Heck palladium nanoparticles

Palladium nanoparticles coated with a fluorous-derivatized surfactant (or stabilizer) have been used to catalyse Heck and Suzuki reactions in a C8F17Br-benzene biphase [11], The reaction between phenylboronic acid and cinnamyl bromide was investigated and the reaction is illustrated in Scheme 10.10. 

As detailed elsewhere, the fluorous palladacycle acetates and hahdes 7 and 8 were synthesized [38,39]. These feature three Rfg ponytails, and were poorly soluble in common organic solvents at room temperature, and insoluble in DMF. However, they were very soluble in DMF at higher temperatures. All were effective catalyst precursors for Heck reactions (100-140 °C), and precipitated (as the halides) upon cooling. However, a number of control experiments established that 7 and 8 served as steady-state sources of colloidal palladium nanoparticles, formed anew with each cycle imtil the palladacycles were exhausted. These, or low-valent Pd(0) species derived therefrom, were the true catalysts. 

Fig. 27. Heck coupling of substituted arylhalides to acrylates in the presence of sc(C02) soluble PPI dendrimers containing palladium nanoparticles is highly selective for the non-standard 1,1-substituted product...

Meier MAR, FUali M, Gohy J-F, Schubert US (2006) Star-shaped block copolymer stabilized palladium nanoparticles for effident catalytic Heck cross-coupling reactions. J Mater Chem 16 3001-3006... 

Multi-nuclear (3H, 29Si and 31P) MAS NMR techniques were employed to study the local structure of catalysators. Pertici and co-workers,201 using 31P SS NMR, confirmed the higher stability towards the agglomeration of the palladium nanoparticles deposited on polydimethylphosphazene (PDMP). The Pd/PDMP system demonstrated much higher catalytic activity in Heck-type C-C coupling reaction. 31P NMR data highlighted the almost complete structural invariability of... 

It has been suggested that active sites in heterogeneous Heck reactions are actually dissolved palladium species.1901 Such a hypothesis is in agreement with a mechanism where palladium nanoparticles suspended in an ionic liquid are suggested to act as reservoir to a molecular catalyst, which is formed upon oxidative addition, as shown in Figure 6.4.1311... 

The first studies that intentionally used colloidal nanocatalysts were reported independently by Beller et al. [50] and Reetz et al. [51] using chemical reduction and electrochemical techniques, respectively, to synthesize colloidal palladium nanoparticles for the Heck reaction. Both Beller and Reetz concluded that the solution-phase catalysis occurred on the surface of the nanoparticle, without confirming that a homogeneous catalytic pathway was nonexistent. Le Bars et al. [52] demonstrated an inverse relationship between the size of Pd nanoparticles and the TOF (normalized to the total number of surface atoms) for the Heck reaction (Fig. 18.4a). After normalizing the rate to the density of defect sites (for each nanoparticle size) (Fig. 18.4b), the TOF for all particle sizes was identical. Colloidal PVP-capped palladium nanoparticles synthesized by ethanol reduction are effective catalysts for Suzuki cross-coupling reactions in aqueous solution [53]. The El-Sayed group reported that the initial rate of reaction increased linearly with the concentration of Pd nanoparticles [53] and the catalytic activity was inversely proportional to the... 

While there is no catalytic activity for the Suzuki and Heck reactions in filtered solutions (which should only contain dissolved molecular Pd species) [61] recent work by Thathagar et al. [6] indicates that leached Pd species are critical participants in the catalysis. Their experimental setup consisted of a cell divided into two sections by a membrane with a pore size of 10 nm. The insoluble base, NaOAc, which is necessary for the reaction to occur, was positioned on one side of the membrane, while 15 nm colloidal palladium nanoparticles (which were larger than the 10 nm... 

In this chapter, we discuss selected literature on palladium catalysts (or catalyst precursors) for the Heck, Suzuki and Sonogashira reactions. The review covers simple homogeneous palladium complexes, Ugand-free palladium catalytic systems, stable palladium colloids and particles and supported palladium catalysts. It focuses on the role of palladium nanoparticles (as catalyst precursors or formed in situ during the course of the reaction) from a mechanistic point of view. 

Fig. 10.9 Heck Reaction catalysed by palladium nanoparticles in a two-chamber reactor showing that palladium becomes detached from the particles during the catalytic cycle. Reprinted from Ref [33] with permission from Wiley.

