Bifunctional One-Component Catalysts for the Addition of Carbon Dioxide to Epoxides

Bifunctional dye-based organocatalysts with enhanced activity in the conversion of CO2 into cyclic carbonates

A series of novel dye-based organocatalysts was designed, synthesised and tested in the cycloaddition of CO2 to styrene oxide yielding styrene carbonate under mild reaction conditions (45 °C, 10 bar CO2). Tuning the acid strength and the geometry of the -OH functional group in the modified dyes allowed the generation of a tailored bifunctional catalyst (RhB-Ethyl-PhOH-I) with enhanced catalytic activity.

Catalytic Strategies for the Cycloaddition of CO2 to Epoxides in Aqueous Media to Enhance the Activity and Recyclability of Molecular Organocatalysts

The cycloaddition of CO2 to epoxides to afford versatile and useful cyclic carbonate compounds is a highly investigated method for the nonreductive upcycling of CO2. One of the main focuses of the current research in this area is the discovery of readily available, sustainable, and inexpensive catalysts, and of catalytic methodologies that allow their seamless solvent-free recycling. Water, often regarded as an undesirable pollutant in the cycloaddition process, is progressively emerging as a helpful reaction component. On the one hand, it serves as an inexpensive hydrogen bond donor (HBD) to enhance the performance of ionic compounds; on the other hand, aqueous media allow the development of diverse catalytic protocols that can boost catalytic performance or ease the recycling of molecular catalysts. An overview of the advances in the use of aqueous and biphasic aqueous systems for the cycloaddition of CO2 to epoxides is provided in this work along with recommendations for possible future developments.

Functionalized β-Cyclodextrins Catalyzed Environment-Friendly Cycloaddition of Carbon Dioxide and Epoxides

Ammonium, imidazole, or pyridinium functionalized β-cyclodextrins (β-CDs) were used as efficient one-component bifunctional catalysts for the coupling reaction of carbon dioxide (CO₂) and epoxide without the addition of solvent and metal. The influence of different catalysts and reaction parameters on the catalytic performance were examined in detail. Under optimal conditions, Im-CD1-I catalysts functionalized with imidazole groups were able to convert various epoxides into target products with high selectivity and good conversion rates. The one-component bifunctional catalysts can also be recovered easily by filtration and reused at least for five times with only slight decrease in catalytic performance. Finally, a possible process for hydroxyl group-assisted ring-opening of epoxide and functionalized group- induced activation of CO₂ was presented.

Alumina-Based Bifunctional Catalyst for Efficient CO2 Fixation into Epoxides at Atmospheric Pressure

The quest toward sustainability and decarbonization demands the development of methods for efficient carbon dioxide capture and utilization. The nonreductive CO₂ fixation into epoxides to prepare cyclic carbonates has gained attention in recent years. In this work, we report the development of guanidine hydrochloride-functionalized γ alumina (γ-Al₂O₃), prepared using green solvents, as an efficient bifunctional catalyst for CO₂ fixation. The resulting guanidine-grafted γ-Al₂O₃ (Al-Gh) proved to be an excellent catalyst to prepare cyclic carbonates from epoxides and CO₂ with high selectivity. The nitrogen-rich Al-Gh shows increased CO₂ adsorption capacity compared to that of γ-Al₂O₃. The as-prepared catalyst was able to carry out CO₂ fixation at 85 °C under atmospheric pressure in the absence of solvents and external additives (e.g., TBAI or KI). The material showed negligible loss of catalytic activity even after five cycles of catalysis. The catalyst successfully converted many epoxides into their respective cyclic carbonates under the optimized conditions. The gram-scale synthesis of commercially important styrene carbonates from styrene oxide and CO₂ using Al-Gh was also achieved. Density functional theory (DFT) calculations revealed the role of alumina in activating the epoxide. This activation facilitated the chloride ion to open the ring to react with CO₂. The DFT studies also validated the role of alumina in stabilizing the electron-rich intermediates during the course of the reaction.

Novel hydroxyl-functionalized ionic liquids as efficient catalysts for the conversion of CO2 into cyclic carbonates under metal/halogen/cocatalyst/solvent-free conditions

Environmentally friendly synthesis route to carbonates from CO2 and epoxides catalysed by novel hydroxyl-functionalized ionic liquids under metal/halogen/cocatalyst/solvent-free conditions.

