Article pubs.acs.org/Macromolecules
New Method for Asymmetric Polymerization: Asymmetric Allylic Substitution Catalyzed by a Planar-Chiral Ruthenium Complex Naoya Kanbayashi,* Taka-aki Okamura, and Kiyotaka Onitsuka* Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan S Supporting Information *
ABSTRACT: A new type of asymmetric polymerization reaction by asymmetric allylic substitution catalyzed by planar-chiral cyclopentadienyl-ruthenium (Cp′Ru) complexes is described. Achiral AB-type monomers, which feature both the allylic chloride and N-alkoxyamide moieties in a single molecule, are smoothly polymerized by the catalysis of chiral ruthenium complexes with quantitative monomer conversion to afford optically active polyamides in good yields. The resulting polymers are characterized by optical rotation, circular dichroism, and 1H NMR analyses, which clearly show that the polymerization system works in a highly stereoselective manner to afford optically active polyamides with high regio- and enantioselectivities. The results revealed that the planar-chiral Cp′Ru complexes work as a highly efficient catalyst to control the stereochemistry even in the polymerization reaction.
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INTRODUCTION In recent years, precise polymer synthesis has received much attention for developing versatile polymeric materials.1 Stereospecific polymerization in particular is one of the current topics in polymer synthesis because precise control of the stereoregularity of the polymer backbone changes the physical properties in conventional polymer materials.2,3 For instance, the stereoselective polymerization by metallocene4,5 and nonmetallocene6,7 catalysts of transition metals, which can control the relative stereochemistry of the polymer main chain, has garnered a great deal of attention over the past several decades. In contrast, most naturally occurring polymers are optically active, and the polymers are formed by controlling the absolute configuration of the asymmetric centers on the main chain. The stereoselectivity of a natural polymer represents a significant foundation for the construction of three-dimensional structures that exhibit the biological functions of the natural polymer. On this point, the precise synthesis of optically active polymers,8−12 which are controlled by the absolute chiral configuration of the main chain, has received a great deal of attention. The current goal is not only to understand the structures and properties of optically active polymers but also to develop a novel foundation for the polymeric synthesis of characteristic functional materials.12−18 The asymmetric polymerization of achiral monomers is one of most the promising methods of introducing stereogenic centers onto the main chain and synthesizing optically active polymers.8,10,11 However, when common vinyl monomers are used, the resultant stereoregular polymer does not show any optical activity, even if a significant asymmetric induction occurs at a stereogenic carbon of the main chain because these vinyl polymers possess mirror © 2014 American Chemical Society
symmetry in a direction perpendicular to the translational axis (Figure 1a). In fact, long-chain stereoregular polymers of α-
Figure 1. (a) General isotactic vinyl polymer. (b) Isotactic polymer used to remove translational symmetry by inserting an unsymmetrically monomer unit.
olefins produced by optically active catalysts did not show any detectable optical activity in solution,19,20 although short-chain polymers showed measurable optical activity.21−23 Thus, the design of the polymer-base structure remains important for the synthesis of optically active polymer. In naturally occurring polymers, such as polypeptides, each stereogenic carbon is connected to an unsymmetrical amide group and the resulting polymer is chiral. To obtain chiral polymers from an isotactic polymer, the introduction of the unsymmetrical unit is necessary to remove translational symmetry (Figure 1b).9 Two synthesis approaches for asymmetric polymerization are shown in Figure 2. One is (a) asymmetric alternating copolymerization of two kinds of monomers, and the other is Received: April 25, 2014 Revised: June 12, 2014 Published: June 23, 2014 4178
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Scheme 1. Asymmetric Allylic Substitutions Catalyzed by 1
Figure 2. Asymmetric polymerization using (a) alternating copolymerization of two kinds of monomers and (b) a single unsymmetrical AB-type monomer.
system. Because the Cp′Ru-catalyzed allylic substitution reactions exhibit high reactivity and selectivity, we have attempted to extend our system to an asymmetric polymerization reaction via polycondensation. We present herein a new method for asymmetric polymerization by means of asymmetric allylic substitution catalyzed by planar-chiral cyclopentadienyl ruthenium complex 1, which affords a new type of optically active polymer from several types of main-chain structures.
