A DFT Study on the Mechanism of the Cycloaddition Reaction of CO2to Epoxides Catalyzed by Zn(Salphen) Complexes

Stable Fe(iii) phenoxyimines as selective and robust CO2/epoxide coupling catalysts

Air- and moisture-stable Fe(iii)-phenoxyimine complexes catalyse CO₂ coupling reactions with a range of epoxides to selectively form cyclic carbonates.

Cycloaddition of carbon dioxide to epoxides catalysed by supported ionic liquids

Simple hybrid silica-imidazolium based ILs as highly active and selective catalysts for the carbonation of epoxides in batch and continuous flow systems under mild reaction conditions.

Formylation or methylation: what determines the chemoselectivity of the reaction of amine, CO2, and hydrosilane catalyzed by 1,3,2-diazaphospholene?

DFT computations have been performed to gain insight into the mechanisms of formylation/methylation of amines (e.g. methylaniline (1a)/2,2,4,4-tetramethylpiperidine (2a)) with CO2 and hydrosilane ([Si]H2, [Si] = Ph2Si), catalyzed by 1,3,2-diazaphospholene ([NHP]H). Different from the generally proposed sequential mechanism for the methylation of amine with CO2, i.e. methylation proceeds via formylation, followed by further reduction of formamide to give an N-methylated amine, the study characterized a competition mechanism between formylation and methylation. The chemoselectivity originates from the competition between the amine and [NHP]H hydride to attack the formyloxy carbon of [Si](OCHO)2 (the insertion product of CO2 into [Si]H2). When the attack of an amine (e.g.1a) wins, the transformation affords formamide (1b) but would otherwise (e.g.2a) result in an N-methylated amine (2c). The reduction of formamide by [Si]H2 or [NHP]H is highly unfavorable kinetically, thus we call attention to the sequential mechanism for understanding the methylation of amine with CO2. In addition, the study has the following key mechanistic findings. The activation of CO2 by [NHP]H establishes an equilibrium: [NHP]H + CO2 ⇄ [NHP]OCHO ⇄ [NHP]+ + HCO2-. The ions play catalytic roles to promote formylation via HCO2- or methylation via[NHP]+ . In 1a formylation, HCO2- initiates the reaction, giving 1b and silanol byproducts. However, after the initiation, the silanol byproducts acting as hydrogen transfer shuttles are more effective than HCO2- to promote formylation. In 2a methylation, [NHP]+ promotes the generation of the key species, formaldehyde and a carbocation species (IM17+ ). Our experimental study corroborates our computed mechanisms.

Recent Developments in the Synthesis of Cyclic Carbonates from Epoxides and CO2

The use of CO₂ as a C1 building block will be of essential importance in the future. In this context the synthesis of cyclic carbonates from epoxides and CO₂ gained great attention recently. These products are valuable compounds in a variety of chemical fields. The development of new catalysts and catalytic systems for this atom-economic, scalable, and industrially relevant reaction is a highly active research field. Over the past 17 years great advances have been made in this area of research. This chapter covers the survey of the important known classes of homogeneous catalysts for the addition of CO₂ to epoxides. Besides pioneering work, recent developments and procedures that allow this transformation under mild reaction conditions (reaction temperatures of ≤100 °C and/or CO₂ pressures of 0.1 MPa) are especially emphasized.

Metallosalen-Based Ionic Porous Polymers as Bifunctional Catalysts for the Conversion of CO2 into Valuable Chemicals

A series of new metallosalen-based ionic porous organic polymers (POPs) were synthesized for the first time using a simple unique strategy based on the free-radical copolymerization reaction. Various techniques were used to characterize the physicochemical properties of these catalysts. These well-designed materials endowed high surface area, hierarchical porous structures, and enhanced CO₂ /N₂ adsorptive selectivity. Moreover, these POPs having both metal centers (Lewis acid) and ionic units (nucleophile) could serve as bifunctional catalysts in the catalytic conversion of CO₂ into high value-added chemicals without any additional co-catalyst under mild and solvent-free conditions, for example, CO₂ /epoxides cycloaddition and Nformylation of amines from CO₂ and hydrosilanes. The results demonstrated that the irregular porous structure was very favorable for the diffusion of substrates and products, and the microporous structural property resulted in the enrichment of CO₂ near the catalytic centers in the CO₂ -involved transformations. Additionally, the superhydrophobic property could not only enhance the chemoselectivity of products but also promote the stability and recyclability of catalysts.

