Showing papers on "Heck reaction published in 2007"

Palladium nanoparticles as efficient green homogeneous and heterogeneous carbon-carbon coupling precatalysts: a unifying view.

Abstract: Pd catalysis of C-C bond formations is briefly reviewed from the angle of nanoparticles (NPs) whether they are homogeneous or heterogeneous precatalysts and whether they are intentionally preformed or generated from a Pd derivative such as Pd(OAc)2. The most studied reaction is the Heck coupling of halogenoarenes with olefins that usually proceeds at high temperature (120-160 degrees C). Under such conditions, the PdII precursor is reduced to Pd0, forming PdNPs from which Pd atom leaching, subsequent to oxidative addition of the aryl halide onto the PdNP surface, is the source of very active molecular catalysts. Other C-C coupling reactions (Suzuki, Sonogashira, Stille, Negishi, Hiyama, Corriu-Kumada, Ullmann, and Tsuji-Trost) can also be catalyzed by species produced from preformed PdNPs. For catalysis of these reactions, leaching of active Pd atoms from the PdNPs may also provide a viable molecular mechanistic scheme. Thus, the term "PdNP catalysis of C-C coupling" used in this review refers to this function of PdNPs as precursors of catalytically active Pd species (i.e., the PdNPs are precatalysts of C-C coupling reactions).

Ion‐ and Atom‐Leaching Mechanisms from Palladium Nanoparticles in Cross‐Coupling Reactions

Abstract: 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 [PdIIArI] 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.

Mizoroki−Heck Coupling Using Immobilized Molecular Precatalysts: Leaching Active Species from Pd Pincers, Entrapped Pd Salts, and Pd NHC Complexes

Abstract: The Mizoroki−Heck reaction is a palladium-catalyzed reaction of both practical importance and scientific significance. Two challenges currently faced by practitioners of the Heck and other palladium-catalyzed coupling reactions are (i) minimizing the costs associated with this reaction by developing high TON catalysts or facilitating catalyst recovery and (ii) elucidating the nature of the active species when using various different precatalysts. These two challenges, one practical and one fundamental, served as our motivation in our studies of immobilized molecular palladium(II) complexes as precatalysts in Mizoroki−Heck reactions. This Forum Article describes our investigations in this area, highlighting our approach to the elucidation of the active catalyst by combining kinetic and poisoning studies of several different related precatalysts, our development of new, selective catalyst poisons, and our quest for stable, recyclable supported Heck, Suzuki, and Sonogashira coupling catalysts. Under high-tem...

Genesis of coordinatively unsaturated palladium complexes dissolved from solid precursors during heck coupling reactions and their role as catalytically active species

Abstract: The Forum Article critically summarizes investigations and discussions on the nature and role of potential active species in C−C coupling reactions of the Heck type using catalyst systems with “ligand-free” inorganic salts, simple inorganic complexes, and supported and nonsupported (colloidal) Pd particles. From a series of experiments and reports, it can be concluded that the “active species” is generated in situ in catalytic systems at higher temperature conditions (>100 °C). In all heterogeneous systems with solid Pd catalysts, Pd is dissolved from the solid catalyst surface under reaction conditions by a chemical reaction (complex formation and/or oxidative addition of the aryl halide), forming extremely active coordinatively unsaturated Pd species. Pd is partially or completely redeposited onto the support at the end of the reaction when the aryl halide is used up. The Pd dissolution−redeposition processes correlate with the reaction rate and are strongly influenced by the reaction conditions. Skille...

Supported Ultra Small Palladium on Magnetic Nanoparticles Used as Catalysts for Suzuki Cross-Coupling and Heck Reactions

Abstract: Application of transition metal nanoparticles as catalysts for organic transformations has been attracting wide interest as nanoparticle-based catalytic systems might exhibit superior catalytic activities than the corresponding bulk materials. However, the main difficulty is that such small particles are almost impossible to separate by conventional means, which can lead to the blocking of filters and valves by the nanoparticle catalyst. We have conveniently prepared ultra small palladium(0) particles (less than 1 nm) on magnetic nanoparticle supports, in which immobilized auxiliaries are used as stabilizing ligands. These catalysts were active for Suzuki cross-coupling and Heck reactions giving isolated yields of 83 % and 56 %, respectively. In addition, the catalyst can be easily separated using a magnet and reused several times with sustained activity.

