Electroreductive Cleavage of Substituted 9,10-Anthraquinones in 50percent Aqueous THF Buffers: A pH-Dependent Process

Article abstract of DOI:10.1021/jo00248a024

A variety of substituted 9,10-anthraquinones with acetate leaving groups (4a-7a) were reduced at a glassy carbon electrode in 50percent aqueous THF prepared from aqueous buffers at pH 6, 7, and 8.Cyclic voltammograms of these compounds exhibited a single reduction wave with E_p values of -450 to -530 mV (vs Ag/AgCl/0.10 M Cl^-) at pH 7.E_p shifted to more negative values with increasing pH (45-50 mV/pH unit) consistent with a 2e^-/2H⁺ reduction process which converts 4a-7a to their corresponding anthrahydroquinones.Constant potential reduction of acetates 4a-7a at -800 mV at pH 6 or 7 gave n values of 2.2-2.4.Air oxidation of the catholytes, a procedure that converts anthrahydroquinones to anthraquinones, led to a 56-89percent recovery of the acetates and 7-32percent yields of the reductive cleavage products 4b-7b.In contrast, electroreduction of 4a-7a at pH 8 gave much higher yields (50-73percent) of 4b-7b with n values of 3.5-3.8.This pH-dependent process suggests that 4a-7a cleave much slower via their intermediate anthrahydroquinones than the conjugate bases of their anthrahydroquinones, which are present in relatively high concentration at higher pH.NaOH titration curves of the anthrahydroquinones of 5a and 5b support this mechanistic picture.