Stereoselective Catalysts Research Articles

Asymmetric hydrogenation of diketones catalyzed by magnesium oxide‐supported chitosan–Rh complex

AbstractMgO‐supported chitosan–Rh complex (MgO‐CS‐Rh) has been prepared and found to be a highly stereoselective catalyst for the asymmetric hydrogenation of some diketones to corresponding chiral alcohols, such as 2,3‐butanedione to (2S,3S)‐(+)‐2,3‐butanediol and 2,4‐pentanedione to (2S,4S)‐(+)‐2,4‐pentanediol at room temperature and under 1 atm H2. The optical yield was greatly affected by the Rh content in the complex, the solvent and the reaction temperature. The optical yields of (2S,3S)‐(+)‐2,3‐butanediol and (3S,4S)‐(+)‐2,4‐pentanediol obtained were 87% and 81.2%, respectively. The chiral chitosan–Rh complex catalyst was very easy to prepare, and could be reused without noticeable change in its optical catalytic activity. Copyright © 2003 John Wiley & Sons, Ltd.

A simple and effective synthetic approach to chiral 4-pyridinyl proline derivatives

A rapid approach has been developed to provide a novel series of 4-pyridinyl proline derivatives as potential stereoselective catalysts. Nucleophilic displacement at the 4-pyridinyl position proved straightforward using proline but required microwave heating to proceed when using the more bulky α-methyl proline. Total control of enantioselectivity was obtained for the synthesis of proline derivatives.

Heck Arylation of Conjugated Alkenes Catalysed by a Silica-Supported Poly-γ-Methylselenopropylsiloxane Palladium(0) Complex, the First Polymeric Organoselenium Palladium Complex

A silica-supported poly-γ-methylselenopropylsiloxane palladium(0) complex was prepared from poly-γ-chloro-propylsiloxane by treatment with sodium methyl-selenolate, followed by reaction with palladium chloride and then reduction with hydrazine hydrate. The first polymeric organoselenium palladium complex is a highly active and stereoselective catalyst for the arylation of conjugated alkenes.

DNA-Catalyzed Polymerization

Native DNA oligomers are shown to be stereoselective catalysts for the polymerization of 5'-amino-3'-acetaldehyde-modified thymidine/adenosine nucleosides through reductive amination. The reaction follows step-growth kinetics to read the encoded sequence and chain-length information in the antiparallel direction. Single mismatches in the template are selected against at a level of >100:1. A method is therefore established to translate biopolymer-encoded information stereoselectively into sequence- and chain-length specific synthetic polymers.

Direct resolution of the enantiomers of new diphosphine and diphosphine oxide ligands by high-performance liquid chromatography

Resolution of the enantiomers of new racemic diphosphines, which are very useful ligands for stereoselective catalysts, and of the corresponding phosphine oxides has been investigated by high-performance liquid chromatography (HPLC) on four different chiral stationary phases (CSP)—Chiralpak AD, Whelk-01, and Supelcosil naphthylurea and phenylurea columns. The mobile phases were optimized to achieve separation of the enantiomers. α andRS values ranged from 1.05 to 5.17 and from 0.37 to 6.57, respectively, for the Chiralpak AD and (R,R)-Whelk-01 columns. For the Supelcosil LC-(R)-phenylurea and Supelcosil LC-(S)-naphthylurea columns α values ranged from 1.05 to 1.62 andRS from 0.35 to 3.61.

MCM-41 supported aminopropylsiloxane palladium(0) complex: a highly active and stereoselective catalyst for Heck reaction

A new MCM-41 supported aminopropylsiloxane palladium(0) complex was prepared and characterized. The catalyst is highly active and stereoselective for Heck reaction at 70 °C.

MCM‐41 Supported Aminopropylsiloxane Palladium Acetate Complex: A Highly Active and Stereoselective Catalyst for Heck Reaction

AbstractA new MCM‐41 supported aminopropylsiloxane palladium acetate was prepared and characterized. This catalyst is highly active and stereoselective for Heck reaction at 70°C.