(b) the use of a single unsymmetrical bifunctional, i.e., an ABtype monomer. The former has been reported as alternating copolymerizations of α-olefins and CO using chiral palladium catalysts, producing a single enantiomer of isotactic polyketones to control the enantioselectivity of each elementary reaction.24−27 Until now, several efficient systems for this method have been reported.28−32 In most cases, however, the mainchain structures of the resultant chiral polymers are limited because addition polymerization was used. The latter is the asymmetric polycondensation or polyaddition of AB-type monomers, which connects each unsymmetrical monomer backbones by asymmetrical centers (Figure 2(b)). Because the polymer backbone can be modified by using various monomers in this method, the number of candidate polymer structures is vastly larger than that in addition polymerization. Thus, the introduction of various backbones into the polymer main chain represents a great method for developing tailor-made optically active polymer materials with the possibility of developing a completely new type of optically active polymer. Itsuno and coworkers reported asymmetric polycondensation and polyaddition by asymmetric C−C bond formation reactions such as the asymmetric Sakurai-Hosomi reaction33 and the Mukaiyama aldol reaction34 and prepared various asymmetric polymers with different backbones.35 Generally, the synthesis of optically active polymers by asymmetric polycondensation or polyaddition is fairly difficult. To obtain high-molecular-weight polymer by successive polycondensation, a high extent of reaction is necessary; if not produced, it results in a mixture of small oligomers. Moreover, the chiral catalysts of the polycondensation must recognize exactly the small reaction terminal moiety of a long polymer, and each reaction should proceed with high enantioselectivity. Therefore, a highly reactive and enantioselective catalyst is indispensable to achieving satisfactory asymmetric polymerization. Allylic substitution catalyzed by an organometallic complex is a powerful method for creating carbon−carbon and carbon− heteroatom bonds.36−40 In polymer synthesis, a few polycondensation reactions by repeating allylic substitutions have been reported.41 For instance, Nomura and co-workers reported the polymerization of bifunctional monomers by allylic substitutions catalyzed by a transition-metal catalyst,42−45 which tolerates various backbones in the main chain. However, asymmetric polymerization via allylic substitution has not been reported. We have already shown that planar-chiral cyclopentadienyl-ruthenium (Cp′Ru) complexes (1)46−49 are proficient catalysts for regio- and enantioselective allylic substitutions of monosubstituted allylic halides with nucleophiles (Nu) containing carbon,50 oxygen,51−54 and nitrogen55 atoms (Scheme 1). The fact that these complexes tolerate a wide scope of allylic derivatives is a key attribute of the present
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RESULTS AND DISCUSSION We began our investigation by applying allylic substitution catalyzed by Cp′Ru (1) to a new type of asymmetric polymerization. As a model reaction, asymmetric allylic amidation was selected.55 To substantiate the asymmetric polymerization, we designed AB-type monomer 2 as an unsymmetrical monomer, which has both the allylic chloride and N-hexyloxyamide moieties in a single molecule, i.e., an unsymmetrical monomer (Scheme 2(a)). To obtain a highScheme 2. (a) Asymmetric Polymerization Using the Asymmetric Allylic Amidation of Monomer 2a and (b) Model Reaction of the Asymmetric Polymerization of 2a
molecular-weight polymer by polycondensation, strict adjustments to the stoichiometry of the two reacting groups were necessary. First, we conducted a model reaction as the initial step toward the polymerization of 2a. A reaction between allylic chloride (3) and N-hexyloxybenzamide (4) was performed in the presence of (S)-1a (Ar = Ph) to afford an expected branched allylic amide (5) in 99% yield with 97% ee (Scheme 2(b)). The polymerization of monomer 2a was carried out under reaction conditions similar to those employed for the model reaction mentioned above, i.e., in the presence of 1 mol % (S)1a (Ar = Ph) as a catalyst, KHCO3 as a base, and molecular 4179
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sieves 3A (MS 3A) at 30 °C in THF (0.5 M) (Scheme 3).56 The polymerization of monomer 2a proceeded smoothly with Scheme 3. Asymmetric Polymerization Using Asymmetric Allylic Amidation Catalyzed by Cp′Ru Complex (S)-1a
an almost quantitative monomer conversion after 36 h, and the usual workup of the reaction mixture afforded a crude polymer (Mn = 9200) with a broad molecular-weight distribution (Mw/ Mn = 2.4). The crude product was purified to remove the catalyst by column chromatography on silica gel using dichloromethane and then ethyl acetate as eluents. The polymer (poly-2a-((S)-1a)) was obtained in 65% yield from the first fraction eluted by dichloromethane; the SEC analysis of poly-2a-((S)-1a) showed Mn = 17 000 and a comparatively narrow Mw/Mn = 1.5.57,58 The second fraction eluted by ethyl acetate afforded residual oligomers, indicating that the effective purification of the polymerization product may be performed by column chromatography on silica gel. Poly-2a-((S)-1a) showed an optical rotational power of [Φ]D = +24, which may indicate that the polymerization proceeded via repeating the asymmetric allylic substitution reaction in the expected manner, affording optically active polyamides with chiral carbons in the main chain. Characterization of Poly-2a-((S)-1a) by 1H NMR. The 1 H NMR spectral analysis may offer valuable information about the main-chain structure of the polymer. We also used the 1H NMR spectral method for the structural analysis of poly-2a((S)-1a) by comparing it to the branched allylic compound (6) prepared by a model asymmetric amidation reaction using catalyst (S)-1a. The 1H NMR spectra of poly-2a-((S)-1a) and model compound 6 in CDCl3 at 298 K are shown in Figure 3. In the spectrum of poly-2a-((S)-1a) (Figure 3(b)), there are multiplet peaks in the 5.34−5.55 ppm (CHCHCH2) region because of the terminal olefinic protons assigned to a branched allylic structure, which is almost identical to the spectrum of model branched allylic compound 6 (Figure 3(a)). The minor peaks (δ 6.65 (d, CHCHCH2, 1H), 6.45−6.38 (m, CH CHCH2, 1H), 4.53 (br, CHCHCH2, 2H), and 3.74 (t, OCH2CH2, 2H) ppm) assignable to the linear allylic structure were detected, although they were negligible compared to those of the branched structure. Additionally, methylene protons as the polymer terminal moiety were observed at around 4.01 ppm.59 The NMR molecular weight based on the terminal moiety was calculated as Mn = 14 000, which approximately agreed with SEC data (Mn = 17 000). These results suggest that the polymerization catalyzed by 1 proceeded with high regioselectivity (x/y > 20/1), and thus, the catalytic system using 1 can be applied to the asymmetrical polymerization of
Figure 3. 1H NMR (500 MHz, in CDCl3 at 298 K) spectra of (a) model compound 6 and (b) poly-2a-((S)-1a).
