Regenerable and reusable dendritic catalysts have been synthesized by the ionic assembly of polyammonium dendrimers and polyoxometalate (POM) trianionic units. These dendrimers (see example depicted, in which the POM is [PO4{WO(O2)2}4]3−) are found to catalyze, at ambient temperature, the quantitative epoxidation of olefins by H2O2 in water/CDCl3 and the selective and quantitative oxidation of thioanisole to its sulfone.
Metallodendritic catalysts combine the advantages of homogeneous and heterogeneous catalysts: they are soluble and perfectly well defined on the molecular level, and yet they can be recovered by precipitation, ultra-filtration or ultra-centrifugation (as biomolecules) and recycled several times. In this article, we summarize our recent work in this field with examples operating under ambient conditions in metathesis, Pd-catalyzed Sonogashira coupling, redox catalysis of nitrate and nitrite cathodic reduction to ammonia and various oxidation reactions by H 2 O 2 catalyzed by polyoxometallates. The dendritic effects on the catalytic efficiencies are scrutinized, i.e., the comparison of the metallodentritic catalysts with their monomeric models and among the dendrimer generations. It is concluded that metallostars or low-generation metallodendrimers usually are optimized catalysts in terms of efficiency and recovery/re-use.
A series of 3- and 9-armed dendrons, functionalized at the focal position to quaternary ammonium salts, were synthesized and characterized. The reaction of these ammonium dendrons with the heteropolyacid H(3)PW(12)O(40) in the presence of hydrogen peroxide led to a family of 9- and 27-armed air-stable polyoxometalate (POM)-cored dendrimers containing a catalytically active trianionic POM species [PO(4)[WO(O(2))(2)](4)](3-) in the core. These POM-cored dendrimers are air-stable, efficient, recoverable, and reusable catalysts for the selective oxidation of alkenes to epoxides, sulfides to sulfones, and alcohols to ketones, in an aqueous/CDCl(3) biphasic system with hydrogen peroxide as the primary oxidant. A study of the countercation effects showed that the dendritic structure increased the stability of the POM species and facilitated the recovery of the catalyst up to the eighth cycle, whereas the increased bulkiness around the POM center led to a negative kinetic dendritic effect. Within the 9-armed POM-cored dendrimer series, the reaction kinetics were susceptible to the nature of the peripheral endgroups. Indeed, the 9-armed n-propyl-terminated POM-cored dendrimer was identified as the most active catalyst. In addition, the results obtained with POM-cored dendrimers versus tetraalkylammonium POMs ([[n-(C(8)H(17))(3)NCH(3)](+)](3)[PO(4)[WO(O(2))(2)](4)](3-) and [[nC(18)H(37)(75 %) + nC(16)H(33)(25 %)](2)N(CH(3))(2)](+)](3)[PO(4)[WO(O(2))(2)](4)](3-)) clearly reveal that the dendritic structures are more stable than their nondendritic counterparts. After the reactions were complete, the dendrimer catalysts were easily recovered and recycled without a discernable lost of activity, whereas attempts to recover tetraalkylammonium POMs gave unsatisfactory results. A significant advantage of the dendritic structures is that they enable the recovery and recyclability of the POM catalyst, in contrast to the other tetraalkylammonium POMs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.