Wei-Min Ren

CO2 Copolymers from Epoxides: Catalyst Activity, Product Selectivity, and Stereochemistry Control

The use of carbon dioxide as a carbon source for the synthesis of organic chemicals can contribute to a more sustainable chemical industry. Because CO(2) is such a thermodynamically stable molecule, few effective catalysts are available to facilitate this transformation. Currently, the major industrial processes that convert CO(2) into viable products generate urea and hydroxybenzoic acid. One of the most promising new technologies for the use of this abundant, inexpensive, and nontoxic renewable resource is the alternating copolymerization of CO(2) and epoxides to provide biodegradable polycarbonates, which are highly valuable polymeric materials. Because this process often generates byproducts, such as polyether or ether linkages randomly dispersed within the polycarbonate chains and/or the more thermodynamically stable cyclic carbonates, the choice of catalyst is critical for selectively obtaining the expected product. In this Account, we outline our efforts to develop highly active Co(III)-based catalysts for the selective production of polycarbonates from the alternating copolymerization of CO(2) with epoxides. Binary systems consisting of simple (salen)Co(III)X and a nucleophilic cocatalyst exhibited high activity under mild conditions even at 0.1 MPa CO(2) pressure and afforded copolymers with >99% carbonate linkages and a high regiochemical control (∼95% head-to-tail content). Discrete, one-component (salen)Co(III)X complexes bearing an appended quaternary ammonium salt or sterically hindered Lewis base showed excellent activity in the selectively alternating copolymerization of CO(2) with both aliphatic epoxides and cyclohexene oxide at high temperatures with low catalyst loading and/or low pressures of CO(2). Binary or one-component catalysts based on unsymmetric multichiral Co(III) complexes facilitated the efficient enantioselective copolymerization of CO(2) with epoxides, providing aliphatic polycarbonates with >99% head-to-tail content. These systems were also very efficient in catalyzing the terpolymerization of cyclohexene oxide, propylene oxide and CO(2). The resulting terpolymer had a single glass-transition temperature and a single thermolysis peak. This Account also provides a thorough mechanistic understanding of the high activities, excellent selectivities, and unprecedented stereochemical control of these Co(III)-based catalysts in the production of CO(2) copolymers . The catalysis occurs through a cooperative monometallic mechanism, in which the Lewis acidic Co(III) ion serves as electrophile to activate then epoxide and the nucleophilic counterion or cocatalyst serves as a nucleophile to initiate polymer-chain growth. The high activity and excellent regioselectivity observed in the epoxide ring-opening reactions results from epoxide activation through the moderate electrophilicity of the Co(III) ion, the fast insertion of CO(2) into the Co-O bond, and the facile dissociation of the propagating carboxylate species from the central metal ion. The reversible intra- or intermolecular Co-O bond formation and dissociation helps to stabilize the active Co(III) species against reversion to the inactive Co(II) ion. We also describe our laboratorys recent preparation of the first crystalline CO(2)-based polymer via highly stereospecific copolymerization of CO(2) and meso-cyclohexene oxide and the selective synthesis of perfectly alternating polycarbonates from the coupling of CO(2) with epoxides bearing an electron-withdrawing group.

Mechanistic Aspects of the Copolymerization of CO2 with Epoxides Using a Thermally Stable Single-Site Cobalt(III) Catalyst

