Substituent Effects on Reductive Cleavage of N-Methylarenesulfonanilides. Cleavage by Sodium Anthracene and Electrochemically at the Vitreous Carbon Electrode

The relative rates of cleavage of ten para-substituted N-methylbenzenesulfonanilides by sodium anthracene in tetrahydrofuran at 25 °C were determined. The rates of those with less electronegative substituents (p-dimethyl-amino through p-fluoro) give a moderately good correlation with σ constants, p = 1.91 (r = 0.987). More strongly electron-withdrawing substituents, however, result in a cleavage rate much slower than expected due to reduction of the substituent rather than of the sulfonyl group. Electrochemical reduction in acetonitrile solution at a vitreous carbon electrode proceeds via an irreversible two-electron process. The peak potentials of all the sulfonamides give an excellent correlation with σn constants, p = 1.07 V (r = 0.995). Whether this is an eec or ece process is discussed, as well as possible causes for the large differences between homogeneous and electrochemical reduction. A suggested value of σn for the p-methanesulfinyl group is +0.54.