Enantioselective electrocatalytic epoxidation of olefins by chiral manganese Schiff-base complexes

Asymmetric electrocatalytic epoxidation of olefins has been achieved with chiral manganese Schiff-base complexes immobilized on a glassy carbon electrode surface using molecular dioxygen as oxidant. The electrocatalytic system gives moderate enantiomeric excess (ee) values (65–77%) for the epoxidation of cis-stilbene, trans-stilbene and styrene. Our results indicated that the catalyst turnover number is significantly improved when the manganese complexes are immobilized on the electrode surface, which can be attributed to the suppression of the formation of inactive manganese dimer when the active metal centres are attached to the polymer network.     

Tethering of nickel(II) Schiff-base complex onto mesoporous silica: An efficient heterogeneous catalyst for epoxidation of olefins

A new hybrid catalyst has been prepared by tethering a nickel(II) Schiff-base complex via post-synthesis modification of mesoporous silica, MCM-41. The Schiff-base has been derived from salicylaldehyde and 3-aminopropyltriethoxysilane (3-APTES) which is chemically anchored on MCM-41 via silicon alkoxide route. The anchored Schiff-bases imposed a stable planar coordination geometry around the central nickel ions. The catalyst has been characterized by elemental analysis, FT-IR, UV-Vis, small angle X-ray diffraction (SAX) and transmission electron microscopy (TEM) studies. The SAX and TEM measurement showed the mesoporosity of the catalyst. The activity of the catalyst has been assessed in the epoxidation of olefins using tert-butyl-hydroperoxide (tert-BuOOH) as oxidant in heterogeneous condition. Immobilized nickel catalyst was found to be catalytically more active and selective compared to the similar type of nickel(II) complex as well as Ni(NO₃)₂·6H₂O in homogeneous media. The catalyst can be recycled and reused several times without significant loss of activity.     

Comparison of Ta–MCM-41 and Ti–MCM-41 as catalysts for the enantioselective epoxidation of styrene with TBHP

Chiral Ti–MCM-41 and Ta–MCM-41 catalysts have been prepared by grafting of Ti(OⁱPr)₄ and Ta(OEt)₅ and the modification with R-(+)-diethyl l-tartrate or R-(+)-diisopropyl l-tartrate. In general, the solid catalysts are more active and selective than their homogeneous counterparts in the epoxidation of styrene with tert-butyl hydroperoxide. The enantioselectivities depend on both the nature of the chiral ligand and the calcination temperature of the support, as it is supposed this controls the type of surface species that are formed. The best result of 71% ee is obtained with DIPT–Ta–MCM₅₅₀ and is the first example of the use of a Ta catalyst for the enantioselective epoxidation of unfunctionalized alkenes. Nonetheless, the recovered Ta catalysts are less active and selective.     

Synthesis,characterization and pronounced epoxidation activity of cis-dioxo-molybdenum(VI) tridentate Schiff base complexes using tert-butyl hydroperoxide

The synthesis and catalytic performance of novel cis-dioxo-Mo(VI) complexes containing simple ONO tridentate Schiff base ligands in the epoxidation of various olefins using tert-butyl hydroperoxide in desired times with excellent chemo- and stereoselectivity have been described. The study of turnover numbers and the UV–Vis spectra of the Mo complexes in the present epoxidation system indicate well the high efficiency and stability of the catalysts during the reaction. The electron-deficient and bulky groups on the salicylidene ring of the ligand promote the effectiveness of the catalyst.     

Preparation and catalysis of NaY-encapsulated Mn(III) Schiff-base complex in presence of molecular oxygen

The new NaY zeolite (DMY), whose cages' size had been expanded, was used as supports for encapsulating the large Schiff-base complexes. The catalysts were used in the aerobic epoxidation of 1-octene. The catalyst showed high activity and selectivity for the substrate and products. These catalysts proved to be more active than other kind of metal ion or supports. The effect of temperature, organic acid or base, and the amount of catalyst was discussed.     