Chiral metallocenes: the synthesis and X-ray crystal structures of TiCl2η5:η5-C5Me4SiMe2C5H3R*) (R* = menthyl or neomenthyl) and related compounds

The syntheses of the chiral ansa-metallocene complexes TiCl2(η5:η5-C5Me4SiMe2C5H3R*) (R* = menthyl (4a) or neomenthyl (4b)) are reported; initially 4a was obtained as a 3:1 mixture of (R:S) diastereoisomers, which differ only in which face of the asymmetrically substituted cyclopentadienyl ring is bonded to the titanium (chiral descriptor shown). The major diastereomer 4aR was crystallized out optically pure from the initial reaction mixture, whereas the 4aS diastereoisomer was isolated after isomerizing a racemic mixture of 4a to a 1:3 mixture of (R:S) diastereoisomers using UV irradiation. The corresponding neomenthyl complex 4b was obtained as a 1.3:1 mixture of diastereoisomers that could not be separated. The optically pure 4aR was converted stereoselectively into the corresponding (R)-TiMe2(η5:η5-C5Me4SiMe2C5H3R*) (R* = menthyl). Syntheses of the related indenyl ligand system C5Me4SiMe2C9H7 (3) is reported but complexation to titanium proved to be problematic although ZrCl2(η5:η5-C5Me4SiMe2C9H6) (6) was isolated. The crystal structures of both 4aR and 4aS are reported and compared. Crystals of 4aR are orthorhombic, a = 27.857(11), b = 9.985(5), and c = 9.596(4) Å, Z = 4, space group P212121 (D24, No. 19), and those of 4aS are monoclinic, a = 8.5810(10), b = 38.679(4), and c = 8.5842(10) Å, β = 113.001(2)°, Z = 4, space group P21 (C22, No. 4). The structures were solved by the Patterson method and 4aR was refined by blocked-cascade least-squares procedures to R = 0.0628 (Rω = 0.0503) for 902 reflections with |F|/σ(|F|) [Formula: see text] 3.0, whereas 4aS was refined by full-matrix least-squares procedures to R = 0.0646 (wR2 = 0.1829) for 5734 reflections with |F|/σ(|F|) [Formula: see text] 4.0. Both diastereomers of 4a catalyze hydrosilylation of ketones, but as expected from a comparison of the two crystal structures, the 4aR isomer is the more stereoselective catalyst, i.e., hydrosilylation of acetophenone followed by hydrolysis gives 82% enantiomeric excess (ee) of (S)-PhCH(Me)OH with 4aR whereas only 16% ee of (R)-PhCH(Me)OH with 4aS.Key words: titanium, metallocene, chiral, structure, catalyst.

Catalytic behaviors of silica-supported chitin–platinum complex for asymmetric hydrogenation of α-phenylethanol

Silica-supported chitin–platinum complex (SiO 2–CT–Pt) has been prepared and found to be a high stereoselective catalyst for asymmetric hydrogenation of racemic α-phenylethanol to R-(+)-1-cyclohexyl ethanol at 30°C and under an atmospheric hydrogen pressure. The optical selectivity of R-(+)-1-cyclohexyl ethanol amounted to 100% when a fitting condition was selected. In this case, the yield and optical selectivity were affected greatly by many factors, such as Pt content in SiO 2–CT–Pt complex, the reaction temperature, the solvent, and the amount of the additive of hydrochloric acid, etc. SiO 2–CT–Pt catalyst also has shown high stability. The optical selectivity for R-(+)-1-cyclohexyl ethanol was almost keeping at 100% when the catalyst was reused for many times. Moreover, SiO 2–CT–Pt complex is an economical chiral catalyst. The chitin is an abundant natural biopolymer and the procedure for preparation of the complex is very simple.

