A Brønsted acidic ionic liquid anchored to magnetite nanoparticles as a novel recoverable heterogeneous catalyst for the Biginelli reaction

In this study, simple and effective methods were used for the preparation of an ionic liquid that immobilized magnetite nanoparticles. Fe₃O₄ nanoparticles were prepared via a chemical co-precipitation method. Then, a SiO₂ shell was coated on the magnetic core via the Stober method. Finally, CPTES (chloropropyltriethoxysilane) and morpholine were coated on the SiO₂ shell. Morpholine sulfate, an acidic ionic liquid, was successfully bound to magnetite nanoparticles (Mag@Morph-AIL) and this was used as an efficient catalyst for the preparation of 3,4-dihydropyrimidinones. Compared to previous works, the easy separation of the nanocatalyst using an external magnet and the recyclability, non-toxicity, versatility, and high stability of the catalyst, combined with low reaction times and excellent yields, make the present protocol very useful for the synthesis of the title products. The synthesized products and catalyst were confirmed via¹H-NMR, ¹³C-NMR, FT-IR, scanning electron microscope, X-ray diffraction, and elemental analysis.

In this study, morpholine sulfate acidic ionic liquid bonded on magnetite nanoparticles (Mag@Morph-AIL) has been used as a catalyst for the preparation of 3,4-dihydropyrimidinones.     

Urazolium diacetate as a new,efficient and reusable Brønsted acid ionic liquid for the synthesis of novel derivatives of thiazolidine-4-ones

Urazolium diacetate catalyzed synthesis of new derivatives of 1,3-thiazolidine-4-ones (azo dispersive dyes family) via multicomponent reaction of various aldehydes, thioglycolic acid and 4-aminoazobenzene under solvent-free reaction was reported. This avenue for the synthesis of new derivatives of thiazolidine-4-one has advantages as: short reaction times, high yields, green aspect of chemistry and environmental friendliness, easy workup, solvent-free conditions and convenient operation.

Urazolium diacetate catalyzed synthesis of new derivatives of 1,3-thiazolidine-4-ones (azo dispersive dyes family) via multicomponent reaction of various aldehydes, thioglycolic acid and 4-aminoazobenzene under solvent-free reaction was reported.     

Silica-immobilized ionic liquid Brønsted acids as highly effective heterogeneous catalysts for the isomerization of n-heptane and n-octane

Metal-free imidazolium-based ionic liquid (IL) Brønsted acids 1-methyl imidazolium hydrogen sulphate [HMIM]HSO₄ and 1-methyl benzimidazolium hydrogen sulphate [HMBIM]HSO₄ were synthesized. Their physicochemical properties were investigated using spectroscopic and thermal techniques, including UV-Vis, FT-IR, ¹H NMR, ¹³C-NMR, mass spectrometry, and TGA. The ILs were immobilized on mesoporous silica gel and characterized by FT-IR spectroscopy, scanning electron microscopy, Brunauer–Emmett–Teller analysis, ammonia temperature-programmed desorption, and thermogravimetric analysis. [HMIM]HSO₄@silica and [HMBIM]HSO₄@silica have been successfully applied as promising replacements for conventional catalysts for alkane isomerization reactions at room temperature. Isomerization of n-heptane and n-octane was achieved with both catalysts. In addition to promoting the isomerization of n-heptane and n-octane (a quintessential reaction for petroleum refineries), these immobilized catalysts are non-hazardous and save energy.

Metal-free imidazolium-based ionic liquid (IL) Brønsted acids 1-methyl imidazolium hydrogen sulphate [HMIM]HSO₄ and 1-methyl benzimidazolium hydrogen sulphate [HMBIM]HSO₄ were synthesized.     

Metal- and solvent-free synthesis of aniline- and phenol-based triarylmethanes via Brönsted acidic ionic liquid catalyzed Friedel-Crafts reaction

A beneficial, scalable and efficient methodology for the synthesis of aniline-based triarylmethanes has been established through the double Friedel-Crafts reaction of commercial aldehydes and primary, secondary or tertiary anilines using Brönsted acidic ionic liquid as a powerful catalyst, namely [bsmim][NTf₂]. This protocol was successfully performed under metal- and solvent-free conditions with a broad range of substrates, giving the corresponding aniline-based triarylmethane products in good to excellent yields (up to 99%). In addition, alternative aromatic nucleophiles such as phenols and electron-rich arenes were also studied using this useful approach to achieve a diversity of triarylmethane derivatives in high to excellent yields.