Aluminum-Mediated Formation of Cyclic Carbonates: Benchmarking Catalytic Performance Metrics

We report a comparative study on the activity of a series of fifteen binary catalysts derived from various reported aluminum-based complexes. A benchmarking of their initial rates in the coupling of various terminal and internal epoxides in the presence of three different nucleophilic additives was carried out, providing for the first time a useful comparison of activity metrics in the area of cyclic organic carbonate formation. These investigations provide a useful framework for how to realistically valorize relative reactivities and which features are important when considering the ideal operational window of each binary catalyst system.

Mechanistic Insights into the Carbon Dioxide/Cyclohexene Oxide Copolymerization Reaction: Is One Metal Center Enough?

A detailed study on the mechanism for the alternating copolymerization of cyclohexene oxide (CHO) and CO₂ mediated by an [Al{amino-tri(phenolate)}]/NBu₄ I binary catalyst system was performed by using DFT-based methods. Four potential mechanisms (one monometallic and three bimetallic) were considered for the first propagation cycle of the CHO/CO₂ copolymerization. The obtained Gibbs free energies provided a rationale for the relative high activity of a non-covalent dimeric structure formed in situ and thus for the feasibility of a bimetallic mechanism to obtain polycarbonates quantitatively. Gibbs free energies also indicated that the alternating copolymerization was favored over the cyclic carbonate formation.

A unique zinc-organic framework constructed through in situ ligand synthesis for conversion of CO2 under mild conditions and as a luminescence sensor for Cr2O72−/CrO42−

A novel 3D pillared-layer-like zinc-organic framework was synthesized through an in situ generated tetrazole ligand, displaying significant catalyst activity for the conversion of CO₂ and the luminescence detection of Cr₂O₇²⁻/CrO₄²⁻.

Metal–organic frameworks with Lewis acidity: synthesis, characterization, and catalytic applications

In this highlight, we review the recent development in the design and synthesis of metal–organic frameworks with Lewis acidity, the characterization techniques of Lewis acid sites, and their applications in heterogeneous catalysis.

Catalytic Coupling of Carbon Dioxide with Terpene Scaffolds: Access to Challenging Bio-Based Organic Carbonates

The challenging coupling of highly substituted terpene oxides and carbon dioxide into bio-based cyclic organic carbonates catalyzed by Al(aminotriphenolate) complexes is reported. Both acyclic as well as cyclic terpene oxides were used as coupling partners, showing distinct reactivity/selectivity behavior. Whereas cyclic terpene oxides showed excellent chemoselectivity towards the organic carbonate product, acyclic substrates exhibited poorer selectivities owing to concomitant epoxide rearrangement reactions and the formation of undesired oligo/polyether side products. Considering the challenging nature of these coupling reactions, the isolated yields of the targeted bio-carbonates are reasonable and in most cases in the range 50-60 %. The first crystal structures of tri-substituted terpene based cyclic carbonates are reported and their stereoconnectivity suggests that their formation proceeds through a double inversion pathway.

Novel iminopyridine derivatives: ligands for preparation of Fe(ii) and Cu(ii) dinuclear complexes

A series of imino- and amino-pyridine ligands based on dihydrobenzofurobenzofuran (BFBF) and methanodibenzodioxocine (DBDOC) backbones have been synthesized. These ligands form exclusively dinuclear complexes with metals such as iron(II) and copper(II). The structures for complexes 15, 16, 18, 19, 20, 21, 23, and 24 were determined by X-ray crystallography. The complexes show large distances for the metal nuclei and different geometries depending on the nature of the metal. An octahedral geometry was observed for the iron(II) complexes, while copper(II) complex 24 showed a distorted trigonal bipyramidal geometry. The iron(II) complexes showed activity as catalysts in the cycloaddition of CO2 to epoxides, obtaining moderate yields of cyclic carbonates.

The mechanism of CO2 hydration: a porous metal oxide nanocapsule catalyst can mimic the biological carbonic anhydrase role

The mechanism for the hydration of CO2 within a Keplerate nanocapsule is presented. A network of hydrogen bonds across the water layers in the first metal coordination sphere facilitates the proton abstraction and nucleophilic addition of water. The highly acidic properties of the polyoxometalate cluster are crucial for explaining the catalysed hydration.