Alkyl exchange reactions in trifluoroacetic acid. t-Butyl trifluoroacetate, an efficient alkylation agent for aromatic compounds

Abstract

Activated aromatic compounds are rapidly alkylated by t-butyl trifluoroacetate in trifluoroacetic acid. The reaction proceeds in quantitative yield and is conveniently carried out by mixing the reagents at room temperature and evaporating the solvent after a few hours. The reaction is accelerated by strong acids, and in the presence of the latter ortho-substituted anisoles rearrange to the para-isomers. Debutylation of ortho-t-butylphenols occurs readily at room temperature in neat trifluoroacetic acid. A kinetic isotope effect, k_H/k_D= 5·5, was found for the dealkylation of 2,4,6-tri-t-butylphenol. No dealkylation of the corresponding anisole was observed under comparable conditions and a mechanism for the debutylation is proposed which involves ketonisation to give the cyclohexa-2,4-dienone.