Heck carbon support

Discussion and Conclusions. In the present work a large variety of Pd/C catalysts with different properties was studied as catalysts in Heck reactions of aryl bromides with olefins. The activity of the catalysts strongly depends on the Pd dispersion, the Pd oxidation state in the fresh catalyst, the water content (wet or dry catalysts) and the catalyst preparation conditions (in regnation method, pretreatment conditions). The effects are significant, i.e. Pd on the same activated carbon support is eitiier found to be a nearly inactive catalyst or, in the other extreme, a catalyst with the highest activity ever reported up to now for heterogeneous systems for the conversion of aryl bromides (Table 6.)... 

Gordon and Holmes32 used a supported triphenylphosphine-Pd(II) complex as an effective catalyst for Heck and Suzuki couplings in supercritical carbon dioxide (entry 27). After optimization of the amine base for the reaction, the final products have been isolated in good yields and high purity with no traces of metal. 

The bond between carbon and rhodium is extremely stable, thus allowing mono- or biphasic hydroformylations without any excess of ligands. Due to this stability, for the first time ever an anchoring to a polymer support seems possible without leaching. These N-heterocyclic carbenes appear to be excellent ligands to stabilize catalytically active metals even under harsh temperature conditions, e. g. Heck C-C-coupling reactions at 130 °C [152 b]. 

This reaction type differs from die three-component reaction reported by Grigg et al. Thus, Grigg et al. [53] (Scheme 7) immobihzed 3-iodo-4-(N-acetyl-N-(2-methyl-2-propenyl)amino)benzoate (36) onto a sohd support. In the presence of suitable Pd salts, Pd substituted the iodide function of the aromatic. The proximal isopropyhdene group trapped the resulting metalated species in an intramolecular Heck reaction. The resulting alkyl palladium species (37) could then react with a suitable carbanion equivalent. The authors used vinylstaimanes or boronates for this purpose, which they obtained in situ from alkynes by hydroboration or hydro-starmylation. The latter procedure allowed them to attach the same vinylic species via its terminal carbon (boronate) (41) and its subterminal carbon (stannane) (39). 

Since the renaissance of solid-phase organic chemistry in 1992, carbon-carbon bond formation reactions on solid support have probably been the best studied reactions. Many different facets of the Suzuki, Heck and Stille reactions have been evaluated. The influence of linkers, catalyst, solvents, microwave, polymer-bound aryl halides or polymer-bound arylboronic acids (or stannanes) have been studied in detail. 

A new type of soluble polystyrene-supported palladium complex was synthesised (Figure 6.1) as an excellent and recyclable palladacycle catalyst for carbon-carbon bond formation in Heck, Suzuki and Sonogashira reactions to give high yields of the desired products. 

K Kohler, M Wagner, L Djakovitch. Supported palladium as catalyst for carbon-carbon bond construction (Heck reaction) in organic synthesis. Catal Today 66 105-114,2001. 

With the aim of preparing elaborate molecules in the solid phase, chemists are searching for robust and versatile methods to assemble complex carbon frameworks in the insoluble support. One of the predominant sequences for creating carbon-carbon bonds in the solid phase is Pd-catalysed coupling reactions. The Heck reaction, for instance, is a convenient way to diversify activated alkenes (Scheme 5a). This coupling involves an arene with iodide, bromide, triflate or diazonium on one hand, and an alkene (generally electron poor) on the other, in the presence of a catalyst and a base. 

Table 5.4 Supported Pd catalysts on carbon tested in Mizoroki-Heck reactions.

Table 5.4), prepared from reduction of Pd(II) salts with potassium graphite. The results suggested that this catalyst was not very active. However, some years later Jikei and Kakimoto [73] prepared a more active Pd/CGr based on a smaller crystallite size. In 2002, Kohler et al. [74] studied a variety of Pd/C catalysts with different properhes (Pd dispersion, oxidation state, water content, conditions of catalysts preparation etc.) in the Heck reaction of aryl bromides with olefins (entry 4, Table 5.4). The authors pointed out the hypothesis that the leached Pd from the support is the active species and the solid Pd/C catalyst acts as a reservoir that delivers catalytically active Pd species into solution. All catalysts were obtained by wet impregnation (5% Pd loading). The Heck reaction can also be conducted in ionic liquids through promotion by microwave irradiation. Moreover the reaction of iodobenzene with methylacrylate in NMP was reported to be accelerated by ultrasound [75]. The ionic liquid containing the catalyst system was used five consecutive times with only a slight loss of activity (entry 5, Table 5.4) [76]. Perosa [77] reported the addition of a phase transfer catalyst to an ionic liquid as a method to accelerate the C-C coupling reaction. As far as we know, only by using ionic liquids has Pd on carbon been recovered and reused with success.

Supported catalysts involving palladium on carbon and dendrimer-encapsulated palladium and a polymer-supported phosphine palladium catalyst have facilitated C-C coupling reactions in SCCO2. Polymer-tethered substrates or amine bases have also been successfully used for the Mizoroki-Heck and Suzuki-Miyaura reactions in SCCO2. For example, REM resin underwent a Mizoroki-Heck reaction with iodobenzene to yield, after cleavage, ( )-methyl cinnamate 48 (74%) (Scheme 88). It is assumed that SCCO2 acts as a good solvent that swells the polymers and exposes reactive sites. 

The concept of SI L catalyst has been developed quickly in the last decade. Holderich et al. [4] added acidic chloroaluminate ILs to various types of supports, and the catalytic activities of the immobilized ILs were found to be higher than those of the conventional catalysts under the same conditions. Inspired by this work, SIL catalysts have been widely used in the coupling reactions for olefin hydroformylation [5], olefin metathesis [6], Heck reactions [7], and hydroamination [8], and so on. SIL catalytic systems have also been reported for some other reactions, such as water-gas shift reaction [9], dihydroxylation of olefins [10], and hydrogenation [1 Ij. The solid supports used include magnetic NPs [12], mesoporous molecular sieves [13], soluble organic ions [14], noncovalently solid-phase [15], IL-functionalized carbon nanotubes [16], polymer cocktail [17], and so on. 

Pt particles with the IL protects the catalyst from oxidation, which can be utilized to protect air-sensitive catalysts [35]. Further examples of heterogeneous reactions on metal immobilized in porous supports using ILs are Heck C-C coupling on Pd [36] benzene hydrogenation on nanoscale Ru-catalysts [37] selective acetylene hydrogenation on Pd-nanoparticles, for example, dissolved in [BMlM][PFg] [38] and citral hydrogenation on Pd immobilized by different ILs on an active carbon cloth [39]. 

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