Design, synthesis, and application of tartaric acid derived N-spiro quaternary ammonium salts as chiral phase-transfer catalysts

Isothiourea-Catalyzed Enantioselective Functionalisation of Glycine Schiff Base Aryl Esters via 1,6- and 1,4-Additions

The enantioselective α-functionalisation of glycine Schiff base aryl esters through isothiourea catalysis is successfully demonstrated for 1,6-additions to para-quinone methides (21 examples, up to 95:5 dr and 96:4 er) and 1,4-additions to methylene substituted dicarbonyl or disulfonyl Michael acceptors (17 examples, up to 98:2 er). This nucleophilic organocatalysis approach gives access to a range of α-functionalised α-amino acid derivatives and further transformations of the activated aryl ester group provide a straightforward entry to advanced amino acid-based esters, amides or thioesters.

Synthesis and Catalytic Activity of Bifunctional Phase-Transfer Organocatalysts Based on Camphor

Ten novel bifunctional quaternary ammonium salt phase-transfer organocatalysts were synthesized in four steps from (+)-camphor-derived 1,3-diamines. These quaternary ammonium salts contained either (thio)urea or squaramide hydrogen bond donor groups in combination with either trifluoroacetate or iodide as the counteranion. Their organocatalytic activity was evaluated in electrophilic heterofunctionalizations of β-keto esters and in the Michael addition of a glycine Schiff base with methyl acrylate. α-Fluorination and chlorination of β-keto esters proceeded with full conversion and low enantioselectivities (up to 29% ee). Similarly, the Michael addition of a glycine Schiff base with methyl acrylate proceeded with full conversion and up to 11% ee. The new catalysts have been fully characterized; the stereochemistry at the C-2 chiral center was unambiguously determined.

(Thio)urea containing chiral ammonium salt catalysts

(Thio)-urea-containing bifunctional quaternary ammonium salts emerged as powerful non-covalently interacting organocatalysts over the course of the last decade. The most commonly employed catalysts in this field are either based on Cinchona alkaloids, α-amino acids, or trans-cyclohexane-1,2-diamine. Our group has been heavily engaged in the design and use of such catalysts, i. e. trans-cyclohexane-1,2-diamine-based ones for around 10 years now, and it is therefore the intention of this short personal account to provide an overview of the, at least in our opinion, most significant and pioneering achievements in this field by looking on catalyst design and asymmetric method development, with a special focus on our own contributions.

Enantioselective organocatalytic syntheses of α-selenated α- and β-amino acid derivatives

Selenium-containing amino acids are valuable targets but methods for the stereoselective α-selenation of simple amino acid precursors are rare. We herein report the enantioselective electrophilic α-selenation of azlactones (masked α-amino acid derivatives) and isoxazolidin-5-ones (masked β-amino acids) using Cinchona alkaloids as easily accessible organocatalysts. A variety of differently substituted derivatives was accessed with reasonable levels of enantioselectivities and further studies concerning the stability and suitability of these compounds for further manipulations have been carried out as well.

Enantioselective Synthesis of Acyclic Orthogonally Functionalized Compounds Bearing a Quaternary Stereocenter Using Chiral Ammonium Salt Catalysis

We herein report an asymmetric protocol to access a series of orthogonally functionalized acyclic chiral target molecules containing a quaternary stereogenic center by carrying out the enantioselective α-alkylation of novel orthogonally functionalized dioxolane-containing cyanoacetates under chiral ammonium salt catalysis. By using just 1 mol % of Maruoka's spirocyclic ammonium salt catalysts enantioselectivities up to e.r.=97.5 : 2.5 could be achieved and further functional group manipulations of the products were carried out as well.

Asymmetric phase-transfer catalysed β-addition of isoxazolidin-5-ones to MBH carbonates

A novel high yielding, enantio- and diastereoselective protocol for the synthesis of α-allylated highly functionalised β-amino acid derivatives by adding isoxazolidin-5-ones to MBH carbonates under asymmetric phase-transfer catalysis has been developed.

