Internal Epoxides Research Articles

Synthesis of Cyclic Carbonates Catalyzed by CaI2–Et3N and Studies on Their Biocatalytic Kinetic Resolution

Herein, we report a simple catalytic system consisting of CaI2 in combination with Et3N as a ligand for the efficient cycloaddition of CO2 to a variety of epoxides. Terminal (bio-based) carbonates were obtained under mild conditions (23 °C, 1 bar CO2 pressure) in excellent yields up to 97%. Under elevated temperature (70 °C) and higher CO2 pressure (10 bar), even challenging internal epoxides were converted, leading to the desired carbonates in yields up to 90%. 1H NMR experiments indicated the in situ coordination of the amine to the Lewis acidic calcium center as well as the activation of the epoxide. Furthermore, the enzymatic kinetic resolution of seven cyclic carbonates using readily available pig liver esterase was investigated. Styrene carbonate proved to be the most suitable substrate for the kinetic resolution, leading to (R)-styrene carbonate in 99% ee.

Markovnikov alcohols via epoxide hydroboration by molecular alkali metal catalysts

SummarySynthesis of branched “Markovnikov” alcohols is crucial to various chemical industries. The catalytic reduction of substituted epoxides under mild conditions is a highly attractive method for preparing such alcohols. Classical methods based on heterogeneous or homogeneous transition metal-catalyzed hydrogenation, hydroboration, or hydrosilylation usually suffer from poor selectivity, reverse regioselectivity, limited functional group compatibility, high cost, and/or low availability of the catalysts. Here we report the discovery of highly regioselective hydroboration of nonsymmetrical epoxides catalyzed by ligated archetypal reductants in organic chemistry ‒ alkali metal triethylborohydrides. The chemoselectivity and turnover efficiencies of the present catalytic approach are excellent. Thus, terminal and internal epoxides with ene, yne, aryl, and halo groups were selectively and quantitatively reduced under a substrate-to-catalyst ratio (S/C) of up to 1000. Mechanistic investigations point to a mechanism reminiscent of frustrated Lewis pair action on substrates in which a nucleophile and Lewis acid act cooperatively on the substrate.

Ammonium zincates as suitable catalyst for the room temperature cycloaddition of CO2 to epoxides

We have recently shown that simple ammonium ferrates are competent catalyst for the cycloaddition reaction of CO2 to epoxides under moderate reaction conditions (T = 100°C, P(CO2) = 0.8 MPa). We report here that ammonium zincates of general formulae [TBA]2 [ZnX4] (TBA = tetrabutylammonium), simply obtained by treating an ethanolic solution of an appropriate zinc(II) salt with two equivalents of tetrabutylammonium halides, outperform ammonium ferrates in the synthesis of cyclic carbonates under milder reaction conditions (room temperature and atmospheric CO2 pressure). Using [TBA]2[ZnBr4] complex as homogeneous catalyst at 100°C and P(CO2) = 0.8 MPa a 52% conversion of styrene oxide with complete selectivity in styrene carbonate in just 15 min was observed, corresponding to a Turnover frequency (TOF) of 416 h−1. The same catalyst proved to be very active even at room temperature and atmospheric or very moderate CO2 pressures (0.2 MPa), with a quite broad range of substrates, especially in the case of terminal epoxides, with high selectivity towards cyclic carbonate products. The difference in reactivity of terminal and internal epoxides could be exploited using 4-vinylcyclohexene dioxide, where the endocyclic epoxide remained untouched when reacted at room temperature and the formation of the di-carbonate product was observed only at harsher conditions. A multigram scale conversion of propylene oxide was achieved (46 mmol) and the catalyst also proved to be recyclable (3 cycles) by distillation of the product and subsequent addition of fresh reagent, maintaining high conversion values and complete selectivity for propylene carbonate. This simple zinc-based catalytic system, which outperform the recently reported iron-based one by working at much milder conditions, could represent a valuable prospect in both laboratory and industrial scale, combining an inherent cheapness and synthetic easiness that should be deeply considered when the goal is to give value to a waste product as CO2.

