A Novel Mechanism for the NIH-Shift

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RESUMO

Kinetics of aromatization of 1,4-dimethylbenzene oxide (I) to 2,5-dimethylphenol (II) and 2,4-dimethylphenol (III), the latter arising by an NIH-Shift of a methyl group, as measured in the pH range 1-12, follow the equation -d[I]/dt = [I][k0 + (k[unk] + k[unk])aH], where k0 = 4.8 × 10-3 sec-1, k[unk] = 7.3 × 102 M-1 sec-1, and k[unk] = 5.3 × 102 M-1 sec-1. The ratio of products II to III at pH ≥ 6 in the spontaneous rearrangement (k0) is 13 to 87, and changes to 54 to 46 in the acid-catalyzed rearrangement (k[unk] and k[unk]). While no intermediate is detectable in the acid-catalyzed rearrangement of the arene oxide by pathway k[unk], simultaneous addition of water (and other nucleophiles) by pathway k[unk] leads, via the intermediate 1,4-dimethyl-2,5-cyclohexadiene-1,4-diol (IV), to the phenols II and III. This new mechanism for the NIH-Shift serves as a model for the ease of nucleophilic addition to other arene oxides, such as those of the polycyclic aromatic hydrocarbons recently implicated in mechanisms of carcinogenesis.

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