Formation and Fate of Benzimidazole-Based Quinone Methides. Influence of pH on Quinone Methide Fate

Article abstract of DOI:10.1021/jo00048a020

The influence of pH on quinone methide fate was assessed from a comparative hydrolytic study of benzimidazole hydroquinones and their O-methylated analogues.Elimination of a leaving group from the hydroquinones affords the carbocation or the quinone methide depending on the pH.The O-methylated analogues, on the other hand, can only afford the carbocation species.Evidence is presented herein that the quinone methide species is reversibly protonated to afford the carbocation species.The acid dissociation constant for this equilibrium is pK_a 5.5.Above pH 5.5, the quinone methide species traps both nucleophiles and the proton.Below pH 5.5, the quinone methide species is protonated to afford the carbocation species, which exclusively traps nucleophiles.Therefore, the carbocation acid dissociation constant can be used to predict quinone methide fate as a function of pH.