Quaternary Ammonium Salt Catalysts Research Articles

Chiral Quaternary Ammonium Salt‐Catalyzed Enantioselective Addition Reactions of Hydantoins

AbstractWe herein report a protocol for the asymmetric 1,4‐addition of hydantoins to various Michael acceptors by utilizing Cinchona alkaloid‐based chiral quaternary ammonium salt catalysts. Various products were obtained with moderate to good enantioselectivities and accompanying computational investigations helped to identify key interactions responsible for the observed selectivity. DFT calculations along with non‐covalent interaction plots reveal the presence of numerous stabilizing non‐classical hydrogen bonding interactions along with other non‐covalent interactions between the chiral quaternary ammonium salt and hydantoin molecule in the C−C bond forming transition states leading to the formation of the products. In addition, a first proof‐of‐concept for an analogous a‐sulfanylation reaction, albeit with poor selectivity, is reported as well.

Controlled ring-opening copolymerization of epoxides and cyclic anhydrides through bifunctional thiourea–quaternary ammonium salt catalysts

Bifunctional thiourea–quaternary ammonium salt catalysts exhibited significantly higher activity over a two-component system with a similar structure at a low catalyst loading for the ring-opening copolymerization of epoxides and cyclic anhydrides.

Towards an asymmetric β-selective addition of azlactones to allenoates.

We herein report the asymmetric organocatalytic addition of azlactones to allenoates. Upon using chiral quaternary ammonium salt catalysts, i.e., Maruoka's binaphthyl-based spirocyclic ammonium salts, the addition of various azlactones to allenoates proceeds in a β-selective manner with moderate levels of enantioselectivities (up to 83:17 er). Furthermore, the obtained products can be successfully engaged in nucleophilic ring opening reactions, thus giving highly functionalized α-amino acid derivatives.

Enantioselective β-Selective Addition of Isoxazolidin-5-ones to Allenoates Catalyzed by Quaternary Ammonium Salts.

The enantioselective addition of isoxazolidin-5-ones to the β-carbon of allenoates has been carried out by using a novel spirobiindane-based quaternary ammonium salt catalyst. This protocol, which proceeds under classical liquid-solid phase-transfer conditions, gives access to unprecedented highly functionalized β2,2-amino acid derivatives with good enantioselectivities and in high yields, and further manipulations of these products have been carried out as well.

A Remarkably Efficient Phase-Transfer Catalyzed Amination of α-Bromo-α, β-Unsaturated Ketones in Water

Tandem conjugate addition–alkylation reaction of various amines with α-bromo-α, β-unsaturated ketones resulted in near-quantitative conversions into the corresponding aziridines when the reaction was carried out in the presence of 10 mol% of phase-transfer, PT catalysts in water. Some chiral quaternary ammonium salts derived from Cinchona alkaloids were investigated as water-stable PT catalysts. The scope and limitations of the reaction have also been investigated. The catalytic performances were significantly improved in comparison with the corresponding ordinary quaternary ammonium salt catalysts, and excellent yields (81%–96%) were obtained. Although an increase in the rate of aziridination has been accomplished, no stereoselectivity was observed. The positive values of the protocol have been confirmed.

Catalytic Enantioselective γ‐Selective Additions of 2‐Allylazaarenes to Activated Ketones

AbstractReported herein is the first example of 2‐allylazaarenes in asymmetric catalysis. Highly γ‐selective allylation was demonstrated for activated ketones, including isatins and trifluoromethyl ketones. In the presence of either an amino‐acid‐based tertiary amine or quaternary ammonium salt catalyst, two series of tertiary hydroxy‐containing moieties were installed at the remote δ‐position of azaarenes in good chemical yields, excellent enantioselectivities, and E/Z ratios. The success of current γ‐selective reactions should provide inspiration for expansion to other allylazaarene derivatives and would open up new paradigms for the synthesis of chiral γ‐ and/or δ‐functionalized azaarenes.

Catalytic Enantioselective γ-Selective Additions of 2-Allylazaarenes to Activated Ketones.

