Homogeneous Analogues Research Articles

Novel Ruthenium‐Based Metathesis Catalysts Containing Electron‐Withdrawing Ligands: Synthesis, Immobilization, and Reactivity.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic oxidation of a thioether and dibenzothiophenes using an oxorhenium(V) dithiolate complex tethered on silica

A silica-tethered version of the oxorhenium(V) dithiolate oxidation catalyst [–S(CH 2) 3S–]Re(O)(Me)(PPh 3) was prepared and characterized by solid state 13C and 31P NMR spectroscopy. This tethered complex (SiO 2-RTA)Re(O)(Me)(PPh 3) ( Fig. 2) catalyzes the selective oxidation of methyl( p-tolyl)sulfide (MTS), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-Me 2DBT)) to their corresponding sulfoxides and sulfones by tert-butylhydroperoxide (TBHP) and can be recycled at least four times without loss in activity. Although the rate of oxidation catalyzed by the immobilized catalyst is about a factor of three times lower than the homogeneous analog [–S(CH 2) 3S–]Re(O)(Me)(PPh 3), the supported catalyst is much more long-lived achieving approximately 3000 turnovers in the oxidation of DBT at 100 °C. The immobilized catalyst effectively catalyzes the oxidation of DBT and the hindered 4,6-Me 2DBT to their sulfoxides or sulfones in simulated petroleum feedstocks.

Catalytic activity of halogenated iron porphyrins in alkene and alkane oxidations by iodosylbenzene and hydrogen peroxide

A poly-halogenated iron porphyrin, Fe(PCl8)Cl, has been synthesised and used as a catalyst in hydrocarbon oxidations by iodosylbenzene and hydrogen peroxide both in solution and covalently bound to aminopropylated silica. The poly-chlorinated iron porphyrin shows the same efficiency of the related Fe(P)Cl, in the epoxidation of alkenes but higher efficiency in the hydroxylation of alkanes by iodosylbenzene, with increased preference for the oxidation of secondary carbon in adamantane and primary carbon in the oxidation of pentane. These selectivities may reflect the steric constraints around the oxo-iron species or, alternatively, it may arise from the greater reactivity of the active oxidant from Fe(PCl8)Cl. The supported iron(III) porphyrin showed lower activity as compared with the homogeneous analogue and the related supported Fe(P)Cl. The poly-chlorinated iron porpyrin is a poor catalyst with hydrogen peroxide. Excessive substitutuion by electron withdrawing groups on the porphyrin periphery eventually prohibits the formation of the key intermediate in catalytic oxidations. The alternative oxidation mechanism could involve radical participation.

Ethylene polymerization and ethylene/1-hexene copolymerization using homogeneous and heterogeneous unbridged bisindenyl zirconocene catalysts

Unbridged bis-substituted-indenyl zirconocene complexes, [(2,4-Me 2Ind) 2ZrCl 2, Met-1; (2,4,6-Me 3Ind) 2ZrCl 2, Met-2], were supported on silica and montmorillonite carriers (resulting in silica-supported catalysts MS-1 and MS-2, and montmorillonite-supported catalyst MT-1). Ethylene polymerization by homogeneous and heterogeneous catalysts showed high activity, affording polyethylenes with high molecular weight. The catalytic activity and the molecular weight of the polymer were improved using the heterogeneous systems. The activities for the ethylene/1-hexene copolymerization by heterogeneous systems were lower than those using homogeneous systems, however, the comonomer was incorporated efficiently into polymer in both the homo- and the heterogeneous systems, and moreover, the microstructure of the copolymer derived from the heterogeneous catalysts showed different characteristics from those resulting from the homogeneous systems. The r E r H values of the heterogeneous catalysts (1.82 for MS-1 and 0.70 for MS-2), are quite different from those of their homogeneous analogues (1.25 for Met-1 and 1.26 for Met-2).

Novel Ruthenium-Based Metathesis Catalysts Containing Electron- Withdrawing Ligands: Synthesis, Immobilization, and Reactivity

The syntheses and reactivity of seven different ruthenium-based metathesis catalysts are described. Ru(CF3COO)2(PCy3)(=CH-2-(2-PrO)C6H4) (1), Ru(CF3COO)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (2), and Ru(CF3COO)2(PCy(3))(1,3-dimesityldihydroimidazolin-2-ylidene)(=CHC6H5) (3) were prepared via chlorine exchange by reacting RuCl2(PCy3)2(=CH-2-(2-PrO)C6H4), RuCl2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4), and RuCl2(PCy3)(1,3-dimesityldihydroimidazolin-2-ylidene)(=CHC6H5), respectively, with silver trifluoroacetate (Cy =cyclohexyl). In analogy, Ru(CF3CF2COO)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (4) and Ru(CF3CF2CF2COO)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (5) were prepared from RuCl2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) via reaction with CF3CF2COOAg and CF3CF2CF2COOAg, respectively. Ru(C6F5COO)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (6) and Ru(C6F5O)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (7) were prepared from RuCl2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) via reaction with C6F5COOTl and C6F5OTl, respectively. Supported catalysts Ru(PS-DVB-CH2OOCCF2CF2CF2COO)(CF3COO)(PCy3)(1,3-dimesityldihydroimidazolin-2-ylidene)(=CHC6H5) (8), Ru(PS-DVB-CH2OOCCF2CF2CF2COO)(CF3COO)(PCy3)(=CH-2-(2-PrO)C6H4) (9), and Ru(PS-DVB-CH2OOCCF2CF2CF2COO)(CF3COO)(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (10) were synthesized by reaction of RuCl2(PCy3)(1,3-dimesityldihydroimidazolin-2-ylidene)(=CHC6H5), RuCl2(PCy3)(=CH-2-(2-PrO)C6H4), and RuCl2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4), respectively, with a perfluoroglutaric acid-derivatized poly(styrene-co-divinylbenzene) (PS-DVB) support (silver form). Halogen exchange in PCy3-containing systems had to be carried out in dichloromethane in order to suppress precipitation of AgCl.PCy3. The reactivity of all new catalysts in ring-closing metathesis (RCM) of hindered electron-rich and -poor substrates, respectively, at elevated temperature (45 degrees C) was compared with that of existing systems. Diethyl diallylmalonate (DEDAM, 11), diethyl allyl(2-methylallyl)malonate (12), N,N-diallyl-p-toluenesulfonamide (13), N-benzyl-N-but-1-en-4-ylbut-2-enecarboxylic amide (14), and N-allyl-N-(1-carboxymethyl)but-3-en-1-yl-p-toluenesulfonamide (15) were used as educts. Supported catalysts were prepared with high loadings (2.4, 22.1, and 160 mg of catalyst/g PS-DVB for 8, 9, and 10, respectively). Catalyst 8 showed higher and catalysts 9 and 10 sowed significantly reduced activities in RCM compared to their homogeneous analogues. Thus, with 8, turnover numbers (TONs) up to 4200 were realized in stirred-batch (carousel) RCM experiments. To elucidate the nature of the bound species, catalysts 8-10 were subjected to 13C- and 31P-MAS NMR spectroscopy. These investigations provided evidence for the proposed structures. Leaching of ruthenium into the reaction mixture was low, resulting in ruthenium contents <85 ppb (ng/g) in the final RCM-derived products.

Polymer-Supported Bis(oxazolines) and Related Systems: Toward New Heterogeneous Enantioselective Catalysts

Polymer-supported bis(oxazoline) and related ligands have been prepared by grafting and by polymerization methodologies under a variety of conditions. The corresponding Cu complexes have been assayed as catalysts for the cyclopropanation of styrene in the presence of diethyl azoacetate. Data reveal that insoluble systems prepared by polymerization are much more effective that those prepared by grafting. The best performances, in particular in terms of selectivity, were obtained for tert-butylglycine-derived systems. In some cases, functional resins that are more active, chemoselective, and enantioselective than the corresponding homogeneous analogue systems have been obtained. The presence of additional functionalities in the polymeric matrix is, in general, detrimental for the performance of the supported catalyst, even if the presence of more flexible cross-linkers enhances the accessibility of the functional sites. The Copper bis(oxazoline) polymeric systems are easily recycled and reused and provide materials of high utility for their use in facilitated chemical synthesis.

Polysiloxane‐Bound Ligand Accelerated Catalysis: A Modular Approach to Heterogeneous and Homogeneous Macromolecular Asymmetric Dihydroxylation Ligands.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Polystyrene-supported dicyclohexylphenylphosphine adducts of amine- and phosphite-based palladacycles in the Suzuki coupling of aryl chlorides

The polystyrene-immobilised palladacyclic complexes [Pd(TFA)(kappa2-N,C-C6H4CH2NMe2){P(C6H4-4-PS)Cy2}] and [PdCl(kappa2-P,C-{P(OC6H2-2,4-tBu2)(OC6H3-2,4-tBu2)2}{P(C6H4-4-PS)Cy2}](PS = polystyrene) and the homogeneous analogues [Pd(TFA)(kappa2-N,C-C6H4CH2NMe2)(PPhCy2)] and PdCl(kappa2-P,C-{P(OC6H2-2,4-tBu2)(OC6H3-2,4-tBu2)2}(PPhCy2)] were synthesised and characterised. The X-ray structure of one of the homogeneous analogues, [Pd(TFA)(kappa2-N,C-C6H4CH2NMe2)(PPhCy2)] was determined. All the complexes have been tested and show good activity in the Suzuki coupling of aryl chloride substrates. While the polystyrene-immobilised complexes are not recyclable, they are easily extracted and show low levels of palladium leaching.

Peroxotungstate Immobilized on Ionic Liquid-Modified Silica as a Heterogeneous Epoxidation Catalyst with Hydrogen Peroxide

Peroxotungstate immobilized on ionic liquid-modified SiO2 is capable of heterogeneously epoxidizing a wide range of olefins with the maintenance of the catalytic activity of homogeneous analogue. The epoxidation was immediately stopped by the removal of the catalyst, and no tungsten species could be found in the filtrate after the removal of the catalyst. These results can rule out any contribution to the observed catalysis from the tungsten species that leached into the reaction solution, and the observed catalysis is truly heterogeneous in nature. Furthermore, the catalyst was reusable without the loss of the catalytic performance.

Grafted Metallocalixarenes as Single-Site Surface Organometallic Catalysts

Metallocalixarenes were grafted onto silica using a surface organometallic approach and shown to be active and selective catalysts for epoxidation of alkenes using organic hydroperoxides. Calixarene-Ti(IV) precursors were anchored at surface densities from 0.1 to near-monolayer coverages (0.025-0.25 calixarene nm(-2)). Several spectroscopic methods independently detected calixarene-Ti(IV) connectivity before and after epoxidation catalysis. Kinetic analyses of cyclohexene epoxidation confirmed that the active sites were anchored on the silica surface and were significantly more active than their homogeneous analogues. The steric bulk and multidentate binding of the calixarenes led to structural stability and to single-site behavior during epoxidation catalysis. Rate constants were independent of surface density for cyclohexene epoxidation with tert-butyl hydroperoxide (11.1 +/- 0.3 M(-2) s(-1)) or cumene hydroperoxide (25 +/- 2 M(-2) s(-1)). The materials and methods reported here allow the assembly of robust surface organometallic structures in which the active sites behave as isolated species, even near saturation monolayer coverages. In turn, this makes possible the rational design and synthesis of a class of heterogeneous oxide catalysts with atomic-scale precision at the active site.

Novel Silica Gel Supported Chiral Biaryl‐Diphosphine Ligands for Enantioselective Hydrogenation.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Use of Nanosized Catalysts for Transformation of Chloro-Organic Pollutants

A new method to transform anthropogenic, chloro-organic compounds (COC) by use of nanosized molecular catalysts immobilized in sol-gel matrixes is presented. COC represent a serious threat to soil and groundwater quality. Metalloporphyrinogens are nanometer sized molecules that are known to catalyze degradation of COC by reduction reactions. In the current study, metalloporphyrinogens were immobilized in sol--gel matrixes with pore throat diameters of nanometers. The catalytic activity of the matrix arrays for anaerobic reduction of tetrachloroethylene (PCE), trichloroethylene (TCE), and carbon tetrachloride (CT) was examined. Experiments were performed under conditions pertinent to groundwater systems, with titanium citrate and zero-valent iron as electron donors. All chloroorganic compounds were reduced in the presence of several sol-gel-metalloporphyrinogen hybrids (heterogeneous catalysts). For example, cobalt-5,10,15,20-(4-hydroxyphenyl)-21H,23H-porphine (TP(OH)P-Co) and cyanocobalamin (vitamin B12) reduced CT concentrations to less than 5% of their initial values in a matter of hours. Cyanocobalamin was found to reduce PCE to trace amounts in less than 48 h and TCE to less than 25% of its initial concentration in 144 h. The reactions were compared to their homogeneous (without sol-gel matrix) analogues. The reduction activity of COC for the homogeneous and heterogeneous systems ranged between similar reactivity in some cases to lower reduction rates for the heterogeneous system. These lower rates are, however, compensated by the ability to encapsulate and reuse the catalyst. Experiments with cyanocobalamin showed that the catalyst could be reused over at least 12 successive cycles of 24 h each.

The volume operator in spherically symmetric quantum geometry

The spherically symmetric volume operator is discussed and all its eigenstates and eigenvalues are computed. Even though the operator is more complicated than its homogeneous analogue, the spectra are related in the sense that the larger spherically symmetric volume spectrum adds fine structure to the homogeneous spectrum. The formulae of this paper complete the derivation of an explicit calculus for spherically symmetric models which is needed for future physical investigations.

Improved Palladium and Nickel Catalysts Heterogenised on Oxidic Supports (Silica, MCM‐41, ITQ‐2, ITQ‐6)

AbstractTwo series of solid catalysts in which a chiral palladium or nickel complex with Schiff bases as ligands have been immobilized on ordered mesoporous silica supports (MCM‐41), delaminated ITQ‐2, ITQ‐6 zeolites and amorphous silica have been prepared. Hydrogenation of alkenes and imines was studied with the homogeneous as well as with the counterpart heterogenized catalysts. The high accessibility introduced by the structure of the supports allows the preparation of highly efficient immobilized catalysts with TOFs of 1,000,000 h−1. A moderate acidity in the support increases the catalytic activity considerably, and the easy recoverable immobilized catalysts can duplicate the activity of the homogeneous analogues. No deactivation of the catalysts was observed after repeated recycling.

Effect of site isolation on the preparation and performance of silica-immobilized Ti CGC-inspired ethylene polymerization catalysts

Titanium constrained-geometry-inspired complexes (CGCs) are assembled using several different synthetic protocols on aminosilica scaffolds. In particular, a new synthetic method is reported that utilizes spatially patterned amines on the silica surface to create Ti–CGC sites that are substantially more active for ethylene polymerization than materials prepared using traditional methods. The materials are characterized using multiple techniques including thermogravimetric analysis (TGA), nitrogen physisorption, FT-Raman spectroscopy, 29Si and 13C cross-polarization magic-angle spinning (CPMAS) NMR spectroscopy, and diffuse–reflectance UV–visible (UV–vis) spectroscopy. While the new patterning protocol allows for quantitative addition of the cyclopentadienyl group to the surface amines and near quantitative metallation with the titanium source, these steps result in subquantitative additions on aminosilica surfaces prepared via traditional techniques (at high amine loadings) and excess titanium addition (at low amine loadings). Using methylaluminoxane (MAO) as a co-catalyst, the patterned catalyst is more productive than the control materials that were prepared using traditional techniques. However, use of MAO causes significant leaching of the metal complex from the solid support. Using a tris(pentafluorophenyl)borane/trialkylaluminum system as co-catalyst alleviates the leaching problem, allowing for the productivity of the immobilized species to be evaluated. In the polymerization of ethylene, the patterned catalyst is shown to be up to 10 times more productive than the solid control materials. The patterned catalyst is also more active than the homogeneous analogue.

