Phospho Sulfonic Acid: A Highly Efficient and Novel Catalyst for Formation of Bis(Indolyl)Alkanes from Aldehydes and Indole under Aqueous Conditions

Abstract

Bis(indolyl)alkanes are a class of alkaloids that possess significant biological activities. Every year, the varieties of bis(indolyl)alkanes isolated from natural sources are increasing. Nevertheless, the deficiency of natural products from natural sources led to a decrease in the exploration of natural products for biological investigations. Corresponding to this fact, there is a demand to develop efficient protocols for the construction of bis(indolyl)alkanes. In this regards, we have prepared phospho sulfonic acid (PSA), which is a non-corrosive, highly reactive, inexpensive and low toxic catalyst, and have applied to construct bis(indolyl)alkanes. The catalyst was prepared by reaction of diammonium hydrogen phosphate with chloro sulfuric acid and was fully characterized by FTIR spectrometry. PSA has been used as a solid acid catalyst to promote the electrophilic substitution reaction of indole with aldehydes under water to furnish a library of bis(indolyl)alkanes in excellent yields over short reaction times. The present method eliminates use of toxic catalyst and solvent, and tolerates a series of functional groups. The catalyst can be reused several times without any important activity loss. The methodology has several advantages, for example, simple experimental procedure, cost efficiency (use of inexpensive catalyst), ease of preparation and handling of the catalyst and no side reactions.