Reported herein is the first example of 2-allylazaarenes in asymmetric catalysis. Highly γ-selective allylation was demonstrated for activated ketones, including isatins and trifluoromethyl ketones. In the presence of either an amino-acid-based tertiary amine or quaternary ammonium salt catalyst, two series of tertiary hydroxy-containing moieties were installed at the remote δ-position of azaarenes in good chemical yields, excellent enantioselectivities, and E/Z ratios. The success of current γ-selective reactions should provide inspiration for expansion to other allylazaarene derivatives and would open up new paradigms for the synthesis of chiral γ- and/or δ-functionalized azaarenes.

Enantioselective fluorination of β-keto ester using a PEG-bound urea-containing chiral quaternary ammonium salt catalyst

Enantioselective fluorination of β-keto ester using a PEG-bound urea-containing chiral quaternary ammonium salt catalyst

Third-Liquid Phase Transfer Catalysis for Horner–Wadsworth–Emmons Reactions of “Moderately Acidic” and “Weakly Acidic” Phosphonates

The Horner–Wadsworth–Emmons (HWE) reaction was investigated in a green third-liquid phase-transfer catalysis (TLPTC) system. With 6% equiv of the catalyst and 50% NaOH aqueous solution, the third phase was generated. In this TLPTC system, HWE reactions of benzaldehydes bearing an electron-donating group with “weakly” or “moderately” acidic phosphonates proceeded in high yield (>90%) with an all E-isomer product (stilbene). Teraoctyl ammonium bromide (TOAB) afforded a high yield of 93% for the HWE reaction of benzaldehydes with electron-withdrawing groups. It was found that the ratio of E-stilbene to Z-stilbene was also influenced by the steric hindrance of the achiral quaternary ammonium salt catalyst. The isolated yield and geometric selectivity for the HWE reaction remained unchanged in four consecutive runs of both the third and the aqueous phase. A highly efficient and practical hydroxide-initiated TLPTC system was developed for the HWE reaction.

ChemInform Abstract: Syntheses and Applications of (Thio)Urea‐Containing Chiral Quaternary Ammonium Salt Catalysts.

AbstractThe enantioselective α‐fluorination of β‐keto esters is optimized with respect to the chiral urea catalyst used.

Syntheses and Applications of (Thio)Urea-Containing Chiral Quaternary Ammonium Salt Catalysts.

We herein report our efforts to obtain a new class of systematically modified bifunctional (thio)urea-containing quaternary ammonium salts based on easily obtainable chiral backbones. Among the different classes of catalysts that were successfully synthesized, those based on trans-1,2-cyclohexane diamine were found to be the most powerful for the asymmetric α-fluorination of β-keto esters. Selectivities up to 93:7 could be obtained by using only 2 mol-% of the optimized catalyst. The importance of the bifunctional nature of these catalysts was demonstrated by control experiments using simplified monofunctional catalyst analogues, which gave almost racemic product only.

ChemInform Abstract: Towards Tartaric‐Acid‐Derived Asymmetric Organocatalysts

AbstractReview: <50 refs.

Towards Tartaric‐Acid‐Derived Asymmetric Organocatalysts

Tartaric acid is one of the most prominent naturally occurring chiral compounds. Whereas its application in the production of chiral ligands for metal-catalysed reactions has been exhaustively investigated, its potential to provide new organocatalysts has been less extensively explored. Nevertheless, some impressive results, such as the use of TADDOLs as chiral H-bonding catalysts or of tartrate-derived asymmetric quaternary ammonium salt catalysts, have been reported over the last decade. The goal of this article is to provide a representative overview of the potential and the limitations of tartaric acid or TADDOLs in the creation of new organocatalysts and to highlight some of the most spectacular applications of these catalysts, as well as to summarize case studies in which other classes of chiral backbones were better suited.

ChemInform Abstract: Bifunctional Chiral Quaternary Ammonium Salt Catalysts: A Rapidly Emerging Class of Powerful Asymmetric Catalysts

AbstractReview: 37 refs.

