SCS-pincer palladium complex

Another fluorous palladium complex that was applied in a Mizoroki-Heck reaction is the SCS pincer palladium complex 24 (Table 15.1, entry 3) [67]. It was applied under thermal and microwave heating. No fluorous solvent was used and the insoluble catalyst dissolved at the reaction temperature of 140 °C. The catalyst was recovered after 30 to 45 min by solid-phase extraction with a fluorous silica gel. Depending on whether activated or nonactivated substrates were coupled, the yields ranged between 76 and 94%. 

An easy recycling method involving both catalyst and reaction medium was achieved in a Mizoroki-Heck arylation reaction of acrylic acid, using a fluorous carbene complex (prepared in situ fl om a fluorous ionic liquid and palladium acetate) as the catalyst and a fluorous ether solvent (F-626) as the reaction medium. Because of the very low solubility of arylated carboxylic acids in F-626, the products precipitated during the course of the reaction. After separation of the products and amine salts by filtration, the filtrate, which contained the fluorous Pd catalyst, could be recycled for several runs (Scheme 13). The Mizoroki-Heck reaction was effectively promoted by a fluorous SCS pincer palladium, which is discussed in Section 3.4.5. 

Reinhoudt and coworkers studied the synthesis of hyper-branched polymers composed of organopalladium complexes with an SCS pincer ligand [11]. Removal of acetonitrile ligands on palladium led to the self-assembly of dinu-clear palladium complex (9) to give hyper-branched polymer (10), which was... 

Random polynorbomene-based terpolymer 5 (Fig. 5.8) functionalized with SCS palladated pincer complexes, dibenzo[24]crown-8 (DB24C8) rings, and 2,6-diaminopyridine units was synthesized by ROMP (South et al. 2006). The palladium complex serves as the anchoring unit for metal coordination, 2,6-diaminopyridine as the DAD hydrogen bonding moiety, and DB24C8 as the precursor for pseudorotaxane formation, which has been smdied extensively by Stoddart and his group... 

Several studies have appeared that show beyond doubt that PCP and SCS pincer complexes also decompose during the Heck reaction and lead to the formation of colloidal palladium [121-124]. Evidence was based on immobilisation studies and on the application of the extensive Hg poisoning protocol developed by Finke, which proved the presence of palladium colloids [125]. 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

Lewis acids and Bu4NI catalyzed allylboration with potassium allyl- and crotyltrifluoroborates (Equation (154)).30,40 620,621 The reaction of pinacol ester derivatives was very slow even at room temperature, but Sc(OTf)3 smoothly catalyzed the addition at — 78 °C with high diastereoselectivity (Equation (155)622 and (156)623,624). A palladium pincer complex catalyzed the addition of trifluoroborate to tosylimines (Equation (157)).625... 

To examine the differences in reactivity of various immobilized palladium pincer catalysts, small molecule, polymer-supported (poly(norbomene) - soluble, and Merrifield resin - insoluble), and silica-supported (SBA-15) pincer complexes were synthesized (Fig. 20.9) [44 6]. Both sulfur-containing (SCS) and phosphorous-containing (PCP) ligands were studied. 

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... 

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