O emprego de lipases como agentes de resolução cinética de enantiômeros em síntese orgânica: aspectos gerais sobre a influência do solvente

In organic synthesis, lipases are the most frequently used biocatalysts. They are efficient stereoselective catalysts in the kinetic resolution of a wide variety of chiral compounds. The discovery that enzymes possess catalytic activity in organic solvents has made it possible to address the question of reaction medium influence on enzymatic specificity. Perhaps the most exciting and significant development in this emerging area is the discovery that enzyme specificity, in particular enantioselectivity, can be affected by changing from one organic solvent to another. This article discusses the scope and possible mechanistic models of this phenomenon in hydrolases, specially lipases, as well as directions of future research in the area.

Hydroxynitrile lyase catalyzed enantioselective HCN addition to O-protected α-hydroxyaldehydes

Various O-protected glycol- and racemic lactaldehydes 3 and 6 as well as O-allyl protected racemic α-hydroxyaldehydes 7 (R 1=Et, Pr, Bu) have been prepared to investigate and perform a stereoselective Kiliani–Fischer synthesis by hydroxynitrile lyase (HNL) catalyzed addition of HCN. From all protecting groups investigated the allyl moiety was most suitable. ( R)-PaHNL from bitter almonds ( Prunus amygdalus), yielding the (2 S)-cyanohydrins 8– 10, was found to be a more stereoselective catalyst than ( S)-MeHNL from maniok ( Manihot esculenta). While ( R)-PaHNL led to enantiomeric excesses ≥93%, with ( S)-MeHNL the (2 R)-cyanohydrins 8– 10 were obtained with enantiomeric excesses ≤78%.

Synthesis, Characterization, and Catalysis of β3-[(CoIIO4)W11O31(O2)4],10- the First Keggin-Based True Heteropoly Dioxygen (Peroxo) Anion. Spectroscopic (ESR, IR) Evidence for the Formation of Superoxo Polytungstates

Reactions of hydrogen peroxide with several lacunary polyoxometalates of the 1:11 series, XW11O39m- (X = Co3+, Ga3+, Fe3+, Si4+, and P5+), are reported. Synthetic pathways to new polyoxotungstates incorporating dioxygen moieties (peroxo and/or superoxo) are developed. The key step involves treating lacunary precursors with H2O2 in strongly buffered aqueous solutions. Upon reaction of H2O2 with α-[Co3+W11O39],9- (a) the central tetrahedral Co3+ is reduced to Co2+ and (b) each of the four unshared oxygens surrounding the vacancy are replaced by a peroxide group, yielding salts of the tetraperoxide anion β3-[(Co2+O4)W11O31(O2)4]10- (1). These results are unequivocally established by a combination of elemental analysis, spectroscopy (UV−Vis−near-IR and IR), magnetic moment determination, and complete X-ray crystal structure analysis of (NH4)9K[(Co2+O4)W11O31(O2)4]·5H2O. The dioxygen O−O bonds are 1.41 and 1.44 A, typical of peroxo complexes. Salts of 1 are excellent stereoselective catalysts for the oxidation...

Asymmetric catalysis. Part 127: Enantioselective desymmetrization of 2- n-butyl-4,7-dihydro-1,3-dioxepin with (η 6-arene)ruthenium(II) half-sandwich complexes

(η 6-Arene)ruthenium half-sandwich complexes are highly active and stereoselective catalysts in the enantioselective desymmetrization of 2- n-butyl-4,7-dihydro-1,3-dioxepin to give 2- n-butyl-4,5-dihydro-1,3-dioxepin. Enantioselectivities up to 61% ee were achieved. The temperature and solvent dependence of the catalysis as well as the activation of the catalyst were investigated.

ChemInform Abstract: Silica‐Supported Poly‐γ‐mercaptopropylsiloxane Palladium(0) Complex: A Highly Active and Stereoselective Catalyst for Arylation of Styrene and Acrylic Acid.

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.

Coordinative polymerization of p‐substituted phenyl glycidyl ethers, 1. Effect of electron‐donating groups

AbstractA series of p‐substituted glycidyl phenyl ethersIUPAC nomenclature: 2,3‐epoxypropyl phenyl ether. with electron donor groups was prepared using a multistep synthetic method. These compounds were polymerized using four of the most common aluminium‐based coordinative initiators, (C2H5)3Al/H2O (the Vandenberg catalyst), (C2H5)3Al/CH3COCH2COCH3/H2O (the Vandenberg chelate catalyst), [(CH3)2CHO&]2AlOZnOAl[OCH(CH3)2]2 (the Teyssié catalyst) and [(CH3)2CHO]3AV/ZnCI2 (the Price catalyst), all with different Lewis acid character. The influence of the electron‐donor group on the characteristics and conversion was studied. Generally speaking, both the Vandenberg and the Teyssié catalysts yield mostly an insoluble polymer fraction, whereas the Price catalyst was shown to be the most stereoselective catalyst. Moreover, the Vandenberg chelate catalyst gave the highest molecular weight, the Price catalyst the lowest. Thermal characteristics of polymers were studied by means of differential thermal analysis to obtain glass transition temperatuures, melting temperatures and fusion enthalpies.

Silica-Supported Poly-γ-Mercaptopropylsiloxane Palladium(0) Complex: A Highly Active and Stereoselective Catalyst for Arylation of Styrene and Acrylic Acid

Silica-Supported Poly-γ-Mercaptopropylsiloxane Palladium(0) Complex: A Highly Active and Stereoselective Catalyst for Arylation of Styrene and Acrylic Acid

Catalytic oxidation with hydrogen peroxide catalyzed by ‘sandwich’ type transition metal substituted polyoxometalates

The oxidation of organic substrates catalyzed by ‘sandwich’ type transition metal substituted polyoxometalates of the general formula, Na x M 2Zn 3W 19O 68, (M = Ru, Mn, Zn, Pd, Pt, Co, Fe, Rh) was examined in three different reaction media. The manganese analog was dissolved in a 1,2-dichloroethane phase using a lipophilic quaternary ammonium counter cation. Various organic substrates were oxidized with 30% aqueous H 2O 2. Alkenes reactivity increased as a function of the nucleophilicity of the double bond, but decreased as a function of steric crowding in the cyclohexene series. Alkenols with primary hydroxyl groups reacted chemo- and stereoselectively to form the corresponding epoxy alcohols. On the other hand, alkenols with secondary hydroxyl units did not react chemoselectively; both ketones and epoxy alcohols were formed. Diols were oxidized in most cases to ketols, except for 1,4-butanediol which yielded γ-butyrolactone. Secondary amines yielded hydroxyl amines except for piperidine which reacted with the solvent. A manganese containing catalyst supported on a functionalized silica particle was as active and selective as the organic solvent containing biphasic system for the oxidation of alkenes and alkenols. Reactions were also carried out by dissolving Na x M 2Zn 3W 19O 68 in aqueous solutions of 30% H 2O 2, 70% t-butylhydroperoxide or 0.02 M potassium persulfate in the absence of solvent. Hydrogen peroxide degraded all the TMSP compounds. One degradation product was an effective and chemo- and stereoselective catalyst for the epoxidation of primary alkenols. In alcohol oxidation only the ruthenium precursor was active. For oxidations with 70% t-butylhydroperoxide all compounds were stable but only the Na 11Ru 2Zn 3W 19O 68 compound was active. Alcohols were oxidized selectively, however, alkenols yielded a mixture of products. With persulfate, some catalytic effects were observed in double bond oxidation.

