Fabrication of heterogeneous-based lacunary polyoxometalates as efficient catalysts for the multicomponent and clean synthesis of pyrazolopyranopyrimidines

Abstract

In continues efforts to the synthesis of polyoxometalate (POM) as catalysts, two lacunary POM-based heterogeneous catalysts (TBA)9[BW11O39]·11H2O ((TBA)9BW11) and (TBA)6[Zr(OH)BW11O39]·8H2O ((TBA)6BZrW11) (TBA+: tetrabutylammonium) were synthesized and fully characterized using different techniques. The role of the cationic alkyl chains (TBA+) not only leads to the formation of hydrophobic POM catalyst, but also reactants can be trapped near the POM center, which leads to higher catalytic efficiencies. Both catalysts can act as Lewis base due to their terminal oxygen atoms, while (TBA)6BZrW11 have accessible Lewis acid sites (due to substituting zirconium site). Consequently, the catalytic utilities of these samples were investigated in the multicomponent synthesis of pyrazolopyranopyrimidine derivatives, which were attained in short times with high yields. These results established the efficiency of this protocol in terms of being environmentally friendly, high product yields, and being completed in short reaction times. Meanwhile, the synergetic effect between the catalysts components also played an essential role during the catalytic process.