Electrophilic substitution with rearrangement. Part 9. Dienones derived from brominations of o-, m-, and p-cresol

Abstract

Regiospecific protodebromination of ring-substituted bromophenols derived from 2-, 3-, or 4-methylphenol can be effected by heating them with aqueous hydrogen iodide; the synthetic scope of this reaction has been explored. These di- and poly-bromophenols can generally be converted by further bromination in aqueous acetic acid into dienones, which have now been shown to have the 4-bromo-2,5-dienone rather than the 2-bromo-3,5-dienone structure. The rearrangements of these dienones to ring-substituted polybromophenols by treatment with sulphuric acid have been investigated; where more than one product is formed, the regioselectivity differs from that prevailing in the corresponding direct bromination of the phenol with liquid bromine. The alternative rearrangements of these dienones in aprotic solvents with and without illumination have been compared with results obtained by reaction of the methylphenols with bromine under the same conditions. Characteristic differences between the behaviours of 2-, 3-, and 4-methyl-substituted compounds reflect the specific reactions available to the particular dienones.