Aromatic rearrangements in the benzene series. Part 3. Rearrangement of isotopically labelled phenyl benzoates: intermolecularity of the ortho-directed rearrangement. Criteria for determining the intra-/inter-molecularity of aromatic rearrangements

Abstract

The rearrangement of a mixture of Ph¹³COOPh and PhCO¹⁸OPh (with some unlabelled ester) catalysed by anhydrous AlBr₃ in homogeneous solution in chlorobenzene has been examined. Contrary to expectations, more of the 2-hydroxybenzophenone is formed intermolecularly than of the 4-hydroxybenzophenone at the beginning of the reaction, contradicting earlier views that a high ortho : para ratio in such reactions indicates the operation of an intramolecular mechanism. A mechanism is proposed for this stage of the rearrangement involving a cyclic six-membered transition state originating from one molecule of phenyl benzoate and one of Ph–C(OPh)=[graphic omitted]–ĀlBr₃, allowing C-acylation by the benzoyl moiety of the catalyst–ester complex to compete with bimolecular ester interchange. This C-acylation can only occur at the ortho-position of the phenoxy-ring of the (uncomplexed) ester molecule, because of steric requirements. This step of the rearrangement is thus necessarily intermolecular. Criteria for judging the intra- or inter-molecularity of aromatic rearrangements are also briefly discussed.