Synthesis and characterization of an unsymmetric salicylaldimine ligand derived from 1-(2-Aminoethyl) piperazine and investigation of its analytical properties for the extraction and preconcentratoion of some divalent cations

The synthetic, structural, spectroscopic and analytical properties of steric hindered Schiff-base ligand [N-(3,5-di-tert-butylsalicylaldimine)-1-(2-Aminoethyl) piperazine (HL)] and its mononuclear Cu(II), Co(II) and Ni(II) complexes are described. The new unsymmetric steric hindered Schiff base ligand containing a donor set of NONO was prepared by the reaction of 1-(2-Aminoethyl) piperazine with 3,5-di-tert-butylsalicylaldehyde. Certain metal complexes of this ligand were synthesized by treating an ethanolic solution of the ligand with an equimolar amount of metal salts. The ligand and its metal complexes were characterized by FT-IR, UV-Vis, ¹H NMR, elemental analysis, molar conductivity and magnetic susceptibility techniques. The reaction of this ligand in a 1:2 mole ratio with metal acetate afforded mononuclear metal complexes. The molar conductivity (??_M) values of the metal complexes of Ni(II), Co(II) and Cu(II) were in the range of 6.4 to 9.8 ??^?1 cm² mol^?1 at room temperature. Preconcentration and separation of Cu²⁺ from aqueous solution using N-(3,5-di-tert-butylsalicylaldimine)-1-(2-Aminoethyl) piperazine (HL) as a new extractant were studied. The extraction experiments were carried out at various pHs. While Cu²⁺ showed the highest extractability and selectivity at pH 7.0, extractions of Co²⁺ and Ni²⁺ were unsuccessful due to precipitate formation.     

Crystal structure of an unsymmetrical Schiff base, immobilization of its cobalt and manganese complexes on a silica support, and catalytic studies

An unsymmetrical tetradentate Schiff base (H₂L) was synthesized by the reaction of 3-methoxysalicylaldehyde, o-phenylenediamine, and salicylaldehyde in EtOH. H₂L was characterized by single-crystallographic X-ray analysis. Its Co(II) and Mn(III) complexes ([CoL] and [MnLCl]) were prepared and immobilized on 3-aminopropyltriethoxysilane functionalized silica gel. The immobilized materials were found to be efficient catalysts for epoxidation of styrene in the presence of tert-BuOOH in acetonitrile at 40 °C. The catalysts can be reused several times without significant loss of performance.     

Nanoscaled Copper Metal–Organic Framework (MOF) Based on Carboxylate Ligands as an Efficient Heterogeneous Catalyst for Aerobic Epoxidation of Olefins and Oxidation of Benzylic and Allylic Alcohols

Aerobic epoxidation of olefins at a mild reaction temperature has been carried out by using nanomorphology of [Cu₃(BTC)₂] (BTC=1,3,5‐benzenetricarboxylate) as a high‐performance catalyst through a simple synthetic strategy. An aromatic carboxylate ligand was employed to furnish a heterogeneous copper catalyst and also serves as the ligand for enhanced catalytic activities in the catalytic reaction. The utilization of a copper metal–organic framework catalyst was further extended to the aerobic oxidation of aromatic alcohols. The shape and size selectivity of the catalyst in olefin epoxidation and alcohol oxidation was investigated. Furthermore, the as‐synthesized copper catalyst can be easily recovered and reused several times without leaching of active species or significant loss of activity.     

Transition-metal ligands bound onto the micelle-templated silica surface

This paper reviews recent works on the design of immobilized transition-metal ligands on solid supports. After an overview of some results concerning ligand anchorage on polymeric supports and encapsulation of transition-metal complexes inside layered or zeolitic minerals, the grafting of ligands onto the silicic wall surface of micelle-templated silicas (MTS) is reported. MTS silicas featuring a regular mesoporous system of pore-monodispersed size and exhibiting larger pores than zeolites, provide a new opportunity to allow anchorage of organic moieties through the silanation procedure. Mn(III) Salpr and tSalpr complexes bound onto the MTS surface are active in epoxidation reaction using PhIO as oxygen donor. Anchorage of (1R,2S)-ephedrine has been also investigated with the aim to obtain benefit from the MTS structure effect. These new supported chiral catalysts are active in alkylation of benzaldehyde with diethylzinc although less enantioselective than the corresponding homogeneous catalyst. The effect of dispersion of active sites and of surface passivation has been investigated and discussed in terms of the nature of the support surface.     

Synthesis and catalytic activity of heterogeneous rare-earth metal catalysts coordinated with multitopic Schiff-base ligands

Four multitopic Schiff-base ligand precursors were synthesized via condensation of 4,4'-diol-3,3'-diformyl-1,1'-diphenyl or 1,3,5-tris(4-hydroxy-5-formylphenyl)benzene with 2,6-diisopropylaniline or 2,6-dimethylaniline. Amine elimination reactions of Ln[N(SiMe(3))(2)](3) (Ln = La, Nd, Sm or Y) with these multitopic ligand precursors gave ten heterogeneous rare-earth metal catalysts. These heterogeneous rare-earth metal catalysts are active for intramolecular hydroalkoxylation of alkynols, and the catalytic activities are influenced by the ligand and metal ion. The recycling experiment on the most active heterogeneous catalyst showed the catalyst has a good reusability.     

Chiral bis(2-oxazolinyl)xanthene (xabox)/transition-metal complexes catalyzed 1,3-dipolar cycloaddition reactions and Diels-Alder reactions

A series of chiral 4,5-bis(2-oxazolinyl)-(2,7-di-tert-butyl-9,9-dimethyl)-9H-xanthenes (xabox) and their transition-metal complexes were synthesized. The X-ray analysis of xabox-RhCl₃ complex shows a unique facial type structure. Xabox-Bn-Mn(II) and xabox-Bn-Mg(II) complexes were found to be efficient catalysts in nitrone 1,3-dipolar cycloaddition (1,3-D.C.) reaction resulting in good to excellent enantioselectivities ranging from 96:4 to >99:1of endo/exo ratio and 91-96% ee for the endo adduct. The correlation between enantiomeric excess of the ligand and the product in the nitrone 1,3-D.C. reaction shows a clear linear relationship, which suggests xabox-metal catalyst worked as a single molecular catalyst. In addition, xabox-i-Pr-Mn(II) complex was also found to be an active catalyst for Diels-Alder (D-A) reaction of acryloyloxazolidinone and cyclopentadiene affording the corresponding cycloadduct in quantitative yield along with 82% ee and 98:2 endo/exo ratio.     

Pore tuned activated carbons as supports for an enantioselective molecular catalyst

The Jacobsen catalyst was immobilized onto four activated carbons with different average pore sizes, achieved by a gasification process followed by molecular oxygen oxidation. The influence of the textural properties of the activated carbon in the immobilization process and in the catalytic performance of the Mn(III) heterogeneous catalysts was investigated in detail. Three different catalytic systems were studied: styrene epoxidation using m-chloroperoxybenzoic acid; 6-CN-2,2-diMeChromene epoxidation using NaOCl and iodosylbenzene (PhIO) as oxidants. The catalysts tested were active and enantioselective in the three systems studied. Selectivity towards the desired epoxide only decreases in the case of the material with smaller pores, remaining identical to that of the homogeneous phase in all the other materials. The enantiomeric excess values (%ee) for alkene epoxidation increase with the pore size of the heterogeneous catalysts, and these values are even higher than the homogeneous counterparts in the styrene epoxidation reaction. Total Mn(III) loadings increase with the pore size, as well as their distribution within the carbon porous matrix. Characterization of the activated carbons bearing the immobilized manganese(III) complexes by TPD and XPS point to reaction between carbon surface phenolate groups and the manganese(III) complexes through axial coordination of the metal centers to these groups.     