Bifunctional Chiral Quaternary Ammonium Salt Catalysts: A Rapidly Emerging Class of Powerful Asymmetric Catalysts

AbstractChiral quaternary ammonium salt catalysts have emerged as one of the outstanding catalytic principles over the last decades. Their ability to activate nucleophiles in a noncovalent fashion makes them unique. However, over the last years it has become obvious that in a variety of difficult transformations activation and control of nucleophiles alone does not result in sufficient selectivities, and the same holds true for the sole control of electrophiles by other catalytic motifs. Thus, the use of bifunctional ammonium salt catalysts has become one of the most promising strategies to carry out demanding reactions in a highly stereoselective fashion under mild reaction conditions, and many of these transformations are not possible by using other methods. This review will highlight some of the most impressive recent examples in this field and compare the performances of these catalysts with those of other commonly employed ones.

Transport Limitations during Phase Transfer Catalyzed Ethyl-Benzene Oxidation: Facts and Fictions of “Halide Catalysis”

The mechanistic interpretation of the catalytic effect of phase transfer catalysts in the selective oxidation of ethyl benzene is hampered by mass transfer effects. We demonstrate that proper experiments lead to a more correct interpretation of the role of the quaternary ammonium salt (QAS) and its counterion. Specifically, experiments unequivocally show that the main action of the counterion is to enhance physical mass transfer processes, while its catalytic effect is limited to a shift in selectivity, not activity. The QAS as a whole accelerates the induction process in the ethyl benzene (EB) oxidation by degenerate branching of its hydroperoxide (EBHP). Proper mechanistic understanding of these phenomena in QAS catalysts is especially crucial under industrially relevant conditions

Production of dimethyl carbonate from ethylene carbonate and methanol using immobilized ionic liquids on MCM-41

Ionic liquids on ordered mesoporous silica were prepared and their catalytic performance in the synthesis of dimethyl carbonate (DMC) was investigated. The ionic liquids were immobilized on chloropropyl-functionalized MCM-41 (CP-MS41) through the quaternization of trialkylamines. The supported ionic liquids were proven to be an effective heterogeneous catalyst for the synthesis of DMC from transesterification of ethylene carbonate (EC) with methanol. The immobilized quaternary ammonium salt (QCl-MS41) catalysts with longer alkyl chains showed higher EC conversion and turnover number (TON). Higher temperatures and longer reaction times were favorable for the reactivity of QCl-MS41. However, carbon dioxide pressure showed a maximum for catalytic activity. The catalyst can be reused for reactions in up to three consecutive runs with only a slight decrease in catalytic activity.

Oxidation of cyclopentene catalyzed by phosphotungstic quaternary ammonium salt catalysts

A series of phosphotungstic quaternary ammonium salts, Q 3 (PW 12O 40) and Q 3(PW 4O 16) [Q = (C 5H 5)N +(C 16H 33), (C 16H 33)N +(CH 3) 3, (C 4H 9) 4N +, and (CH 3) 4N +], were used as the catalysts in oxidation of cyclopentene. The catalysts [(C 5H 5)N(C 16H 33)] 3(PW 4O 16) and [(C 16H 33)N(CH 3) 3] 3(PW 4O 16) showed high catalytic activity in the selective oxidation of cyclopentene while using H 2O 2 (50%) as an oxidant and 2-propanol as a solvent. The oxidation products mainly consisted of glutaraldehyde, cis-1,2-cyclopentanediol and trans-1,2-cyclopentanediol. The above-mentioned two catalysts were dissolved completely in the reaction medium during the catalysis process and precipitated themselves from the reaction system after reaction, showing the characteristics of reaction-controlled phase-transfer catalysis. The types of quaternary ammonium cations and the phosphotungstic anions in phosphotungstic quaternary ammonium salts affected catalytic activity.

A Yb(OTf)3/PEG-Supported Quaternary Ammonium Salt Catalyst System for a Three-Component Mannich-Type Reaction in Aqueous Media

A one-pot, three-component Mannich reaction of cyclohexanone with aromatic aldehyde and aromatic amine was efficiently catalyzed by Yb(OTf)3/PEG-supported quaternary ammonium salt system in water at room temperature. This reaction afforded the corresponding β-amino carbonyl compounds in good yields with high anti selectivity, and the catalyst system was recyclable.

Selective production of cyclic carbonate over polycarbonate using a double metal cyanide–quaternary ammonium salt catalyst system

A double metal cyanide complex, an efficient catalyst for the copolymerization of CO2 with epoxide, was successfully utilized for the cycloaddition of CO2 to various epoxides in the presence of quaternary ammonium salts without any solvents.