Synthesis of Both Enantiomers of Chiral Phenylalanine Derivatives Catalyzed by Cinchona Alkaloid Quaternary Ammonium Salts as Asymmetric Phase Transfer Catalysts

A practical synthesis of both enantiomers of unnatural phenylalanine derivatives by using two pseudoenantiomeric phase transfer catalysts is described. Through asymmetric α-alkylation of glycine Schiff base with substituted benzyl bromides and 1-(bromomethyl)naphthalene under the catalysis of O-allyl-N-(9-anthracenmethyl) cinchoninium bromide (1f) and O-allyl-N-(9-anthracenylmethyl)cinchonidium bromide (1i), respectively, a series of both (R)- and (S)-enantiomers of unnatural α-amino acid derivatives were obtained in excellent yields and enantioselectivity. The synthetic method is simple and scalable, and the stereochemistry of the products is fully predictable and controlled: the cinchonine-type phase transfer catalyst 1f resulted in (R)-α-amino acid derivatives, and the cinchonidine-type phase transfer catalyst 1i afforded (S)-α-amino acid derivatives.

Chiral phase-transfer catalysis in the asymmetric α-heterofunctionalization of prochiral nucleophiles

Chiral phase-transfer catalysis is one of the major catalytic principles in asymmetric catalysis. A broad variety of different catalysts and their use for challenging applications have been reported over the last decades. Besides asymmetric C–C bond forming reactions the use of chiral phase-transfer catalysts for enantioselective α-heterofunctionalization reactions of prochiral nucleophiles became one of the most important field of application of this catalytic principle. Based on several highly spectacular recent reports, we thus wish to discuss some of the most important achievements in this field within the context of this review.

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.

Enantioselective Alkylation of Amino Acid Derivatives Promoted by Cyclic Peptoids under Phase-Transfer Conditions

The effects of substituents and cavity size on catalytic efficiency of proline-rich cyclopeptoids under phase-transfer conditions were studied. High affinity constants (Ka) for the sodium and potassium cations, comparable to those reported for crown ethers, were observed for an alternated N-benzylglycine/L-proline hexameric cyclopeptoid. This compound was found to catalyze the alkylation of N-(diphenylmethylene)glycine cumyl ester in values of enantioselectivities comparable with those reported for the Cinchona alkaloid ammonium salts derivatives (83-96% ee), and with lower catalyst loading (1-2.5% mol), in the presence of a broad range of benzyl, allyl and alkyl halides.

A new generation of chiral phase-transfer catalysts

Phase-transfer catalysis has long been recognized as a versatile method for organic synthesis. In particular, over more than the past three decades, asymmetric phase-transfer catalysis based on the use of structurally well-defined chiral catalysts has become a topic of great scientific interest. Although various effective chiral catalysts have already been reported and these catalysts were utilized for practical asymmetric transformations, further design and development of new chiral phase-transfer catalysts are still attractive research subjects in organic chemistry due to the high utility and practicability of phase-transfer-catalyzed reactions. This review focuses on the recent examples of newly designed effective chiral phase-transfer catalysts.

9-Amino-(9-deoxy)cinchona alkaloid-derived new chiral phase-transfer catalysts

A new class of 9-amino-(9-deoxy)cinchona alkaloid-derived chiral phase-transfer catalysts bearing amino groups was developed by using known cinchona alkaloids as the starting materials. Due to the transformation of the 9-hydroxyl group into a 9-amino functional group, the catalytic performances were significantly improved in comparison with the corresponding first generation phase-transfer catalysts, and excellent yields (92-99%) and high enantioselectivities (87-96% ee) were achieved in the benchmark asymmetric α-alkylation of glycine Schiff base. Based on the special contribution of the amino group to the high yield and enantioselectivity, the possible catalytic mechanism was conjectured.

