Aromatic substitution with rearrangement. Part V. Base-catalysed prototropic rearrangement of 4-bromo-2,6-di-t-butylcyclohexa-2,5-dien-one in acetic acid

Abstract

The prototropic rearrangement of 4-bromo-2,6-di-t-butylcyclohexa-2,5-dienone to 4-bromo-2,6-di-t-butylphenol has been studied in acetic acid and in aqueous acetic acid under catalysis by added sodium acetate. The kinetic form is consistent with a rate-determining attack of ionic species, including triple ions, on the unprotonated dienone. The reaction is subject to a relatively large primary isotope effect, which varies with the solvent; the maximum value recorded (k₁^4–H:k₁^4–D= 8·8) was in acetic acid containing 2·6M-water. The reaction is also subject to a small solvent deuterium isotope effect, being slightly faster in deuterioacetic acid than in acetic acid. Measurements of the extent of protonation of p-bromo-NN-dimethylaniline and of p-(NN-dimethylamino)azobenzene have been recorded for the solvents used for the kinetic measurements. The mechanistic implications of the observations are discussed.