Brönsted acidic ionic liquid catalyzed the synthesis of aniline- and phenol-based triarylmethanes via Friedel-Crafts reaction under metal- and solvent-free conditions.     

Brønsted acidic ionic liquids for cellulose hydrolysis in an aqueous medium: structural effects on acidity and glucose yield

The conversion of cellulose into valuable chemicals has attracted much attention, due to the concern about depletion of fossil fuels. The hydrolysis of cellulose is a key step in this conversion, for which Brønsted acidic ionic liquids (BAILs) have been considered promising acid catalysts. In this study, using BAILs with various structures, their acidic catalytic activity for cellulose hydrolysis assisted by microwave irradiation was assessed using the Hammett acidity function (H₀) and theoretical calculations. The glucose yields exceeded 10% when the H₀ values of the BAIL aqueous solutions were below 1.5. The highest glucose yield was about 36% in 1-(1-octyl-3-imidazolio)propane-3-sulfonate (Oimps)/sulfuric acid (H₂SO₄) aqueous solution. A long alkyl side chain on the imidazolium cation, which increased the hydrophobicity of the BAILs, enhanced the glucose yield.

Using Brønsted acidic ionic liquids with various structures, their acidic catalytic activity for cellulose hydrolysis assisted by microwave irradiation was assessed using the Hammett acidity function (H₀) and theoretical calculations.     

Isobutane/2-butene alkylation catalyzed by Brønsted–Lewis acidic ionic liquids

The alkylation reaction of isobutane with 2-butene to yield C₈-alkylates was performed using Brønsted–Lewis acidic ionic liquids (ILs) comprising various metal chlorides (ZnCl₂, FeCl₂, FeCl₃, CuCl₂, CuCl, and AlCl₃) on the anion. IL 1-(3-sulfonic acid)-propyl-3-methylimidazolium chlorozincinate [HO₃S-(CH₂)₃-mim]Cl-ZnCl_{2 (x=0.67)} exhibited outstanding catalytic performance, which is attributed to the appropriate acidity, the synergistic effect originating from its double acidic sites and the promoting effect of water on the formation and transfer of protons. The Lewis acidic strength of IL played an important role in improving IL catalytic performance. A 100% conversion of 2-butene with 85.8% selectivity for C₈-alkylate was obtained under mild reaction conditions. The IL reusability was good because its alkyl sulfonic acid group being tethered covalently, its anion [Zn₂Cl₅]⁻ inertia to the active hydrogen, and its insolubility in the product. IL [HO₃S-(CH₂)₃-mim]Cl-ZnCl₂ had potential applicability in the benzene alkylation reaction with olefins and halohydrocarbons.

The alkylation reaction of isobutane with 2-butene to yield C₈-alkylates was performed using Brønsted–Lewis acidic ionic liquids (ILs) comprising various metal chlorides (ZnCl₂, FeCl₂, FeCl₃, CuCl₂, CuCl, and AlCl₃) on the anion.     

Synthesis of α-indolylacrylates as potential anticancer agents using a Brønsted acid ionic liquid catalyst and the butyl acetate solvent

In this study, new α-indolylacrylate derivatives were synthesized by the reaction of 2-substituted indoles with various pyruvates using a Brønsted acid ionic liquid catalyst in butyl acetate solvent. This is the first report on the application of pyruvate compounds for the synthesis of indolylacrylates. The acrylate derivatives could be obtained in good to excellent yields. A preliminary biological evaluation revealed their promising anticancer activity (IC₅₀ = 9.73 μM for the compound 4l) and indicated that both the indole core and the acrylate moieties are promising for the development of novel anticancer drugs. The Lipinski''s rule and Veber''s parameters were assessed for the newly synthesized derivatives.

4lNew α-indolylacrylate derivatives were synthesized by reaction of 2-substituted indoles with various pyruvates using a Brønsted acid ionic liquid catalyst in butyl acetate solvent. This is the first application of pyruvate compounds for the synthesis of indolylacrylates.     