Ru(Schiff-base)/Y复合材料对苯加氢反应的催化性能研究

一种新的低温苯加氢催化剂Ru(Schiff-base)金属配合物, 通过自由配体法封装于Y型沸石的孔腔中。使用XRD、N2吸附、FT-IR、DRS、DTA对催化剂进行表征。结果表明, 复合催化剂中希夫碱(Schiff-base)配体改变了中心离子的电子结构，使其更容易与反应物分子形成配位过渡态，络合活化的反应物分子更容易转化为产物。与离子交换法制备的母体Ru/Y相比，对纯苯的催化加氢性能明显提高。希夫碱配体的几何尺寸对复合材料的催化性能也有很大影响。     

Lanthanum heterocyclic Schiff-base complex initiated ring-opening polymerization of ε-caprolactone

Lanthanum complex supported by the heterocyclic Schiff-base ligand of N-(2-pyridyl)-3,5-di-tert-butyl-salicylaldimine was prepared and employed for the ring-opening polymerization(ROP)ofε-caprolactone(ε-CL).The polymers obtained with this initiator showed a unimodal molecular weight distribution implied that only one active species was present.Mechanism study revealed that the polymerization proceeds via acyl-oxygen bond cleavage.     

Rhodium-catalyzed hydroformylation of olefins: Effect of [bis(2,4-di-tert-butyl) pentaerythritol] diphosphite (alkanox P-24) on the regioselectivity of the reaction

Rhodium (I) associated with [bis(2,4-di-tert-butyl) pentaerythritol] diphosphite (I) as a ligand represents an active catalyst system for highly regioselective hydroformylation of various alkenes. The commercially available bis(2,4-di-tert-butyl)pentaerythritol diphosphite (alkanox P-24) (I), which has been used so far as an antioxidant in the stabilization of polymers, was used as a diphosphite ligand for the selective hydroformylation reaction of olefins. Excellent selectivity towards linear aldehydes and excellent conversions were achieved in the hydroformylation of alkenes. The hydroformylation reaction was applied to various olefinic substrates including the internal alkenes.     

手性过渡金属(Mn,Co,Ni)-Salen配合物催化NaOCl不对称环氧化苯乙烯的反应研究

本文论述了由手性1,2-二苯基乙二胺与适当取代的邻羟基苯甲醛生成的希佛碱作为过渡金属Co(Ⅱ)、Ni(Ⅲ)、Mn(Ⅲ)的配体合成的6种过渡金属-Salen配合物在NaOCl下均相催化苯乙烯不对称环氧化反应的效果。对环氧化反应进行控制性研究,我们发现环氧化反应与中心金属离子的相关性可能主要源于配体结构、金属离子的选择及二者结合而形成的配合物的空间构型。好的不对称环氧化反应催化剂要求中心金属离子具有适当的氧化还原电位,对底物烯烃因其取代基的大小,催化剂结构的空间构型应有好坏之分,我们得到的循环伏安数据及实验结果部分说明这一点。同时对手性镍(Ⅱ)-Salen及手性锰(Ⅲ)-Salen配合物的催化反应机理配合实验现象分别作出假设,都经过自由基历程,但却是截然不同的活性氧化物种,同时,由于起主要作用的不对称诱导因素不同,不对称诱导方向有异。对同一催化剂在不同pH值下起作用的活性氧化物种类别也可能不同。     

Cu(II), Co(II), Ni(II), Mn(II), and Fe(II) metal complexes containing N,N′-(3,4-diaminobenzophenon)-3,5-But2-salicylaldimine ligand: Synthesis,structural characterization,thermal properties,electrochemistry, and spectroelectrochemistry

The synthesis, structure, spectroscopic and electro-spectrochemical properties of steric hindered Schiff-base ligand [N,N′-(3,4-benzophenon)-3,5-Bu^t₂-salicylaldimine (LH₂)] and its mononuclear Cu(II), Co(II), Ni(II), Mn(II) and Fe(II) complexes are described in this work. The new dissymmetric steric hindered Schiff-base ligand containing a donor set of NONO was prepared through reaction of 3,4-diaminobenzophenon with 3,5-Bu^t₂-salicylaldehyde. Certain metal complexes of this ligand were synthesized by treating an ethanolic solution of the ligand with an equimolar amount of metal salts. The ligand and its complexes were characterized by FT-IR, UV–vis, ¹H NMR, elemental analysis, molar conductivity and thermal analysis methods in addition to magnetic susceptibility, electrochemistry and spectroelectrochemistry techniques. The tetradentate and mononuclear metal complexes were obtained by reacting N,N′-(3,4-benzophenon)-3,5-Bu^t₂-salicylaldimine (LH₂) with some metal acetate in a 1:1 mole ratio. The molar conductance data suggest metal complexes to be non-electrolytes.     

