Improving the Catalytic Performance of (S)-Proline as Organocatalyst in Asymmetric Aldol Reactions in the Presence of Solvate Ionic Liquids: Involvement of a Supramolecular Aggregate

Sustainable preparation of 2-acylbenzothiazoles under the cooperation of ionic liquids and microwave irradiation

Microwave irradiation (MW) and ionic liquids (ILs) are two of the most promising relatively greener synthetic approaches to preparing value-added chemicals. Herein, a series of 2-acylbenzothiazole derivatives were synthesized for the first time from commercially available α-bromoacetophenones and disulfane-diyl-dianilines through the cooperation of ionic liquids and microwave irradiation under metal- and extra-additives-free conditions. A plausible mechanism involving the successive IL-induced enolation has been proposed.

Highly efficient morpholine-based organocatalysts for the 1,4-addition reaction between aldehydes and nitroolefins: an unexploited class of catalysts

Many studies have demonstrated how the pyrrolidine nucleus is more efficient than the corresponding piperidine or morpholine as organocatalysts in the condensation of aldehydes with electrophiles via enamine. Focussing on morpholine-enamines, their low reactivity is ascribed to the presence of oxygen on the ring and to the pronounced pyramidalisation of nitrogen, decreasing the nucleophilicity of the enamine. Thus, the selection of efficient morpholine organocatalysts appears to be a difficult challenge. Herein, we reported on the synthesis of new organocatalysts belonging to the class of ß-morpholine amino acids that were tested in a model reaction, i.e., the 1,4-addition reaction of aldehydes to nitroolefins. Starting from commercially available amino acids and epichlorohydrin, we designed an efficient synthesis for the aforementioned catalysts, controlling the configuration and the substitution pattern. Computational studies indeed disclosed the transition state of the reaction, explaining why, despite all the limitations of the morpholine ring for enamine catalysis, our best catalyst works efficiently, affording condensation products with excellent yields, diastereoselection and good-to-exquisite enantioselectivity.

Chiral Graphene Hybrid Materials: Structures, Properties, and Chiral Applications

Chirality has become an important research subject. The research areas associated with chirality are under substantial development. Meanwhile, graphene is a rapidly growing star material and has hard-wired into diverse disciplines. Rational combination of graphene and chirality undoubtedly creates unprecedented functional materials and may also lead to great findings. This hypothesis has been clearly justified by the sizable number of studies. Unfortunately, there has not been any previous review paper summarizing the scattered studies and advancements on this topic so far. This overview paper attempts to review the progress made in chiral materials developed from graphene and their derivatives, with the hope of providing a systemic knowledge about the construction of chiral graphenes and chiral applications thereof. Recently emerging directions, existing challenges, and future perspectives are also presented. It is hoped this paper will arouse more interest and promote further faster progress in these significant research areas.

New Mesoporous Silica-Supported Organocatalysts Based on (2S)-(1,2,4-Triazol-3-yl)-Proline: Efficient, Reusable, and Heterogeneous Catalysts for the Asymmetric Aldol Reaction

Novel organocatalytic systems based on the recently developed (S)-proline derivative (2S)-[5-(benzylthio)-4-phenyl-(1,2,4-triazol)-3-yl]-pyrrolidine supported on mesoporous silica were prepared and their efficiency was assessed in the asymmetric aldol reaction. These materials were fully characterized by FT-IR, MS, XRD, and SEM microscopy, gathering relevant information regarding composition, morphology, and organocatalyst distribution in the doped silica. Careful optimization of the reaction conditions required for their application as catalysts in asymmetric aldol reactions between ketones and aldehydes afforded the anticipated aldol products with excellent yields and moderate diastereo- and enantioselectivities. The recommended experimental protocol is simple, fast, and efficient providing the enantioenriched aldol product, usually without the need of a special work-up or purification protocol. This approach constitutes a remarkable improvement in the field of heterogeneous (S)-proline-based organocatalysis; in particular, the solid-phase silica-bonded catalytic systems described herein allow for a substantial reduction in solvent usage. Furthermore, the supported system described here can be recovered, reactivated, and reused several times with limited loss in catalytic efficiency relative to freshly synthesized organocatalysts.

Novel Methodologies for Chemical Activation in Organic Synthesis under Solvent-Free Reaction Conditions

One central challenge for XXI century chemists is the development of sustainable processes that do not represent a risk either to humanity or to the environment. In this regard, the search for more efficient and clean alternatives to achieve the chemical activation of molecules involved in chemical transformations has played a prominent role in recent years. The use of microwave or UV-Vis light irradiation, and mechanochemical activation is already widespread in many laboratories. Nevertheless, an additional condition to achieve "green" processes comes from the point of view of so-called atom economy. The removal of solvents from chemical reactions generally leads to cleaner, more efficient and more economical processes. This review presents several illustrative applications of the use of sustainable protocols in the synthesis of organic compounds under solvent-free reaction conditions.

