Investigation of catalytic properties of two new orthopalladated complexes supported on montmorillonite: Synthesis,characterization and application in aerobic oxidation of alcohols

Two new complexes, [Pd(L₁)(C,N)]NO₃ ( 1 ) and [Pd(L₂)(C,N)]NO₃ ( 2 ) (L₁ = 5‐nitro‐1,10‐phenanthroline, L₂ = 4‐methyl‐1,10‐phenanthroline, C,N = benzylamine), have been synthesized and characterized using infrared and NMR spectroscopies and elemental analysis. Montmorillonite (MMT‐K10 clay) was used as a solid support for incorporating the cationic part of complexes 1 and 2 to produce catalysts 1 and 2 , respectively, as heterogeneous catalysts. Catalyst 1 was identified using powder X‐ray diffraction and scanning and transmission electron microscopies, and the content of palladium obtained from inductively coupled plasma analysis. By changing the electron‐donating group on the L₁ ligand with an electron‐withdrawing one, a minor improvement was observed in the catalytic properties. Catalyst 1 showed better efficiency for oxidation of benzyl alcohol in comparison with catalyst 2 , so catalyst 1 was used for the aerobic oxidation of alcohols to corresponding aldehydes or ketones without over‐oxidation (with and without bubbling of air). This catalytic system showed high activity towards alcohols under mild conditions. Finally, the reusability of catalyst 1 was investigated with multiple reuses of the supported catalyst in subsequent alcohol oxidation reactions.     

Molybdenum hexacarbonyl supported on amine modified multi‐wall carbon nanotubes: an efficient and highly reusable catalyst for epoxidation of alkenes with tert‐butylhydroperoxide

In the present work, highly efficient epoxidation of alkenes catalyzed by Mo(CO)₆ supported on multi‐wall carbon nanotubes modified by 2‐aminopyrazine, APyz‐MWCNTs, is reported. The prepared catalyst was characterized by elemental analysis, scanning electron microscopy, FT IR and diffuses reflectance UV–vis spectroscopic methods. This new heterogenized catalysts, [Mo(CO)₆@APyz‐MWCNT], was used as a highly efficient catalyst for epoxidation of alkenes with tert‐BuOOH. This robust catalyst was reused several times without loss of its catalytic activity. Copyright © 2010 John Wiley & Sons, Ltd.     

Highly Enantioselective Iron‐Catalyzed cis‐Dihydroxylation of Alkenes with Hydrogen Peroxide Oxidant via an FeIII‐OOH Reactive Intermediate

The development of environmentally benign catalysts for highly enantioselective asymmetric cis‐dihydroxylation (AD) of alkenes with broad substrate scope remains a challenge. By employing [Fe^II(L)(OTf)₂] (L=N,N′‐dimethyl‐N,N′‐bis(2‐methyl‐8‐quinolyl)‐cyclohexane‐1,2‐diamine) as a catalyst, cis‐diols in up to 99.8 % ee with 85 % isolated yield have been achieved in AD of alkenes with H₂O₂ as an oxidant and alkenes in a limiting amount. This “[Fe^II(L)(OTf)₂]+H₂O₂” method is applicable to both (E)‐alkenes and terminal alkenes (24 examples >80 % ee, up to 1 g scale). Mechanistic studies, including ¹⁸O‐labeling, UV/Vis, EPR, ESI‐MS analyses, and DFT calculations lend evidence for the involvement of chiral Fe^III‐OOH active species in enantioselective formation of the two C?O bonds.     

A Copper(I)‐Catalyzed Enantioselective γ‐Boryl Substitution of Trifluoromethyl‐Substituted Alkenes: Synthesis of Enantioenriched γ,γ‐gem‐Difluoroallylboronates

The first catalytic enantioselective γ‐boryl substitution of CF₃‐substituted alkenes is reported. A series of CF₃‐substituted alkenes was treated with a diboron reagent in the presence of a copper(I)/Josiphos catalyst to afford the corresponding optically active γ,γ‐gem‐difluoroallylboronates in high enantioselectivity. The thus obtained products could be readily converted into the corresponding difluoromethylene‐containing homoallylic alcohols using highly stereospecific allylation reactions.     

