The intermediates in the interaction of phenols with N-bromosuccinimide

Abstract

Phenols react with N-bromosuccinimide to generate 2-bromocyclohexadienone-type intermediates that enolize to 2-bromophenols thermally or photolytically with a wide range of rate constants depending on the parent phenol. A trace amount of acids not only accelerates the rates of the formation and decomposition of the 2-bromodienones, it also promotes the rearrangement of the 2-bromodienones to cross-conjugated 4-bromocyclohexadienones. The enolization of these last compounds affords 4-bromophenols. The occurrence of autocatalysis in dienone decompositions arising from more acidic bromophenols is also established. The reaction is assumed to occur by a bromophilic attack of phenols on the N—Br bond to form unstable hypobromites that rapidly rearrange to the 2-bromocyclohexadienone intermediates. The quantum yield of the photodecomposition of 2-bromo-(2H)-naphthalene-1-one in the vicinity of 365 nm was determined to be unity.