radiation (λ = 1.54178 Å). The crystals were kept at 150 or 160 K during data collection. Using Olex2,21a the structure of 4 was solved with the ShelXS21b structure solution program by direct methods and refined with the ShelXL21b refinement package using least squares minimization. The solution of the structures and the refinement of 1, 2 and 3 were carried out with the SHELXTL program package.21c The non-coordinate lattice solvent molecules of cages 1, 2 and 4 are highly disordered, and attempts to locate and refine the solvent peaks were unsuccessful. Contributions to scattering due to these solvent molecules were removed using the SQUEEZE routine of PLATON.21d The structures were then refined again using the data generated. The positions of the hydrogen atoms were generated geometrically. A summary of the crystal structure refinement data is shown in Table 1, and selected bond angles and distances are listed in Table S1.† Crystallographic data for the structures reported in this paper have been deposited in the Cambridge Crystallographic Data Center with CCDC reference numbers 1038148 (1), 1038149 (2), 1038150 (3) and 1038151 (4).
3. Results and discussion 3.1
Ligand considerations
Amino acids, especially natural ones, are commonly used as chiral adducts in asymmetric catalysis, due to the accessibility.
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Table 1
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Formula Fw T/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° Volume/Å3 Z ρcalc/g cm−3 μ/mm−1 F(000) Total/unique Rint Goodness-of-fit on F2 R1, wR2 [I ≥ 2σ (I)] R1, wR2 [all data] Flack parameter
1·2HClO4
2·2HClO4
3·3CH3CN
4
C72H66N8O43Cu4Cl2 2056.39 150 Trigonal P321 26.9516(2) 26.9516(2) 13.0398(2) 90.00 90.00 120.00 8202.96(15) 3 1.249 2.024 3144 16 318/8177 0.0408 1.054 0.0613, 0.1678 0.0637, 0.1710 0.02(3)
C72H66N8O43Cu4Cl2 2056.39 150 Trigonal P321 26.9979(4) 26.9979(4) 13.0953(2) 90 90 120 8266.2(2) 3 1.239 2.008 3144 38 856/9279 0.0447 1.060 0.0807, 0.2077 0.0824, 0.2092 0.00(6)
C103H120N16O33Cu4 2364.31 160 Trigonal P3221 16.74461(11) 16.74461(11) 34.2624(3) 90.00 90.00 120.00 8319.52(10) 3 1.416 1.587 3690 76 532/8491 0.0302, 3.0.0302 1.073 0.0247, 0.0643 0.0249, 0.0645 −0.006(15)
C300H345N24O105Cu12 6729.46 150 triclinic P1 17.37579(16) 18.2874(2) 32.7446(2) 77.6702(8) 78.5238(7) 64.3532(10) 9096.12(17) 1 1.228 1.417 3501 150 437/58 612 0.0434 1.045 0.0449, 0.1183 0.0494, 0.1226 0.009(6)
They can react with transition metal salts to form metal carboxylates, which are found to be excellent catalysts. Due to the good coordination ability of amino groups to the active metal centers, which then might poison the catalysts, the amino groups in these amino acids need to be protected. Usually, amino acids firstly react with cyclic anhydrides, and are transformed to imides. Two famous chiral ligand series based on amino acids are N-phthaloyl-amino acids and N-naphthoylamino acids, which were developed by the Hashimoto group22a and the Müller group,22b respectively, and were obtained from the reactions of amino acids with phthalic anhydride and 1,8-naphthalic anhydride, respectively. N-Phthaloyl-amino acids and N-naphthoyl-amino acids have been successfully employed as the chiral ligands of dirhodium(II) carboxylate catalysts, which promote a large number of organic reactions, such as aziridination, cyclopropanation, and C–H bond activation reactions, with excellent diastereoselectivities and enantioselectivities.22 On the other hand, ditopic