Supported C‐Scorpionate Vanadium(IV) Complexes as Reusable Catalysts for Xylene Oxidation

C‐Scorpionate vanadium(IV) [VO_xCl_3−x {κ³‐RC(pz)₃}] [pz=pyrazol‐1‐yl; x= 0, R=SO₃ ( 1 ); x= 1, R=CH₂OH ( 2 ) or CH₂OSO₂Me ( 3 )] complexes supported on functionalized carbon nanotubes (CNTs) are the first V‐scorpionate catalysts used so far for the neat oxidation of o ‐, m ‐ or p ‐xylene, with TBHP (70 % aqueous solution), to the corresponding toluic acids (main products), tolualdehydes and methylbenzyl alcohols. Remarkably, a p ‐toluic acid yield of 43 % (73 % selectivity, TON=1.34×10³) was obtained with 2 @CNT in a simple microwave‐assisted mild oxidation procedure, using a very low catalyst charge (3.2×10⁻² mol ^% vs. substrate). Further, this occurred in the absence of any bromine source, what is significant towards the development of a greener and more sustainable process for oxidation of xylenes. Moreover, reuse of catalysts with preservation of their activity was found for up to six consecutive cycles. The effects of reaction parameters, such as reaction time, temperature, amount of catalyst or type of heating source, on the performance of the above catalytic systems are reported and discussed.     

Ruthenium‐Catalyzed Oxidation of Alkenes,Alkynes, and Alcohols to Organic Acids with Aqueous Hydrogen Peroxide

A protocol that adopts aqueous hydrogen peroxide as a terminal oxidant and [(Me₃tacn)(CF₃CO₂)₂Ru^III(OH₂)]CF₃CO₂ ( 1 ; Me₃tacn=1,4,7‐trimethyl‐1,4,7‐triazacyclononane) as a catalyst for oxidation of alkenes, alkynes, and alcohols to organic acids in over 80 % yield is presented. For the oxidation of cyclohexene to adipic acid, the loading of 1 can be lowered to 0.1 mol %. On the one‐mole scale, the oxidation of cyclohexene, cyclooctene, and 1‐octanol with 1 mol % of 1 produced adipic acid (124 g, 85 % yield), suberic acid (158 g, 91 % yield), and 1‐octanoic acid (129 g, 90 % yield), respectively. The oxidative C?C bond‐cleavage reaction proceeded through the formation of cis‐ and trans‐diol intermediates, which were further oxidized to carboxylic acids via C? C bond cleavage.     

Gold nanoparticle-catalyzed environmentally benign deoxygenation of epoxides to alkenes

We have developed a highly efficient and green catalytic deoxygenation of epoxides to alkenes using gold nanoparticles (NPs) supported on hydrotalcite [HT: Mg(6)Al(2)CO(3)(OH)(16)] (Au/HT) with alcohols, CO/H(2)O or H(2) as the reducing reagent. Various epoxides were selectively converted to the corresponding alkenes. Among the novel metal NPs on HT, Au/HT was found to exhibit outstanding catalytic activity for the deoxygenation reaction. Moreover, Au/HT can be separated from the reaction mixture and reused with retention of its catalytic activity and selectivity. The high catalytic performance of Au/HT was attributed to the selective formation of Au-hydride species by the cooperative effect between Au NPs and HT.     

Homogeneous [RuIII(Me3tacn)Cl3]‐Catalyzed Alkene cis‐Dihydroxylation with Aqueous Hydrogen Peroxide

A simple and green method that uses [Ru(Me₃tacn)Cl₃] ( 1 ; Me₃tacn=N,N′,N′′‐trimethyl‐1,4,7‐triazacyclononane) as catalyst, aqueous H₂O₂ as the terminal oxidant, and Al₂O₃ and NaCl as additives is effective in the cis‐dihydroxylation of alkenes in aqueous tert‐butanol. Unfunctionalized alkenes, including cycloalkenes, aliphatic alkenes, and styrenes (14 examples) were selectively oxidized to their corresponding cis‐diols in good to excellent yield (70–96 %) based on substrate conversions of up to 100 %. The preparation of cis‐1,2‐cycloheptanediol (119 g, 91 % yield) and cis‐1,2‐cyclooctanediol (128 g, 92 % yield) from cycloheptene and cyclooctene, respectively, on the 1‐mol scale can be achieved by scaling up the reaction without modification. Results from Hammett correlation studies on the competitive oxidation of para‐substituted styrenes (ρ=?0.97, R=0.988) and the detection of the cycloadduct [(Me₃tacn)ClRuHO₂(C₈H₁₄)]⁺ by ESI‐MS for the 1 ‐catalyzed oxidation of cyclooctene to cis‐1,2‐cyclooctanediol are similar to those of the stoichiometric oxidation of alkenes by cis‐[(Me₃tacn)(CF₃CO₂)Ru^VIO₂]⁺ through [3+2] cycloaddition (W.‐P. Yip, W.‐Y. Yu, N. Zhu, C.‐M. Che, J. Am. Chem. Soc. 2005 , 127, 14239).     

