Engineering Polymer‐Enhanced Bimetallic Cooperative Interactions in the Hydrolytic Kinetic Resolution of Epoxides

Through systematic variations of the length of oligo(ethylene glycol)‐based linkers and the catalyst density of poly(styrene)‐supported cobalt‐salen catalysts, we have elucidated an optimal catalyst flexibility and density of polymeric Co‐salen catalysts for the hydrolytic kinetic resolution (HKR) of racemic terminal epoxides that follows a bimetallic cooperative pathway. The optimized polymeric catalyst brings the two cooperative Co‐salen units to a favorable proximity efficiently and hence displays significantly improved catalytic performance in the HKR compared with its monomeric small molecule analogue. Complex Co(5b) , representing the most active poly(styrene)‐supported HKR catalyst known so far, can effect the resolution of a variety of epoxides to reach ≥98 % ee in 6–24 h with a low cobalt loading of 0.01–0.1 mol %.     

Hydrolytic Kinetic Resolution of Epoxides Catalyzed by Chromium(III)‐endo,endo‐2,5‐diaminonorbornane‐salen [Cr(III)‐DIANANE‐salen] Complexes. Improved Activity,Low Catalyst Loading

The hydrolytic kinetic resolution (HKR) of terminal epoxides, using chiral chromium(III )‐salen catalysts based on DIANANE (endo,endo‐2,5‐diaminonorbornane), was studied. A broad substrate scope was found for the chromium(III )‐DIANANE catalysts, and very low loadings (down to 0.05 mol %) were needed to achieve high enantiomeric purities of both the remaining epoxides and the product diols (up to >99 % ee). Besides monosubstituted epoxides, 2‐methyl‐2‐n‐pentyloxirane, which is an example for 2,2‐disubstituted epoxides, could be ring‐opened in an asymmetric fashion with water in the presence of an electronically tuned chromium(III )‐DIANANE complex.     

Cooperative Activation in the Hydrolytic Kinetic Resolution of Epoxides by a Bis‐Cobalt(III)salen‐Calix[4]arene Hybrid

A chiral, bimetallic cobalt(III)salen‐calix[4]arene hybrid structure was prepared and tested in the hydrolytic kinetic resolution (HKR) of racemic epoxides. Kinetic studies have revealed that the two catalytic units on the upper rim of the calixarene scaffold are able to activate the reactants in a cooperative and primarily intramolecular mode. High enantioselective behaviour was observed and besides, a higher stability was found for the bimetallic catalyst as compared to a monometallic reference complex.     

Platinum(II) Diphosphinamine Complexes for the Efficient Hydration of Alkynes in Micellar Media

Highly active monomeric bis‐cationic platinum(II) catalysts bearing small bite angle diphosphinamine [N,N‐bis(diarylphosphino)amine] ‘PNP’ ligands efficiently catalyze Markovnikov hydration of terminal and internal alkynes to the corresponding ketones in water. Catalyst solubilization in water is achieved via ion pairing with anionic micelles formed by surfactant addition. The micelles ensure dissolution of apolar alkynes and promote the intimate contact between reagents and catalyst, while in organic‐water media in the absence of surfactants the reaction is sluggish. Hydration products can be isolated by means of extraction with an apolar solvent and the catalyst, that remains confined in the aqueous phase, can be recycled up to four times without loss of catalytic activity.     

Trichloroisocyanuric Acid: A Convenient Oxidation Reagent for Phase‐Transfer Catalytic Epoxidation of Enones under Non‐Aqueous Conditions

Trichloroisocyanuric acid (TCCA) is a cheap, safe and readily available alternative to the commonly used hydrogen peroxide and hypochlorite for the phase‐transfer catalytic epoxidation of α,β‐enones under non‐aqueous conditions. A variety of chalcone derivatives give the corresponding epoxides with quantitative conversion and satisfactory yields in just a few hours under mild conditions. An asymmetric variant of the epoxidation can be carried out in the presence of chiral N‐anthracenylmethylcinchonidine bromide catalyst giving 73–93% ees and 76–94% yields.     

Poly(4‐vinylimidazolium)s/Diazabicyclo[5.4.0]undec‐7‐ene/Zinc(II) Bromide‐Catalyzed Cycloaddition of Carbon Dioxide to Epoxides

Poly(4‐vinylimidazolium)s, derived from the self‐immobilization of 4‐vinylimidazoliums, with diazabicyclo[5.4.0]undec‐7‐ene (DBU) and zinc bromide (ZnBr₂) are used as a highly efficient catalyst for the chemical fixation of carbon dioxide. This catalytic system has been applied for the preparation of cyclic carbonates from terminal epoxides and carbon dioxide. Many functional groups, including chloro, vinyl, ether, and hydroxy groups are well tolerated in the reactions. Moreover, the catalytic system was found to catalyze the conversion of more sterically congested epoxides which are generally considered to be challenging substrates for fabricating the cyclic organic carbonates. In addition, the disubstituted epoxides are found to react with retention of configuration. The polymer precatalyst is easily recovered and reused. A plausible reaction mechanism is proposed.

