Letter pubs.acs.org/macroletters
Planar-to-Axial Chirality Transfer in the Polymerization of Phenylacetylenes Zhiyuan Zhao, Sheng Wang, Xichong Ye, Jie Zhang, and Xinhua Wan* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of MOE, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China S Supporting Information *
ABSTRACT: A pair of enantiomerically pure planar chiral phenylacetylenes, R- and S-2′-ethynyl-1,10-dioxa[10]-paracyclophane, were prepared and polymerized under the catalysis of Rh(nbd)BPh4 and MoCl5, respectively. The resultant polymers had high cis-structure contents and took dominant cis−transoid helical conformations with an excess screw sense as revealed by 1H NMR, Raman, polarimetry, circular dichroism spectroscopy, and computational simulation, manifesting the effective guidance of the planar chirality of monomers to the growth of the polymer main chains. The rigid ansa-structure of monomer unit made the helical structure of polymer backbone stable toward grinding and thermal treatments. The stereoselective interactions between these chiral polymers and the enantiomers of racemic ethynyl-1,10-dioxa[10]paracyclophane and cobalt(III) acetylacetonate were observed. This work demonstrated the first planar-to-axial chirality transfer in the polymerization of acetylenes and offered a new strategy to prepare chiral materials based on optically active helical polymers.
S
was brominated with NBS in acetonitrile, followed by Pd(II)catalyzed Sonogashira coupling and the removal of trimethylsilyl group, to yield the racemic monomer, rac-M. The chemical structure of rac-M was characterized by 1H-/13C NMR and high resolution mass spectroscopy (Figures S1−S3). The assignments of resonance absorptions were made in terms of their integrals and splits as well as heteronuclear single quantum correlation (HSQC) and heteronuclear multiple bond correlation (HMBC; Figures S4 and S5). The restricted rotation of the phenyl ring through the aliphatic cavity makes its two stereoisomers (i.e., R-M and S-M) resolvable. The HPLC spectrum recorded by using Chiralcel CHIRAPAK IB as the chiral stationary phase and the mixture of dichloromethane/n-hexane (20/80, v/v) as the mobile phase displayed two baseline separated peaks with equal areas (Figure S6a). By using a preparative chiral HPLC, both enantiomeric monomers were obtained with a specific optical rotation [α]D25 of +94° for the first eluted isomer and that of −95° for the second one, which presented mirror images of circular dichroism (CD) spectra (Figure S6b). To determine the absolute spatial configuration of each enantiomer, the CD spectra of R-M and S-M were calculated by Gaussian 09 with density functional theory (DFT) and GaussSum 2.2 (Figure S7). By a comparison with the experimental results, the R configuration was assigned to the
ynthetic helical polymers have attracted much attention for their structural similarity to the messengers in nature like DNA and proteins.1 Among them, the point chirality in monomers,2−11 solvents,12−15 initiators,16−18 catalysts,19−21 and additives22,23 are frequently employed to dictate the twisting direction of polymer main chains. There are also a few examples that the screw sense is induced by the axial chirality of spiro pendant moieties24,25 and helical side chains.26−29 However, the role of planar chirality has been ignored in this realm. Planar chirality is one kind of stereogenicity resulting from the arrangement of out-of-plane groups with respect to a reference plane.30 Because of its strong steric hindrance, tunable conformation, and excellent molecular recognition ability, the small planar chiral molecules have found wide applications in asymmetric catalysis,31−35 chiral discrimination,36,37 and molecular devices, such as brake,38−40 hinge,41 and motor.42 We envisioned that the planar-to-axial chirality transfer in polymerization would not only induce a dominant screw sense of polymer backbone, but also lead to novel chiral materials. Herein, we report the preparation and helix-senseselective polymerization of a pair of enantiomerically pure planar chiral monomers, R- and S-2′-ethynyl-1,10-dioxa[10]paracyclophane (R- and S-M). The first example of helical poly(phenylacetylene)s induced by planar chirality was obtained. Their optical activities and chiral separation properties were investigated. The monomer synthesis started with hydroquinone and 1,8dibromooctane (Scheme 1). After two times of etherification, 1,10-dioxa[10]-paracyclophane was produced. This compound © XXXX American Chemical Society
Received: November 22, 2016 Accepted: February 10, 2017
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ACS Macro Letters Scheme 1. Synthesis of 2′-Ethynyl-1,10-dioxa[10]-paracyclophane (rac-M)
Figure 1. Molecular structures of R-M (left) and S-M (right) determined by X-ray analysis.
