Schiff base complex of Cu immobilized on MCM-41 as reusable catalyst for efficient oxidative coupling of thiols and oxidation of sulfides using hydrogen peroxide

Schiff base complex of copper-functionalized MCM-41 (Cu-complex@MCM-41) was synthesized and used as an efficient and novel heterogeneous catalyst for the oxidative coupling of thiols into corresponding disulfides and oxidation of sulfides to sulfoxides using hydrogen peroxide (H₂O₂) as the oxidant. An aliphatic and aromatic series of sulfides and thiols including various functional groups were successfully converted into corresponding products. The all products were obtained in good to excellent yields. The mesoporous catalyst is characterized by FT-IR spectroscopy, BET, XRD, SEM, EDS and TGA. Recovery of the catalyst is easily achieved by simple filtration and reused for several consecutive runs without significant loss of its catalytic efficiency.     

Tethering of Cu(II) Schiff base metal complex on mesoporous material MCM-41: catalyst for Ullmann-type coupling reactions

The siloxane-functioned Cu(II) complex derived from 3-aminopropyltrimethoxysilane, terephthaldehyde and 2-aminophenol was anchored on hexagonal mesoporous silica MCM-41. The MCM-41 and its Schiff base MCM-41 complex was characterized by FTIR, small-angle X-ray diffraction, SEM-EDX, N₂ adsorption and desorption analysis and TGA. The synthesized Cu-Schiff base MCM-41 catalyst has been successfully applied as a catalyst for Ullmann-type coupling reaction of the aryl halides with aryl halides, phenols, amines and N-heterocyclic amines. In the coupling reaction the yields of the products were good and the catalyst was recovered by simple filtration method and can be reused.     

Mild and highly efficient transformation of thiols to symmetrical disulfides using urea–hydrogen peroxide catalyzed by a Mn(III)–salen complex

The oxidative coupling of aromatic thiols into the corresponding disulfides with urea–hydrogen peroxide using a Mn(III)–salen complex as catalyst under mild conditions is described. This system provides an efficient, convenient and practical method for the syntheses of symmetrical disulfides.     

Oxidative coupling of aromatic thiols to corresponding disulfides using magnetic particle-supported sulfonic acid catalyst and hydrogen peroxide under mild conditions

A green and efficient method for chemoselective oxidation of aromatic thiols to the corresponding disulfides is reported using a recoverable supported iron oxide nanoparticle (γ-Fe₂O₃–SO₃H) catalyst in the presence H₂O₂ as the oxidant and methanol as the solvent at room temperature. This reaction is operationally simple with an easy work-up, utilizes mild reaction conditions, is high-yielding, clean, and exhibits high selectivity toward aromatic disulfides with no side reactions. The supported iron oxide nanoparticles can be easily recovered using an external magnet from the reaction mixture and reused several times. Reaction progress was monitored by potentiometric titration.     

Zr (IV)-ninhydrin supported MCM-41 and MCM-48 as novel nanoreactor catalysts for the oxidation of sulfides to sulfoxides and thiols to disulfides

Modification of MCM-41 and MCM-48 mesoporous materials with bonded aminosilane species, Schiff base preparation by ninhydrin and finally complexation with zirconium, has attracted much attention in order to design catalyst with advanced applications in the oxidation of sulfides to sulfoxides and thiols to disulfides in the presence of hydrogen peroxide. In all oxidation of sulfides to sulfoxids 0.4 mL H₂O₂ used as oxidant in the presence of Zr(IV)-ninhydrin supported MCM-41 (0.01 g) or Zr(IV)-ninhydrin supported MCM-48 (0.005 g) at room temperature and solvent-free condition. Also the best conditions for oxidation of thiols to disulfides with 0.4 mL H₂O₂ were 0.005 g Zr(IV)-ninhydrin supported MCM-41 or Zr(IV)-ninhydrin supported MCM-48 at room temperature and in ethanol. These catalysts are characterized by SEM, XRD, TGA, FT-IR, EDS, ICP and BET analysis. Also the Turn over frequency (TOF) and Turn over number (TON) of catalysts are calculated. Obtained results by these heterogeneous catalysts revealed several advantages including short reaction times, simple workup, easy isolation and reusability.     

