Aromatic rearrangements in the benzene series. Part 6. The Fries rearrangement of phenyl benzoate: the role of tetrabromoaluminate ion as an aluminium bromide transfer agent

Abstract

The ester PhC¹⁷O¹⁷OPh was prepared, and its ¹⁷O NMR spectra in PhCl were recorded alone, in the presence of AlBr₃ and in the presence of AlBr₃+ AlBr₄^–. Under similar conditions, ¹³C NMR spectra of unlabelled PhCO₂Ph were recorded, as were FTIR spectra of PhCO₂Ph and benzoates of three 2,6-disubstituted phenols. The spectra show clearly that formation of the carbonyl-oxygen-coordinated complex PhC(OPh)O–ĀlBr₃ from PhCO₂Ph and AlBr₃ is partially reversed in the presence of AlBr₄^–(presumably by capture of AlBr₃ to give Al₂Br₇^–), with no detectable formation of any new intermediate. As this cannot increase the rate of rearrangement of PhCO₂Ph by AlBr₃, which is the experimentally observed consequence of the addition of AlBr₄^– to the 1 : 1 rearrangement reaction (1 AlBr₃ : 1 PhCO₂Ph in homogeneous solution in PhCl), the AlBr₄^– must play an additional role. It is suggested that the AlBr₄^– effectively transfers AlBr₃ from the carbonyl oxygen of the catalyst-ester complex (above) to the phenoxy oxygen, via a process involving a cyclic transition state, simultaneously resulting in fragmentation of the complex to give the ion-pair (PhCO·PhOĀlBr₃), previously invoked as the intermediate involved in the second-stage reaction of the two-stage 1 : 1 rearrangement reaction (above).