acids, in which two –COOH groups are linked together by an organic bridge with an angle of 0, 60, 90 or 120°, are commonly used for the syntheses of coordination cages.23 To design a chiral dicarboxylate ligand, we can connect two enantiopure amino acids by a dianhydride bridge with varying angles. In order to construct a cage structure, flexible dianhydrides might be better, considering that the resulting ligands have an adaptive ability to match the symmetry required for cage formation, whereas the rigid bridges like pyromellitic dianhydride often prefer to assemble infinite frameworks.24 In this work, we designed ligands S,SL1, S,SL2 and S,SL3 and R,R L1 for the construction of homochiral coordination cages, and their bridges are based on the flexible bicyclo[2,2,2]oct-7ene-2,3,5,6-tetracarboxylic dianhydride. S,SL1 and R,RL1 are a pair of enantiomers, whereas S,SL1, S,SL2 and S,SL3 are in the same absolute chirality but are different in steric hindrance
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(e.g. methyl, isopropyl and tert-butyl groups are attached to the chiral carbons). It is known that steric hindrance is of remarkable importance for the resulting enantiomeric excess of the catalytic reactions. 3.2
Syntheses and crystal structures
The reaction of S,SL1 and Cu(ClO4)2 in EtOH–DMF (50 : 1) gave rise to blue block crystals of cage [Cu4(S,SL1)4(H2O)3] (1). Single-crystal X-ray diffraction analysis revealed that 1 crystallizes in the chiral space group P321. Each of the two Cu(II) cations is bridged by four carboxylate groups to form a Cu2(CO2)4 paddle-wheel secondary building unit (SBU), and two such SBUs are further connected by four S,SL1 ligands to result in a discrete (Cu2)2L4 cage structure (Fig. 1a). The Cu–Cu distance (2.574 and 2.566 Å) in the dinuclear Cu2(CO2)4 SBU is well below the sum of van der Waals radii of two Cu atoms (2.8 Å) and is slightly longer than the Cu–Cu distance of 2.56 Å in metallic.25 If the Cu–Cu contact was neglected, each Cu atom pointing away from the center of the cage is pentacoordinated to four O atoms of carboxylate groups from four S,SL1 ligands in the equatorial plane and one water molecule in the axial direction. The τ values of 0 for the two outer Cu atoms indicate an almost ideal square-pyramid coordination environment.26 The distance of Cu–Oaxial (2.089 Å for the shortest) is definitely longer than that of Cu–Oequatorial (
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Homochiral coordination cages assembled from dinuclear paddlewheel nodes and enantiopure ditopic ligands: syntheses, structures and catalysis† Lianfen Chen, Jian Kang, Hao Cui, Yingxia Wang, Lan Liu, Li Zhang* and Cheng-Yong Su* A series of homochiral metal–organic cages (MOCs) have been obtained from self-assembly of Cu(II) salts with chiral N,N’-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)-bis-amino acids. Single-crystal X-ray diffraction
Received 10th December 2014, Accepted 19th February 2015 DOI: 10.1039/c4dt03782k www.rsc.org/dalton
1.
analyses reveal that these compounds show a lantern-type cage structure, in which one pair of Cu2(CO2)4 paddlewheels is linked by four diacid ligands. The resulting homochiral cages have been fully characterized by EA, TOF-MS, TGA, VTPXRD, IR, UV, and CD measurements. The catalytic tests reveal that these Cu(II) cages are effective in cyclopropanation with excellent diastereoselectivity (up to 99 : 1 E/Z). In addition, the cage catalysts can promote the aziridination reaction with PhIvNNs.
