ACID-CATALYZED AROMATIZATIONS OF ARENE OXIDES AND ARENE HYDRATES - ARE ARENE OXIDES HOMOAROMATIC

Measurements are reported of rates of acid-catalyzed dehydration of water adducts (hydrates) of benzene, naphthalene, anthracene, and phenanthrene. Benzene hydrate (2,4-cyclohexadienol) reacts in aqueous acetic acid buffers and is subject to specific acid catalysis consistent with rate-determining formation of a cyclohexadienyl cation intermediate which is rapidly deprotonated to benzene. The mechanism of dehydration differs from that of simple alcohols for which deprotonation of the carbocation is rate-determining, reflecting the lower stability of an alkene than an arene product. For 9,10-phenanthrene hydrate (9,10-dihydro-9-hydroxyphenanthrene), product studies from solvolysis of 9-phenanthryl carboxylate esters show that an appreciable fraction (20%) of the 9-phenanthrenonium ion intermediate reforms the hydrate in competition with deprotonation. This indicates that deprotonation is partially rate-determining in the dehydration reaction, consistent with the smaller gain in resonance energy accompanying the formation of phenanthrene rather than benzene or naphthalene as products. Relative reactivities of arene hydrates are strongly influenced by benzoannelation: benzene hydrate is 500 times more reactive than 1-hydroxy-1,2-dihydronaphthalene (alpha-naphthalene hydrate) which is 100 times more reactive than 9,10-phenanthrene hydrate. This reactivity order reflects relative stabilities of carbocation intermediates and hydrate reactants. Comparison of benzene hydrate with benzene oxide shows, surprisingly, that the hydrate is more reactive toward acid despite acid-catalyzed carbon-oxygen bond breaking in simple epoxides occurring 10(6)-10(7) times more readily than in structurally related alcohols. For a series of arene oxides and hydrates, oxide/hydrate aromatization rate ratios are inversely related to the resonance energy of the aromatized ring. This behavior is tentatively ascribed to homoaromatic stabilization of the arene oxides.