Dramatic rate enhancement of asymmetric phase-transfer-catalyzed alkylations

Recent Asymmetric Phase-Transfer Catalysis with Chiral Binaphthyl-Modified and Related Phase-Transfer Catalysts over the Last 10 Years

In this personal account, we describe our recent advances in the three types of phase-transfer catalysis for various transformations including asymmetric induction: Firstly, asymmetric phase-transfer catalysis with Maruoka-type C₂ -symmetric chiral biaryl-modified tetraalkylammonium salts and phosphonium salts; Secondly, asymmetric phase-transfer catalysis under base-free and neutral conditions; Thirdly, hydrogen-bonding catalysis using tetraalkylammonium and trialkylsulfonium salts. These three different strategies are illustrated by using various phase-transfer catalyzed transformations.

Nucleophilic Reactions Using Alkali Metal Fluorides Activated by Crown Ethers and Derivatives

We review crown ether-facilitated nucleophilic reactions using metal salts, presenting the studies using kinetic measurements and quantum chemical methods. We focus on the mechanistic features, specifically on the contact ion-pair (CIP) mechanism of metal salts for nucleophilic processes promoted by crown ethers and derivatives. Experimental verification of the CIP form of the metal salt CsF complexed with [18-Crown-6] by H-NMR spectroscopy is described. The use of chiral crown ethers and derivatives for enantioselective nucleophilic processes is also discussed.

A General Method for High-Pressure-Promoted Postfunctionalization of Unclosed Cryptands: Potential Phase-Transfer Catalysts

We report a high-pressure approach to facile late-stage functionalization of unclosed cryptands (UCs) (11 examples, yield up to 99%). Direct comparison of classic and high-pressure conditions of the quaternization reaction in a sterically crowded intraannular position is investigated, and differences in the reactivity of tertiary amine substrates are discussed. Finally, we demonstrated the application of UCs as catalysts for synthetically important alkylation reactions under phase-transfer 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.

Synthesis of a Calixarene-Based Chiral Phase-Transfer Catalyst Derived from Cinchonine and its Application in Asymmetric Alkylation

The synthesis of a novel calixarene-based chiral phase-transfer catalyst derived from cinchonine has been achieved in four steps from p-t-butylcalix[4]arene. Its catalytic properties were evaluated over a range of conditions by carrying out the phase-transfer alkylation of N-(diphenylmethylene)glycine t-butyl ester with benzyl bromide. Under the optimal conditions the alkylated product was formed in 96% yield with 91% ee.

Rate enhancement of phase transfer catalyzed conjugate additions by CsCl

A rate enhancement of phase transfer catalyzed conjugate additions was accomplished using a catalytic amount of CsCl. The utility of this method was demonstrated in the asymmetric synthesis of glutamic acid derivatives by using a chiral phase transfer catalyst.

Camphor sulfonyl hydrazines (CaSH) as organocatalysts in enantioselective Diels-Alder reactions

Cyclic sulfonyl hydrazine was demonstrated for the first time as a new functionality for organocatalysis. A series of six-membered cyclic hydrazines derived from camphor sulfonic acid were synthesized. With trichloroacetic acid as cocatalyst, they are efficient organocatalysts for enantioselective Diels-Alder reactions with good chemical yields and up to 96% ee. The reactions took place in brine at 0 degrees C to room temperature.

Design of Binaphthyl-Modified Symmetrical Chiral Phase-Transfer Catalysts: Substituent Effect of 4,4′,6,6′-Positions of Binaphthyl Rings in the Asymmetric Alkylation of a Glycine Derivative

A series of symmetrical chiral phase-transfer catalysts with 4,4',6,6'-tetrasubstituted binaphthyl units have been designed, and these aryl- and trialkylsilyl-substituted phase-transfer catalysts, which included a highly fluorinated catalyst, were prepared. The chiral efficiency of these chiral phase-transfer catalysts was investigated in the asymmetric alkylation of tert-butylglycinate-benzophenone Schiff base under mild phase-transfer conditions, and the eminent substituent effect of the 4,4',6,6'-positions of the binaphthyl units on enantioselection was observed. In particular, the OctMe2Si-substituted catalyst was found to be highly efficient for the phase-transfer alkylation of tert-butylglycinate-benzophenone Schiff base with various alkyl halides, including sec-alkyl halides. The highly fluorinated catalyst was also utilized as a recyclable chiral phase-transfer catalyst by simple extraction with fluorous solvents.

Recent advances in asymmetric phase-transfer catalysis

The use of chiral nonracemic onium salts and crown ethers as effective phase-transfer catalysts have been studied intensively primarily for enantioselective carbon-carbon or carbon-heteroatom bond-forming reactions under mild biphasic conditions. An essential issue for optimal asymmetric catalysis is the rational design of catalysts for targeted reaction, which allows generation of a well-defined chiral ion pair that reacts with electrophiles in a highly efficient and stereoselective manner. This concept, together with the synthetic versatility of phase-transfer catalysis, provides a reliable and general strategy for the practical asymmetric synthesis of highly valuable organic compounds.

Practical Stereoselective Synthesis of β-Branched α-Amino Acids through Efficient Kinetic Resolution in the Phase-Transfer-Catalyzed Asymmetric Alkylations

Phase-transfer-catalyzed alkylation of glycinate Schiff base with racemic secondary alkyl halides proceeded with excellent levels of syn- and enantioselectivities under the influence of chiral quaternary ammonium bromide 1d and 18-crown-6. The alkylation product can be selectively converted to the corresponding anti isomer, allowing the preparation of all the stereoisomers of beta-alkyl-alpha-amino acid derivatives, an extremely valuable chiral building block.

Design of chiral organocatalysts for practical asymmetric synthesis of amino acid derivatives

A series of structurally rigid, chiral quaternary ammonium salts and several chiral sec-amine catalysts derived from commercially available (R)- or (S)-binaphthol have been designed as new C(2)-symmetric chiral phase-transfer catalysts and chiral bifunctional amino-catalysts. These chiral organocatalysts have been successfully applied to the highly practical asymmetric synthesis of various amino acid derivatives.

Synthesis, Structure, and Computational Studies of Soluble Conjugated Multidentate Macrocycles

[Chemical reaction: See text] Conjugated, shape-persistent macrocycles based on [3 + 3] Schiff-base condensation are of interest for supramolecular materials. In an effort to develop new discotic liquid crystals based on these compounds, a series of macrocycles with peripheral alkoxy groups of varying length have been prepared. The synthesis and mechanism of formation have been probed by isolation of oligomeric intermediates. A single-crystal X-ray diffraction study of one macrocycle revealed a nonplanar, strongly hydrogen-bonded structure. To our surprise, even with very long substituents, the macrocycles were not liquid crystalline. This has been rationalized by ab initio calculations that indicate the macrocycles are undergoing rotation of the dihydroxydiiminobenzene rings that may not allow a stable discotic liquid crystalline phase. These results provide new insight into the formation and properties of these large macrocycles and may provide guidance to developing stable liquid crystalline materials in the future.