Introduction
Assemblies of metal–organic containers, including so-called metal–organic polyhedra (MOPs) or metal–organic cages (MOCs), rings and tubes,1 have attracted significant research attention in the past two decades due to their applications in the stabilization or protection of sensitive species,2 drug delivery,3 selective recognition or separation of species from a mixture,4 catalysis,5,6 the formation of unusual products under common conditions,7 etc. Since the creation of a chiral environment can facilitate the synthesis and separation of enantiomerically pure compounds,8 assembly of chiral MOCs is of understandable importance. Chirality may be introduced from achiral precursors by symmetry breaking during the selfassembly process, either from the metal center9 or the twisted ligand.10 For example, Saalfrank and co-workers reported tetrahedral M4L6 (M = MnII, CoII, NiII) MOCs with the chirality arising from the configuration of the octahedral metal centers (Δ or Λ).9a Robson described a capsule M12L8 (M = CuII) in which the ligands served as the corners and the metals as the edges, and the chirality was based on the twisted ligands.10a
MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry and Chemical Engineering, Sun yat-sen University, Guangzhou, 510275, P. R. China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: TOF-MS spectra, PXRD and CD spectra of the cage compounds. CIF files giving crystallographic data. CCDC 1038148–1038151. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03782k
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Alternatively, chirality can also be inherently embedded in MOCs by using chiral organic ligands as linkers11 or an auxiliary attached to the metal center.12 However, due to difficulties in synthesizing enantiomerically pure bridging ligands or auxiliaries, homochiral MOCs based on chiral starting materials are much fewer than racemizing MOCs prepared by the first method. Among limited examples, Stack et al. reported an M4L6 tetrahedral cluster assembled from GaIII and an enantiopure bis(catecholate) ligand.11a Fujita’s group utilized PdII with (R,R)- or (S,S)-diaminocyclohexane as an auxiliary for the self-assembly of optically active M6L4 truncated tetrahedra.12a Despite the availability of many chiral MOCs synthesized by the above two methods,13,14 only a limited number of chiral MOCs have been further applied in asymmetric catalysis.15 Fujita et al. have utilized homochiral M6L4 octahedra based on chiral diamine auxiliaries for asymmetric [2 + 2] olefin cross photoaddition, in which enantiomeric excesses of up to 50% were achieved.15a Raymond and coworkers have developed enantiopure M4L6 tetrahedra either by resolution of racemate or by chiral ligand directed assembly, and used them in enantioselective catalysis, such as Aza-Cope rearrangement and carbonyl-ene cyclization, yielding up to 78% and 69% ee, respectively.15b,c Duan and Zhang et al. have reported a chiral M3L2 cylinder-like cage, which was then applied in the asymmetric cyanosilylation of imines, resulting in modest asymmetric induction.15d Meanwhile, copper complexes are commonly used catalysts due to their diverse catalytic reactivity and low cost. The catalytic asymmetric cyclopropanation of alkenes in the presence of
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Scheme 1 The enantiopure boxylic)-bis-amino acids.
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N,N’-(bicyclo[2,2,2]oct-7-ene-tetracar-
chiral copper complexes shows significant potential as a general method for the synthesis of optically pure cyclopropanes, which is of great importance in organic and biological chemistry. Investigations of enantioselectivity and diastereoselectivity in intermolecular cyclopropanation reactions have principally focused on diazoacetate derivatives. Exceptional enantiocontrol (up to 99% ee) has been achieved in reactions of diazoacetates with alkenes using chiral salicylaldimine-, semicorrinato-, and bis-oxazolidine-liganded Cu(II) catalysts.16 Nevertheless, high diastereoselectivity was obtained in few cases and the control of E-/Z-diastereoselectivity still remains a challenge.16f,g In addition, transition metal-catalyzed aziridination reactions have also been studied widely, considering that aziridine rings have drawn special attention as important organic intermediates and bioactive natural compounds.17 Considering the fact that a large number of copper-based MOCs have been synthesized,18,19 and few of them have been tested in carbene/nitrene transfer reactions,5c in this work, a series of homochiral Cu-containing coordination cages have been synthesized from the self-assembly of Cu(II) salts with enantiopure N,N′-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)-bisamino acids (Scheme 1). The catalytic capabilities of the obtained chiral cages in carbene/nitrene transfer reactions have been studied. To the best of our knowledge, no coordination cages have been examined in such asymmetrical cyclopropanation/aziridination reactions.
2. Experimental 2.1
General materials and methods
All the reagents in the present work were obtained from commercial sources and were used directly without further purification. The elemental analyses were performed with a PerkinElmer 240 elemental analyzer. HRESI-MS was performed using a Bruker Daltonics ESI-Q-TOF maXis4G, the data analyses of ESI-TOF mass spectra were processed using Bruker Data Analysis software and the simulations were performed using Bruker Isotope Pattern software. GC-MS data were obtained on an Agilent 7890A GC instrument coupled with a 5975 mass detector. Infrared spectra on KBr pellets were collected with a Nicolet/Nexus-670 FT-IR spectrometer in the region of 4000–400 cm−1. UV-Vis spectra were tested on a Shimadzu/ UV-3600 spectrophotometer. 1H NMR spectra were recorded
Dalton Trans.
