Efficient Phase Transfer Catalyst Research Articles

Ion Pair Catalyst - Pentanidinium.

In this account, we further describe our already developed N-sp2 hybrid guanidinium as an efficient phase-transfer catalyst and ion pair catalysis based on N-sp2 hybrid pentanidinium and its application in some new reactions. The sp3 hybrid quaternary ammonium salt has a tetrahedral structure, which means that three sides of it can be effectively steric, allowing the remaining side to be close to the substrate. However, the sp2 hybrid ammonium salt allows the substrate to form ion pairs from both directions respectively, so it is a greater challenge to control the stereoselectivity of the reaction. Van der Waals forces, such as hydrogen bonds and interactions, have been used to make electrophiles approach from a certain direction, leading to a higher enantioselectivity. Based on the above idea, we designed an N-sp2 hybrid phase-transfer catalyst, pentanidinium. Pentanidinium has five conjugated nitrogen atoms, one of which has a formal positive charge, which is necessary for it to become an ion pair catalyst. We have confirmed that pentanidinium can catalyze α-hydroxylation of 3-substituted-2-oxindoles, Michael addition of 3-alkyloxindoles with vinyl sulfone, and alkylation reactions of sulfenate anions and dihydrocoumarins, desymmetrization of pro-chiral sulfinate to afford enantioenriched sulfinate esters. Pentanidinium with side chain structure changes can also be catalyzed efficiently with enantioconvergent halogenophilic nucleophilic substitution, including azidation and thioesterification. In the reaction catalyzed by pentanidinium, it always attracts us with the advantages of low catalytic load and good enantioselectivity.

Crowning Lithium Ions in Hole-Transport Layer toward Stable Perovskite Solar Cells.

State-of-the-art perovskite solar cells (PSCs) exhibit comparable power conversion efficiency (PCE) to that of silicon photovoltaic devices. However, the device stability remains a major obstacle that restricts widespread application. Doping-induced hygroscopicity, ion diffusion, and use of polar solvents in the hole-transport layer are detrimental factors for performance degradation of PSCs. Here, phase-transfer-catalyzed LiTFSI doping in Spiro-OMeTAD is developed to address these negative impacts. 12-Crown-4as an efficient phase-transfer catalyst promotes the dissolution of LiTFSI without requiring acetonitrile. A combined experimental and theoretical study demonstrates the host-guest interaction between Li+ ions and 12-crown-4. Crowning Li+ ions by forming more stable and less diffusive crown-ether-Li+ complexes retards the generation of hygroscopic lithium oxides and mitigates Li+ -ion migration. Optimized PSCs deliver enhanced PCE and significantly improved stability under humid and thermal conditions compared with a control device. This method can also be applied to dope π-conjugated polymer. The findings provide a facile avenue to improve the long-term stability of PSCs.

Triphenyl methyl phosphonium tosylate as an efficient phase transfer catalyst for ultrasound-assisted oxidative desulfurization of liquid fuel.

The novel phosphonium-based ionic liquid (IL), triphenyl methyl phosphonium tosylate ([TPMP][Tos]), has been synthesized and applied as a phase transfer catalyst (PTC) in the ultrasound-assisted oxidative desulfurization (UAODS). Oxidation of model fuel (MF) containing dibenzothiophene (DBT) was carried out using an equimolar mixture of H2O2-CH3COOH as an oxidant at 40-70 °C in the presence of IL. The sulfur compound is converted into polar sulfone, and the maximum desulfurization efficiency was examined. The effect of process parameters such as reaction temperature, reaction time, molar ratio of oxidant to sulfur (n(O/S)), and the mass ratio of ionic liquid to model fuel (m(IL/MF)) was studied, and the conditions for maximizing the DBT conversion rate were found. Maximum conversion (> 99%) was obtained at a temperature of 70 °C with m(IL/MF) of 0.8. The oxidation reactivity of various sulfur compounds was studied at different time intervals. To verify the effect of ionic liquid and ultrasound irradiation, extractive desulfurization (EDS), oxidative desulfurization (ODS), and UAODS in the presence of IL were carried out. The experimental results show that the UAODS process gives the highest desulfurization efficiency. A kinetic study was performed to estimate the rate constant and the order of oxidation reaction.

