纳米碳纤维负载钯催化剂在Heck反应中的应用——载体相互作用的影响

使用多元醇还原法制备了均匀分散的钯纳米颗粒.将钯纳米颗粒负载于板式、鱼骨式和管式纳米碳纤维,得到稳定、可重复使用的非均相催化剂.实验结果表明,钯纳米胶粒同载体之间的电位差对钯在载体上的负载量、粒子大小以及Heck反应中钯的溶失量有很大的影响.在制备过程中,增加钯纳米胶粒同纳米碳纤维表面的电位差能够大大降低钯在Heck反应中的流失.催化剂的反应活性随钯粒子的增大而降低.     

First palladium-catalyzed Heck reactions with efficient colloidal catalyst systems

For the first time it has been shown that palladium colloids are effective and active catalysts for the olefination of aryl bromides (Heck reaction). Worthy of note are the high activities of the catalyst system for activated aryl bromides under optimized reaction conditions, which are better than or comparable with “classical” palladium phosphine complexes. Addition of phosphines strongly retards the reaction rate of the colloid catalyst. Nevertheless, this type of catalyst is not suitable for the activation of non-activated substrates, especially technically interesting aryl chlorides.     

Solid-supported cross-coupling catalysts derived from homogeneous nickel and palladium coordination complexes

Solid-supported catalysts derived from homogeneous nickel(II) and palladium(II) non-symmetrical salen-type coordination complexes have been prepared and shown to be effective in the heterogeneous catalysis of carbon-carbon cross-coupling reactions. The nickel catalyst has been used in room-temperature Tamao-Kumada-Corriu reactions and the palladium catalyst in the Heck reaction at elevated temperatures. The complexes were prepared by improved methods and characterised by spectroscopic techniques. Comparisons between the solid-supported catalysts and their homogeneous analogues are reported. The single-crystal structure determination of the nickel and palladium complexes [M(salenac-OH)][M = Ni, Pd; salenac-OH = 9-(2',4'-dihydroxyphenyl)-5,8-diaza-4-methylnona-2,4,8-trienato](2-)] is reported.     

Catalytic activity of palladium supported on single wall carbon nanotubes compared to palladium supported on activated carbon Study of the Heck and Suzuki couplings, aerobic alcohol oxidation and selective hydrogenation

Nanoparticles (2–10 nm) of palladium have been deposited on single wall carbon nanotubes (SWNT) by spontaneous reduction from Pd(OAc)₂ or from oxime carbapalladacycle. These catalysts exhibit higher catalytic activity than palladium over activated carbon (Pd/C) for the Heck reaction of styrene and iodobenzene and for the Suzuki coupling of phenylboronic and iodobenzene. This fact has been attributed as reflecting the dramatic influence of the size particle on the activity of the palladium catalyst for CC bond forming reactions as compared to other reaction types less demanding from the point of view of the particle size. Thus, in contrast to the Heck and Suzuki reactions, Pd/C is more active than palladium nanoparticles deposited on SWNT for the catalytic oxidation by molecular oxygen of cinnamyl alcohol to cinnamaldehyde and for the hydrogenation of cinnamaldehyde to 3-phenylpropionaldehyde.     

非钯催化剂催化Heck反应研究进展

非钯催化剂用于催化Heck反应有较多的报道.与钯催化剂相比,一些非钯催化剂(如镍、铜、钴等)显示了更优越的性能.近年来,铂、铑、钌、铁等催化体系也被用于Heck反应.对近年来非钯催化剂用于催化Heck反应的研究进行了归纳总结.     

Design of highly active heterogeneous palladium catalysts for the activation of aryl chlorides in Heck reactions

In situ generation of highly active palladium species by intermediate dissolution of Pd from solid supported catalysts has been demonstrated to be a very successful approach for the activation of aryl chlorides in Heck reactions. The new ‘heterogeneous’ Pd catalysts act as reservoir for molecular Pd species with unsaturated coordination sphere in solution. Crucial Pd leaching and re-deposition onto the support can be controlled by optimization of reaction conditions and by the properties of the catalysts. Pd is re-deposited onto the support at the end of the reaction. The catalysts, palladium supported on activated carbon, on various metal oxides or fluorides and Pd complexes in zeolites, are easy to prepare, though the preparation conditions are crucial. The catalysts convert all aryl bromides completely within minutes (TON 100,000). Aryl chlorides (even deactivated ones) are converted with high yields, within 2-6 h. The catalysts belong to the most active ones in Heck reactions at all (including best homogeneous systems) and fulfill all relevant requirements for practical applications in laboratory and industry.     

