Asymmetric Alternating Copolymerization of Cyclohexene Oxide and CO₂ with Dimeric Zinc Complexes

Moore D.R., Cheng M., Lobkovsky E.B., Coates G.W.

Kumagai T., Itsuno S.

Darensbourg D.J., Yarbrough J.C.

The air-stable, chiral (salen)Cr(III)Cl complex (3), where H(2)salen = N,N'-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexene diamine, has been shown to be an effective catalyst for the coupling of cyclohexene oxide and carbon dioxide to afford poly(cyclohexenylene carbonate), along with a small quantity of its trans-cyclic carbonate. The thus produced polycarbonate contained >99% carbonate linkages and had a M(n) value of 8900 g/mol with a polydispersity index of 1.2 as determined by gel permeation chromatography. The turnover number (TON) and turnover frequency (TOF) values of 683 g of polym/g of Cr and 28.5 g of polym/g of Cr/h, respectively for reactions carried out at 80 degrees C and 58.5 bar pressure increased by over 3-fold upon addition of 5 equiv of the Lewis base cocatalyst, N-methyl imidazole. Although this chiral catalyst is well documented for the asymmetric ring-opening (ARO) of epoxides, in this instance the copolymer produced was completely atactic as illustrated by (13)C NMR spectroscopy. Whereas the mechanism for the (salen)Cr(III)-catalyzed ARO of epoxides displays a squared dependence on [catalyst], which presumably is true for the initiation step of the copolymerization reaction, the rate of carbonate chain growth leading to copolymer or cyclic carbonate formation is linearly dependent on [catalyst]. This was demonstrated herein by way of in situ measurements at 80 degrees C and 58.5 bar pressure. Hence, an alternative mechanism for copolymer production is operative, which is suggested to involve a concerted attack of epoxide at the axial site of the chromium(III) complex where the growing polymer chain for epoxide ring-opening resides. Preliminary investigations of this (salen)Cr(III)-catalyzed system for the coupling of propylene oxide and carbon dioxide reveal that although cyclic carbonate is the main product provided at elevated temperatures, at ambient temperature polycarbonate formation is dominant. A common reaction pathway for alicyclic (cyclohexene oxide) and aliphatic (propylene oxide) carbon dioxide coupling is thought to be in effect, where in the latter instance cyclic carbonate production has a greater temperature dependence compared to copolymer formation.

Liu Z., Torrent M., Morokuma K.

The mechanism of copolymerization of CO2 with cyclohexene oxide catalyzed by the Zn(II) organometallic compound (BDI)ZnOCH3 (BDI = N(2,6-iPr2C6H3)C(Me)CHC(Me)N(2,6-iPr2C6H3) chelating β-diimine ligand) has been studied with the hybrid molecular orbital (MO) method ONIOM, combining the density functional method B3LYP/LANL2DZ(d) with the semiempirical MO method PM3. In particular, the insertions of CO2 and cyclohexene oxide/ethylene oxide into zinc−alkoxyl and zinc−carbonate bonds have been investigated in detail. The insertion of CO2 into either a zinc−alkoxyl (epoxide + CO2 alternating insertion) or zinc−carbonate (consecutive CO2 insertion) bond has been found to be thermodynamically less favorable but is in general kinetically favored over the insertion of epoxide, due to a high barrier for the latter. This high barrier is associated with a rather asynchronous transition state where the ring opening has taken place and yet the C−O bond is not formed. However, only in the case of insertion of sterically ...

Isobe Y., Onimura K., Tsutsumi H., Oishi T.

Asymmetric anionic homopolymerizations of N-1-anthrylmaleimide (1-AMI) were performed with diethylzinc (Et2Zn)-chiral ligand complexes to obtain optically active polymers. The optical activity of poly(1-AMI) was influenced by polymerization conditions such as temperature, solvents and structures of chiral ligands. Poly(1-AMI) obtained with Et2Zn/(–)-2,2’-(1-ethylpropylidene)bis(4-benzyl-2-oxazoline) (Bnbox) in THF at 0°C showed the highest specific optical rotation ([α]D25=+92.5°). CD spectra of (+)-poly(1-AMI) exhibited negative exciton chirality around 230–300 nm, indicating that 1Bbtransition moments of anthryl groups were twisted counterclockwise. Chiral recognition ability of poly(1-AMI) was investigated by 1H NMR and HPLC, and the polymer coated on silica gel optically resolved 1, 1’-bi-2-naphthol.

