Alkoxide Catalyst Research Articles

ChemInform Abstract: Multiple Bonds Between Main-Group Elements and Transition-Metals. Part 100 (Section 2). Methyltrioxorhenium as a Catalyst for Olefin Oxidation.

ChemInform Abstract: Multiple Bonds Between Main-Group Elements and Transition-Metals. Part 100 (Section 2). Methyltrioxorhenium as a Catalyst for Olefin Oxidation.

ChemInform Abstract: Asymmetric Kharasch Reaction: Catalytic Enantioselective Allylic Oxidation of Olefins Using Chiral Pyridine Bis(diphenyloxazoline)-Copper Complexes and tert-Butyl Perbenzoate.

Copper complexes of chiral pyridine bis(diphenyloxazoline)-type ligands have been studied as catalysts for the enantioselective allylic oxidation of olefins. Using 2.5−5 mol % of these chiral catalysts and tert-butyl perbenzoate as oxidant, optically active allylic benzoates were obtained in up to 86% ee. A variety of copper salts was studied under different conditions and in different solvents. Acetone was found to be a superior solvent for the reaction. Use of phenylhydrazine in conjunction with the chiral copper complex played a crucial role in increasing the rate of the reaction. Use of 4 A molecular sieves increased the optical yield of product in almost every case.

Ethylene Epoxidation on a Au Nanoparticle versus a Au(111) Surface: A DFT Study

The heterogeneously catalyzed epoxidation of alkenes is experimentally challenging, theoretically interesting, and technologically of vital importance. Recent experimental studies show that small gold nanoparticles supported on inert materials are efficient and robust catalysts for the selective oxidation of alkenes. The reasons for the outstanding catalyst of Au nanoparticles have been investigated and compared with the Au(111) surface by means of density functional theory. The nanoparticle is intrinsically much more selective than the surface in the epoxidation. The fundamental cause is the inversion in the ordering of activation barriers for the competing pathways to epoxide formation versus acetaldehyde formation. On the nanoparticle, epoxide formation is less activated than acetaldehyde formation, whereas the opposite is true on the (111) surface. This behavior is associated with a late transition state to epoxidation on the nanoparticle (i.e., product-like) compared to an early (reactant-like) transition state to epoxidation on the (111) surface.

Polymer-supported titanate as catalyst for the transesterification of acrylic monomers

Transesterification of (meth)acrylic esters catalysed by alkyl titanates is an important industrial reaction. The current processes are homogeneous but heterogeneous alternatives are attractive for cost and environmental reasons. We have developed a straightforward and cost-effective synthesis of a polymer-supported titanium alkoxide catalyst involving cheap, readily available and relatively non-toxic chemicals, solvents and workup procedures. Efficiency and stability of the catalyst were tested in a laboratory-scale continuous flow reactor under equilibrium conditions for the preparation of N,N-dimethylaminoethylacrylate, a chemical of industrial interest. The results obtained indicate a good stability of the system to metal leaching on the long term.

Effective Oxidation of Secondary Amines to Nitrones with Alkyl Hydroperoxides Catalysed by (Trialkanolaminato)titanium(IV) Complexes

AbstractThe effective catalytic oxidation of secondary amines to nitrones with alkyl hydroperoxides as the primary oxidants is described. The titanium alkoxide catalysts are protected from the water co‐product by the combined use of a tightly binding trialkanolamine ligand and molecular sieves. Nitrones can be obtained in high yields (up to 98 %) under homogeneous, anhydrous conditions and even in the absence of solvent. The reactions are fast (2–7 h) and good selectivity and complete conversion can be achieved with as little as 1 % catalyst.

