ipso Halogenation. II. Bromination of phenols, isomerization and disproportionation of bromophenols, and dienone–phenol rearrangement of bromodienones

Abstract

Bromination of p-cresol, 2-bromo-p-cresol, 3,4-dimethylphenol, and mesitol in trifluoromethanesulfonic acid gives as the main product the bromo derivative with bromine meta to hydroxyl, a result attributed to the intermediate formation of a bromodienone and its rearrangement. Phenol does not give m-bromophenol in trifluoromethanesulfonic acid. 4-Bromo-2,4,6-trimethylcyclohexa-2,5-dienone rearranges to 3-bromomesitol in trifluoromethanesulfonic acid and, similarly, 2,4,6-tribromo-4-methylcyclohexa-2,5-dienone rearranges to 2,3,6-tribromo-4-methylphenol. Under appropriate conditions debromination of bromodienones is competitive with rearrangement. Tetramethylammonium bromide in trifluoromethanesulfonic acid is an effective reagent for isomerization and disproportionation of bromophenols. Tetramethylammonium iodide in trifluoromethanesulfonic acid is an effective reagent for selective debromination of bromophenols at the ortho and para positions.