The Use of Sustainable Transition Metals for the Cycloaddition of Epoxides and CO2 under Mild Reaction Conditions

AbstractFollowing green chemistry principles, we present a one pot synthesis of zinc curcumin complex as a homogeneous, sustainable catalyst for cyclic carbonate (CC) synthesis from CO2 and a variety of terminal and internal epoxides under 1 bar CO2 at 60 °C using DMSO as a solvent. Zinc curcumin (Zn‐Cur) complex was identified via 1H/13C NMR and ATR‐FTIR spectroscopies, elemental analysis, ionic conductivity, and quantitative silver nitrate test to confirm a 1 : 1 molar ratio of metal to the ligand. Interestingly, Zn‐Cur showed an outstanding performance in catalysing the synthesis of renewable CCs including dimeric pinene and limonene carbonates. In this context, other metal complexes, including magnesium (MgII), calcium (CaII), and nickel (NiII) complexes showed good to excellent catalytic activity. Furthermore, a plausible reaction mechanism was investigated by density functional theory (DFT) calculations. The reaction was found to be overall exergonic by 39 kJ/mol and the rate determining step was CO2 insertion, which is in good agreement with the experimental findings.

ONSN]-type chromium complexes catalyzed coupling of CO2 with epoxides

A new family of [ONSN]-type chromium complexes bearing hybrid coordination site (O, N, S) have been developed for the coupling of epoxides with CO2. The catalytic system resulting from the complex C2 in combination with TBAB shows the high activity for the cycloaddition of epoxides with CO2 with a broad range of substrates including terminal epoxides and internal epoxides. When catalytic system is switched from complex C2 and TBAB to complex C3 and PPNCl as well as regulation of the reaction conditions, which causes the variation of chemo-selectivity and allows the highly selective synthesis of poly(cyclohexene carbonate) by employing cyclohexene oxide as substrate. The spectroscopic analysis, including UV–vis, FT-IR, and ESIMS, pointed to the formation of new chromium complex, which may be an active species in the reaction.

Enzyme-Catalyzed Meinwald Rearrangement with an Unusual Regioselective and Stereospecific 1,2-Methyl Shift.

The Meinwald rearrangement is a synthetically useful reaction but often lacks regioselectivity and stereocontrol. A significant challenge in the Meinwald rearrangement of internal epoxides is the non-regioselective migration of different substituents to give a mixture of products. Herein, an enzyme-catalyzed regioselective and stereospecific 1,2-methyl shift in the Meinwald rearrangement of internal epoxides is reported. Styrene oxide isomerase (SOI) catalyzed the unique isomerization of internal epoxides through 1,2-methyl shift without 1,2-hydride shift to give the corresponding aldehydes and a cyclic ketone as the sole product. SOI-catalyzed isomerization showed high stereospecificity, fully retaining the stereoconfiguration. The synthetic utility of this enzymatic Meinwald rearrangement was demonstrated by its incorporation into three new types of enantioselective cascades, to convert trans-β-methyl styrenes into the corresponding R-configured alcohols, acids, or amines in high ee and yield.

Enzyme‐Catalyzed Meinwald Rearrangement with an Unusual Regioselective and Stereospecific 1,2‐Methyl Shift

AbstractThe Meinwald rearrangement is a synthetically useful reaction but often lacks regioselectivity and stereocontrol. A significant challenge in the Meinwald rearrangement of internal epoxides is the non‐regioselective migration of different substituents to give a mixture of products. Herein, an enzyme‐catalyzed regioselective and stereospecific 1,2‐methyl shift in the Meinwald rearrangement of internal epoxides is reported. Styrene oxide isomerase (SOI) catalyzed the unique isomerization of internal epoxides through 1,2‐methyl shift without 1,2‐hydride shift to give the corresponding aldehydes and a cyclic ketone as the sole product. SOI‐catalyzed isomerization showed high stereospecificity, fully retaining the stereoconfiguration. The synthetic utility of this enzymatic Meinwald rearrangement was demonstrated by its incorporation into three new types of enantioselective cascades, to convert trans‐β‐methyl styrenes into the corresponding R‐configured alcohols, acids, or amines in high ee and yield.

Catalytic Hydrogenation of Epoxides to Alcohols.

