Article pubs.acs.org/cm
Chiral Periodic Mesoporous Organosilicas: Probing Chiral Induction in the Solid State Michael W. A. MacLean,† Thomas K. Wood,† Gang Wu,† Robert P. Lemieux,† and Cathleen M. Crudden*,†,‡ †
Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6 Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa, Nagoya 464-8602, Japan
‡
S Supporting Information *
ABSTRACT: The production of a variety of PMO materials made with 1,1′-biphenylene monomers, and the inclusion of 1,1′binaphthalene dopants is described. The effect of the point of attachment with regard to the biaryl axis was studied, and it was found that the highest level of chirality transfer is obtained when the biaryl axis is not restricted by the polymerization point. In addition, the actual loading of dopant was found to vary considerably depending on the synthesis procedure, and the effect of dopant, even at low level, was magnified under basic conditions when more ordered materials are produced.
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INTRODUCTION Porous solid-state materials that possess chirality have many potential applications in catalysis and chromatography,1,2 and thus well-understood methods for the incorporation of chirality into such materials are crucial.3−5 Since periodic mesoporous organosilica materials (PMOs) are made entirely of organosilica monomers,3−5 they permit incorporation of a substantial amount of the chiral source, which should increase the effective chiral functionality of the resulting material. Currently there are two main methods for the incorporation of chirality into PMO materials; the first and most widely studied involves the use of a single enantiomer of a bis-siloxane precursor bridged by a chiral organic group (Figure 1).6−9 An alternative strategy involves
the production of PMO materials using catalytic amounts of a resolved, chiral monomer along with a prochiral monomer whose conformational distribution can be perturbed by the chiral “dopant”.10 This strategy has the advantage that chirality can be transmitted through the material by employing only smaller quantities of chiral dopant, similar to the sergeants and soldiers effect in liquid crystals.11−13 For example, the biphenyl-bridged siloxane precursor 1 has chiral conformations about the biaryl axis; however, because of the low barrier to rotation (10−15 kcal/mol), 14 the enantiomeric conformations equilibrate at room temperature.14,15 Molecules with higher rotational barriers about the central C−C biaryl bond coexist as two atropisomers16 that are stable at room temperature and higher, and thus their enantiomeric forms can be resolved.17 The inclusion of bissiloxanes derived from such biaryls into silica materials permits incorporation of chirality into the material. Furthermore, the presence of resolved chiral biaryl species such as 2−9 in a bulk PMO comprised of fluxionally chiral monomers such as 1 provides an intriguing possibility to transfer of chirality in the solid state from dopant to “host”.10,18,19 We first reported this Received: May 21, 2014 Revised: September 10, 2014 Published: September 14, 2014
Figure 1. Production of chiral PMOs from chiral bridged siloxane precursors. © 2014 American Chemical Society
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concept in 2008 using the resolvable diester 2 as a chiral dopant and monomer 1 as host.10 Dopant 2 was shown to be effective at chirality transfer in the solid state; however, the synthesis of this monomer was tedious and not scalable.10,20 We thus turned to the 1,1′-binaphthalene family of molecules since they are considered to have “privileged” structures in terms of chirality transmission in asymmetric catalysis and materials chemistry.21,22 Several examples of 1,1′-binaphthalene-bridged siloxane precursors used in the formation of PMO materials can be found in the literature,8,9,18,19,23−30 and selections of these are shown in Figure 2.8,9,18,19,23,24 These include compounds such as 4,8 5,23 Figure 3. Cartoon representations of a 1,1′-binaphthalene skeleton with attachment points at the 3,3′- (a), 6,6′- (b), 5,5′- (c), and 4,4′(d) positions. In (d), the range of conformations accessible at room temperature is shown, with the green conformation being the most favorable.
in the gas phase, conformers with dihedral angles between 30 to 150° are all within less than 10 kcal/mol of each other. Thus, introduction of the siloxane groups along the biaryl axis, as in 9, provides the critical opportunity for the molecule to equilibrate about its most favorable dihedral angle and would presumably maximize the chirality transfer in the solid state. In this report, we present the synthesis of 1,1′-binaphthalenebridged siloxane precursor 9 (Figure 2), which features the point of attachment into the material along the biaryl axis at the 4 and 4′-positions of the 1,1′-binaphthalene skeleton. We also report a variety of materials made using siloxane precursor 9 with both the biphenyl-bridged siloxane precursor 1 (BTEBp) and the inorganic tetraethoxysilane (TEOS) as the major constituents of the siloxane materials under both alkaline and acidic conditions. By comparing the resulting chiral materials with similar materials made using siloxane precursor 6, we are able to gain new insights into the importance of the anchoring point as well as the relative importance of meso and molecular scale periodicity on chirality transfer in the solid state. Although many materials have been made using biaryl structures, this point has been largely ignored prior to this study.