Synthesis of Chiral 3-Substituted Indanones via an Enantioselective Reductive-Heck Reaction

Abstract: An efficient intramolecular palladium-catalyzed, asymmetric reductive-Heck reaction has been developed, which allowed for the synthesis of either enantiomerically enriched 3-substituted indanones or alpha-exo-methylene indanones depending on the base used.

Efficient palladium(II) catalysis under air. Base-free oxidative heck reactions at room temperature or with microwave heating.

Abstract: Scope and limitations of the base-free oxidative Heck reaction with arylboronic acids have been explored. Under our conditions, the dmphen−palladium(II)-catalyzed arylation proceeded with air or p-benzoquinone as reoxidants of palladium(0). We found that ambient temperature and mild aerobic conditions allow for the use of substrates sensitive to palladium(II)-catalyzed oxidation. Oxidative Heck couplings, employing different arylboronic acids, were smoothly and regioselectively conducted with both electron-rich and electron-poor olefins, providing high yields even with disubstituted butyl methacrylate, sensitive acrolein, and a vinylboronate ester. Controlled microwave processing was used to reduce reaction times from hours to minutes both in small scale and in 50 mmol scale batch processes.

Synthesis of dibenzo[g,p]chrysenes from bis(biaryl)acetylenes via sequential ICl-induced cyclization and Mizoroki-Heck coupling.

Abstract: We report a facile synthesis of functionalized dibenzo[g,p]chrysenes via initial ICl-promoted cyclization of bis(biaryl)acetylenes, followed by the Mizoroki−Heck coupling reaction. This new approach works well for various bis(biaryl)acetylenes to afford dibenzo[g,p]chrysenes bearing various functionalities. With substrates of one special type including 4‘-methoxy-2-ethynylbiphenyls, we found that the ICl treatment led to ipso cyclization to give bicyclic spirocyclohexadienones. In the presence of MeOH/H2SO4, these spiroketone products undergo rearrangement to give 9-iodophenanthrenes through a selective 1,2-alkenyl migration. We prepared various 4‘-methoxy-2-ethynylbiphenyl compounds to show the generalization of such an ipso cyclization and 1,2-alkenyl shift. This ipso-cyclization approach can be extended to the preparation of dibenzo[g,p]chrysenes.

N-Type Conjugated Materials Based on 2-Vinyl-4,5-dicyanoimidazoles and Their Use in Solar Cells

Abstract: The high-yield syntheses of a new class of n-type conjugated materials based on the Heck coupling of 2-vinyl-4,5-dicyanoimidazole with selected dibromoaromatics is reported. The resultant materials have tunable HOMO−LUMO levels and photoluminescence ranging from deep blue to red. Solution-processed organic solar cells with external quantum efficiencies exceeding 14% could be prepared by blending selected novel materials with poly-(3-hexylthiophene) (P3HT).

Studies on the Heck Reaction with Alkenyl Phosphates: Can the 1,2-Migration Be Controlled? Scope and Limitations

Abstract: A catalyst system was identified which promotes the Heck coupling of nonactivated vinyl phosphates with electron deficient alkenes providing a new entry to diene products from simple and readily accessible starting materials. In contrast to our earlier work exploiting P(t-Bu)3 as the ligand in the presence of PdCl2(COD), the application of Buchwald's dialkylbiarylphosphines, X-Phos, effectively promoted the vinylic substitution with a wide range of alkenyl phosphates in the presence of 10 equiv of lithium chloride. Importantly, these reaction conditions suppressed 1,2-migration of the alkenyl palladium(II) intermediate. Further studies are also reported with the catalytic system which encourages isomerization in order to determine the range of vinyl phosphates that may participate in these coupling reactions. The extent of the 1,2-migration was dependent on the C1-substituent where best results were noted for substrates possessing a C1-alkyl quaternary carbon. Hence, with certain members of this class of ...