Experimental and theoretical study of ZrMo-KIT-6 solid acid catalyst with abundant Brønsted acid sites

Given their excellent reusability and environmental friendliness, solid acid catalysts have drawn considerable attention in acid-catalyzed reactions. However, the rational design and synthesis of solid acid catalysts with abundant Brønsted acid sites remains a challenge. In this paper, KIT-6, Zr-KIT-6, Mo-KIT-6, and ZrMo-KIT-6 solid acid catalysts are designed and synthesized. The textural properties, chemical bonds, and acidic properties of these catalysts are explored. Theoretical calculations are conducted to explore the formation mechanism of Brønsted acid sites. The theoretical trend of acidity is consistent with the experimental result of acidity and further demonstrates that the synergistic effect of Zr and Mo species improves the formation of Brønsted acid sites. The as-obtained ZrMo-KIT-6 solid acid catalysts are employed in Friedel–Crafts benzylation reaction, and the outstanding catalytic performance of the ZrMo-KIT-6 catalyst indicates that it is an excellent Brønsted solid acid catalyst.

Synergistic effect of Zr and Mo species in the formation of Brønsted acid sites is investigated by experimental and theoretical study.     

Brønsted acid-promoted thiazole synthesis under metal-free conditions using sulfur powder as the sulfur source

A Brønsted acid-promoted sulfuration/annulation reaction for the one-pot synthesis of bis-substituted thiazoles from benzylamines, acetophenones, and sulfur powder has been developed. One C–N bond and multi C–S bonds were selectively formed in one pot. The choice of the Brønsted acid was the key to the high efficiency of this transformation under metal-free conditions.

A Brønsted acid-promoted protocol for the synthesis of 2,4-disubstituted thiazoles from benzylamines, acetophenones, and sulfur powder under metal-free conditions is described.

At least 50% of the biologically active compounds have a heterocyclic skeleton.¹ Among these, the thiazole ring is an important five-membered aromatic heterocycle with nitrogen and sulfur atoms, and the unique structure has led to many applications in different pharmaceuticals and biological processes.² For example (Fig. 1), antimicrobial (Abafungin),³ antihypertension (Arotinolol),⁴ anti-inflammatory (Meloxicam),⁵ and immunomodulatory (Fanetizole)⁶ drugs are prevalent among the drugs based on thiazole that have reached the marketplace.⁷Open in a separate windowFig. 1Selected commercial drugs based on thiazole.In view of this, great efforts have been invested in the development of novel synthetic protocols to facilitate the construction of thiazole derivatives. The typical procedure for the synthesis of thiazoles involves the reaction of α-haloketones with thioureas/thioamides using catalysts such as cyclodextrin,⁸ iodine,⁹ silica chloride,¹⁰ baker''s yeast,¹¹ and others¹² (Scheme 1a). Besides, Wu¹³ and co-workers developed a catalyst-free protocol for the construction of polysubstituted thiazoles from α-haloketones and thioureas/thioamides. Togo¹⁴ reported the efficient synthesis of thiazoles via a base-promoted 1H-1-(1′-alkynyl)-5-methyl-1,2,3-benziodoxathiole 3,3-dioxide reaction with thioamides. Recently, Kshirsagar¹⁵ and co-workers developed NIS-mediated intermolecular cyclization of styrenes and thioamides using water as the solvent. On the other hand, the transition metal-catalyzed direct coupling of pre-existing thiazole compounds provides an alternative approach (Scheme 1b).¹⁶ Very recently, Jiao¹⁷ and co-workers developed a novel Cu-catalyzed aerobic oxidative approach to obtain thiazoles using elemental sulfur as the sulfur source via a multiple Csp³–H bond cleavage strategy (Scheme 1c). In spite of synthetic efficiency, these methods suffer from limitations with respect to special substrates and transition-metal catalysts. Therefore, the development of efficient methods for the synthesis of thiazoles from simple and readily available substrates under metal-free conditions is highly desirable. It is well-known that the sulfur element is cheap, stable, and easy to handle and thus, it is an ideal sulfur source for C–S bond construction.¹⁸ In our continuing efforts on using elemental sulfur for the synthesis of sulfur-containing heterocycles under simple conditions,¹⁹ we describe a three-component strategy for thiazole formation from readily available acetophenones, benzylamines, and sulfur powder under metal-free conditions (Scheme 1d).Open in a separate windowScheme 1Synthesis of 2,4-disubstituted thiazoles.We commenced our investigation using acetophenone (1a), benzylamine (2a), and sulfur powder as the model system (

