Comparative study of chemically immobilized and conventional homogeneous ionic liquids as phase-transfer catalysts for the N-alkylation of heterocyclic compounds

Various ionic liquids (ILs) were screened for their phase-transfer catalytic (PTC) activity using the N-alkylation of nitrogen heterocycles as the model reaction. Immobilized ILs behaved extremely well and proved to be far better catalysts than conventional homogeneous PTCs in terms of their stability, easy recovery, and reusability. The investigation also demonstrated that quaternary tetraalkylammonium salts offer very high catalytic activity, whereas aromatic heterocyclic tetravalent nitrogen catalysts (imidazolium- and pyridinium-based salts) were poorly active.     

Synthesis and Modifications of Dendrimers on Polymer System Supported on Montmorillonite and Their Use in Organic Synthesis

ABSTRACT

Molecules with dendrimeric structure (cascade molecules) were attached to a polymer-montmorillonite system. The supported dendrimeric molecules were modified to produce ammonium and phosphonium salts. The catalytic phase-transfer activities of these systems have been investigated. The results from the catalytic activity studies showed that the dendrimeric system is highly activating the selected organic reactions. This was attributed to the dendrimeric structure of the system which consequently increase the weight efficiency. It is worth mentioning that this is the first report in the literature describing the formation of dendrimers on polymer-montmorillonite system and their use as phase-transfer catalysts.

     

Tetraalkylammonium Salts as Hydrogen‐Bonding Catalysts

Although the hydrogen‐bonding ability of the α hydrogen atoms on tetraalkylammonium salts is often discussed with respect to phase‐transfer catalysts, catalysis that utilizes the hydrogen‐bond‐donor properties of tetraalkylammonium salts remains unknown. Herein, we demonstrate hydrogen‐bonding catalysis with newly designed tetraalkylammonium salt catalysts in Mannich‐type reactions. The structure and the hydrogen‐bonding ability of the new ammonium salts were investigated by X‐ray diffraction analysis and NMR titration studies.     

Spectroscopic study on anion recognition properties of calix[4]pyrroles: effects of tetraalkylammonium cations

The solution binding properties of calix[4]pyrroles with anion (added as tetraalkylammonium salts) were investigated using UV-vis spectroscopic techniques. The obvious red-shift of absorption maximum band of calix[4]pyrrole in EtOH in the presence of the tetramethylammonium (TMA(+)) or tetraethylammonium (TEA(+)) salts were observed. These results displayed in electronic absorption spectra indicated calix[4]pyrrole receptors linking anionic species through multiple hydrogen bonding interactions are capable of using the periphery electron-rich "walls" for selectively binding electron-deficient tetraalkylammonium cation subunits by cation-pi charge-transfer interaction. It was seen that the stability of the calix[4]pyrrole-anion complex depends strongly on the cation. The meso-alkyl groups of the calix[4]pyrrole, the affinity for the anion subunits and the structure of tetraalkylammonium cations have considerable effects on the formation of cation-pi charge-transfer interaction.     

Recent Progress in Asymmetric Ion-Pairing Catalysis with Ammonium Salts

Quaternary ammonium salts play an important role in asymmetric catalysis. In this Minireview, how asymmetric ion-pairing catalysis with ammonium ions has been utilized in organic synthesis is explained, particularly in the design of novel catalytic cycles. This includes the use of chiral ammonium-based catalysts for the construction of challenging stereogenic centers. Ammonium-derived electrophilic reagents, typically formed in situ and in the context of phase-transfer catalysis (PTC), have also been utilized in asymmetric bond-forming reactions. Furthermore, ammonium salts have been employed as substrates in several stereocontrolled C−N bond cleavage processes, leading to enantioenriched products by using novel asymmetric induction modes. In addition, merging ammonium ion-pairing catalysis with other catalytic approaches has also emerged as a new platform for achieving previously less straightforward reactions, thereby allowing new synthetic applications.     

Cocatalysis in phase-transfer catalyzed fluorination of alkyl halides and sulfonates

Phase-transfer catalyzed (PTC) fluorination of alkyl halides and sulfonates with solid KF proceeds efficiently when cocatalyst triphenyltin fluoride is used. The cocatalytic action of the tin compound consists in continuous formation of difluorotriphenylstannate anion that as the tetraalkyloammonium salt enter the solution where it reacts with alkyl halides to produce alkyl fluorides. The cocatalytic system was used to synthesis of 1,1-difluoroalkanes in two steps from aldehydes. A new kind of PTC was elaborated in which Ph₃SnF acts as phase transfer catalyst via continuous formation of potassium salts of diflurotriphenylstannate anions soluble in dipolar aprotic solvents. A new, simple and general method of synthesis of tetraalkylammonium and potassium salts of difluorotriorgano-tin, silicon and germanium anions is reported.     

Cover Picture: Recent Asymmetric Phase-Transfer Catalysis with Chiral Binaphthyl-Modified and Related Phase-Transfer Catalysts over the Last 10 Years (Chem. Rec. 7/2023)

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.     

