Superelectrophilic Activation in Zeolites

The term Superelectrophile was suggested by Olah in the early seventies when it was noticed that acylation of chlorobenzene by acetyl tetrafluoroborate was substantially accelerated in the presence of the corresponding Brpnsted acid.  

Superelectrophilic intermediates are typically generated when a cationic electrophile is further protonated or coordinated by a Lewis acid to produce a dicationic species. 

Similarly, superelectrophilic solvation may also involve weaker interactions without forming limiting dications resulting in enhanced electrophilic reactivity. In order to be involved in superelectrophihc activation the starting molecule is best bifunctional. 

In the last two decades many useful reactions and synthetic approaches involve superelectrophihc activation of reagents mediated by hquid superacids or by excess of aluminium halides. As an example, 0 ,j3-unsaturated aldehydes, ketones, acids or amides, readily condense with aromatics such as benzene and dichlorobenzene which are poor nucleophiles, but only in the presence of a large excess of aluminium chloride or triflic acid. This reactivity has been interpreted by the intermediacy of dicationic species. 

Reaction kinetics, theoretical calculations and finaUy the observation of such species by NMR in superacids as stable dications point out the rehabihty of this mechanistical hypothesis under superacidic conditions. 

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