Brønsted acid-promoted hydroamination of unsaturated hydrazones: access to biologically important 5-arylpyrazolines

A novel and efficient Brønsted acid-promoted hydroamination of hydrazone-tethered olefins has been developed. A variety of pyrazolines have been easily obtained in good to excellent yields with high chemo- and regioselectivity under simple and mild conditions. This method represents a straightforward, facile, and practical approach toward biologically important 5-arylpyrazolines, which are difficult to access by previously reported radical hydroamination of β,γ-unsaturated hydrazones.

An efficient, chemo- and regioselective Brønsted acid-promoted hydroamination reaction of hydrazone-tethered olefins towards 5-arylpyrazolines was developed.     

Green synthesis of coumarin derivatives using Brønsted acidic pyridinium based ionic liquid [MBSPy][HSO4] to control an opportunistic human and a devastating plant pathogenic fungus Macrophomina phaseolina

An eco-friendly simple protocol has been devised for the preparation of coumarin derivatives using doubly Brønsted acidic task specific ionic liquid (TSIL) as a catalyst. Solvent-free conditions were employed for the reaction of different substituted phenols with β-ketoester in TSIL to produce corresponding substituted coumarin derivatives in good to excellent yields at ambient conditions; at room temperature and with reduced reaction times. The ionic liquid catalyst can be recycled and reused up to five times. All the synthesized coumarins were evaluated for their antifungal activities against Macrophomina phaseolina, a plant as well as an opportunistic human pathogenic fungus affecting more than 500 plant species worldwide and with no registered commercial fungicide available against it, to date. Amongst all the coumarins tested, compounds 3f and 3i showed excellent antifungal activity comparable to reference fungicide mancozeb. The current methodology provides an easy and expedient way to access the coumarin core in search of potential fungicides for sustainable agriculture.

An eco-friendly simple protocol has been devised for the preparation of coumarin derivatives using doubly Brønsted acidic task specific ionic liquid (TSIL) as a catalyst.     

A non-metal route to realize the bio-based polyester of poly(hexylene succinate): preparation conditions,side-reactions and mechanism in sulfonic acid-functionalized Brønsted acidic ionic liquids

A biodegradable linear bio-based polyester of poly(hexylene succinate) was effectively prepared in non-metal sulfonic acid-functionalized Brønsted acidic ionic liquids (SFBAILs) as both the catalyst and the polymerization medium, and the processes of polycondensation and post-polycondensation in SFBAILs were also investigated. In addition, the side reactions which were detrimental to the growth of M_w of poly(hexylene succinate) were evaluated and the synthesis mechanism of poly(hexylene succinate) catalyzed by SFBAILs was discussed with the help of DFT calculations. The result shows that both the imidazole ring and the sulfonic group on cations of SFBAILs play an important role in the catalytic process.

Biobased poly(hexylene succinate) synthesized in sulfonic acid-functionalized Brönsted acidic ionic liquids was monitored by ¹H NMR spectra and DFT calculations.     

Tuning Brønsted and Lewis acidity on phosphated titanium dioxides for efficient conversion of glucose to 5-hydroxymethylfurfural

5-Hydroxymethylfurfural (HMF) derived from cellulosic sugars has become increasingly important as a platform chemical for the biorefinery industry because of its versatility in the conversion to other chemicals. Although HMF can be produced in high yield from fructose dehydration, fructose is rather expensive because it requires multiple processing steps. On the other hand, HMF can be produced directly from highly abundant glucose, which could reduce time and cost. However, an effective and multifunctional catalyst is needed to selectively promote the glucose-to-HMF reaction. In this work, we report a bifunctional phosphated titanium dioxide as an efficient catalyst for such a reaction. The best catalyst exhibits excellent catalytic performance for the glucose conversion to HMF with 72% yield and 83% selectivity in the biphasic system. We achieve this by tuning the solvent system, controlling the amount of Brønsted and Lewis acid sites on the catalyst, and modification of the reaction setup. From the analysis of acid sites, we found that the addition of phosphate group (Brønsted acid site) onto the surface of TiO₂ (Lewis acid site) significantly enhanced the HMF yield and selectivity when the optimum ratio of Brønsted and Lewis acid sites is reached. The high catalytic activity, good reusability, and simple preparation method of the catalyst show a promise for the potential use of this catalytic system on an industrial scale.