using a Bruker AVANCE III 400 (400 MHz). Chemical shifts were quoted in parts per million ( ppm) referenced to the appropriate solvent peak or 0 ppm for TMS. HPLC spectra were obtained on Agilent 1200 series. PXRD patterns were recorded using a Bruker D8 Advance diffractometer at 40 kV and 40 mA with a Cu target tube and a graphite monochromator. Thermogravimetric (TG) analyses were performed under an N2 atmosphere at a heating rate of 10 °C min−1 using a NETZSCH TG 209 system. CD spectra were recorded on a JASCO J-810 circular dichroism spectrometer. Cautions! Although we have not experienced any problem in the handling of these compounds (e.g. methyl phenyldiazoacetate and copper(II) perchlorate), extreme care should be taken when manipulating them due to their explosive nature. 2.2
Syntheses of ligands
Four enantiopure N,N′-(bicyclo[2,2,2]oct-7-ene-tetracarboxylic)bis-amino acids were synthesized by a similar procedure as exampled by S,SL120: A solution of bicyclo[2,2,2]oct-7-ene2,3,5,6-tetracarboxylic dianhydride (1.000 g, 4.03 mmol) and L-alanine (0.718 g, 8.06 mmol) in acetic acid (50 mL) was stirred at room temperature overnight and then refluxed for 4 h. After the solution was cooled down at 0 °C overnight, a large amount of white solid was precipitated from the solution, and then washed with diethyl ether and dried under vacuum. Yield, 1.190 g (76%). 1H NMR (400 MHz, d6-DMSO) δ 12.82 (br, 2H), 6.06–5.82 (m, 2H), 4.54 (q, J = 7.1 Hz, 2H), 3.38 (s, 2H), 3.22 (m, 4H), 1.25 (d, J = 7.2 Hz, 6H). FTIR (KBr) v 2947–3413 (m, br), 1774–1753 (w), 1698 (s), 1398 (s), 1219 (m), 1126 (m), 1077 (w), 971 (w), 787 (w), 673 (w), 616 (m) cm−1. HRESI-MS ([M − H]−) Calcd for C18H17N2O8: 389.0999, found: 389.0970. R,R L1. The procedure is similar to that of S,SL1, and 1 D-alanine is used instead. Yield, 1.324 g (84%). H NMR (400 MHz, d6-DMSO) δ 12.82 (br, 2H), 6.06–5.82 (m, 2H), 4.54 (q, J = 7.1 Hz, 2H), 3.38 (s, 2H), 3.22 (m, 4H), 1.25 (d, J = 7.2 Hz, 6H). FTIR (KBr, cm−1): 2947–3413 (m, br), 1752 (w), 1697 (s), 1454 (w), 1398 (s), 1308 (w), 1221 (m), 1127 (m), 971 (w), 843 (w), 788 (w), 674 (w), 617 (m). HRESI-MS ([M − H]−) Calcd for C18H17N2O8: 389.0990, found: 389.0991. S,S L2. The procedure is similar to that of S,SL1, and L-valine is used instead. Yield, 1.450 g (81%). 1H NMR (400 MHz, d6DMSO) δ 12.81 (br, 2H), 6.07–6.01 (m, 2H), 4.14 (d, J = 7.8 Hz, 2H), 3.45–3.36 (m, 2H), 3.26 (m, 2H), 2.41–2.30 (m, 2H), 0.94 (d, J = 6.7 Hz, 6H), 0.68 (d, J = 6.8 Hz, 6H). FTIR (KBr, cm−1): 3432 (m, br), 2971 (m), 1702 (s), 1389 (s), 1200 (m), 1121 (m), 1061 (w), 874 (w), 697 (w), 637 (w), 556 (m). HRESI-MS ([M − H]−) Calcd for C22H25N2O8: 445.1616, found: 445.1620. S,S L3. The procedure is similar to that of S,SL1, and L-tertleucine is used instead. Yield, 1.383 g (72%). 1H NMR (400 MHz, d6-DMSO) δ 12.67 (br, 2H), 6.04 (s, 2H), 4.16 (s, 2H), 3.43 (s, 2H), 3.29 (m, 4H), 0.95 (s, 18H). FTIR (KBr, cm−1): 3415 (m, br), 2965 (m), 1784 (m), 1745 (m), 1705 (s), 1685 (s), 1375 (s), 1239 (s), 1152 (s), 982 (w), 878 (w), 706 (w), 634 (w), 593 (m). HRESI-MS ([M − H]−) Calcd for C24H29N2O8: 473.1929, found: 473.1933.
