Heck heterogeneous catalyst

To assess the utility of this resin, we chose to employ it in the evaluation of the heterogeneity of a commercial polymer-entrapped Pd(OAc)2 precatalyst, Pd-EnCat, also sold by Reaxa. This precatalyst was designed with the goal of providing a heterogeneous catalyst that would allow simple removal of palladium from reactions (24-26). PVPy and QTU were first used as poisons in the Heck reaction of iodobenzene and n-butyl acrylate in DMF using PdfC as the palladium... 

Typically, the Pd species for Heck couplings are homogeneous catalysts, stabilized by air-sensitive ligands. They present economic and environmental problems regarding separation, regeneration and reuse. These difficulties can be diminished with heterogeneous catalysts that are more easily recoverable from the reaction mixture. As mentioned in Sect. 2.6, a catalyst consisting of palladium metal deposited on por-... 

Wang HG, Zheng XM, Ping C, Zheng XM. The fabrication of reactive hollow polysiloxane capsules and their application as a recyclable heterogeneous catalyst for the Heck reaction. J Mater Chem 2006 16 4701-4705. 

Heck olefination of a diazonium salt is a key step in the industrial synthesis of sodium 2-(3,3,3-trifluoropropyl)benzenesulfonate, en route to Novartis sulfonylurea herbicide Prosulfuron (Scheme 15) [46]. Pd(dba)2 (0.5-1.5 mol%, prepared in situ from PdCl2), is used as catalyst. After completion of the arylation step, charcoal is added, thus directly providing a heterogeneous catalyst for the subsequent hydrogenation step and enabling easy removal of the now supported catalyst by filtration. 

Palladium on activated carbon has turned out to be a highly versatile, simple heterogeneous catalyst for one-pot multistep syntheses. Recently, Djakovitch and coworkers [42] have demonstrated that low catalyst loadings of Pd on activated carbon efficiently catalyze the Heck reaction of bromo benzene and styrene giving rise to T-stilbene (1) (92%), Z-slilbcnc (1%), and 1,1-diphenylethene (7%). If the Heck products are not isolated but an atmosphere of 20 bar of hydrogen is imposed onto the reaction vessel the sequence furnishes 1,2-diphenylethane (2) in 93% yield (Scheme 1). 

Palladium deposited onto the mesoporous material MCM-41 by vacuum sublimation of CpPd(allyl) gives a heterogeneous catalyst for Heck arylations.87... 

A supported palladium catalyst for the Heck reaction (5.13) could be reused several times with little loss in activity.39 Heterogeneous catalysts for the Heck reaction, such as palladium on silica and platinum on alumina can perform just as well in the Heck reaction as homogeneous catalysts.40... 

Some of the problems associated with the implementation of Heck technology can be removed by the use of heterogeneous catalysts but, although there have been reports stating that Pd/C can promote the Heck arylation, data are limited... 

Semmelhack demonstrated that the addition of alkali metal chloride salts can sometimes markedly increase the yields of coupling products in heterogeneous Pd-catalyzed reactions, especially when the olefin component contains an amide function [15]. It has been claimed that palladium-grafted mesoporous material (MCM-41), designated Pd-TMSll, is one of the most active heterogeneous catalysts for the Heck reaction and enables C-C formation with activated and non-activated aryl substrates [16a,b]. Nanoscale particles of palladium clusters prepared by the ultrasonic reduction of Pd(OAc)2 and NR4X in THF or methanol, were also active for C-C couplings [17]. 

F Zhao, M Shirai, M Aral. Heterogeneous catalyst system for Heck reaction using supported ethylene glycol phase Pd/TPPTS catalyst with inorganic base. J Mol Catal A Chem 154 39-44, 2000. 

Conna, A Garcia, H Leyva, A Primo. A. Basic zeolites containing palladium as bifunctional heterogeneous catalysts for the Heck reaction. Applied Catalysis A General, 2003 247,41 -49. 

Calo and coworkers reported the use of Pd nanopartides (3.3 nm) supported on chitosan as very effident heterogeneous catalysts for the Heck reaction of aryl bromides and activated aryl chlorides with butyl acrylate in tetrabutylammonium... 

In 1997 Ying and coworkers reported that Pd-grafted mesoporous materials are active catalysts for Heck reactions. The Pd-TMSll catalyst (prepared by gas phase reaction of volatile Pd complexes and MCM-41) was one of the very first heterogeneous catalysts that succeeded in the activation of aryl chlorides. With a catalyst amount of 0.1 mol% Pd at least 16% of chlorobenzene could be converted (TON = 64). However, 170 °C and 32 h reaction time were necessary. Only 40% selectivity to the Heck coupling products (= 6% yield) could be achieved [152, 159]. Another catalyst was prepared by Srivastava et al. by ion exchange using SAPO-31 and (NH3)4PdCl2-H20. Up to 70% of chlorobenzene could be converted at 120°C within 70 h (67% yield) [160]. 

A Pd-modified zeolite NaY was utiUzed as a heterogeneous catalyst for Heck reactions of 4-choroacetophenone [161,162]. Entrapped [Pd(NH3)4] gave yields of about 60% of stilbene derivatives with palladium amounts of 0.1mol% (170 °C,... 

Mechanistically, all catalytic systems, including heterogeneous catalysts, pal-ladacydes and pincers operate through the formation of soluble Pd(0) nanoparticles in solution. In the catalytic cycle the aryl halide oxidatively adds to the palladium atoms at the rim, leading to the formation of an anionic aryl-palladium dihalide complex, or possibly its dimer. This anionic species then completes the Heck cycle. 

There is evidence in some cases that C-C coupling by heterogeneous catalysts also occurs by soluble metal colloids (or smaller species) leached from solid supports, such as silica. In the 1970s, the catalytic activity of palladium black for C-C coupling was already reported by Mizoroki et al. [11,12], Various examples of Heck reactions with heterogeneous catalysts have been reported since then [13]. Therefore, while heterogeneous catalysts are not the main focus of this section, they are addressed where appropriate. 

Gedanken et al. prepared palladium nanoparticles by decomposition of [Pd2(dba)3] or [Pd(OAc)2]/Me3(Cj4H29)NBr by extended exposure to ultrasound. In the case of [Pd2(dba)3], the colloidal metal particles were stabilized by amorphous carbon formed from dba. Activities of the soluble colloids in the Heck reaction were comparable to those of the heterogeneous catalyst palladium on charcoal (Table 2) [30, 31]. 

Table 2 Examples of C-C coupling reactions with colloids stabilized by other means than polymers or surfactants and Heck coupling with heterogeneous catalysts. 

Biffis et al. concluded that, in C-C coupling reactions, heterogeneous catalysts are only a source of the actual soluble active species, which is formed under reaction conditions by interaction of the catalyst with one or more of the reagents [50]. They argued that the finding of a correlation of the number of corner and surface atoms with the catalyst activity of colloids in Heck reactions [19] and Suzuki reactions [24,... 

To recycle the palladium catalyst used for the Mizoroki-Heckreaction, a reaction using Pd/C as a heterogeneous catalyst was performed in [bmimJIPFe]. Ethyl cin-namate 86 was extracted simply from the ionic liquid using diethyl ether or hexane (Scheme 32). After the reaction, the Pd/C remained suspended in the ionic liquid, suitable for reuse. Since the EtsN+n formed in the course of the Mizoroki-Heck reaction accumulates in the [bmimJfPFe], slightly lower yields were obtained for successive runs. However, washing the ionic liquid with water removed any iodide salt present. 

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