Aromatic rearrangements in the benzene series. Part 4. Intramolecularity of both the ortho- and para-rearrangements of benzyl phenyl ether as shown by labelling experiments

Abstract

The rearrangement of a mixture of Ph¹³CH₂OPh and PhCH₂¹⁸OPh (with some unlabelled ether) catalysed by AlBr₃ in homogeneous solution in 1,2,4-trichlorobenzene and in nitrobenzene has been examined. The literature report of this reaction was shown to be substantially correct: the reaction is very rapid, and ortho-migration of the benzyl group predominates, but does not (in contrast to the earlier report) occur exclusively. The ortho : para ratio of the rearrangement products is ca. 11 in 1,2,4-trichlorobenzene and ca. 4.7 in nitrobenzene. The ether undergoes rearrangement in both solvents without prior scrambling of isotopic labels. In trichlorobenzene, the reaction appears to be almost completely intramolecular, even for para-rearrangement: o-benzylphenol is unscrambled, the para-isomer almost so. In nitrobenzene, o-benzylphenol is virtually unscrambled, the para-isomer ca. 25% so. A mechanism involving a tight ion-pair (from which some leakage occurs in nitrobenzene) is proposed, and possible explanations of the high ortho : para ratios are considered.