A New Basic Ionic Liquid Supported on Magnetite Nanoparticles: An Efficient Phase-Transfer Catalyst for the Green Synthesis of 2-Amino-3-Cyano-4H-Pyrans

An efficient and straightforward procedure has been developed for the synthesis of 2-amino-3-cyano-4H-pyrans via the one-pot three-component reaction of benzaldehydes, malononitrile and 1,3-dicarbonyl compounds using a new nanomagnetite-supported organocatalyst (MNP@PEG-ImOH) in aqueous media. The MNPs are fully characterized by various techniques including Fourier transform infrared spectroscopy (FT-IR), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDAX), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and vibrating sample magnetometry (VSM). The catalyst could readily be separated from the reaction mixture using an external magnet and could be reused in subsequent runs without a notable decrease of activity.

The Synthesis of Hydrobenzoin-Based Monoaza Crown Ethers and Their Application as Recyclable Enantioselective Catalysts

New recyclable monoaza-15-crown ethers have been synthesized starting from (R,R)-(+)- and (S,S)-(−)-hydrobenzoin. These macrocycles proved to be efficient and reusable phase transfer catalysts in a few asymmetric reactions under mild conditions. The asymmetric epoxidation of trans-chalcone took place with up to 81% ee, while using other chalcone derivatives, the products were formed with 68–88% ee. The hydrobenzoin-based lariat ethers were also tested in the cyclopropanation of a few electron deficient olefins using diethyl bromomalonate to afford the product with good enantioselectivities (54–75% ee). The catalysts were recovered by salt formation, followed by extraction, and were reused without the loss of the activity and effect on the enantioselectivity.Graphic The synthesis of hydrobenzoin-based monaza crown ethers and their application as recyclable enantioselective catalysts.

Improved Access to Chiral Tetranaphthoazepinium-Based Organocatalysts Using Aqueous Ammonia as Nitrogen Source.

The class of 3,3′-diaryl substituted tetranaphthobisazepinium bromides has found wide application as highly efficient C2-symmetrical phase-transfer catalysts (PTCs, Maruoka type catalysts). Unfortunately, the synthesis requires a large number of steps and hampers the build-up of catalyst libraries which are often desired for screening experiments. Here, we present a more economic strategy using dinaphthoazepine 7 as the common key intermediate. Only at this stage various aryl substituents are introduced, and only two individual steps are required to access target structures. This protocol was applied to synthesize ten tetranaphthobisazepinium compounds 1a–1j. Their efficiency as PTCs was tested in the asymmetric substitution of tert-butyl 2-((diphenylmethylene)amino)acetate. Enantioselectivities up to 92% have been observed with new catalysts.

Brønsted Acids of Anionic Chiral Cobalt(III) Complexes as Catalysts for the Iodoglycosylation or Iodocarboxylation of Glycals

Brønsted acids of anionic chiral Co(III) complexes were found to act as efficient phase-transfer catalysts for the diastereoselective iodoglycosylation or iodocarboxylation of glycals with a variety of alcohols or carboxylic acids, respectively, with N-iodosuccinimide as the iodo cation source. The corresponding 2-deoxy-2-iodoglycosides, including monosaccharides and disaccharides, and 2-deoxy-2-iodoglycosyl carboxylates, which are of high synthetic and biological importance, were obtained in high yields (up to 88%) with good diastereoselectivities (up to 9:1 dr).

L-tert-Leucine derived urea-ammonium salts: Efficient bifunctional phase transfer catalysts for highly diastereo- and enantioselective aza-Henry reaction of isatin-derived N-Boc ketimines with α-aryl nitromethanes

An efficient way of aza-Henry reaction between isatin-derived N-Boc ketimines and α-aryl nitromethanes catalyzed by bifunctional phase transfer catalysts with a quaternary ammonium center derived from L-tert-Leucine has been developed. A series of 3-substituted 3-amino-oxindoles were constructed by this catalytic protocol in excellent yields (90–99%), with high enantioselectivities (83–95%) and diastereoselectivities (79:21–97:3). The asymmetric aza-Henry reaction of N-Boc amidosulfones and α-aryl nitromethanes were also investigated and gave the corresponding products in high to excellent yields (72–97%) with high enantioselectivities (up to 99%) and diastereoselectivities (up to >99:1).

