Ring-methylation of pyrrole and indole using supercritical methanol

The ring-methylation of pyrrole or indole using supercritical methanol proceeded at 623 K without the further addition of catalysts. Pyrrole produced a mixture of unreacted pyrrole and mono-, di-, tri-, and tetra-methylpyrroles at the reaction time of 8 h. On the other hand, indole was selectively methylated at the C3 position to afford 3-methylindole in 79% yield at the reaction time of 5 h. The ring-methylation of indole using supercritical methanol was claimed to proceed via (1H-indol-3-yl)methanol. The conversion of indole to (1H-indol-3-yl)methanol would be achieved by the electrophilic aromatic substitution between the indol-1-ide (indole anion) and H₂C⁺–OH. The (1H-indol-3-yl)methanol must be reduced to 3-methylindole in the presence of supercritical methanol.