iminium

An iminium ion is a positively charged nitrogen-containing species that features a carbon–nitrogen double bond. The typical representation is R2C=NR′ with a positive charge on nitrogen, often isolated as salts such as R2C=NR′ X−. In practice, iminium ions arise when a carbonyl compound (an aldehyde or ketone) reacts with a secondary amine and undergoes dehydration to form an iminium salt; they can also be generated by protonating neutral imines (Schiff bases).

Structure and reactivity: The nitrogen bears the positive charge, and the carbon of the C=N bond is highly electrophilic, making the adjacent carbon prone to nucleophilic attack. Iminium ions are typically stabilized as salts with counterions such as BF4−, Cl−, or OTf−. They are key reactive intermediates in many transformations, including Mannich-type reactions, electrophilic amination, and organocatalytic cycles in which an aldehyde or ketone reacts with a secondary amine to form the activated iminium species that then reacts with nucleophiles or is reduced to amines.

Formation and applications: Formation commonly occurs under acidic conditions or with dehydrating reagents from aldehydes/ketones and secondary amines. In organocatalysis, iminium catalysis involves reversible formation of activated iminium species to enable stereoselective carbon–carbon bond formation. Iminium ions can be reduced to amines using hydride donors, or hydrolyzed to restore the carbonyl and amine.

See also: imine, Schiff base, enamine.