Preparation and characterization of a titanium(IV) silsesquioxane epoxidation catalyst anchored into mesoporous MCM-41.

This paper describes the synthesis and spectroscopic study of a titanium(IV) silsesquioxane complex, which is heterogenised in the pores of an MCM-41 host material. Its immobilization is performed via chemical bonding, not by means of physical adsorption. As a linking molecule between the MCM-41 carrier and the silsesquioxane, (3-glycidyloxypropyl)trimethoxysilane is used. Characterization is performed by using nitrogen adsorption techniques, TGA, diffuse reflectance infrared spectroscopy (DRIFT) and ICP-MS. Also its catalytic activity towards the epoxidation of alkenes shows interesting results.     

Oxovanadium(IV) salicylidene Schiff base complex anchored on mesoporous silica MCM-41 as hybrid materials: a robust catalyst for the oxidation of sulfides

Oxovanadium(IV) Schiff base complex have been anchored onto the surface of purely siliceous MCM-41 and tested for its activity as catalyst for the oxidation of sulfides. This catalyst could alter this oxidation reaction extremity, exhibiting excellent yields with 100 % selectivity. The intercalation of the complex inside the silica matrix was supported by various characterization techniques like X-ray diffraction, differential thermogravimetric (TG-DTA), BET measurements, UV–Vis diffuse reflectance spectroscopy and Fourier transform infrared spectroscopy (FT-IR). The stability of the catalyst during the course of the reaction was confirmed from its post catalytic FT-IR and XRD analysis. The catalyst could be reused five times without notable loss of its catalytic activity and efficiency which indicates that the metal-Schiff base moiety is intact and the coordination environments are not altered during the reaction.     

Diphosphino-functionalized MCM-41 anchored palladium(0) complex as an efficient catalyst for Heck arylation of conjugated alkenes with aryl halides

Diphosphino-functionalized MCM-41 anchored palladium(0) complex (denoted as MCM-41-2P-Pd(0)) was conveniently synthesized from commercially available and cheap γ-aminopropyltriethoxysilane via immobilization on MCM-41, followed by reacting with diphenylphosphinomethanol and palladium chloride and then the reduction with hydrazine hydrate. XRD and XPS spectroscopies were employed to characterize the title palladium complex. It was found that this complex is a highly active and stereoselective catalyst for Heck arylation of conjugated alkenes with aryl halides and can be reused many times without loss of activity.     

Synthesis of tetradentate N4 Schiff base dioxomolybdenum (VI) complex within MCM-41 as selective catalyst for epoxidation of olefins

Covalent grafting of MCM-41 with 3-chloropropyl trimethoxysilane and subsequent reactions respectively with pypr [N,N′-bis(2-pyrrolmethylidenaminopropyl)amine] and complexation with MoO₂(acac)₂ afforded MoO₂pyprMCM-41. X-ray diffraction and nitrogen sorption analyses revealed the preservation of the textural properties of the support as well as accessibility of the channel system despite sequential reduction in surface area, pore volume and pore size. Elemental analyses showed nearly complete complexation of the supported ligands and the presence of 0.24 mmol molybdenum per gram of the catalyst. Epoxidation of cyclooctene, cyclohexene, 1-hexene and 1-octene in the presence of MoO₂pyprMCM-41 with tert-butyl hydroperoxide (TBHP) were carried out with 21–98% conversion under the mild reaction conditions.     

Epoxidation of DL-limonene using an indenyl molybdenum(II) tricarbonyl complex as catalyst precursor

The complex IndMo(CO)₃Me (Ind = η⁵-C₉H₇) is an effective catalyst precursor for the epoxidation of DL-limonene using the following oxidant solutions: (i) commercial tert-butylhydroperoxide in decane (TBHPdec), (ii) commercial aqueous TBHP (TBHPaq) pre-mixed with limonene (TBHP_lim), or (iii) TBHPaq pre-mixed with 1,2-dichloroethane (TBHPdce); simple pre-drying treatments of the reaction solutions were applied prior to feeding the catalyst precursor to the batch reactor. The best results were found for the efficiently pre-dried reaction system (iii), which gave higher 1,2-epoxy-p-meth-8-ene yield at 35 min/55 °C than (i) (82% and 73% yield, respectively). This approach avoids the undesirable partial oxidation of decane, which would imply costly work-up procedures to remove high boiling point impurities from the epoxides. These results together with studies on the reactivity of different olefins indicate fairly high regioselectivity toward the epoxidation of the internal double bonds.     