Tunable Lewis and Brønsted acid sites on P–TiO₂ tandem catalysts for glucose-to-HMF conversion providing high HMF yield (72%) and selectivity (83%).     

Silica gel-immobilised chiral 1,2-benzenedisulfonimide: a Brønsted acid heterogeneous catalyst for enantioselective multicomponent Passerini reaction

A chiral heterogeneous catalyst derivative of (−)-4,5-dimethyl-3,6-bis(1-naphthyl)-1,2-benzenedisulfonimide is proven here to be efficient in a three-component asymmetric Passerini protocol, carried out in a deep eutectic solvent. Reaction conditions are mild and green, while enantioselectivity is excellent. The catalyst was easily recovered and reused with no decrease in its catalytic activity.

A chiral heterogeneous catalyst derivative of (−)-4,5-dimethyl-3,6-bis(1-naphthyl)-1,2-benzenedisulfonimide is proven here to be efficient in a three-component asymmetric Passerini protocol, carried out in a deep eutectic solvent.     

Synthesis of six-membered spirooxindoles via a chiral Brønsted acid-catalyzed asymmetric intramolecular Friedel–Crafts reaction

By means of the direct condensation of N-aminoethylpyrroles and isatins, followed by a chiral phosphoric acid-catalyzed asymmetric intramolecular Friedel-Crafts reaction, a new class of valuable chiral 3′,4′-dihydro-2′H-spiro[indoline-3,1′-pyrrolo[1,2-a]pyrazin]-2-ones bearing a quaternary carbon stereocenter were successfully synthesized in good to excellent yields and with moderate to good enantioselectivities under mild reaction conditions.

A chiral phosphoric acid-catalyzed asymmetric intramolecular Friedel–Crafts reaction for the synthesis of 3′,4′-dihydro-2′H-spiro[indoline-3,1′-pyrrolo[1,2-a]pyrazin]-2-ones.