A DFT-design of single component bifunctional organocatalysts for the carbon dioxide/propylene oxide coupling reaction

The aim of this work is to develop single-component bifunctional organic catalysts capable of effective coupling reactions between CO2 and propylene epoxide (PO) under mild conditions using density functional theory (DFT) calculations. The dual functionalities of the target catalysts come from their inclusion of a hydroxyl-containing electrophile and the nucleophilicity of iodide ion. In this respect, a series of hydroxyl-functionalized quaternary onium-based ionic liquids were studied using M062X-D3/def2-TZVP//M062X-D3/def2-SVPP model chemistry. The design of catalysts was based on tailoring two structural factors; the first one is the onium center of pnictogens (N, P, As, Sb and Bi), and the second one is the number of hydrogen bond donor groups (n = 1-3). The proposed catalysts were examined by investigation of their catalytic mechanisms to afford the cyclic carbonate. Additionally, the highest active transition state, along with the potential energy difference, was examined using non-covalent interaction (NCI) analysis. Also, the activation strain model (ASM) was used to explain the kinetic behavior of PO activation. The findings showed that the ring-opening step of PO is always the critical step of the reaction. Among the suggested catalysts, the results indicated that the dihydroxyl ammonium-based catalyst (2OH-NI) is a good choice for this catalysis under mild and solvent-free conditions.

Microwave-Assisted Electrostatically Enhanced Phenol-Catalyzed Synthesis of Oxazolidinones

An electrostatically enhanced phenol is utilized as a straightforward, sustainable, and potent one-component organocatalyst for the atom-economic transformation of epoxides to oxazolidinones under microwave irradiation. Integrating a positively charged center into phenols over a modular one-step preparation gives rise to a bifunctional system with improved acidity and activity, competent in rapid assembly of epoxides and isocyanates under microwave irradiation in a short reaction time (20-60 min). A careful assessment of the efficacy of various positively charged phenols and anilines and the impact of several factors, such as catalyst loading, temperature, and the kind of nucleophile, on catalytic reactivity were examined. Under neat conditions, this one-component catalytic platform was exploited to prepare more than 40 examples of oxazolidinones from a variety of aryl- and alkyl-substituted epoxides and isocyanates within minutes, where up to 96% yield and high degree of selectivity were attained. DFT calculations to achieve reaction barriers for different catalytic routes were conducted to provide mechanistic understanding and corroborated the experimental findings in which concurrent epoxide ring-opening and isocyanate incorporation were proposed.

A Strained Ion Pair Permits Carbon Dioxide Fixation at Atmospheric Pressure by C-H H-Bonding Organocatalysis

The cycloadditions of carbon dioxide into epoxides to afford cyclic carbonates by H-bond donor (HBD) and onium halide (X) cocatalysis have emerged as a key strategy for CO₂ fixation. However, if the HBD is also a halide receptor, the two will quench each other, decreasing the catalytic activity. Here, we propose a strained ion pair tris(alkylamino)cyclopropenium halide (TAC·X), in which TAC repels X. TAC possesses a positively charged cyclopropenium core that makes the vicinal C-H or N-H a nonclassical HBD. The interionic strain within TAC·X makes TAC a more electrophilic HBD, allowing it to activate the oxygen of the epoxide and making X more nucleophilic and better able to attack the methylene carbon of the epoxide. NMR titration spectra and computational studies were employed to probe the mechanism of the cycloaddition of CO₂ to epoxides reactions under the catalysis of TAC·X. The ¹H and ¹³C{¹H}NMR titration spectra of the catalyst with the epoxide substrate unambiguously confirmed H-bonding between TAC and the epoxide. DFT computational studies identified the transition states in the ring-opening of the epoxide (TS1) and in the ring-closure of the cyclic carbonate (TS2).

Bifunctional Catalysts Containing Zn(II) and Imidazolium Salt Ionic Liquids for Chemical Fixation of Carbon Dioxide

Zn(II) can efficiently promote the catalytic performance of imidazolium salt ionic liquids (imi-ILs) for the chemical fixation of CO₂ into epoxides. To obtain sustainability, immobilized bifunctional catalysts containing both imi-ILs and Zn(II) were prepared using bimodal mesoporous silica (BMMs) as carrier, through grafting of Zn(OAc)₂ and 1-(trimethoxysilyl)propyl-3-methylimidazolium chloride (Si-imi) separately in the nanopores. The catalysts, named as BMMs-Zn&ILs, were identified as efficient catalysts for cycloaddition reaction of CO₂ into epoxides under solvent-free conditions. BMMs-Zn&ILs showed good catalytic activity, which increased with the increase of the molar ratio of Zn(II) to Si-imi. As a comparison, different catalytic systems including homogeneous imi-IL, BMMs-ILs and BMMs-Zn were studied to demonstrate different cooperation behaviors. Furthermore, the kinetics studies of homogeneous and heterogeneous bifunctional catalysts were employed to confirm the differences, as well as to support the proposed cooperative catalysis mechanism in the nanopores.