Figure 2. Representative organic bridged siloxane precursors with biaryl structures.
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and 718 in which the polymerizable silane is bound directly to the aromatic framework, as well as several examples where this group is bound through a more flexible tether, i.e., 38 and 8.24 Due to synthetic limitations, all of the 1,1′-binaphthalenebridged dopants described thus far (3−8) feature the polymerizable silane (i.e., Si(OEt)3) at positions off the biaryl (atropisomeric) axis. This is potentially problematic since, although the two atropisomers cannot interconvert in highly substituted biaryls such as these, the specific dihedral angle between the two aryl rings is still critically important for the transmission of chirality31,32 and in catalytic applications since the dihedral angle can have a significant effect on catalytic activity.33,34 Thus, having the polymerizable group in positions not aligned with the rotational axis creates the possibility of freezing the dopant molecules with significantly different dihedral angles, which would likely result in poor chiral induction due to a plurality of conformations. This is shown schematically in Figure 3, where the black rods represent the anchoring points. Interestingly as shown in Figure 3d, the structure shown in green represents the optimal conformation of the molecule (see Supporting Information), but yellow and even red conformations are all accessible at room temperature. Remarkably, although a dihedral angle of close to 90° is optimal
EXPERIMENTAL SECTION
All reagents and solvents were purchased from Aldrich, Acros, or Tokyo Chemical Industry and used as received. Triethylamine and diethyl ether were dried over calcium hydride and sodium, respectively, and then distilled prior to use. Solution NMR spectra were recorded on Bruker Avance 400 (BBFO probe) or Bruker Avance 500 (BBFO probe) instruments and shifts are reported in ppm relative to the residual solvent. High-resolution mass-spectra (HRMS) were measured by the Queen’s Mass Spectrometry and Proteomics Unit (MSPU) at Queen’s University, Kingston, Ontario, Canada. Mass spectra were measured on Applied Biosystems/MDS Sciex QStar XL QqTOF or Waters ZQ Single Quad. Fragment signals are given in mass-to-charge ratios (m/z). TEM measurements were taken at the Microscopy and Microanalysis facility at the University of New Brunswick, Fredericton, New Brunswick, Canada. Images were taken from the thin edges of particles supported on a porous carbon grid using a JEOL 2010 STEM instrument operating at 200 keV. The sample was suspended in ethanol using ultrasound, after which a droplet of the suspension was dried on the grid. Nitrogen adsorption/desorption isotherms were measured at 77 K with a Micromeritics ASAP 2010. The surface area was calculated by the Brunauer−Emmett−Teller (BET) method, and the pore size was obtained using a DFT method with medium fit. Solid-state CP/MAS 13C and 29Si NMR measurements were recorded on a Bruker Avance 600 spectrometer operating at 150.9 5853
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and 119.2 MHz for 13C and 29Si, respectively, and using a Bruker 4 mm CP MAS probe. A cross-polarization contact time of 2 ms was used to acquire 29Si and 13C CP/MAS spectra with a repetition delay of 2 s. Circular dichroism (CD) spectra were obtained on solid samples measured through diffuse reflectance with powder samples of undiluted materials being held against a quartz window at the back of an integrating sphere. The spectra were obtained using a JASCO J815 CD Spectrometer with DRCD 466L attachment. X-ray diffraction data were collected at the McMaster Analytical Xray Diffraction Facility (MAX), Hamilton, Ontario, Canada. Data were collected using a SMART6000 area detector with a fixed chi, three circle goniometer, parallel beam optics, and Rigaku Cu rotating anode operating at 50 kV and 90 mA. Powder samples were loaded between two thin Mylar polymer sheets supported by a steel frame. Data were obtained in transmission