Stamp L.M., Mang S.A., Holmes A.B., Knights K.A., de Miguel Y.R., McConvey I.F.

A new polymer-supported chromium porphyrin has been prepared and fully characterised; its catalytic activity and recyclability were investigated for the ring-opening copolymerisation of 1,2-cyclohexene oxide (CHO) and carbon dioxide (CO2).

Kumagai T., Itsuno S.

We present a study of the asymmetric polymerization of dialdehyde and bis(allylsilane) based on the enantioselective addition of allylsilane to aldehyde in the presence of chiral Lewis acid. Chiral (acyloxy)borane (CAB) derived from enantiopure tartaric acid was used as a chiral Lewis acid to promote the asymmetric polymerization. Repetitive allylation reaction between these monomers in the presence of CAB yielded optically active polymers having asymmetric carbons in their main chain. The corresponding model asymmetric reaction of β-substituted allylsilane with benzaldehyde was also studied.

Cheng M., Moore D.R., Reczek J.J., Chamberlain B.M., Lobkovsky E.B., Coates G.W.

Synthetic routes to zinc beta-diiminate complexes are reported. The synthesis of 11 beta-diimine [(BDI)-H] ligands, with varying N-aryl substituents and bridging structures, is described. These ligands are converted to (BDI)ZnX complexes (X = OAc, Et, N(SiMe3)2, Br, Cl, OH, OMe, O(i)Pr). X-ray structural data revealed that all zinc complexes examined exist as micro-X-bridged dimers in the solid state, with the exception of the zinc ethyl and amido complexes which were monomeric. Complexes of the form (BDI)ZnOR (R = alkyl, acyl) and (BDI)ZnN(SiMe3)2 are highly active catalysts for the alternating copolymerization of epoxides and CO2. Copolymerizations of cyclohexene oxide (CHO) and CO2 with (BDI-1)ZnX [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene)] and (BDI-2)ZnX [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene)], where X = OAc, Et, N(SiMe3)2, Br, Cl, OH, OMe, O(i)Pr, were attempted at 50 degrees C and 100 psi CO2. Complexes with X = OAc, N(SiMe3)2, OMe, O(i)Pr all produced polycarbonate by the alternated insertion of CHO and CO2 with similar catalytic activities, comparable molecular weights, and narrow molecular weight distributions (MWD approximately 1.1), indicating the copolymerizations are living. Furthermore, ligand effects were shown to dramatically influence the polymerization activity as minor steric changes accelerated or terminated the polymerization activity.

Nakano K., Nozaki K., Hiyama T.

Signals of 13C NMR spectra of poly[cyclohexene oxide-alt-carbon dioxide] in a carbonyl region split into two parts:  153.7 and 153.3−153.1 ppm. To assign these signals, isotactic and syndiotactic model dimers and tetramers were synthesized. The signals of isotactic oligomers appeared at 153.8 ppm, whereas the signals of syndiotactic isomers appeared at higher field and shifted to higher field (up to 153.1 ppm) on going from dimer to tetramer. These results led to the final assignment of the copolymer:  the signal at 153.7 ppm includes isotactic diads, and the ones at 153.3−153.1 ppm include syndiotactic diads.

Koning C., Wildeson J., Parton R., Plum B., Steeman P., Darensbourg D.J.

