Mechanism of Oxidative Alkoxyamine Cleavage: The Surprising Role of the Solvent and Supporting Electrolyte

In this work, we show that the nature of the supporting electrolyte and solvent can dramatically alter the outcome of the electrochemically mediated cleavage of alkoxyamines. A combination of cyclic voltammetry experiments and quantum chemistry is used to study the oxidation behavior of TEMPO-i-Pr under different conditions. In dichloromethane, using a noncoordinating electrolyte (TBAPF₆), TEMPO-i-Pr undergoes reversible oxidation, which indicates that the intermediate radical cation is stable toward mesolytic fragmentation. In contrast, in tetrahydrofuran with the same electrolyte, oxidized TEMPO-i-Pr undergoes a rapid and irreversible fragmentation. In nitromethane and acetonitrile, partially irreversible oxidation is observed, indicating that fragmentation is much slower. Likewise, alkoxyamine oxidation in the presence of more strongly coordinating supporting electrolyte anions (BF₄^-, ClO₄^-, OTf^-, HSO₄^-, NO₃^-) is also irreversible. These observations can be explained in terms of solvent- or electrolyte-mediated S_N2 pathways and indicate that oxidative alkoxyamine cleavage can be "activated" by introducing coordinating solvents or electrolytes or be "inhibited" through the use of noncoordinating solvents and electrolytes.