Metal–Support Cooperative Catalysts for Environmentally Benign Molecular Transformations

Metal–support cooperative catalysts have been developed for sustainable and environmentally benign molecular transformations. The active metal centers and supports in these catalysts could cooperatively activate substrates, resulting in high catalytic performance for liquid‐phase reactions under mild conditions. These catalysts involved hydrotalcite‐supported gold and silver nanoparticles with high catalytic activity for organic reactions such as aerobic oxidation, oxidative carbonylation, and chemoselective reduction of epoxides to alkenes and nitrostyrenes to aminostyrenes using alcohols and CO/H₂O as reducing reagents. This high catalytic performance was due to cooperative catalysis between the metal nanoparticles and basic sites of the hydrotalcite support. To increase the metal–support cooperative effect, core–shell nanostructured catalysts consisting of gold or silver nanoparticles in the core and ceria supports in the shell were designed. These core–shell nanocomposite catalysts were effective for the chemoselective hydrogenation of nitrostyrenes to aminostyrenes, unsaturated aldehydes to allyl alcohols, and alkynes to alkenes using H₂ as a clean reductant. In addition, these solid catalysts could be recovered easily from the reaction mixture by simple filtration, and were reusable with high catalytic activity.     

Polymer‐supported Ti(IV) and Mn(III) asymmetric alkene epoxidation catalysts

Epoxides represent a very important group of speciality and fine chemicals because they are derived directly from alkenes, a primary petrochemical source, and because of the breadth of opportunity they offer the organic synthetic chemist in terms of the highly selective reactions they undergo, often requiring only very mild conditions. Since most epoxides also bear at least one stereogenic centre the strategic importance of these molecules in synthesis is even higher. The most important asymmetric alkene epoxidation catalyst systems that have been discovered are those reported by Sharpless and his co‐workers utilising tartrate ester complexed Ti(IV) centres¹ and by Jacobsen and his co‐workers utilising chiral Mn(III) salen complexes.² The former system provides high conversions and high enantioselectivity (enantiomeric excess, ee%) in the case of allylic alcohol substrates, while the latter is likewise effective in the case of non‐functional cis‐internal alkenes, especially cyclic systems. Both catalytic systems are homogeneous and exploitation of both involve rather laborious work‐up procedures. Generally no attempt is made to recover and re‐use these catalysts. The potential advantages in converting a process catalysed by a homogeneous metal complex into one involving a heterogeneous polymer‐supported analogue have been well rehearsed.³ Suffice to say that on a laboratory scale supported metal complex catalysts considerably facilitate product work‐up and isolation, while on a large scale such heterogeneous species allow processes to be run continuously using packed or fluidised bed columns with considerable financial advantages both in terms of capital expenditure on plant and with regard to recurrent costs.     

Ruthenium‐Immobilized Periodic Mesoporous Organosilica: Synthesis,Characterization, and Catalytic Application for Selective Oxidation of Alkanes

Periodic mesoporous organosilica (PMO) is a unique material that has a crystal‐like wall structure with coordination sites for metal complexes. A Ru complex, [RuCl₂(CO)₃]₂, is successfully immobilized onto 2,2’‐bipyridine (BPy) units of PMO to form a single‐site catalyst, which has been confirmed by various physicochemical analyses. Using NaClO as an oxidant, the Ru‐immobilized PMO oxidizes the tertiary C?H bonds of adamantane to the corresponding alcohols at 57 times faster than the secondary C?H bonds, thereby exhibiting remarkably high regioselectivity. Moreover, the catalyst converts cis‐decalin to cis‐9‐decalol in a 63 % yield with complete retention of the substrate stereochemistry. The Ru catalyst can be separated by simple filtration and reused without loss of the original activity and selectivity for the oxidation reactions.     

