A test set of 14 TEMPO-based alkoxyamines was studied via a combination of cyclic voltammetry (CV) and accurate quantum chemistry to assess the effect of substituents on electrochemical cleavage. The experimental oxidation potentials of the alkoxyamines fell into the range of 1.1-1.6 V versus Ag/AgCl in acetonitrile, were well reproduced by theory (MAD 0.04V), with values showing good correlation with the sR Hammett parameters of both the R group and the OR group in TEMPO-R. Importantly, most of the studied alkoxyamines underwent oxidative cleavage to form either TEMPO • and R + or TEMPO + and R • , with the former favored by electron donating substituents on R (e.g., 2-oxolane, Ac, CH(CH3)Ph, i-Pr, t-Bu) and the latter by electron withdrawing substituents (Bn, allyl, CH(CH3)C(O)OCH3, C(CH3)2C(O)OCH3, CH(CH3)CN). Where R is not stabilized (e.g. R = CH2C(O)OCH3, Me, