Methanolysis of K-region arene oxides: Comparison between acid-catalyzed and methoxide ion addition reactions

Article abstract of DOI:10.1021/ja00058a015

Reactions of K-region arene oxides of benz[a]anthracene (BA-O) and its 1-(1-MBA-O), 4- (4-MBA-O), 7-(7-MBA-O), 11- (11-MBA-O), 12-methyl- (12-MBA-O), 7,12-dimethyl-(DMBA-O), and 7-bromo- (7-BrBA-O) substituted derivatives, benzo[a]pyrene (BaP-O), benzo[c]phenanthrene (BcP-O), benzo[e]pyrene (BeP-O), benzo[g]chrysene (BgC-O), chrysene (Chr-O), dibenz[a,h]anthracene (DBA-O), phenanthrene (Phe-O), 3-bromophenanthrene (3-BrPhe-O), and pyrene (Pyr-O) with acid and methoxide ion in methanol, are compared. For the acid-catalyzed reaction, products consist of cis- and trans-methanol adducts and phenols. There is no isotope effect on the ratio of phenols to solvent adducts produced from Phe-O or BcP-O when deuterium is substituted for the hydrogen that migrates. This observation is consistent with a mechanism in which product distribution in acid is determined by the relative rates of solvent capture and conformational inversion of a carbocation intermediate. As expected, only trans-methanol adducts, consisting of regioisomeric mixtures for unsymmetrical arene oxides, are formed from the reaction of methoxide ion with K-region arene oxides. The use of methanol permits the identification of products formed at each benzylic position of unsymmetrical arene oxides. Rate data for reactivity at each position could be fitted to the equation log k_MeO/k_MeO^Phe-O = m(log k_H/k_H^Phe-O) + b, where k_MeO and k_H are the second-order rate constants of the methoxide ion addition and acid-catalyzed reaction, respectively, and k_MeO^Phe-O and k_H^Phe-O are the corresponding rate constants for the reference compound phenanthrene oxide. A plot of log k_MeO/k_MeO^Phe-O vs log k_H/k_H^Phe-O for the reaction of 1-MBA-O, 12-MBA-O, DMBA-O, BcP-O, and BgC-O, which have either a methyl substituent in the bay region or a sterically crowded fjord region which affects the planarity of the molecules, defined one line (m = 0.31 ± 0.02, b = 0.67 ± 0.09), whereas a plot of the data for the reaction of the nearly planar arene oxides BA-O, 4-MBA-O, 7-MBA-O, 11-MBA-O, BaP-O, BeP-O, Chr-O, DBA-O, Phe-O, and Pyr-O defined a different line (m = 0.33 ± 0.07, b = -0.05 ± 0.05). The nonzero intercept for the sterically crowded, nonplanar arene oxides indicates a steric acceleration of their rates of methoxide ion addition. The positive slopes of both lines are consistent with an S_N2 mechanism with an unsymmetrical transition state in which the epoxide C-O bond breaking is more advanced than the formation of the new C-O bond to methoxide ion, such that a partial positive charge is developed on the aromatic moiety.