Acid-catalyzed rearrangement of K-region arene oxides: Observation of ketone intermediates and a sterically induced change in rate-determining step

Article abstract of DOI:10.1021/ja00058a016

K-region arene oxides rearrange to phenols in acetonitrile in two acid-catalyzed steps: rapid rearrangement of the arene oxide to positionally isomeric keto tautomers of K-region phenols, followed by slow enolization. Accumulation of the ketones, proposed intermediates in the acid-catalyzed solvolyses of arene oxides in aqueous solution, allowed their direct spectroscopic observation and characterization for the first time under solvolytic conditions. Rate constants and products are reported for the K-region arene oxides of benz[a]anthracene, its 1-, 4-, 7-, 11-, 12-methyl, and 7,12-dimethyl substituted derivatives, benzo[a]pyrene, benzo[c]phenanthrene, 3-bromophenanthrene, chrysene, dibenz[a,h]anthracene, phenanthrene, and pyrene. No primary kinetic isotope effect is observed for ketone formation from phenanthrene 9,10-oxide. A linear correlation with a slope of 1.07 is observed between the logarithm of the second-order rate constants for acid-catalyzed reaction of the arene oxide at each K-region position in acetonitrile (first step) and in methanol (where ketone does not accumulate). Negative deviations from this correlation are observed for the formation of ketones in which the carbonyl oxygen is peri to a methyl substituent. These results are discussed in terms of a mechanism in which pseudoaxial opening of the epoxide gives an initial carbocation that must undergo conformational isomerization in order to produce phenolic products by migration of a pseudoaxial hydrogen. For compounds that follow the correlation, the rate-determining step in both methanol and acetonitrile is formation of the carbocation. For those compounds that deviate from the correlation and whose carbocations have their hydroxyl group in a peri position to a methyl ring substituent, the rate-determining step changes from formation of the carbocation (methanol) to its conformational inversion (acetonitrile). With few exceptions, NMR and kinetic evidence show that the regioisomeric K-region keto tautomers from a given arene oxide enolize with very similar rates (k_slow). Rates of enolization are decreased by electron withdrawing groups and by steric factors that favor nonplanarity of the ring system. A large primary kinetic isotope effect (k_H/k_D = 4.4) is observed for the acid-catalyzed enolization of the K-region ketone derived from phenanthrene. Slow abstraction of a proton by the solvent acetonitrile from the α-methylene group of the O-protonated ketone is proposed to account for these results and for the fact that ketone does not accumulate in more basic solvents. The major driving force for enolization (k_slow) is development of aromaticity in the phenol. For unsubstituted keto tautomers, a linear relationship, log k_slow = 31.8-39.2 (X), is observed, where X is the Hu?ckel π-bond character for the K-region bond of the parent hydrocarbon.