Formyl dication

Although it has been reported in a Molecular Modelling study that the formation of a formyl dication (HCOH ) by the action of HF/SbFs on CO is associated with extremely high activation energy, this species has been proposed by several authors as the active electrophile in formylation reactions utilising CO and HF/SbFs. The formation of a formyl dication could, therefore, also be invoked as plausible explanation for electrophilic aromatic substitution reactions in case of full protonation of both CO and the aromatic substrate by the HF/BF3 super acid (Eq 2.24). 

Carbon monoxide, hydrogen cyanide, and nitriles also react with aromatic compounds in the presence of strong acids or Friedel-Crafts catalysts to introduce formyl or acyl substituents. The active electrophiles are believed to be dications resulting from diprotonation of CO, HCN, or the nitrile.64 The general outlines of the mechanisms of these reactions are given below. 

Attempts were made to formylate and acetylate (230) but decomposition was extensive and no product could be isolated. Treatment of a solution of (230) in benzene with bromine vapor gave a stable blue cyclazine cation bromide, which was converted on further treatment with bromine into a greenish brown dication dibromide. 

The dication concept can also be extended to the formation of the anisole- and phenol dication as reported by Takezaki " who isolated an anisole/FIF/BFs complex containing a 1 1 2 ratio of reactants (Scheme 2.3). Protonation of the pro-electrophile CO via the intracomplex mechanism is followed by electrophilic aromatic substitution with formation of the 71- and a -complex respectively followed by aldehyde formation through the loss of a proton and X. The corresponding rate of formylation is expected to be slower than in the case of toluene due to the lower nucleophilicity of the protonated anisole. 

Scheme 2.3 Dication formation with resulting intracomplex formylation to the aldehyde.

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