     

Multicomponent Synthesis of 1,2,3‐Triazoles in Water Catalyzed by Copper Nanoparticles on Activated Carbon

Copper nanoparticles on activated carbon have been found to effectively catalyze the multicomponent synthesis of 1,2,3‐triazoles from different azide precursors, such as organic halides, diazonium salts, anilines and epoxides in water. The first one‐pot transformation of an olefin into a triazole is also described. The catalyst is easy to prepare, very versatile and reusable at a low copper loading.     

Mixtures of Ionic Liquids and Water as a Medium for Efficient Enantioselective Hydrogenation and Catalyst Recycling

In a screening of ligands, ionic liquids and reaction conditions in the Rh‐catalyzed hydrogenation of enamides, a novel multi‐phase reaction system consisting of an ionic liquid (IL) and water (wet ILs) was found to give the most promising results. In many cases such IL/water combinations were superior compared to conventional organic solvents and biphasic ILs/organic co‐solvents media with respect to catalytic performance as well as to catalyst separation and recycling. So far, the best results were obtained with Rh‐ferrocenyl‐diphosphine catalysts (>99% ee). Generally, somewhat lower ees were observed at higher pressure. However, this effect was less pronounced with wet ILs than with conventional solvents. It is shown that IL/water combination allow repeated catalyst recycling without significant loss of activity and that industrially relevant turnover numbers of >10,000 can be obtained.     

Preparation of polymer‐supported polyethylene glycol and phase‐transfer catalytic activity in benzoate synthesis

The crosslinked polymeric microspheres (GMA/MMA) of glycyl methacrylate (GMA) and methyl methacrylate (MMA) were prepared by suspension polymerization. Polyethylene glycol (PEG) was grafted on GMA/MMA microsphers via the ring‐opening reaction of the epoxy groups on the surfaces of GMA/MMA microspheres, forming a polymer‐supported triphase catalyst, PEG‐GMA/MMA. The Phase‐transfer catalytic activity of PEG‐GMA/MMA microspheres was evaluated using the esterification reaction of n‐chlorobutane in organic phase and benzoic acid in water phase as a model system. The effects of various factors on the phase transfer catalysis reaction of liquid–solid–liquid were investigated. The experimental results show that the PEG‐GMA/MMA microspheres are an effective and stable triphase catalyst for the esterification reaction carried out between oil phase and water phase. The polarity of the organic solvent, the ratio of oil phase volume to water phase volume and the density of the grafted PEG on PEG‐GMA/MMA microspheres affect the reaction rate greatly. For this investigated system, the solvent with high polarity is appropriate, an adequate volume ratio of oil phase to water phase is 2:1, and the optimal PEG density on the polymeric microspheres is 15 g/100 g. Triphase catalysts offer many advantages associated with heterogeneous catalysts such as easy separation from the reaction mixture and reusability. The activity of PEG‐GMA/MMA microspheres is not nearly decreased after reusing of 10 recycles. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Polymerization of epoxides catalyzed by a mixed‐valent iron trifluoroacetate [Fe3O(O2CCF3)6(H2O)3]

The readily available mixed‐valent iron trifluoroacetate complex [Fe₂^IIIFe^II(µ₃‐O)(O₂CCF₃)₆(H₂O)₃] is an effective catalyst for the polymerization of epoxides. A very small amount of the catalyst (1.0–0.01 mol%) could initiate the polymerization of cyclohexene oxide, cyclopentene oxide and epichlorohydrin. Based on quantitative end‐group analysis by ¹⁹F NMR spectroscopy, a Lewis acid (LA^⊕) catalyzed anionic reaction mechanism is proposed. Copyright © 2012 Society of Chemical Industry     

Ionic Liquid Effect on the Reversal of Configuration for the Magnesium(II) and Copper(II) Bis(oxazoline)‐Catalysed Enantioselective Diels–Alder Reaction

Ionic liquids have been used to support a range of magnesium‐ and copper‐based bis(oxazoline) complexes for the enantioselective Diels–Alder reaction between N‐acryloyloxazolidinone and cyclopentadiene. Compared with reaction performed in dichloromethane or diethyl ether, an enhancement in ee is observed with a large increase in reaction rate. In addition, for non‐sterically hindered bis(oxazoline) ligands, that is, phenyl functionalised ligands, a reversal in configuration is found in the ionic liquid, 1‐ethyl‐3‐methylimidazolium bis[(trifluoromethanesulfonyl)imide], compared with molecular solvents. Supported ionic liquid phase catalysts have also been developed using surface‐modified silica which show good reactivity and enantioselectivity for the case of the magnesium‐based bis(oxazoline) complexes. Poor ees and conversion were observed for the analogous copper‐based systems. Some drop in ee was found on supporting the catalyst due a drop in the rate of reaction and, therefore, an increase in the contribution from the uncatalysed achiral reaction.     

