An efficient and green synthetic protocol for the preparation of bis(indolyl)methanes catalyzed by H6P2W18O62•24H2O, with emphasis on the catalytic proficiency of Wells-Dawson versus Keggin heteropolyacids

A simple, atom economy, and a highly well-organized green protocol has been developed for the synthesis of bis(indolyl)methanes. Wells-Dawson diphosphooctadecatungsticacid (H6P2W18O62 center dot 24H(2)O) was employed as a proficient, a highly water tolerant, and green heteropolyacid catalyst for the electrophilic substitution reactions of indole with various aldehydes and ketones to afford the corresponding bis(indolyl)methane derivatives under solvent-free condition. Presumably, discrepancies observed in the reactivity patterns of various heteropolyacids would be explained concerning differences in the acidity, structural diversity, and dissimilarities in the approach of the substrates to the bulk of the heteropolyacid catalysts. It is confirmed that the identity of the catalyst was retained almost completely during the catalytic reaction. (C) 2011 Elsevier B.V. All rights reserved.