Polyoxometalate–molybdenylacetylacetonate hybrid complex: A reusable and efficient catalyst for oxidation of alkenes with tert-butylhydroperoxide

The hybrid complex consist of molybdenylacetylacetonate complex covalently linked to a lacunary Keggin-type polyoxometalate, was synthesized and characterized by elemental analysis, SEM, diffuse reflectance UV–Vis and FT-IR spectroscopic methods. The hybrid complex, [MoO 2 (acac)–POM], was used for alkene epoxidation with tert -BuOOH in 1,2-dichloroethane as solvent. This heterogeneous catalyst was reused several times in the oxidation of cyclooctene. The hybrid complex consist of molybdenylacetylacetonate complex covalently linked to a lacunary Keggin-type polyoxometalate, K 8 [SiW 11 O 39 ] (POM), was synthesized and characterized by elemental analysis, SEM, XRD, diffuse reflectance UV–Vis and FT-IR spectroscopic methods. The hybrid complex, [MoO 2 (acac)–POM] ( 1 ), was used for alkene epoxidation with tert -BuOOH in 1,2-dichloroethane as solvent. The complex ( 1 ) can catalyze epoxidation of various olefins including non-activated terminal olefins. The effect of reaction parameters such as oxidant, solvent, and temperature on the epoxidation of cyclooctene was also investigated. This heterogeneous catalyst was reused several times in the oxidation of cyclooctene.

Kinetics of catalytic Meerwein-Ponndorf-Verley reduction of aldehydes and ketones using boron triethoxide

AbstractCatalytic Meerwein-Ponndorf-Verley (MPV) reduction of various aliphatic, aromatic, and unsaturated aldehydes and ketones to corresponding alcohols (analyzed by GC-MS) in the presence of boron triethoxide (B(OEt)3) were studied. Kinetics of this reduction reaction was also studied and the respective rate constants were determined. It was found that B(OEt)3 catalyzes the reduction of aliphatic aldehydes and ketones to alcohols at room temperature while aromatic aldehydes and ketones were not reduced under the same conditions. In addition, MPV reduction using B(OEt)3 was found to be chemoselective as unsaturated aldehydes and ketones afforded the corresponding alcohols without affecting unsaturated groups. The mechanism proposed involves a six-membered transition state in which both the alcohol and the carbonyl are coordinated to the same boron centre of a boron alkoxide catalyst.

Heteropolyacid-conjugated chitosan matrix for triphase catalyst

H 3PW 12O 40 (12-phosphotungstic acid; PWA) is efficient catalysts for the selective oxidation of alkanes, aldehydes, and perfluorinated acids. However, PWA molecules are high solubility in water and this property has been making it difficult to utilize as an industrial catalyst. In this study, the water-insoluble PWA-conjugated chitosan matrix was prepared by mixing the aqueous PWA and chitosan solutions. This conjugate matrix was constructed by the electrostatic interaction between amino group of chitosan and negative charge of PWA and showed the thermal stability at <100 °C. In addition, the PWA-conjugated chitosan matrix indicated the property for triphase catalyst as epoxidation of allylic alcohols, such as cinnamyl alcohol, farnesol, and so on. The isolated yield of epoxidation with the conjugate matrix was as same as that of pure PWA. Furthermore, since this conjugate matrix did not dissolve in water, matrices could easily separate from the reaction solvent and recycle more than 3 times. Therefore, the PWA-conjugated chitosan matrices may have a potential to utilize the novel catalyst for the epoxidation of various allylic alcohols.

Silica‐Supported Zirconium Complexes and their Polyoligosilsesquioxane Analogues in the Transesterification of Acrylates: Part 2. Activity, Recycling and Regeneration