Fixation of CO2 into Cyclic Carbonates by Halogen-Bonding Catalysis

Halogen bonding, parallel to hydrogen bonding, was introduced into the catalytic cycloaddition of carbon dioxide into epoxide (CCE) reactions. A series of halogen-bond donor (XBD) catalysts of N-iodopyridinium halide featured with N-I bond were synthesized and evaluated in CCE reactions. The optimal XBD catalyst, 4-(dimethylamino)-N-iodopyridinium bromide ([DMAPI]Br), under screened conditions at 100 °C, ambient pressure, and 1 mol % catalyst loading, realized 93 % conversion of styrene oxide into cyclic carbonate in 6 h. The substrate scope was successfully extended with excellent yields (mostly ≥93 %) and quantitative selectivity (more than 99 %). ¹ H NMR spectroscopy of the catalyst [DMAPI]Br on substrate epoxide certified that the N-I bond directly coordinated with the epoxide oxygen. A plausible mechanism of halogen-bonding catalysis was proposed, in which the DMAPI cation functioned as halogen-bond donor to activate the epoxide, and the counter anion bromide attacked the methylene carbon to initiate the ring-opening of the epoxide. CCE reactions promoted by N-iodopyridinium halide, exemplify a first case of halogen-bonding catalysis in epoxide activation and CO₂ transformation.

Crosslinked Resin-Supported Bifunctional Organocatalyst for Conversion of CO2 into Cyclic Carbonates

The development of solvent-free, metal-free, recyclable organic catalysts is required for the current chemical fixation of carbon dioxide converted into cyclic carbonates. With the goal of reducing the cost, time, and energy consumption for the coupling reaction of CO₂ and epoxides, a series of highly active heterogeneous catalysts, based on a thiourea and quaternary ammonium salt system, are synthesized by using a thiol-ene click reaction under ultraviolet light. Benefitting from synergistic interactions of the electrophilic center (thiourea) and the nucleophilic site (ammonium bromide), the catalysts exhibit excellent catalytic selectivity (99 %) for the cycloaddition of carbon dioxide with a diverse range of epoxides under mild conditions (1.2 MPa, 100 °C). Moreover, the catalyst can be easily recycled by facile filtration and reused for 5 times without noticeable loss of activity and selectivity. This work provides a potential heterogeneous catalyst for the conversion of carbon dioxide into high value-added chemicals with the combined advantages of low cost, easy recovery, and satisfactory catalytic properties.

An Aminopyridinium Ionic Liquid: A Simple and Effective Bifunctional Organocatalyst for Carbonate Synthesis from Carbon Dioxide and Epoxides

An aminopyridinium ionic liquid is presented as a green, tunable, and active metal-free one-component catalytic system for the atom-efficient transformation of oxiranes and CO₂ to cyclic carbonates. Inclusion of a positively charged moiety into aminopyridines, through a simple single-step synthesis, provides a one-component ionic liquid catalytic system with superior activity; effective in ring opening of epoxide, CO₂ inclusion, and stabilization of oxoanionic intermediates. An efficiency assessment of a variety of positively charged aminopyridines was pursued, and the impact of temperature, catalyst loading, and the kind of nucleophile on the catalytic performance was also investigated. Under solvent-free conditions, this bifunctional organocatalytic system was used for the preparation of 18 examples of cyclic carbonates from a broad range of alkyl- and aryl-substituted oxiranes and CO₂ , where up to 98 % yield and high selectivity were achieved. DFT calculations validated a mechanism in which nucleophilic ring-opening and CO₂ inclusion occur simultaneously towards cyclic carbonate formation.