mode, and a blank, consisting of two Mylar sheets with no powder in between was collected and subtracted from each sample frame obtained. Dopant incorporation levels were determined from solid-state 13C CP/MAS NMR spectra obtained under the nonquaternary suppression (NQS) condition. A 13C π pulse of 7.0 μs was inserted in the midpoint of the dipolar dephasing period to ensure proper phases in all spinning sidebands. The dipolar dephasing period was also synchronized with the rotor period (Tr). A sample spinning frequency of 8.0 kHz (Tr = 125 μs) was chosen to avoid spectral overlaps between the methoxy and aromatic carbon signals. For the organosilica samples examined in this study, the optimal dipolar dephasing period was found to be 500 μs (4 × Tr). Monomer Synthesis. 2,2′-Dimethoxy-4,4′-bis(2-triethoxysilyl)ethynyl-1,1′-binaphthalene (9). In a typical procedure, 2,2′dimethoxy-4,4′-dibromo-1,1′-binaphthalene (21, 120 mg, 0.25 mmol, see Supporting Information for synthesis), bis[tri(o-tolyl)phosphine]palladium(II) dichloride (4.5 mg, 0.006 mmol), tri(o-tolyl)phosphine (3.5 mg, 0.012 mmol) and tetra(n-butyl)ammonium bromide (17 mg, 0.053 mmol) were combined in a glass vessel that could be sealed with a Teflon tap equipped with a Teflon stir bar. DMF (1.75 mL), triethylamine (0.25 mL, 1.8 mmol), water (1 μL, 0.055 mmol), and vinyltriethoxysilane (0.15 mL, 0.7 mmol) were added. The mixture was then thoroughly degassed, and the atmosphere was replaced with argon and then sealed with the Teflon tap and heated at 125 °C for 48 h with stirring. Upon cooling, the liquid was separated from the ammonium salt crystals by washing with a minimal amount (0.25 mL) of dry diethyl ether. The solvents were then removed by Kugelrohr distillation, dry diethyl ether (10 mL) was added to the mixture, and it was sonicated briefly and then filtered through Celite. The product was purified by column chromatography on silica gel, using dichloromethane (DCM) as the eluent (ca. 400 mL) before changing to 10% THF in DCM to elute the product which was isolated as a clear yellow oil in 73% yield (130 mg, 0.19 mmol). 1H NMR (500 MHz, CDCl3): δH 8.18 (d, J = 8 Hz, 1H), 8.14 (d, J = 19 Hz, 1H), 7.67 (s, 1H), 7.37 (app t, J = 8 Hz, 1H), 7.22 (app t, J = 8 Hz, 1H), 7.13 (d, J = 8 Hz, 1H), 6.38 (d, J = 19 Hz, 1H), 4.00 (q, J = 7 Hz, 6H), 3.80 (s, 3H), 1.35 (t, J = 7 Hz, 9H). 13C NMR (125 MHz, CDCl3): δC 154.87, 146.52, 137.29, 134.44, 127.07, 126.52, 126.04, 124.12, 123.63, 122.11, 120.92, 112.00, 58.92, 57.04, 18.52. HRMS (EI+): m/z calcd 690.3044 (M+), found 690.3062 (M+). 2,2′-Dimethoxy-3,3′-bis(2-triethoxysilyl)ethynyl-1,1′-binaphthalene (6). The procedure described above was adapted using 2,2′dimethoxy-3,3′-diiodo-1,1′-binaphthalene (15, 136 mg, 0.24 mmol) to give the desired compound as a clear colorless oil in 71% yield (118 mg, 0.17 mmol). 1H NMR (500 MHz, CDCl3): δH 8.21 (s, 1H), 7.91 (d, J = 8 Hz, 1H), 7.75 (d, J = 19 Hz, 1H), 7.39 (app t, J = 8 Hz, 1H), 7.22 (app t, J = 8 Hz, 1H), 7.10 (d, J = 8 Hz, 1H), 6.48 (d, J = 19 Hz, 1H), 3.93 (q, J = 7 Hz, 6H), 3.37 (s, 3H), 1.29 (t, J = 7 Hz, 9H). 13C NMR (125 MHz, CDCl3): δC 154.57, 144.51, 134.41, 131.72, 128.59, 126.91, 126.77, 125.81, 125.27, 125.23, 120.45, 61.51, 58.82, 18.48. HRMS (EI+): m/z calcd 690.3044 (M+), found 690.3071 (M+). Materials Synthesis. Basic Conditions. For the synthesis of BTEBp-B material, trimethylstearylammonium chloride (OTACl) was used as the structure-directing agent under alkaline conditions (molar