手性过渡金属(Mn, Co, Ni)-Salen配合物催化NaOCl不对称环氧化苯乙烯的反应研究

本文论述了由手性1, 2-二苯基乙二胺与适当取代的邻羟基苯甲醛生成的希佛碱作为过渡金属Co(II)、Ni(II)、Mn(II)的配体合成的6种过渡金属-Salen配合物在NaOCl下均相催化苯乙烯不对称环氧化反应的效果。对环氧化反应进行控制性研究, 我们发现环氧化反应与中心金属离子的相关性可能主要源于配体结构、金属离子的选择及二者结合形成的配合物的空间构型。好的不对称环氧化反应催化剂要求中心金属离子具有适当的氧化还原电位, 对底物烯烃因其取代基的大小, 催化剂结构的空间构型应有好坏之分, 我们得到的循环伏安数据及实验结果部分说明这一点。同时对手性镍(II)-Salen及手性锰(III)-Salen配合物的催化反应机理配合实验现象分别作出假设, 都经过自由基历程, 但却是截然不同的活性氧化物种, 同时, 由于起主要作用的不对称诱导因素不同, 不对称诱导方向有异。对同一催化剂在不同pH值下起作用的活性氧化物种类别也可能不同。     

An efficient hybrid, nanostructured, epoxidation catalyst: titanium silsesquioxane-polystyrene copolymer supported on SBA-15

A novel interfacial hybrid epoxidation catalyst was designed with a new immobilization method for homogeneous catalysts by coating an inorganic support with an organic polymer film containing active sites. The titanium silsesquioxane (TiPOSS) complex, which contains a single-site titanium active center, was immobilized successfully by in-situ copolymerization on a mesoporous SBA-15-supported polystyrene polymer. The resulting hybrid materials exhibit attractive textural properties (highly ordered mesostructure, large specific surface area (>380 m2 g-1) and pore volume (>or==0.46 cm3 g-1)), and high activity in the epoxidation of alkenes. In the epoxidation of cyclooctene with tert-butyl hydrogen peroxide (TBHP), the hybrid catalysts have rate constants comparable with that of their homogeneous counterpart, and can be recycled at least seven times. They can also catalyze the epoxidation of cyclooctene with aqueous H2O2 as the oxidant. In two-phase reaction media, the catalysts show much higher activity than their homogeneous counterpart due to the hydrophobic environment around the active centers. They behave as interfacial catalysts due to their multifunctionality, that is, the hydrophobicity of polystyrene and the polyhedral oligomeric silsesquioxanes (POSS), and the hydrophilicity of the silica and the mesoporous structure. Combination of the immobilization of homogeneous catalysts on two conventional supports, inorganic solid and organic polymer, is demonstrated to achieve novel heterogeneous catalytic ensembles with the merits of attractive textural properties, tunable surface properties, and optimized environments around the active sites.     

Facile nucleophilic fluorination by synergistic effect between polymer-supported ionic liquid catalyst and tert-alcohol reaction media system

A highly efficient method was developed for nucleophilic fluorination using an alkali metal fluoride through the synergistic effect of the polymer-supported ionic liquid (PSIL) as a catalyst and tert-alcohol as an alternative reaction media. This PSIL/tert-alcohol system not only enhances the reactivity of alkali metal fluorides and reduces the formation of by-products but also allows the use of a polymer-supported catalyst protocol. As an example, the nucleophilic fluorinations of the model compound, 2-(3-bromopropoxy)naphthalene, with CsF using only tert-amyl alcohol as solvent (for 2 h reaction time), 0.5 equiv of PS[hmim][BF₄] in CH₃CN (for 12 h reaction time), and 0.5 equiv of PS[hmim][BF₄] in tert-amyl alcohol (which is a PSIL/tert-alcohol system for the synergistic effect; for 2 h reaction time) provided 18, 40, and 84% yield, respectively. The characteristics of the nucleophilic fluorination reactions of some halo- and alkanesulfonyloxyalkane systems to the corresponding fluoroalkanes using various alkali metal fluorides are also reported.