Spirooxindoles, containing a spirocyclic system, are unique structural motifs found in a wide range of natural products and bioactive compounds.¹ For instance, Spirotryprostatin B (Fig. 1) was isolated from the fermentation broth of Aspergillus fumigatus and was shown to completely inhibit the G2/M progression of cell division in mammalian tsFT210 cells.²Gelsemium alkaloids (e.g. gelsenicine, gelsedine and gelsedilam) were isolated from the ancient medicine Yakatsu stored in the Shosoin Repository and exhibit a wide range of biological activities, including analgesic, anti-inflammatory, and antitumor effects.³ Besides, a considerable number of spirooxindoles display anticancer activity. For example, APG-115 and MI77301, which can effectively block the MDM2-p53 protein–protein interaction in cells as MDM2 inhibitors, are under clinical trials as promising anticancer drugs.⁴Open in a separate windowFig. 1Selected examples of natural and artificial spirooxindoles.Consequently, the unique structure of those compounds has attracted much attention from synthetic chemists.⁵ Some investigated strategies to access this skeleton include intramolecular reactions of imines via Pictet–Spengler reaction synthesized from tryptamines or tryptophans with isatin (Scheme 1a),⁶ oxidative rearrangement of tetrahydro-β-carbolines prepared via Pictet–Spengler reaction (Scheme 1b),⁷ metal or small-molecule catalyzed 1,3-dipolar cycloaddition of imino esters with methyleneindolinones (Scheme 1c),⁸ Rh-catalyzed [4 + 1] cycloaddition of azocompound with vinyl isocyanates (Scheme 1d),⁹ chiral iodoarene catalytic oxidative spirocyclization (Scheme 1e),¹⁰ Pd-catalyzed intramolecular addition and domino spirocyclization,¹¹ intramolecular nucleophilic addition¹² and so on.¹³ Despite of several methods developed for the construction of this skeleton, however, less work finished catalytic asymmetric synthesis and preparation of novel chiral spirooxindoles are warmly anticipated. Herein, we report an efficient protocol via an intermolecular condensation/intramolecular Friedel–Crafts reaction to synthesize a new class of 3′,4′-dihydro-2′H-spiro[indoline-3,1′-pyrrolo[1,2-a]pyrazin]-2-ones in an asymmetric way.Open in a separate windowScheme 1Catalytic synthesis of spirooxindoles.At the outset of this study, we envisaged that reaction of N-aminoethylpyrroles with isatins catalyzed by chiral phosphoric acids might undergo direct condensation, followed by intramolecular Friedel–Crafts reactions under very mild reaction conditions.¹⁴ We first began our investigation with 1-benzylindole-2,3-dione (1.0 equiv.) and N-aminoethylpyrrole (1.2 equiv.) as the substrates, BINOL-derived chiral phosphoric acid 4a as the catalyst, dichloromethane (DCM) as the solvent, 4 Å MS as a desiccant. The reaction was performed at room temperature (20 °C). To our delight, the cascade reaction proceeded smoothly providing 3′,4′-dihydro-2′H-spiro[indoline-3,1′-pyrrolo[1,2-a]pyrazin]-2-one 3aa in 99% yield but with a poor enantioselectivity (5% ee, 15 we turned to employ another kind of spiro catalysts¹⁶ and investigated their function. Delightingly, catalyst (R)-PA 5a was found to significantly improve the ee from 49% to 88% and product 3aa was given in 85% yield under mild reaction conditions in DCM. Then we evaluated the feasibility of the reaction in other solvents ( EntrySubstrate(R)-PASolventTemp. [°C]Yield^b [%]ee^c [%]11a4aDCMr.t.99521a4bDCMr.t.99031a4cDCMr.t.72541a4dDCMr.t.99051a4eDCMr.t.992161a4fDCMr.t.99071a4gDCMr.t.99081a4hDCMr.t.97091a4iDCMr.t.9649 10 1a5a DCM r.t. 85 88 111a5bDCMr.t.7019121a5aTHFr.t.9980131a5aMeCNr.t.6862141a5aDMFr.t.2844151a5aEt₂Or.t.8179161a5aCHCl₃r.t.9078171a5a1,4-Dioxaner.t.8383181a5aDCEr.t.8487191b5aDCMr.t.8939201c5aDCMr.t.6868211d5aDCMr.t.9774221e5aDCMr.t.8474231f5aDCMr.t.9269241a5aDCM07179251a5aDCM309080Open in a separate window^aReaction conditions: 1a (0.2 mmol), 2a (1.2 equiv.), (R)-PA (10 mol%), 4 Å MS (50 mg), 2.0 mL of solvent, r.t. = 20 °C, overnight.^bIsolated yield.^cee was determined by chiral HPLC.With the set of optimized reaction conditions in hand, we focused our attention on the substrate scope of this catalytic asymmetric intramolecular Friedel–Crafts reaction. At first, substituted isatin derivatives were screened as the substrates in combination with N-aminoethyl pyrrole 2a, affording chiral products 3′,4′-dihydro-2′H-spiro[indoline-3,1′-pyrrolo[1,2-a]pyrazin]-2-ones in moderate to excellent yields ( Open in a separate window^aReaction conditions: 1a (0.2 mmol), 2a (1.2 equiv.), (R)-PA 5a (10 mol%), 4 Å MS (50 mg), 2.0 mL of solvent, overnight.^bIsolated yield.^cee was determined by chiral HPLC.Next, the substrate scope of N-aminoalkylpyrroles was also investigated. As shown in Open in a separate window^aReaction conditions: 1a (0.2 mmol), 2a (1.2 equiv.), (R)-PA 5a (10 mol%), 4 Å MS (50 mg), 2.0 mL of solvent, overnight.^bIsolated yield.^cee was determined by chiral HPLC.Furthermore, a scale up reaction of 1a and 2a was performed, generating product 3aa in 89% yield and 82% ee (Fig. 2).Open in a separate windowFig. 2Scale-up reaction.Finally, based on the experimental results, together with related studies on CPA-catalyzed reactions,^5f–5i we proposed a possible reaction pathway to explain the stereochemistry of the formation of spirooxindoles 3 (Scheme 2). Isatins 1 initially participated in a Mannich reaction with N-aminoethylpyrroles 2, affording transient intermediates 6 under the catalysis of Brønsted acid. Through the dual-hydrogen-bond, (R)-CPA 5a interacted with intermediates 6 to realize their catalysis and stereocontrol. The enantioenriched spirooxindoles 3 were subsequently yielded via the intramolecular Friedel–Crafts reaction of intermediates 6.Open in a separate windowScheme 2Proposed reaction pathway and activation mode.     

