Dendrimers water-soluble, dendritic catalysts

Four water-soluble dendritic phosphonated ligands were first synthesized based on poly(amidoamine) (PAMAM) (generation 3 with 32 cascade end groups), with the hydrophilic amine or sulfonic acid groups on the surface of the dendrimer. Their Rh(I) complexes were used as the catalysts in the two-phase hydroformylation of styrene and 1-octene. The results showed that these new water-soluble dendritic catalysts exhibited high catalytic activity [64]. 

A water-soluble hydroformylation catalyst was developed by Xi and co-workers [65]. Third generation PAMAM dendritic ligands, with hydrophilic amine or sulfonic acid end groups, were phosphonated and the rhodium complexes thus formed were found to catalyse efficiently the hydroformylation of 1-octene and styrene, under very mild conditions. Water-soluble dendritic cobalt phthalocyanines that exhibited catalytic activities and oxidised thiols in the presence of oxygen, have been synthesised by Kimura and co-workers [66]. The catalytic activity of the phthalocyanines was influenced by a egation of the catalytic sites that results fi om strong intermolecular cohesive forces. It was proposed that steric isolation, enforced by the addition of a bulky dendritic coaf around the active phthalocyanine unit, could improve the catalytic activity. Acid terminated polyamide dendrimers were coupled to a phthalocyanine core to produce the desired water-soluble cobalt phthalocyanines, which were tested subsequently for catalytic activity and stability. The results obtained showed that the aggregation of phthalocyanines was reduced the catalytic activity was improved and the stability of the catalyst was improved by addition of the dendritic substituents. 

Reetz et al.59 have introduced polypropylenimine (PPI) dendrimers as the core for building phosphine-coated constructs that can complex with Rh(COD) BF4, where COD = 1,5-cyclooctadiene, to instill the desired catalytic character. Hydroformylation of 1-octene with these metallodendrimers was shown to have turnover numbers that were comparable to those of monomeric analogs. It was pointed out that these catalysts could be easily recovered by means of membrane separation technology.60 Gong et al. have used water-soluble, phosphonated dendritic... 

Poly(ethylene oxide) (PEO) has been employed frequently as a water-soluble catalyst support [9]. Further water-soluble polymers investigated include other linear polymers such as poly(acrylic acid) [10], poly(N-alkylacrylamide)s [11], and copolymers of maleic anhydride and methylvinylether [12], as well as dendritic materials such as poly(ethyleneimin) [10a, c] or PEO derivatives of polyaryl ethers [13]. The term dendritic refers to a highly branched, tree-like structure and includes perfectly branched dendrimers as well as statistically branched, hyperbranched macromolecules. 

The water-soluble phosphine, PTA, was grafted to dendrimers and reacted with [RuCl2(p-cymene)]2- The resulting dendritic complexes were applied to the redox isomerization of l-octen-3-ol in water/re-heptane biphasic systems at 75°C. Albeit the reactions were slow, there was a large positive dendritic effect The catalytic activity increased with increasing size of the dendritic support, and the catalyst derived from the third generation dendrimers afforded 98% conversion whereas use of the soluble monomeric catalyst led to only 38% conversion under the same conditions. The catalyst was recycled in the aqueous phase by decantation with no or negligible loss of the catalytic activity (138). 

Star polymers and dendrimers have been synthesized by Astruc using cyclopentadienyliron-mediated peralkylation, benzylation, and allylation reactions of cationic tri-, tetra-, and /iexa-methylbenzene cyclopentadienyliron com-plexes. " " These star and dendritic polymers contained cationic cyclopentadienyliron moieties at the core and/or the periphery. The cathodic reduction of nitrates and nitrites to ammonia has been achieved using a water-soluble dendrimer containing six cationic cyclopentadienyliron moieties as a redox catalyst. " The octametallic star (41) was prepared by deprotonation of permethylated iron complexes. ... 

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