Journal of Molecular Catalysis & Organic Synthesis

SnCl2/Montmorillonite KSF is an highly efficient heterogeneous reusable catalyst for the selective alkylation of indoles with α, β unsaturated carbonyl compounds under solvent free conditions

Jakkidi Krishna Reddy, Suryakiran Navath

The indole ring system is the most common heterocycle moiety in the preparation of many biologically and pharmacologically active compounds.1 Among them the 3-substituted indoles are the building blocks for the preparation of such compounds. In the recent years many methods have been employed for the synthesis of 3- substituted and various other indole derivatives.2 Friedel crafts reaction is the one of the effective transformation for the formation of C-C bonds by using Lewis acids as promoters. In the earlier reports several Lewis acids like GaCl3, InBr3, InCl3,  ZnCl2, CeCl3.6H2O, BiCl3, FeCl3, AlCl3, ZrCl2, SmI2, Cu and Sc salts were found  to catalyse the conjugate addition of  indoles  to  electron deficient olefins for the preparation of  3-substituted indoles derivatives. But the main limitation of these synthetic processes is there environmental incompatibility because many of the procedures involve expensive salts, low yields,3 low selectivity, non-reusable catalysts and solvent feasible reactions that disturb the ecological balance.

To ovecome these problems zeolites and clays4 are used as efficient heterogeneous catalysts in various organic reactions.5-12 In the present investigation we have studied the indole alkylation over zeolites, clays and metal modified clays. Among those SnCl2/montmorillonite13 KSF clay is highly acidic, from our experiments it is clearly observed that SnCl2 itself is an highly effective Lewis acid catalyst for the alkylation of indoles, but the major disadvantage in using this catalyst is, SnCl2 is a homogeneous catalyst and thus separation of the catalyst from the mixture requires lot of organic solvent that are expensive and hazardous for the environment, and it is a time taking process.  In order to overcome these major disadvantages and to develop more efficient and environmentally benign methods in organic synthesis we have prepared SnCl2/Montmorillonite supported catalyst which is highly efficient, heterogeneous reusable and   selective catalyst for the alkylation of indoles at 3-position in solvent free condition at room temperature, side reactions like dimerizations and trimerizations are not absorved which indicates that catalyst is highly selective.