Chiral Quaternary Ammonium Salts Research Articles

Chiral Tertiary Sulfonium Salts as Effective Catalysts for Asymmetric Base‐Free Neutral Phase‐Transfer Reactions

AbstractAlthough chiral quaternary ammonium and phosphonium salts are commonly used for asymmetric organocatalysis, the catalytic ability of chiral tertiary sulfonium salts has yet to be demonstrated in asymmetric synthesis. Herein, we show that chiral bifunctional trialkylsulfonium salts catalyze highly enantioselective conjugate additions of 3‐substituted oxindoles to maleimides under base‐free neutral phase‐transfer conditions.

Chiral Tertiary Sulfonium Salts as Effective Catalysts for Asymmetric Base-Free Neutral Phase-Transfer Reactions.

Although chiral quaternary ammonium and phosphonium salts are commonly used for asymmetric organocatalysis, the catalytic ability of chiral tertiary sulfonium salts has yet to be demonstrated in asymmetric synthesis. Herein, we show that chiral bifunctional trialkylsulfonium salts catalyze highly enantioselective conjugate additions of 3-substituted oxindoles to maleimides under base-free neutral phase-transfer conditions.

Asymmetric Synthesis of Chiral Bimetallic [Ag28Cu12(SR)24]4- Nanoclusters via Ion Pairing.

In this work, a facile ion-pairing strategy for asymmetric synthesis of optically active negatively charged chiral metal nanoparticles using chiral ammonium cations is demonstrated. A new thiolated chiral three-concentric-shell cluster, [Ag28Cu12(SR)24]4-, was first synthesized as a racemic mixture and characterized by single-crystal X-ray structure determination. Mass spectrometric measurements revealed relatively strong ion-pairing interactions between the anionic nanocluster and ammonium cations. Inspired by this observation, the as-prepared racemic mixture was separated into enantiomers by employing chiral quaternary ammonium salts as chiral resolution agents. Subsequently, direct asymmetric synthesis of optically active enantiomers of [Ag28Cu12(SR)24]4- was achieved by using appropriate chiral ammonium cations (such as N-benzylcinchoninium vs N-benzylcinchonidinium) in the cluster synthesis. These simple strategies, ion-pairing enantioseparation and direct asymmetric synthesis using chiral counterions, may be of general use in preparing chiral metal nanoparticles.

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

ChemInform Abstract: Chiral Quaternary Phosphonium Salts as Phase‐Transfer Catalysts for Environmentally Benign Asymmetric Transformations

AbstractReview: [37 refs.

ChemInform Abstract: Organic Base‐Promoted Enantioselective Electrophilic Cyanation of β‐Keto Esters by Using Chiral Phase‐Transfer Catalysts.

AbstractHighly enantioselective cyanation of β‐keto esters such as (I), (IV), and (VI) is achieved using hypervalent iodine reagent (II) as electrophilic cyanating reagent and a cinchona alkaloid‐based chiral quaternary ammonium salt as catalyst.

ChemInform Abstract: Design of Chiral Urea‐Quaternary Ammonium Salt Hybrid Catalysts for Asymmetric Reactions of Glycine Schiff Bases.

AbstractThe new catalysts are tested in asymmetric Michael addition reactions and aldol‐initiated cascade reactions.

ChemInform Abstract: Asymmetric Michael Addition Reaction of α‐Aryl‐Substituted Lactams Catalyzed by Chiral Quaternary Ammonium Salts Derived from Cinchona Alkaloids: A New Short Synthesis of (+)‐Mesembrine.

AbstractThe utility of this method is demonstrated in the synthesis of (+)‐mesembrine (VI).