AbstractThe catalytic activity of both supported and soluble molecular zirconium complexes was studied in the transesterification reaction of ethyl acrylate by butanol. Two series of catalysts were employed: three well defined silica‐supported acetylacetonate and n‐butoxy zirconium(IV) complexes linked to the surface by one or three siloxane bonds, (SiO)Zr(acac)3 (1) (SiO)3Zr(acac) (2) and (SiO)3Zr(O‐n‐Bu) (3), and their soluble polyoligosilsesquioxy analogues (c‐C5H9)7Si8O12(CH3)2Zr(acac)3 (1′), (c‐C5H9)7Si7O12Zr(acac) (2′), and (c‐C5H9)7Si7O12Zr(O‐n‐Bu) (3′). The reactivity of these complexes were compared to relevant molecular catalysts [zirconium tetraacetylacetonate, Zr(acac)4 and zirconium tetra‐n‐butoxide, Zr(O‐n‐Bu)4]. Strong activity relationships between the silica‐supported complexes and their polyoligosilsesquioxane analogues were established. Acetylacetonate complexes were found to be far superior to alkoxide complexes. The monopodal complexes 1 and 1′ were found to be the most active in their respective series. Studies on the recycling of the heterogeneous catalysts showed significant degradation of activity for the acetylacetonate complexes (1 and 2) but not for the less active tripodal alkoxide catalyst, 3. Two factors are thought to contribute to the deactivation of catalyst: the lixivation of zirconium by cleavage of surface siloxide bonds and exchange reactions between acetylacetonate ligands and alcohols in the substrate/product solution. It was shown that the addition of acetylacetone to the low activity catalyst Zr(O‐n‐Bu)4 produced a system that was as active as Zr(acac)4. The applicability of ligand addition to heterogeneous systems was then studied. The addition of acetylacetone to the low activity solid catalyst 3 produced a highly active catalyst and the addition of a stoichiometric quantity of acetylacetone at each successive batch catalytic run greatly reduced catalyst deactivation for the highly active catalyst 1.

Synthesis, characterization and crystal structure of a dioxomolybdenum(VI) complex with a N,O type bidentate Schiff base ligand as a catalyst for homogeneous oxidation of olefins

The synthesis of a Mo(VI) Schiff base complex, cis-[MoO 2{(4,6-bis( tert-butyl)-2-{(benzyl)iminomethyl}phenolate) 2}], cis-[MoO 2(L) 2] where L = 4,6-bis( tert-butyl)-2-{(benzyl)iminomethyl}phenol, derived from benzylamine and 3,5-di- tert-butylsalycilaldehyde is reported. Full characterization of this complex was accomplished with elemental analyses, spectroscopic studies (NMR, IR and electronic) and X-ray structure analysis. This complex was tested as a catalyst for the homogeneous oxidation of olefins. The Mo(VI) complex is catalytically active for the epoxidation of aliphatic substrates at 80 °C, yielding the epoxide as the sole product in yields up to 100% and turnover numbers up to 5000. Under the optimized conditions styrene was oxidized in an 81% conversion to produce styrene oxide, benzaldehyde, and acetophenone.

Poly(propylene imine) dendrimer complexes as catalysts for oxidation of alkenes

Complexes of poly(propylene imine) dendrimers D8[DAB-dendr-(NH 2) 8] and D32 [DAB-dendr-(NH 2) 32] were prepared by interaction of the dendrimers with transition metal salts such as FeCl 3.6H 2O; CoCl 2.6H 2O; CuCl 2.2H 2O; VOSO 4.5H 2O; Na 2MoO 4.2H 2O and Na 2WO 4.2H 2O at room temperature in aqueous solutions. The content of metal ions in the complexes was found to be from 8.2 to 69.6 mg metal ion/g polymer carrier. The complexes were characterized by using IR, UV–VIS, Moessbauer spectroscopy and EPR. The anticipated co-ordination structure of the compounds was suggested. It was found that the order of the catalytic activity of the complexes of poly(propylene imine) dendrimers D8 and D32 in the reaction of epoxidation of cyclohexene with organic hydroperoxides such as tert-butyl hydroperoxide ( t-BHP), ethylbenzene hydroperoxide (EBHP) and cumene hydroperoxide (CHP) was as follows: D32-MoО 2 2+>D32-VО 2+>D32-WО 2 2+ > D32-Co 2+ > D32-Cu 2+>D32-Fe 3+. The order of reactivity of organic hydroperoxides in the reaction studied was: t-BHP > EBHP > CHP.

