Mechanism of the solvent effect on the rate of the cyclohexene hydroxymethoxycarbonylation catalyzed by bis(triphenylphosphine)palladium

Kinetic data of the cyclohexene hydroxymethoxycarbonylation catalyzed by bis(triphenylphosphine) palladium Pd(PPh₃)₂ were processed and considered on the basis of the quantum-chemical calculations. By the method of density functional DFT PBE/3z we found that among the possible catalyst moleculs based on the tetrakis(triphenylphosphine)palladium the most stable is Pd(PPh₃)₂ with the coordination number of palladium equal 2. The interaction energy of Pd(PPh₃)₂ with acetone, acetonitrile, dichloroethane, 1,4-dioxane, nitromethane, and tetrahydrofuran calculated by PM3 method was found to correlate linearly with the reaction rate logarithm. The mechanism of the solvent effect on the reaction rate consists in a specific complexation with the catalyst depending on the molecule rigidity and the creation of energetic and steric constraints for the substrate to access the catalyst.     

A practical synthesis of highly functionalized aryl nitriles through cyanation of aryl bromides employing heterogeneous Pd/C: in quest of an industrially viable process

Preparation of aryl nitrile 2a through classical Rosenmund-von Braun reaction of aryl bromide 1a resulted in a poor yield (61%) due to a high reaction temperature (165 °C) and a lack of efficient procedure for separating 2a from a large quantity of heavy metal waste (Cu salts). To address these issues, a practical synthesis of multifunctional aryl nitriles through cyanation of aryl bromides has been developed with heterogeneous Pd/C used as the catalyst. Treatment of aryl bromides 1 with Zn(CN)₂ in the presence of Pd/C, Zn, ZnBr₂ and PPh₃ in DMA provided aryl nitriles 2 involving those carrying sterically demanding electron-rich substituent in good yields and in highly reproducible manner. The activity of Pd/C is highly dependent on the properties of the Pd/C. Oxidic thickshell type catalyst Pd/C D5 was found to furnish the highest rate acceleration and yield. The use of heterogeneous Pd/C might anchor and disperse Pd over the solid support of the catalyst, at least in the initial stage of the reaction, to assure the formation of monomeric Pd complex without precipitating to inactive Pd black. The use of a slightly excess of Zn(CN)₂ (0.6 equiv) and air oxidation of phosphine ligand, after end of the reaction, converted Pd species to insoluble phosphine-free Pd cyanides, from which Pd was recovered in high yield through simple filtration followed by usual recovery process involving combustion.     

Structure of Complexes and Catalytic Oxidation of Triarylphosphine in the Reaction of 9-Phenyl-9-phosphafluorene with Bis(acetylacetonato)palladium

Complex formation of 9-phenyl-9-phosphafluorene with bis(acetylacetonato)palladium in benzene and acetonitrile was studied by means of NMR, IR, and UV spectroscopy. The structure of the resulting complexes, as well as specific features of catalytic oxidation of the arylphosphine and the reduction of Pd(II) to Pd(0) to form the complex Pd(PC_{1 8}H_{1 3})₄ were explored. 9-Phenyl-9-phosphafluorene gives two types of complexes with Pd(acac)₂). The first ones are similar to triphenylphosphine complexes, and the others ( complexes) are formed by coordination of the planar aromatic system of the phosphine to the plane of Pd(acac)₂.     

Palladium complexes containing ligands with hydrogen-bonding functionalities. Reactivity and catalytic studies with CO and olefins