ratio: Si/water/NaOH/OTACl = 1:1320:12.16:1.28). In a typical synthesis, H2O (17.84 g), OTACl (334 mg), and 6 N NaOH (1.52 g) were combined in a jar equipped with PTFE lid and Teflon lined stirbar. This mixture was stirred vigorously on a magnetic stirrer at 600 rpm for approximately 15 min. For materials containing 15% of the dopant, 9 (78 mg 0.11 mmol) was weighed into a small vial with THF (80 mg) and BTEBp (304 mg, 0.63 mmol), and then the mixture of the silanes was mixed until homogeneous and this was added to the surfactant mixture while stirring at 600 rpm. The mixture immediately became cloudy. The mixture was then stirred at 600 rpm at room temperature for 16 h, and this resulted in a clear and colorless solution. The stir bar was then removed, and the vessel was sealed and aged at 95 °C for an additional 24 h. The resulting solids were then collected via vacuum filtration and washed twice with hot water. The resulting white powder was then put back into a jar with a Teflon lined cap and stirbar, and extracting solution (20 mL of a solution of 6 g of 12 M HCl in 180 g of EtOH) was added and the mixture was stirred for 6 h at 65 °C. The material was then collected via vacuum filtration and washed with an additional portion of hot extracting solution (acidic ethanol as above, 20 mL) and subsequently by a portion of EtOH (20 mL). The materials were then placed into a vacuum oven at 80 °C overnight prior to characterization. Acidic Conditions. Brij-76 was employed as the structure-directing agent under acidic conditions (molar ratio: Si/water/HCl/NaCl/Brij76 = 1:511:12.14:15.04:0.44). In a typical synthesis, H2O (6.9 g), Brij76 (234 mg), and HCl (500 mg of 12 M aqueous solution) were combined in a vial with a Teflon lined cap and stir bar and heated to 60 °C for 1 h while stirring with a magnetic stirrer at 600 rpm. NaCl (660 mg) was then added, and the mixture was allowed to stir an additional 3 h at 600 rpm and 60 °C. For materials containing 15% of the dopant, 9 (78 mg, 0.11 mmol) was weighed into a small vial with THF (80 mg), and BTEBp (304 mg, 0.63 mmol) was added; then the mixture of the silanes was mixed until homogeneous, and this was added to the surfactant mixture. While stirring at 600 rpm, the mixture immediately became cloudy. The mixture was then allowed to stir at 600 rpm and 60 °C for an additional 16 h. The stir bar was then removed, and the vessel was sealed and aged at 95 °C for an additional 24 h. The resulting solids were then collected via vacuum filtration and washed twice with hot water. The surfactant was removed by Soxhlet extraction with EtOH for 16 h. The materials were then placed in a vacuum oven at 80 °C overnight prior to characterization. The above procedures were adapted to make materials with X% resolved 9 to give material (R or S)-9X·BTEBp-A or B, as well as a variety of materials using racemic 9 at various loadings ((rac)-9(X)· BTEBp-A or B where X is the loading of the dopant, A denotes acidic conditions, and B stands for basic). To determine overall incorporation of the chiral monomers, materials were made with a 98% isotopically enriched 13C methoxy labeled analogue of 9 at a loading of 20% to give material (rac)-9*(20)·BTEBp-A. (S)-9(15)-TEOS-B materials were made using the inorganic building block tetraethoxysilane (TEOS) and dopant 9 at 15% via an adaption of the above procedure in which the ratios were modified (molar ratio: Si/Water/NaOH/OTACl = 1:81:0.11:0.25) and the silicon component was normalized to 0.113 mmol of 9 to 1.28 mmol TEOS for a 0.75 mmol scale.
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RESULTS AND DISCUSSION 1,1′-Binaphthyl-bridged siloxane precursors have been widely used to create a variety of organosilica materials;18,23,24 however, the use of a 1,1′-binaphthyl-bridged siloxane precursor in which the anchoring point to the siloxane framework is along the biaryl axis was virtually unknown at the start of our work.10 This is because the attachment of condensable silanes to an aromatic carbon skeleton is typically achieved by converting C−halogen bonds into C−Si bonds. Electronic effects in the 1,1′-binaphthalene skeleton dictate the substitution pattern for the incorporation of these halogen atoms, and neither electron poor nor electron rich systems 5854
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Figure 4. Routes to functionalization of 1,1′-binaphthyl derivatives at the (a) ortho and pseudo-para, (b) pseudo-meta, and (c) meta, desired on-axis functionalization. The bond formed is shown in bold and the numbering system is shown in the inset.
