Electrophilic aromatic substitution acyl transfer

In the gas phase, alkylation of five-membered heterocycles by alkyl cations usually occurs via the usual addition-elimination mechanism of aromatic electrophilic substitution. The phenyl cation behaves differently, however although its substrate discrimination is limited, in accord with its exceedingly high reactivity, it has marked selectivity for the a position, which does not conform with the hard character of this cation. It has, therefore, been suggested that an electron-transfer mechanism is followed this is thermodynamically allowed for the phenylium, and likewise for the methyl cation, but not for other alkyl cations (Scheme 28). This SET mechanism applies also for acyl cations [87]. 

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