Design and synthesis of a magnetic hierarchical porous organic polymer: A new platform in heterogeneous phase‐transfer catalysis

Recyclable phase transfer catalysts containing magnetic nanoparticles (MNPs) have been known as a major trend towards sustainable catalysts. In this study, a novel class of magnetic porous polymer on the basis of calix[4]resorcinarene was synthesized starting from silica‐coated Fe3O4 core‐shell nanoparticles. This compound was found as an efficient phase transfer catalyst to the conversion of benzyl halides into benzyl azides and cyanides in good yields. The catalyst could be used at least for five consecutive cycles without appreciable loss in the catalytic activity.

Cluster Preface: Asymmetric Brønsted Base Catalysis

Choon-Hong Tan is a professor at the Division of Chemistry and Biological Chemistry, Nanyang Technological University, Singapore. He received his BSc (Hons) First Class from the National University of Singapore (NUS) and his Phd from the University of Cambridge. He underwent postdoctoral training at the Department of Chemistry and Chemical Biology, Harvard University and the Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School. He began his independent career at the Department of Chemistry, National University of Singapore in 2003. Choon Hong has focused on the development of organocatalytic Brønsted base reactions that can be catalyzed with chiral guanidines. He has also demonstrated that pentanidiums (conjugated guanidiniums) are efficient phase-transfer catalysts. Recently, he described the use of chiral organic cations such as bisguanidiniums to modulate and activate anionic metallic salts. Benjamin List has been a director at the Max-Planck-Institut für Kohlenforschung since 2005. He obtained his Ph.D. in 1997 (Frankfurt). From 1997 until 1998 he conducted postdoctoral research at The Scripps Research Institute in La Jolla (USA) and became an assistant professor there in January 1999. In 2003 he joined the Max-Planck-Institut für Kohlenforschung. He has been an honorary professor at the University of Cologne since 2004. Ben List’s research focuses on organic synthesis and catalysis. He has contributed fundamental concepts to chemical synthesis including aminocatalysis, enamine catalysis, and asymmetric-counteranion-directed catalysis (ACDC). His latest work deals with chiral counteranions in asymmetric catalysis. This remarkably general strategy for asymmetric synthesis has recently found widespread use in organocatalysis, transition-metal catalysis, and Lewis acid catalysis.

Phase-Transfer and Ion-Pairing Catalysis of Pentanidiums and Bisguanidiniums.

