Imines, diprotonated

Vicinal 1,2-carbodicationic systems are some of the most important and thoroughly studied superelectrophiles. They include inter alia 1,2-ethylene dications, related carbon-nitrogen superelectrophiles (diprotonated imines and nitriles), protosolvated carboxonium ions, and superelectrophilic tri-halomethyl cations. It is understood that many of these systems involve extensive charge delocalization, and in a sense may not be considered formal 1,2-dicationic systems. For example, diprotonated 2,3-butanedione (34) may be represented formally as a 1,2-ethylene dication (35a, a gitonic superelectrophile), but even in monocationic carboxonium ions, there is a significant amount of double-bond character retained in the carbon-oxygen bond (eq 5).22... 

In superacidic reactions, diprotonated imines form gitonic superelectrophiles.48 As described in Chapter 2, kinetic experiments have shown that diprotonated intermediates are involved in these conversions. Other experiments showed that the reaction provides higher yields in stronger acid systems (eq 32),... 

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