Studies of Ring-Opening Reactions of Styrene Oxide by Chromium

Jan 27, 2013 - common ring-opening reactions in organic chemistry and the recent report on the cobalt-catalyzed formation of PSC by. Darensbourg and ...
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Studies of Ring-Opening Reactions of Styrene Oxide by Chromium Tetraphenylporphyrin Initiators. Mechanistic and Stereochemical Considerations Nicole D. Harrold, Yang Li, and Malcolm H. Chisholm* Department of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210, United States S Supporting Information *

ABSTRACT: Chromium(III) tetraphenylporphyrin chloride, TPPCrCl, in the presence of bis(triphenylphosphine)iminium chloride, PPN+Cl−, is shown to react catalytically with styrene oxide, SO, under various conditions to lead to poly(styrene oxide), PSO, poly(styrene carbonate), PSC, styrene carbonate, SC, poly(styrene succinate), PSS, and β-hydroxyester. The stereochemical consequences of the ring-opening event in this coordination catalysis have been investigated by NMR spectroscopy, polarimetry, and chiral HPLC, employing both rac- and R-SO. The ring-opening event occurs by attack at both the methylene and methine carbons and nucleophilic attack at the methine occurs preferentially with inversion. These findings concerning coordinate catalysis are compared with the more common ring-opening reactions in organic chemistry and the recent report on the cobalt-catalyzed formation of PSC by Darensbourg and co-workers.

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INTRODUCTION Epoxides, oxiranes, are known to undergo ring-opening reactions by both electrophilic and nucleophilic attack. Polymerization by electrophilic attack tends to yield low molecular weight oligomers and rings via carbonium reactions.1 Higher molecular weight straight chain polymers are formed by base promoted reactions though molecular weights may be limited by competing proton abstraction reactions leading to olefinic end-groups.2 The highest molecular weight polymers are generally formed in coordinate catalysis, such as was originally developed by Union Carbide in their calcium−amide alkoxide catalytic production of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO.3 Strongly Lewis acidic metal centers can induce ring-opening in a cationic manner, leading to low molecular weight oligomers and cycles while weaker Lewis acidic metal centers can promote backside nucleophilic attack as was demonstrated by Jacobsen’s kinetic resolution of epoxides employing chiral chromium catalysts.4 The recent interest in the copolymerization of epoxides and carbon dioxide to give polyalkenecarbonates is another example of a metalpromoted ring-opening of epoxides, and various bimetallic mechanisms have been invoked.5 However, the pioneering work of Darensbourg showed that in the copolymerization of cyclohexene oxide and CO2 the propagating sequence involved only a single chromium center though the initial ring-opening event involved two chromium centers.6 He proposed the reaction depicted in Scheme 1. Subsequently, we observed that the homopolymerization of PO and the copolymerization of PO and CO2 by porphyrin aluminum(III) complexes was also first order in aluminum.7 Given the relative rigidity of the porphyrin ligand, the epoxide © 2013 American Chemical Society

Scheme 1

must be enchained by a near side attack of the type indicated in Scheme 1. We also found that the formation of head−head (HH) junctions in the polymer chain could occur with retention or inversion of configuration at the methine carbon, and the outcome was sensitive to the nature of the metal center. Furthermore, for the reaction involving R,R-salen chromium(III) with S-PO retention was favored while R-PO favored inversion. It should also be noted that the ratio of HH:TT:HT junctions is quite sensitive to the given catalytic system, and certain cobalt(III) catalysts yield nearly exclusive HT junctions. While much attention has been given to the copolymerization of propylene oxide, PO, and cyclohexene oxide with CO2, styrene oxide has received little attention except for the recent cobalt-catalyzed production of polystyrene carbonate, PSC, and cyclic carbonates by Lu and Darensbourg.8,9 Styrene oxide, SO, is notably less reactive than PO and other aliphatic and alicyclic Received: December 4, 2012 Revised: January 9, 2013 Published: January 27, 2013 692