Towards an asymmetric organocatalytic α-cyanation of β-ketoesters

This communication describes the first proof of concept for an asymmetric α-cyanation of β-ketoesters using a hypervalent iodine-based electrophilic cyanide-transfer reagent. A series of different organocatalysts has been investigated and it was found that the use of naturally occurring Cinchona alkaloids allows obtaining the target products in good yields and with moderate enantioselectivities up to er = 76:24 under operationally simple conditions.

Enantioselective synthesis of benzoindolizidine derivatives using chiral phase-transfer catalytic intramolecular domino aza-Michael addition/alkylation

An efficient and enantioselective strategy to synthesize benzoindolizidinesviadomino intramolecular aza-Michael addition/alkylation was developed.

Facile one-pot fabrication of a silica gel-supported chiral phase-transfer catalyst—N-(2-cyanobenzyl)-O(9)-allyl-cinchonidinium salt

A novel silica gel-supported cinchona alkaloid-based PTC catalyst was prepared by one-pot synthesis for the first time. Excellent enantioselectivities (72.0–96.9% ee) and 80–96% yields in enantioselective α-alkylation were achieved without significant loss in the catalytic performance for five runs.

Asymmetric cyclopropanation of chalcones using chiral phase-transfer catalysts

The first phase-transfer catalyzed cyclopropanation reaction of chalcones using bromomalonates as the nucleophiles in a Michael Initiated Ring Closing reaction (MIRC) was developed. Key to success was the use of a free OH-containing cinchona alkaloid ammonium salt catalyst and carefully optimized liquid/liquid reaction conditions. The reaction performed well for electron neutral and electron deficient chalcones giving the products in yields up to 98% and with enantiomeric ratios up to 91:9.

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.

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.

Application scope and limitations of TADDOL-derived chiral ammonium salt phase-transfer catalysts

We have recently introduced a new class of chiral ammonium salt catalysts derived from easily available TADDOLs. To get a full picture of the scope of application and limitations of our catalysts we tested them in a variety of different important transformations. We found that, although these compounds have recently shown their good potential in the asymmetric α-alkylation of glycine Schiff bases, they clearly failed when we attempted to control more reactive nucleophiles like b-keto esters. On the other hand, when using them to catalyse the addition of glycine Schiff bases to different Michael acceptors it was found necessary to carefully optimize the reaction conditions for every single substrate class, as seemingly small structural changes sometimes required the use of totally different reaction conditions. Under carefully optimized conditions enantiomeric ratios up to 91:9 could be achieved in the addition of glycine Schiff bases to acrylates, whereas acrylamides and methyl vinyl ketone gave slightly lower selectivities (up to e.r. 77:23 in these cases). Thus, together with additional studies towards the syntheses of these catalysts we have now a very detailed understanding about the scope and limitations of the synthesis sequence to access our PTCs and about the application scope of these catalysts in asymmetric transformations.

Recent developments in asymmetric phase-transfer reactions

Phase-transfer catalysis has been recognized as a powerful method for establishing practical protocols for organic synthesis, because it offers several advantages, such as operational simplicity, mild reaction conditions, suitability for large-scale synthesis, and the environmentally benign nature of the reaction system. Since the pioneering studies on highly enantioselective alkylations promoted by chiral phase-transfer catalysts, this research field has served as an attractive area for the pursuit of "green" sustainable chemistry. A wide variety of asymmetric transformations catalyzed by chiral onium salts and crown ethers have been developed for the synthesis of valuable organic compounds in the past several decades, especially in recent years.

Investigations Concerning the Syntheses of TADDOL-Derived Secondary Amines and Their Use To Access Novel Chiral Organocatalysts

A structurally carefully diversified library of novel TADDOL-derived chiral secondary amines was synthesized and investigated for their applicability to obtain new organocatalysts like chiral Lewis bases and chiral phase-transfer catalysts. The scope and limitations of the developed syntheses routes to access these catalysts as well their catalytic performance in different benchmark reactions were systematically investigated. The most powerful of the catalysts prepared was found to be highly useful for the phase-transfer catalyzed α-alkylation of glycine Schiff base (high yields and up to 93% ee).