Donald J. Darensbourg

Catalysts for the reactions of epoxides and carbon dioxide

Abstract Oxiranes and carbon dioxide are known to cycloadd and/or copolymerize in the presence of a variety of catalysts. Indeed, cyclic carbonates are prepared on a technical scale by coupling epoxides and carbon dioxide. The fact that cyclic carbonate formation represents one of the few examples of successful carbon dioxide utilization, coupled with the high reactivity of epoxides, has resulted in many papers which reveal a remarkable variety of active catalysts for the CO2/epoxide coupling processes. Catalysts include simple alkali metal salts, ammonium salts, phosphines, main-group metal complexes, and both non-oxidative and oxidative transition-metal complexes. The purpose of this review is to compile the different catalysts into their general groups of similarity, with the hopes of shedding light on some of the important differences in reaction pathways. There generally appears to be a lack of detailed mechanistic studies; therefore, it is hoped that this review will emphasize where mechanistic clarification is most importantly needed. Furthermore, we anticipate that this review will provide insight into cyclic carbonate vs. polycarbonate production from the CO2/epoxide coupling process.

Construction of Ultrastable Porphyrin Zr Metal–Organic Frameworks through Linker Elimination

A series of highly stable MOFs with 3-D nanochannels, namely PCN-224 (no metal, Ni, Co, Fe), have been assembled with six-connected Zr6 cluster and metalloporphyrins by a linker-elimination strategy. The PCN-224 series not only exhibits the highest BET surface area (2600 m(2)/g) among all the reported porphyrinic MOFs but also remains intact in pH = 0 to pH = 11 aqueous solution. Remarkably, PCN-224(Co) exhibits high catalytic activity for the CO2/propylene oxide coupling reaction and can be used as a recoverable heterogeneous catalyst.

Chemistry of carbon dioxide relevant to its utilization: a personal perspective.

This presentation provides a review of the authors research groups contributions to the organometallic chemistry of carbon dioxide with a focus on incorporating this information into the development of an effective means for producing chemicals from this greenhouse gas. The types of chemistry addressed include insertion reactions of CO(2) into M-H, M-C, and M-O bonds as well as seminal contributions related to the copolymerization reactions of CO(2) and cyclic ethers. Relevant to this latter subject, efforts were made to place the authors achievements into perspective with the accomplishments of other investigators, but these efforts were not meant to be exhaustive.

What's new with CO2? Recent advances in its copolymerization with oxiranes

The present review highlights recent advances in the copolymerization of carbon dioxide (CO2) and epoxides (oxiranes) to produce polycarbonates with specific regard for catalysts designed to afford high selectivity for copolymer versus cyclic carbonate formation.

Ring-opening polymerization of lactides catalyzed by natural amino-acid based zinc catalysts.

A series of chiral NNO-tridentate Schiff base ligands derived from natural amino acids were reacted with zinc(bis-trimethylsilylamide)(2) to provide metal complexes which have been fully characterized. One of these derivatives was further reacted with p-fluorophenol to yield a phenoxide complex. X-ray crystallographic studies reveal the zinc Schiff base amide complexes to be monomeric, whereas, the p-fluorophenolate complex was shown to be dimeric with bridging phenoxide ligands. All zinc complexes were shown to be very effective catalysts for the ring-opening polymerization (ROP) of lactides at ambient temperature, producing polymers with controlled and narrow molecular weight distributions. These enantiomerically pure zinc complexes did not show selectivity toward either L- or D-lactide, that is, k(D(obsd))/k(L(obsd)) approximately = 1. However, steric substituents on the Schiff base ligands exhibited moderate to excellent stereocontrol for the ROP of rac-lactide. Heterotactic polylactides were produced from rac-lactide with P(r) values ranging from 0.68 to 0.89, depending on the catalyst employed and the reaction temperature. The reactivities of the various catalysts were greatly affected by substituents on the Schiff base ligands, with sterically bulky substituents being rate enhancing.

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.

Ring-opening polymerization of cyclic esters and trimethylene carbonate catalyzed by aluminum half-salen complexes.

A series of ONO-tridentate Schiff base ligands derived from chiral and achiral amino alcohols and amino acids were synthesized and reacted with AlEt(3) to provide dimeric aluminum complexes. These complexes were tested for the ring-opening polymerization (ROP) of rac-lactide at 70 °C in toluene, producing poly(lactide) with up to 82% isotacticity. The most active of these aluminum complexes was chosen to perform ring-opening homopolymerizations of rac-lactide, trimethylene carbonate (TMC), rac-β-butyrolactone (rac-β-BL), δ-valerolactone (δ-VL), and ε-caprolactone (ε-CL). Kinetic parameters were investigated, and each polymerization was found to be first order with respect to monomer concentration. Fractional orders were observed with respect to catalyst concentration, indicating catalyst aggregation during the polymerization processes. Activation parameters were determined for all monomers, with their ΔG(‡) values at 90 °C being in the order rac-lactide ≈ rac-β-BL > δ-VL > TMC ≈ ε-CL. Fineman-Ross and kinetic studies of the copolymerization of rac-lactide and δ-VL both indicate that the rate of rac-lactide enchainment is higher than that of δ-VL, resulting in a tapered copolymer. In addition, single crystals of one of these aluminum complexes were grown in the presence of rac-lactide and characterized using X-ray crystallography. The unit cell contains two lactide monomers, one D- and one L-lactide, adding further proof that polymerization takes place via an enantiomorphic site control mechanism.

