Pushing the Limits with Squaramide-Based Organocatalysts in Cyclic Carbonate Synthesis

Cholinium Pyridinolate Ionic Pair-Catalyzed Fixation of CO2 into Cyclic Carbonates

A halide-free ionic pair organocatalyst was proposed for the cycloaddition of CO2 into epoxide reactions. Cholinium pyridinolate ionic pairs with three different substitution positions were designed. Under conditions of temperature of 120 °C, pressure of 1 MPa CO2, and catalyst loading of 5 mol %, the optimal catalyst cholinium 4-pyridinolate ([Ch]+[4-OP]-) was employed. After a reaction time of 12 h, styrene oxide was successfully converted into the corresponding cyclic carbonate, and its selectivity was improved to 90%. A series of terminal epoxides were converted into cyclic carbonates within 12 h, with yields ranging from 80 to 99%. The proposed mechanism was verified by 1H NMR and 13C NMR titrations. Cholinium cations act as a hydrogen bond donor to activate epoxides, and pyridinolate anions combine with carbon dioxide to form intermediate carbonate anions that attack epoxides as nucleophiles and lead to ring opening. In summary, a halide-free ionic pair organocatalyst was designed and the catalytic mechanism in the cycloaddition of CO2 into epoxides reactions was proposed.

Epoxide/CO2 Cycloaddition Reaction Catalyzed by Indium Chloride Complexes Supported by Constrained Inden Schiff-Base Ligands

Cyclic carbonates have received significant interests for uses as reagents, solvents, and monomers. The coupling reaction of epoxides with carbon dioxide (CO2 ) to produce cyclic carbonate is an attractive route which can significantly reduce greenhouse gas emissions and environmental hazards. Herein, a series of five indium chloride complexes supported by inden Schiff-base ligands were reported along with four X-ray crystal structures. The constrained five-membered rings were added to the ligands to enhance the coordination of epoxides to the In metal. From the catalyst screening, In inden complex having tert-butyl substituents and propylene backbone in combination with tetrabutylammonium bromide (TBAB) exhibited the highest catalytic activity (TON up to 1017) for propylene oxide/CO2 coupling reaction with >99 % selectivity for cyclic carbonate under solvent-free conditions. In addition, the catalyst was shown to be active at atmospheric pressure of CO2 at room temperature. The catalyst system can be applied to various internal and terminal epoxide substrates to exclusively produce the corresponding cyclic carbonates.

Catalysis Conversion of Carbon Dioxide and Epoxides by Tetrahydroxydiboron To Prepare Cyclic Carbonates

A binary catalytic system comprising tetrahydroxydiboron and tetrabutylammonium iodide (TBAI) was used to catalyze the cycloaddition of carbon dioxide (CO2) with epoxides. The tetrahydroxydiboron catalyst (9 mol %), in combination with the use of TBAI (13.5 mol %) as a nucleophile, is capable of catalyzing the cycloaddition of CO2 with various terminal epoxides under room temperature and a CO2 balloon. In addition, a range of internal epoxides, including sterically hindered bicyclic epoxides and vegetable oil-based epoxides, were suitable for the catalytic system, affording a series of cyclic carbonates in moderate to high yields. The tetrahydroxydiboron/TBAI cooperative catalytic mechanism was elucidated using Fourier transform infrared spectroscopy, nuclear magnetic resonance, and electrospray ionization-high-resolution mass spectrometry. Results reveal that the tetrahydroxydiboron catalyst exhibits dual effects, activating both CO2 and epoxides; initially, it underwent the insertion of CO2 to form a boron-CO2 adduct and subsequently activated the epoxides through interaction of the B-O bond.