Application of KBigraphite intercalation compounds as stereoselective catalysts of cyclohexanol conversion

The second stage of KBigraphite intercalation compounds with interplanar distance of 1320 pm was used as a catalyst for cyclohexanol conversion. Intercalation of a KBi alloy into graphite caused a two-fold increase in activity for cyclohexanol dehydrogenation to cyclohexanone and dehydration to cyclohexene in the temperature range 200–400 °C. The KBiGICs catalyst was also stereoselective; complete inactivity in the dehydrogenation to benzene was found. A correlation was found between the catalytic activity and thermally stimulated electron emission from the KBiGICs in the temperature range of 300–350 °C

Aryloxy complexes and cyclometallated aryloxy alkylidene complexes of tungsten (VI). Application to the metathesis of functionalized olefins

In this review article, the synthesis of aryloxy complexes of tungsten with the general formula W(OAr) x Cl 6− x is described together with that of aryloxy (chloro)neopentylidene-tungsten complexes. The properties of these compounds in the metathesis of non-functionalized and functionalized olefins are reviewed. The aryloxy complexes of tungsten are good air-stable precursor catalysts when combined with organotin or -lead cocatalysts. In particular, depending on the cocatalyst and substituents on the aryloxide ligands, one can achieve either linear or cross-linked polydicyclopentadienes. One can also govern the induction period for the obtention of cross-linked polydicyclopentadiene, a phenomenon compatible with the requirements of the RIM processing. The cyclometallated neopentylidene tungsten complex 1 can be synthesized by two different routes, either from the W(OAr) 2Cl 4 (OAr = -O-2,6-C 6H 3Cl 2 or -O-2,6-C 6H 3Ph 2) precursor or from the Schrock carbyne W(CCMe 3)(Cl 3)(dme). In both cases, when OAr = O-2,6-C 6H 3Ph 2, a metallation occurs by cleavage of the CH bond in one substituting phenyl group. Complex 1 is probably one of the most active and stereoselective catalysts for metathesis of cis- and trans-2-pentene (total retention of the configuration of the starting olefin). Similarly, it selectively transforms 1-methyl-norbornene to 100% cis, 100% headtail syndiotactic poly-1-methyl-norbornene. Complex 1 is also one of the most active catalysts for metathesis of functionalized olefins: with ethyl oleate 1 converts approximatively 250 equivalents of ethyl oleate into 9-octadecene and diethyl-9-octadecenedioate at 25°C. Compound 1 converts approximatively 100 equivalents of unsubstituted diallyl ether into the corresponding 2,5-dihydrofurane, and it (as well as the bicomponent catalysts) is able to realize metathesis of ω-unsaturated glucosides into the corresponding di-glucosides. It is also able to convert cyclic and acyclic double bonds with thio-ether functionalities to the corresponding polymer or cyclic products. For example, 1 converts diallyl sulphide or related compounds to the corresponding 2,5-dihydrothiophene derivatives. Similarly, phosphorus, silicon and tin diallyl derivatives can be transformed into the corresponding cyclic compound ECH 2CHCHC H 2, E = P, Si, Sn.

Solid‐State NMR characterization of a superactive supported ziegler–natta catalyst

AbstractThe formation of CH‐type catalysts has been investigated by high‐resolution and solid‐state NMR. These catalysts are prepared from a soluble MgCl2 and 2‐ethyl‐1‐hexanol adduct (MgCl2·3EH) by reaction with phthalic anhydride (PA) to form dioctylphthalate (DOP) and then with TiCl4 in the presence of di‐i‐butylphthalate (BP). In the model systems MgCl2·3EH/PA, MgCl2/BP, and MgCl2/TiCl4/BP, the ester is complexed with MgCl2 and /or TiCl4 in two or more states. Only single‐ester CO and OCH2 resonances are seen in TiCl4/BP, probably due to exchange of ester coordinations. CH‐catalysts prepared by three different procedures exhibit a single mode of bonding for the ester. The chemical shift values are consistent for ester complexed with MgCl2. The most active and stereoselective catalyst has the most shielded chemical shift values for the CO and —OCH2— carbons, shortest TH1 and TH1p, and longest TCH relaxation times. These parameters change monotonically with the decrease of activity and stereoselectivity of the catalyst preparation. © 1993 John Wiley & Sons, Inc.