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

•Chiral 24-armed dendritic polyoxometalates (POM) were prepared and compared to 12-armed analogous.•Catalytic efficiency evaluated in the enantioselective oxidation of thioanisole.•The increased bulkiness around the POM decreased ee values, with improved selectivity to sulfoxide.•Four catalytic cycles were performed.

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 {PO4[WO(O2)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.

Graphical abstract24-Armed enantiopure DENDRI-POM hybrids were prepared and tested as recoverable catalysts in the selective oxidation of thioanisole to the corresponding chiral sulfoxide with up to 10% ee. The comparison with the 12-armed n-propyl DENDRI-POM analogous indicates a negative effect on the reaction rate and the enantioselectivity when the size of the dendritic structure is increased. However, the selectivity to the chiral sulfoxide is improved.Figure optionsDownload full-size imageDownload as PowerPoint slide