Novel silica gel supported chiral biaryl-diphosphine ligands for enantioselective hydrogenation

The synthesis of functionalized Biphemp and MeO-Biphep biaryl diphosphine ligands and their covalent attachment to silica gel are described. The catalytic performance of the immobilized ligands was tested in the asymmetric hydrogenation of methyl acetamidocinnamate (MAC) with Rh and of methyl phenylglyoxylate with Ru and compared with that of the homogeneous analogues. With the exception for a Rh catalyzed hydrogenation, where an increase of ee from 29% for the unfunctionalized ligand, to 40% for the functionalized ligand and 45% for the immobilized ligand was observed, functionalization and immobilization did not significantly affect the catalytic properties. The best ees of 90% were obtained for the Ru catalyzed hydrogenation of methyl phenylglyoxylate with the immobilized MeO-Biphep ligand and are comparable with those of the homogeneous catalyst. Recycling of the immobilized catalysts resulted in a significant drop in activity for the Rh catalysts, whereas the Ru catalysts were much more robust and could be used in >10 catalytic runs.

Heterogeneous Bifunctional Catalysts for the Synthesis of Chiral Vicinal Diols

Heterogeneous bifunctional catalysts consisting of palladate–osmate and osmate–tungstate developed by the ion exchange on quaternary ammonium salts covalently bound to resin, and their homogeneous bimetallic analogs are evaluated for the synthesis of chiral vicinal diols. The heterogeneous bifunctional catalyst (resin-PdOs) and its homogeneous analog are used in the tandem Heck-asymmetric dihydroxylation (AD) to afford diols with excellent yields and enantiomeric excesses (ee's) in the presence of various co-oxidants. The other bifunctional catalyst (resin-OsW) and its homogeneous analog are used in a simultaneous asymmetric dihydroxylation-N-oxidation of a wide range of olefins to obtain chiral vicinal diols with higher yields and ee's using H2O2 as terminal oxidant. The bifunctional resin catalysts are recovered quantitatively by a simple filtration and reused for a number of cycles with consistent activity.

Carbonylative ring expansion of aziridines to beta-lactams with rhodium-complexed dendrimers on a resin.

Rhodium-complexed dendrimers, supported on a resin, were evaluated as catalysts for the carbonylative ring expansion reactions of a variety of aziridines with carbon monoxide to give beta-lactams. The effects of reaction temperature, solvent, time, and pressure of carbon monoxide on this transformation were also investigated. The dendritic catalysts showed comparable activity to the homogeneous analogue. More importantly, this catalytic system can be easily recovered by simple filtration and recycled without significant loss of activity.

Hybrid organic—inorganic catalysts: a cooperative effect between support, and palladium and nickel salen complexes on catalytic hydrogenation of imines

A method for immobilizing chiral salen palladium and nickel complexes (salen = (R,R)- N′, N′- bis-(3,5-di- tert-butylsalicylidene)-1,2-cyclohexanediamine) on ordered mesoporous silica supports (MCM-41), delaminated ITQ-2 and ITQ-6 zeolites, and amorphous silica has been developed. Hydrogenation of imines was studied with the homogeneous as well as with the heterogenized counterpart catalysts. The high accessibility introduced by the structure of the supports allows the preparation of highly efficient immobilized catalysts with TOFs of 200×10 3 h −1. A moderate acidity in the support increases the catalytic activity by stabilizing the charged transition state and can duplicate the activity of the homogeneous analogues. No deactivation of the catalysts was observed after repeated recycling.

Facile and rapid immobilization of copper(II) bis(oxazoline) catalysts on silica: application to Diels–Alder reactions, recycling, and unexpected effects on enantioselectivity

Two chiral copper(II) bis(oxazoline) complexes have been immobilized on silica via electrostatic interactions using a remarkably straightforward procedure. The immobilized catalysts were tested in a standard Diels–Alder reaction and gave surprising results. Where the immobilized Cu(( S, S)-phenyl-box)(OTf) 2 catalyst was used, the predominant enantiomer formed was the opposite of that produced using the same catalyst in a homogeneous reaction. This is a startling result given that the only difference is the electrostatic immobilization of the catalyst on amorphous silica. The activity of the catalyst in a hetero Diels–Alder reaction was also tested. This catalyst was also recycled, successfully maintaining a similar activity to the homogeneous analogue through a number of cycles.