The mechanism of the copolymerization of CO(2) and epoxides to afford the corresponding polycarbonates catalyzed by a highly active and thermally stable cobalt(III) complex with 1,5,7-triabicyclo[4,4,0] dec-5-ene (designated as TBD, a sterically hindered organic base) anchored on the ligand framework has been studied by means of electrospray ionization mass spectrometry (ESI-MS) and Fourier transform infrared spectroscopy (FTIR). The single-site, cobalt-based catalyst exhibited excellent activity and selectivity for polymer formation during CO(2)/propylene oxide (PO) copolymerization even at temperatures up to 100 degrees C and high [epoxide]/[catalyst] ratios, and/or low CO(2) pressures. The anchored TBD on the ligand framework plays an important role in maintaining thermal stability and high activity of the catalyst. ESI-MS and FTIR studies, in combination with some control experiments, confirmed the formation of the carboxylate intermediate with regard to the anchored TBD on the catalyst ligand framework. This analysis demonstrated that the formed carboxylate intermediate helped to stabilize the active Co(III) species against decomposition to inactive Co(II) by reversibly intramolecular Co-O bond formation and dissociation. Previous studies of binary catalyst systems based on Co(III)-Salen complexes did not address the role of these nucleophilic cocatalysts in stabilizing active Co(III) species during the copolymerization. The present study provides a new mechanistic understanding of these binary catalyst systems in which alternating chain-growth and dissociation of propagating carboxylate species derived from the nucleophilic axial anion and the nucleophilic cocatalyst take turns at both sides of the Co(III)-Salen center. This significantly increases the reaction rate and also helps to stabilize the active SalenCo(III) against decomposition to inactive SalenCo(II) even at low CO(2) pressures and/or relatively high temperatures.

Perfectly Alternating Copolymerization of CO2 and Epichlorohydrin Using Cobalt(III)-Based Catalyst Systems

Selective transformations of carbon dioxide and epoxides into biodegradable polycarbonates by the alternating copolymerization of the two monomers represent some of the most well-studied and innovative technologies for potential large-scale utilization of carbon dioxide in chemical synthesis. For the most part, previous studies of these processes have focused on the use of aliphatic terminal epoxides or cyclohexene oxide derivatives, with only rare reports concerning the synthesis of CO(2) copolymers from epoxides containing electron-withdrawing groups such as styrene oxide. Herein we report the production of the CO(2) copolymer with more than 99% carbonate linkages from the coupling of CO(2) with epichlorohydrin, employing binary and bifunctional (salen)cobalt(III)-based catalyst systems. Comparative kinetic studies were performed via in situ infrared measurements as a function of temperature to assess the activation barriers for the production of cyclic carbonate versus copolymer involving two electronically different epoxides: epichlorohydrin and propylene oxide. The relative small activation energy difference between copolymer versus cyclic carbonate formation for the epichlorohydrin/CO(2) process (45.4 kJ/mol) accounts in part for the selective synthesis of copolymer to be more difficult in comparison with the propylene oxide/CO(2) case (53.5 kJ/mol). Direct observation of the propagating polymer-chain species from the binary (salen)CoX/MTBD (X = 2,4-dinitrophenoxide and MTBD = 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene) catalyst system by means of electrospray ionization mass spectrometry confirmed the perfectly alternating nature of the copolymerization process. This observation in combination with control experiments suggests possible intermediates involving MTBD in the CO(2)/epichlorohydrin copolymerization process.

Asymmetric copolymerization of CO2 with meso-epoxides mediated by dinuclear cobalt(III) complexes: unprecedented enantioselectivity and activity.

Unprecedented enantioselectivity and catalytic activity was observed in the asymmetric copolymerization of CO2 with meso-epoxides (including the less reactive cyclopentene oxide) mediated by the dinuclear Co(III) complex (S,S,S,S)-1 under mild conditions. The resultant copolymers possess more than 99 % carbonate linkages and a perfectly isotactic structure.