Heck reaction catalyzed by PD-modified zeolites.

[Pd]-exchanged NaY zeolites have been prepared, characterized, and applied for the first time for catalytic carbon-carbon coupling reactions. The catalysts exhibit a high activity and selectivity toward the Heck reaction of aryl bromides with olefins for small palladium concentrations (< or =0.1 mol % of Pd). The catalysts can easily be separated from the reaction mixture and reused after washing without loss in activity. No limitation to the diffusion of adducts in the zeolite cages was observed (for linear alkenes). The electronic nature of the aryl bromides and the olefins has a dominating effect on the reaction yield and selectivity. The heterogeneous catalysts quantitatively convert all types of all aryl bromide (complete conversion of bromobenzene within 30 min) and activated aryl chlorides under standard reaction conditions. Product form selectivity is observed in the Heck reaction with cyclic olefins.     

Palladium–N-heterocyclic carbene complexes for the Mizoroki–Heck reaction: An appraisal

This article presents a review of the most significant developments with N-heterocyclic carbene (NHC)–palladium catalytic systems used for the Heck reaction. For more than the past two decades, a large number of new NHC–Pd complexes have been synthesized and characterized. These studies focused on NHCs as a phosphine analogue, but the current review highlights the differences with particular ligands so as to attain a suitable balance between the electronic and bulky environments around the metal. NHCs have gained wide recognition as these ligands act as excellent σ-donors that form stable metal–NHCs with strong metal–carbon bonds. For this reason, metal–NHCs are commonly used as they are highly reactive and can selectively serve as catalysts for various chemical transformations. The objective of our article is to highlight significant recent progress in NHC–Pd(II) complexes and provide an overview of their extensive interaction in the Mizoroki–Heck reaction.     

Heck reactions of iodobenzene and methyl acrylate with conventional supported palladium catalysts in the presence of organic and/or inorganic bases without ligands

The vinylation of iodobenzene with methyl acrylate has been studied with several supported palladium catalysts in N-methylpyrrolidone in the presence of triethylamine and/or sodium carbonate. The reaction can be performed in air without any solubilizing or activating ligands. It was found that significant amounts of palladium leach out into the solvent and these dissolved Pd species essentially catalyze the reaction. It is interesting, however, that almost all the palladium species in the solution can redeposit onto the surface of the supports after the reaction has been completed (at 100% conversion of iodobenzene). Thus, the catalysts were recyclable without loss of activity. The use of both inorganic and organic bases is very effective in the promotion of the palladium redeposition as well as in the enhancement of the reaction rate. For Heck reactions with bromobenzene and chlorobenzene it was found that the use of triethylamine and sodium carbonate increases the selectivity of the Heck coupling product (benzene is also produced for these two substrates), but the mixed bases do not affect the overall rate of reaction as much.     

Silica-supported polymeric palladium (0) complexes as catalysts for Heck reaction in organic synthesis

Silica-supported palladium (0) complexes have been prepared from γ-chloropropyl triethoxysilica and γ-aminopropyl triethoxysilica via immobilization on fumed silica, followed by reacting with ethylenediamine and salicylaldehyde, and then the reaction with palladium chloride in ethanol and reduction with KBH₄ in ethanol. They are efficient catalysts for Heck reaction of aryl iodides with alkene at 90^oC. These polymeric palladium (0) catalysts can be recovered and reused without loss in activity. This revised version was published online in June 2006 with corrections to the Cover Date.     

A ligand-free Heck reaction catalyzed by the in situ-generated palladium nanoparticles in PEG-400

<正>A ligand-free Heck reaction catalyzed by in situ-generated palladium nanoparticles in PEG-400 has been developed.This catalytic system is a simple and active protocol for the Heck reaction of aryl halides under mild conditions.Comparative experiments demonstrated that the Heck reaction catalyzed by the palladium nanoparticles in situ-generated under the Heck reaction conditions was carried out much quicker than that by the in ex situ-generated ones.     

Cyclopalladated complexes of 2-phenylaniline and their catalytic activity in Suzuki and Heck reactions under mild conditions

A series of palladium complexes bearing monodentate and bidentate ligands were applied in the Suzuki reaction of aryl halides and the Heck reaction of styrene with phenylboronic acid. The complexes were found to be effective catalysts for these reactions affording the cross-coupled products in moderate to excellent yields.     