Atom-economical catalytic reactions are a highly enticing strategy because all atoms of the starting materials are incorporated into the products. Catalytic hydrogenation of epoxides to alcohols is an attractive and alternative protocol to other synthetic methodologies for the synthesis of alcohols from alkenes. In the last two decades, catalytic hydrogenation of epoxides to alcohols has made remarkable progress in chemical synthesis. In this review, an overview of the catalytic hydrogenation of both terminal and internal epoxides to the corresponding alcohols is presented. An outline of both homogeneous and heterogeneous hydrogenation of epoxides to the corresponding alcohols is provided. Moreover, the selectivity, efficiency, and the reaction mechanisms of these epoxide hydrogenation reactions are highlighted.

Utilization of CO2-Available Organocatalysts for Reactions with Industrially Important Epoxides

Recent knowledge in chemistry has enabled the material utilization of greenhouse gas (CO2) for the production of organic carbonates using mild reaction conditions. Organic carbonates, especially cyclic carbonates, are applicable as green solvents, electrolytes in batteries, feedstock for fine chemicals and monomers for polycarbonate production. This review summarizes new developments in the ring opening of epoxides with subsequent CO2-based formation of cyclic carbonates. The review highlights recent and major developments for sustainable CO2 conversion from 2000 to the end of 2021 abstracted by Web of Science. The syntheses of epoxides, especially from bio-based raw materials, will be summarized, such as the types of raw material (vegetable oils or their esters) and the reaction conditions. The aim of this review is also to summarize and to compare the types of homogeneous non-metallic catalysts. The three reaction mechanisms for cyclic carbonate formation are presented, namely activation of the epoxide ring, CO2 activation and dual activation. Usually most effective catalysts described in the literature consist of powerful sources of nucleophile such as onium salt, of hydrogen bond donors and of tertiary amines used to combine epoxide activation for facile epoxide ring opening and CO2 activation for the subsequent smooth addition reaction and ring closure. The most active catalytic systems are capable of activating even internal epoxides such as epoxidized unsaturated fatty acid derivatives for the cycloaddition of CO2 under relatively mild conditions. In case of terminal epoxides such as epichlorohydrin, the effective utilization of diluted sources of CO2 such as flue gas is possible using the most active organocatalysts even at ambient pressure.

Oxovanadium(V) catalysts for the CO2 fixation into cyclic carbonates under ambient pressure

AbstractDeveloping feasible strategies to utilize CO2 as a C1 synthon for constructing various valued‐added chemical feedstock is crucial for alleviating the global warning and achieving a carbon‐neutral cycle. The coupling between epoxide and carbon dioxide is a promising direction due to the large driving force for the formation of thermodynamically stable cyclic carbonate and its 100% atom‐economy efficiency. Despite numerous metal‐based complexes that have been reported to facilitate this coupling reaction catalytically, harsh conditions (high temperature and pressure) are usually required. In this study, the synthesis and structural/spectroscopic characterizations of oxovanadium(V) complexes 2a–2f chelated with one‐step synthesized hydrazone‐imidate ligands were reported. The catalytic performance of the synthesized oxovanadium(V) complexes regarding the coupling reaction between epoxide and CO2 was successfully demonstrated with only 0.5 mol% loading of both oxovanadium(V) catalyst and [NBu4][Br] at 80°C under 1 bar of CO2. The substrate scope containing both terminal epoxide derivative and internal epoxide derivative indicates that the oxovanadium(V) catalysts could exert efficient catalytic coupling reaction of terminal epoxide (63 ~ 85% conversion rates) under ambient pressure of CO2.

Halide-free pyridinium saccharinate binary organocatalyst for the cycloaddition of CO2 into epoxides

