Heterogeneous Chiral Catalysts Research Articles

Surface confinement effects in enantioselective catalysis: Design of new heterogeneous chiral catalysts based on C 1-symmetric bisoxazolines and their application in cyclopropanation reactions

Surface confinement effects are extremely important in determining the stereochemical course of cyclopropanation reactions catalyzed by C 2-symmetric bisoxazoline–copper complexes immobilized on laponite. In the case of the PhBox ligand, complete reversal of trans/ cis-diastereoselectivity is observed, as well as a reversal in the absolute configuration of the major cis-cyclopropane. Furthermore, the enantioselectivity obtained with the immobilized catalyst is better than that obtained in the homogeneous phase, indicating that support effects can be beneficial to the stereoselectivity of the reaction. The design of C 1-symmetric bisoxazoline ligands specifically for heterogeneous catalysis allows improvement of the cis-selectivity of the benchmark cyclopropanation reaction up to 91% cis-cyclopropanes. This finding is in agreement with a closer complex surface approach. Experiments carried out with reagents with a greater steric demand highlight the existence of reaction channels in which the ester group points toward the support surface, a factor that was disregarded in the former stereoinduction model.

ChemInform Abstract: Noncovalently Supported Heterogeneous Chiral Amine Catalysts for Asymmetric Direct Aldol and Michael Addition Reactions.

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 of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Noncovalently Supported Heterogeneous Chiral Amine Catalysts for Asymmetric Direct Aldol and Michael Addition Reactions

A new strategy for the immobilization of asymmetric organocatalysts by combining polystyrene (PS)/sulfonic acids and chiral amines in situ through acid-base interactions is presented. The PS/sulfonic acids play a dual role as catalyst anchors and modulators for activity and stereoselectivity. Different types of polymeric sulfonic acids were examined and 1% divinylbenzene (DVB) cross-linked PS/sulfonic acid 1 e with a medium loading of sulfonic acid moieties was found to be the optimal support. Furthermore, the noncovalency of this system allows combinatorial screening of optimal catalysts for the targeted reactions. In this regard, highly efficient and enantioselective heterogeneous catalysts were identified for the asymmetric direct aldol and Michael addition reactions. The catalysts could be easily recovered by filtration and reused for six cycles with similar stereoselectivity but slightly decreased activity. Significantly, the deactivated catalysts could be regenerated following an acidic washing/amine recharging procedure.

Green chemistry and catalysis

Green chemistry and catalysis

Asymmetric transfer hydrogenation of prochiral ketone catalyzed over Fe-CS/SBA-15 catalyst

A heterogeneous chiral catalyst Fe(III)-CS (chitosan) complex/mesoporous molecular sieve SBA-15 (Santa Barbara Amorphous) was prepared. The asymmetric transfer hydrogenations of prochiral acetophenone and 4-methyl-2-pentanone to corresponding chiral alcohols were carried out on Fe-CS/SBA-15 at atmosphere pressure using 2-propanol as hydrogen donor. Effects of Fe content in catalyst, reaction temperature, reaction time and promoter KOH concentration on the conversion of substrates and enantio-selectivity were investigated. Fe-CS/SBA-15 with 2.2% mass fraction Fe exhibits considerable enantioselectivity and catalytic activity for the asymmetric transfer hydrogenations of aromatic ketone and aliphatic ketone. Under optimal reaction conditions: KOH concentration 0.03 mol/L, reaction temperature 70°C and reaction time 4 h, enantiomer excess (ee) of (R)-1-phenylethanol and conversion of acetophenone can reach 87.4% and 27.7%, respectively. Under the above KOH concentration and reaction temperature and reaction time of 8 h, the ee of (R)-4-methyl-2-pentanol and conversion 4-methyl-2-pentanone amounted to 50.2% and 25.5%, respectively.

Pybox Monolithic Miniflow Reactors for Continuous Asymmetric Cyclopropanation Reaction under Conventional and Supercritical Conditions

Supported catalysts having pybox chiral moieties were prepared as macroporous monolithic miniflow systems. These catalysts are based on styrene-divinylbenzene polymeric backbones having different compositions and pybox chiral moieties. Their corresponding ruthenium complexes were tested for the continuous flow cyclopropanation reaction between styrene and ethyldiazoacetate (EDA) under conventional conditions and in supercritical carbon dioxide (scCO2). Ru-Pybox monolithic miniflow reactors not only provided a highly efficient and robust heterogeneous chiral catalyst but also allowed us to develop more environmental reaction conditions without sacrificing the global efficiency of the process.