The synthesis of the new bidentate N-N ligand 1-(2-(1-(pyridin-2-yl)ethylideneamino)ethyl)-3-ethylurea (PyUr) with a urea substituent attached to the imine nitrogen is reported. This ligand has been used to form palladium complexes and study the potential influence of the urea group (as a hydrogen bonding unit and a hemilabile ligand) in the insertion of CO and olefins into Pd-C bonds. The reaction of PyUr with [Pd(CH₃)(Cl)(COD)] to yield [Pd(CH₃)Cl(PyUr)] (1) is reported. A crystallographic study of this complex was carried out showing that the urea moieties are involved in a series of intermolecular hydrogen bonding interactions. Upon removal of the chloride from the coordination sphere of 1 (by addition of AgBF₄) the urea group of PyUr coordinates to the palladium centre stabilizing an otherwise coordinatively unsaturated complex. The reaction of these complexes with CO to yield [Pd{C(O)CH₃}Cl(PyUr)] (3) and [Pd{C(O)CH₃}(PyUr)][BF₄] (4) is also discussed. Following on from these reactions, the copolymerization of CO and styrene using 1 as a catalyst was studied and is herein reported. The copolymers synthesized using 1 as a catalyst were obtained in moderate yields and showed to have a narrow size distribution. The same reaction was performed using a palladium complex coordinated by an analogous pyridine ligand but without a hydrogen bonding substituent. The results of the copolymerization reactions showed that, although slightly better yields and larger molecular weights were obtained with the PyUr-containing catalyst, the hydrogen bonding groups in PyUr have little influence on the course of the reaction. To explore further the reactivity of the palladium complexes, the reaction between [Pd(CH₃)Cl(PyUr)][BF₄] (2) and CH₂CHCH₂OH was carried out to yield the allyl complex [Pd(η³-CH₂CHCH₂)(PyUr)] (6). The crystal structure of this complex is also reported.     

Bis(triphenylphosphine)palladium(II)succinimide as a precatalyst for Suzuki cross-coupling—subtle effects exerted by the succinimide ligand

A new palladium(II) precatalyst for Suzuki cross-coupling of aryl halides and organoboronic acids has been identified, namely bis(triphenylphosphine)palladium(II)succinimide [(Ph₃P)₂Pd(N-Succ)₂] 2. The precatalyst is easily prepared from palladium(0) precursors, such as (Ph₃P)₄Pd or Pd₂dba₃·CHCl₃/Ph₃P and succinimide, is air, light and moisture stable, and may be employed with a variety of substrates to give the cross-coupled products, in good yields and in reasonable time, at relatively low catalyst loadings.     

CO2‐promoted Water‐medium Ullmann Reaction over a Pd/Phenyl‐SBA‐15 Mesoporous Catalyst

Water‐medium Ullmann reaction was carried out in CO₂ atmosphere over the mesoporous Pd/Ph‐SBA‐15 catalyst exhibiting high activity and selectivity owing to the uniform dispersion of Pd particles and hydrophobilic mesoporous channels which facilitate the diffusion and adsorption of organic molecules, especially in an aqueous medium. The CO₂ also shows promoting effect on activity and selectivity, which could be understood by considering the role of H⁺ in the mechanism of Ullmann reaction. The optimum Ph‐Ph yield (84.0%) was obtained at p=0.8 MPa and V=6.0 mL and could remain almost unchanged even after the catalyst has been used repetitively for 5 times.     

A simple catalyst system for the palladium-catalyzed coupling of aryl halides with terminal alkynes

A convenient catalyst system consisting of Pd(OAc)₂, PPh₃, K₃PO₄ and DMSO was found to be effective for the coupling reaction of aryl halides with terminal alkynes as well as the deacetonative coupling reaction using a 4-aryl-2-methylbut-3-yn-2-ol as a terminal alkyne precursor. An iminophosphine as a ligand worked more effectively for some combination of substrates than triphenylphosphine.     

Palladium-catalyzed trans-selective alkynylation-alkylation tandem process for the synthesis of (E)-3-alkyl-1-trialkylsilyl-3-alken-1-ynes

The Pd-catalyzed trans-selective monoalkynylation of 1,1-dihalo-1-alkenes with XZnCCSiMe₃, where X is Br or Cl, can proceed generally in excellent yields in the presence of Pd(DPEphos)Cl₂ or Pd(dppf)Cl₂, and subsequent alkylation with methyl- and ethylzincs can proceed in excellent yields with ≥98% retention of configuration in the presence of Pd(^tBu₃P)₂ as a catalyst.     

A practical procedure for the synthesis of multifunctional aldehydes through the Fukuyama reduction and elucidation of the reaction site and mechanism

A highly efficient heterogeneous Pd/C catalyst D1 was found to effect the reduction of thiol esters 1 to the corresponding aldehydes 2 with such a low catalyst loading as 0.5-1.0 mol %. The chemical properties of the Pd/C catalysts together with the XRF analysis reveal that the reduction is most likely to proceed on the solid surface of the Pd/C catalyst rather than in the solution phase outside the pores. A reaction mechanism through oxidative addition of Pd to the thiol esters 1 was postulated by detection of the oxidative addition intermediate by React IR analysis. A practical purification of 2 was accomplished by conversion to water-soluble bisulfite adducts 7.     

