Mechanistic Studies of the Cathodic Cleavage of Diphenylacetaldehyde Derivatives

At the 2014 Electrochemical Society Orlando meeting, we reported the electrochemical oxidation of 1,1-diphenylacetaldehyde (1) in the presence of an equimolar quantity of any of several alcohols (2) in acetonitrile in an undivided cell containing two carbon electrodes (Scheme 1). When the electrolysis was interrupted at its mid-point it was possible to isolate a-alkoxy aldehydes (3) in good yield. When the electrolysis was continued past this point, compounds 3 were further converted to benzhydryl ethers (4) in excellent yields. Since 1 is converted to 3 at the anode via 2, we initially assumed that the ethers 4 are also produced at the anode by subsequent oxidation of 3 to 4. However, two control experiments disproved this hypothesis. First, 3 (independently synthesized by reaction of alpha-bromo-diphenylacetaldehyde² with cyclohexanol in the presence of silver tetrafluoroborate³) was placed in the anode compartment of a divided cell. It was found to be stable to anodic oxidation. On the other hand, when it was placed in the cathode compartment, it was quickly decarbonylated to 4a. Likewise, when electrolysis of 1 was carried out in the anode compartment of a divided cell, 3 was produced but was found to be stable to further oxidation. On the other hand, when 3 was placed in the cathode compartment of the divided cell it was readily converted to 4 (Scheme 1). The mechanism of this unusual mild cathodic cleavage reaction is the subject of this report. We hypothesized that the reaction involves initial attack on the carbonyl group by a nucleophilic component of the medium  (X), followed by ejection of a benzhydryl anion (6) from the initial adduct 5. Protonation of 6 by solvent would then afford 4 We believe that X is superoxide ion, generated by reduction of cathodic reduction of traces of oxygen in the medium. Indeed, electrolysis of 3 in the cathode compartment while passing oxygen through the solution quickly converted  3 to 4. Furthermore, this suggests that it should be possible to convert 3 to 4 in a separate non-electrochemical experiment. Indeed, addition of potassium superoxide to a solution of 3 in acetonitrile was found to produce 4 in high yield (Scheme 2). Interestingly, benzophenone (8) is produced when 1 is either (a) electrolyzed in the cathode compartment with oxygen or (b) treated with potassium superoxide. This conversion probably involves initial hydroperoxidation of 3 at the a-position, followed by nucleophilic addition to the carbonyl group and fragmentation. Experiments testing the further scope of these cathodic decarbonylative cleavages will be described.