Organocatalytic carbon dioxide fixation to epoxides by perfluorinated 1,3,5-triols catalysts

In order to improve epoxides conversion to carbonates by fixation of CO2 a new type of perfluorinated triol catalysts was developed. These simple acyclic scaffolds of enhanced acidity are efficient for catalysis through selective H-bonding activation of the epoxide. In combination with TBAI as co-catalyst, this useful transformation is performed under only 1 atmosphere of CO2 and between 30 to 80 °C. Both the 1,3,5-triol motif and the perfluorinated side chains are crucial in order to observe this epoxide opening under such mild conditions. In addition, the stereochemistry of the starting epoxide can efficiently be conserved during the carbonate formation.

The Role of Water Revisited and Enhanced: A Sustainable Catalytic System for the Conversion of CO2 into Cyclic Carbonates under Mild Conditions

The role of water as highly effective hydrogen-bond donor (HBD) for promoting the coupling reaction of CO₂ with a variety of epoxides was demonstrated under very mild conditions (25-60 °C, 2-10 bar CO₂ ). Water led to a dramatic increase in the cyclic carbonate yield when employed in combination with tetrabutylammonium iodide (Bu₄ NI) whereas it had a detrimental effect with the corresponding bromide and chloride salts. The efficiency of water in promoting the activity of the organic halide was compared with three state-of-the-art hydrogen bond donors, that is, phenol, gallic acid and ascorbic acid. Although water required higher molar loadings compared to these organic hydrogen-bond donors to achieve a similar degree of conversion of CO₂ and styrene oxide into the corresponding cyclic carbonate under the same, mild reaction conditions, its environmental friendliness and much lower cost make it a very attractive alternative as hydrogen-bond donor. The effect of different parameters such as the amount of water, CO₂ pressure, reaction temperature, and nature of the organic halide used as catalyst was investigated by using a high-throughput reactor unit. The highest catalytic activity was achieved with either Bu₄ NI or bis(triphenylphosphine)iminium iodide (PPNI): with both systems, the cyclic carbonate yield at 45 °C with different epoxide substrates could be increased by a factor of two or more by adding water as a promoter, retaining high selectivity. Water was an effective hydrogen-bond donor even at room temperature, allowing to reach 85 % conversion of propylene oxide with full selectivity towards propylene carbonate in combination with Bu₄ NI (3 mol %). For the conversion of epoxides in which PPNI is poorly soluble, the addition of a cyclic carbonate as solvent allowed the formation of a homogeneous solution, leading to enhanced product yield.

Novel porous organocatalysts for cycloaddition of CO2 and epoxides

Three classes of organosilicas (DMO, OMOs and PMOs) containing immobilized multi-hydroxyl bis-(quaternary ammonium) iodide salts were prepared and tested in the cycloaddition of CO₂ and epoxides. Owing to its higher surface area, pore volume and optimum nucleophilicity of the iodide ion, OMO-2 with two hydroxyl groups was found to be the most active catalyst. For substrates that are easy to activate such as propylene oxide, 1,2-epoxybutane and epichlorohydrin, excellent yields and selectivities were obtained under mild reaction conditions (0.5 MPa CO₂, 50 °C and 10-15 h). Moreover, OMO-2 showed very good catalytic properties (yield ≥ 93% and selectivity ≥ 98%), and excellent chemical and textural stability in the synthesis of 1,2-butylene carbonate over 5 cycles.

Design Principles of Ionic Liquids for Transdermal Drug Delivery

Ionic liquids (ILs) and deep eutectic solvents have shown great promise in drug delivery applications. Choline-based ILs, in particular choline and geranic acid (CAGE), have been used to enhance the transdermal delivery of several small and large molecules. However, detailed studies outlining the design principles of ILs for transdermal drug delivery are still lacking. Using two model drugs of differing hydrophilicities, acarbose and ruxolitinib and 16 ILs, the dependence of skin penetration on the chemical properties of ILs is examined. First, the impact of ion stoichiometry on skin penetration of drugs is assessed using CAGE, which evidences that a molar ratio of 1:2 of choline to geranic acid yields the highest delivery. Subsequently, variants of CAGE are prepared using anions with structural similarity to geranic acid and cations with structural similarity to choline at a ratio of 1:2. Mechanistic studies reveal that the potency of ILs in enhancing transdermal drug delivery correlates inversely with the inter-ionic interactions as determined by 2D NMR spectroscopy. Using this understanding, a new IL is designed, and it provides the highest delivery of ruxolitinib of all ILs tested here. Overall, these studies provide a generalized framework for optimizing ILs for enhancing skin permeability.

