A. Geissman in 1950 ( DOI ). Reaction of 1-allylbenzylamine with formic acid and formic acid did not yield the expected 1-allylbenzyldimethylamine (the result of methylation ) but benzaldehyde and 1-dimethylamino-3-butene. Mechanism : Addition of formaldehyde to the starting amine gives an iminium ion which reacts in a sigmatropic reaction to