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2.3
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Syntheses of cage compounds
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S,S
R,R
Enantiomeric cages [Cu4( L1)4(H2O)3] (1) and [Cu4( L1)4(H2O)3] (2) were synthesized by a similar procedure as exampled by 1: Cu(ClO4)2·6H2O (0.037 g, 0.1 mmol) and S,SL1 (0.020 g, 0.05 mmol) were dissolved in 5 mL of mixed solvent of ethanol and DMF (50 : 1, v/v). Blue block crystals were obtained by vapour diffusion of diethyl ether into the solution after a few days (0.013 g, 55% yield, calculated based on the ligand). The obtained cages were found to crystallize as adducts of solvents and perchloric acid. Anal. Calcd for C74H92Cl2N8O51Cu4 (1·2HClO4·7H2O·C2H5OH): C, 39.77; H, 4.15, N, 5.01%; found: C, 39.89; H, 4.54; N, 5.49%. FTIR (KBr) v 3434 (s, br), 1766 (w), 1696 (s), 1637 (s), 1461 (w), 1398 (s), 1353 (m), 1303 (w), 1208 (m), 1122 (m), 974 (w), 805 (w), 679 (w), 647 (w) cm−1. HRMS (ESI+) Calcd for Cu4C72H65N8O32 [Cu4(R,RL1)4 + H]+: 1807.0880; found: 1807.0887. Cage [Cu4(R,RL1)4(H2O)3] (2) was prepared by the reaction with R,RL1 instead (0.011 g, 49% yield, calculated based on the ligand). Anal. Calcd for C80H94N10Cl2O47Cu4 (2·2HClO4·H2O· C2H5OH·2DMF): C, 42.28; H, 4.17, N, 6.16%; found: C, 42.18; H, 4.55; N, 6.27%. FTIR (KBr) v 3435 (m, br), 1764 (w), 1693 (s), 1638 (s), 1461 (w), 1400 (s), 1354 (m), 1304 (w), 1208 (m), 1121 (w), 973 (w), 804 (w), 778 (w), 682 (w), 645 (w), 612 (w) cm−1. HRMS (ESI+) Calcd for Cu4C72H65N8O32 [Cu4(R,RL1)4 + H]+: 1807.0880; found: 1807.0947. Cage [Cu4(S,SL2)4(CH3CN)2(H2O)] (3). Cu(NO3)2·3H2O (0.025 g, 0.1 mmol) and S,SL2 (0.022 g, 0.05 mmol) were dissolved in 5 mL acetonitrile. Blue block crystals were obtained by slow evaporation after two days (0.009 g, 35% yield, calculated based on the ligand). Anal. Calcd for C100H124N14O37Cu4 (3·4H2O·4CH3CN): C, 50.71; H, 5.28; N, 8.28%; found: C, 50.80; H, 5.20; N, 8.30%. FTIR (KBr) v 3448 (m, br), 2965 (w), 1770 (w), 1701 (s), 1639 (s), 1474 (w), 1388 (m), 1199 (m), 1068 (w), 821 (w), 792 (w), 756 (w), 691 (w), 658 (w), 605 (w) cm−1. HRMS (ESI+) Calcd for Cu4C88H97N8O32 [Cu4(S,SL2)4 + H]+: 2031.3389; found: 2031.3324. Cage [Cu4(S,SL3)4(EtOH)2(H2O)] (4). Cu(CF3SO3)2·4H2O (0.043 g, 0.1 mmol) and S,SL3 (0.024 g, 0.05 mmol) were dissolved in 5 mL ethanol. Blue block crystals were obtained by slow evaporation after one week (0.004 g, 16% yield, calculated based on the ligand). Anal. Calcd for C100H136N8O40Cu4 (4·5H2O): C, 51.23; H, 5.85, N, 4.78%; found: C, 51.12; H, 5.85; N, 5.21%. FTIR (KBr) v 3432 (m, br), 2962 (w), 1705 (s), 1639 (s), 1483 (w), 1382 (m), 1351 (m), 1305 (w), 1181 (m), 1088 (w), 883 (w), 825 (w), 801 (w), 764 (w), 691 (w), 658 (w), 596 (w), 473 (w) cm−1. HRMS (ESI+) Calcd for Cu4C96H113N8O32 [Cu4(S,SL3)4 + H]+: 2145.4611; found: 2145.4648. 2.4
General procedure for catalytic cyclopropanation
