Mild Phase-transfer Conditions Research Articles

Enantioselective cyclopropanation of conjugated cyanosulfones using carbohydrate-based crown ether catalysts

A few new d-galactose- and d-glucose-based monoaza-15-crown-5 type lariat ethers have been synthesized. These macrocycles and their derivatives proved to be efficient catalysts in the cyclopropanation of (E)-3-phenyl-2-(phenylsulfonyl)acrylonitrile performed with diethyl bromomalonate under mild phase transfer conditions. Among the catalysts tested, the macrocycle having methyl α-d-galactopyranoside unit generated the highest asymmetric induction (80% ee). In the reactions of the aryl-substituted phenylsulfonyl-acrylonitrile derivatives, the cyclopropanation of the meta- and para-substituted starting materials took place with high ee values (75–84% ee). The cyclopropane derivatives synthesized from analogous α,β-unsaturated cyanosulfones containing naphthyl, pyridyl, furyl and thienyl groups were obtained with enantioselectivities up to 85%, and in excellent yields.

Enantioselective Michael addition of malonic esters to benzalacetophenone by using chiral phase transfer catalysts derived from proline-mandelic acid/tartaric acid

Herein, we have explored the enantioselective Michael addition of various malonate esters to benzalacetophenone by successful utilization of chiral phase transfer catalysts derived from proline, mandelic acid and tartaric acid under mild phase transfer conditions. The obtained results signify that these chiral phase transfer catalysts are efficacious towards enantioselective Michael addition as the use of it resulted in good enantioselectivity and appreciable chemical yields. SYNOPSIS Highly enantioselective Michael addition of dialkyl malonates to benzalacetophenone has been established under mild phase transfer conditions, by successful utilization of chiral phase transfer catalysts derived from proline, mandelic acid and tartaric acid (I–VI) with appreciable chemical yields.

Synthesis of chiral crown ethers derived from d-galactose and their application in enantioselective reactions

A few new α- and β-d-galactopyranoside-based chiral lariat ethers of monoaza-15-crown-5 type were synthesized. These macrocycles proved to be efficient catalysts in a few asymmetric reactions under mild phase transfer conditions. The catalytic effect of the lariat ethers with methoxy, ethoxy, isopropoxy and aryloxy substituents on the C-1 of the sugar unit in both α and β positions was compared. In the presence of β-d-galactopyranoside-based crown ethers, the asymmetric Darzens condensation of α-chloroacetophenone and benzaldehyde, the epoxidation of trans-chalcone, the cyclopropanation (MIRC reaction) of benzylidenemalononitrile and 2-benzylidene-1,3-indandione with diethyl bromomalonate were performed with enantioselectivities of 61%, 64%, 86% and 96%, respectively. In all reactions, the β-anomers were more efficient in terms of enantioselectivity than the α forms.

Tandem Alkylation–Second-Order Asymmetric Transformation Protocol for the Preparation of Phenylalanine-Type Tailor-Made α-Amino Acids

In this work, we disclose an advanced general process for the synthesis of tailor-made α-amino acids (α-AAs) via tandem alkylation–second-order asymmetric transformation. The first step is the alkylation of the chiral Ni(II) complex of glycine Schiff base, which is conducted under mild phase-transfer conditions allowing the structural construction of target α-AAs. The second step is based on the methodologically rare second-order asymmetric transformation, resulting in nearly complete precipitation of the corresponding (SC,RN,RC)-configured diastereomer, which can be collected by a simple filtration. The operational convenience and potential scalability of all experimental procedures, coupled with excellent stereochemical outcome, render this method of high synthetic value for the preparation of various tailor-made α-AAs.

Sulfinate‐Organocatalyzed (3+2) Annulation of Allenyl Sulfones with 1,1‐Dicyano Olefins in the Presence of a Quaternary Ammonium Phase Transfer Agent

AbstractThe benzenesulfinate‐catalyzed (3+2) annulation between allenyl sulfones and aryl(alkyl)idenemalononitriles has been developed under mild phase transfer conditions, affording a breadth of functionalized sulfonyl cyclopentenes in good to excellent yields (22 examples, 49–99%) and high diastereoselectivities. These adducts were likely generated, via an allylsulfone anion featuring a quaternary ammonium cation.magnified image

Synthesis and Photoswitching Studies of OPE-Embedded Difurylperfluorocyclopentenes

We report the synthesis and photochemical behavior of five photochromic molecular switches 7a-e with attached molecular wires based on differently substituted oligo(phenylene ethynylene) (OPE) building blocks. The switchable molecular wires 7a-e were built in a convergent approach from substituted iodotolans 6a-e and 1,2-bis(2-methyl-5-ethynylfuran-3-yl)perfluorocyclopentene 5 by 2-fold Sonogashira coupling. Compound 5 was prepared from the corresponding bis-aldehyde 2 by Wittig-type olefination with [PPh3CHBr2]Br·CH3CN, followed by elimination to the bromoalkyne under mild phase-transfer conditions at 0 °C. Halogen-metal exchange with i-PrMgCl·LiCl and hydrolysis furnished 5 in good overall yield. Substituents R(1) and R(2) in the OPE portion were either electron-withdrawing or electron-donating, and their influence on the photostability and photoswitching characteristics of 7a-e was studied. All resulting molecules show reversible photochromism between the colorless off and the deeply colored on states when irradiated with light of 313 and 576 nm wavelengths, respectively. The quantum yields of these photoreactions increased when electron-withdrawing groups were used. This was further corroborated by reversible protonation/deprotonation of 7e (R(1) = NMe2, R(2) = H) for which the ring-closing quantum yield increased 10-fold upon switching off the donor by protonation.