Enhanced Asymmetric Induction for the Copolymerization of CO2 and Cyclohexene Oxide with Unsymmetric Enantiopure SalenCo(III) Complexes: Synthesis of Crystalline CO2-Based Polycarbonate

Enantiopure metal-complex catalyzed asymmetric alternating copolymerization of CO(2) and meso-epoxides is a powerful synthetic strategy for preparing optically active polycarbonates with main-chain chirality. The previous studies regarding chiral zinc catalysts provided amorphous polycarbonates with moderate enantioselectivity, and thus, developing highly stereoregular catalysts for this enantioselective polymerization is highly desirable. Herein, we report the synthesis of highly isotactic poly(cyclohexene carbonate)s from meso-cyclohexene oxide using dissymmetrical enantiopure salenCo(III) complexes in conjunction with bis(triphenylphosphine)iminium chloride (PPNCl) as catalyst. The presence of a chiral induction agent such as (S)-propylene oxide or (S)-2-methyltetrahydrofuran significantly improved the enantioselectivity regarding (S,S)-salenCo(III) catalyst systems. Up to 98:2 of RR:SS was observed in the resultant polycarbonates obtained from the catalyst system based on (S,S)-salenCo(III) complex 4d bearing an adamantyl group on the phenolate ortho position, in the presence of (S)-2-methyltetrahydrofuran. Primary ONIOM (DFT:UFF) calculations, which were performed to investigate the effect of the competitive coordination of (S)-induction agent versus cyclohexene oxide to Co(III) center on enantioselectivity, suggest that the (S)-C-O bond in cyclohexene oxide is more favorable for cleavage, due to the interaction between oxygen atom of (S)-induction agent and (S)-C-H of the coordinated cyclohexene oxide. The highly isotactic poly(cyclohexene carbonate) is a typical semicrystalline polymer, possessing a melting point of 216 °C and a decomposition temperature of 310 °C.

Bifunctional Aluminum Catalyst for CO2 Fixation: Regioselective Ring Opening of Three-Membered Heterocyclic Compounds

Regioselective ring opening of three-membered heterocyclic compounds (epoxides or N-substituted aziridines) at various temperatures was observed in coupling reactions with CO2 by the use of an aluminum-salen catalyst in conjunction with intramolecular quaternary ammonium salts as cocatalysts, affording the corresponding five-membered cyclic products with complete configuration retention at the methine carbon. Notably, this bifunctional aluminum-based catalyst exhibited nearly 100% regioselectivity for the ring opening at the methylene C-O bond for various terminal epoxides. This was true for those bearing an electron-withdrawing group, such as styrene oxide or epichlorohydrin, thereby affording the synthesis of various enantiopure cyclic carbonates that have previously been obtained only rarely by other methods. An intramolecular cooperative catalysis is suggested to contribute to the high activity and excellent stereochemistry control observed. Surprisingly, the highly selective ring opening at the methine carbon of N-substituted aziridines was found in the coupling with CO2, predominantly giving 5-substituted oxazolininones with retention of configuration as a result of double inversion at the methine carbon.

Binuclear chromium–salan complex catalyzed alternating copolymerization of epoxides and cyclic anhydrides

Mono- and bi-nuclear chromium(III)–salan complexes are efficient catalysts for the alternating copolymerization of terminal epoxides [such as epichlorohydrin (ClPO) and glycidyl phenyl ether (GO)] and cyclic anhydrides [e.g. maleic anhydride (MA) and succinic anhydride (SA)] to afford the corresponding copolymers with >99% ester content. The binuclear complex c bearing a binaphthol linker showed significantly higher activity than the mononuclear chromium–salan complexes a and b. For example, the catalytic activities (based on chromium concentration) of complex c for MA/ClPO and MA/GO copolymerizations are 4.1 and 7.1 times that of complex a, respectively. An intramolecular bimetallic synergistic effect, in which the alternating chain-growth and dissociation of propagating copolymer species take turns at the two metal ions of the binuclear catalyst c, was suggested to make a contribution to the enhanced catalytic activity. Importantly, when using the binuclear complex c as a catalyst for MA/(S)-GO copolymerization, a highly regioregular ring-opening step was observed with a concomitant >99% retention of configuration at the methine carbon center of epoxide incorporated into the polyester.