achiral monomers, which may be utilized to introduce regular arrangements of chiral carbons in the main chains of polymers. Optimization of Reaction Conditions. To attain highly enantioselective polymerization, we investigated the reaction conditions for the polymerization. In the optimization experiments, the stereochemistry of the main-chain structure was evaluated by optical rotation measurement. The experimental results obtained from the polymerization of monomer 2a using catalyst (S)-1a in THF are summarized in Table 1. Initially, we Table 1. Screening of the Reaction Conditionsa entry
concentration (M)
temp (°C)
yield (%)b
1 2 3 4 5 6 7 8 9f 10g
0.25 0.33 0.50 0.80 0.50 0.50 0.50 0.50 0.50 0.50
30 30 30 30 35 40 25 20 25 25
60 60 65 60 73 65 63 61 80 70
Mnc,d 11 11 17 10 19 14 15 11 11 14
000 000 000 000 000 000 000 000 000 000
Mw/Mnc,d
[Φ]De
1.4 1.5 1.5 1.5 1.3 1.3 1.5 1.4 2.1 1.6
+24 +24 +24 +24 +23 +23 +25 +24 +24 +27
a
(S)-1a (0.005 mmol), 2a (0.50 mmol), and KHCO3 (0.60 mmol) in THF, stirred for 36 h. bIsolated yield. cEstimated by SEC analysis using a polystyrene standard. dAfter purification with silica gel chromatography, eluted with dichloromethane to remove the catalyst. e Molar rotation analysis in CHCl3 at 21 °C. fNa2CO3 was used instead of KHCO3. g(S)-1b was used instead of (S)-1a.
evaluated the effect of the monomer concentration in the asymmetric polymerization. When the monomer concentration was reduced, the yield and molecular weight of the resulting polymers were slightly decreased (entries 1 and 2 vs entry 3). In contrast, when the monomer concentration was increased up to 0.8 M, the reaction mixture solidified after 12 h, and the 4180
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Figure 4. CD and UV spectra of (a) poly-2a-((S)-1b) and model compound 5 (in CHCl3 at 298 K). The vertical axis is normalized for the benzene unit. (b) Spectra of poly-2a-((S)-1b) and poly-2a-((R)-1a). Poly-2a-((S)-1b) and poly-2a-((R)-1a) were prepared by the polymerization of monomer 2a using catalysts (S)-1b and (R)-1a, respectively.
Generally, determining the optical purity of stereogenic carbons in the main chain of polymers is very difficult. One of the direct methods for the analysis of the enantiomerical purity of the repeat unit in a chiral polymer is to degrade the polymer into the repeat unit and then to determine the enantiomeric purity of the unit.30,32 This method has been successfully applied to certain polycarbonates and polyesters; however, the method requires the highly efficient degradation of the polymer backbone and the protection of the stereogenic centers in the polymer against racemization during the degradation process. In the case of our polyamide, the degradation method was inapplicable because we could not find a suitable procedure under mild reaction conditions for cleaving the C−N bonds in the polymer main chain. Therefore, we tried to evaluate the stereochemistry of the polymer on the basis of spectral methods which are frequently used to analyze the chiral polymers.32,60 Enantioselectivity in the asymmetric polymerization of monomer 2a has been evaluated by means of ultraviolet (UV) and circular dichroism (CD) spectroscopic methods. The UV and CD spectra of poly-2a along with those of model dimer (5) measured in CHCl3 at 298 K are shown in Figure 4. The CD spectrum of 5 was normalized for the benzene unit (Figure 4(a)). As seen from Figure 4(a), 5 (97% ee) exhibited a positive Cotton effect in the region of 240−260 nm in the CD spectrum, which is assignable to the π−π* transition of the benzene chromophore. Poly-2a-((S)-1b) also displayed a CD spectral pattern quite similar to that of 5. Because the CD spectrum is believed to reflect the local conformation of poly2a, the result of the spectral analysis indicates that the stereogenic carbon atom in poly-2a has the same absolute configuration as model compound 5. In poly-2a-((S)-1b), the UV absorption band appeared at around 280 nm, which has no relation to the chirality. In addition, poly-2a-((R)-1a), which was prepared by the polymerization of 2a with (R)-1a, exhibited a CD spectrum in a mirror image of poly-2a-((S)1b) (Figure 4(b)). These facts indicate that the new stereogenic centers of poly-2a are generated by asymmetric allylic amidation using catalyst 1, and high stereoselectivity is achieved.