Lewis Basic Ionic Liquids‐Catalyzed Conversion of Carbon Dioxide to Cyclic Carbonates

A series of easily prepared Lewis basic ionic liquids were developed for cyclic carbonate synthesis from epoxide and carbon dioxide at low pressure without utilization of any organic solvents or additives. Notably, quantitative yields together with excellent selectivity were attained when 1,8‐diazabicyclo[5.4.0]undec‐7‐enium chloride ([HDBU]Cl) was used as a catalyst. Furthermore, the catalyst could be recycled over five times without appreciable loss of catalytic activity. The effects of the catalyst structure and various reaction parameters on the catalytic performance were investigated in detail. This protocol was found to be applicable to a variety of epoxides producing the corresponding cyclic carbonates in high yields and selectivity. Therefore, this solvent‐free process thus represents an environmentally friendly example for the catalytic conversion of carbon dioxide into value‐added chemicals by employing Lewis basic ionic liquids as catalyst. A possible catalytic cycle for the hydrogen bond‐assisted ring‐opening of epoxide and activation of carbon dioxide induced by the nucleophilic tertiary nitrogen of the ionic liquid was also proposed.     

PdCl2(bipy) complex—An efficient catalyst for Heck reaction in glycol–organic biphasic medium

The Palladium complex PdCl₂(bipy) (1) was found to be efficient in catalyzing the Heck reaction of aryl halides in a biphasic system comprising ethylene glycol along with an organic solvent. A proper tailoring of the organic phase ensured that the leaching of the catalyst to the organic phase was negligible. The catalyst was active for the Heck reaction of a number of olefins, aryl halides, in presence of organic and inorganic bases. The PdCl₂(bipy) catalyst was stable and could be recycled for five times without any loss in activity and selectivity. A major advantage of this work is that unlike hitherto reported biphasic systems; it does not employ a water-soluble ligand to achieve biphasic catalysis.     

Hydroaminomethylation of the Renewable Limonene with Ammonia in an Aqueous Biphasic Solvent System

With the hydroaminomethylation of the natural compound limonene with ammonia an atom‐economic method for the synthesis of primary amines is described. This tandem reaction allows the direct conversion of the unfunctionalized monoterpene to a valuable amine product. For the first time, ammonia served as substrate to result in a maximum primary amine yield of 25 %. To overcome unwanted side reactions, a biphasic solvent system was used, consisting of an aqueous catalyst phase and an organic product phase. As catalyst the water‐soluble transition metal complex [Rh(cod)Cl]₂/triphenylphosphine trisulfonate was chosen. In combination with the surfactant hexadecyltrimethylammonium chloride it provided a good phase interaction and the possibility for easy phase separation after the reaction.     

3,5‐Bis(n‐perfluorooctyl)benzyltriethylammonium Bromide (F‐TEBA): An Efficient,Easily Recoverable Fluorous Catalyst for Solid‐Liquid PTC Reactions

A readily available 3,5‐bis(perfluorooctyl)benzyl bromide and triethylamine were reacted under mild conditions to give 3,5‐bis(n‐perfluorooctyl)benzyltriethylammonium bromide ( F‐TEBA ), an analogue of the versatile phase‐transfer catalyst, benzyltriethylammonium chloride (TEBA), containing two fluorous ponytails. This perfluoroalkylated quaternary ammonium salt was successfully employed as a catalyst in a variety of reactions run under solid‐liquid phase‐transfer catalysis (SL‐PTC) conditions. Thus, being both hydrophobic and lipophobic, F‐TEBA could be quickly recovered in quantitative yields, and reused without loss of activity over several reaction cycles.     

