NaHSO4-SiO2: An Efficient Reusable Green Catalyst for Selective C-3 Propargylation of Indoles with Tertiary Propargylic Alcohols

Although several methods for the preparation of 4o propargyl indole derivatives have been published, this synthetic transformation is complicated by the tendency of 3o propargyl alcohols to form allenium intermediates in acidic media.  It is therefore a challenge to find an efficient method for the C-3 propargylation of indoles with 3° propargylic alcohols.  In this paper we wish to report on the successful application of silica gel impregnated with NaHSO4 as catalyst for the preparation of 4o propargyl indole derivatives.