Aryl to Aryl Palladium Migration in the Heck and Suzuki Coupling of o-Halobiaryls

Abstract: A novel 1,4-palladium migration between the o- and o'-positions of biaryls has been observed in organopalladium intermediates derived from o-halobiaryls. The organopalladium intermediates generated by this migration have been trapped either by a Heck reaction employing ethyl acrylate or by Suzuki cross-coupling using arylboronic acids. This palladium migration can be activated or deactivated by choosing the appropriate reaction conditions. Chemical and computational evidence supports the presence of an equilibrium that correlates with the C-H acidity of the available arene positions.

One-step synthesis of palladium/SBA-15 nanocomposites and its catalytic application

Abstract: Well-dispersed palladium nanoparticles in mesoporous SBA- 15 SiO2 were prepared in a facile one-step approach during sol-gel route under reductive atmosphere. X-ray diffraction (XRD) results indicate that as-synthesized nanocomposites basically remain ordered two-dimensional hexagonal mesostructure while transmission electron microscopy (TEM) study exhibits a well dispersion of palladium nanoparticles within the mesoporous SBA-15 channels. The size of Pd nanoparticles is approximately in the range of 5-10nm. However, the resulting nanocomposites exhibit a highly catalytic activity and reused ability at least after five recycles without ligand in air for both the Suzuki and Heck coupling reactions.

Porphyrin pincer complexes: peripherally cyclometalated porphyrins and their catalytic activities controlled by central metals.

Abstract: A series of heterobimetallic porphyrin pincer palladium complexes have been synthesized through direct cyclometalation of β,β‘-dipyridylporphyrin. X-ray analyses elucidated their highly distorted structures of porphyrin cores induced by the outer metalation. The catalytic activity has been investigated in the typical Heck reaction of iodobenzene with butyl acrylate. These complexes exhibit markedly different catalytic activities in the initial phase of the reaction, depending on the central metals.

Pd(quinoline-8-carboxylate)(2) as a low-priced, phosphine-free catalyst for Heck and Suzuki reactions.

Abstract: N,O-Bidentate compounds were systematically evaluated as phosphine-free ligands for Pd-catalyzed C−C bond-formation reactions through kinetic measurements. Pd(quinoline-8-carboxylate)2 was identified as one of the most efficient, yet still low-priced, phosphine-free catalysts for Heck as well as Suzuki reactions of unactivated aryl bromides with high turnover numbers up to ca. 10,000.

Enantioselective Synthesis of (S)- and (R)-Tolterodine by Asymmetric Hydrogenation of a Coumarin Derivative Obtained by a Heck Reaction

Abstract: An efficient and short enantioselective synthesis of (S)- and (R)-tolterodine was performed by asymmetric hydrogenation of a coumarin intermediate, easily obtained by a Heck reaction from inexpensive and commercially available starting materials.

Palladium-catalyzed heck and suzuki coupling in glycerol

Abstract: The Heck coupling of halobenzenes with various alkenes and the Suzuki cross coupling of halobenzenes with phenylboronic acid were successfully performed in glycerol as the reaction solvent using homogeneous and heterogeneous palladium catalysts. Glycerol is a renewable and recyclable green solvent that is able to dissolve organic substrates, inorganic bases, and palladium complexes, and that allows easy isolation of the reaction product by simple extraction with glycerol-immiscible solvents such as diethyl ether, hexane, and dichloromethane.

Screening of a Modular Sugar‐Based Phosphite–Oxazoline Ligand Library in Asymmetric Pd‐Catalyzed Heck Reactions

Abstract: We have synthesised a library of phosphite-oxazoline ligands derived from readily available D-glucosamine. These ligands have been successfully screened in the palladium-catalysed Heck reaction of several substrates with high regio- (up to 99 %) and enantioselectivities (ee's up to 99 %) as well as with improved activities under standard thermal conditions. The results indicate that the catalytic performance is highly affected by the oxazoline and biarylphosphite substituents and the axial chirality of the biaryl moiety of the ligand. The Heck reactions were also performed under microwave irradiation conditions, allowing a considerably shorter reaction time (full conversion in minutes) maintaining the excellent regio- and enantioselectivities.