Hydrogenation on Granular Palladium-containing Catalysts: I. Hydrogenation of Tertiary Acetylene Alcohols

Commercial granular palladium catalyst (0.5% of Pd on -Al₂O₃) was modified by treating with zinc acetate (type 1) or successively with zinc acetate and ammonia (type 2). The treatment significantly increased the hydrogenation selectivity for a triple bond into a double bond: to 85.3-93.1% with the type 1 catalyst and to 96.3-97.8% with the type 2 catalyst. A construction of an autoclave with a fixed bed of the granular catalyst is described.     

Effect of the ligand nature in cobalt complexes on the selectivity of the reaction of carbon dioxide and propylene oxide

The reaction of carbon dioxide with propylene oxide in the presence of the (salen)CoCl or (TPP)CoCl (salen = bis(3,5-di-tert-butyl-salicylidene)-1,2-diaminocyclohexane, TPP = 5,10,15,20-tetraphenylporphyrin) catalyst and the PPNCl (bis(triphenylphosphine)iminium chloride) cocatalyst has been carried out at 20–60°С and a СО₂ pressure of 0.6 MPa to investigate the effect of the ligand nature on the reaction rate and selectivity. The change in the reaction rate and selectivity in relation to the temperature and cocatalyst/catalyst ratio has been studied. The activation energy of the copolymerization of СО₂ with propylene oxide catalyzed by the (salen)CoCl complex have been obtained.     

Synthesis and application of ortho-palladated complex of (4-phenylbenzoylmethylene)triphenylphosphorane as a highly active catalyst in the Suzuki cross-coupling reaction

The ortho-metallated complex [Pd(μ-Cl){κ₂(C,C)-[C₆H₄(PPh₂CHC(O)C₆H₄-Ph-4)}]₂ (2a) was prepared by refluxing of Pd(OAc)₂ and {(Ph)₃PCHCOC₆H₄-Ph-4} (PhBPPY) of in CH₂Cl₂ followed by addition of an excess of KCl in MeOH Complex (2a) reacts with triphenylphosphine to give the mononuclear derivative [Pd(Cl)(PhC₆H₄COCHPPh₂C₆H₄)(PPh₃)] (3a) whose crystal structure has been determined by single crystal X-ray diffraction. The Suzuki reactions of aryl bromides and chlorides of varying electron density using complex (3a) as an efficient catalyst were performed, giving the cross-coupling products in good to excellent yields.     

Partial Oxidation of Ethane to Syngas over Supported Metal Catalysts

The reaction performance for C₂H₆-O₂ to syngas over different supported metal catalysts was investigated in a flow-reactor. The activated behavior of ethane is different from that of methane over the supported nickel catalysts. Although there may exist a gas phase reaction at high temperatures, over a Ni (or Rh)/-Al₂O₃ catalyst, the partial oxidation of ethane to syngas is a heterogeneous process, while over a Pt (or Pd)/-Al₂O₃ catalyst, it may be a homo-heterogeneous process. The Ni/-Al₂O₃ and Rh/-Al₂O₃ catalysts are suitable for partial oxidation of ethane to syngas at high temperatures.     

NOx-Sorbing Metal Oxides, MnOx–CeO2. Oxidative NO Adsorption and NOx–H2 Reaction

(n)MnO_x–(1–n)CeO₂ binary oxides have been studied for the sorptive NO removal and subsequent reduction of NO_x sorbed to N₂ at low temperatures (150 °C). The solid solution with a fluorite-type structure was found to be effective for oxidative NO adsorption, which yielded nitrate (NO^– ₃) and/or nitrite (NO^– ₂) species on the surface depending on temperature, O₂ concentration in the gas feed, and composition of the binary oxide (n). A surface reaction model was derived on the basis of XPS, TPD, and DRIFTS analyses. Redox of Mn accompanied by simultaneous oxygen equilibration between the surface and the gas phase promoted the oxidative NO adsorption. The reactivity of the adsorbed NO_x toward H₂ was examined for MnO_x–CeO₂ impregnated with Pd, which is known as a nonselective catalyst toward NO–H₂ reaction in the presence of excess oxygen. The Pd/MnO_x–CeO₂ catalyst after saturated by the NO uptake could be regenerated by micropulse injections of H₂ at 150 °C. Evidence was presented to show that the role of Pd is to generate reactive hydrogen atoms, which spillover onto the MnO_x–CeO₂ surface and reduce nitrite/nitrate adsorbing thereon. Because of the lower reducibility of nitrate and the competitive H₂–O₂ combustion, H₂–NO reaction was suppressed to a certain extent in the presence of O₂. Nevertheless, Pd/MnO_x–CeO₂ attained 65% NO-conversion in a steady stream of 0.08% NO, 2% H₂, and 6% O₂ in He at as low as 150 °C, compared to ca. 30% conversion for Pd/–Al₂O₃ at the same temperature. The combination of NO_x-sorbing materials and H₂-activation catalysts is expected to pave the way to development of novel NO_x-sorbing catalysts for selective deNO_x at very low temperatures.     