Catalysts accelerate biological processes and organic reactions in a controlled and selective fashion. There are continuing efforts in asymmetric catalysis to develop efficient catalysts with broad reaction scope and industrial practicability. Among the various modes of asymmetric catalysis, phase-transfer catalysis has attracted intense interest due to its facile scale up and low catalyst loading. Chiral quaternary ammonium and phosphonium salts are well-studied classes of chiral phase-transfer catalysts, and they are typically composed of sp3-hybridized quaternary onium salts. In this Account, we describe our recent attempts to develop N-sp2-hybridized guanidinium-type salts as efficient phase-transfer catalysts as well as ion-pair catalysis based on N-sp2 hybridized bisguanidinium-type salts. The sp2-quaternized ammonium salts, pentanidiums, which contain five nitrogen atoms in conjugation, displayed remarkable phase-transfer catalytic efficiency. We have shown that pentanidium can catalyze Michael additions of tert-butyl glycinate-benzophenone Schiff bases with various α,β-unsaturated acceptors, such as vinyl ketones, acrylates, and chalcones, in high enantioselectivities. The structurally amendable pentanidium phase-transfer catalysts supply diverse reactivity and selectivity to various other organic transformations, such as α-hydroxylation of 3-substituted-2-oxindoles, Michael addition of 3-alkyloxindoles with vinyl sulfone, and alkylation reactions of sulfenate anions and dihydrocoumarins. Pentanidium salts are applicable to enantioselective transformations on a preparative scale at low catalyst loading, allowing for the synthesis of a broad range of enantiopure compounds. From computational and experimental results, we also proposed that the halogenated pentanidium catalysts participated in halogen bonding and that this contributed to the excellent stereocontrol in alkylation reactions. Subsequently, we determined that chiral cations can direct functional anions besides basic anions in traditional Brønsted basic phase-transfer reactions, including metal-centered anions. We identified dicationic bisguanidinium as an excellent ion-pairing catalyst, first demonstrating that bisguanidinium formed an ion pair with permanganate and directed the anion in enantioselective dihydroxylation and oxohydroxylation of a,β-unsaturated esters. This initial success led us to explore chiral cationic ion-pairing catalysis as a general mode of catalysis. This mode of catalysis is at the interphase between organocatalysis, phase-transfer catalysis and organometallic catalysis. We then identified bisguanidinium diphosphatobisperoxotungstate and bisguanidinium dinuclear oxodiperoxomolybdosulfate ion pairs as the active catalysts in enantioselective sulfoxidations using aqueous H2O2 as the oxidant. The structure of the bisguanidinium dinuclear oxodiperoxomolybdosulfate ion pair was elucidated using single-crystal X-ray analysis. Bisguanidinium-catalyzed sulfoxidations emerged as a practical methodology for the synthesis of enantioenriched sulfoxides including armodafinil and lansoprazole, which are commercial drugs. Finally, we are also able to show that pentanidium and bisguanidinium hypervalent silicates are intermediates in enantioselective alkylations using silylamide as a Brønsted probase.

Catalytic Alkylation of 2-Aryl-2-oxazoline-4-carboxylic Acid Esters Using Cyclopeptoids; Newly Designed Phase-Transfer Catalysts

Nonionic, chiral macrocyclic peptoids are efficient phase-transfer catalysts in the C-4 enantioselective alkylation of 2-[4-(trifluoromethyl)phenyl]-2-oxazoline-4-carboxylic acid esters. Screening of the structural features of cyclic peptoids, namely the ring size, symmetry elements­, number of proline residues, and substituents on the side chains, showed that the alternated <i>N</i>-(4-methoxybenzyl)glycine/<sc>l</sc>-proline cyclohexapeptoid is the optimal catalyst (good yields and up to 75% ee) for the stereocontrolled construction of α-alkylated serine <i>tert</i>-butyl esters.

Crown ether derived from d-glucose as an efficient phase-transfer catalyst for the enantioselective Michael addition of malonates to enones

A monoaza-15-crown-5 lariat ether derived from d-glucose 1 has been applied as a chiral phase-transfer catalyst in the Michael addition of diethyl, dimethyl, diisopropyl and dibenzyl malonates to enones under mild conditions to afford the adducts with good to excellent enantioselectivities. In the reaction of diethyl malonate with substituted trans-chalcone, the adducts with in enantioselectivities up to 89% ee. Among the reactions of substituted diethyl malonates, those of diethyl acetoxymalonate gave the best results (96% ee). The effect of the substituents of the chalcone was also investigated in reactions with diethyl acetoxymalonate. Among the chalcones substituted on the β-aryl, the para-substituted compounds resulted in the highest enantioselectivities (88–97% ee). The substituents on the benzoyl aryl of chalcone caused a decrease in the enantioselectivity compared to the unsubstituted case. The adducts with furyl or thiophenyl substituents were formed with >99% ee. The glucose-based catalyst also proved to be effective in the cases of diisopropyl and dibenzyl acetoxymalonates (including ees up to 99% ee). The reactions of diethyl acetoxymalonate with cyclic enones gave the corresponding Michael adducts with enantioselectivities up to 83%. The absolute configuration of one of the new Michael adducts was determined by CD spectroscopy.