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At room temperature the ratio of III to IV was 2:3 when R = Ph or n-octyl. At 60−70 °C the catalytic reaction is faster, and the ratio of III to IV is 3:2. Thus, higher temperatures favor attack at the methylene carbon while lower temperatures the methine. We had difficulty in separating the isomers III and IV, which obviated our desire to examine the stereochemistry of the methine carbon in IV. Consequently, we oxidized both isomers with CrO3 in acetone/H2SO4 at 0 °C. Isomer III yielded a ketone and IV a carboxylic acid which were now more readily separable by column chromatography. The oxidation of IV is known not to change the stereochemistry at the methine carbon.12 When the reaction was carried out with R-SO, the resultant carboxylic acid, V, was found to have an R to S ratio of 1:2 as determined by polarimetry.

epoxides but is known to undergo ring-opening with acids and bases. The acid and base hydrolyses of SO was studied by Lin and Whalen, whose results indicated attack at both the methine and methylene carbon and thus conflicted with previous findings.10 The metal-assisted coordination ring-opening of an epoxide would seem to invoke a combination of both mechanisms. The metal behaves as a Lewis acid, and the attendant ligand acts as a nucleophile. This is akin to a cismigratory insertion process as commonly seen in metal−alkyl olefin polymerizations and in the ring-opening of cyclic esters by metal alkoxide ligands.11 In order to probe further into the mechanism of this coordination assisted ring-opening of epoxide, we elected to investigate the role and stereochemical influence of the metal in the activation of SO.

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RESULTS AND DISCUSSION The Initial Ring-Opening Event. The stoichiometric reaction between tetraphenylporphyrin chromium chloride, TPPCrCl, and SO in the presence of 1 equiv of PPN+Cl− at room temperature followed by reaction with acetic acid yields the chloro alcohols shown in I and II in the ratio 2:1, respectively. Formation of I implies chloride attack at the methylene carbon while formation of II attack at the methine carbon.

This implies that the attack at the methine carbon shows preference for inversion of configuration which contrasts with that reported for both acid- and base-catalyzed hydrolyses and methanolysis of SO. Styrene Carbonate Formation. At 60−70 °C SO and CO2 (30 bar) in the presence of TPPCrCl yield exclusively styrene carbonate, SC. At lower temperatures the slower formation of SC along with polystyrene carbonate, PSC, (vide infra) is seen. At room temperature R-SO and CO2 yielded formation of 70% R-SC and 30% S-SC along with polystyrene carbonate, PSC. The formation of cyclic carbonates in reactions involving epoxides and CO2 by coordinate catalysis is commonly seen and is believed to occur by a backbiting mechanism.8 Backbiting can occur by one of two processes as shown in Schemes 2 and 3. Note that backbiting by the alkoxide ligand (Scheme 2) involves the attack of the carbonate carbon with expulsion of SC and re-formation of the alkoxide to metal bond. This reaction does not change the stereochemistry of the ring-opened epoxide. Thus, alkoxide backbiting will retain directly the information on the stereochemistry of the ringopening event. In contrast, backbiting by the alkylcarbonate (Scheme 3) ligand formally displaces the alkoxide anion and involves an intramolecular nucleophilic attack on an sp3hybridized carbon. We anticipate that this ring closure would involve inversion at the carbon being attacked by the carbonate. If the original ring-opening involved attack at the methylene carbon, the original stereochemistry would be retained. However, if the original ring opening involved attack at the

While the initial ring-opening of the epoxide by the chloride ligand is relatively fast, the subsequent further ring-opening of SO is slow at room temperature. However, upon heating to 70 °C the reaction to produce polystyrene oxide, PSO, occurs slowly with a turnover frequency, TOF, of less than 1 per hour. The PSO formed in this reaction is regiorandom in contrast to the reaction between SO and NaOEt at 70 °C which yields regioregular (HT)nPSO (see Figure S1 in the Supporting Information). When SO and a carboxylic acid are present with the porphyrin chromium chloride complex, there is a catalytic ringopening reaction to yield the β-hydroxy esters shown in III and IV.

