Pyrrole-Mannich-Formation of an iminium Ion

NOTE: Important charges and non-bonding electrons are shown throughout the animation except during the transition phase

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The initial step has to be protonation of the formaldehyde so it can react with the amine. The formation of the iminium ion can be reversed if water is added to the final product. The processes would occur in the same way but in the reverse order.

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M. Arend, B. Westermann and N. Risch, Angew. Chemie Int. Ed., 1998, 37, 1044–1070.