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

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 H(3+n)PMo(12-n)V(n)O(40), kind of support, cesium content in Cs(x)H(3-x)PW(12)O(40) and nature of the metal cation in [(n-C(4)H(9))(4)N]((7-n))PMo(2)W(9)(M(n+)center dot H(2)O)O(39). The product yield was strongly dependent on the acidic characteristics which, in turn, depend on chemical composition of catalysts. In the presence of H(3+n)PMo(12-n)V(n)O(40) and Cs(x)H(3-x)PW(12)O(40), the catalytic activity and acidity decreased with increasing V atom or cesium content. Among supported H(3+n)PMo(12-n)V(n)O(40). H(5)PMo(10)V(2)O(40)/gamma-Al(2)O3 is a weaker catalyst than bulk one. [(n-C(4)H(9))(4)N]((7-n))PMo(2)W(9)(M(n+)center dot H(2)O)O(39) 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. (C) 2009 Published by Elsevier B.V.