Enantiopure 24-armed dendritic polyoxometalates: Synthesis and evaluation as recoverable catalysts for asymmetric sulfide oxidation

Abstract Enantiopure 24-armed dendritic polyoxometalate (DENDRI-POM) hybrids were prepared by ionic coupling of enantiopure 8-armed n -propyl dendritic ammoniums with a catalytically active peroxophosphotungstate trianion {PO 4 [WO(O 2 ) 2 ] 4 } 3− . The catalytic properties of these DENDRI-POM hybrids were evaluated in the oxidation of thioanisole as a model reaction and compared to those of the 12-armed n -propyl analogous previously reported in our group. Up to 10% enantiomeric excess ( ee ) was obtained, indicating a negative effect on the reaction rate and the enantioselectivity, whereas the selectivity to the chiral sulfoxide versus sulfone was improved. This study aids understanding of how the structure and size of the dendritic wedge around the POM can influence its catalytic properties, especially regarding enantioselectivity. Four catalytic cycles were performed without any obvious change in the activity, selectivity and enantioselectivity.