Trifluoroacetic acid-catalysed transacylation of arenes by acylpentamethylbenzene

Abstract

Facile transacylation between acylpentamethylbenzene and activated arenes such as anisole was found to occur in boiling trifluoroacetic acid (TFA). The mechanism for the transacylation between acetylpentamethylbenzene (AcPMB) and anisole with TFA was elucidated by means of product isolation and kinetics. The reaction proceeds via reversible protodeacetylation of AcPMB involving an ipso-protonated intermediate B to give pentamethylbenzene and acetic trifluoroacetic anhydride followed by irreversible acetylation of anisole by the mixed anhydride. The mechanism resulting in an ipso-protonated intermediate B was deduced from the reaction of acetylmesitylene with [²H]TFA as well as from the rate of deacetylation of 3,5-dideuterioacetylmesitylene with TFA. The formation of such an intermediate was also independently confirmed by ¹³C n.m.r. spectroscopic studies on AcPMB in superacid solutions under stable ion conditions.