SBA immobilized phosphomolybdic acid: Efficient hybrid mesostructured heterogeneous catalysts

SBA-15 and SBA-3 mesoporous silicas are synthesised by P123 and CTAB surfactants via hydrothermal and liquid phase deposition procedures, respectively. An inorganic–organic hybrid mesoporous material is then synthesised by functionalization of SBA-15 with aminopropyl functional groups via grafting method. After characterization, effect of immobilizing support and functional groups on intercalation of phosphomolybdic acid (H 3PMo 12O 40) is taken into consideration. The immobilization pattern is discussed and supported H 3PMo 12O 40 catalysts are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), EDX, inductively coupled plasma (ICP), FT-IR, and UV–vis analysis. The newly synthesised hybrid catalysts are investigated for epoxidation of cyclooctene in presence of hydrogen peroxide as oxidant. The reaction mechanism is discussed. Furthermore, effects of different immobilizing supports and functionalization on catalyst activity, stability, and reusability are taken into consideration. Similar catalytic reactions are carried out with pristine supports and neat H 3PMo 12O 40 (homogeneous). Results reveal that the mesostructured phosphomolybdic acid based catalysts are shown to be efficient and selective heterogeneous catalysts for oxidation of alkenes.

Ring-opening polymerization ofl-lactide with silica supported titanium alkoxide catalysts

TiCl(O-i-Pr)3/SiO2 and Ti(O-i-Pr)4/SiO2 were prepared by immobilizing chlorotitanium (IV) isopropoxide (TiCl(O-i-Pr)3) and titanium (IV) isopropoxide (Ti(O-i-Pr)4), to pretreated silica. The effect of the polymerization reaction conditions on the catalytic activity and characteristics of the resulting PLA were investigated. The catalytic conversion, molecular weight and polydispersity index (PDI) of the PLA produced on the titanium alkoxide supported catalysts increased proportionally with the reaction temperature. When the PLA was synthesized in bulk polymerization, the PLA produced with the supported catalysts had higher molecular weight than those with homogeneous catalysts. The melting temperature of the polymer produced with silica supported alkoxide catalysts was approximately 170-180 °C.

Cerium(IV) Catalysts for the Ring-Opening Polymerization of Lactide

A rare example of a cerium(IV) alkoxide catalyst for lactide polymerization is reported. The lactide polymerization activity of the new cerium(IV) complex supported by a ferrocene Schiff base ligand, salfen, is compared to the activity of the yttrium analogue and to that of Ce(O(t)Bu)(4)(THF)(2). The complex Ce(salfen)(O(t)Bu)(2) is less active than Ce(O(t)Bu)(4)(THF)(2) and the corresponding yttrium(III) alkoxide, Y(salfen)(O(t)Bu)(THF). The different activity was correlated with the Mulliken charges calculated by density functional theory for the two complexes.

Surface-Initiated Titanium-Mediated Coordination Polymerization from Catalyst-Functionalized Single and Multiwalled Carbon Nanotubes

Single (SWNTs) and multiwalled (MWNTs) carbon nanotubes were functionalized with a titanium alkoxide catalyst through a Diels−Alder cycloaddition reaction. The catalyst-functionalized carbon nanotubes (CNTs) were used for the surface initiated titanium-mediated coordination polymerizations of l-lactide (l-LA), ϵ-caprolactone (ϵ-CL) and n-hexyl isocyanate (HIC) employing the “grafting from” technique. 1H NMR, IR and Raman spectra showed that the precursor catalyst was successfully synthesized and covalently attached on the CNTs surface. Thermogravimetric analysis (TGA) revealed that the grafted poly(l-lactide) (PLLA) content could be controlled with time. The final polymer-grafted CNTs were readily dissolved in organic solvents as compared to the insoluble pristine and catalyst-functionalized CNTs. The presence of thick layers of polymers around the CNTs was observed through transmission electron microscopy (TEM). Differential scanning calorimetry (DSC) proved that the glass transition (Tg) and melting (Tm...

Verkade super base-catalysed transesterification of propylene carbonate with methanol to co-produce dimethyl carbonate and propylene glycol

The co-generation of dimethyl carbonate (potential fuels additive, environmentally friendly replacement for more toxic methylating agents, etc.) and propylene glycol via the transesterification reaction of propylene carbonate and methanol was studied using base catalysis. Verkade super bases (proazaphosphatranes) proved especially effective at low catalyst loadings (0.5%) for the rapid transesterification of propylene carbonate with methanol under mild conditions and with high product selectivity (typically >99.5%). These bases neatly avoid the use of alkoxide catalysts which generate by-products in their neutralisation. The kinetics of the reactions were studied using data obtained from in situ NMR reaction monitoring. The influences of several reaction optimisation parameters are also dealt with.