A Simple and Efficient Protocol for Proline-Catalysed Asymmetric Aldol Reaction

The proline-catalysed asymmetric aldol reaction is usually carried out in highly dipolar aprotic solvents (dimethylsulfoxide, dimethylformamide, acetonitrile) where proline presents an acceptable solubility. Protic solvents are generally characterized by poor stereocontrol (e.g., methanol) or poor reactivity (e.g., water). Here, we report that water/methanol mixtures are exceptionally simple and effective reaction media for the intermolecular organocatalytic aldol reaction using the simple proline as the catalyst.

Green Asymmetric Organocatalysis

Asymmetric organocatalysis is becoming one of the main tools for the synthesis of chiral compounds that are needed as medicines, crop protection agents, and other bioactive molecules. It can be effectively combined with various green chemistry methodologies. Intensification techniques, such as ball milling, flow, high pressure, or light, bring not only higher yields, faster reactions, and easier product isolation, but also new reactivities. More sustainable reaction media, such as ionic liquids, deep eutectic solvents, green solvent alternatives, and water, also considerably enhance the sustainability profile of many organocatalytic reactions.

Enantioselective Direct anti-Selective Mannich-type Reactions Catalyzed by 3-Pyrrolidinecarboxylic Acid in the Presence of Potassium Carbonate: Addition of Potassium Carbonate Improves Enantioselectivities

Mannich-type reactions of cyclohexanone and related six-membered-ring ketones with N-p-methoxyphenyl-protected imines of arylaldehydes catalyzed by 3-pyrrolidinecarboxylic acid in the presence of K₂CO₃ that afford anti-isomers of the Mannich products with high diastereo- and enantioselectivities are reported. Addition of K₂CO₃ improved the enantioselectivities of the reactions catalyzed by 3-pyrrolidinecarboxylic acid while retaining the anti-selectivity of the reaction. Thus, the use of K₂CO₃ expands the scope of these organocatalytic reactions for providing the products with high enantioselectivities.

Understanding the effects of solvate ionic liquids as solvents on substitution processes

The effects of solvate ionic liquids as solvents have been considered for two substitution processes where the solvent effects of typical ionic liquids have been extensively investigated previously; the bimolecular nucleophilic substitution (SN2) reaction between pyridine and benzyl bromide and the nucleophilic aromatic substitution (SNAr) reaction between ethanol and 1-fluoro-2,4-dinitrobenzene. It was found that use of solvate ionic liquids gave rise to similar trends in the activation parameters for both substitution processes as typical ionic liquids, implying the microscopic interactions responsible for the effects were the same. However, different effects on the rate constants compared to typical ionic liquids were observed due to the changes in the balance of enthalpic and entropic contributions to the observed rate constants. From these data it is clear that the reaction outcome for both of these substitution reactions fall within the 'predictive framework' established in previous studies with a cautionary tale or two of their own to add to the general knowledge of ionic liquid solvent effects for these processes, particularly with respect to potential reactivity of the solvate ionic liquids themselves.

Remarkable Effect of [Li(G4)]TFSI Solvate Ionic Liquid (SIL) on the Regio- and Stereoselective Ring Opening of α-Gluco Carbasugar 1,2-Epoxides

Carba analogues of biologically relevant natural carbohydrates are promising structures for the development of future drugs endowed with enhanced hydrolytic stability. An open synthetic challenge in this field is the optimization of new methodologies for the stereo- and regioselective opening of α-gluco carbasugar 1,2-epoxides that allow for the preparation of pseudo mono- and disaccharides of great interest. Therefore, we investigated the effect of Lewis acids and solvate ionic liquids (SILs) on the epoxide ring opening of a model substrate. Of particular interest was the complete stereo- and regioselectivity, albeit limited to simple nucleophiles, toward the desired C(1) isomer that was observed using LiClO4. The results obtained with SILs were also remarkable. In particular, Li[NTf2]/tetraglyme ([Li(G4)]TFSI) was able to function as a Lewis acid and to direct the attack of the nucleophile preferentially at the pseudo anomeric position, even with a more complex and synthetically interesting nucleophile. The regioselectivity observed for LiClO4 and [Li(G4)]TFSI was tentatively ascribed to the formation of a bidentate chelating system, which changed the conformational equilibrium and ultimately permitted a trans-diaxial attack on C(1). To the best of our knowledge, we report here the first case in which SILs were successfully employed in a ring-opening process of epoxides.