Styrene (87 μL, 0.75 mmol), Cu cage (0.015 mmol, 10 mol% relative to the diazo substrate) and solvent (0.1 mL) were added to a Schlenk tube and the mixture was heated to 40 °C (or 80 °C). A solution of methyl phenyldiazoacetate (0.026 g, 0.15 mmol) in a solvent (0.4 mL) was added to the reaction mixture over a period of 6 h through a syringe pump, and then
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the whole reaction mixture was stirred for an additional 12 h. The reaction mixture was subjected to GC-MS analysis directly, collecting information about the diastereoselectivities. To obtain the NMR yield of the cage-catalyzed reactions, the reaction solution was filtered through a pad of silica gel, and the filtrate was evaporated to dryness. Then the NMR yield was obtained by the addition of CH2Br2 (10.4 μL, 0.15 mmol) as the internal standard. The enantiomeric excesses of the isolated E-cyclopropanes were determined by HPLC with a Daicel Chiralcel OJ column (hexane–iPr = 90 : 10, flowing speed: 0.8 ml min−1, detected at 254 nm). 2.5
General procedure for catalytic aziridination
A solution of [N-(p-nitrophenylsulfonyl) imino]phenyliodinane (PhIvNNs, 0.040 g, 0.1 mmol) and Cu cage (0.01 mmol, 10 mol%) in styrene (1 mL) was heated at 80 °C under a nitrogen atmosphere. After 24 h, the reaction mixture was filtered through a pad of silica gel, and the filtrate was evaporated to dryness. The NMR yield was obtained through the addition of CH2Br2 (6.9 μL, 0.10 mmol) as the internal standard. The enantiomeric excesses of the isolated aziridines were determined by HPLC with a Daicel Chiralcel OD-H column (hexane–iPrOH = 90 : 10, flowing speed: 0.8 ml min−1, detected at 254 nm.) 2.6
X-ray crystallography
The X-ray diffraction data were collected with an Oxford Gemini S Ultra diffractometer for cages 1 and 3 and an Agilent Technologies SuperNova X-ray diffractometer system for cages 2 and 4 equipped with Cu Kα radiation (λ = 1.54178 Å). The crystals were kept at 150 or 160 K during data collection. Using Olex2,21a the structure of 4 was solved with the ShelXS21b structure solution program by direct methods and refined with the ShelXL21b refinement package using least squares minimization. The solution of the structures and the refinement of 1, 2 and 3 were carried out with the SHELXTL program package.21c The non-coordinate lattice solvent molecules of cages 1, 2 and 4 are highly disordered, and attempts to locate and refine the solvent peaks were unsuccessful. Contributions to scattering due to these solvent molecules were removed using the SQUEEZE routine of PLATON.21d The structures were then refined again using the data generated. The positions of the hydrogen atoms were generated geometrically. A summary of the crystal structure refinement data is shown in Table 1, and selected bond angles and distances are listed in Table S1.† Crystallographic data for the structures reported in this paper have been deposited in the Cambridge Crystallographic Data Center with CCDC reference numbers 1038148 (1), 1038149 (2), 1038150 (3) and 1038151 (4).
3. Results and discussion 3.1
Ligand considerations
Amino acids, especially natural ones, are commonly used as chiral adducts in asymmetric catalysis, due to the accessibility.