Organocatalytic Polymers from Affordable and Readily Available Building Blocks for the Cycloaddition of CO2 to Epoxides

The catalytic cycloaddition of CO₂ to epoxides to afford cyclic carbonates as useful monomers, intermediates, solvents, and additives is a continuously growing field of investigation as a way to carry out the atom-economic conversion of CO₂ to value-added products. Metal-free organocatalytic compounds are attractive systems among various catalysts for such transformations because they are inexpensive, nontoxic, and readily available. Herein, we highlight and discuss key advances in the development of polymer-based organocatalytic materials that match these requirements of affordability and availability by considering their synthetic routes, the monomers, and the supports employed. The discussion is organized according to the number (monofunctional versus bifunctional materials) and type of catalytically active moieties, including both halide-based and halide-free systems. Two general synthetic approaches are identified based on the postsynthetic functionalization of polymeric supports or the copolymerization of monomers bearing catalytically active moieties. After a review of the material syntheses and catalytic activities, the chemical and structural features affecting catalytic performance are discussed. Based on such analysis, some strategies for the future design of affordable and readily available polymer-based organocatalysts with enhanced catalytic activity under mild conditions are considered.

Ionic Conjugated Polymers as Heterogeneous Catalysts for the Cycloaddition of Carbon Dioxide to Epoxides to Form Carbonates under Solvent- and Cocatalyst-Free Conditions

The generation of cyclic carbonates by the cycloaddition of CO₂ with epoxides is attractive in the industry, by which CO₂ is efficiently used as C1 source. Herein, a series of catalysts were developed to efficient mediate the cycloaddition of CO₂ with epoxides to generate carbonates. The catalysts were easily synthesized via the amine-formaldehyde condensation of ethidium bromide with a variety of linkers. The newly prepared heterogeneous catalysts have high thermal stability and degradation temperatures. The surface of the catalysts is smooth and spherical in shape. The effect of temperature, pressure, reaction time and catalyst dosage on the cycloaddition of CO₂ with epoxide were investigated. The results show that the catalyst with 1,3,5-tris(4-formylphenyl)benzene as the linker can achieve 97.4 % conversion efficiency at the conditions of 100 °C, reaction time of 12 h, and the reaction pressure of 1.2 MPa in a solvent-free environment. Notably, the polymers serve as homogeneous catalysts during the reaction (reaction temperature above T_g ) and can be separated and recovered easily as homogeneous catalysts at room temperature. In addition, the catalyst is not only suitable for a wide range of epoxide substrates, but also can be recycled many times. Furthermore, DFT calculations show that the coordination between the electrophilic center of the catalyst and the epoxide reduces the energy barrier, and the reaction mechanism is proposed based on the reaction kinetic studies and DFT calculations.

Squaramide-Tethered Sulfonamides and Coumarins: Synthesis, Inhibition of Tumor-Associated CAs IX and XII and Docking Simulations

(1) Background: carbonic anhydrases (CAs) are attractive targets for the development of new anticancer therapies; in particular, CAs IX and XII isoforms are overexpressed in numerous tumors. (2) Methods: following the tail approach, we have appended a hydrophobic aromatic tail to a pharmacophore responsible for the CA inhibition (aryl sulfonamide, coumarin). As a linker, we have used squaramides, featured with strong hydrogen bond acceptor and donor capacities. (3) Results: Starting from easily accessible dimethyl squarate, the title compounds were successfully obtained as crystalline solids, avoiding the use of chromatographic purifications. Interesting and valuable SARs could be obtained upon modification of the length of the hydrocarbon chain, position of the sulfonamido moiety, distance of the aryl sulfonamide scaffold to the squaramide, stereoelectronic effects on the aromatic ring, as well as the number and type of substituents on C-3 and C-4 positions of the coumarin. (4) Conclusions: For sulfonamides, the best profile was achieved for the m-substituted derivative 11 (Ki = 29.4, 9.15 nM, CA IX and XII, respectively), with improved selectivity compared to acetazolamide, a standard drug. Coumarin derivatives afforded an outstanding selectivity (Ki > 10,000 nM for CA I, II); the lead compound (16c) was a strong CA IX and XII inhibitor (Ki = 19.2, 7.23 nM, respectively). Docking simulations revealed the key ligand-enzyme interactions.

