An efficient and green synthetic protocol for the preparation of bis(indolyl)methanes catalyzed by H 6P 2W 18O 62·24H 2O, with emphasis on the catalytic proficiency of Wells-Dawson versus Keggin heteropolyacids

Abstract

A simple, atom economy, and a highly well-organized green protocol has been developed for the synthesis of bis(indolyl)methanes. Wells-Dawson diphosphooctadecatungsticacid (H 6P 2W 18O 62·24H 2O) 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.