Cycloaddition of CO2 into epoxides (CCE) reaction was the few chemical processes viable commercially in valorizations of carbon dioxide. The produced cyclic carbonate was valuable in electrolyte of lithium batteries and so on, however, halide anion was monopoly as cocatalyst in catalytic CCE reactions. To avoid the harmful halide in corrosion of process equipment, the detrimental halide in electrolytes, and the waste halide as environmental burden, halide-free catalysis for CCE was highly desirable. In design of halide-free catalytic platform, we propose binary adducts of food sweetener saccharin and strong pyridine base in ionic pair pyridinium saccharinate exemplified by PPy·Sac (3). CCE reaction of epichlorohydrin at mild conditions of 40 °C, 0.1 MPa, by 10 mol% loading of catalyst 3 in 24 h were evaluated. Terminal epoxides including glycidyl ethers, alky epoxides, and styrene oxide were transformed into their corresponding cyclic carbonates by excellent yields (80 to 90% up). Internal epoxides of hexene and stilbene were not viable under these mild conditions. The key role of saccharinate anion in cocatalysis in both the nucleophilic attack on epoxide in initiating the ring-opening, and as good leaving group in ring-closure was validated. 1H NMR titration experiment of PPy·Sac on epoxide verified the activation of the substrate by H-bonding, the capture of intermediates in mass spectroscopy certified the ring-opening by Sac, and the control experiment by switching the cation and anion of PPy·Sac supported the indispensable of the cation and anion in the cocatalysis. In view of the wide application of halide anions in nucleophilic catalysis and substitutional reactions, diversity design of anions of appropriate nucleophilicity and nucleofugality would be useful in practical catalysis.

Closing the loop in the synthesis of heteroscorpionate-based aluminium helicates: catalytic studies for cyclic carbonate synthesis.

Novel polynuclear helical aluminium complexes supported by bulky heteroscorpionate ligands have been developed and characterised. The use of bulkier ligands has allowed the isolation of unprecedented intermediates for the preparation of helical aluminium complexes. The catalytic activity of these aluminium complexes for cyclic carbonates formation has also been investigated under mild reaction conditions. The combination of complex 16 and Bu4NBr catalysed the synthesis of a broad range of monosubstituted cyclic carbonates from their corresponding epoxides and CO2 at 25 °C and one bar of CO2 pressure. This catalyst system also showed good catalytic activity for the preparation of disubstituted cyclic carbonates from internal epoxides and CO2.

Synthesis of carbonates from CO2 and epoxides catalyzed by the system of N‐heterocyclic carbene, hydrogen bond donor, CrCl2, and tetrabutylammonium bromide

AbstractA three‐component catalytic system including pyridine‐bridged benzimidazolium salts, CrCl2, and tetrabutylammonium bromide (TBAB) was developed. Based on the control experiments and spectroscopic measurements, the role of the three components in the catalytic process was clarified, in which benzimidazolium salts were used as N‐heterocyclic carbene precursor, a new Cr complex generating from the coordination of CrCl2 with pyridine nitrogen and pyrazole nitrogen bearing benzimidazolium salts was employed as hydrogen bond donor, TBAB was used as nucleophilic reagent, respectively. Under mild conditions (50°C and 1 bar CO2), the terminal epoxides displayed high reactivity in the three‐component catalytic system. The catalytic system showed also high catalytic activity for the internal epoxides by increasing the temperature and CO2 pressure and/or prolonging the reaction time.

Enantioselective Resolution Copolymerization of Racemic 2,3‐Disubstituted cis‐Epoxides with CO2 Mediated by Binuclear Cobalt(III) Catalyst†

Main observation and conclusionEnantioselective resolution copolymerization of racemic internal epoxides with carbon dioxide (CO2) is a challenging issue because of their poor reactivity and complicated regio/stereoselectivity. Herein, we describe the first enantioselective resolution copolymerization of racemic aromatic 2,3‐disubstituted cis‐epoxides and CO2 using enantiopure dinuclear cobalt(III) complexes as catalyst under mild conditions, affording the corresponding polycarbonates with completely alternating structure and good enantioselectivity in the range of 70% to 97% ee. The isotactic polycarbonate from β‐methylstyrene oxide is a typical semicrystalline material, possessing a melting point of 241 oC; while other isotactic‐enriched copolymers from 2,3‐disubstituted cis‐epoxides bearing a substitute group on the aromatic ring are amorphous, having glass transition temperatures between 86 oC and 124 oC. Interestingly, the copolymer selectivity is correlated well with the Hammett substituent constant, and the highest polymer selectivity of 98% was found in the system using the epoxides bearing an electron‐donating group on the benzene ring.