High Performance Pd,PdNi, PdSn and PdSnNi Nanocatalysts Supported on Carbon Nanotubes for Electrooxidation of C2C4 Alcohols

Nanocatalysts Pd, Pd₈Ni₂, Pd₈Sn₂ and Pd₈Sn₁Ni₁ supported on multi‐walled carbon nanotubes (MWCNTs) were successively synthesized by the chemical reduction method in the glycol‐water mixture solvent. Transmission electron microscopy results show that the prepared Pd, Pd₈Ni₂, Pd₈Sn₂ and Pd₈Sn₁Ni₁ nanoparticles are uniformly dispersed on the surface of MWCNTs. The average particle sizes of the nanocatalysts are 3.5–3.8 nm. Electroactivity of the prepared catalysts towards oxidation of ethanol, 1‐propanol, 2‐propanol, n‐butanol, iso‐butanol and sec‐butanol (C2? C4 alcohols) in alkaline medium was studied by cyclic voltammetry and chronoamperometry. The current density obtained for the electrooxidation of C2? C4 alcohols depends on the catalysts and the various structures of the alcohols. Addition of Sn or/and Ni to Pd nanoparticles enhances the electroactivity of the Pd/MWCNT catalyst. Furthermore, the ternary Pd₈Sn₁Ni₁/MWCNT catalyst presents the highest electroactivity for the oxidation of C2? C4 alcohols among the prepared catalysts. Electrocatalytic activity order among propanol isomers and butanol isomers is as follows respectively: 1‐propanol > 2‐propanol, and n‐butanol > iso‐butanol > sec‐butanol > tert‐butanol. This is consistent with the Mulliken charge value of the carbon atom bonded with hydroxyl group in the corresponding alcohol molecule.     

An Improved Ruthenium Catalyst for the Environmentally Benign Amination of Primary and Secondary Alcohols

The N‐alkylation of amines in the presence of different ruthenium catalysts generated in situ was investigated. Among the various catalysts tested, the combination of [Ru₃(CO)₁₂] and N‐phenyl‐2‐(dicyclohexylphosphanyl)pyrrole showed the best performance. By applying this novel catalyst, a variety of functionalized alcohols and amines were converted into the corresponding secondary amines in high yield.     

Photocatalytic Dehydrogenative Cross‐Coupling of Alkenes with Alcohols or Azoles without External Oxidant

Direct cross‐coupling between alkenes/R‐H or alkenes/RXH is a dream reaction, especially without external oxidants. Inputting energy by photocatalysis and employing a cobalt catalyst as a two‐electron acceptor, a direct C−H/X−H cross‐coupling with H₂ evolution has been achieved for C−O and C−N bond formation. A new radical alkenylation using alkene as the redox compound is presented. A wide range of aliphatic alcohols—even long chain alcohols—are tolerated well in this system, providing a new route to multi‐substituted enol ether derivatives using simple alkenes. Additionally, this protocol can also be used for N ‐vinylazole synthesis. Mechanistic insights reveal that the cobalt catalyst oxidizes the photocatalyst to revive the photocatalytic cycle.     

Iron‐Catalyzed Highly Enantioselective cis‐Dihydroxylation of Trisubstituted Alkenes with Aqueous H2O2

Reliable methods for enantioselective cis‐dihydroxylation of trisubstituted alkenes are scarce. The iron(II) complex cis‐α‐[Fe^II(2‐Me₂‐BQPN)(OTf)₂], which bears a tetradentate N₄ ligand (Me₂‐BQPN=(R,R)‐N,N′‐dimethyl‐N,N′‐bis(2‐methylquinolin‐8‐yl)‐1,2‐diphenylethane‐1,2‐diamine), was prepared and characterized. With this complex as the catalyst, a broad range of trisubstituted electron‐deficient alkenes were efficiently oxidized to chiral cis‐diols in yields of up to 98 % and up to 99.9 % ee when using hydrogen peroxide (H₂O₂) as oxidant under mild conditions. Experimental studies (including ¹⁸O‐labeling, ESI‐MS, NMR, EPR, and UV/Vis analyses) and DFT calculations were performed to gain mechanistic insight, which suggested possible involvement of a chiral cis‐Fe^V(O)₂ reaction intermediate as an active oxidant. This cis‐[Fe^II(chiral N₄ ligand)]²⁺/H₂O₂ method could be a viable green alternative/complement to the existing OsO₄‐based methods for asymmetric alkene dihydroxylation reactions.     