Table 1. Polymerization Results of Monomers run 1 2 3 4 5 6
monomer R-M S-M rac-M R-M S-M rac-M
catalyst a
Rh(nbd)BPh4 Rh(nbd)BPh4a Rh(nbd)BPh4a MoCl5/Ph4Snb MoCl5/Ph4Snb MoCl5/Ph4Snb
polymer
yield (%)
Mnc (10−3)
PDIc
[α]58925 (deg)d
R-P S-P rac-P R-O S-O rac-O
65 70 85 19 26 10
7.0 6.5 7.2 2.4 2.4 4.4
1.50 1.54 1.72 1.46 1.21 1.85
−3385 +3670 −2182 +3208
Reaction condition: solvent, THF; temperature, 30 °C; monomer to catalyst molar ratio, 60. bReaction condition: solvent, toluene; temperature, 60 °C; monomer to catalyst molar ratio, 15. cDetermined by GPC run at 35 °C using THF as the eluent and calibrated against polystyrene standards. d Specific optical rotation of polymer measured in CHCl3 at a concentration of 0.01 g/dL. a
first eluted enantiomer, while the S configuration to the second one. This tentative speculation was further supported by X-ray crystallographic analysis (Figure 1). The space group of each crystal was orthorhombic P212121 with four molecules per unit cell. The eight bond angles of C−C−C and O−C−C in the aliphatic chain were observed as 114.9° on average, much larger than the normal value of 109.5°, pointing to the evident ring strain in the molecule. The phenyl ring exhibited a boat-like conformation. The phenyl carbons connected to the oxygen atom slightly tilted upward from the phenyl plane for 7.45° on average, with the corresponding C−O bond tilted upward for
2.89° as well, indicating a very tight structure, as a specific character of ansa compounds.43 The polymerizations of racemic and enantiomeric pure monomers were carried out in tetrahydrofuran (THF) and toluene by using either MoCl5−Ph4Sn or Rh(nbd)BPh4 as the catalyst, respectively (Table 1). The reaction mixture under the catalysis of MoCl5−Ph4Sn remained homogeneous throughout the polymerization but only the polymers with relatively low molecular weights, that is, R-O, S-O, and rac-O, were obtained at low yields. When Rh(nbd)BPh4 was used as the catalyst, the reaction mixture of rac-M still kept homogeneous. However, 206
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ACS Macro Letters
structures. This might also account for the width increases and intensity decreases of the Raman peaks of rac-P at 1524 and 837 cm−1 (Figure S8) as well as the presence of the peak at 1324 cm−1, which could be assigned to the C−C bond in a trans-polyene backbone.44−47 S-M and R-M showed specific optical rotations ([α]D25) of −95° and +94°, respectively, while the corresponding polymers S-P and R-P displayed more than 35× larger values of 3670 and −3385°, respectively. The remarkable strength enhancement and opposite direction in the optical rotation of the polymers to the monomers evinced the formation of helical backbones with an excess screw sense. The UV−vis absorption and CD spectra of R-M and S-M as well as their corresponding polymers, R-P and S-P, are depicted in Figure 3. The absorption spectrum of
dark red solids precipitated during the polymerization process of either R-M or S-M. These pristine solids were insoluble in all the solvents tested but fortunately turned to red solids soluble in THF and chloroform (CHCl3) by grinding. It has been known that the grinding or compression can cause polyacetylene derivatives to undergo cis to trans conformation transition,44−50 which may improve polymer solubility. To exclude this possibility and rationalize the solubility improvement, Raman, FTIR spectra, and X-ray diffraction patterns of the polymers were recorded before and after grinding. The peak at 1524 cm−1 in the Raman spectrum of S-P, as obtained from polymerization, was assigned to the stretching cis-CC bond, while that at 837 cm−1 to cis-C−H bond (Figure S8).44,45,47,51 The much lower C−H band than other cis-polyene scaffold implied that the backbone of S-P was more stretched. After grinding, the sample showed almost identical Raman spectrum, indicating such an operation caused no structural variation. FTIR experiments led to a consistent conclusion (Figure S9). Furthermore, the as-obtained polymers showed five XRD reflections attributed to (100), (110), (200), (210), and (300), respectively, indicating a two-dimensional hexagonal lattice with a parameter αhex = 1.30 nm (Figure S10). However, after grinding, these diffraction peaks became broader and vaguer, implying less ordered packing of polymer chains. It was therefore considered that the polymer formed during polymerization packed orderly and had strong interchain interactions, which led to its poor solubility. The grinding disturbed the ordered packing of polymer chains and enabled its dissolution in THF and chloroform. The grinding was also applied to S-M and caused no chemical or stereochemical structure change, indicating that the pendants of S-P were stable toward mechanical force. It seemed that the shear deformation was important to cause such a supramolecular structural variation since a compression with more than 100 kg/cm2 pressure exerted no improvement in solubility. Figure 2 exhibits 1H NMR spectra of S-M, S-P, and rac-P measured in CDCl3 at 25 °C. The resonance peak of acetylenic
Figure 3. UV−vis absorption and CD spectra of monomers and polymers in chloroform at a concentration of 4.0 × 10−5 mol/L.