Ionic cobalt complexes derived from an amine-bis(benzotriazole phenolate) ligand as bifunctional catalysts for copolymerization of epoxides and anhydrides

This study reports that a series of structurally well-defined mono- and di-nuclear cobalt complexes containing the amine-bis(benzotriazole phenolate) ligand (amine-BiBTP) have been synthesized and fully characterized. The one-pot reaction of the amine-BiBTP ligand precursor with cobalt nitrate salt in the presence of triethylamine under refluxing methanolic solution generated di-nuclear cobalt(II) complexes [(C1NNBiBTP)2Co2] (1). The mono-Co(II) complex [(C1NNBiBTP)CoR] (R = DMAP (2), tBuPy (3)) could be prepared by 1 with a stoichiometric amount of 4-(dimethylamino)pyridine (DMAP) or 4-tert-butylpyridine (tBuPy) to yield a five-coordinated mononuclear cobalt species. Interestingly, the Co(III) complexes 4–7 resulted from the treatment of 2 or 3 with silver nitrate/trifluoroacetate as the oxidizing agent, and an ionic and mononuclear species formulated as [(C1NNBiBTP)CoR2]X (R = DMAP, X = NO3 (4), R = DMAP, X = CF3COO (5), R = tBuPy, X = NO3 (6), R = tBuPy, X = CF3COO (7)) could be formed. Versatile catalysis towards copolymerization of internal epoxides with anhydride catalyzed by these Co(II)/Co(III) was systematically studied under mild conditions. Experimental results revealed that complex 4 was shown to effectively catalyze copolymerization of cyclohexene oxide (CHO) with phthalic anhydride (PA), resulting in copolymers with a high alternating microstructure possessing ≥ 99% ester-linkage contents.

N-Heterocyclic carbene-nitrogen molybdenum catalysts for utilization of CO2

Three new N-heterocyclic carbene-nitrogen molybdenum complex was synthesized, and its catalytic activity was evaluated in the cycloaddition of epoxides with CO2. The molybdenum complex combined with tetrabutyl ammonium iodide (TBAI) resulted in a catalytic system for efficient conversion of a wide range of terminal and internal epoxides under 80 °C and 5–7 bar pressure for CO2. The cooperative catalysis mechanism between molybdenum complex and TBAI was elucidated, in which molybdenum complex was used as Lewis acid, and TBAI was employed as nucleophilic reagent. In addition, the NHC-Mo catalytic system was also successfully applied for the direct carboxylation of terminal alkynes with CO2.

WO3–SiO2 nanomaterials synthesized using a novel template-free method in supercritical CO2 as heterogeneous catalysts for epoxidation with H2O2

A series of tungsten oxide-silica (WO3–SiO2) composite nanomaterials were synthesized through a novel, template-free sol-gel method, in which supercritical-CO2 (scCO2) was utilized as synthesis medium. The efficacy of the synthesis method stems from a tailored reactor design that allows the contact of the reactants only in the presence of scCO2. Selected synthetic parameters were screened with the purpose of enhancing the performance of the resulting materials as heterogeneous catalysts in epoxidation reactions with H2O2 as environmentally friendly oxidant. A cyclooctene conversion of 73% with epoxide selectivity of > 99% was achieved over the best WO3–SiO2 catalyst under mild reaction conditions (80 °C), equimolar H2O2 amount (1:1) and low WO3 loading (~2.5 wt%). The turnover number achieved with this catalyst (TON = 328), is significantly higher than that of a WO3–SiO2 prepared via a similar sol-gel route but without supercritical CO2, and that of commercial WO3. A thorough characterization with a combination of techniques (ICP-OES, N2-physisorption, XRD, TEM, STEM-EDX, SEM-EDX, FT-IR and Raman spectroscopy, XPS, TGA and FT-IR analysis of adsorbed pyridine) allowed correlating the physicochemical properties of the WO3–SiO2 nanomaterials with their catalytic performance. The high catalytic activity was attributed to: (i) the very high surface area (892 m2/g) and (ii) good dispersion of the W species acting as Lewis acid sites, which were both brought about by the synthesis in supercritical CO2, and (iii) the relatively low hydrophilicity, which was tuned by optimizing the tetramethyl orthosilicate concentration and the amount of basic solution used in the synthesis of the materials. Our optimum catalyst was also tested in the reaction of cyclohexene with H2O2, resulting in cyclohexane diol as main product due to the presence of strong Brønsted acid sites in the catalyst, whereas the reaction with limonene yielded the internal epoxide as the major product and the corresponding diol as side product. Importantly, the catalyst did not show leaching and could be reused in five consecutive runs without any decrease in activity.