Palladium-triazine aminoalcohol nanocomposite, its reactivity on Heck reaction

Several studies on dendrimer synthesis and reactivity have been carried out in order to control the size and functionality of compounds. From such studies, it has been suggested that these molecules may be used as ligands to synthesize potential homogeneous catalysts, firstly, in order to get the benefits of both homo- and heterogeneous catalysis (i.e. high activity and/or selectivity, good reproducibility, accessibility of the metal site, intermediaries detection, etc.); secondly, because, unlike other polymeric species, they can be readily recoverable after reaction. In this paper, following our interest in homogeneous catalysts, we would like to present our findings from studies on the synthesis and characterization of a prime molecule, triazine aminoalcohol, as starting or zero generation dendrimer and its interaction-reaction with palladium nanoparticles as well as our results on the reactivity on Heck type catalysis.     

Ion- and atom-leaching mechanisms from palladium nanoparticles in cross-coupling reactions

Leaching of palladium species from Pd nanoparticles under C--C coupling conditions was observed for both Heck and Suzuki reactions by using a special membrane reactor. The membrane allows the passage of palladium atoms and ions, but not of species larger than 5 nm. Three possible mechanistic scenarios for palladium leaching were investigated with the aim of identifying the true catalytic species. Firstly, we examined whether or not palladium(0) atoms could leach from clusters under non-oxidising conditions. By using our membrane reactor, we proved that this indeed happens. We then investigated whether or not small palladium(0) clusters could in fact be the active catalytic species by analysing the reaction composition and the palladium species that diffused through the membrane. Neither TEM nor ICP analysis supported this scenario. Finally, we tested whether or not palladium(II) ions could be leached in the presence of PhI by oxidative addition and the formation of [Pd(II)ArI] complexes. Using mass spectrometry, UV-visible spectroscopy and 13C NMR spectroscopy, we observed and monitored the formation and diffusion of these complexes, which showed that the first and the third mechanistic scenarios were both possible, and were likely to occur simultaneously. Based on these findings, we maintain that palladium nanoparticles are not the true catalysts in C--C coupling reactions. Instead, catalysis is carried out by either palladium(0) atoms or palladium(II) ions that leach into solution.     

Rationally designed pincer-type heck catalysts bearing aminophosphine substituents: Pd IV intermediates and palladium nanoparticles

The aminophosphine-based pincer complexes [C6H3-2,6-(XP(piperidinyl)2)2Pd(Cl)] (X=NH 1; X=O 2) are readily prepared from cheap starting materials by sequential addition of 1,1',1'-phosphinetriyltripiperidine and 1,3-diaminobenzene or resorcinol to solutions of [Pd(cod)(Cl)2] (cod=cyclooctadiene) in toluene under N2 in "one pot". Compounds 1 and 2 proved to be excellent Heck catalysts and allow the quantitative coupling of several electronically deactivated and sterically hindered aryl bromides with various olefins as coupling partners at 140 degrees C within very short reaction times and low catalyst loadings. Increased reaction temperatures also enable the efficient coupling of olefins with electronically deactivated and sterically hindered aryl chlorides in the presence of only 0.01 mol % of catalyst. The mechanistic studies performed rule out that homogeneous Pd 0 complexes are the catalytically active forms of 1 and 2. On the other hand, the involvement of palladium nanoparticles in the catalytic cycle received strong experimental support. Even though pincer-type Pd IV intermediates derived from 1 (and 2) are not involved in the catalytic cycle of the Heck reaction, their general existence as reactive intermediates (for example, in other reactions) cannot be excluded. On the contrary, they were shown to be thermally accessible. Compounds 1 and 2 show a smooth halide exchange with bromobenzene to yield their bromo derivatives in DMF at 100 degrees C. Experimental observations revealed that the halide exchange most probably proceeded via pincer-type Pd IV intermediates. DFT calculations support this hypothesis and indicated that aminophosphine-based pincer-type Pd IV intermediates are generally to be considered as reactive intermediates in reactions with aryl halides performed at elevated temperatures.     

