Synthesis of 9-and 27-armed tetrakis (diperoxotungsto)phosphate-cored dendrimers and their use as recoverable and reusable catalysts in the oxidation of alkenes, sulfides, and alcohols with hydrogen peroxide

摘要

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 H3PW12O40 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 [PO4{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/CDCl3 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 recover), 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-(CsH17)(3)NCH3](+)}(3)[PO4{WO(O-2)(2)}(4)](3-) and [{nC(18)H(37)(75%) + nC(16)H(33)(25%)](2)N-(Ch(3))(2)}(+)](3)[PO4{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.