The Activation of Carbon Dioxide by Metai Complexes

Publisher Summary Chemistry of one-carbon-atom molecules (C 1 chemistry) is an important area of research for the organometallic chemist. The motivation for these efforts stems from the belief that the raw material base for commercial organic chemicals will shift from oil to coal, owing to both economic reasons and declining petroleum reserves. Prior to a discussion of CO 2 insertion reactions into M–H and M–C bonds, it is useful to review some of the known coordination chemistry of carbon dioxide, because the activation of CO 2 by metal centers is assumed to be of significance in most of these processes. In principle, the insertion of CO 2 into a transition-metal hydrogen bond can result in either M–O or M–C bond formation––that is, production of metalloformate or metallocarboxylic acid derivatives. Some very interesting work on the reversible insertion of carbon dioxide into palladium–carbon bonds has been reported recently by Braunstein and coworkers. The reactions of CO 2 , with metal –hydroxides, –alkoxides, and –amides to provide metallobicarbonates, –alkyl carbonates, and –carbarnates in general do not involve activation of carbon dioxide by prior coordination to the metal center. The use of carbon dioxide as an industrial source of chemical carbon has been limited mainly to the production of organic carbonates, carboxylic acids, and ureas. The chapter discusses recent literature relevant to the coordination chemistry of carbon dioxide and its use as a source of chemical carbon.

Mechanistic Studies of the Copolymerization Reaction of Oxetane and Carbon Dioxide to Provide Aliphatic Polycarbonates Catalyzed by (Salen)CrX Complexes

Chromium salen derivatives in the presence of anionic initiators have been shown to be very effective catalytic systems for the selective coupling of oxetane and carbon dioxide to provide the corresponding polycarbonate with a minimal amount of ether linkages. Optimization of the chromium(III) system was achieved utilizing a salen ligand with tert-butyl groups in the 3,5-positions of the phenolate rings and a cyclohexylene backbone for the diimine along with an azide ion initiator. The mechanism for the coupling reaction of oxetane and carbon dioxide has been studied. Based on binding studies done by infrared spectroscopy, X-ray crystallography, kinetic data, end group analysis done by (1)H NMR, and infrared spectroscopy, a mechanism of the copolymerization reaction is proposed. The formation of the copolymer is shown to proceed in part by way of the intermediacy of trimethylene carbonate, which was observed as a minor product of the coupling reaction, and by the direct enchainment of oxetane and CO 2. The parity of the determined free energies of activation for these two processes, namely 101.9 kJ x mol (-1) for ring-opening polymerization of trimethylene carbonate and 107.6 kJ x mol (-1) for copolymerization of oxetane and carbon dioxide supports this conclusion.

Water-soluble organometallic compounds. 5. The regio-selective catalytic hydrogenation of unsaturated aldehydes to saturated aldehydes in an aqueous two-phase solvent system using 1,3,5-triaza-7-phosphaadamantane complexes of rhodium☆

The water-soluble phosphine complex of Rh(I), [Rh(PTAH)(PTA)2Cl]Cl(1), has been synthesized from RhCl3 and 1,3,5,-triaza-7-phosphaadamantane (PTA) in 96% ethanol. This complex is an effective catalyst for the regioselective reduction of unsaturated aldehydes to saturated aldehydes. The rate of hydrogenation of trans-cinnamaldehyde with sodium formate as reductant was studied as a function of catalyst, substrate, and sodium formate concentration. The presence of an excess of PTA was found to inhibit hydrogenations completely. This reaction was also found to be partially inhibited by cyclo-octatetraene and Hg(0), leading to the conclusion that both heterogenous and homogenous mechanisms are operating. Recycling experiments show complex 1 to be quite robust, with minimal leaching into the organic phase in a biphasic system. The complex resulting from the addition of an excess of PTA to 1, [Rh(PTAH)3(PTA)Cl]Cl3 (2) has been prepared and structurally characterized by a single crystal X-ray diffraction study. 31P and 1H NMR, IR and UV-VIS spectroscopy and pH titrametric measurements were employed to study the reactivity of the various rhodium PTA complexes in aqueous solutions.