Phase-Transfer-Catalyzed Asymmetric Conjugate Cyanation of Alkylidenemalonates with KCN in the Presence of a Brønsted Acid Additive

Highly enantioselective conjugate cyanation of alkylidenemalonates with KCN was achieved under mild phase-transfer conditions with a bifunctional phase-transfer catalyst. The effect of Brønsted acid additives was examined based on the hypothetical catalytic cycle, and the Brønsted acid additive was found to be essential to promote the conjugate cyanation efficiently.

Stereoselective Synthesis of Unsaturated and Functionalized l-NHBoc Amino Acids, Using Wittig Reaction under Mild Phase-Transfer Conditions

The stereoselective synthesis of a new amino acid phosphonium salt was described by quaternization of melting triphenylphosphine with the γ-iodo NHBoc-amino ester, derived from L-aspartic acid. The deprotection of the carboxylic acid function to afford the phosphonium salt with a free carboxylic acid group was achieved by a palladium-catalyzed desallylation reaction. This phosphonium salt was used in the Wittig reaction with aromatic or aliphatic aldehydes and trifluoroacetophenone, under solid-liquid phase-transfer conditions in chlorobenzene and in the presence of K(3)PO(4) as weak base, to afford the corresponding unsaturated amino acids without racemization. Thus, the reaction with substituted aldehydes allows to graft various functionalized groups on the lateral chain of the amino acid, such as trifluoromethyl, cyano, nitro, ferrocenyl, boronato, or azido. In addition, the reaction of the amino acid Wittig reagent with α,β-unsaturated aldehydes leads to amino acids bearing a diene on the lateral chain. Finally, this amino acid phosphonium salt appears to be a new powerful tool for the preparation of unsaturated and non-proteinogenic α-amino acids, directly usable for the synthesis of customized peptides.

Practical asymmetric synthesis of α-methylserine derivatives under mild phase-transfer conditions

The enantioselective methylation reaction of phenyloxazoline tert-butyl ester 5 using ( R)- 4b as a catalyst under mild phase-transfer conditions provides optically active α-methylserine derivatives in moderate yields with high enantioselectivity. Other α-alkylated serine derivatives are also achievable in high yields with high enantioselectivity by using catalytic amount of ( R)- 4b.

ChemInform Abstract: 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.

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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.

Highly Enantioselective Monoalkylation of p‐Chlorobenzaldehyde Imine of Glycine tert‐Butyl Ester under Mild Phase‐Transfer Conditions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Highly enantioselective monoalkylation of p-chlorobenzaldehyde imine of glycine tert-butyl ester under mild phase-transfer conditions

The selective monoalkylation of glycine tert-butyl ester aldimine Schiff base 3 has been realized in high chemical yield with excellent enantioselectivity under mild liquid–liquid phase-transfer conditions by the use of binaphthyl-derived chiral quaternary ammonium bromides 7 and 8 as catalysts. This achievement demonstrates that 3 can be used as a cost-effective substrate for the preparation of optically active α-alkyl-α-amino acid derivatives by chiral phase-transfer catalysis.

Importance of Chiral Phase‐Transfer Catalysts with Dual Functions in Obtaining High Enantioselectivity in the Michael Reaction of Malonates and Chalcone Derivatives.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Importance of Chiral Phase-Transfer Catalysts with Dual Functions in Obtaining High Enantioselectivity in the Michael Reaction of Malonates and Chalcone Derivatives

[reaction: see text]. Highly enantioselective Michael addition of diethyl malonate to chalcone derivatives has been achieved under mild phase-transfer conditions by the successful utilization of N-spiro C(2)-symmetric chiral quaternary ammonium bromide 1 as a catalyst, which possesses diarylhydroxymethyl functionalities as a recognition site for the prochiral electrophile. This simple asymmetric Michael addition process was found to be quite effective for various chalcone derivatives, including those with heteroaromatic substituents.

Mild Electrophilic Halogenation of Chloropyridines Using CCl4 or C2Cl6 under Basic Phase Transfer Conditions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Design of New Chiral Phase‐Transfer Catalysts with Dual Functions for Highly Enantioselective Epoxidation of α,β‐Unsaturated Ketones.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic Asymmetric Synthesis of α‐Amino Acid Derivatives and Peptides Using Chiral Phase‐Transfer Catalysts

AbstractFor Abstract see ChemInform Abstract in Full Text.

Mild electrophilic halogenation of chloropyridines using CCl 4 or C 2Cl 6 under basic phase transfer conditions

Effective chlorination of 2,3,5,6-tetrachloropyridine to pentachloropyridine was realized under mild phase transfer conditions with carbon tetrachloride and 50% NaOH or solid K 3PO 4. Mechanistic study of this reaction indicated the possibility of an aromatic carbanionic intermediate. Hexachloroethane was established as a more selective electrophilic chlorination agent.

Design of new chiral phase-transfer catalysts with dual functions for highly enantioselective epoxidation of alpha,beta-unsaturated ketones.

A new chiral ammonium bromide, 1-Br, possessing diarylmethanol functionality as a substrate recognition site has been designed as a promising, dual-functioning catalyst for the highly enantioselective epoxidation of alpha,beta-unsaturated ketones under mild phase-transfer conditions. For instance, vigorous stirring of a mixture of chalcone, 1-Br (3 mol %), and 13% NaOCl in toluene at 0 degrees C for 24 h gave rise to epoxy chalcone quantitatively with 96% ee. A variety of alpha,beta-unsaturated ketones can also be epoxidized with rigorous stereochemical control, clearly demonstrating the effectiveness and utility of the present system. Further, a successful single-crystal X-ray diffraction analysis of 1-PF6 uncovered its distinctive three-dimensional molecular architecture and provided useful information for postulating the transition state.