Scheme 2. Mechanism of SC Formation from Backbiting of the [Cr]-Alkoxide

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Scheme 3. Mechanism of SC Formation from Backbiting of the [Cr]-Alkylcarbonate

methine carbon, the ring closure could lead to a “double inversion” or retention of stereochemistry if the ring-opening went by inversion. If we assume that the ring-opening of the epoxide by an alkylcarbonate is similar to that involving the carboxylate then at room temperature, we expect that the alkylcarbonate will prefer ∼60% attack at the methine carbon and ∼40% at the methylene carbon. The latter attack will retain the R-SO in the SC. Based on the stereochemistry of V, which results from ringopening attack at the methine carbon, we expect that of the 60% two-thirds will go with inversion. Based on these data, if the formation of SC were to proceed via backbiting from the alkoxide ligand, we would expect the resultant ratio of R- to SSC to be about 3:2. However, if the formation of SC were to proceed via backbiting from the alkylcarbonate ligand, we would expect the resultant ratio to be 4:1 R- to S-SC. Our observed ∼70% R-SC and 30% S-SC is readily accounted for if both mechanisms are occurring in equal propensity. It should also be noted that neither S- nor R-SC is racemized by TPPCrCl/PPN+Cl− under the conditions just described as determined by both polarimetry and chiral HPLC. Experimentally, we observed an initial burst of SC formation followed by a gradual slow increase of SC throughout the remainder of the reaction. We hypothesized that the initial SC formation occurs by chloride ion displacement from the carbonate ligand formed upon the initial ring-opening of SO, namely [Cr]−O2CO(SO)−Cl. This implies that the stereochemistry of the initially formed SC would follow from that for I and II, subject to the stereochemical outcome of backbiting by the alkyl carbonate (Scheme 3). Thus, we expect 33% R-SC via I plus 44% (two-thirds × 66%) R-SC contribution via II by double inversion. The experimentally observed ratio of R- to SSC was 78% R-SC to 22% S-SC, which is in agreement with this hypothesis. Subsequent to chain growth, SC formation would occur without re-forming the Cr−Cl bond.

Polystyrene Carbonate Formation. As noted above, at room temperature SO and CO2 (30 bar) react slowly to yield a mixture of SC and polystyrene carbonate, PSC. The polymer is not soluble in hot hexanes, which readily allows removal of SC. The PSC can then be extracted with chloroform and precipitated with cold acidic (HCl) methanol, yielding a powdery fluffy solid. For the reaction involving 1000:1 SO to TPPCrCl/PPN+Cl− the yield of PSC was 40% after 70 h with a Mw ∼ 10 000 Da and PDI = 1.48. The 1H and 13C{1H} NMR spectra of the PSC revealed that it was free from ether rich linkages; i.e., the homopolymerization of SO does not effectively compete with the copolymerization. The 1H NMR spectrum is given in the Supporting Information (Figure S10), and the 13C{1H} NMR of the carbonate region is shown in Figure 1. The NMR reveals a HT:TT:HH (Scheme 4) ratio of

Figure 1. Carbonate region of the 13C{1H} NMR of PSC formed from the reaction of SO and CO2 catalyzed by TPPCrCl/PPN+Cl−. 694

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Scheme 4. Head−Head (HH), Tail−Tail (TT), and Head− Tail (HT) Junctions of PSC

Figure 2. Carbonate region of the 13C{1H} NMR of PSC formed from the reaction of R-SO and CO2 catalyzed by TPPCrCl/PPN+Cl−.

In the formation of PSC there must be an enhancement of attack at the methylene carbon. Polystyrene Succinate Formation. Recognizing that the rate of ring-opening of propylene oxide, PO, by TPPAl−X bonds follows the order X = O2CR > O2COR > OR,5d,7 we investigated the reaction involving SO and succinic anhydride, SA, catalyzed by TPPCrCl/PPN+Cl−. At 70 °C this catalytic reaction proceeds with a TOF of ∼200 per hour to give regiorandom polystyrene succinate, PSS (vide infra). In reactions involving SO to [Cr] ratios of 100:1, 200:1, 500:1, and 1000:1 where the SA:SO ratio was greater than 1, the polymers formed had a very narrow PDI in the range of 1.04−1.11 with a Mw ∼ 2000−9000 Da. When the ratio of SA:SO was exactly 1:1, the polymers formed had much larger molecular weights in the range of 10 000−24 000 Da. We attribute the lower Mw of PSS when the SA:SO > 1 to the increased presence of phenylacetaldehyde, PA, as confirmed by 1 H NMR (see Figure S5). Succinic anhydride, even after careful sublimation, contains trace amounts of acid that can ring-open SO to its isomer PA, which after tautomerization to the corresponding enol, can act as a chain-transfer agent (Scheme 5).13 Thus, even minimal formation of PA can dramatically

approximately 3:1:1 at 125 MHz (13C NMR), and there is little stereochemical information to be gained beyond the apparent splitting of the HT resonance into two signals. This is presumably due to the presence of i and s diad sequences in the HT junction as depicted in VI and VII.