Enantioselective hydrogenation over immobilized rhodium diphosphine complexes on mesostructured materials

New heterogeneous chiral catalysts were prepared from rhodium diphosphine complexes [Rh(P–P)COD]Cl ((P–P) = diphosphine ligand and COD = cyclooctadiene), and Al–MCM-41, Al–MCM-48, and Al–SBA-15, respectively. Impregnation of the mesoporous Al–MCM-41, Al–MCM-48, and Al–SBA-15 with the organometallic complexes in dichloromethane led to strongly bonded hydrogenation catalysts. The catalysts were characterised with XRD, FT-IR and MAS-NMR, as well as thermoprogrammed desorption of ammonia, thermogravimetric analysis, and nitrogen sorption experiments. The hydrogenation of dimethyl itaconate, methyl α-acetamidoacrylate, and methyl α-acetamidocinnamate were studied as test reactions. The immobilized catalysts showed high activities and excellent chemo- and enantioselectivities. Up to 98% e.e., >99% conversion and 99% selectivity were observed in the case of studied prochiral olefins. The catalysts could be reused without a loss of catalytic activity. Leaching of the homogeneous complex out of the mesoporous framework was not observed.

Heterogeneous asymmetric hydroformylation of olefins on chirally modified Rh/SiO 2 catalysts

Heterogeneous chiral catalysts were prepared by modifying silica-supported rhodium (Rh/SiO 2) with chiral phosphorus ligands. The chirally modified Rh/SiO 2 catalysts exhibited high activity, regioselectivity, and enantioselectivity for the asymmetric hydroformylation of styrene and vinyl acetate. Up to 72% ee and 100% selectivity of branched aldehyde for the hydroformylation of vinyl acetate were obtained for ( R)-BINAP–Rh/SiO 2 catalysts. It is noteworthy that the modification of Rh/SiO 2 with ( S, S)-DIOP resulted in increased activity for the hydroformylation of vinyl acetate and gave a TOF of 128 h −1, even higher than that of the unmodified Rh/SiO 2 catalyst (90 h −1). It is found that chiral modifiers with bidentate phosphines and an optimized modifier/rhodium molar ratio close to 1.0 were prerequisites for chiral induction on the chirally modified catalysts. P 31 MAS NMR results and IR spectra of adsorbed CO indicated that the chiral modification via the coordination of phosphines to rhodium produces chirally active sites on the Rh/SiO 2 catalysts.

Self‐Supported Chiral Catalysts for Heterogeneous Enantioselective Reactions

The development of heterogeneous chiral catalysts for enantioselective reactions is highly desirable in order to overcome some drawbacks of homogeneous catalysts. Different from the conventional approaches by using various types of supports or biphasic systems for the recovery and reuse of homogeneous catalysts, a conceptually new strategy for heterogenization of homogeneous chiral catalysts, that is, a "self-supporting" approach, has been developed to use homochiral metal-organic coordination polymers generated by the self-assembly of chiral multitopic ligands with metal ions, and thus obviates the use of any support. In this concept article, the success of this "self-supporting" strategy will be exemplified in heterogeneous catalysis of asymmetric carbonyl-ene, sulfoxidation, epoxidation, and asymmetric hydrogenation reactions.

New chiral basic heterogeneous catalyst based on Csβ zeolite

A new basic chiral catalyst Met-Csβ has been synthesised for preparing optically active products of the Michael reaction and tandem transformations by the interaction of starting achiral α,β-unsaturated carbonyl compounds with malononitrile.

Enantioselective hydrogenation over immobilized rhodium diphosphine complexes on aluminated SBA-15

A series of heterogeneous chiral catalysts was prepared from rhodium diphosphine complexes [Rh(L-L)COD]Cl ((L-L) = diphosphine ligand and COD = cyclooctadiene) and aluminated SBA-15. Impregnation of mesoporous Al-SBA-15 with organometallic complexes led to strongly bonded hydrogenation catalysts. The complexes were bound to the carrier by the interaction of the cationic rhodium of the organometallic complex with the anionic host framework, and between Al Lewis acid sites and P Lewis basic sites. The hydrogenation of dimethyl itaconate and that of methyl α-acetamidoacrylate were studied as test reactions. The immobilized catalysts showed high activities and excellent chemo- and enantioselectivities. Up to 93% e.e., >99% conversion, and 99% selectivity were observed in the case of studied prochiral olefins. The catalysts could be reused without loss of catalytic activity. These new catalysts were stable toward leaching of the homogeneous complex from the solid framework.

