Acylium cation reacting with aromatics

Depending on the specific reaction conditions, complex 4 as well as acylium ion 5 have been identified as intermediates with a sterically demanding substituent R, and in polar solvents the acylium ion species 5 is formed preferentially. The electrophilic agent 5 reacts with the aromatic substrate, e.g. benzene 1, to give an intermediate cr-complex—the cyclohexadienyl cation 6. By loss of a proton from intermediate 6 the aromatic system is restored, and an arylketone is formed that is coordinated with the carbonyl oxygen to the Lewis acid. Since a Lewis-acid molecule that is coordinated to a product molecule is no longer available to catalyze the acylation reaction, the catalyst has to be employed in equimolar quantity. The product-Lewis acid complex 7 has to be cleaved by a hydrolytic workup in order to isolate the pure aryl ketone 3. 

Aromatic rings are usually not good enough nucleophiles to react with this sink. If a Brpnsted acid or a Lewis acid is added to improve the electron sink, then electrophilic aromatic substitution occurs, usually via the acylium cation as an electrophile. 

Alkynes react with acylium tetrafluoroborates in aromatic solvents to give the product corresponding to trans-addition of the acyl cation and the aryl nucleophile to the triple bond (Scheme 57). ... 

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