Studies on the activation of hydrogen peroxide for color removal in the presence of a new Cu(II)-polyampholyte heterogeneous catalyst

In this work we describe the application of a new non-soluble and non-porous complex with copper ion based on ethylene glycol diglycidyl ether (EGDE), methacrylic acid (MAA) and 2-methylimidazole (2MI) in the decolorization of an azo dye Methyl Orange (MO) as a model pollutant at room temperature.The complex with copper ion was studied by ESR and SEM and was tested as a heterogeneous catalyst for H₂O₂ activation. A possible mechanism of interaction involves the production of hydroxyl radicals (confirmed by ESR), dioxygen and water.The Cu(II)-polyampholyte/H₂O₂ system acted efficiently in the color removal of MO. The adsorption and oxidative degradation of the azo-based dye followed pseudo-first-order kinetic profiles, and the rate constant for degradation had a second-order dependence on copper ion content in the mixture.A removal of MO higher than 90% was achieved in 20 min at pH 7.0, combining 0.8 mM of complexed copper ions in the mixture with 24 mM hydrogen peroxide.The dye adsorbed on the polyampholyte following a L4-type isotherm with 4.9 μmol g⁻¹ maximum loading capacity and 3.1 μM dissociation constant for the first monolayer.     

Heterogenization of [Cu(NCCH3)4][BF4]2 on mesoporous AlMCM-41/AlMCM-48 and its application as cyclopropanation catalyst

[Cu(NCCH₃)₄][BF₄]₂ can be heterogenized by an ion exchange method on mesoporous NaAlMCM-41 and NaAlMCM-48 materials. The heterogenized complex and the homogeneous Cu compound are applicable for the cyclopropanation of olefins at 303 K. Comparable yields are observed for non-bulky olefins, olefins with bulkier substituents lead to lower yields. [Cu(NCCH₃)₄][BF₄]₂ grafted on mesoporous surfaces (as heterogeneous catalyst) are relatively stable and recyclable for several catalytic runs with only slight decrease in catalytic activity.     

Diethanol amine‐functionalized polymer‐supported palladium (0) complex as catalyst for Suzuki cross‐coupling reaction in water

Diethanol amine‐functionalized polymer‐supported palladium (0) complex as catalyst for Suzuki cross‐coupling reaction in water was synthesized and characterized. The catalyst exhibits excellent catalytic activity and stability in the Suzuki cross‐coupling reaction. Various aryl bromides were coupled with aryl boronic acids in water, under air, and in the presence of 0.5 mol % of the catalyst to afford corresponding cross‐coupled products in high yields at 100°C. Furthermore, the heterogeneous catalyst can be readily recovered by simple filtration and reused for several times only with a slight decrease in its activity. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Novel transition metal free oxidation of aromatic aldehydes to carboxylic acids using N-methylpyrrolidin-2-one hydrotribromide (MPHT) as catalyst and hydrogen peroxide as oxidant

N-Methylpyrrolidin-2-one hydrotribromide (MPHT) was found to be an efficient catalyst for the oxidation of various aromatic aldehydes to carboxylic acids using aq. hydrogen peroxide as oxidant in excellent yields under mild reaction conditions.     

Copper-alginates: a biopolymer supported Cu(II) catalyst for 1,3-dipolar cycloaddition of alkynes with azides and oxidative coupling of 2-naphthols and phenols in water

Copper(II) ions immobilized onto a biopolymer (sodium alginate) is an effective heterogeneous catalyst for 1,3-dipolar cycloaddition of alkynes with azides and oxidative coupling of 2-naphthols and phenols in water to afford the 1,4-disubstituted 1,2,3-triazoles and biaryl compounds respectively in good yields. The catalyst was recovered quantitatively by simple filtration and reused for several times without significant loss of activity.     