<Emphasis Type="Italic">N</Emphasis>-doped carbon supported Pd catalysts for <Emphasis Type="Italic">N</Emphasis>-formylation of amines with CO<Subscript>2</Subscript>/H<Subscript>2</Subscript>

Using mesoporous N-doped carbons (NCs) derived from glucose and melamine as the supports, a series of Pd/NC catalysts were prepared, in which Pd nanoparticles with average size<2.0 nm were uniformly distributed on the supports. It was indicated that the resultant Pd/NC catalysts were effective for N-formylation of amines with CO₂ and H₂ in ethanol without any additives. Especially, the catalyst Pd/NC-800-6.9% containing quaternary N showed the best performance, affording a series of formylamides in good or even excellent yields. Further investigation reveals that the interaction between the Pd nanoparticles and quaternary nitrogen in the NC support was responsible for the good performance of the catalyst.     

Hydrogenation on Granular Palladium-containing Catalysts: II. Hydrogenation of Nitroheterocyclic Compounds

Nitroheterocyclic compounds were reduced in a classical reactor with an agitator equipped with a cell for fixed bed of catalyst. As catalysts were applied granular palladium catalysts (0.5% of Pd on -Al₂O₃ and 2% of Pd on granulated carbon). Anilines and 3-amino-2(1H)-pyridones were obtained in high yield at reduction of the appropriate nitro compounds, and the activity of the catalyst samples only slightly decreased. Yet aminopyrazoles and aminoimidazoles obtained by hydrogenation on palladium were very sensitive to the presence of air even as hydrochlorides. In the course of hydrogenation of nitropyrazoles and nitroimidazoles the activity of the catalyst markedly decreased.     

Domino allylation and cyclization of ortho-alkynylbenzaldehydes with allyltrimethylsilane catalyzed by Pd(II)-Cu(II) bimetallic systems

The reaction of ortho-alkynylated benzaldehydes 1 with allyltrimethylsilane under the Pd(OAc)₂-CuCl₂ catalyst system gave the isochromene derivatives 2 together with the chlorinated products 3. When the reaction was conducted in the presence of half equiv of H₂O, the formation of 3 was suppressed and 2 was obtained in good to high yields. When the reaction of 1a was carried out with trimethylsilylcyanide instead of allylsilane, the cyano group-substituted isochromene 9 was obtained in 94% yield.     

Catalytic activity of Pd(II) and Pd(II)/DAB-R systems for the Heck arylation of olefins

Palladium-catalyzed reactions of aryl bromides with various olefins involving Pd(II)/diazabutadiene (DAB-R) systems have been investigated. The scope of a coupling process using Pd(II) sources and an α-diimine as ligand in the presence of Cs₂CO₃ as base was tested using various substrates. The Pd(OAc)₂/DAB-Cy (1, DAB-Cy=1,4-dicyclohexyl-diazabutadiene) system presents the highest activity with respect to electron-neutral and electron-deficient aryl bromides in coupling with electron rich olefins. The synthesis and X-ray characterization of a Pd(II)-diazabutadiene ligand is reported. Extensive optimization experiments showed that another Pd(II) source, Pd(acac)₂ (acac=acetylacetonate), proved to activate aryl bromides at high temperatures, low catalyst loadings when the appropriate concentration of ⁿBu₄NBr additive was employed. The effect of the DAB-Cy ligand is important at very low catalyst loadings and high temperatures. Pd(acac)₂ and Pd(acac)₂/DAB-Cy precatalysts were very effective for the arylation of various olefins with aryl bromides with respect to reaction rate, catalyst loadings, and functional group tolerance.