coupling yielding 13 in four steps and 50% overall yield (Figure 4c).37,38 The two atropisomers formed during the oxidative coupling are then separated by preparative chiral HPLC. Having installed the halogens at the desired positions on the biaryl axis, the next step was to convert these groups into a condensable silane. Neither the Masuda coupling39 with triethoxysilane nor organometallic routes through Mg or Li exchange were effective.40 However, the Mizoroki−Heck reaction afforded the target compound in reasonable yields (Scheme 1). For a direct comparison of the effect of position of polymerization of the dopant in the materials, the exact 3,3′analogue was also synthesized using our previously reported method23 and the polymerizable siloxane introduced using our Heck strategy to yield the 1,1′-binaphthyl-bridged siloxane precursor 6 in three steps and reasonable yield (Scheme 1).
favor halogenation at the 4,4′ positions. As shown in Figure 4a, electron rich substituents such as hydroxy or alkoxy groups at the 2,2′-positions as in 10 are strong directing groups for halogen incorporation at the 6 and 6′ (pseudo para) positions using electrophilic aromatic substitution (EAS) chemistry. The directing effect comes from stabilization of the cationic intermediate by the oxygen substituent en route to 14 as shown in Figure 4a.35 Introduction of halogen at the ortho position would be similarly stabilized, but steric effects hinder this reaction. However, the ortho position relative to the directing group can be readily accessed by anionic chemistry, since coordination of strong bases such as organolithium species to heteroatoms directs lithiation to this site, providing access to 15 by the well-studied directed ortho metalation reaction (DoM).36 In order to access the meta position, a deactivating (electron poor) group can be installed at the 2,2′ position as in 11 (Figure 4b). This serves to destabilize cation formation at the ortho and para positions relative to the directing group, and thus halogens are introduced in the meta position. In practice, however, the pseudo-meta position (5 or 5′) in naphthalene systems often predominates due to steric effects, giving, for example, analogue 16 rather than 17.18 In order to prepare the desired 4,4′-disubstituted monomer with the polymerization point on the atropisomeric axis, we needed to completely redesign the synthesis and employ a route in which the binaphthyl structure is brought together af ter functionalization at the desired position. Thus, the starting point became commercially available 1-naphthylamine 18, which can be readily converted to bromonaphthol 12 on multigram scales (Figure 4c).37 In the key step, the binaphthyl skeleton is then assembled via a Cu-promoted oxidative
Scheme 1. Synthesis of Chiral Dopants 9 and 6
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the materials (Figure 5 b,e) confirm that Si−C bond cleavage has not occurred, and the 13C CP/MAS NMR spectra (Supporting Information) showed the materials largely resemble the parent BTEBp as expected. Nitrogen adsorption/desorption isotherms showed type-IV isotherms for (rac)-9(15)·BTEBp-B, (rac)-9(15)·BTEBp-A, and (rac)-6(15)·BTEBp-B, which are typical of mesoporous materials prepared under these conditions. The Brunauer− Emmett−Teller (BET) surface areas, pore volumes, and density functional theory (DFT)-calculated pore diameters are summarized in Table 2, which also includes the TEOS−based
With monomers 6 and 9 in hand, a variety of materials were produced with different levels of dopant loadings under various conditions (Table 1). BTEBp and TEOS were used as the bulk Table 1. Chiral Materials Synthesized with Chiral Dopants 9 and 6
Table 2. Physical Characteristics of Chiral PMOs
dopant (loading) 9 9 6 9
(5−20%) (5−20%) (15%) (15%)
bulk
conditions
material
BTEBp BTEBp BTEBp TEOSc
basica acidicb basic basic
9(5−20)·BTEBp-B 9(5−20)·BTEBp-A 6(15)·BTEBp-B 9(15)·TEOS-B
a
materials
SAa (m2 g−1)
pore size (nm)
PVb (cm3 g−1)
(rac)-9(15)·BTEBp-B (rac)-9(15)·BTEBp-A (rac)-9(15)·TEOS-B (rac)-6(15)·BTEBp-B
806 955 990 786
2.7 3.4 3.4 2.7
0.524 0.584 0.725 0.554
BET surface area. bPore volume.
material (rac)-9(15)·TEOS-B. The physical properties of these materials are comparable with the parent PMO materials, indicating that the inclusion of small amounts (