Scheme 5. Chain-Transfer Agent Formation from SO

The 13C{1H} NMR spectrum of PSC formed from R-SO is shown in Figure 2, which reveals that one of the diads is now present in significant excess. If we again assume that the ringopening of SO by the alkylcarbonate ligand is comparable in stereochemical outcome to that of the carboxylate at room temperature, then attack at the methine carbon is favored 3:2 over that at the methylene. Attack at the methylene leads to retention of stereochemistry while attack at the methine yields a 2:1 inversion to retention. This combination would favor the isotactic diad over the syndiotactic diad by an order of 3:2. Inspection of Figure 2, however, implies a ratio on the order of 4:1. Degradation with NaOEt of the PSC formed from R-SO revealed an R- to S- ratio of 4:1 as determined by chiral HPLC and polarimetry. This is consistent with our suggested assignment of the isotactic diad as the peak in significant excess in Figure 2. Thus, the expectation of the 3:2 ratio of isoto-syndio-tactic diads based on the assumption that an alkylcarbonate behaves as carboxylate is an oversimplification.

influence the Mw of the resultant PSS. Also at this time we believe that the presence of adventitious water not only leads to succinic acid formation but also results in chain transfer of the carboxylate end groups. In order to test our theory on the role of PA in the PSS polymerization, we performed a series of experiments to verify the correlation between the amount of PA present and PSS Mw. In the first set of NMR experiments we added various equivalents of SA to a set amount of SO, minus any catalyst, and heated for several hours. Upon completion, we measured the amount of PA present by integrating its aldehyde resonance in the 1H NMR. As anticipated, we observed a linear relationship between the amount of SA added and PA formed, 695

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thus confirming our suspicion that minimal acid present in the SA is ring-opening a small amount of SO to PA. In the second set of experiments, we added SO and SA in a 1:1 ratio, the catalyst TPPCrCl/PPN+Cl−, and various quantities of PA. These polymerizations were stirred overnight in an oil bath at 70 °C. Upon work-up, the Mw of the polymers indicated that as amount of added PA increased, Mw of the resultant polymer decreased. See Supporting Information for these data (Figures S14−S17). When SA is employed as a limiting reagent, the polymer that is formed is a block copolymer H(SO)n(SO-SA)m-Cl upon hydrolysis and work-up (see Figure S4). The PSO is observed to be regioirregular by 1H and 13C{H} NMR, in contrast to the related polymerization of PO by TPPCrCl/PPN+Cl− which yields regioregular (HT)n poly(propylene oxide), PPO.14 These findings complement the report be Coates et al. on the copolymerization of epoxides and cyclic anhydrides by zinc catalysts and other more recent reports by Duchateau and Darensbourg.15 The reaction of SA with SO when the SA:SO ratio is greater than or equal to 1 resulted in perfect alternating copolymers without measurable side products, i.e., cyclic esters or polyether. The carboxylate region of PSS is shown in Figure 3 and clearly shows the presence of four resonances

Scheme 6. Tail−Tail (TT), Head−Tail (HT), and Head− Head (HH) Junctions of PSS

Figure 3. Carbonate region of the 13C{1H} NMR of PSS formed from the reaction of SO and SA catalyzed by TPPCrCl/PPN+Cl−.

