Synthesis of organic carbamates as polyurethane raw materials from CO2: the quest for metal alkoxides as regenerable reagents

It is well known that the utilization of carbon dioxide (CO₂) for chemical materials is attracting research attention from the viewpoint of the carbon cycle. To contribute to the reduction of CO₂ emission through such CO₂ utilization reactions and counteract global climate change, the target compounds should be core chemical products that are distributed in large quantities and used for a long time. One such synthetic target is isocyanates that are used as raw materials for the production of polyurethanes, which are versatile polymeric materials with a service life of approximately 10 years. However, since direct synthesis of isocyanate from CO₂ is quite difficult due to equilibrium constraints, a method via the use of its alcohol adduct, organic carbamate, as a precursor has been proposed. In this Perspective, we present regenerative metal alkoxide reactants, such as tin alkoxide, titanium alkoxide, and alkoxysilane, as environmentally benign reactants for the synthesis of organic carbamates from CO₂. We also present a practical and environmentally friendly method for the highly efficient synthesis of various organic carbamates, including industrially important diisocyanate precursors, from 1 atm CO₂ using alkoxysilanes. Furthermore, prospects for the practical application of these carbamate synthesis reactions are also discussed.

Enhancement in the active site exposure in a porphyrin-based PIL/graphene composite catalyst for the highly efficient conversion of CO2

Poly(ionic liquid)s (PILs) have gained widespread attention in recent years due to their excellent properties similar to both ionic liquids and polymers. However, their further applications are limited because abundant and flexible ions easily block nanopores in the PIL catalysts, thus blocking the active sites and ultimately leading to decreased catalytic activity. This work reports the synthesis of a PIL/graphene composite catalyst (iPOP-ZnTPy@GNFs) based on an in situ surface preparation strategy, which effectively controlled the particle size and dispersion state of ionic liquids. The iPOP-ZnTPy@GNFs exhibited a larger surface area and more exposed active sites, which intensified the catalytic activity in the CO₂ cycloaddition reaction with propylene oxide with almost double the reaction rate as compared to that of iPOP-ZnTPy-2 at 100 °C and S/C = 1000. As expected, the iPOP-ZnTPy@GNF catalyst efficiently converted epoxides to cyclic carbonates at room temperature or atmospheric pressure, which can significantly reduce the process cost. In addition, iPOP-ZnTPy@GNFs exhibited excellent broad substrate scope, catalytic diversity, and remarkable reusability. The reaction mechanism of CO₂ cycloaddition was studied via density functional theory calculations and was validated by experimental findings. This work provides a feasible method for improving the utilization of active sites in PILs as a highly robust catalyst for CO₂ cycloaddition and can be further extended to other types of catalytic reactions in practical applications.

Cycloaddition of di-substituted epoxides and CO2 under ambient conditions catalysed by rare-earth poly(phenolate) complexes

Lanthanum complex 1/TBAI is the first catalyst to achieve the cycloaddition of 1,2-disubstituted epoxides with 1 bar CO2 at room temperature. A DFT study discloses that the poly(phenolato) ligand plays a key role in the product dissociation step.

Paramagnetic Rhenium Iodide Cluster with N-Heterocyclic Carbene

triangulo-Trirhenium nonaiodide Re₃I₉ reacts with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) to produce the novel 13-electron paramagnetic cluster Re₃I₈(IMes)₂, which was characterized by means of X-ray diffraction analysis, ESR spectroscopy, magnetometry, and quantum chemistry.

Cycloaddition of carbon dioxide and epoxides catalyzed by rare earth metal complexes bearing a Trost ligand

Rare earth metal complexes containing Trost ligands were used to catalyze the cycloaddition reaction of epoxides with CO₂. A series of epoxides were successfully converted into the corresponding cyclic carbonates under mild conditions.

Copolymerization of CO2 and epoxides mediated by zinc organyls

Herein we report the copolymerization of CHO with CO2 in the presence of various zinc compounds R2Zn (R = Et, Bu, iPr, Cy and Ph). Several zinc organyls proved to be efficient catalysts for this reaction in the absence of water and co-catalyst. Notably, readily available Bu2Zn reached a TON up to 269 and an initial TOF up to 91 h-1. The effect of various parameters on the reaction outcome has been investigated. Poly(ether)carbonates with molecular weights up to 79.3 kg mol-1 and a CO2 content of up to 97% were obtained. Under standard reaction conditions (100 °C, 2.0 MPa, 16 h) the influence of commonly employed co-catalysts such as PPNCl and TBAB has been investigated in the presence of Et2Zn (0.5 mol%). The reaction of other epoxides (e.g. propylene and styrene oxide) under these conditions led to no significant conversion or to the formation of the respective cyclic carbonate as the main product.

A Simple Zinc Catalyst for Carbamate Synthesis Directly from CO2

Several zinc salts were employed as catalysts for the synthesis of carbamates directly from aromatic amines, CO₂ , and silicate esters. Zn(OAc)₂ offered the best performance among the salts tested. The addition of an N-donor ligand such as 1,10-phenanthroline increased the yield. The best catalytic performance of Zn(OAc)₂ can be explained by carboxylate-assisted proton activation. The interaction between the substrate and the catalyst can be observed by chemical shifts in ¹ H and ¹⁵ N NMR spectra. Isocyanate was a key intermediate, which was generated from amine and CO₂ . Silicate ester was finally converted to siloxane, which was determined by ²⁹ Si NMR. The commercially available catalyst system could be reused. The yield of isolated carbamate could reach up to 96 % with various substrates, and the catalytic reaction was amine-selective in the presence of other functional groups.

Recyclable Single-Component Rare-Earth Metal Catalysts for Cycloaddition of CO2 and Epoxides at Atmospheric Pressure

Ionic rare-earth metal complexes 1-4 bearing an imidazolium cation were synthesized, which, as single-component catalysts, showed good activity in catalyzing cyclic carbonate synthesis from epoxides and CO₂. In the presence of 0.2 mol % catalyst, monosubstituted epoxides bearing different functional groups were converted into cyclic carbonates in 60-97% yields under atmospheric pressure. In addition, bulky/internal epoxides with low reactivity yielded cyclic carbonates in 40-95% yields. More importantly, the readily available samarium complex 2 was reused for six successive cycles without any significant loss in its catalytic activity. This is the first recyclable rare-earth metal-based catalyst in cyclic carbonate synthesis.