Suzuki‐ and Heck‐Type Cross‐Coupling with Palladium Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes

We report on a systematic study of the use of palladium nanoparticles immobilized on spherical polyelectrolyte brushes – Pd@SPB – for Heck‐ and Suzuki‐type coupling reactions. The spherical polyelectrolyte brush particles serving as carriers for the palladium nanoparticles consist of a solid polystyrene core with a radius of 46 nm onto which long chains of cationic polyelectrolytes are grafted. The palladium nanoparticles have directly been generated within this brush layer and the stabilization of the nanoparticles is effected by the colloidal carriers, no further surface stabilization is necessary. We demonstrate that these composite particles present robust catalysts for the Heck‐ and Suzuki‐type coupling reactions. This was shown by carrying out the Suzuki‐ and Heck‐type coupling reactions at relatively low temperatures (Suzuki reaction: 50 °C, Heck reaction: 70 °C). We demonstrate that the catalytic composite particles are not changed by these reaction conditions and retain their full activity for at least four runs. The yields obtained for both reactions are good to excellent. The mild operation conditions of the palladium nanoparticles are traced back to the absence of surface stabilization. Further mechanistic implications are discussed.     

Hybrid Organic‐Inorganic Materials from Di‐(2‐pyridyl)methylamine‐Palladium Dichloride Complex as Recoverable Catalysts for Suzuki,Heck and Sonogashira Reactions

Hybrid silica materials containing the di‐(2‐pyridyl)methylamine‐palladium dichloride complex, prepared by sol‐gel cogelification, are efficient recyclable catalysts for Suzuki (aryl bromides and chlorides), Heck (aryl bromides) and Sonogashira reactions (aryl iodides and bromides). Formation of palladium(0) nanoparticles is observed in the Suzuki and Heck reactions but not in the Sonogashira coupling.     

Palladacycles as Precatalysts in Heck and Cross‐Coupling Reactions

A variety of palladacycles are shown to catalyze Heck alkenylations with similar efficiencies. The new oxapalladacycle 1 was shown to insert and transmetalate under mild conditions with n‐butyl acrylate and vinyltributylstannane, respectively. This palladacycle acts as an active (pre)catalyst for cross‐coupling and Heck reactions.     

Catalytic performance of PVC‐triethylene‐tetramine supported palladium complex for Heck reaction

Polyvinyl chloride‐triethylene‐tetramine supported palladium complex (PVC‐TETA‐Pd) was prepared from polyvinyl chloride via simple method and the production cost of the complex was remarkably low. The complex was an efficient catalyst for Heck reaction. It was active for low activity substrates such as aryl bromides and aryl chlorides. The coupling of bromobeneze with styrene catalyzed by PVC‐TETA‐Pd afforded 99.8% yield of stilbene under the optimized reaction conditions. PVC‐TETA‐Pd could catalyze the Heck reactions in the presence of several different kinds of acid‐binding agents. Furthermore, the good reusability of PVC‐DTA‐Pd was also found for Heck reaction. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Feeding the Heck Reaction with Alcohol: One‐Pot Synthesis of Stilbenes from Aryl Alcohols and Bromides

Aryl alcohols are employed as feedstock for the Heck reaction. Keggin‐type heteropolyacids catalyse the selective dehydration of the alcohols to styrenes, which, in one‐pot, undergo palladium‐catalysed Heck arylation with aryl bromides, affording broadly functionalised stilbenes. The choice of solvent is critical for the cascade dehydration–Heck reaction, with electron‐rich aryl alcohols preferring a basic medium while electron‐deficient ones demanding solvents of lower basicity.     

One‐Pot Palladium‐Catalyzed Synthesis of Selectively Substituted Phenanthridines by Sequential Aryl‐Aryl and Heck Couplings,Aza‐Michael and Retro‐Mannich Reactions

A catalytic synthesis of selectively substituted phenanthridines is achieved through a reaction sequence involving palladium/norbornene‐catalyzed unsymmetrical aryl‐aryl and Heck couplings followed by aza‐Michael and retro‐Mannich reactions. In spite of the many steps involved the method is very simple and allows the formation of selectively substituted phenanthridines under mild conditions in a straightforward one‐pot reaction starting from readily available aryl iodides and bromides.     

Development of a Method for Preparing a Highly Reactive and Stable,Recyclable and Environmentally Benign Organopalladium Catalyst Supported on Sulfur‐Terminated Gallium Arsenide(001): A Three‐Component Catalyst, {Pd}‐S‐GaAs(001), and its Properties

We have developed a method for preparing a recyclable and environmentally benign organopalladium catalyst for the Heck reaction supported on sulfur‐terminated gallium arsenide(001). This three‐component catalyst, {Pd}‐S‐GaAs(001), exhibited high stability and activity, furthermore, it tolerated reuse in 10 runs of the Heck reaction (average yield, 97 %) under aerobic conditions. The sulfur layer was very important to stabilize this catalyst. Only trace amounts of Pd were leached from this catalyst to the reaction mixture, as measured by ICP‐mass. The valence of immobilized Pd was zero by XPS spectrometry.     