Efficient access to general α-tertiary amines via water-accelerated organocatalytic multicomponent allylation

A tetrasubstituted carbon atom connected by three sp³ or sp²-carbons with single nitrogen, i.e., the α-tertiary amine (ATA) functional group, is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. The synthetic approach toward ATA structures is intricate, therefore, a straightforward catalytic method has remained a substantial challenge. Here we show an efficient water-accelerated organocatalytic method to directly access ATA incorporating homoallylic amine structures by exploiting readily accessible general ketones as useful starting material. The synergistic action of a hydrophobic Brønsted acid in combination with a squaramide hydrogen-bonding donor under aqueous condition enabled the facile formation of the desired moiety. The developed exceptionally mild but powerful system facilitated a broad substrate scope, and enabled efficient multi-gram scalability.

The α-tertiary amine functional group is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. Here the authors show an efficient water-accelerated organocatalytic method to access α-tertiary amines incorporating homoallylic amine structures by exploiting ketones as useful starting material.

Molecular Cavity for Catalysis and Formation of Metal Nanoparticles for Use in Catalysis

The employment of weak intermolecular interactions in supramolecular chemistry offers an alternative approach to project artificial chemical environments like the active sites of enzymes. Discrete molecular architectures with defined shapes and geometries have become a revolutionary field of research in recent years because of their intrinsic porosity and ease of synthesis using dynamic non-covalent/covalent interactions. Several porous molecular cages have been constructed from simple building blocks by self-assembly, which undergoes many self-correction processes to form the final architecture. These supramolecular systems have been developed to demonstrate numerous applications, such as guest stabilization, drug delivery, catalysis, smart materials, and many other related fields. In this respect, catalysis in confined nanospaces using such supramolecular cages has seen significant growth over the years. These porous discrete cages contain suitable apertures for easy intake of substrates and smooth release of products to exhibit exceptional catalytic efficacy. This review highlights recent advancements in catalytic activity influenced by the nanocavities of hydrogen-bonded cages, metal-ligand coordination cages, and dynamic or reversible covalently bonded organic cages in different solvent media. Synthetic strategies for these three types of supramolecular systems are discussed briefly and follow similar and simplistic approaches manifested by simple starting materials and benign conditions. These examples demonstrate the progress of various functionalized molecular cages for specific chemical transformations in aqueous and nonaqueous media. Finally, we discuss the enduring challenges related to porous cage compounds that need to be overcome for further developments in this field of work.

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.

Azo-Functionalized Zirconium-Based Metal-Organic Polyhedron as an Efficient Catalyst for CO2 Fixation with Epoxides

Chemical fixation of CO₂ as C1 source at ambient temperature and low pressure is an energy-saving way to make use of the green-house gas, but it still remains a challenge since efficient catalyst with high catalytic active sites is required. Here, a novel monoclinic azo-functionalized Zr-based metal-organic polyhedron (Zr-AZDA) has been prepared and applied in CO₂ fixation with epoxides. The inherent azo groups not only endow Zr-AZDA with good solubilization, but also act as basic sites to enrich CO₂ showing efficient synergistic catalysis as confirmed by TPD-CO₂ analysis. XPS results demonstrate that the Zr active sites in Zr-AZDA possess suitable Lewis acidity, which satisfies both substrates activation and products desorption. DFT calculation indicates the energy barrier of the rate-determining step in CO₂ cycloaddition could be reduced remarkably (by ca. 60.9 %) in the presence of Zr-AZDA, which may rationalize the mild and efficient reaction condition employed (80 °C and 1 atm of CO₂ ). The work provides an effective multi-functional cooperative method for improvement of CO₂ cycloaddition.