水杨酸功能化聚苯乙烯的制备及其与铕(Ⅲ)离子配合物荧光特性研究

以氯甲基聚苯乙烯(CMPS)为出发物质,通过两步改性反应,相转移催化的酯化反应(乙醛酸钠为反应试剂)与均相的Schiff碱反应,制备了侧链键合水杨酸(ASA)配基的聚苯乙烯ASAPS,并使之与Eu(Ⅲ)离子配位螯合,制备了高分子-稀土配合物ASAPS-Eu(Ⅲ).重点研究了相转移催化的酯化反应,也初步考察了配合物ASAPS-Eu(Ⅲ)的发光性能.研究结果表明,在季铵盐相转移催化剂作用下,溶解于有机溶剂中的CMPS可有效地与水相中的乙醛酸钠发生酯化反应,形成醛基(AL)化改性的聚苯乙烯ALPS.相转移催化剂的结构与溶剂的极性对相转移催化反应有显著的影响;配合物ASAPS-Eu(Ⅲ)具有Eu(Ⅲ)的特征荧光发射,并产生显著的Antenna效应.     

Phase-Transfer Catalyzed Two-Phase Reactions in Preparative Organic Chemistry

Quaternary ammonium and phosphonium salts catalyze reactions between substances located partly in an aqueous and partly in an organic phase. Use of such phase-transfer catalysts simplifies and accelerates numerous reactions traditionally conducted in nonaqueous media. These reactions include carbene reactions, nucleophilic substitutions, alkylations of ketones and nitriles, Wittig and Darzens reactions, formation of ethers and esters. Other reactions such as hydrolysis and oxidation can be accelerated.     

Photophysics of xanthene dyes in surfactant solution

The spectral (both absorption and fluorescence) and photoelectrochemical studies of some anionic xanthene dyes namely erythrosine B, rose bengal and eosin have been carried out in micellar solution of cationic cetyl trimethyl ammonium bromide (CTAB), anionic sodium dodecyl sulphate (SDS) and neutral triton X-100 (TX-100). The results show that all these dyes form 1:1 electron-donor-acceptor (EDA) or charge-transfer (CT) complexes with TX-100, which acts as an electron donor. There is no interaction of these dyes with SDS, whereas the interaction with CTAB is mainly electrostatic in nature. In presence of TX-100, these dyes show enhancement of fluorescence intensity with a red shift and develop photovoltage in a photoelectrochemical cell. A good correlation has been found among the photovoltage generation in the systems consisting of these dyes and TX-100, spectral shift due to complex formation and thermodynamic properties of these complexes.     

Chiral quaternary phosphonium salts: a new class of organocatalysts

Phase-transfer catalysis has widely been used as a prime synthetic tool for both laboratory and industrial processes. During the last twenty years, asymmetric phase-transfer catalysis using chiral organocatalysts has attracted widespread interest. However, the scope of chiral phase-transfer catalysis has been limited mostly to the quaternary ammonium salts. As an emerging area, the recent developments in the application of quaternary phosphonium salts as chiral phase-transfer catalysts are discussed in this article.     

Synthesis of Benzyl Esters of Glycyrrhizic Acid in the Presence of Phase-Transfer Catalysts

Benzyl esters of glycyrrhizic acid were synthesized using PhCH₂Br in DMSO-CH₂Cl₂-NaOH in the presence of phase-transfer catalysts, such as tetraalkylammonium salts and 2-methyl-2-propanol.     

Effects of charge separation, effective concentration, and aggregate formation on the phase transfer catalyzed alkylation of phenol

The factors that influence the rate of alkylation of phenol under phase transfer catalysis (PTC) have been investigated in detail. Six linear, symmetrical tetraalkylammonium cations, Me(4)N(+), Et(4)N(+), (n-Pr)(4)N(+), (n-Bu)(4)N(+), (n-Hex)(4)N(+), and (n-Oct)(4)N(+), were examined to compare the effects of cationic radius and lipophilicity on the rate of alkylation. Tetraalkylammonium phenoxide·phenol salts were prepared, and their intrinsic reactivity was determined from initial alkylation rates with n-butyl bromide in homogeneous solution. The catalytic activity of the same tetraalkylammonium phenoxides was determined under PTC conditions (under an extraction mechanism) employing quaternary ammonium bromide catalysts. In homogeneous solution the range in reactivity was small (6.8-fold) for Me(4)N(+) to (n-Oct)(4)N(+). In contrast, under PTC conditions a larger range in reactivity was observed (663-fold). The effective concentration of the tetraalkylammonium phenoxides in the organic phase was identified as the primary factor influencing catalyst activity. Additionally, titration of active phenoxide in the organic phase confirmed the presence of both phenol and potassium phenoxide aggregates with (n-Bu)(4)N(+), (n-Hex)(4)N(+), and (n-Oct)(4)N(+), each with a unique aggregate stoichiometry. The aggregate stoichiometry did not affect the PTC initial alkylation rates.     