molecular weight of the resulting polymer was decreased (entry 4). In all cases, no loss of optical rotation was observed. Next, to investigate the effect of the reaction temperature, the polymerization reactions of 2a (0.5 M) were carried out in the temperature range of 20−40 °C (entries 3 and 5−8). As previously reported, in the asymmetric allylic amidation, the products in allylic substitution catalyzed by 1 exhibited high enantioselectivity within 20−40 °C.51−55 For the polymerization at 25 and 30 °C, the resulting polymer exhibited a slightly higher optical rotation at high molecular weight (entries 3 and 7). When the base was changed to Na2CO3, a broad Mw/ Mn was obtained (entry 9). To improve the enantioselectivity of the polymer, we examined the effect of the aryl groups on the phosphine group of Cp′Ru catalyst 1. We have already shown that the aryl group in 1 plays an important role in the enantioselectivity of allylic substitution.53 Thus, we conducted the reaction using (S)-1b (Ar = 3,5-Me2C6H3) as the catalyst under optimum conditions ([2a] = 0.5 M, 25 °C, KHCO3). The resulting polymer exhibited a relatively large optical rotational value of [Φ]D = +27 (entry 10). Evaluation of the Stereochemistry of the Main Chain. As shown above, the polymerization of 2a using planar-chiral ruthenium catalysts with an S configuration afforded optically active polymers showing plus optical rotation. The use of planar-chiral ruthenium catalyst (R)-1a possessing an opposite absolute configuration to that of (S)-1a resulted in the formation of optically active poly-2a-((R)-1a) with the opposite sign of the optical rotational power, [Φ]D = −25. Of course, the use of a racemic catalyst (a mixture of equivalent of (S)- and (R)-1a) apparently afforded an optically inactive poly-2a-((rac)-1a) with [Φ]D = 0. These facts suggest that the polymerization of 2a catalyzed by (S)- or (R)-planar-chiral Ru complexes can be featured as an asymmetric polymerization. Additionally, the reaction between allylic chloride 3 and Nhexyloxyamide 4, which may correspond to the initial step in the polymerization of 2a (Scheme 2), was performed in the presence of (S)-1a to give expected branched allylic amide 5 in 99% yield with 97% ee. Therefore, it is likely that, even in polymerization, allylic amidation catalyzed by the Cp′Ru (1) complex proceeds in a highly regio- and enantioselective manner. 4181
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Determination of the Optical Purity of the Main Chain. To determine the optical purity of the main chain formed by the polymerization using (S)-1b as a catalyst, we conducted the NMR analysis of a chiral polymer ((S)-poly-7((S)-1b)), which was prepared from optically active monomer ((S)-7) bearing an (S)-2-octyloxy group (Scheme 4(a)).
was used as a catalyst, (S)-9-((S)-1b) was obtained with 96% ee, which was determined by HPLC analysis using the chiral stationary phase. The 1H NMR spectrum of (S)-9-((S)-1b) is shown in Figure 5(a), in which mainly one peak (3.75 ppm) was observed, indicating the degree of enantioselectivity of the present asymmetric allylic amidation reaction to be 94% ee, which is consistent with the value of optical purity obtained by the HPLC analysis. Next, we applied this method to determine the optical purity of the main chain of poly-7. The polymerization of (S)-7 was carried out in the presence of (S)-1b under optimized conditions (vide supra) to afford poly-7-((S)-1b) with quantitative monomer conversion (Mn = 6300, Mw/Mn = 2.2). The 1H NMR spectra of poly-7-((S)-1b) are shown in Figure 6a, in which mainly one peak was observed in the region
Scheme 4. (a) Diastereoselective Polymerization of Optically Active Monomer (S)-7 and (b) Model Reaction of Diastereoselective Polymerization
If the asymmetric polymerization of (S)-7 takes place by the catalysis of (S)-1b, then the repeat unit of the resulting (S)poly-7-(rac) should have two chiral carbons whose configuration is either (S,S) or (S,R). Therefore, we attempted to determine the optical purity of the main chain by this diastereomeric method using chiral monomer (S)-7. Initially, we carried out the model reaction between cinnamyl chloride and (S)-N-2-octyloxybenzamide ((S)-8) with (rac)-1a and analyzed the product by 1H NMR spectroscopy (Figure 5). The reaction proceeded to afford the expected branched allylic amide ((S)-9-((rac)-1a)) in good yield. The 1H NMR spectrum revealed that the diastereomeric methine protons on the 2-octyloxy group were equivalently separated into two peaks at 3.75 and 3.50 ppm, indicating that kinetic resolution did not occur during the reaction (Figure 5(b)). When (S)-1b
Figure 6. 1H NMR spectra (400 MHz, in CDCl3 at 298 K) of poly-7 using (a) (S)-1b and (b) (rac)-1a as catalysts.