Metal-Cocatalyst Interaction Governs the Catalytic Activity of MII-Porphyrazines for Chemical Fixation of CO2

Chemical fixation of CO₂ to produce cyclic carbonates can be a green and atomic efficient process. In this work, a series of porphyrazines (Pzs) containing electron-withdrawing groups and central M^II ions (where M = Mg, Zn, Cu, and Co) were synthesized and investigated as catalysts for the cycloaddition of CO₂ to epoxides. Then, the efficiency of the Pzs was tested by varying cocatalyst type and concentration, epoxide, temperature, and pressure. Mg^IIPz bearing trifluoromethyl groups (1) showed the best conversion, producing, selectively, 78% of propylene cyclic carbonate (PCC), indicating that a harder and stronger Lewis acid is more effective for epoxide activation. Moreover, cocatalyst variation showed a notable effect on the reaction yields. Spectrophotometric titrations, MALDI-TOF mass spectra, and theoretical calculations suggest poisoning of the catalyst when tetrabutylammonium chloride (TBAC) and large amounts of tetrabutylammonium bromide (TBAB) were used in the system. The same was not observed for tetrabutylammonium iodide (TBAI), indicating that the metal-cocatalyst interaction may govern the reaction rate. In addition, two rare examples of crystalline structures were obtained, proving the distorted square pyramidal geometry with water molecule as axial ligand. This is one of the first studies reporting Pzs as catalysts for the chemical fixation of CO₂, and we believe that the intricate balance between cocatalyst concentration and conversion efficiency shown here may aid future studies in the area.

Asymmetric organocatalysis: an enabling technology for medicinal chemistry

The efficacy and synthetic versatility of asymmetric organocatalysis have contributed enormously to the field of organic synthesis since the early 2000s. As asymmetric organocatalytic methods mature, they have extended beyond the academia and undergone scale-up for the production of chiral drugs, natural products, and enantiomerically enriched bioactive molecules. This review provides a comprehensive overview of the applications of asymmetric organocatalysis in medicinal chemistry. A general picture of asymmetric organocatalytic strategies in medicinal chemistry is firstly presented, and the specific applications of these strategies in pharmaceutical synthesis are systematically described, with a focus on the preparation of antiviral, anticancer, neuroprotective, cardiovascular, antibacterial, and antiparasitic agents, as well as several miscellaneous bioactive agents. The review concludes with a discussion of the challenges, limitations and future prospects for organocatalytic asymmetric synthesis of medicinally valuable compounds.

Catalytic Performance of CPM-200-In/Mg in the Cycloaddition of CO2 and Epoxides

Crystalline porous materials (CPM)-200-In and CPM-200-In/Mg metal-organic frameworks (MOFs) were synthesized by a solvothermal method and were characterized by using powder X-ray diffraction (PXRD), FT-IR, Brunauer–Emmett–Teller (BET), temperature programmed desorption (TPD), TGA, XPS, and SEM-EDS. They were used as heterogeneous catalysts for the cycloaddition of CO2 with epoxides and found to be highly efficient toward the cycloaddition reaction at moderate reaction conditions under solvent-free conditions. The catalyst was easily separated by a simple filtration and can be reused up to five consecutive times without any considerable decrease of its initial activity. CPM-200-In/Mg showed excellent catalytic performance in the cycloaddition reaction due to the synergistic role of the acidic sites and basic sites. A plausible reaction mechanism for the CPM-200-In/Mg MOF catalyzed cycloaddition reaction is proposed based on the experimental results and our previously reported DFT (Density Functional Theory) studies.

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).

Efficient and Reusable Iron Catalyst to Convert CO2 into Valuable Cyclic Carbonates

The production of cyclic carbonates from CO2 cycloaddition to epoxides, using the C-scorpionate iron(II) complex [FeCl2{κ3-HC(pz)3}] (pz = 1H-pyrazol-1-yl) as a catalyst, is achieved in excellent yields (up to 98%) in a tailor-made ionic liquid (IL) medium under mild conditions (80 °C; 1-8 bar). A favorable synergistic catalytic effect was found in the [FeCl2{κ3-HC(pz)3}]/IL system. Notably, in addition to exhibiting remarkable activity, the catalyst is stable during ten consecutive cycles, the first decrease (11%) on the cyclic carbonate yield being observed during the 11th cycle. The use of C-scorpionate complexes in ionic liquids to afford cyclic carbonates is presented herein for the first time.