Using [Zn(2,6-difluorophenoxide)2]2 (THF)2 as the catalyst, poly(cyclohexane carbonate) (PCHC) was synthesized from CO2 and cyclohexene oxide. The ether content of the polymer was limited to a few mol%. The molecular weight distribution of the linear polycarbonate was broad, with Mn=42 and M w =252 kg/mol. The recorded Tg was 115°C, which is in excellent agreement with the reported value of 116°C. In spite of its high molar mass, PCHC behaves like a brittle polymer, with an elongation at break of 1–2%. On the other hand, the tensile modulus of PCHC (3600 MPa) is much higher than the corresponding value for bisphenol-A polycarbonate (BP-A PC) (2400 MPa). Like the extremely tough BP-A PC, the PCHC exhibits a γ-transition around −110°C, the presence of which has been related to toughness. The magnitude of this γ-transition is lower than the corresponding value for BP-A PC, which indicates that the main chain of PCHC is less flexible than that of BP-A PC. Moreover, the low temperature relaxation of PCHC is probably related to chair–chair transitions of the cyclohexane side group. The brittle behavior of PCHC is expected from the relatively low plateau modulus of PCHC in the melt, from which a relatively high average molecular weight between entanglements (Me) of ca. 15,000 g/mole was estimated, which is in the same order of magnitude as the Me of the brittle polystyrene.

Darensbourg D.J., Rainey P., Yarbrough J.

A series of salicylaldimine ligands of the general formula (NR2C7H5-x(R1)xOH) [x = 1 or 2; R1 = Me, tBu, Cl, OMe; R2 = 2,6-iPr2C6H3, or 3,5-(CF3)2C6H3] have been synthesized and characterized via 1H and 13C NMR, elemental analysis, and X-ray crystallography. The concomitant series of zinc bis(salicylaldiminato) complexes of the general formula have been synthesized and characterized in the solid state by X-ray crystallography. All complexes crystallized as four coordinate monomers with distorted tetrahedral geometry about the zinc center. The O−Zn−O angles range between 105 and 112.5°, and the N−Zn−N bond angles were more obtuse spanning the range 122.9−128.9°. The only deviation from distorted tetrahedral geometry occurred when R2 = 3,5-(CF3)2C6H3 which crystallized as a distorted trigonal bipyramidal dimeric species with Oax−Zn−Oax bond angles of 165.00(15)°. The equatorial angles approach 120° except for the Neq−Zn−Neq angle of 110.54(16)° which is attributed to the strain of the bridging ligands. The ...

Darensbourg D.J., Wildeson J.R., Yarbrough J.C., Reibenspies J.H.

Dimeric phenoxide derivatives of zinc and cadmium have been synthesized from the reaction of the corresponding metal bistrimethylsilylamide and two equivalents of 2,6-F2C6H3OH in tetrahydrofuran. The zinc analogue, [Zn(O-2,6-F2C6H3)2·THF]2 (1), has been characterized in the solid state via X-ray crystallography, where the zinc centers are shown to possess distorted tetrahedral geometry containing two bridging phenoxides and a terminal phenoxide and THF ligand. The distance between the metal centers (Zn···Zn) was found to be 3.059 A, and the THF ligands lie on opposite sides of the plane formed by the two zinc atoms and two bridging phenoxide ligands' oxygen atoms. There are several Zn···F nonbonding distances involving the bridging phenoxide ligands that are less than the van der Waals internuclear distance. In addition, both the zinc and cadmium dimeric derivatives have been prepared such that the labile THF ligands are replaced by the sterically encumbering basic phosphine, PCy3. The solid-state structu...

Sarbu T., Styranec T., Beckman E.J.

Liquid and supercritical carbon dioxide have attracted much interest as environmentally benign solvents1, but their practical use has been limited by the need for high CO2 pressures to dissolve even small amounts of polar, amphiphilic, organometallic, or high-molecular-mass compounds2,3,4. So-called ‘CO2-philes’ efficiently transport insoluble or poorly soluble materials into CO2 solvent, resulting in the development of a broad range of CO2-based processes, including homogeneous and heterogeneous polymerization, extraction of proteins and metals, and homogeneous catalysis5,6,7,8,9,10,11. But as the most effective CO2-philes are expensive fluorocarbons, such as poly(perfluoroether), the commercialization of otherwise promising CO2-based processes has met with only limited success. Here we show that copolymers can act as efficient, non-fluorous CO2-philes if their constituent monomers are chosen to optimize the balance between the enthalpy and entropy of solute–copolymer and copolymer–copolymer interactions. Guided by heuristic rules regarding these interactions, we have used inexpensive propylene and CO2 to synthesize a series of poly(ether-carbonate) copolymers that readily dissolve in CO2 at low pressures. Even though non-fluorous polymers are generally assumed to be CO2-phobic, we expect that our design principles can be used to create a wide range of non-fluorous CO2-philes from low-cost raw materials, thus rendering a variety of CO2-based processes economically favourable, particularly in cases where recycling of CO2-philes is difficult.

