Divergent reactivity of intramolecular cycloadditions of keteniminium ions with alkynes: [4+2] or [2+2]?

We describe here divergent reactivity in the intramolecular cyclizations of a family of keteniminium ions (KIs) that contain a tethered alkyne. The KI precursors were tertiary amides having (i) unsaturation (arene or alkene) located β,γ to the amide carbonyl and (ii) an alkyne tethered through the amide nitrogen atom. The KIs were generated by the action of triflic anhydride and a pyridine base at 0 °C. Substrates having a three-atom linker between the central carbon atom of the KI undergo unprecedented [4 + 2] cycloadditions between the alkyne and the styrenic/dienic subunit of the conjugated KI leading to indoline or carbazole derivatives. DFT computations suggest that the reaction proceeds by a stepwise mechanism. In contrast, substrates with a four- or five-atom tether undergo [2 + 2] cycloaddition to afford isolable, fused, cyclobutenyl iminium ions further useful as synthons.