Imidazole and Imidazoline Derivatives as N‐Donor Ligands for Palladium‐Catalyzed Mizoroki–Heck Reaction

Imidazole and imidazoline (dihydroimidazole) derivatives can serve as efficient and simple ligands for the palladium‐catalyzed Mizoroki–Heck reaction. Among the imidazole and imidazoline derivatives in our investigations, the 2‐methylimidazoline‐palladium(II) chloride complex exhibited the highest catalytic activity.     

Palladium‐Catalyzed,Chelation‐Assisted Stereo‐ and Regioselective Synthesis of Tetrasubstituted Olefins by Oxidative Heck Arylation

An efficient synthesis of tetrasubstituted olefins was achieved via a palladium‐catalyzed, chelation‐assisted oxidative Heck arylation protocol from trisubstituted olefins bearing a tether with a directing group in a completely stereo‐ and regioselective manner. The stereo‐ and regioselectivity as well as excellent yields of tetrasubstituted olefins originated from the stabilization of a palladium intermediate by chelation between the palladium center and a directing group.     

Efficient Heterogeneously Palladium‐Catalysed Heck Arylation of Acrolein Diethyl Acetal. Selective Synthesis of Cinnamaldehydes or 3‐Arylpropionic Esters

A heterogeneous tetrakis(ammine)palladium‐NaY zeolite {[Pd(NH₃)₄]/NaY} catalyst was applied successfully to the Heck arylation of acrolein diethyl acetal using a large variety of aryl and heteroaryl bromides. Depending on the reaction conditions (Heck versus Cacchi) good to high selectivities toward the 3‐arylpropionic esters or to the cinnamaldehydes were achieved, respectively. Under classical Heck conditions, while the catalyst was found to be stable over the two first runs, it showed significant loss of activity from the third cycle. Under Cacchi conditions, the catalyst could not be reused as it led to high dehalogenation rates. All results indicate that the reactions proceed through dissolved palladium species in the bulk solution (leaching). As observed by transmission electronic microscopic (TEM) analyses, while these species can be trapped and stabilised by the zeolite framework under the Heck conditions, they tend to form large palladium(0) aggregates under the Cacchi conditions leading to dehalogenation rather than to the expected Heck coupling.     

Synthesis of Dibenzothiophenes and Carbazoles by Sequential ‘Tetra‐Fold Heck/6 π‐Electrocyclization/Dehydrogenation’ Reactions of Tetrabromothiophene and Tetrabromo‐N‐methylpyrrole

Dibenzothiophenes and carbazoles were prepared by tetrafold Heck reactions of tetrabromothiophene and N‐methyltetrabromopyrrole and subsequent 6π‐electrocyclization and dehydrogenation. A number of reactions could be successfully carried out as domino reactions in only one synthetic step.     

Ligand‐Free Reusable Palladium‐Catalyzed Heck‐Type Coupling Reactions of Hypervalent Iodine Reagents under Mild Conditions

Highly effective palladium‐catalyzed Heck‐type couplings of hypervalent iodine reagents are reported for the first time. It is noteworthy that such reactions could be carried out in the absence of ligand at 40 °C and the catalytic system could be easily reused for five times.     

An Expedient Variant of Heck Reaction of Alkenyl Nonaflates: Homogeneous Ligand‐Free Palladium Catalysis at Room Temperature

A mechanistic study on the ligand‐free room‐temperature Heck reaction of alkenyl nonafluorobutanesulfonates (nonaflates) is described. Kinetic data obtained from poisoning experiments, centrifugation and variation of catalyst loading consistently provide evidence for a homogeneous palladium catalysis unprecedented in Heck chemistry. The Heck reaction of alkenyl perfluorobutanesulfonates represents a remarkably robust, active and efficient catalytic system generally applicable to the coupling with a broad range of terminal olefins including non‐activated ones under ambient conditions. It features insensitivity towards atmospheric oxygen and moisture, furnishing uniformly high yields of the anticipated coupling products without the necessity to purify commercial reagents and solvents.     

Practical Synthesis of Vinyl‐Substituted p‐Phenylenevinylene Oligomers and Their Triethoxysilyl Derivatives

Luminescent semiconducting organic compounds are widely used as active layers in electro‐optical devices. Apart from conjugated polymers, monodisperse oligomers also represent attractive materials. The synthesis of stilbenoid oligomers with polymerizable end groups is presented. Oligo(phenylenevinylene)s with terminal vinyl groups 17 – 19 are prepared in good yields by Horner–Emmons olefinations or by the Heck reaction of the iodo‐substituted oligomers 15 , 16 with compressed ethene. Triethoxysilyl groups can be linked via rigid 1,2‐vinylene units to the chromophores 26 – 30 , either in the direct reaction of 14 , 24 with silanes 21 , 22 or by cross‐metathesis of 17 – 19 with the vinylsilanes 21 , 22 using Grubbs catalyst.     