Covalent anchoring of Mo(VI) Schiff base complex into SBA-15 as a novel heterogeneous catalyst for enhanced alkene epoxidation

Molybdenum(VI) Schiff base complexes modified mesoporous SBA-15 hybrid heterogeneous catalysts were synthesized by the reaction of MoO₂(acac)₂ with mesoporous SBA-15 functionalized by grafting procedures of 3-aminopropyl-triethoxysilane and salicylaldehyde, respectively. The physico-chemical properties of the as-synthesized catalysts were analyzed by ICP-AES, XRD, N₂ adsorption–desorption, FT-IR, SEM, TEM and EDX. The as-synthesized catalysts were effective in the catalytic epoxidation of cyclohexene. The catalytic activity can be further enhanced by silylation of the residual Si–OH groups using Me₃SiCl, which was largely due to the higher content of Mo active sites. The conversion and selectivity reached to 97.78 and 93.99 % using tert-butyl hydroperoxide as oxidant for Mo–CH₃–SA–NH₂–SBA-15, while 81.97 and 89.41 % in conversion and selectivity for Mo–SA–NH₂–SBA-15. At the same time, the catalytic performances of the hybrid materials were further systematically investigated under various reaction conditions (solvent, oxidants and alkenes, etc.). Mo–CH₃–SA–NH₂–SBA-15 catalyst can be recycled effectively and reused four cycles with little loss in activity. In addition, the results from hot filtration demonstrated that the catalytic activity mostly resulted from the heterogeneous catalytic process.     

Oxidation of hydrophobic alcohols using aqueous hydrogen peroxide over amphiphilic silica particles loaded with titanium(IV) oxide as a liquid–liquid phase-boundary catalyst

Phase-boundary catalysis (PBC), a new concept of a heterogeneous catalytic system for oxidation of various hydrophobic alcohols with aqueous hydrogen peroxide (H₂O₂), has been investigated. A part the external surface of silica (SiO₂) particles loaded with titanium(IV) oxides was modified with hydrophobic alkyl groups to obtain amphiphilic particles, having both hydrophobic and hydrophilic surfaces on each particle. The amphiphilic particles were spontaneously assembled at interfaces between dual phase mixtures of aqueous solutions and water-immiscible organic compounds. Upon addition to a dual phase mixture of aqueous H₂O₂ and toluene-containing hydrophobic alcohols, these particles acted as an efficient catalyst for the reaction, to produce corresponding aldehydes and ketones selectively. Notable features of the PBC system are that the oxidation proceeds even without agitation and that only a few percent of titanium species was detected as dissolved species. Productions of aldehydes and ketones were also observed when titanium loaded SiO₂ without modification with alkyl groups was employed for the reaction. However, a large amount of titanium loaded on the material was leached during the reaction. These results indicate that surface-covered alkyl groups not only bring about effective contact with hydrophobic alcohols in the organic phase but also give stability against leaching, leading to heterogeneous catalytic functions.     

Introduction of Cu(II) complex into transparent synthetic resin and optical function of the resin as a near-infrared ray cutoff filter

A mole ratio method was conducted to determine a necessary condition to solubilize Cu(II) ion into an organic solvent. Copper benzoate anhydrous (CB) and acetone was used as a Cu(II) ion supplier and a solvent, respectively. Methacryloyloxy-ethyl phosphate (PMOE) and di(2-ethylhexyl) phosphate (DIEHP) were used asligands for solubilization. Cu(II) ion was solubilized with only two PMOE molecules, although six molecules were needed for DIEHP. PMOE formed an intermediate layer which surrounded the Cu(II) ion with two molecules between the ion and solvent and made it possible to solubilize Cu(II) ion into the organic solvent. In the case of DIEHP, however, six molecules were needed to form such a layer for solubilization. Furthermore, Cu(II) ion was introduced homogeneously into poly(methyl methacrylate) (PMMA) by copolymerization of PMOE with MMA, although the polymer was opaque in the case of DIEHP. The ligand having a methacryloyl group was considered to participate in the polymerization process, which avoided exclusion of the Cu(II) complex from polymer phase. The intermediate zone formed by PMOE was considered to keep compatibility of Cu(II) ion with the polymer matrix even after polymerization. The near-infrared ray cutting-off filter made of resin was realized by introducing Cu(II) ion at higher concentration through complexation with the proper ligand. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:903–912, 1998