Facile condensation of indole with benzaldehyde over Keggin-type heteropoly compounds: An initial effort toward catalyst design Original Research Article

A series of catalysts derived from Keggin-type heteropoly compounds (HPCs) were prepared and used as catalysts for the reaction of indole with benzaldehyde to afford bis(3-indolyl)phenylmethane. The results showed that the difference in catalytic activity of HPCs in this reaction depends on the number of introduced vanadium atoms in H3+nPMo12−nVnO40, kind of support, cesium content in CsxH3−xPW12O40 and nature of the metal cation in [(n-C4H9)4N](7−n)PMo2W9(Mn+·H2O)O39. The product yield was strongly dependent on the acidic characteristics which, in turn, depend on chemical composition of catalysts. In the presence of H3+nPMo12−nVnO40 and CsxH3−xPW12O40, the catalytic activity and acidity decreased with increasing V atom or cesium content. Among supported H3+nPMo12−nVnO40, H5PMo10V2O40/γ-Al2O3 is a weaker catalyst than bulk one. [(n-C4H9)4N](7−n)PMo2W9(Mn+·H2O)O39 catalysts exhibit a high electron-acceptor character and interesting structural and coordination properties. Catalytic activity of these compounds is related to the difference in their ability to coordinate substrates.