Heterogeneous Chiral Catalysts Research Articles

Green chemistry and catalysis

Preface. Foreword. 1 Introduction: Green Chemistry and Catalysis. 1.1 Introduction. 1.2. E Factors and Atom Efficiency. 1.3 The Role of Catalysis. 1.4 The Development of Organic Synthesis. 1.5 Catalysis by Solid Acids and Bases. 1.6 Catalytic Reduction. 1.7 Catalytic Oxidation. 1.8 Catalytic C-C Bond Formation. 1.9 The Question of Solvents: Alternative Reaction Media. 1.10 Biocatalysis. 1.11 Renewable Raw Materials and White Biotechnology. 1.12 Enantioselective Catalysis. 1.13 Risky Reagents. 1.14 Process Integration and Catalytic Cascades. References. 2 Solid Acids and Bases as Catalysts. 2.1 Introduction. 2.2 Solid Acid Catalysis. 2.2.1 Acidic Clays. 2.2.2 Zeolites and Zeotypes: Synthesis and Structure. 2.2.3 Zeolite-catalyzed Reactions in Organic Synthesis. 2.2.3.1 Electrophilic Aromatic Substitutions. 2.2.3.2 Additions and Eliminations. 2.2.3.3 Rearrangements and Isomerizations. 2.2.3.4 Cyclizations. 2.2.4 Solid Acids Containing Surface SO3H Functionality. 2.2.5 Heteropoly Acids. 2.3 Solid Base Catalysis. 2.3.1 Anionic Clays: Hydrotalcites. 2.3.2 Basic Zeolites. 2.3.3 Organic Bases Attached to Mesoporous Silicas. 2.4 Other Approaches. References. 3 Catalytic Reductions. 3.1 Introduction. 3.2 Heterogeneous Reduction Catalysts. 3.2.1 General Properties. 3.2.2 Transfer Hydrogenation Using Heterogeneous Catalysts. 3.2.3 Chiral Heterogeneous Reduction Catalysts. 3.3 Homogeneous Reduction Catalysts. 3.3.1 Wilkinson Catalyst. 3.3.2 Chiral Homogeneous Hydrogenation Catalysts and Reduction of the C= C Double Bond. 3.3.3 Chiral Homogeneous Catalysts and Ketone Hydrogenation. 3.3.4 Imine Hydrogenation. 3.3.5 Transfer Hydrogenation using Homogeneous Catalysts. 3.4 Biocatalytic Reductions. 3.4.1 Introduction. 3.4.2 Enzyme Technology in Biocatalytic Reduction. 3.4.3 Whole Cell Technology for Biocatalytic Reduction. 3.5 Conclusions. References. 4 Catalytic Oxidations. 4.1 Introduction. 4.2 Mechanisms of Metal-catalyzed Oxidations: General Considerations. 4.2.1 Homolytic Mechanisms. 4.2.1.1 Direct Homolytic Oxidation of Organic Substrates. 4.2.2 Heterolytic Mechanisms. 4.2.2.1 Catalytic Oxygen Transfer. 4.2.3 Ligand Design in Oxidation Catalysis. 4.2.4 Enzyme Catalyzed Oxidations. 4.3 Alkenes. 4.3.1 Epoxidation. 4.3.1.1 Tungsten Catalysts. 4.3.1.2 Rhenium Catalysts. 4.3.1.3 Ruthenium Catalysts. 4.3.1.4 Manganese Catalysts. 4.3.1.5 Iron Catalysts. 4.3.1.6 Selenium and Organocatalysts. 4.3.1.7 Hydrotalcite and Alumina Systems. 4.3.1.8 Biocatalytic Systems. 4.3.2 Vicinal Dihydroxylation. 4.3.3 Oxidative Cleavage of Olefins. 4.3.4 Oxidative Ketonization. 4.3.5 Allylic Oxidations. 4.4 Alkanes and Alkylaromatics. 4.4.1 Oxidation of Alkanes. 4.4.2 Oxidation of Aromatic Side Chains. 4.4.3 Aromatic Ring Oxidation. 4.5 Oxygen-containing Compounds. 4.5.1 Oxidation of Alcohols. 4.5.1.1 Ruthenium Catalysts. 4.5.1.2 Palladium-catalyzed Oxidations with O 2 . 4.5.1.3 Gold Catalysts. 4.5.1.4 Copper Catalysts. 4.5.1.5 Other Metals as Catalysts for Oxidation with O 2 . 4.5.1.6 Catalytic Oxidation of Alcohols with Hydrogen Peroxide. 4.5.1.7 Oxoammonium Ions in Alcohol Oxidation. 4.5.1.8 Biocatalytic Oxidation of Alcohols. 4.5.2 Oxidative Cleavage of 1,2-Diols. 4.5.3 Carbohydrate Oxidation. 4.5.4 Oxidation of Aldehydes and Ketones. 4.5.4.1 Baeyer-Villiger Oxidation. 4.5.5 Oxidation of Phenols. 4.5.6 Oxidation of Ethers. 4.6 Heteroatom Oxidation. 4.6.1 Oxidation of Amines. 4.6.1.1 Primary Amines. 4.6.1.2 Secondary Amines. 4.6.1.3 Tertiary Amines. 4.6.1.4 Amides. 4.6.2 Sulfoxidation. 4.7 Asymmetric Oxidation. 4.7.1 Asymmetric Epoxidation of Olefins. 4.7.2 Asymmetric Dihydroxylation of Olefins. 4.7.3 Asymmetric Sulfoxidation. 4.7.4 Asymmetric Baeyer-Villiger Oxidation. 4.5 Conclusion. References. 5 Catalytic Carbon-Carbon Bond Formation. 5.1 Introduction. 5.2 Enzymes for Carbon-Carbon Bond Formation. 5.2.1 Enzymatic Synthesis of Cyanohydrins. 5.2.1.1 Hydroxynitrile Lyases. 5.2.1.2 Lipase-based Dynamic Kinetic Resolution. 5.2.2 Enzymatic Synthesis of &alpha -Hydroxyketones (Acyloins). 5.2.3 Enzymatic Synthesis of &alpha -Hydroxy Acids. 