Palladium dendrimers

These cyclometalation compounds are van Koten s carbosilane pincer nickel dendrimer 9.54 [109, 111], pincer SCS palladium dendrimer 9.55 [110, 112], azobenzene platinum chloro-bridged liquid crytal 9.56 [113, 114], and N,N-dimethylnaphthylene palladium resolving agent 9.57 [117-119] as shown in Fig. 9.10, and photosensitizers for hydrogen production, bis(2-phenylpyridine-4-methyl,4 -fluoride) 9.59 and other photosensitizers with bis(2-phenylpyridine) derivatives 9.60 as shown in Fig. 9.11 [120,121]. 

Andres R, de Jesus E, Hores JC. Catalysts based on palladium dendrimers. New J Chem 2007 31 1161-91. 

Gomez MV, Giuerra J, Velders AH, Crooks RM (2009) NMR characterization of fourth-generation PAMAM dendrimers in the presence and absence of palladium dendrimer-encapsulated nanoparticles. J Am Chem Soc 131 341-350... 

One of the main applications of dendrimers is in catalysis allowing easy recycling of the homogeneous catalyst by means of nanofiltration. Carbosilane dendrimers functionalized with diphenylphosphine groups at the periphery have been synthesized and characterized. Palladium complexes of these dendrimers have been used as catalysts in the allylic alkylation reaction. These dendrimeric catalysts can be used in a continuous process using a membrane reactor.509... 

Dubois et al. [184] reported the synthesis of phosphorus based dendrimers terminated with multivalent palladium complexes. Iterative free radical addition... 

A non-covalently functionalized dendrimer was also applied in a continuous allylic amination reaction.[33] PPI dendrimers functionalized with urea adamantyl groups can act as host molecules for phosphorus ligands equipped with acetyl urea groups (Figure 4.18). The so formed supramolecular complex was reacted with a palladium precursor... 

One of the first results on the use of phosphine dendrimers in catalysis was reported by Dubois and co-workers [16]. They prepared dendritic architectures containing phosphorus branching points which can also serve as binding sites for metal salts. These terdentate phosphine-based dendrimers were used to incorporate cationic Pd centers in the presence of PPh3. Such cationic metalloden-dritic compounds were successfully applied as catalysts for the electrochemical reduction of C02 to CO (e.g. 9, Scheme 9) with reaction rates and selectivities comparable to those found for analogous monomeric palladium-phosphine model complexes suggesting that this catalysis did not involve cooperative effects of the different metal sites. 

Beside these catalytically active metallophosphine dendrimers (see above), preliminary studies on the chemical properties of phoshorus-based dendrimers complexed to metals such as platinum, palladium and rhodium have been described by Majoral, Caminade and Chaudret [21], They showed that these macromolecules (see Scheme 13) could be useful for the (in situ) generation of metallodendrimer catalysts. 

Another example is the palladium catalyzed allylic substitution of 3-phenyl-2-propenyl-carbonic acid methyl ester to yield iV-(3-phenyl-2-propenyl)morpho-line reported by Reetz, Kragl and co-workers. This reaction was performed in the presence of phosphino-terminated amine dendrimers [17, 18] loaded with Pd11 cations as shown in Scheme 10. For this particular dendrimer with a molecular weight of 10 212 g/mol, a retention of 0.999 per residence time [35] was estimated in a membrane reactor with a SELRO MPF-50 membrane. It must be noted that a very high retention is a prerequisite for a continuous operating system, since a small leaching of the dendrimer leads to an exponential decrease in the amount... 

The use of heterogeneous catalysts in this reaction has also been achieved palladium-montmorillonite clays [93] or palladium/activated carbon [94] in the presence of dppb transformed 2-allylphenols into lactones, the regiose-lectivity of the reaction being largely dependant on the nature of the support. Very recently, palladium complexes immobilized onto silica-supported (polyaminoamido)dendrimers were used as catalysts in the presence of dppb for the cyclocarbonylation of 2-allylphenols, 2-allylanilines, 2-vinylphenols, and 2-vinylanilines affording five-, six-, or seven-membered lactones and lactams. Good conversions are realized and the catalyst can be recycled 3-5 times [95]. 

Scheme 1. Second generation of polyphenylene dendrimers based on palladium catalyzed coupling (1) and Diels-Alder cycloaddition (2)...

Scheme 6. Palladium catalyzed synthesis of polyphenylene dendrimers...

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...

A. 1.1. Covalently Functionalized Dendrimers Applied in a CFMR The palladium-catalyzed allylic substitution reaction has been investigated extensively in the preceding decades and provides an important tool for the formation of carbon—carbon and carbon—heteroatom bonds 14). The product is formed after attack of a nucleophile to an (f/ -allyl)Pd(II) species, formed by oxidative addition of the unsaturated substrate to palladium(0) (Scheme 1). To date several nucleophiles have been used, mostly resulting in the formation of carbon—carbon and... 

Better results were obtained by using in situ prepared palladium complexes of a G4 dendrimer (calculated molecular weight 20 564 Da for 100% palladium loading of the 32 diphosphines). After 100 residence times, the conversion had decreased from 100% to approximately 75% (Fig. 3). A small amount of palladium was leached from the catalyst during this experiment (0.14% per residence time), which only partly explains the decrease in conversion. The formation of inactive PdCl2 was proposed to account for the additional drop in activity. A sound conclusion about the effect of this dendritic catalyst requires more experiments. 

De Groot et al. (18) prepared phosphine-functionalized carbosilane dendrimers of different generations (4, 8, 24, and 36 phosphine end groups) and used their palladium complexes as catalysts for the allylic alkylation of allyl trifluoroacetate with diethyl sodio-2-methylmalonate. 

The palladium catalyst supported on the dendrimer with 24 phosphine end groups (2) was used in a CFMR. In the continuous process a solution of allyl trifluoroacetate and sodium diethyl 2-methylmalonate in THF (including -decane as an internal standard) was pumped through the reactor. Figure 4 shows the conversion as a function of the amount of substrate solution (expressed in reactor volumes) pumped through the reactor. The reaction started immediately after the addition of the catalyst, and the maximum conversion was reached after two reactor volumes had passed, whereupon a drop in conversion was observed. It was inferred from the retention of the dendrimer (99.7% in dichloromethane) that the decrease was not caused by dendrimer depletion, and it was therefore ascribed to the... 

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