of methine protons (3.76 ppm). In contrast, when (rac)-1a was used as a catalyst for poly-7-((rac)-1a) (Mn = 6500, Mw/Mn = 2.3),61 two peaks at 3.77 and 3.58 ppm with a ratio of 50:50 were observed because of the diastereomers (Figure 6(b)). Additionally, on changing the catalyst from (S)-1b to (R)-1a, a single peak due to a methine proton with the opposite absolute configuration was observed. These facts clearly indicated that the stereogenic centers of poly-7 were highly controlled by the asymmetric allylic amidation when (S)-1b was used as the catalyst. The results reveal that the polymerization proceeds in highly stereoselective manner to afford optically active polymers with the expected stereochemistry. Polymerization of Other Bifunctional Monomers. Our polymerization method can synthesize polymers possessing a variety of backbones. To investigate the influence of the backbone of monomer (2) in asymmetric polymerization, the polymerization of several bifunctional monomers 2b-e bearing allylic chloride and N-alkoxyamide moieties was carried out in the presence of (S)-1b (Scheme 5). The results are summarized in Table 2. Monomer 2b bearing an alkynyl group, which may be used to build a rigid backbone, was polymerized in quantitative monomer conversion to afford poly-2b in good yield with a slightly lower regioselectivity (x/y = 10/1). The
Figure 5. 1H NMR (400 MHz, in CDCl3 at 298 K) spectra of (S)-9 using (a) (S)-1b and (b) (rac)-1a as catalysts. 4182
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(poly-2d). All of the polymers obtained here showed optical activities, indicating that the planar-chiral Cp′Ru complexes can catalyze an asymmetric polymerization of allylic chloride derivatives with a variety of backbones via allylic substitution reactions. Reaction Mechanism of Asymmetric Polymerization. For asymmetric allylic substitution catalyzed by 1, the reaction mechanism was studied. The reaction involves the oxidative addition of an allylic moiety to the (S)-1 complex, affording a πallyl intermediate ((S)-10) with high diastereoselectivity at the chiral metal center and planar chirality of the π-allyl ligand. Subsequent inside attack of the nucleophile via the Ru complex ((S)-11), which is generated by the substitution of chloride with nucleophile, affords a branched allylic compound with high enantioselectivity (Figure 7). Therefore, it is necessary to control the metal-center chirality of (S)-10 and (S)-11 in the propagation steps of polymerization in order to realize the asymmetric polymerization. Because the CD spectrum is believed to reflect a local conformation of poly-2a, the result of the spectral analyses indicates that the stereogenic carbons in poly-2a have the same absolute configuration as that of model compound 5. Thus, it is likely that the asymmetric polymerization proceeds through the same pathway as described above with control over the metal-center chirality even in polymerization.
Scheme 5. Asymmetric Polymerization of Several Bifunctional Monomers
Table 2. Screening of Monomersa entry
2
yield (%)b
x/yc
Mnd,e
Mw/Mnd,e
[Φ]Df
1 2 3
2b 2c 2d
67 66 82
10/1 >20/1 >20/1
14 000 12 000 8000
1.5 1.4 1.8
+42 +61 +42
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a
(S)-1b (0.005 mmol), 2 (0.5 mmol), and KHCO3 (0.60 mmol) in THF, stirred for 36 h. bIsolated yield. cEstimated from the 1H NMR spectrum. dEstimated by SEC analysis using a polystyrene standard. e After purification with silica gel chromatography, eluted with dichloromethane to remove the catalyst. fMolar rotational analysis in CHCl3 at 21 °C.
CONCLUSIONS We demonstrated asymmetric polymerization by repeating the asymmetric allylic amidation catalyzed by planar-chiral Cp′Ru complexes. The structure of the resulting polymer was analyzed by 1H NMR, UV, and CD spectroscopic methods, and it was proved that the polymerization proceeds with high regio- and enantioselectivity. The results indicate that the planar-chiral Cp′Ru complexes work as a highly efficient catalyst to control the stereochemistry even in a polymerization reaction. This is
polymerization of monomer 2c, bearing an ester group, was accomplished with good selectivity to afford poly-2c. Moreover, monomer 2d, having a 1,3-phenylene group in a monomer backbone, was also polymerized to afford the desired polymer
Figure 7. Plausible reaction mechanism of asymmetric polymerization. 4183
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the first asymmetric polymerization reaction using an asymmetric allylic substitution that affords novel optically active polymers with several types of main-chain structures. The asymmetric polymerization system is expected to be used to develop the carbon−carbon and carbon−heteroatom bondforming reactions48−53 further using the Cp′Ru catalyst. Additionally, as the resulting optically active polymer still features a terminal double bond per monomer unit, the polymer may be transformed to a variety of functional materials. Therefore, the resultant polymer has great potential for developing characteristic functional materials.