Chiral Vanadyl Salen Complex Anchored on Supports as Recoverable Catalysts for the Enantioselective Cyanosilylation of Aldehydes. Comparison Among Silica, Single Wall Carbon Nanotube, Activated Carbon and Imidazolium Ion as Support.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Chiral vanadyl salen complex anchored on supports as recoverable catalysts for the enantioselective cyanosilylation of aldehydes. Comparison among silica, single wall carbon nanotube, activated carbon and imidazolium ion as support

The activity and enantiomeric excess (ee) (in some cases >85%) obtained for the asymmetric addition of trimethylsilyl cyanide to aldehydes using different heterogeneous chiral catalysts are compared. A library of recoverable catalysts was developed by immobilization of a chiral vanadyl salen complex having a terminal carbon–carbon double bond onto a series of scaffolds including silica, single-wall carbon nanotubes, activated carbon and room-temperature ionic liquids. The covalent linkage has been achieved by radical initiated addition of mercapto groups to C C. The highest enantiomeric excesses, similar to those obtained in the homogeneous phase, were achieved using silica as support or with the homogeneous tetra- tert-butyl salen catalyst dissolved in an imidazolium ionic liquid. The use of silica as support permits an easier separation and reuse of the catalyst from the reaction media.

The role of binding constants in the efficiency of chiral catalysts immobilized by electrostatic interactions: the case of azabis(oxazoline)-copper complexes.

As shown by theoretical calculations, azabis(oxazoline)-copper complexes are considerably more stable than the analogous bis(oxazoline)-copper complexes. This enhanced stability allows them to be efficiently immobilized by means of electrostatic interactions to different anionic supports, such as clays and nafion-silica nanocomposites, without the loss of a ligand, as is observed for bis(oxazolines). As a result, enantioselectivities of around 90 % ee are obtained in the cyclopropanation reaction between styrene and ethyl diazoacetate. Moreover, the solid catalyst is easily recoverable.

Asymmetric Hydrogenation of Furan- containing Ketones over Tartaric Acid-modified Raney Nickel Catalyst

The hydrogenation of β-keto esters containing a furan unit at the conjugated position to the (3-carbonyl group was carried out over a chiral heterogeneous catalyst, tartaric acid-modified Raney nickel. The hydrogenation first proceeds at the carbonyl to give optically active alcohols, but a simple substrate undergoes further hydrogenation of the furan part to produce a diastereomeric mixture of alcohols having a tetrahydrofuran moiety. This over-reduction was efficiently suppressed by substitutions of a substituent at the furan part. The optical yield at the hydroxy group is in the range of 40-90%.

Internal/External use of dendrimer in catalysis

Dendrimers, well-defined hyper-branched macromolecules with characteristic globular structures, have inspired chemists and chemical engineers to develop new materials and several applications have been explored. The dendritic molecule has emerged as an attractive material in the field of catalysis and various dendrimer catalysts have been applied not only to catalytic reactions but also to non-catalytic ones such as nanoscale reactor systems. This article presents a review of research work on the dendrimer-based catalysis involving the rational design of homogeneous or heterogeneous chiral dendrimer catalysts for enantioselective reactions and the synthesis of catalytically active bimetallic nanoparticles in the presence of dendrimer as a template.