5.2.4 Enzymatic Synthesis of Aldols (&beta -Hydroxy Carbonyl Compounds). 5.2.4.1 DHAP-dependent Aldolases. 5.2.4.2 PEP- and Pyruvate-dependent Aldolases. 5.2.4.3 Glycine-dependent Aldolases. 5.2.4.4 Acetaldehyde-dependent Aldolases. 5.2.5 Enzymatic Synthesis of &beta -Hydroxynitriles. 5.3 Transition Metal Catalysis. 5.3.1 Carbon Monoxide as a Building Block. 5.3.1.1 Carbonylation of R-X (CO Insertion/R-migration). 5.3.1.2 Aminocarbonylation. 5.3.1.3 Hydroformylation or Oxo Reaction. 5.3.1.4 Hydroaminomethylation. 5.3.1.5 Methyl Methacrylate via Carbonylation Reactions. 5.3.2 Heck-type Reactions. 5.3.2.1 Heck Reaction. 5.3.2.2 Suzuki and Sonogashira Reaction. 5.3.3 Metathesis. 5.3.3.1 Metathesis involving Propylene. 5.3.3.2 Ring-opening Metathesis Polymerization (ROMP). 5.3.3.3 Ring-closing Metathesis (RCM). 5.4 Conclusion and Outlook. References. 6 Hydrolysis. 6.1 Introduction. 6.1.1 Stereoselectivity of Hydrolases. 6.1.2 Hydrolase-based Preparation of Enantiopure Compounds. 6.1.2.1 Kinetic Resolutions. 6.1.2.2 Dynamic Kinetic Resolutions. 6.1.2.3 Kinetic Resolutions Combined with Inversions. 6.1.2.4 Hydrolysis of Symmetric Molecules and the meso-trick. 6.2 Hydrolysis of Esters. 6.2.1 Kinetic Resolutions of Esters. 6.2.2 Dynamic Kinetic Resolutions of Esters. 6.2.3 Kinetic Resolutions of Esters Combined with Inversions. 6.2.4 Hydrolysis of Symmetric Esters and the meso-trick. 6.3 Hydrolysis of Amides. 6.3.1 Production of Amino Acids by (Dynamic) Kinetic Resolution. 6.3.1.1 The Acylase Process. 6.3.1.2 The Amidase Process. 6.3.1.3 The Hydantoinase Process. 6.3.1.4 Cysteine. 6.3.2 Enzyme-catalysed Hydrolysis of Amides. 6.3.3 Enzyme-catalysed Deprotection of Amines. 6.4 Hydrolysis of Nitriles. 6.4.1 Nitrilases. 6.4.2 Nitrile Hydratases. 6.5 Conclusion and Outlook. References. 7 Catalysis in Novel Reaction Media. 7.1 Introduction. 7.1.1 Why use a solvent? 7.1.2 Choice of Solvent. 7.1.3 Alternative Reaction Media and Multiphasic Systems. 7.2 Two Immiscible Organic Solvents. 7.3 Aqueous Biphasic Catalysis. 7.3.1 Olefin Hydroformylation. 7.3.2 Hydrogenation. 7.3.3 Carbonylations. 7.3.4 Other C-C Bond Forming Reactions. 7.3.5 Oxidations. 7.4 Fluorous Biphasic Catalysis. 7.4.1 Olefin Hydroformylation. 7.4.2 Other Reactions. 7.5 Supercritical Carbon Dioxide. 7.5.1 Supercritical Fluids. 7.5.2 Supercritical Carbon Dioxide. 7.5.3 Hydrogenation. 7.5.4 Oxidation. 7.5.5 Biocatalysis. 7.6 Ionic Liquids. 7.7 Biphasic Systems with Supercritical Carbon Dioxide. 7.8 Thermoregulated Biphasic Catalysis. 7.9 Conclusions and Prospects. References. 8 Chemicals from Renewable Raw Materials. 8.1 Introduction. 8.2 Carbohydrates. 8.2.1 Chemicals from Glucose via Fermentation. 8.2.2 Ethanol. 8.2.2.1 Microbial Production of Ethanol. 8.2.2.2 Green Aspects. 8.2.3 Lactic Acid. 8.2.4 1,3-Propanediol. 8.2.5 3-Hydroxypropanoic Acid. 8.2.6 Synthesizing Aromatics in Nature's Way. 8.2.7 Aromatic -Amino Acids. 8.2.7 Indigo: the Natural Color. 8.2.8 Pantothenic Acid. 8.2.9 The &beta -Lactam Building Block 7-Aminodesacetoxycephalosporanic Acid. 8.2.9 Riboflavin. 8.3 Chemical and Chemoenzymatic Transformations of Carbohydrates into Fine Chemicals and Chiral Building Blocks. 8.3.1 Ascorbic Acid. 8.3.2 Carbohydrate-derived C3 and C4 Building Blocks. 8.3.3 5-Hydroxymethylfurfural and Levulinic Acid. 8.4 Fats and Oils. 8.4.1 Biodiesel. 8.4.2 Fatty Acid Esters. 8.5 Terpenes. 8.6 Renewable Raw Materials as Catalysts. 8.7 Green Polymers from Renewable Raw Materials. 8.8 Concluding Remarks. References. 9 Process Integration and Cascade Catalysis. 9.1 Introduction. 9.2 Dynamic Kinetic Resolutions by Enzymes Coupled with Metal Catalysts. 9.3 Combination of Asymmetric Hydrogenation with Enzymatic Hydrolysis. 9.4 Catalyst Recovery and Recycling. 9.5 Immobilization of Enzymes: Cross-linked Enzyme Aggregates (CLEAs). 9.6 Conclusions and Prospects. References. 10 Epilogue: Future Outlook. 10.1 Green Chemistry: The Road to Sustainability. 10.2 Catalysis and Green Chemistry. 10.3 The Medium is the Message. 10.4 Metabolic Engineering and Cascade Catalysis. 10.5 Concluding Remarks. References. Subject Index.