Transition metal/Brønsted acid cooperative catalysis enabled facile synthesis of 8-hydroxyquinolines through one-pot reactions of ortho-aminophenol,aldehydes and alkynes

A convenient and straightforward three-component one-pot strategy has been developed for the synthesis of 8-hydroxyquinoline derivatives. Under the cooperative catalysis of silver(i) triflate and trifluoroacetic acid, ortho-aminophenol reacted with a range of aldehydes and alkynes under mild reactions, affording the corresponding 8-hydroxyquinoline derivatives with good to excellent yields. These transformations exhibited exceptional substrate generality and functional group compatibility.

An efficient and cooperative catalytic one-pot synthetic methodology for 8-hydroxyquinoline compounds has been developed.     

Sulfonated carbon derived from the residue obtained after recovery of essential oil from the leaves of Cinnamomum longepaniculatum using Brønsted acid ionic liquid,and its use in the preparation of ellagic acid and gallic acid

A Brønsted acid ionic liquid, 3-methyl-1-(4-sulfonylbutyl) imidazolium hydrogensulfate ([HO₃S(CH₂)₄mim]HSO₄), was used for the first time for the preparation of a sulfonated carbon catalyst. The catalyst was prepared from the residue obtained after recovery of the essential oil from the leaves of Cinnamomum longepaniculatum. The sulfonated carbon catalyst with an amorphous structure attained high acidic efficiency at a sulfonation temperature of 200 °C for 2 h of sulfonation time, and was characterised. SEM morphologies revealed that the carbon catalyst consisted of uniform carbon microspores. FTIR analysis, elemental analysis, and X-ray photoelectron spectroscopy revealed that the sulfonic acid group was successfully introduced on the surface of the sulfonated carbon catalyst. The result of TG analysis showed that the obtained sulfonated carbon catalyst has high thermal stability. Good acid and catalytic activity of the obtained sulfonated carbon catalyst were observed for the preparation of ellagic acid and gallic acid, which is comparable to those of diluted sulfuric acid and a sulfonated carbon catalyst that had been prepared with concentrated sulfuric acid. The excellent reusability of the sulfonated carbon catalyst was also confirmed by repeated experimental trials. In summary, the sulfonated catalyst derived from the residue obtained after recovery of essential oil from the leaves of C. longepaniculatum is an economic, eco-benign and promising substitute for traditional mineral acid catalysts for acidic catalysis in industrial applications.

A Brønsted acid ionic liquid, 3-methyl-1-(4-sulfonylbutyl) imidazolium hydrogensulfate ([HO₃S(CH₂)₄mim]HSO₄), was used for the first time for the preparation of a sulfonated carbon catalyst.     

Cascade reaction engineering on zirconia-supported mesoporous MFI zeolites with tunable Lewis–Brønsted acid sites: a case of the one-pot conversion of furfural to γ-valerolactone

