Tetramethyl orthosilicate as a sharp-selective catalyst of C3-methylation of indole by supercritical methanol

► Methylation of indole in supercritical methanol catalyzed by TMOS and TEOS at 350 °C. ► Selectivity on skatole formation (3-MI) was 95-96 mol% with using catalyst. ► Combination of TMOS-methanol is real methylating system. ► Water content influences on indole conversion and regioselectivity of methylation. ► No reaction was observed in sc-ethanol or sc-isopropanol with and without catalyst.