Cycloaddition mechanisms of CO2 and epoxide catalyzed by salophen – an organocatalyst free from metals and halides

The coupling mechanism of CO₂ and epichlorohydrin catalyzed by salophen is computed. A neutral concerted bifunctional mechanism of phenolate as nucleophile and phenol as H-bonding donor in epoxide ring-opening and CO₂ addition is suggested.

Cycloaddition of CO2 to epoxides by highly nucleophilic 4-aminopyridines: establishing a relationship between carbon basicity and catalytic performance by experimental and DFT investigations

New highly nucleophilic homogeneous and heterogeneous catalysts based on the 3,4-diaminopyridine scaffold are reported for the halogen-free cycloaddition of CO₂ to epoxides.

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.

Squaric acid analogues in medicinal chemistry

In this review, we summarize the published data on squaric acid analogues with a special focus on their use in medicinal chemistry and as potential drugs. Squaric acid is an interesting small molecule with an almost perfectly square shape, and its analogues have a variety of biological activities that are enabled by the presence of significant H-bond donors and acceptors. Unfortunately, most of these compounds also exhibit reactive functionalities, and this deters the majority of medicinal chemists and pharmacologists from trying to use them in drug development. However, this group of compounds is experiencing a renaissance, and large numbers of them are being tested for antiprotozoal, antibacterial, antifungal, and antiviral activities. The most useful of these compounds exhibited IC₅₀ values in the nanomolar range, which makes them promising drug candidates. In addition to these activities, their interactions with living systems were intensively explored, revealing that squaric acid analogues inhibit various enzymes and often serve as receptor antagonists and that the squaric acid moiety may be used as a non-classical isosteric replacement for other functional groups such as carboxylate. In summary, this review is focused on squaric acid and its analogues and their use in medicinal chemistry and should serve as a guide for other researchers in the field to demonstrate the potential of these compounds based on previous research.

Zinc Imine Polyhedral Oligomeric Silsesquioxane as a Quattro-Site Catalyst for the Synthesis of Cyclic Carbonates from Epoxides and Low-Pressure CO2

In the present research, the synthesis, spectroscopic characterization, and structural investigations of a unique Zn^II complex of imine-functionalized polyhedral oligomeric silsesquioxane (POSS) is designed, and hereby described, as a catalyst for the synthesis of cyclic carbonates from epoxides and CO₂. The uncommon features of the designed catalytic system is the elimination of the need for a high pressure of CO₂ and the significant shortening of reaction times commonly associated with such difficult transformations like that of styrene oxide to styrene carbonate. Our studies have shown that imine-POSS is able to chelate metal ions like Zn^II to form a unique coordination complex. The silsesquioxane core and the hindrance of the side arms (their steric effect) influence the construction process of the homoleptic Zn₄@POSS-1 complex. The compound was characterized in solution by NMR (¹H, ¹³C, ²⁹Si), ESI-MS, UV/Vis spectroscopy and in the solid state by thermogravimetric/differential thermal analysis (TG-DTA), elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), cross-polarization magic angle spinning (CP MAS) NMR (¹³C, ²⁹Si) spectroscopy, and X-ray crystallography.

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.

Intertwined chemistry of hydroxyl hydrogen-bond donors, epoxides and isocyanates in the organocatalytic synthesis of oxazolidinones versus isocyanurates: rational catalytic investigation and mechanistic understanding

The efficiency and chemoselectivity of the cycloaddition of isocyanates to epoxides to afford oxazolidinones were investigated using hydroxyl hydrogen-bond donors as organocatalysts.