Aromatic rearrangements in the benzene series. Part 5. The Fries rearrangement of phenyl benzoate: the rearranging species. The effect of tetrabromoaluminate ion on the ortho/para ratio: the non-involvement of the proton as a co-catalyst

Abstract

Kinetic studies show that in the rearrangement of phenyl benzoate catalysed by anhydrous AlBr₃ in homogeneous solution in chlorobenzene, the actual species undergoing rearrangement is PhCO₂Ph·AlBr₃(when a 1 : 1 molar ratio of catalyst: ester is used). Addition of AlBr₄^– to the reaction mixture (as Bu₄N⁺AlBr₄^– which itself causes no rearrangement) gives lower ortho/para ratios than are found in the absence of this ion, the ratio decreasing as the quantity of AlBr₄^– increases. In the absence of deliberately added AlBr₄^–, ²⁷Al n.m.r. spectroscopy shows that AlBr₄^– is undetectable at, the beginning of the 1 : 1 rearrangement reaction (1 AlBr₃:1 PhCO₂Ph), though it accumulates during the reaction; but that ca. 0.8% of the AlBr₃ is present as this ion at the start of the closely related 1 : 1 : 1 acylation reaction (1 AlBr₃: 1 PhCOBr: 1 PhOH), accumulating during the course of this reaction also. Thus, the different behaviour of the rearrangement and acylation reactions (as indicated by their o:p ratios, and the variation of these with time) is explained by the initial absence of AlBr₄^– from, or its initial presence in, the various reaction mixtures. Rearrangement reactions carried out with a stream of nitrogen bubbled through them to remove HBr show o:p ratios which support the above conclusion, but more rigorous proof comes from acylation reactions involving PhCOBr and PhO^–Na}(rather than PhOH), which give only PhCO₂Ph and NaBr, i.e. H⁺ and (soluble) Br^– are not formed. Under the influence of AlBr₃, these reactions then mimic the 1 : 1 rearrangement reactions, but if Bu₄N⁺Br^– is also added with the AlBr₃, the subsequent rearrangements mimic the 1 : 1 : 1 acylation reactions. This excludes H⁺ as a co-catalyst in the 1 : 1 rearrangement reaction. Finally, calorimetric measurements provide data supporting the proposed mechanism of the first stage of the 1 : 1 rearrangement (set out in Part 3 of this series), and investigations of the initial stages of the 1 : 1 rearrangement and 1 : 1 : 1 acylation reactions are described.