Nakano T., Tamada D., Miyazaki J., Kakiuchi K., Okamoto Y.

Cheng M., Darling N.A., Lobkovsky E.B., Coates G.W.

Enantiomerically pure zinc complexes have been designed and synthesized for the enantioselective copolymerization of cycloalkene oxides and CO2.

Rusconi Y., D'Alterio M.C., De Rosa C., Coates G.W., Talarico G.

The factors contributing to the enantioselective ring opening copolymerization of CO2 and meso-epoxide have been unveiled by DFT calculations combined with activation strain model and noncovalent interaction analysis.

Shi F., Qian B., Yang S., Liu Y., Li P., Wang H., Cui X.

AbstractCarbon dioxide based degradable polycarbonate can be obtained through the copolymerization reaction of carbon dioxide with epoxide in the presence of a catalyst. This polymer has attracted much attention in recent years owing to its environmentally friendly and sustainable characteristics, and excellent material properties. Due to its unique properties, CO2-based polycarbonate has a wide range of applications in many fields such as electronic and electrical parts, automotive parts, medical devices, aerospace equipment, power electronic equipment, and radiation protection products. Therefore, numerous catalytic systems have been explored for the CO2/epoxide copolymerization process, in which zinc catalyst has the longest history and the greatest variety. In this short review, the significant advances in zinc catalysts for the copolymerization transformation of CO2 with epoxide are demonstrated, covering both heterogeneous and homogeneous catalysts. Moreover, both benefits and drawbacks of zinc catalytic system are described, and the outlook for large-scale industrial applicati ons in the future is also represented.1 Introduction2 Heterogeneous Zinc Catalysts3 Homogeneous Zinc Catalysts4 Overview of Heterogeneous and Homogeneous Zinc Catalysts5 Conclusion

Ghosh D., Dabas S., Kalose S.A., Gotgi N.M., Subramanian S.

The utilization of carbon dioxide as a C1 feedstock to construct molecules has been extensively studied and successfully employed in diverse catalytic processes. In this context, asymmetric organic transformations leveraging...

Deng Z., Yuan D., Yao Y.

Yang G., Xie R., Zhang Y., Xu C., Wu G.

Singha Roy S., Sarkar S., Srinivasan N., Antharjanam P.K., Kunnikuruvan S., Chakraborty D.

Maurya Y., Singh A., Kumar V., Ul Nisa M., Chatterjee S.

In this review we have tried to present our view on the synergy between nature and chemists' toolbox toward discovery of new reactions. Toward the search for new chemical reactivity, not only chemists learn from nature but nature's evolutionary pathway perhaps has footprints of chemist's innovative discoveries and beyond. Taking CO2 as a representative example of an abundant energy rich small molecule resource, we have tried to explore the probable strategies of activating the molecule using first row transition metal catalysts for sustainable organic transformation. Our study has tried to unfold nature's strategy of CO2 activation and recycling which eventually motivated chemists toward discovery of new chemical reactions and further inspired to expand their toolbox for discovering new reactivities beyond the natural enzyme repertoire. The basic chemical CO2 activation strategy which have been focused in this review are using CO2 as a C1 building block in organic synthesis; catalytic reductive formylation and methylation of amines; carboxylation reactions via CO2 insertion. Additionally, carbonylation reactions with CO2 as CO surrogate has also been discussed. The focus of this review is to correlate nature's chemical strategies with chemist's approach to understand the concept involved in the chemical activation and utilization process of CO2 for unlocking new reaction paradigm, both for chemists' and for nature's catalytic repertoire.

Thierry T., Geiger Y., Bellemin-Laponnaz S.