4‐Aryl‐2‐quinolones through a Pseudo‐Domino Heck/Buchwald–Hartwig Reaction in a Molten Tetrabutylammonium Acetate/ Tetrabutylammonium Bromide Mixture

4‐Aryl‐2‐quinolones can be prepared from readily available o‐bromocinnamamide and aryl iodides using phosphine‐free palladium(II ) acetate as the precatalyst and a molten tetra(n‐butyl)ammonium acetate/tetra(n‐butyl)ammonium bromide mixture as the reaction medium. The reaction proceeds through a pseudo‐domino process involving two mechanistically independent, sequential catalytic cycles: a Heck reaction followed by an intramolecular Buchwald–Hartwig C N bond forming reaction.     

The Clicked Pyridyl‐Triazole Ligand: From Homogeneous to Robust,Recyclable Heterogeneous Mono‐ and Polymetallic Palladium Catalysts for Efficient Suzuki–Miyaura,Sonogashira, and Heck Reactions

Various mono‐ and polymetallic palladium complexes containing a 2‐pyridyl‐1,2,3‐triazole (pyta) ligand or a nonabranch‐derived (nonapyta) ligand have been synthesized by reaction of palladium acetate with these ligands according to a 1:1 metal‐ligand stoichiometry and used as catalysts for carbon‐carbon cross‐coupling including the Suzuki–Miyaura, Sonogashira and Heck reactions. The unsubstituted monopalladium and nonapalladium complexes were insoluble in all the reaction media, whereas tri‐ and tetranuclar palladium complexes were soluble, which allowed conducting catalysis under either homogeneous or heterogeneous conditions. The organopalladium complexes were characterized by standard analytical and spectroscopic methods and by thermogravimetry showing decomposition above 110 °C. Both types of catalysts showed excellent activity for these cross carbon‐carbon bond formations involving aryl halides including activated aryl chlorides or acyl chloride. Besides the comparison between homogeneous and heterogeneous catalysis, the key feature of these catalysts is their remarkable robustness that allowed recycling at least ten times in the example of the Heck reaction with excellent yields and without significant reduction of the conversion.     

Iron‐Catalyzed Mizoroki–Heck Cross‐Coupling Reaction with Styrenes

In the presence of iron(II) chloride (FeCl₂; 20 mol%) and potassium tert‐butoxide (t‐BuOK; 4 equiv.) in dimethyl sulfoxide (DMSO), aryl and heteroaryl iodides undergo stereoselective Mizoroki–Heck C C cross‐coupling reactions with styrenes at 60 °C giving the corresponding (E)‐alkenes. The best yields are obtained upon adding a ligand (80 mol%) such as proline or picolinic acid. Aryl bromides and pyridinyl bromides are also coupled with styrenes but in lower yields.     

Microwave‐Assisted One‐Step Synthesis of Acetophenones via Palladium‐Catalyzed Regioselective Arylation of Vinyloxytrimethylsilane

The regiochemistry of the palladium‐mediated arylation (Heck arylation) of enol ethers is sensitive to the structure of the enol ether, the arylating agent and the catalytic system. In this study, an effective and practical method was successfully developed for the synthesis of acetophenones with high regioselectivity under palladium‐catalyzed conditions. A variety of acetophenones was readily prepared from aryl iodides in good to excellent yields under microwave irradiation in a single step. The key feature of our new protocol is the use of vinyloxytrimethylsilane as a highly regioselective acylation reagent.     

Palladium‐Catalyzed Oxidative Heck‐Type Allylation of β,β‐Disubstituted Enones with Allyl Carbonates

The palladium‐catalyzed oxidative Heck‐type allylation of β,β‐disubstituted enones, i.e., α‐oxoketene dithioacetals, was efficiently realized with allyl carbonates, providing a concise route to highly functionalized dienes. The present synthetic methodology utilizes the substrate activation strategy to activate the C H bond of β,β‐disubstituted enones by introduction of a 1,2‐dithiolane functionality to make the enone substrate highly polarized and thus increase its reactivity, demonstrating rare examples for transition metal‐catalyzed allylic substitution of ß,ß‐disubstituted enones through a Heck‐type allylation process.

     

A highly efficient polymer‐anchored palladium(II) complex catalyst for hydrogenation,Heck cross‐coupling and cyanation reactions

BACKGROUND: Precise architectures of steric and electronic properties of palladium species play a crucial role in designing highly functionalized catalyst systems responsible for target organic transformations. Pd catalysts supported on polymer materials have been employed extensively as catalysts not only for hydrogenation but also for coupling reactions in the production of fine chemicals. RESULTS: A new polymer‐anchored Pd(II) complex has been synthesized and characterized. The catalyst shows high catalytic activity in the hydrogenation of styrene oxide, Heck cross‐coupling and cyanation reactions of aryl halides. The effect of various reaction parameters were investigated to optimize reaction conditions. The catalytic system shows good activity in the hydrogenation of styrene oxide (conversion 98%) with a selectivity to 2‐phenylethanol (93%) which is higher than its homogeneous analogues. The catalyst also exhibits excellent catalytic activity for the Heck cross‐coupling and cyanation reactions of various substituted and non‐substituted aryl halides. CONCLUSIONS: Results demonstrate that the catalyst is robust and stable and can be recovered quantitatively by simple filtration and reused several times without loss of activity. Copyright © 2010 Society of Chemical Industry