Metal catalysts Heck reactions

In a first test reaction, the polymer support was used for the hydroformylation of 1-octene in water in the presence of Rh(CO)2acac. Within a reaction time of 10 h at 120°C and a H2/CO (1 1) pressure of 5 MPa, conversion of up to 94% was obtained. The n iso ratio was 3 1, sometimes reaching 9 1. The quantities of 3-and 4-nonanal resulting from olefin isomerization, were below 5% [74]. The versa-tihty of the polymer support was further demonstrated with the Heck reaction of styrene and iodobenzene in the presence of Pd(OAc)2 as the metal catalyst The reaction was again performed in pure water at 50 °C and product yield of up to 80% was obtained after 20 h reaction time [75]. 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

Studies on heterogeneous Pd metal catalysts for the arylation of alkenes with aryl halides (the Mirozoki-Heck reaction often reported as Heck reaction) [28] continue to... 

Palladium metal catalysts supported on organic resins containing tertiary amino, cyano, carboxyl, and pyridyl groups have been recently investigated in some Heck reactions, such as the coupling of iodobenzene with methyl acrylate and methyl vinyl ether (Scheme 11) [31]. 

Palladium-catalyzed carbon-carbon cross-coupling reactions are among the best studied reactions in recent decades since their discovery [102, 127-130], These processes involve molecular Pd complexes, and also palladium salts and ligand-free approaches, where palladium(O) species act as catalytically active species [131-135]. For example, the Heck reaction with aryl iodides or bromides is promoted by a plethora of Pd(II) and Pd(0) sources [128, 130], At least in the case of ligand-free palladium sources, the involvement of soluble Pd NPs as a reservoir for catalytically active species seems very plausible [136-138], Noteworthy, it is generally accepted that the true catalyst in the reactions catalyzed by Pd(0) NPs is probably molecular zerovalent species detached from the NP surface that enter the main catalytic cycle and subsequently agglomerate as N Ps or even as bulk metal. 

Supported palladium catalysts for fine chemicals synthesis are generally based on metal particles. Nevertheless, a few examples are reported of the use of supported complexes as catalysts for the Heck reaction (see Chapter 9.6). Nearly all the possible immobilization methods have been tested for this reaction. 

Although only a dozen known metal complexes were tested in this manner, proof of principle was demonstrated. The test revealed Wilkinson s catalyst to be the most active hydrosilylating agent, its use in this type of reaction being known. However, the study also led to the discovery that a palladacycle, [Pd (o-tolyl)2PC6H4 (OAc)]2, which is usually considered to be potent in Heck reactions, is also an excellent hydrosilylation catalyst.37,38 Control experiments showed that the relative order of catalyst activity is the same when conventional substrates are used in place of the dyes (8). 

Metal-catalyzed cross-couplings are key transformations for carbon-carbon bond formation. The applicability of continuous-flow systems to this important reaction type has been shown by a Heck reaction carried out in a stainless steel microreactor system (Snyder et al. 2005). A solution of phenyliodide 5 and ethyl acrylate 6 was passed through a solid-phase cartridge reactor loaded with 10% palladium on charcoal (Scheme 2). The process was conducted with a residence time of 30 min at 130°C, giving the desired ethyl cinnamate 7 in 95% isolated yield. The batch process resulted in 100% conversion after 30 min at 140°C using a preconditioned catalyst. 

Herrmann WA, Brossmer C, Reisinger CP, Riermaier T, Ofele K, Beller M (1997) Coordination chemistry and mechanisms of metal-catalyzed C-C coupling reactions. Part 10. Palladacycles efficient new catalysts for the Heck vinylation of aryl halides. Chem Eur J 3 1357-1364 Iyer S, Jayanthi A (2001) Acetylferrocenyloxime palladacycle-catalyzed Heck reactions. Tetrahedron Lett 42 7877-7878 Iyer S, Ramesh C (2000) Aryl-Pd covalently bonded palladacycles, novel amino and oxime catalysts di- x-chlorobis(benzaldehydeoxime-6-C,AT)dipalla-dium(II), di- x-chlorobis(dimethylbenzylamine-6-C,A)dipalladium(II) for the Heck reaction. Tetrahedron Lett 41 8981-8984 Jeffery T (1984) Palladium-catalysed vinylation of organic halides under solid-liquid phase transfer conditions. J Chem Soc Chem Commun 1287-1289 (b) idem,... 

Analogously, over the years, Arai and co-workers have investigated silica-supported ethylene glycol as a catalyst-philic phase, which contained a metal precursor, for C-C bond-forming reactions, such as the Heck reaction. They describe a multiphasic system with an organic phase (solvent) that contains only reactants and products without any catalyst. The products could be recovered by simple filtration, and the catalyst recycled many times without deactivation, since it did not precipitate, thus making the catalytic system stable and reusable (Figure 6.7). °... 

Other successful examples of catalysts containing NHC ligands are found in palladium- and nickel-catalyzed carbon-carbon bond formations. The catalyst development with these metals has focused in particular on Heck-type reactions, especially the Mizoroki-Heck reaction itself [Eq. (42)] and various cross coupling reactions [Eq. (43)], e.g., the Suzuki-Miyaura reaction ([M] = and the Kumada-Corriu reaction ([M] = MgBr). " Related reactions like the Sonogashira coupling [Eq. (44)]326-329 Buchwald-... 

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