Electrophilic catalysis, by acyl cations

Progress was made by the discovery of electrophilic catalysis by acyl cations in carbonyl reactions (91ZOR1588). This catalysis type consists in the conversion of aldehydes or ketones into highly active acyloxycarbo-cations 51 by the addition of acyl cations regardless of the origin of the latter. In contrast to related hydroxy- and alkoxycarbocations (R R —OR ... 

Electrophilic catalysis by acyl cations in reactions of ketones... 

In the course of acyloxycarbocation investigations112 it has been noted that the reactions of both aldehydes and ketones follow an unusual course or are strongly accelerated if either acylium ions RCO+ are present in the reaction mixtures or conditions to generate them in situ arise. These observations are explained by a transformation of carbonyl compounds into the highly reactive acyloxycarbocations 163 which easily react with weak nucleophiles such as vinyl ethers, vinyl esters, etc. Hence the electrophilic catalysis by acyl cations in carbonyl reactions takes place regardless of the origin of the latter. This catalysis was used in the reaction of ketones with nitriles. 

This does not occur in the case of catalyst and reactants here described. With Bronsted-type catalysis, the reaction between the benzoyl cation, Ph-C" =0, and the hydroxy group in phenol is quicker than the electrophilic substitution in the ring. This hypothesis has been also confirmed by running the reaction between anisole and benzoic acid in this case the prevailing products were (4-methoxy)phenylmethanone (the product of para-C-benzoylation) and methylbenzoate (obtained by esterification between anisole and benzoic acid, with the co-production of phenol), with minor amounts of phenylbenzoate, phenol, 2-methylphenol and 4-methylphenol. Therefore, when the 0 atom is not available for the esterification due to the presence of the substituent, the direct C-acylation becomes the more favored reaction. 

Condensations of pyrroles with aldehydes and ketones oceur easily by acid catalysis, but the resulting pyrrolyl-carbinols cannot usually be isolated, for under the reaction conditions proton-catalysed loss of water produces 2-alkylidene-pyrrolium cations that are themselves reactive electrophiles. Thus, in the case of pyrrole itself, reaction with aliphatic aldehydes in acid inevitably leads to resins, probably linear polymers. Reductive trapping of these cationic intermediates, producing alkylated pyrroles, can be synthetically useful, however all free positions react acyl and alkoxycarbonyl-substituents are unaffected. ... 

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