In 1997, Antonietti et al. reported on catalytically active palladium nanoparticles prepared by reduction of palladium(II) compounds in inverse block copolymer micelles, namely polystyrene-ib-poly(4-vinylpyridine) (PS-b-P4VP). Activated aryl bromides were coupled reproducibly in Heck reactions [18]. Small partide sizes were a prerequisite for high conversions, as indicated by qualitative TEM investigations. Very high total turnovers were reported (0.0012 mol% palladium, 68% conversion in five days, corresponding to 56 000 TO) (Table 1). Catalyst activity was found to be dependent on the structure of the block copolymer employed, which was attributed to a better accessibility of the metal particles in smaller micelles with a high surfacer area and thinner polystyrene layer. 

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]. 

Using a fluorous palladacycle catalyst 10 originating from the corresponding fluorous Schiff base and palladium acetate, a fluorous Mizoroki-Heck reaction was achieved with an excellent turnover number (Scheme 12). A homogeneous catalytic reaction system was obtained when DMF was used as the solvent. After the reaction, perfluorooctyl bromide was added to facilitate the separation of DMF (containing the products and amine salts) from the catalyst phase. The resulting lower fluorous layer was condensed under vacuum and the catalyst residue was used in a second run. In this reaction, the palladacycle catalyst appears to act as a source of palladium nanoparticles, which are thought to be the dominant active catalyst. 

The Heck-type reaction (see Figure 1.6 and Table 1.2) using palladium nanoparticles as a catalyst was investigated using eight different aryl halides. With a... 

Figure 1.6. Test reaction for (a) Suzuki-type and (h) Heck-type cross-coupling using palladium nanoparticles stahihzed in SPB particles as a catalyst.

The reproducibility was found to be very good for palladium nanoparticles as catalyst of both Suzuki- and Heck-type reactions. In four runs the products were removed by ether and new starting materials were added to the water phase. We found that Pd SPB could be used repeatedly without loss of activity. After these four... 

Chen FR, Huang MM, Li YQ (2014) Synthesis of a novel cellulose microencapsulated palladium nanoparticle and its catalytic activities in Suzuki-Miyaura and Mizoroki-Heck reactions. Ind Eng Chem Res 53 8339-8345... 

Mechanistic studies performed with Freeh s pincer catalyst in the Heck reaction excluded catalytic cycles with the involvement of homogeneous palladium(O) species, as indicated by the results obtained from the (recently developed) dibenzyl-test, which is directly applicable under the reactions conditions applied [24aj. Dibenzyl formation was - in contrast to Heck reactions catalyzed by palladium(O) complexes of type [Pd(PR3)2, where Pd /Pd" cycles are operative - not detectable by gas chromatography-mass spectrometry (GC/MS) when reaction mixtures of aryl bromide, olefin, benzyl chloride ( 10 mol% relative to aryl bromide), catalyst, and base were thermally treated. On the other hand, experimental observations, such as quantitative poisoning experiments with metallic mercury and CS2, which were shown to eflfidently inhibit catalysis, as well as analysis of the reaction profiles showed sigmoidal-shaped kinetics with induction periods and hence indicated that palladium nanoparticles are the catalytically active form... 

Even though aminophosphine- and phosphite-based pincer complexes, as well as other systems such as SCS-based pincer-type Heck catalysts, were shown to serve as stable and clean sources of palladium nanoparticles in Heck reactions [24a, 25], and also xylene-derived phosphine-based systems were found to decompose under Heck reaction conditions in the presence of organic bases and hence palladium nanoparticles generally are considered to be the catalytically active form of palladium pincer Heck catalysts [27], catalytic cycles with the involvement of Pd intermediates could not have been excluded to be operative in pincer-catalyzed versions of the Heck reaction. In contrast, experimental observations, such as halide exchange reactions... 

Heck Reactions Catalyzed by Palladium Pincer Complexes Pd"/Pd Cycles and/or Palladium Nanoparticle Formation... 

These computational investigations showed for the first time that catalytic cycles with the involvement of Pd intermediates are indeed thermally accessible for palladium pincer complexes under Heck reaction conditions and hence are a true alternative to palladium nanoparticle-catalyzed versions of the Heck reaction. This, however, does not imply that Pd /Pd mechanisms are operative in any case for palladium pincer complexes in the Heck reaction. In contrast, palladium nanoparticles have been often shown to be the catalyticaUy active form of pincer-type Heck catalysts, as it is, for example, the case for the aminophosphine-based palladium pincer Heck catalyst [2,6-CgH3(NHP(piperidinyl)2)2Pd(Cl)] (10) (under the reaction conditions applied) - the pincer complex with the highest electron density on the metal center and thus where the lowest energy path was calculated. Therefore, it is reasonable to anticipate that palladium pincer Heck catalysts exist that operate via Pd /Pd mechanisms whereas others serve as sources of palladium nanoparticles. This hypothesis got strong experimental support from... 

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