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Table 1
Dalton Transactions Crystal data and structure refinement parameters for compounds 1, 2, 3 and 4
Formula Fw T/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° Volume/Å3 Z ρcalc/g cm−3 μ/mm−1 F(000) Total/unique Rint Goodness-of-fit on F2 R1, wR2 [I ≥ 2σ (I)] R1, wR2 [all data] Flack parameter
1·2HClO4
2·2HClO4
3·3CH3CN
4
C72H66N8O43Cu4Cl2 2056.39 150 Trigonal P321 26.9516(2) 26.9516(2) 13.0398(2) 90.00 90.00 120.00 8202.96(15) 3 1.249 2.024 3144 16 318/8177 0.0408 1.054 0.0613, 0.1678 0.0637, 0.1710 0.02(3)
C72H66N8O43Cu4Cl2 2056.39 150 Trigonal P321 26.9979(4) 26.9979(4) 13.0953(2) 90 90 120 8266.2(2) 3 1.239 2.008 3144 38 856/9279 0.0447 1.060 0.0807, 0.2077 0.0824, 0.2092 0.00(6)
C103H120N16O33Cu4 2364.31 160 Trigonal P3221 16.74461(11) 16.74461(11) 34.2624(3) 90.00 90.00 120.00 8319.52(10) 3 1.416 1.587 3690 76 532/8491 0.0302, 3.0.0302 1.073 0.0247, 0.0643 0.0249, 0.0645 −0.006(15)
C300H345N24O105Cu12 6729.46 150 triclinic P1 17.37579(16) 18.2874(2) 32.7446(2) 77.6702(8) 78.5238(7) 64.3532(10) 9096.12(17) 1 1.228 1.417 3501 150 437/58 612 0.0434 1.045 0.0449, 0.1183 0.0494, 0.1226 0.009(6)
They can react with transition metal salts to form metal carboxylates, which are found to be excellent catalysts. Due to the good coordination ability of amino groups to the active metal centers, which then might poison the catalysts, the amino groups in these amino acids need to be protected. Usually, amino acids firstly react with cyclic anhydrides, and are transformed to imides. Two famous chiral ligand series based on amino acids are N-phthaloyl-amino acids and N-naphthoylamino acids, which were developed by the Hashimoto group22a and the Müller group,22b respectively, and were obtained from the reactions of amino acids with phthalic anhydride and 1,8-naphthalic anhydride, respectively. N-Phthaloyl-amino acids and N-naphthoyl-amino acids have been successfully employed as the chiral ligands of dirhodium(II) carboxylate catalysts, which promote a large number of organic reactions, such as aziridination, cyclopropanation, and C–H bond activation reactions, with excellent diastereoselectivities and enantioselectivities.22 On the other hand, ditopic acids, in which two –COOH groups are linked together by an organic bridge with an angle of 0, 60, 90 or 120°, are commonly used for the syntheses of coordination cages.23 To design a chiral dicarboxylate ligand, we can connect two enantiopure amino acids by a dianhydride bridge with varying angles. In order to construct a cage structure, flexible dianhydrides might be better, considering that the resulting ligands have an adaptive ability to match the symmetry required for cage formation, whereas the rigid bridges like pyromellitic dianhydride often prefer to assemble infinite frameworks.24 In this work, we designed ligands S,SL1, S,SL2 and S,SL3 and R,R L1 for the construction of homochiral coordination cages, and their bridges are based on the flexible bicyclo[2,2,2]oct-7ene-2,3,5,6-tetracarboxylic dianhydride. S,SL1 and R,RL1 are a pair of enantiomers, whereas S,SL1, S,SL2 and S,SL3 are in the same absolute chirality but are different in steric hindrance
Dalton Trans.
(e.g. methyl, isopropyl and tert-butyl groups are attached to the chiral carbons). It is known that steric hindrance is of remarkable importance for the resulting enantiomeric excess of the catalytic reactions. 3.2
Syntheses and crystal structures
The reaction of S,SL1 and Cu(ClO4)2 in EtOH–DMF (50 : 1) gave rise to blue block crystals of cage [Cu4(S,SL1)4(H2O)3] (1). Single-crystal X-ray diffraction analysis revealed that 1 crystallizes in the chiral space group P321. Each of the two Cu(II) cations is bridged by four carboxylate groups to form a Cu2(CO2)4 paddle-wheel secondary building unit (SBU), and two such SBUs are further connected by four S,SL1 ligands to result in a discrete (Cu2)2L4 cage structure (Fig. 1a). The Cu–Cu distance (2.574 and 2.566 Å) in the dinuclear Cu2(CO2)4 SBU is well below the sum of van der Waals radii of two Cu atoms (2.8 Å) and is slightly longer than the Cu–Cu distance of 2.56 Å in metallic.25 If the Cu–Cu contact was neglected, each Cu atom pointing away from the center of the cage is pentacoordinated to four O atoms of carboxylate groups from four S,SL1 ligands in the equatorial plane and one water molecule in the axial direction. The τ values of 0 for the two outer Cu atoms indicate an almost ideal square-pyramid coordination environment.26 The distance of Cu–Oaxial (2.089 Å for the shortest) is definitely longer than that of Cu–Oequatorial (