Alternating copolymerization of CO2 and styrene oxide with Co(III)-based catalyst systems: differences between styrene oxide and propylene oxide

A detailed study of the difference in reactivity of the copolymerization reactions of styrene oxidevs.propylene oxide with carbon dioxide utilizing binary (salen)cobalt(III) catalyst systems to provide perfectly alternating copolymers is reported. This investigation focuses on the discrepancy exhibited by these two terminal epoxides for the preference for C–O bond cleavage during the ring-opening process. It was found that the nucleophilic ring-opening of styrene oxide occurs predominantly at the methine Cα–O bond which leads to an inversion of configuration at the methine carbon center. This tendency results in a significantly lower reactivity as well as a deterrent for synthesizing stereoregular poly(styrene carbonate) when compared to the propylene oxide/CO2 process. The chiral environment about the metal center had a notable effect on the regioselectivity of the ring-opening step for styrene oxide, with the methylene Cβ–O bond being preferentially cleaved. Using a binary catalyst system composed of an unsymmetrical (S,S,S)-salenCo(III) complex in conjunction with the onium salt PPNY (PPN = bis(triphenylphosphine)iminium, and Y = 2,4-dinitrophenoxy), a highly regioregular ring-opening step was observed with a concomitant 96% retention of configuration at the methine carbon center.

Role of the co-catalyst in the asymmetric coupling of racemic epoxides with CO2 using multichiral Co(III) complexes: product selectivity and enantioselectivity

The kinetic resolution of racemic terminal epoxides with CO2 as reagent via enantioselective coupling represents an attractive method for affording enantiopure epoxides and optically active organic carbonates. A multichiral cobalt(III) complex in conjunction with an ammonium salt is an efficient catalyst system for the asymmetric coupling reaction of CO2 and racemic epoxides, and up to 97.1% ee for cyclic carbonate product and the highest krel (kinetic resolution coefficient) to date of 75.8 was obtained. The variation of nucleophilic co-catalyst and its relative loading dramatically changes the product selectivity and enantioselectivity. Both the anion and cation of ammonium salts have significant effects on the catalytic kinetic resolution process. An ammonium salt consisting of an anion with poor leaving ability and a bulky cation benefits for improving the enantioselectivity. The excess co-catalyst loading favors selective production of cyclic carbonate via the intramolecular cyclic elimination of the formed linear carbonate. A higher krel was observed in the excess co-catalyst loading, in comparison with one equivalent co-catalyst loading, predominantly resulting in polymer formation at the same temperature. It was also found that the excess co-catalyst loading led to significant increases in the regioselective ring-opening at the methylene carbon of various terminal epoxides, including styrene oxide with an electron-withdrawing group. The present study also offers a detailed mechanistic explanation of the role of the nucleophilic co-catalyst in the asymmetric coupling of racemic epoxides with CO2 using multichiral Co(III) complexes.

STEREOREGULAR POLY（CYCLOHEXENE CARBONATE）S： UNIQUE CRYSTALLIZATION BEHAVIOR

An example of crystalline CO2-based polymer from the asymmetric alternating copolymerization of CO2 and cyclohexene oxide is reported. Isotacticity of poly(cyclohexene carbonate) (PCHC) has the critical influence on the crystallinity, and only copolymers with a isotacticity of more than 90% are crystallizable. The stereoregular PCHC is a typical semi-crystalline thermoplastic, and possesses a high melting point (Tm) of 215–230°C and a decomposition temperature of ca. 310°C. The spherulitic morphology of (R)-PCHC grows in a clockwise spiral from a center, and that of (S)-PCHC is a counterclockwise spiral, while the stereocomplex of (S)-PCHC/(R)-PCHC (1/1 mass ratio) presents lath-like dendritic crystal. The novel crystalline CO2-based polycarbonate represents a rare example of optically active polymers with unique crystallization behavior. Our findings reflect the critical influence of stereoregularity on the crystallization for this kind of polymeric materials, and may lead to developments of thermal-resistance CO2 copolymers for application in engineering thermoplastics.