Substituent Effects upon Peak Potentials and Reductive Cleavage Rate Constants of Hydroxy- and Methoxy-Substituted 9,10-Anthraquinones in 50percent Aqueous CH3CN: Do They Correlate?

Article abstract of DOI:10.1021/jo00004a049

A variety of hydroxy- and methoxy-substituted 2-(acetoxymethyl)-9,10-anthraquinones (2a-7a) were reduced electrochemically and with dithionite (S2O4^2-) in 50percent aqueous CH3CN buffers over a wide pH range.Good to excellent yields of their corresponding reductive cleavage products, the substituted 2-methyl-9,10-anthraquinones 2b-7b, were obtained from most of these anthrachinone acetates, but only at higher pH.Rate constants for the reaction of 2-(acetoxymethyl)-9,10-anthraquinone (1a) with excess dithionite ranged from 1.0 x 10E-4 s^-1 at pH values less than 7 to 4.0 x 10E- 4 s^-1 at a pH of 10, demonstrating that loss of acetate occurs in the rate-determining step and that cleavage occurs slower via the anthrahydroquinone of 1a than the conjugate base of the anthrahydroquinone.Substituent effects upon the reductive cleavage process were determined by measuring rate constants for those acetates that react cleanly with dithionite at pH 8.These effects, which are rationalized on the basis of resonance theory and intramolecular H bonding, correlate fairly well with the peak potentials (E_p) of the reductive cleavage products of these acetates.Thus, electron-donating substituents on an anthraquinone acetate not only make it more difficult to reduce resulting in a more negative E_p but also enhance the rate of acetate cleavage in the corresponding anthrahydroquinone.