Asymmetric catalysis has expanded the range of chiral products readily accessible through increasingly efficient synthetic catalysts. The development of these catalysts often starts with a result obtained by systematic screening of known privileged chiral structures and assumes that the active species would be an isolated monomolecular species. Here we report the study of three proline-derived ligands, diphenyl-N-methyl-prolinol, diphenylprolinol and 5-(hydroxydiphenylmethyl)-2-pyrrolidinone, in the zinc-catalysed alkylation of benzaldehyde. The three ligands exhibit different system-level behaviour, characterized by multiple levels of aggregation that may be catalytically active simultaneously. While diphenyl-N-methyl-prolinol behaves as expected from a mechanistic point of view, diphenylprolinol shows enantiodivergence during the reaction due to an asymmetric autoinduction process. With 5-(hydroxydiphenylmethyl)-2-pyrrolidinone, we were able to establish the possibility of at least trimeric active species in equilibrium with less aggregated active species. Simulations using a mathematical model confirm the possibility of such systems-level behaviour. Parallel study of the three systems reveals three distinct system-level behaviours that are central to the efficiency of the catalytic reaction. Three closely related proline-based ligands give rise to different catalytic systems in asymmetric dialkylzinc addition reactions. Mechanistic studies reveal that monomeric, dimeric and product–catalyst complexes and aggregates larger than dimers are all catalytically active.

Chernikova Elena V., Beletskaya Irina P.

Carbon dioxide (CO2) plays a vital role in organic and polymer chemistry as a source of cheap and available raw material for the synthesis of many valuable products, including polymer materials with a specified set of properties, and as a solvent for chemical reactions. This review is devoted to the synthesis, properties and applications of polycarbonates obtained by copolymerization of CO2 with epoxides, a hot topic that has aroused great interest among the scientific community and industry representatives. The existing data on the catalytic systems used for the synthesis of polycarbonates are analyzed and summarized, depolymerization of polycarbonates, which is a key aspect in the polymer recycling, is discussed, information on the properties and applications of polycarbonates is systematized, and prospects for the development of this area of chemistry are considered.Bibliography — 438 references.

Barman S., Bag J., Das D., Pal K.

A set of new generation N/O donors, maleonitrile tethered, tetradentate heteroscorpionate modified Schiff base type ligands, 2-((E)-2-hydroxybenzylideneamino)-3-(pyridin-2-ylmethylamino)maleonitrile(H2LH), 2-((E)-2-hydroxy-5-nitrobenzylideneamino)-3-(pyridin-2-ylmethylamino)maleonitrile(H2LNO2) and 2-((E)-2-hydroxy-3-methoxybenzylideneamino)-3-(pyridin-2-ylmethylamino)maleonitrile(H2LOMe) were synthesized. These ligands were reacted with stoichiometry amount of...

Bhat G.A., Darensbourg D.J.

The use of carbon dioxide as an alternative feedstock for the production of polymeric materials represents a promising strategy for the valorization of CO2. In this regard, there remains much current attention focusing on the synthesis of polycarbonates from CO2 and epoxides. The use of metal coordination complexes as catalysts for the alternating copolymerization process under mild reaction conditions has significantly advanced our synthetic capability and mechanistic understanding of these coupling reactions. This review’s focal point is the advancements made in the area of CO2/epoxide polymerization processes involving a variety of main group and transition metal complexes as catalysts for the preparation of these thermoplastics. A brief discussion of the development of organocatalysts, whose design is based on their metal counterparts, is also presented.

Xie X., Huo Z., Jang E., Tong R.

AbstractPrecisely controlling macromolecular stereochemistry and sequences is a powerful strategy for manipulating polymer properties. Controlled synthetic routes to prepare degradable polyester, polycarbonate, and polyether are of recent interest due to the need for sustainable materials as alternatives to petrochemical-based polyolefins. Enantioselective ring-opening polymerization and ring-opening copolymerization of racemic monomers offer access to stereoregular polymers, specifically enantiopure polymers that form stereocomplexes with improved physicochemical and mechanical properties. Here, we highlight the state-of-the-art of this polymerization chemistry that can produce microstructure-defined polymers. In particular, the structures and performances of various homogeneous enantioselective catalysts are presented. Trends and future challenges of such chemistry are discussed.

Du P., Li Y., Lu X.

Pedretti B.J., Zhu C., Watanabe H., Aoshima S., Lynd N.A.

Wang Y., Liu Z., Guo W., Zhang C., Zhang X.