Employment of a useful liquid membrane electrode system to characterise the micelles of bile salts and other detergents in pure and mixed states

Ion-pairs or coacervates (formed by the reaction between cationic and anionic surfactants) dissolved in nitrobenzene can behave as surfactant-ion registering devices to respond to both surfactant cation and anion. The complexes of cetyltrimethyl ammonium bromide with sodium dodecyl sulfate, sodium salts of deoxycholic and chenodeoxycholic acids, and Aerosol Orange T have been used in nitrobenzene to generate such useful liquid membranes. The complex of dimethyldioctadecyl ammonium bromide and sodium cholate has been used to study the cholate ion behaviour since its complex with cetyltrimethyl ammonium bromide is water soluble. The electrochemical behaviours of the liquid membranes have been found to be fairly good and reproducible. The membrane potential measurements have been used to determine the critical micelle concentrations of the surfactants in pure as well as in mixed states to evaluate surfactant—surfactant interaction in the micelles of the latter.     

Formation of stable Meisenheimer adduct ion pairs in apolar solvents: implications for stereoselective reactions

[reaction: see text] A detailed study concerning the formation of Meisenheimer adducts in biphasic solvent systems is described. The process relies on utilizing a significantly lipophilic quaternary ammonium salt to transfer a nucleophile (e.g., hydroxide ion) between an aqueous and organic layer containing the electron-deficient aromatic substrate. Provided that the organic layer is sufficiently apolar, the resultant Meisenheimer adduct is considerably stable, likely the result of a strong ion-pairing interaction between the large polarizable anionic complex and the diffusively charged tetraalkylammonium cation. Using the diethylamide of 3,5-dinitrobenzoic acid as a model compound, the influence of ion-pairing reagents and solvents on adduct formation was investigated. Dramatically increased equilibrium formation of the Meisenheimer adduct is observed in the biphasic medium (e.g., benzene/2 M NaOH) relative to the same adduct generated in single-phase systems. Spectroscopic studies on this adduct are consistent with those conducted in single-phase polar or dipolar aprotic solvents. The methodology is extended to performing highly enantioselective biphasic kinetic resolutions of several racemic electron-deficient amides.     

Polymer-supported Cinchona alkaloid-derived ammonium salts as recoverable phase-transfer catalysts for the asymmetric synthesis of alpha-amino acids

Alkaloids such as cinchonidine, quinine and N-methylephedrine have been N-alkylated using polymeric benzyl halides or co-polymerized and then N-alkylated, thus affording a series of polymer-supported chiral ammonium salts which have been employed as phase-transfer catalysts in the asymmetric benzylation of an N-(diphenylmethylene)glycine ester. These new polymeric catalysts can be easily recovered by simple filtration after the reaction and reused. The best ee's were achieved when Merrifield resin-anchored cinchonidinium ammonium salts were employed.     

Kinetics of the reaction between 2-phenylpropionitrile and 2-chloro-5-nitrotrifluoromethylbenzene under phase-transfer catalysis

Phase-transfer catalysis (PTC) is a widely accepted methodology in organic synthesis. Although a great number of organic syntheses were reported in PTC conditions, systematic kinetic studies are scarce. In the present report, a detailed study of the kinetics of the reaction between 2-chloro-5-nitrotrifluoromethylbenzene (CNTFB) and 2-phenylpropionitrile anion (HPP-), under PTC, was performed under several conditions. The reaction was carried out either in toluene or chlorobenzene as the organic phase, in the presence of a concentrated aqueous solution of NaOH using tetraalkylammonium (Q+X-) salts as phase-transfer catalysts. The major product was 2-(4-nitro-2-trifluoromethylphenyl)-2-phenylpropionitrile, and its yield depends on the experimental conditions. Different aspects of the mechanism are discussed and quantified. Kinetic data were explained by means of an interfacial mechanism that involves the deprotonation of the adsorbed nucleophile precursor followed by its catalyst-mediated extraction to the organic phase. A multicomponent Langmuir-type interface was assumed. Although the extraction of OH- by catalyst to the organic phase is usually disregarded, the formation of the substrate hydrolysis product that leads to catalyst poisoning was also investigated. The influence of this side reaction on the yield of the main product was established. A discussion about the influence of this side process on the main reaction and the operating mechanism is presented.     

Synthesis of thioether using dimethyloctyl(3-sulfopropyl)ammonium betaine and di-active site quaternary ammonium salt as new phase-transfer catalysts

The novel phase-transfer catalysts S-8 (dimethyloctyl(3-sulfopropyl) ammonium betaine) and DB-2 (di-active site quaternary ammonium salts) were synthesized and employed as the phase-transfer catalysts for synthesizing thioethers from various alkyl bromides of high conversion. This revised version was published online in August 2006 with corrections to the Cover Date.