Reductive C-alkylation

At 95-115°, paraformaldehyde and hydriodic acid completely C-methylate aromatics such as benzene and phenol. Pyrroles are C-methylated similarly, carbethoxy and acetyl groups being lost. In hydriodic acid at 15-45°, typical pyrroles retain these groups and all free positions are C-alkylated, methylated by paraformaldehyde, or otherwise alkylated by the appropriate carbonyl compound. The alkylation of a 2-free- by a 2-formylpyrrole led to a dipyrrylmethane. With pyrroles, hydriodic acid may be replaced by another strong acid and a reducing agent. This was necessary when a β-free pyrrole gave the iodo-alkyl derivative rather than the expected product.