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

Measurements are reported of rates of acid-catalyzed dehydration of wa ter adducts (hydrates) of benzene, naphthalene, anthracene, and phenan threne. Benzene hydrate (2,4-cyclohexadienol) reacts in aqueous acetic acid buffers and is subject to specific acid catalysis consistent wit h 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 c arbocation is rate-determining, reflecting the lower stability of an a lkene than an arene product. For 9,10-phenanthrene hydrate (9,10-dihyd ro-9-hydroxyphenanthrene), product studies from solvolysis of 9-phenan thryl carboxylate esters show that an appreciable fraction (20%) of th e 9-phenanthrenonium ion intermediate reforms the hydrate in competiti on with deprotonation. This indicates that deprotonation is partially rate-determining in the dehydration reaction, consistent with the smal ler gain in resonance energy accompanying the formation of phenanthren e rather than benzene or naphthalene as products. Relative reactivitie s of arene hydrates are strongly influenced by benzoannelation: benzen e hydrate is 500 times more reactive than 1-hydroxy-1,2-dihydronaphtha lene (alpha-naphthalene hydrate) which is 100 times more reactive than 9,10-phenanthrene hydrate. This reactivity order reflects relative st abilities of carbocation intermediates and hydrate reactants. Comparis on of benzene hydrate with benzene oxide shows, surprisingly, that the hydrate is more reactive toward acid despite acid-catalyzed carbon-ox ygen 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 inver sely related to the resonance energy of the aromatized ring. This beha vior is tentatively ascribed to homoaromatic stabilization of the aren e oxides.