Uridate/pyridyl Pd(II) complexes: Phosphine-free high turnover catalysts for the Heck reaction of deactivated aryl bromides

The synthesis and structure of palladium(II) complexes bearing uridato/pyridyl ligands as an anionic N-donor coordination sites are reported. The complexes have been shown to be highly active catalysts for the Heck reaction of aryl bromides (TON 4.0×10⁴-9.1×10⁴) and moderate activity for the activation of aryl chlorides under phosphine-free conditions.     

Palladium N-methylimidazolium supported complexes as efficient catalysts for the Heck reaction

Different N-methylimidazolium supported ligands have been easily synthesized. The palladium complexes derived from those materials can be used for the catalysis of the Heck reaction giving excellent yields, selectivities and very good TON and TOF values. The supported Pd-pincer complexes show an increased stability, and provide a clear improvement in the recovery and reuse for the supported catalysts.     

Dendrimers Functionalized with Palladium Complexes of N-, N,N-, and N,N,N-Ligands

Pyridine, pyridine imine, and bipyridine imine ligands functionalized by a phenol have been synthesized and characterized, in many cases by X-ray diffraction. Several of these N-, N,N-, and N,N,N,-ligands have been grafted onto the surface of phosphorhydrazone dendrimers, from generation 1 to generation 3. The complexation ability of these monomers and dendrimers towards palladium(II) has been assayed. The corresponding complexes have been either isolated or prepared in situ. In both cases, the monomeric and dendritic complexes have been tested as catalysts in Heck couplings and in Sonogashira couplings. In some cases, a positive dendritic effect has been observed, that is, an increase of the catalytic efficiency proportional to the dendrimer generation.     

Palladium-catalyzed Heck coupling reactions using different fluorinated phosphine ligands in compressed carbon dioxide and conventional organic solvents

Palladium-catalyzed Heck reaction of iodobenzene and styrene was investigated in compressed CO₂ using different fluorinated phosphine compounds as ligands at a temperature of 70 °C. The reaction mixture is a single phase at 12 MPa but biphasic at 8 MPa, a little higher than the critical pressure of pure CO₂ under the reaction conditions used. Although the solubility of fluorinated ligands is very high in dense CO₂, they have marginal improvements in Heck conversion in this medium compared with a non-fluorinated ligand of triphenylphosphine. The activity of palladium complexes strongly depends on the kind of phosphine compound used, in the order of bis(pentafluorophenyl)phenylphosphine (III)>triphenylphosphine (I), tris(pentafluorophenyl)phosphine (IV)>diphenyl(pentafluorophenyl)phosphine (II), tris(p-fluorophenyl)phosphine (V)>tris(p-trifluoromethyl phenyl)phosphine (VI), 1,2-bis[bis(pentafluorophenyl)phosphino]ethane (VII), for the homogeneous reaction at 12 MPa. This order of effectiveness of these ligands is different from those obtained in conventional organic solvents. Hexane, toluene, ethanol, and N-methylpyrrolidone (NMP) showed maximum conversions with the ligands VI, IV, V and VII, respectively. The conversion in CO₂ with the ligand III is comparable with those in polar solvents of ethanol and NMP, and larger than those in hexane and toluene in the presence of the best ligands. The dense CO₂ may affect the specific activity of palladium complex catalysts and/or the reactivity of reacting species. Small quantities of fluorinated products were observed to form at high pressure of CO₂ and this is direct evidence of P–C bond cleavage during Heck reaction in dense CO₂. The activity of palladium complexes with those ligands is higher in more polar solvent.     

Tricyclohexylphosphine-cyclopalladated ferrocenylimine complexes: synthesis, crystal structures and application in Suzuki and Heck reactions

A series of novel tricyclohexylphosphine (PCy(3))-cyclopalladated ferrocenylimine complexes 2c-2g have been easily synthesized. These new palladacycles are thermally stable and are not sensitive to air and moisture. Their detailed structures have been determined by single-crystal X-ray analysis and six different types of intermolecular hydrogen bonds are found to be existed in the crystals of these complexes. The use of 2c-2g as catalysts for Suzuki and Heck reactions was examined. They were found to be very efficient for the Suzuki reaction of aryl chlorides with phenylboronic acid. Typically, using 0.1 mol% of catalyst in the presence of 1.5 equivalent of Cs(2)CO(3) as base in dioxane at 100 degrees C provided coupled products in excellent yields. These complexes also displayed good activity in the Heck reaction of a range of aryl bromides with acrylic acid ethyl ester although they were not particularly useful for the activation of aryl chlorides.