Cationic Carboxylato Complexes of Dirhodium(II) with Oxo Thioethers: Catalysts for Silane Alcoholysis under Homogeneous and Liquid‐Liquid Biphasic Conditions

A set of cationic dirhodium(II) complexes with oxo thioethers was prepared and employed as catalysts for the silane alcoholysis reaction. The complexes were found to be highly active under homogeneous conditions, both in the absence and in the presence of a solvent, including coordinating solvents such as N,N‐dimethylformamide; the catalysts could be conveniently employed in concentrations as low as 0.01 mol %, and a maximum TON of 30000 was recorded after 24 h. The same catalysts were also employed under liquid‐liquid biphasic conditions with an ionic liquid as the catalyst‐containing phase: comparable catalytic activity was observed under these conditions, and the catalyst‐containing phase could be recovered and recycled. A chiral cationic dirhodium(II) complex was also prepared in the frame of this work; kinetic resolution of a racemic alcohol was attempted with this catalyst, unfortunately without success.     

Towards a Greener Epoxidation Method: Use of Water‐Surfactant Media and Catalyst Recycling in the Platinum‐Catalyzed Asymmetric Epoxidation of Terminal Alkenes with Hydrogen Peroxide

Remarkable improvements in enantioselectivity as well as recycle were observed in the catalytic asymmetric epoxidation of terminal alkenes with a chiral, electron‐poor platinum(II ) catalyst with hydrogen peroxide as terminal oxidant in water‐surfactant media.     

Poly(4‐vinylphenol)/tetra‐n‐butylammonium iodide: Efficient organocatalytic system for synthesis of cyclic carbonates from CO2 and epoxides

An efficient polymer‐based catalytic system of poly(4‐vinylphenol) and tetra‐n‐butylammonium iodide was developed for the synthesis of cyclic carbonates from epoxides and CO₂. Owing to the synergistic effects of hydroxyl groups and iodide anions, this commercially available and metal‐free system was highly active for the reaction of various terminal epoxides under environmentally benign conditions, at 25 to 60 °C and atmospheric pressure of CO₂, without the use of any organic solvents. The catalyst system can be easily separated by adding ether, and its ability was recovered by treating it with 40% CH₃CO₂H aq. The recyclability was investigated in detail for three substrates, epichlorohydrin, 1,2‐epoxyhexane, and styrene oxide, using ¹H nuclear magnetic resonance analysis. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45189.     

Transition‐metal trifluoromethane‐sulphonates‐recyclable catalysts for the synthesis of branched fatty derivatives by Diels‐Alder reactions at unsaturated fatty esters

Diels‐Alder reactions of conjugated linoleic acid ethyl ester (1) with different quinones and with a variety of α/βunsaturated aldehydes and ketones are described in this paper. Using Sc(OTf)₃ or Cu(OTf)₂ as catalysts the reactions can be carried out at 25—40 °C with good yields. For the first time in oleochemistry it is possible to prepare Diels‐Alder cycloadditions with catalyst concentrations of 10 mol‐% instead of stoichiometric amounts of Lewis acids. Furthermore, the reaction time was partly shortened drastically. The catalyst Sc(OTf)₃ can be removed by a simple extraction of the organic layer with water. After evaporation of the aqueous phase to dryness the catalyst can be reused without loss of yield.     

Synthesis of n‐butyl phenyl ether by tri‐liquid‐phase catalysis using poly(ethylene glycol)‐600 as a catalyst – analysis of factors affecting the reaction in a batch reactor

As the second part of a series of studies on the synthesis of n‐butyl phenyl ether (ROPh) by tri‐liquid‐phase catalysis, this work examines the factors affecting the reaction between n‐butyl bromide (RBr, organic substrate) and sodium phenolate (NaOPh, aqueous nucleophile) with poly(ethylene glycol)‐600 (PEG‐600) as a phase‐transfer catalyst. The reaction is performed in a batch reactor at 45–85 °C for 2 h while the agitation speed is fixed at 1000 rpm. Experimental results indicate that the individual mole fractions of NaOPh and PEG‐600 slightly affect the reaction, while the total amount of these components exerts significant influence. When the mole fraction of PEG‐600 is 0.5, the reaction rate and the conversion of RBr are the highest. No byproducts are formed in the course of the reaction. The system using a non‐polar organic solvent might obtain a higher conversion compared with a weakly polar one owing to a higher concentration of PEG‐600 in the third liquid phase. Furthermore, adding NaOH facilitates the reaction to obtain a higher reaction rate than adding other kinds of salt because the addition of a base results in the formation of a third liquid phase. The catalytic ability of PEG with average molecular weight of 600 gmol^?1 is far higher than that with average molecular weight of 200, 400 and 1000 because PEG‐600 possesses an appropriate chain length which can tightly associate with Na⁺ to form the complex of PEG‐600‐Na⁺OPh^? for reacting with RBr. In addition, this nucleophilic substitution reaction is found to be pseudo‐first‐order with respect to RBr. © 2001 Society of Chemical Industry