Neighboring Group Participation by Carbonyl Oxygen in Nucleophilic Substitution. Hydrolyses of methyl(p-nitrophenyl)sulfonium Tetrafluoroborate

Article abstract of DOI:10.1021/jo01298a011

Evidence was obtained for participation of a hydration equilibrium of a carbonyl compound in an S_N2 reaction.Hydrolysis of <(9-oxobenzonorbornen-exo-2-yl)methyl>methyl(p-nitrophenyl)sulfonium tetrafluoroborate (10) in aqueous buffer medium afforded in quantitative yield an alcohol of retained structure, (9-oxobenzonorbornen-exo-2-yl)methanol (9), as a result of nucleophilic attack of water at the sterically more hindered methylene carbon α to the sulfonium sulfur.In contrast, the same reactions of the endo-2-yl epimer (11) and isobutylmethyl(p-nitrophenyl)sulfonium perchlorate (15, a model compound) occurred at the less hindered methyl carbon, entirely for 11 and predominantly for 15.The overall rate law for the hydrolysis of 10 is suggested to be rate = (k_H2O + k_H+> + k_OH-> + k_HB + k_B + k_BT,OHT->).All the coefficients k determined were in almost the same order of magnitude as the corresponding k's recently reported for the hydrolysis of the anti-9-hydroxybenzonorbornene derivative (4), which involves participation by hydroxyl oxygen.When k_H2O and k_OH were compared with the corresponding rates of another model, dimethyl(p-nitrophenyl)sulfonium perchlorate (14), the relative reactivities were calculated as 240 and 10⁶, respectively, and the effective molarity (EM) was calculated as 1.3*10⁴ M.