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.

New chiral heterogeneous catalysts based on mesoporous silica: asymmetric diethylzinc addition to benzaldehyde

New heterogeneous catalyst systems have been developed employing a proline-derived ligand immobilized on mesoporous silicas; asymmetric diethylzinc addition to benzaldehyde was carried out using the catalyst systems; the reaction enantioselectivity was found to be largely dependent upon the pore size of the mesoporous silicas, capping of free silanol moieties with trimethylsilyl group and employment of BunLi.

Asymmetric catalysis on solids

The processing advantages associated with heterogeneous catalysis have led to the development of many new chiral heterogeneous catalysts for a variety of important asymmetric reactions. Established systems, such as chirally modified metals, have been further developed, while new strategies for the creation of chiral polymer catalysts and oxide systems, such as zeolites containing chelate complexes, are being explored. Important issues, such as site isolation, stability against leaching, and mass transfer to the active sites, have been addressed. Advances have been made in chiral zeolite frameworks and porous hybrid solids, chiral modifiers in porous hosts, asymmetric catalysts occluded in membranes and dissolved in supported liquid films, and modular chelate complexes on polymer beads.

Synthesis and organization of zeolite-like materials with three-dimensional helical pores

The increasing demand for enantiomerically pure chemicals has stimulated extensive research into the preparation of heterogeneous chiral catalysts or separation media that combine both shape selectivity and enantioselectivity1. Helical pores in inorganic materials might be able to perform such functions, but their occurrence is rare1. Attempts have been reported to synthesize a specific enantiomorph of the chiral zeolite beta2, and chiral metal complexes have been used to assemble inorganic precursors into chiral frameworks3,4. Materials with a fully three-dimensional array of helical structural units are particularly rare, because helical structures (such as quartz) are commonly generated by a uni-dimensional symmetry element acting on an achiral structural subunit5,6. Here we report on a family of zeolite-type materials (which we call UCSB-7) that possess two independent sets of three-dimensional crosslinked helical pores, separated by a gyroid periodic minimal surface13. We have synthesized the UCSB-7 framework for various compositions (zinc and beryllium arsenates, gallium germanate) using either inorganic cations or amines as structure-directing agents. The helical-ribbon motif that we identify might be exploited more widely for developing useful chiral solid-state structures.