Hierarchically nanoporous copolymer with built-in carbene-CO2 adducts as halogen-free heterogeneous organocatalyst towards cycloaddition of carbon dioxide into carbonates

A serial of porous polymers incorporating N‑heterocyclic carbene-CO2 adducts (designated as NHC-CO2 adducts) were facilely synthesized via radical polymerization of N-vinylimidazolate and ethyleneglycol dimethacrylate plus post-treatment with dimethyl carbonate (DMC). The polymers possessed tunable chemical structure, large surface area along with abundant meso-macropores and enhanced CO2 uptake which functioned as metal-halogen-free heterogeneous organocatalysts for solvent-free cycloaddition of CO2 to styrene oxide under 10 bar CO2 pressure, exhibiting highly catalytic activity and stable recyclability. Further, turnover frequency (TOF) of 7.1 h−1 (94.3% yield, 12 h) was obtainable under ambient condition, much higher than those of reported metal-halogen-free heterogeneous catalysts, even comparable with existing halogen-containing ones. Furthermore, the catalyst could efficiently convert various epoxides including internal epoxides into corresponding cyclic carbonates under mild condition (1 bar, 70–120 °C). The excellent catalytic performance was ascribed to the appropriate proportion of NHC-CO2 adducts in porous skeleton structure triggered by DMC. A plausible reaction mechanism was proposed to understand NHC-catalyzed fixation of CO2. This study highlights porous organic polymer is a versatile platform of NHC-CO2 adducts to construct eco-friendly solid catalyst towards efficient CO2 conversion.

Deoxygenation of Epoxides with Carbon Monoxide.

The use of carbon monoxide as a direct reducing agent for the deoxygenation of terminal and internal epoxides to the respective olefins is presented. This reaction is homogeneously catalyzed by a carbonyl pincer‐iridium(I) complex in combination with a Lewis acid co‐catalyst to achieve a pre‐activation of the epoxide substrate, as well as the elimination of CO2 from a γ‐2‐iridabutyrolactone intermediate. Especially terminal alkyl epoxides react smoothly and without significant isomerization to the internal olefins under CO atmosphere in benzene or toluene at 80–120 °C. Detailed investigations reveal a substrate‐dependent change in the mechanism for the epoxide C−O bond activation between an oxidative addition under retention of the configuration and an SN2 reaction that leads to an inversion of the configuration.

A robust Co(II)-MOF with nitrogen-rich micropores for CO2 transformation and protective effect on acute heart failure treatment by increasing the expression of Hif-1α

A 3D neutral porous MOF (1), [Co2(tdc)2(Hdatz)·DMF(H2O)3]n (H2tdc = 2,5-thiophenedicarboxylic acid, Hdatz = 3,5-diamino-1,2,4-triazole), which has aplenty micropores along with free groups of NH2 was assembled via combining 2,5-thiophenedicarboxylic acid ligands and nitrogen-rich 3,5-diamino-1,2,4-triazole as well as Co(II) ion. The activated MOF (1a) has a fine affinity for CO2 molecules and a fine catalytic performance for the conversion of CO2 with internal epoxides along with various terminal ones. Furthermore, the protective activity of 1a on acute heart failure was evaluated by measuring left ventricular ejection fraction and Left ventricular fraction shortening. And the mechanism of this compound was explored by detect the Hif-1α expression level. Synopsis A 3D neutral porous MOF based on 2,5-thiophenedicarboxylic acid and 3,5-diamino-1,2,4-triazole as the organic linkers has been prepared, which has microporous network decorate with free NH2 groups. The activated MOF has a fine affinity for CO2 molecules and a fine catalytic performance for the conversion of CO2 with internal epoxides along with various terminal ones. Furthermore, the protective activity of 1a on acute heart failure was evaluated by measuring LVEF and LVFS. And the mechanism of this compound was explored by detect the Hif-1α expression level.