Catalytic cascade reactions are strongly desired as a potential means of combining multistep reactions into a single catalytic reactor. Appropriate catalysts composed of multi-reactive sites to catalyze cascade reactions in a sequential fashion are central to such efforts. Here, we demonstrate a bifunctional zeolite catalyst with close proximity of Brønsted and Lewis acid sites through the synthesis of a mesoporous ZrO₂[Al]MFI nanosponge (NS). The unique mesopores of the MFI-NS allow the confinement of zirconium oxide clusters (Lewis acid sites, LA) within the few-unit-cell-thin MFI aluminosilicate zeolite wall (Brønsted acid sites, BA). Such a structure is clearly distinct from the conventional MFI zeolite, where the agglomeration of zirconium oxide clusters onto the external surface area within the crystal bulk is not possible, resulting in segregated BA and LA sites on the internal and external zeolite, respectively. By bringing the BA and LA within ZrO₂[Al]MFI-NS 30, we uncovered a more efficient catalytic route for the conversion of furfural (100% within 2 h) to γ-valerolactone (GVL) (83%). This route is only evident when the long molecular diffusion path, in the most extreme case of physically mixed ZrO₂-(LA) and Al-zeolites (BA) (45% of GVL yield), is eliminated. Unlike the bifunctional ZrO₂–Al-beta (GVL yield of 75%), where the BA concentration is greatly compromised at the expense of LA formation, we also show that the ZrO₂[Al]MFI-NS is able to maintain a high density and good stability of both types of acids.

The highly mesoporous ZrO₂[Al]MFI-NS with close proximity of Brønsted and Lewis acid sites exhibited the one-pot conversion of furfural to γ-valerolactone (GVL) and achieved a high yield of 83% GVL.     

Influence of a Lewis acid and a Brønsted acid on the conversion of microcrystalline cellulose into 5-hydroxymethylfurfural in a single-phase reaction system of water and 1,2-dimethoxyethane

5-Hydroxymethylfurfural (HMF) is a typical dehydration product of C6 carbohydrates, and it can be converted into a series of chemicals and liquid fuels. In this study, an advanced low-boiling single-phase reaction system consisting of water and 1,2-dimethoxyethane (DMOE) was proposed for the production of HMF from microcrystalline cellulose (MCC). AlCl₃ and H₃PO₄ were selected as the Lewis acidic catalyst and Brønsted acidic catalyst, respectively, and the influence of these two catalysts on the conversion behavior of MCC was studied. The results showed that MCC could be selectively converted into HMF or levulinic acid (LA) by altering the solvent composition. As for the composition of the catalyst, high AlCl₃ content favored the generation of HMF, whereas high H₃PO₄ content could decrease the HMF yield and promote the formation of glucose and fructose. The highest HMF yield of 49.42% was obtained at an AlCl₃–H₃PO₄ ratio of 1 : 0.8. GC-MS analysis suggested that much MCC was transformed into furans and cyclopentenones in the presence of AlCl₃, while anhydrosugars tended to be generated with a high H₃PO₄ proportion in the catalyst. Besides, FTIR analysis of the insoluble humin formed during MCC conversion indicated that AlCl₃ could also facilitate the depolymerization of MCC.

An advanced single-phase reaction system consisting of water–DMOE solvent, AlCl₃ and H₃PO₄ for high-efficiency HMF production from cellulose.     

Influence of the acid–base stoichiometry and residual water on the transport mechanism in a highly-Brønsted-acidic proton-conducting ionic liquid

In this study, Brønsted-acidic proton conducting ionic liquids are considered as potential new electrolytes for polymer membrane fuel cells with operating temperatures above 100 °C. N-Methyltaurine and trifluoromethanesulfonic acid (TfOH) were mixed at various stoichiometric ratios in order to investigate the influence of an acid or base excess. The proton conductivity and self-diffusion of the “neat” and with 6 wt% water samples were investigated by following electrochemical and NMR methods. The composition change in the complete species and the relative proton transport mechanism based on the NMR results are discussed in detail. During fuel cell operation, the presence of significant amounts of residual water is unavoidable. In PEFC electrolytes, the predominating proton transfer process depends on the cooperative mechanism, when PILs are fixed on the polymer matrix within the membrane. Due to the comparable acidity of the cation [2-Sema]⁺ and the hydroxonium cation, with excess N-methyltaurine or H₂O in the compositions, fast proton exchange reactions between the protonated [2-Sema]⁺ cation, N-methyltaurine and H₂O can be envisaged. Thus, an increasing ratio of cooperative proton transport could be observed. Therefore, for polymer membrane fuel cells operating at elevated temperatures, the highly acidic PILs with excess bases are promising candidates for future use as electrolytes.

There is a transition between prevailing vehicular and cooperative transport mechanism in base-excess Brønsted-acidic proton-conducting ionic liquids depending on stoichiometry.