Asymmetric Direct Aldol Reaction Catalyzed by (1R, 2R)-(+)-1, 2- Diammonium Cyclohexane-L-tartrate in Water

Background:

A cheap and commercially available organocatalyst, (1R, 2R)-(+)-1, 2- diammonium cyclohexane-L-tartrate 1 was applied in direct aldol reaction in water. The organocatalyst 1 afforded aldol products from cyclohexanone and substituted aromatic aldehydes with high yield (up to 90%) and good stereoselectivity (up to 99% ee and up to 11.5:1 dr) in large volume of water (10 ml).

Methods:

The same aldol reaction when carried out in the presence of more expensive organocatalyst e.g. (1R, 2R)-(+)-1,2-diaminocyclohexane and 1,6-hexanediaoic acid as acid additive furnished the aldol products with only 20% yield, 2:1 anti/syn ratio and 92% ee.

Results and Conclusion:

In summary, we have applied a reasonably cheap and commercially available organocatalyst 1 for highly enantioselective direct aldol reaction in water at room temperature.

A Review of Solvate Ionic Liquids: Physical Parameters and Synthetic Applications

Solvate Ionic Liquids (SILs) are a relatively new class of ionic liquids consisting of a coordinating solvent and salt, that give rise to a chelate complex with very similar properties to ionic liquids. Herein is the exploration of the reported Kamlet-Taft parameters, Gutmann Acceptor numbers and the investigation of chelating effects through NMR spectroscopy of multiple atomic nuclei. These properties are related to the application of SILs as reaction media for organic reactions. This area is also reviewed here, including the implication in catalysis for the Aldol and Kabachnik-Fields reactions and electrocyclization reactions such as Diels-Alder and [2+2] cycloaddition. Solvate ILs exhibit many interesting properties and hold great potential as a solvent for organic transformations.

Process Catalyst Mass Efficiency by Using Proline Tetrazole Column-Flow System

Generally, organocatalysts are not decomposed during chemical transformation, which is different from traditional metal catalysts. To improve catalytic processes efficiency, various studies have been applied to flow synthesis for organocatalysis. Furthermore, many immobilized organocatalysts have been used for heterogeneous flow synthesis, which requires huge amounts of immobilized catalyst and requires several steps to prepare. We took advantage of organocatalysts with low-polarity organic solvent and developed a flow system through a packed-bed column with simply proline tetrazole (5-(2-pyrrolidinyl)-1H-tetrazole) for heterogeneous organocatalytic synthesis. Under ambient temperature, this heterogeneous organocatalyst continuous flow-column system with ketones as a donor provides aldol, Mannich, and o-nitroso aldol reactions in up to quantitative yields with excellent enantio- and chemoselectivity values. Our heterogeneous-flow synthesis provides extremely low process catalyst mass efficiency and continuous production without changing the packed-bed catalyst column.

Chiral bisoxazoline catalyzed decarboxylative aldol reactions between β-carbonyl acids and trifluoroacetaldehyde hemiacetals as well as trifluoroacetaldehyde: the mechanism, the origin of enantioselectivity and the role of a catalyst

The role of a catalyst in the decarboxylative aldol reactions between β-carbonyl acids and trifluoroacetaldehyde hemiacetals as well as trifluoroacetaldehyde catalyzed by chiral bisoxazoline were unveiled theoretically.

Polymer ionic liquid network: a highly effective reusable catalyst for one-pot synthesis of heterocyclic compounds

Significant efforts have been devoted to developing immobilized chiral catalysts with high activity, selectivity, and stability. In this present study, a new heterogeneous proline catalyst system was prepared based on strong noncovalent interactions between polymer ionic liquid (PIL) and l-proline. First, pyridine PILs, which can complex with l-proline monomers through noncovalent interactions, were synthesized using reversible addition–fragmentation chain transfer (RAFT) polymerization. The polymer network-supported chiral catalysts were obtained following further free radical polymerization. Different structures were formed in response to different ratios of PIL and chiral monomer, as well as different PIL anions, in the reactions. The new formed layer structures and synergic effects of PIL resulted in heterogeneous catalysts with high catalytic activity and enantioselectivity, thus endowing them with better catalytic performance for the one-pot synthesis of heterocyclic compounds compared to homogeneous catalytic systems. These catalytic systems were able to be reused and recycled five times with no discernible loss in catalytic activity and enantioselectivity. l-Proline was efficiently loaded onto the polymer network simply based on supramolecular interactions, providing a novel method of synthesizing high performance supported catalysts for organic reactions.

Significant efforts have been devoted to developing immobilized chiral catalysts with high activity, selectivity, and stability.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Poly(N-isopropylacrylamide-co-l-proline)-catalyzed Claisen–Schmidt and Knoevenagel condensations: unexpected enhanced catalytic activity of the polymer catalyst

The heterogeneous catalyst is more active than the homogeneous one! The abnormal phenomenon is probably caused by the polymer, which enriches the reactants around the reaction sites.