corresponding to the TT, HT, and HH junctions (Scheme 6). We propose the splitting observed in the resonance at 171.19 ppm is due to the presence of i and s diads of the HH junction. In order to verify this, we prepared a model compound (see VIII) that possesses the same microstructural features as the PSS HH junction. Treatment of cold (−78 °C) dichloromethane solutions of freshly prepared succinic chloride with excess 1-phenylethanol in the presence of a catalytic amount of pyridine results in 80% conversion to the corresponding dicarbonate, bis(1phenylethyl)succinate, VIII, upon warming to room temperature. The 1H of VIII indicated the presence of a pair of isomers, with each isomer possessing a distinct methylene resonance (Figure 4, right). The first compound, (R)-1phenylethyl ((S)-1-phenylethyl) succinate, VIII-a, contains a center of inversion so the methylene region shows a simple pattern. The second compound, bis((S)-1-phenylethyl) succi-

Figure 4. Carbonate region of the 13C{1H} NMR of VIII (left) and methylene region of the 1H NMR of VIII (right).

nate, VIII-b, does not contain a center of inversion so the methylene region shows a more complicated AA′BB′ pattern. Perhaps the most valuable piece of information obtained, however, is the presence of two carbonate signals in the 13 C{1H} NMR spectrum of VIII (Figure 4, left). This indicates that we can, indeed, observe i and s diads of the HH junction in 13 C{1H} NMR of the PSS polymer. Meso compound, VIII-b, was also prepared independently from (S)-1-phenylethanol and succinic chloride, and the AA′BB′ pattern observed in the 1H NMR has been simulated (Figure 5). Only one carbonate signal 696

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is observed in the 13C{1H} NMR, which corresponds to the signal farthest downfield at 171.42 ppm.

this is as described earlier, then attack at the methine carbon would result in 2:1 inversion to retention. Based on these statistics, utilization of R-SO in the copolymerization with SA would result in a decrease in the number of isotactic HH junctions compared to the number of syndiotactic HH junctions. This leads us to conclude the resonance at 171.18 ppm corresponds to the i-HH junction, while the resonance at 171.20 corresponds to the s-HH junction.

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CONCLUSIONS This is one of the first studies of the stereochemical consequences of ring-opening of SO by a coordinate metal catalyst.8b,16 The TPPCrCl/PPN+Cl− system is an effective catalyst in the formation of SC, PPC, and PSS though not as active as some other systems recently reported in the literature. The mechanisms of the reaction can be viewed as an interchange associative nucleophilic attack on the d3-chromium octahedral metal complex by SO which leads to its activation by the electrophilic metal center. The ring-opening event is then comparable to a 1,2-migratory attack by the metal-bound ligand (Cl, OR, O2COR, or O2CR) at an epoxide-activated carbon, somewhat akin to that in olefin polymerization by electrophilic metal centers bearing polar metal−carbon (alkyl) bonds. This attack can occur at either the methylene or methine carbon, and this outcome appears sensitive to the nature of the Cr−X bond involved in the ring-opening event as well as the temperature. Ring-opening at the methine carbon occurs with a significant, though modest, preference for inversion. The system appears well suited for modeling by computational methods, and it will be interesting to see how related metal catalysts perform stereochemically. In addition, these mechanistic studies could assist in the development of catalytic systems that selectively form polycarbonates over cyclic carbonates.

Figure 5. Experimental methylene region of the 1H NMR of VIII-b (bottom) and AA′BB′ pattern simulation (top). See full spectrum in the Supporting Information (Figure SI-9).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and compound characterization data; Figures S1−S17. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. 13

1

Notes

Figure 6. Carbonate regions of the C{ H} NMR of PSS formed from the reaction of rac-SO (red) or (R)-SO (black) and SA catalyzed by TPPCrCl/PPN+Cl−.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Department of Energy, Office of Basic Sciences, Chemistry Division, for financial support of this work.

To distinguish the TT peak from the HT peaks in the C{1H} NMR spectrum of PSS, we prepared a sample of PSS from R-SO and SA catalyzed by TPPCrCl/PPN+Cl− at 60 °C. We observed a decrease in the amount of HH junctions at 171.19 ppm, which allowed us to identify the signal corresponding to the TT junctions as the other peak whose intensity decreased, namely that at 171.76 ppm. Accordingly, the peaks at 171.82 and 171.28 ppm are assigned as the HT junction. Interestingly, exchanging rac-SO to R-SO under the same reaction conditions results in only the single most upfield HH resonance to decrease intensity by approximately two-thirds. The ring-opening of SO in its copolymerization with SA involves the attack of SO by a carboxylate group. If we assume 13

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