Towards robust alkane oxidation catalysts: electronic variations in non-heme iron(ii) complexes and their effect in catalytic alkane oxidation

A series of non-heme iron(ii) bis(triflate) complexes containing linear and tripodal tetradentate ligands has been prepared. Electron withdrawing and electron donating substituents in the para position of the pyridine ligands as well as the effect of pyrazine versus pyridine and sulfur or oxygen donors instead of nitrogen donors have been investigated. The electronic effects induced by these substituents influence the strength of the ligand field. UV-vis spectroscopy and magnetic susceptibility studies have been used to quantify these effects and VT (1)H and (19)F NMR spectroscopy as well as X-ray diffraction have been used to elucidate structural and geometrical aspects of these complexes. The catalytic properties of the iron(ii) complexes as catalysts for the oxidation of cyclohexane with hydrogen peroxide have been evaluated. In the strongly oxidising environment required to oxidise alkanes, catalyst stability determines the overall catalytic efficiency of a given catalyst, which can be related to the ligand field strength and the basicity of the ligand and its propensity to undergo oxidation.

An efficient, practical and cost-effective polymer-supported catalyst for the transesterification of methyl methacrylate by 1-butanol

A straightforward and cost-effective synthesis of a polymer-supported titanium alkoxide catalyst is reported. The described synthesis involves cheap, readily available and relatively non toxic chemicals, solvents and workup procedures. Efficiency and stability of the catalyst were tested in a laboratory-scale continuous flow reactor under equilibrium conditions over a month period. The average daily titanium leaching was estimated to less than 1% of the total amount of titanium engaged. To our knowledge, this result is the best obtained, to date, concerning the use of heterogeneous titanate for the catalysis of transesterification reactions.

Characteristics of heterogeneous titanium alkoxide catalysts for ring-opening polymerization of lactide to produce polylactide

TiCl(O- i-Pr) 3/SiO 2 and Ti(O- i-Pr) 4/SiO 2 were prepared by immobilizing titanium(IV) chloride triisopropoxide (TiCl(O- i-Pr) 3) and titanium(IV) tetraisopropoxide (Ti(O- i-Pr) 4), respectively, over pretreated silica. A series of solution polymerizations of l-lactide were conducted with homogeneous alkoxide catalysts and silica-supported titanium alkoxide catalysts to compare their catalytic activity and the characteristics of the polylactide (PLA) produced. The conversion of l-lactide with the silica-supported alkoxide catalysts was higher than that with homogeneous catalysts even though the catalyst activity was low. In addition, the PLA produced using silica-supported alkoxide catalysts had a higher molecular weight and T m than those produced with the homogeneous catalyst.

Kinetic understanding of asymmetric amplification in amino-alcohol catalyzed organozinc additions

The dynamic features of Noyori’s mechanism for the dimethylaminoisoborneol (DAIB) catalyzed alkylzinc addition to benzaldehyde have been investigated using the whole set of differential equations that describe the evolution of 19 species (starting products, intermediates, final compounds) involved in 36 coupled reversible or non-reversible processes. Using all the data available in the literature regarding the kinetic and thermodynamic parameters, the modeling confirms that the non-linear correlation between the enantiomeric excess of the chiral auxiliary (ee aux) and that of the product (ee prod) originates from the accumulation of the heterochiral alkylzinc amino alkoxide dimer (a reservoir effect). The variations of the enantiomeric excess of the product versus the benzaldehyde conversion are shown. Inhibition by the product can be understood by considering a reversible interaction between the alkoxide produced and the monomeric alkylzinc amino alkoxide catalyst. The effect of the order of the reactant addition on the ee prod and the rate of reaction as observed with 1-piperidino-3,3-dimethyl-2-butanol (PDB) instead of DAIB can be simulated without changing the relative thermodynamic stabilities of the homo- and heterochiral dimers although at the expense of a slowing-down of their equilibration rates.