Supported choline hydroxide (ionic liquid) as heterogeneous catalyst for aldol condensation reactions

Choline hydroxide was used as a basic catalyst for aldol condensation reactions to produce new C-C bonds between several ketones and aldehydes. Choline supported on MgO exhibits higher TOF values than other well known basic catalysts in these reactions.     

Alanine-supported protic ionic liquids as efficient catalysts for aldol condensation reactions

A series of novel protic ionic liquids were immobilized on alanine, a cheap readily available amino acid. These short aliphatic chain ionic liquids have a low cost of preparation, a low toxicity, and need simple synthesis/purification processes. Their catalytic activity was tested for citral–acetone and benzaldehyde–acetone condensations, two reactions with an interest for pharmacological as well as flavor and fragrance industry. Good results were obtained in terms of conversion and selectivity; moreover, the catalysts can be recycled and reused for several consecutive cycles without significant loss of activity.     

Direct catalytic asymmetric cross-aldol reactions in ionic liquid media

Enantioselective proline-catalyzed direct asymmetric cross-aldol reactions with aldehydes were performed in ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate media, which simplified product isolation and catalyst recycling, affording 3-hydroxy aldehydes in high yield with excellent stereoselectivity. In addition, the enhanced reactivity of the asymmetric cross-aldol reactions in ionic liquid allowed the catalyst loading to be lowered significantly.     

Highly enantioselective synthesis of syn-aldols of cyclohexanones via chiral primary amine catalyzed asymmetric transfer aldol reactions in ionic liquid

Chiral primary-tertiary diamine/TfOH was found to catalyze kinetic resolution of racemic syn-aldols of cyclohexanones in ionic liquid effectively, affording the chiral syn-aldols with up to 99:1 syn/anti and 99% ee.     

Stereoselective aldol condensation reactions of iron acetyl enolates

By proper choice of counterion, the enolate of η⁵ - CpFe(CO)(PPh₃)COCH₃ will react with aldehydes to form the aldol products with high stereoselectivity.     

Coumarin syntheses via Pechmann condensation in Lewis acidic chloroaluminate ionic liquid

1-Butyl-3-methylimidazolium chloroaluminate, [bmim]Cl·2AlCl₃ ionic liquid is used as an alternative to conventional acid catalysts in the Pechmann condensation of phenols with ethyl acetoacetate leading to the formation of coumarin derivatives. The reaction time is reduced drastically even at ambient conditions. The ionic liquid plays the dual role of solvent and Lewis acid catalyst providing a quick and efficient route to the syntheses of coumarins.     

Catalytic asymmetric aldol reactions in aqueous media

Nature has perfected the stereospecific aldol reaction by using aldolase enzymes. While virtually all the biochemical aldol reactions use unmodified donor and acceptor carbonyls and take place under catalytic control in an aqueous environment, the chemical domain of the aldol addition has mostly relied on prior transformation of carbonyl substrates, and the whole process traditionally is carried out in anhydrous solvents. The area of aqua-asymmetric aldol reactions has received much attention recently in light of the perception both of its green chemistry advantages and its analogy to eon-perfected enzyme catalysis. Both chiral metal complexes and small chiral organic molecules have been recently reported to catalyze aldol reactions with relatively high chemical and stereochemical efficiency. This tutorial review describes recent developments in this area.     

Functionalized ionic liquids catalyzed direct aldol reactions

A series of functionalized ionic liquids (FILs) incorporated with chiral-pyrrolidine unit have been synthesized and tested as reusable organocatalysts for direct aldol reactions. FIL 1b in combination with acetic acid and water as additives could effectively catalyze direct aldol reactions of various ketone donors in high yields and the FIL catalyst was easily recycled and reused for six times with slight reduction in activity. Based on experimental observations as well as previous reports, we proposed that the reactions occurred via syn-enamine intermediate and the ionic-liquid moiety in the FIL provides some space shielding for the participating aldehyde acceptors that accounts for the modest enantioselectivities observed in the reactions.     

Direct organocatalytic aldol reactions in buffered aqueous media

Organocatalytic cross-aldol reactions catalyzed by cyclic secondary amines in aqueous media provide a direct route to a variety of aldols including carbohydrate derivatives and may warrant consideration as a prebiotic route to sugars.     

Stereoselective aldol reactions via titanium enolates

The species produced by treatment of the lithium enolate of N-propionylpyrroldine with bis(cyclopentadienyl)titanium dichloride adds to simple aldehydes with high stereoselectivity to give anti-aldol products.     

