Aromatic substitution with rearrangement. Part IV. Catalysis by halide ions of the acid-catalysed prototropic rearrangement of 4-bromo-2,6-di-t-butylcyclohexa-2,5-dienone in acetic acid

Abstract

The acid-catalysed 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 in the presence of some added anions. Whereas lithium perchlorate influences the rate only through an environmental effect, lithium bromide exerts specific catalysis; in part by attack on the 4-proton, thus giving rearrangement, and in part by removing positive bromine. Kinetic evidence indicates that the catalytic power is exerted only within an ion pair of bromide ion and the protonated dienone; from the results, the equilibrium constant between ion pairs involving perchlorate and bromide respectively as counter-ions has been evaluated. Primary (k₁^4–H:k₁^4–D) and solvent (k₁^HOAc:k₁^DOAc) isotope effects have been recorded for the various stages of reaction, and the mechanistic consequences are discussed. Lithium chloride also shows specific catalytic power, though to a lesser extent than lithium bromide.