Ultrasound-assisted synthesis of pyrazolo[1,2-b]phthalazines and dihydrospiro[indoline-3,1'-pyrazolo[1,2-b]phthalazines] using TBAF as an efficient phase-transfer catalyst

Tetrabutylammonim fluoride has been used as an efficient catalyst for synthesis of pyrazolo[1,2-b]-phthalazine-5,10-diones and dihydrospiro[indoline-3,1'-pyrazolo[1,2-b]phthalazines] by one-pot three-component reaction of phthalhydrazide, aromatic aldehydes or isatin derivatives, with malononitrile in water under ultrasond irradiation. This rapid method gave the products in high yield.

N-Heterocyclic olefins as efficient phase-transfer catalysts for base-promoted alkylation reactions.

N-Heterocyclic olefins (NHOs) have very recently emerged as efficient promoters for several chemical reactions due to their strong Brønsted/Lewis basicities. Here we report the novel application of NHOs as efficient phase-transfer organocatalysts for synthetically important alkylation reactions on a wide range of substrates, further demonstrating the great potential of NHOs in organic chemistry.

Reusable chiral salen Mn(III) complexes with phase transfer capability efficiently catalyze the asymmetric epoxidation of unfunctionalized olefins with NaClO

A series of chiral salen Mn(III) polymers with build-in phase transfer capability was prepared by bridging the chiral salen Mn(III) units with polyethylene glycol (PEG)-based di-imidazolium ionic liquid (IL) side by side. Technologies of characterization suggested the alternation of intact chiral salen Mn(III) unit with PEG-based dicationic imidazolium IL moiety in the rigid one-dimension polymers. Amphipathic nature of PEG-based di-imidazolium IL moiety allowed the obtained catalysts to undergo inherent phase transfer catalysis in asymmetric epoxidation of unfunctionalized olefins with NaClO, which in turn increased the reaction rate of epoxidation in water–dichloromethane biphasic system. Decreasing total length of polyether chain leads to an increase in built-in phase transfer capability of corresponding complex, which further enhances the catalytic performance. 91–97% of conversion was obtained in the enantioselective epoxidation of styrene, α-methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene catalyzed by the complex where number of ethylene oxide unit is 3 within 60min, which is significant higher than that observed for the neat complex (56–74%) and the ICP (62–85%). High enantiomeric excess (ee) for the epoxides (in the range of 67–93%) was also achieved, except for styrene (ee, 34%) and α-methylstyrene (ee, 41%). Furthermore, the efficient phase transfer catalysts could be easily recovered by solvent precipitation and be recycled for seven times without significant loss of the activity and enantioselectivity.

Catalytic enantioselective alkylation of sulfenate anions to chiral heterocyclic sulfoxides using halogenated pentanidium salts.

We report halogenated pentanidiums as phase-transfer catalysts for the asymmetric alkylation of sulfenate anions to various sulfoxides with high enantioselectivities (up to 99% ee) and yields (up to 99%). This approach gives access to enantioenriched heterocyclic sulfoxides that might not be compatible with strong oxidants or organometallic reagents. Computational studies have revealed that the multiple noncovalent interactions such as halogen bonds and nonclassical hydrogen bonds are involved.

Discovery and Application of Doubly Quaternized Cinchona-Alkaloid-Based Phase-Transfer Catalysts

We report the discovery of novel N,N′-disubstituted cinchona alkaloids as efficient phase-transfer catalysts for the assembly of stereogenic quaternary centers. In comparison to traditional cinchona-alkaloid-based phase-transfer catalysts, these new catalysts afford substantial improvements in enantioselectivity and reaction rate for intramolecular spirocyclization reactions with catalyst loadings as low as 0.3 mol % under mild conditions.

Discovery and application of doubly quaternized cinchona-alkaloid-based phase-transfer catalysts.

We report the discovery of novel N,N'-disubstituted cinchona alkaloids as efficient phase-transfer catalysts for the assembly of stereogenic quaternary centers. In comparison to traditional cinchona-alkaloid-based phase-transfer catalysts, these new catalysts afford substantial improvements in enantioselectivity and reaction rate for intramolecular spirocyclization reactions with catalyst loadings as low as 0.3 mol% under mild conditions.

Highly efficient phase transfer catalyst supported on Janus composite particles: Synthesis, characterization, and applications

Abstract