Efficient nucleophilic substitution reactions of highly functionalized allyl halides in ionic liquid media

Nucleophilic substitution reactions of highly functionalized allyl halides with three anions, N₃⁻, AcO⁻, and PhSO₂⁻, in ionic liquid media were conducted. The ionic liquid, [Bmim][BF₄], was found to be superior to classical organic solvents to give higher yields and faster reaction rates. The resulting products belong to multifunctionalized trisubstituted α,β-unsaturated ketones, which are useful building blocks for organic synthesis.     

Aldol- and Mannich-type reactions via in situ olefin migration in ionic liquid

An aldol-type and a Mannich-type reaction via the cross-coupling of aldehydes and imines with allylic alcohols catalyzed by RuCl(2)(PPh(3))(3) was developed with ionic liquid as the solvent. The solvent/catalyst system could be reused for at least five times with no loss of reactivity.     

Functional ionic liquid from biorenewable materials: synthesis and application as a catalyst in direct aldol reactions

A new room temperature ionic liquid (IL) (2-hydroxyethyl)-trimethyl-ammonium (S)-2-pyrrolidinecarboxylic acid salt ([Choline][Pro]) has been synthesized from biorenewable and nontoxic raw materials (choline chloride and l(−)-proline) in a simple and relative green route. The IL has been demonstrated to be the efficient catalyst of the direct aldol reactions between ketones and aromatic aldehydes in water at room temperature. The aldol products can be obtained with good yields and the IL in aqueous phase can be separated easily and reused.     

Preparation of porous Cu-rich CuNi electrodes via electrochemically dealloying in ionic liquid

Commercial CuNi (55/45 wt%) alloy can be electrochemically dealloyed to be the porous Cu-rich CuNi electrodes, which reveals an enhanced catalytic activity toward nitrate reduction in contrast with that of the mother alloy electrodes. In comparison with the use of aqueous electrolytes, the formation of the porous structures is reproducible if ionic liquid (IL) is used as the electrolyte for the dealloying process. During the dealloying process, a relatively more quantity of Ni but both Ni and Cu were electrochemically oxidized to be Ni(II) and Cu(II) complex ions; different ions seemed to own different coordinating molecules in accordance with the NMR analysis. Cu(II) ions could be reduced to Cu metal and deposited upon the counter electrode during the dealloying process. Ni(II) ions, on the other hand, can be mostly removed from the IL phase by extraction into the immiscible water phase. A sustainable system may be developed for the preparation of nitrate-active electrodes in accordance with the study shown here.     

A new (S)-prolinamide modified by an ionic liquid moiety—a high performance recoverable catalyst for asymmetric aldol reactions in aqueous media

New prolinamide derivatives modified with ionic liquid moieties were synthesized and studied as organocatalysts in asymmetric aldol reactions in water. Aldol reactions between cycloalkanones or methylketones and aromatic aldehydes proceeded under studied conditions with high conversions (yields), diastereo- and enantioselectivities in the presence of a hydrophobic catalyst bearing a PF₆ anion (1-5 mol %). The procedure is scalable and the catalyst retained its diastereo- and enantioselectivity over at least four reaction cycles and its activity over at least three reaction cycles.     

Asymmetric aldol reaction catalyzed by the anion of an ionic liquid

Herein we report the synthesis of a chiral imidazolium salt derived from trans-L-hydroxyproline and its applications as a catalyst for the asymmetric aldol reaction. By performing the aldol reaction in [Bmim]NTf(2) as a solvent, we report excellent isolated yields of the aldol product (up to 99%), as well as modest to excellent selectivities (dr superior to 99:1. ee up to 89%). Mechanistic insights and the origins of the selectivity of the aldol reaction are discussed on the basis of the results obtained with two catalytic imidazolium salts having different H-bonding potential.     

Preparation of substituted allyl acetates and sulfones from Baylis-Hillman adducts in ionic liquid media

Reactions of potassium acetate and the sodium salt of p-toluenesulfinic acid with acetates of Baylis-Hillman adducts produce substituted allyl acetates and allyl sulfones respectively. Ionic liquids are utilized in place of conventional solvents.     

Decarboxylative aldol reactions of allyl beta-keto esters via heterobimetallic catalysis

Mild and selective heterobimetallic-catalyzed decarboxylative aldol reactions involving allyl beta-keto esters have been developed. The reaction is promoted by Pd(0)- and Yb(III)-DIOP complexes at room temperature and involves the in situ formation of a ketone enolate from allyl beta-keto esters followed by addition of the enolate to aldehydes. The reaction is a new example of heterobimetallic catalysis in which the optimized reaction conditions require the addition of both metals.