How Electrophilic Are Keteniminium Ions?

Keteniminium triflates (R ₂ C=C=NMe ₂⁺ TfO ^-) react via classical electrophilic aromatic substitutions with electron-rich arenes forming Wheland intermediates that rapidly undergo triflate-catalyzed tautomerizations to enammonium ions. These ions do not isomerize to the thermodynamically more-stable iminium ions and can quantitatively be converted into the corresponding enamines by treatment with triethylamine. Kinetic studies of keteniminium attack at the arenes provided the electrophilicity parameters of the keteniminium ions E = -2.8 (for R = Me) and E = -1.9 (for R = Ph). These values quantify the one-bond electrophilic reactivity of keteniminium ions and allow us to derive the energy of concert of their [2 + 2]-cycloadditions with alkenes.