Over 98% Optical Yield Achieved by a Heterogeneous Catalysis. Substrate Design and Analysis of Enantio-Differentiating Factors of Tartaric Acid-Modified Raney Nickel Hydrogenation

Abstract Tartaric acid-modified Raney nickel (TA-MRNi) is a chiral heterogeneous catalyst for the hydrogenation of prochiral ketones. An optical yield (OY) of 86% with methyl acetoacetate (1) as a substrate was improved to 94–96% by employing β-keto esters having a proper bulkiness at the γ -position. The γ -bulkiness effect contributes to a high intrinsic enantio-differentiating ability (factor-i) of the TA-MRNi catalysis. Through the study, we found the best substrate, γ -cyclopropyl-β-keto ester, the hydrogenation of which resulted in 98.6% OY. This further improvement in the OY was ascribed to a smaller contribution of non-enantio-differentiating hydrogenation (N-site catalysis) due to the substrate-specific activation of the enantio-differentiating hydrogenation by the chiral modifier. The OY of the hydrogenation of 1 was analyzed by comparing with well-behaved β-keto esters, and the contribution of the factor-i and the N-site to the OY value was evaluated to deduce the origin of the enantiodifferentiation.

Highly efficient asymmetric epoxidation of alkenes with a D(4)-symmetric chiral dichlororuthenium(IV) porphyrin catalyst.

A dichlororuthenium(IV) complex of 5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,2:5,8-dimethanoanthrance-9-yl]porphyrin, [Ru(IV)(D(4)-Por)Cl(2)] (1), was prepared by heating [Ru(II)(D(4)-Por)(CO)(MeOH)] (2) in refluxing CCl(4). Complex 1 is characterized by (1)H NMR (paramagnetically shifted pyrrolic protons at delta(H) = -52.3 ppm), FAB-mass spectroscopies, and magnetic susceptibility measurement (mu(eff) = 3.1 mu(B)). The ruthenium complex exhibits remarkable catalytic activity toward enantioselective alkene epoxidation using 2,6-dichloropyridine N-oxide (Cl(2)pyNO) as terminal oxidant. The Ru(IV)-catalyzed styrene epoxidation is achieved within 2 h (versus 48 h for the 2-catalyzed reaction), and optically active styrene oxide was obtained in 69% ee and 84% yield (875 turnovers). Likewise, substituted styrenes and some conjugated cis-disubstituted alkenes (e.g., cis-beta-methylstyrene, cis-1-phenyl-3-penten-1-yne, 1,2-dihydronaphthalene, and 2,2-dimethylchromenes) are converted effectively to their organic epoxides in 50-80% ee under the Ru(IV)-catalyzed conditions, and more than 850 turnovers of epoxides have been attained. When subjecting 1 to four repetitive uses by recharging the reaction mixture with Cl(2)pyNO and styrene, styrene oxide was obtained in a total of 2190 turnovers and 69% ee. UV-vis and ESI-mass spectral analysis of the final reaction mixture revealed that a ruthenium-carbonyl species could have been formed during the catalytic reaction, leading to the apparent catalyst deactivation. We prepared a heterogeneous chiral ruthenium porphyrin catalyst by immobilizing 1 into sol-gel matrix. The heterogeneous catalyst is highly active toward asymmetric styrene epoxidation producing styrene oxide in 69% ee with up to 10,800 turnovers being achieved. The loss of activity of the Ru/sol-gel catalyst is ascribed to catalyst leaching and/or deactivation. On the basis of Hammett correlation (rho(+) = -1.62, R = 0.99) and product analysis, a dioxoruthenium(VI) porphyrin intermediate is not favored.

Immobilization of Rhodium Diphosphine Complexes on Mesoporous Al-MCM-41 Materials: Catalysts for Enantioselective Hydrogenation

Heterogeneous chiral catalysts for the enantioselective hydrogenation of prochiral alkenes are prepared from chiral rhodium diphosphine complexes and Al-MCM-41. Impregnation of the mesoporous carrier Al-MCM-41 with the organometallic complexes in dichloromethane leads to strong and not leaching hydrogenation catalysts. The catalysts are characterized by spectroscopic methods such as FT-IR and MAS-NMR as well as thermoprogrammed desorption of ammonia, thermogravimetric analysis, and nitrogen sorption experiments. The hydrogenation of dimethylitaconate was studied as a test reaction. The immobilized catalysts showed high activity and excellent regio- and enantioselectivity. Up to 92% ee, 100% conversion, and 99% regioselectivity were observed for the hydrogenation of dimethyl (R)-methylsuccinate. The catalysts could be reused without any regeneration procedure and with no loss of catalytic acitivity. Leaching of the homogeneous complex out of the MCM-41 framework was not observed.

ChemInform Abstract: New Chiral Heterogeneous Catalysts Based on Mesoporous Silica: Asymmetric Diethylzinc Addition to Benzaldehyde.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.