S-M displayed one absorption peak centered at 306 nm, which could be attributed to π−π* transitions of delocalized orbital of phenyl group. Identical absorption spectrum was observed in RM. In this absorption region, R-M and S-M, presented relatively weak but obvious nonconservative Cotton effects, reflecting the influence of planar chirality of molecules. No obvious absorption and Cotton effects were observed above 340 nm. However, after polymerization, the resultant polymers R-P and S-P displayed two absorptions at 306 and 505 nm attributed to pendants and polyene backbones, respectively. The nonconservative Cotton effects in the long wavelength region suggested the formation of a predominant cis−transoid helical main chain, while the conservative Cotton effects in the short wavelength region suggested that the pendants of R-P were arranged in a skewed way. The CD spectra of R-P and S-P in THF and chloroform were basically the same (Figure S11). The racemic polymer rac-P showed no CD signals. Meanwhile, its polyene backbone absorption wavelength was shorter than those of R-P and S-P, probably due to a more disordered chain structure. In addition to the stereostructure stability in bulk against mechanical force, the helical conformations of R-P and S-P were also stable in solution. Their temperature-variable CD spectra were recorded in CHCl3, and no discernible variation was found (Figures S12 and S13), probably ascribed to the strong steric interactions of neighboring pendants, which stabilized the stretched cis−transoid helix. Computer modeling by Material Studio software (COMPASS force field) suggested that both R-P and S-P chains were constituted by two “coaxial helices” defined by the pendants (external) and by the polyene backbones (internal), respectively (Figure S14 and Tables S3
Figure 2. 1H NMR spectra of S-M, S-P, and rac-P measured in CDCl3 at 30 °C.
proton at 3.30 ppm disappeared after polymerization, indicating the complete consumption of monomer molecules. The signal of olefinic proton of S-M was presented at 5.92 ppm. The resonances of other polymer protons became weaker and broader than those of the monomer due to the limited motion of rigid macromolecular scaffold. The cis-structure content of SP was estimated as 92% (90% for R-P) according to the method developed by Percec and co-workers.52−55 It should be noted that each resonance peaks were more confused and widened for rac-P, indicating the incorporation of racemic monomers into a polymer chain might yield some stereo- or regioirregular 207
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and S4).6−8 S-P took a left-handed 2/1 helix of polyene backbone and right handed helix of pendants. The average values of dihedral angles of C−CC−C and CC−CC were 32.6° and −178.7°, respectively, indicating an extended polyene backbone. The senses of external and internal helices were opposite for R-P. One of optically active helical polymers, R-P, was coated on mesoporous silica particles and packed into a steel column as the chiral stationary phase of HPLC. A hexane solution of racM was injected into the columns and eluted with hexane. Although the UV detector monitored at 254 nm exhibited no chiral resolution, the CD detector showed two continuous peaks when the racemates went through, negative for the first fraction and positive for the second one, corresponding to S-M and R-M, respectively (Figure 4). It suggested the stronger
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00901. Materials and measurements, synthesis of rac-M and chiral resolution and characterization of enantiomeric monomers, polymerization procedures, computational simulation, preparation of single crystals of R-M and S-M and the XRD data, preparation of chiral stationary phase (CSP), 1H NMR, 13C NMR, FTMS, HSQC, and HMBC of monomers, simulated monomer structures and CD spectra by Gaussian 09. Raman spectra, IR spectra, WAXD patterns, and CD spectra of polymers, chiral separation results of rac-M, and cobalt(III) acetylacetonate by CSP using R-P as coating material (PDF).
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 86-10-62754187. Fax 86-10-62751708. E-mail: xhwan@ pku.edu.cn. ORCID
Xinhua Wan: 0000-0003-2851-6650 Notes
The authors declare no competing financial interest.
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Figure 4. Chromatographs for the resolution of rac-M with hexane as the eluent detected at 254 nm.
ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (No. 21274003) and the Research Fund for Doctoral Program of Higher Education of MOE (No. 20110001110084) is greatly appreciated.
interaction between R-P and R-M than its enantiomer. The enantioselection was also found for cobalt(III) acetylacetonate (Figure S15). The asymmetrically arranged ansa-substituents along the helical polymer backbone might provide a chiral constrained space, which favorably interacted with one antipode of cobalt(III) acetylacetonate through van der Waals force or the aromatic π−π interactions. One additional interesting observation was that the polymers obtained via MoCl5/Ph4Sn catalized polymerization also had relatively high cis-structure content (70% for R-O and 68% for S-P, Figures S16−S18). Mo-based catalyst usually produces trans-poly(phenylacetylene)s. This contrast might reflect the influence of steric interactions between neighboring structure units on the geometrical structure of polymer. The absorptions of R-O and S-O blue-shifted compared to those of R-P and S-P (Figure S19), due to their lower molecular weights. In summary, the planar-to-axial chirality transfer was demonstrated in the Rh(nbd)BPh4-catalyzed polymerizations of enantiomerially pure phenylacetylene monomers, R- and SM. The resultant polymers adopted extended cis−transoid helical conformations with an excess of single-handed screw sense due to the dictation of the planar chirality of monomer to the growth of polymer main chain. The rigid ansa-substituents made the helical structures stable toward grinding and thermal treatments. The tentative experiments suggested that these polymers could stereoselectively interact with each enantiomer of rac-M and cobalt(III) acetylacetonate. Although, at the current stage, the complete separation of the enantiomers was not achieved, the unique combination of planar and axial chirality would not only pave the way for future fine-tuning of specific separation needs but also offer a new platform to prepare functional chiral materials.
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