Life Cycle Assessment for the Organocatalytic Synthesis of Glycerol Carbonate Methacrylate

Bifunctional ammonium and phosphonium salts have been identified as potential organocatalysts for the synthesis of glycerol carbonate methacrylate (GCMA). Three of these catalysts showed high efficiency and allowed the conversion of glycidyl methacrylate with CO₂ to the desired product in >99 % conversion and selectivity. Subsequently, immobilized analogues of selected catalysts were prepared and tested. A phenol-substituted phosphonium salt on a silica support proved to be a promising candidate in recycling experiments. The same catalyst was used in 12 consecutive runs, resulting in GCMA yields of up to 88 %. Furthermore, a life cycle assessment was conducted for the synthesis of GCMA starting from epichlorohydrin (EPH) and methacrylic acid (MAA). For the functional unit of 1 kg GCMA, 15 wt % was attributed to the incorporation of CO₂ , which led to a reduction of the global warming potential of 3 % for the overall process.

Multifunctional and Sustainable Fe-Iminopyridine Complexes for the Synthesis of Cyclic Carbonates

The use of multifunctional and sustainable Fe catalysts for the formation of cyclic carbonates from epoxides and carbon dioxide at 80 °C and 3 bar pressure is presented. The optimal catalyst possesses a halide counteranion and a hydrogen bond donor to activate the epoxide for ring opening, affording a single-component, cocatalyst-free catalytic system.

An Electrostatically Enhanced Phenol as a Simple and Efficient Bifunctional Organocatalyst for Carbon Dioxide Fixation

An electrostatically enhanced phenol as a simple and competent bifunctional organocatalyst for the atom-economical conversion of epoxides to cyclic carbonates under environmentally benign conditions is described. Incorporating a positively charged center into phenols through a modular one-step synthesis results in a bifunctional system with enhanced acidity and reactivity, capable of epoxide activation, a halide nucleophilic ring-opening process, and CO₂ incorporation in a synergistic fashion. A rational survey of the efficiency of different positively charged phenols and the influence of different parameters, such as temperature, catalyst loading, and the nature of the nucleophile, on catalytic activity was conducted. In addition, the time-dependent conversion of epoxide into the corresponding cyclic carbonate was further explored by FTIR-ATR and ¹ H NMR spectroscopy. This bifunctional catalytic platform is among one of the mildest and most efficient metal-free systems that is capable of converting a variety of epoxides into cyclic carbonates under virtually ambient conditions. The ¹ H NMR titration experiment validated the bifunctional catalytic mechanism wherein both the epoxide activation and the nucleophilic ring-opening process occur in concert en route to carbon dioxide fixation.

Scorpionate Catalysts for Coupling CO2 and Epoxides to Cyclic Carbonates: A Rational Design Approach for Organocatalysts

Novel scorpionate-type organocatalysts capable of effectively coupling carbon dioxide and epoxides under mild conditions to afford cyclic propylene carbonates were developed. On the basis of a combined experimental and computational study, a precise mechanistic proposal was developed and rational optimization strategies were identified. The epoxide ring-opening, which requires an iodide as a nucleophile, was enhanced by utilizing an immonium functionality that can form an ion pair with iodide, making the ring-opening process intramolecular. The CO₂ activation and cyclic carbonate formation were catalyzed by the concerted action of two hydrogen bonds originating from two phenolic groups placed at the claw positions of the scorpionate scaffold. Electronic tuning of the hydrogen bond donors allowed to identify a new catalyst that can deliver >90% yield for a variety of epoxide substrates within 7 h at room temperature under a CO₂ pressure of only 10 bar, and is highly recyclable.

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.

Synthesis and characterization of CO2-sensitive temperature-responsive catalytic poly(ionic liquid) microgels

A class of poly(ionic liquid) microgels exhibiting CO₂-switchable temperature-responsive volume phase transition behavior have been synthesized and used for CO₂ fixation.

Poly(ethylene glycol)s as Ligands in Calcium-Catalyzed Cyclic Carbonate Synthesis

Herein the use of CaI₂ in combination with poly(ethylene glycol) dimethyl ether (PEG DME 500) as an efficient catalyst system for the addition of CO₂ to epoxides is reported. This protocol is based on a nontoxic and abundant metal in conjunction with a polymeric ligand. Fifteen terminal epoxides were converted at room temperature to give the desired products in yields up to 99 %. Notably, this system was also effective for the synthesis of twelve challenging internal carbonates in yields up to 98 %.

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.