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CHCHCH2), 5.42−5.37 (m, 4H, CHCHCH2, CH2OCO), 3.58 (br, 1H, OCH2), 3.29 (br, 1H, OCH2), 1.23−1.00 (m, 8H, CH2), 0.75 (t, 3H, J = 7.0 Hz, CH3). 13C NMR (CDCl3, 100 MHz): δ 169.9, 166.0, 144.0, 138.7, 134.4, 133.7, 130.2, 130.0, 128.8, 128.5, 128.4, 127.6, 119.8, 66.1, 64.0, 31.4, 27.8, 25.4, 22.5, 14.0. Anal. Calcd for C24H27NO4: C, 72.73; H, 6.86; N, 3.49. Found: C, 72.91; H, 6.94; N, 3.55. [α]D21 +15.5 (c 0.1, CHCl3). [Φ]D21 +62 (c 0.1, CHCl3). Poly-2d. 1H NMR (CDCl3, 500 MHz): δ 7.18−7.33 (m, 4H, Ar), 6.33−6.24 (m, 1H, CHCHCH2), 6.09 (br, 1H, CHCHCH2), 5.24−5.35 (m, 2H, CHCHCH2), 3.54 (br, 1H, OCH2), 3.24 (br, 1H, OCH2), 1.25−0.77 (m, 11H, CH2, CH3). 13C NMR (CDCl3, 126 MHz): δ 169.8, 138.4, 134.9, 133.9, 130.4, 127.7, 127.3, 119.2, 76.8, 63.7, 31.4, 27.7, 25.4, 22.5, 13.9. Anal. Calcd for C16H21NO2: C, 74.10; H, 8.16; N, 5.40. Found: C, 73.94; H, 8.14; N, 5.38. [α]D21 +30.2 (c 0.8, CHCl3). [Φ]D21 +79 (c 0.8, CHCl3). (S)-Poly-7. 1H NMR (CDCl3, 400 MHz): δ 7.68−7.62 (m, 2H, Ar), 7.50−7.36 (m, 2H, Ar), 6.37−6.28 (m, 1H, CHCHCH2), 5.85 (br, 1H, CHCHCH2), 5.35 (d, 1H, J = 10.4 Hz, CHCHCH2), 5.28 (d, 1H, J = 17.3 Hz, CHCHCH2), 3.74 (br, 1H, OCH), 1.52−0.70 (m, 16H, CH2, CH3). 13C NMR (CDCl3, 100 MHz): δ 172.1, 141.3, 134.5, 134.2, 128.6, 128.3, 119.3, 81.6, 66.3, 34.6, 31.6, 29.2, 25.2, 22.5, 18.4, 14.0. [α]D21 +28.6 (c 0.5, CHCl3). [Φ]D21 +80.
EXPERIMENTAL SECTION
General. All reactions were carried out under an Ar atmosphere using a Schlenk technique, whereas the workup was performed in air. 1 H and 13C NMR spectra were recorded in CDCl3 on Varian Mercury 300, JEOL JNM-ECS400, and JEOL JNM-ECA500 spectrometers using SiMe4 as an internal standard. The enantiomeric excess was determined with HPLC (Shimadzu LC-10 and SPD-10AV) using a DAICEL Chiralcel OD-H column. The number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of the polymers were determined by size exclusion chromatography (SEC) in tetrahydrofuran at 40 °C with polystyrene gel columns (Shodex; KF805L × 3, molecular weight = 2.0 × 107; flow rate = 1.0 mL/min) connected to Shimadzu LC-6AD and Shimadzu SPD-10A UV−vis detectors. Optical rotation was measured on JASCO DIP-1000. CD spectra were obtained with a JASCO J-720WO. UV−vis spectra were obtained with a Shimadzu UV 3100PC. Thermogravimetric analysis (TGA) was conducted under a nitrogen atmosphere from 25 to 500 °C at a heating rate of 10 °C/min using a Mettler Toledo TGA/DSC 1 STARe System. Materials. All solvents used for reactions were passed through purification columns just before use. Planar-chiral Cp′Ru complexes 1a and 1b were prepared as reported previously.46,47,53 Standard Method of Polymerization. To a solution of (E)-4-(3chloroprop-1-en-1-yl)-N-hexyloxybenzamide (2a, 0.50 mmol) and (S)1b catalyst (3.9 mg, 5 μmol) in THF (1.0 mL) was added potassium bicarbonate (60.1 mg, 0.60 mmol), and the reaction mixture was stirred at 25 °C. After 36 h, dichloromethane (10.0 mL) was added to the reaction mixture. The insoluble part was filtered through Celite. The combined organic layer was washed with water. After drying over Na2SO4, the solvent was removed under reduced pressure. The residue was purified by silica gel column chromatography with CH2Cl2 and dried under vacuum to give a pale-yellow solid. Poly-2a. 1H NMR (CDCl3, 500 MHz): δ 7.69 (d, 2H, J = 8.0 Hz, Ar), 7.49 (d, 2H, J = 8.0 Hz, Ar), 7.69 (ddd, 1H, J = 17.3, 10.2, 7.2 Hz, CHCHCH2), 6.11 (br, 1H, CHCHCH2), 5.40 (d, 1H, J = 10.2 Hz, CHCHCH2), 5.37(d, 1H, J = 17.3 Hz, CHCHCH2), 3.56 (br, 1H, OCH2), 3.28 (br, 1H, OCH2), 1.30−0.98 (m, 8H, CH2), 0.79 (t, 3H, J = 7.2 Hz, CH3). 13C NMR (CDCl3, 126 MHz): δ 169.8, 141.1, 134.0, 133.8, 128.3, 127.9, 119.4, 77.2, 63.7, 31.4, 27.8, 25.4, 22.4, 13.9. Anal. Calcd for C16H21NO2: C, 74.10; H, 8.16; N, 5.40. Found: C, 74.15; H, 8.18; N, 5.40. [α]D21 +10.4 (c 0.3, CHCl3). [Φ]D21 +27 (c 0.3, CHCl3). TGA measurements: poly-2a ((S)-1a) exhibited an onset of weight loss at 230 °C; poly-2a ((rac)-1a) exhibited an onset of weight loss at 200 °C. Poly-2b. 1H NMR (CDCl3, 500 MHz): 7.70−7.67 (m, 2H, Ar), 7.57−7.53 (m, 4H, Ar), 7.48−7.45 (m, 2H, Ar), 6.40 (ddd, 1H, J = 17.2, 9.7, 7.7 Hz, CHCHCH2), 6.01 (br, 1H, CHCHCH2), 5.42 (d, 1H, J = 9.7 Hz, CHCHCH2), 5.39 (d, 1H, J = 17.2 Hz, CHCHCH2), 3.82−3.39 (m, 12H, CH2), 3.28 (s, 3H, CH3). 13C NMR (CDCl3, 126 MHz): δ 169.7, 138.8, 133.7, 132.1, 132.0, 131.9, 131.3, 128.5, 128.4, 128.3, 126.5, 125.8, 122.4, 119.5, 91.0, 89.1, 71.8, 70.5, 68.4, 64.6, 59.0. [α]D21 +10.7 (c 0.1, CHCl3). [Φ]D21 +42 (c 0.1, CHCl3). Poly-2c. 1H NMR (CDCl3, 400 MHz): δ 8.05 (d, 2H, J = 7.8 Hz, Ar), 7.68 (d, 2H, J = 7.8 Hz, Ar), 7.52 (d, 2H, J = 7.8 Hz, Ar), 7.45 (d, 2H, J = 7.8 Hz, Ar), 6.32−6.24 (m, 1H, CHCHCH2), 6.08 (br, 1H,
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ASSOCIATED CONTENT
* Supporting Information S
Experimental procedures and characterization data. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (N.K.). *E-mail:
[email protected] (K.O.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant-in-aid for young scientists (B) (26870344) from JSPS and partially by a JSPS research fellowship.