A practical innovative method for highly selective oxidation of alkenes and alkanes using Fe (III) and Mn (III) porphyrins supported onto multi‐wall carbon nanotubes as reusable heterogeneous catalysts

Functionalized multi‐walled carbon nanotubes were used for covalent immobilization of meso‐tetrakis(4‐carboxyphenyl) porphyrinatoiron (III) chloride [Fe (TCPP)Cl] and meso‐tetrakis(4‐carboxyphenyl) porphyrinatomanganese (III) acetate [Mn (TCPP)OAc]. The full characterization of the hybrid porphyrinic nanomaterials, by Fourier transform‐infrared and UV–Vis spectroscopy, transmission electron microscopy, thermogravimetry and flame atomic absorption spectrometry is described. The oxidation of alkenes and alkanes with molecular oxygen as green oxidant in the presence of Mn‐ and Fe‐catalysts has been studied in a comparative manner. The Fe‐catalyst was shown to have higher catalytic activity compared with the Mn‐catalyst. In addition, both separable solid catalysts can be recovered and reused at least 10 times along with good yields.     

Efficient and selective oxidation of hydrocarbons with tert-butyl hydroperoxide catalyzed by oxidovanadium(IV) unsymmetrical Schiff base complex supported on γ-Fe2O3 magnetic nanoparticles

The catalytic activity of an oxidovanadium(IV) unsymmetrical Schiff base complex supported on γ-Fe₂O₃ magnetic nanoparticles, γ-Fe₂O₃@[VO(salenac-OH)] in which salenac-OH?=?[9-(2′,4′-dihydroxyphenyl)-5,8-diaza-4-methylnona-2,4,8-trienato](-2), was explored in the oxidation of hydrocarbons with tert-butyl hydroperoxide (TBHP, 70% aqueous solution) as oxidant. High catalytic activity and selectivity were demonstrated by this magnetic nanocatalyst in alkane hydroxylation and alkene epoxidation, and the corresponding products were obtained with good to excellent yields in acetonitrile at 50 °C. Reasonable catalytic activity was presented by this supported catalyst in the epoxidation of linear alkenes under optimal reaction conditions. In addition, alkylbenzene derivatives and cycloalkanes can be oxidized to their corresponding alcohols and ketones with good yields in this catalytic system. It is possible to magnetically separate the γ-Fe₂O₃@[VO(salenac-OH)] catalyst and reuse it four times without losing the activity significantly. Moreover, the catalyst structure and morphology do not change after recovery, as indicated by comparing scanning electron microscopy (SEM) image, Fourier transform infrared (FT-IR) and diffuse reflectance spectrum (DRS) of the recovered catalyst with those of the fresh catalyst.

     

Synthesis and polymerization of long alkyl acetylenic compounds

A variety of long alkyl acetylenic compounds was synthesized and polymerized by using Rh(I) catalysts. Particularly, the monomer having a terminal hydroxyl group was converted into the high molecular weight of polymer in good conversion, which was soluble in THF, DMSO, and DMF. The high stereoregularity (cis) in the main chain was confirmed by ¹H‐NMR in DMSO‐d₆ and the rod‐like structure by wide‐angle X‐ray diffraction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3419–3427, 2000     

Lead Ruthenate Pyrochlore Formed in Clay for Sensitive Determination of Dopamine

Natural iron‐intercalated clay (nontronite, SWa‐1) was converted into a more efficient catalyst by formation of lead ruthenate pyrochlore (Py, Pb₂Ru_2?xPb_xO_7?y) directly inside the matrix. The new material can take advantage of the electrocatalytic properties of both Py and nontronite and can even generate new catalytic active sites due to interaction of Py and nontronite. The combination of these two interesting catalysts results in a more reactive electrode (designated as SWa‐1/PyCME) for electroanalytical applications. The preparation and characterization of the SWa‐1/PyCME as well as its electrocatalytic behavior toward the oxidation of dopamine are described in this study. Surface saturation kinetics in terms of Michaelis‐Menten kinetics suited well the oxidation of dopamine on this modified electrode. Under optimized conditions, the detection limit (S/N= 3) was 0.54 nM by square‐wave voltammetry.     

Layered‐Double‐Hydroxide‐Supported Rhodium(0): An Efficient and Reusable Catalyst for the Heck‐Type Coupling of Alkenes and Arylboronic Acids

Layered‐double‐hydroxide‐supported nanorhodium catalyst (LDH? Rh(0)) was employed in the Heck‐type coupling of alkenes 1 with arylboronic acids 2 to afford the corresponding substituted alkenes 3 in good to high yields (Table 1). The catalyst was separated by simple filtration and reused for five cycles with consistent activity.