Iminium ions are potent electrophiles useful for generating C-C bonds in complex molecules. However, the condensation of amines with carbonyl compounds to form iminium ions is often unfavorable, sometimes requiring harsh dehydrating conditions. Thus, alternative methods for iminium ion generation, particularly by oxidation from the corresponding amine, are a valuable synthesis tool. Iminium ions can be generated from activated amines using Ir(dtbbpy)(ppy)PF as a photoredox catalyst. This transformation is proposed to occur by oxidation of the amine to the aminium radical cation by the excited photocatalyst. This is followed by hydrogen atom transfer to a superstoichimetric oxidant, such as trichloromethyl radical (CCl to form the iminium ion). The iminium ion is then quenched by reaction with a nucleophile. Related transformations of amines with a wide variety of other nucleophiles have been investigated, such as cyanide (Strecker reaction), silyl enol ethers (Mannich reaction), dialkylphosphates, allyl silanes (aza-Sakurai reaction), indoles (Friedel-Crafts reaction), and copper acetylides.
Similar photoredox generation of iminium ions has furthermore been achieved using purely organic photoredox catalysts, such as Rose Bengal and Eosin Y.
An asymmetric variant of this reaction utilizes acyl nucleophile equivalents generated by N-heterocyclic carbene catalysis. This reaction method sidesteps the problem of poor enantioinduction from chiral photoredox catalysts by moving the source of enantioselectivity to the N-heterocyclic carbene.