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REFERENCES
(1) Schlüter, A. D.; Hawker, C. J.; Sakamoto, J. Synthesis of Polymers: New Structures and Methods; Wiley-VCH: Weinheim, 2012. (2) Kobayashi, S. Catalysis in Precision Polymerization; John Wiley: Chichester, 1997. (3) Baugh, L. S.; Canich, J. A. M. Stereoselective Polymerization with Single-Site Catalysts; CRC Press: Boca Raton, FL, 2008. (4) Coates, G. W. Chem. Rev. 2000, 100, 1223−1252. (5) Brintzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143−1170. (6) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169−1204. (7) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. Chem. Rev. 2011, 111, 2363−2449. (8) Okamoto, Y.; Nakano, T. Chem. Rev. 1994, 94, 349−372. (9) Wulff, G.; Dhal, P. K. Angew. Chem., Int. Ed. 1989, 28, 196−198. (10) Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H. Comprehensive Asymmetric Catalysis; Springer: New York, 1999; Vol III, p 1329. (11) Ito, S.; Nozaki, K. In Catalytic Asymmetric Synthesis, 3rd ed.; Ojima, I., Ed.; John Wiley: Hoboken, NJ, 2010; p 931. (12) Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Chem. Rev. 2009, 109, 6102−6211. (13) Okamoto, Y.; Honda, S.; Okamoto, I.; Yuki, H.; Murata, S.; Noyori, R.; Takaya, H. J. Am. Chem. Soc. 1981, 103, 6971−6973. (14) Yamamoto, C.; Okamoto, Y. Bull. Chem. Soc. Jpn. 2004, 77, 227−257. 4184
dx.doi.org/10.1021/ma5008623 | Macromolecules 2014, 47, 4178−4185
Macromolecules
Article
(52) Kanbayashi, N.; Onitsuka, K. J. Am. Chem. Soc. 2010, 132, 1206−1207. (53) Kanbayashi, N.; Onitsuka, K. Angew. Chem., Int. Ed. 2011, 50, 5197−5199. (54) Takii, K.; Kanbayashi, N.; Onitsuka, K. Chem. Commun. 2012, 48, 3872−3874. (55) Kanbayashi, N.; Takenaka, K.; Okamura, T.; Onitsuka, K. Angew. Chem., Int. Ed. 2013, 52, 4897−4901. (56) KHCO3 was used as a base in this reaction, water would be formed as a byproduct, and the water should work as a nucleophile in this catalytic system to produce branched allylic alcohol. Therefore, all of the reactions described below have been conducted in the presence of MS 3A as a descant agent. (57) The SEC spectrum of the crude polymer has a small shoulder peak in the oligomer area, which was selectively removed by column chromatography on a silica gel. See the SI. (58) The polymerization of monomer 2a using (rac)-1a as a catalyst was carried out under the same reaction conditions, and the reaction smoothly proceeded with quantitative monomer conversion to give poly-2a-((rac)-1a) in 65% yield (Mn = 8800, Mw/Mn = 1.6) after purification by column chromatography on silica gel. (59) The allylic chloride group of the terminal moiety was not observed because it was converted to allylic alcohol by a slight amount of water as a nucleophile in the presence of the catalyst, which would not be detected due to overlapping with the peaks of the polymer main chain. (60) Kosaka, N.; Nozaki, K.; Hiyama, T.; Fujiki, M.; Tamai, N.; Matsumoto, T. Macromolecules 2003, 36, 6884−6887. (61) In the polymerization catalyzed by (rac)-1a, the molecular weight of the resulting polymer (poly-7-((rac)-1a) was decreased compared to that of poly-7-((S)-1b). Therefore, the reaction time was prolonged to 48 h.