A Highly Diastereoselective and Enantioselective Phase-Transfer Catalyzed Epoxidation of β-Trifluoromethyl-β,β-disubstituted Enones with H2O2

Trifluoromethylated organic compounds, especially chiral quaternary alcohols bearing trifluoromethyl group are of important intermediates in drugs, agrochemicals and etc.An efficient epoxidation of β-CF3-β,β-disubstituted unsaturated ketones (6) has been developed with environmental benign hydrogen peroxide as the oxidant and F5-substituted chiral quaternary ammonium salt (1g or 5) derived from cinchona-alkaloid as the catalyst. Using 3 mol% of the catalyst, both enantiomers of(R,R) and (S,S) β-trifluoromethyl-α,β-epoxy ketones (7, 8) were obtained in excellent diastereoselectivities (up to 100:1 d.r.) and enantioselectivities (up to 99.7% ee). The effects of catalyst structure, catalyst loading, substrate structure, the nature of oxidant, and reaction conditions on the catalyst capacities have been discussed in full length. The reaction mechanism was proposed to explain the origin of chiral induction. By subsequent reduction with zinc the epoxides are exhibited to be converted into trifluoromethylated quaternary alcohols without any loss in enantioselectivities. All new compounds are fully characterized by IR, NMR, elemental analysis and or high resolution mass spectrum.

Asymmetric Michael Addition Reaction of α-Aryl-Substituted Lactams Catalyzed by Chiral Quaternary Ammonium Salts Derived from Cinchona Alkaloids: A New Short Synthesis of (+)-Mesembrine

The enantioselective Michael addition reaction of α-aryl-substituted lactams with electron-deficient olefins was efficiently catalyzed using chiral quaternary ammonium salts derived from cinchona alkaloids. This method was highly useful for the construction of an all-carbon-substituted quaternary carbon stereogenic center at the α-position of lactams in good to high yields and with good enantiomeric excess and could be applied to the short synthesis of (+)-mesembrine.

A Cation-Directed Enantioselective Sulfur-Mediated Michael/Mannich Three-Component Domino Reaction involving Chalcones as Michael Acceptors.

A new approach has been developed for an asymmetric sulfur-mediated three-component intermolecular Michael/Mannich domino reaction using chalcones as Michael acceptors. This reaction is catalyzed by chiral quaternary ammonium salts derived from modified quinine and provides facile access to complex sulfur-containing compounds with three contiguous stereogenic centers in yields of up to 93%, with 95:5 dr and 95% ee. These compounds were further elaborated to give the equivalent of a chiral aza-Morita-Baylis-Hillman reaction involving chalcones and azetidines bearing four chiral centers.

Organic base-promoted enantioselective electrophilic cyanation of β-keto esters by using chiral phase-transfer catalysts.

Highly enantioselective cyanation of β-keto esters using hypervalent iodine(iii) as the electrophilic cyanating reagent induced by cinchona alkaloid-based chiral quaternary ammonium salt was demonstrated. Organic bases, especially DMAP, in the chiral phase-transfer catalysis were used to obtain high ees.

Cinchona alkaloid-based chiral catalysts act as highly efficient multifunctional organocatalysts for the asymmetric conjugate addition of malonates to nitroolefins.

New pentaerythritol tetrabromide-based chiral quaternary ammonium salts acting as organocatalysts (7a and 7b) have been prepared and used as organocatalysts for enantioselective Michael addition reactions between various nitroolefins and Michael donors (malonates) under mild reaction conditions, such as lower concentration of base and catalyst and room temperature, with very good chemical yields (up to 97%) and ee's (up to 99%).

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

AbstractReview: 37 refs.

ChemInform Abstract: Asymmetric Direct α‐Hydroxylation of β‐Oxo Esters Catalyzed by Chiral Quaternary Ammonium Salts Derived from Cinchona Alkaloids.

AbstractThe ammonium salt (I) efficiently promotes the enantioselective α‐hydroxylation of indanone‐ and tetralone‐derived β‐oxo esters under mild conditions.

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.

ChemInform Abstract: Designer Chiral Phase‐Transfer Catalysts for Green Sustainable Chemistry

AbstractReview: 36 refs.

Asymmetric Direct α-Hydroxylation of β-Oxo Esters Catalyzed by Chiral Quaternary Ammonium Salts Derived from Cinchona Alkaloids

Cinchona alkaloid-derived chiral quaternary ammonium organocatalysts were developed. The catalyst with a bulky 1-adamantoyl group at the C-9 position promoted the enantioselective α-hydroxylation of β-oxo esters and resulted in the corresponding products in 35-95% yields and 58-90% ee. The reaction was successfully scaled to a gram-quantity scale with a similar yield without loss of enantioselectivity.