(15) Yamamoto, T.; Suginome, M. Angew. Chem., Int. Ed. 2009, 48, 539−542. (16) Yamamoto, T.; Yamada, T.; Nagata, Y.; Suginome, M. J. Am. Chem. Soc. 2010, 132, 7899−7901. (17) Tang, Z.; Iida, H.; Hu, H.-Y.; Yashima, E. ACS Macro Lett. 2012, 1, 261−265. (18) Iida, H.; Iwahana, S.; Mizoguchi, T.; Yashima, E. J. Am. Chem. Soc. 2012, 134, 18150−18150. (19) Murahashi, S.; Nozakura, S.; Takeuchi, S. Bull. Chem. Soc. Jpn. 1960, 33, 658−659. (20) Fray, G. I.; Robinson, R. Tetrahedron 1962, 18, 261−266. (21) Pino, P.; Cioni, P.; Wei, J. J. Am. Chem. Soc. 1987, 109, 6189− 6191. (22) Pino, P.; Galimberti, M.; Prada, P.; Consiglio, G. Makromol. Chem. 1990, 191, 1677−1688. (23) Kaminsky, W.; Ahlers, A.; M?llerlindenhof, N. Angew. Chem., Int. Ed. Engl. 1989, 28, 1216−1218. (24) Jiang, Z. Z.; Sen, A. J. Am. Chem. Soc. 1995, 117, 4455−4467. (25) Nozaki, K.; Sato, N.; Takaya, H. J. Am. Chem. Soc. 1995, 117, 9911−9912. (26) Brookhart, M.; Wagner, M. I.; Balavoine, G. G. A.; Haddou, H. A. J. Am. Chem. Soc. 1994, 116, 3641−3642. (27) Nakamura, A.; Kageyama, T.; Goto, H.; Carrow, B. P.; Ito, S.; Nozaki, K. J. Am. Chem. Soc. 2012, 134, 12366−12369. (28) Coates, G. W.; Waymouth, R. M. J. Am. Chem. Soc. 1991, 113, 6270−6271. (29) Coates, G. W.; Waymouth, R. M. J. Am. Chem. Soc. 1993, 115, 91−98. (30) Nozaki, K.; Nakano, K.; Hiyama, T. J. Am. Chem. Soc. 1999, 121, 11008−11009. (31) Nakano, K.; Nozaki, K.; Hiyama, T. J. Am. Chem. Soc. 2003, 125, 5501−5510. (32) Nagai, D.; Sudo, A.; Endo, T. Macromolecules 2006, 39, 8898− 8900. (33) Kumagai, T.; Itsuno, S. Macromolecules 2000, 33, 4995−4996. (34) Itsuno, S.; Komura, K. Tetrahedron 2002, 58, 8237−8246. (35) Itsuno, S. Prog. Polym. Sci. 2005, 30, 540−558. (36) Trost, B. M.; Machacek, M. R.; Aponick, A. Acc. Chem. Res. 2006, 39, 747−760. (37) Mohr, J. T.; Stoltz, B. M. Chem.Asian J. 2007, 2, 1476−1491. (38) Lu, Z.; Ma, S. M. Angew. Chem., Int. Ed. 2008, 47, 258−297. (39) Trost, B. M.; Zhang, T.; Sieber, J. D. Chem. Sci. 2010, 1, 427− 440. (40) Kazamaier, U. Transition Metal Catalyzed Enantioselective Allylic Substitution in Organic Synthesis; Springer: New York, 2011. (41) Kiesewetter, M. K.; Edward, J. A.; Kim, H.; Waymouth, R. M. J. Am. Chem. Soc. 2011, 133, 16390−16393. (42) Nomura, N.; Tsurugi, K.; Okada, M. J. Am. Chem. Soc. 1999, 121, 7268−7269. (43) Nomura, N.; Yoshida, N.; Tsurugi, K.; Aoi, K. Macromolecules 2003, 36, 3007−3009. (44) Nomura, N.; Tsurugi, K.; Yoshida, N.; Komiyama, S.; Okada, M. J. Synth. Org. Chem. Jpn. 2007, 65, 334−346. (45) Nomura, N.; Komiyama, S.; Kasugai, H.; Saba, M. J. Am. Chem. Soc. 2008, 130, 812−814. (46) Dodo, N.; Matsushima, Y.; Uno, M.; Onitsuka, K.; Takahashi, S. J. Chem. Soc., Dalton Trans. 2000, 35−41. (47) Matsushima, E.; Komatsuzaki, N.; Ajioka, Y.; Yamamoto, M.; Kikuchi, H.; Takata, Y.; Dodo, N.; Onitsuka, K.; Uno, M.; Takahashi, S. Bull. Chem. Soc. Jpn. 2001, 74, 527−537. (48) Matsushima, Y.; Onitsuka, K.; Kondo, T.; Mitsudo, T.; Takahashi, S. J. Am. Chem. Soc. 2001, 123, 10405−10406. (49) Onitsuka, K.; Matsushima, Y.; Takahashi, S. Organometallics 2005, 24, 6472−6474. (50) Onitsuka, K.; Kameyama, C.; Sasai, H. Chem. Lett. 2009, 38, 444−445. (51) Onitsuka, K.; Okuda, H.; Sasai, H. Angew. Chem., Int. Ed. 2008, 47, 1454−1457. 4185
dx.doi.org/10.1021/ma5008623 | Macromolecules 2014, 47, 4178−4185
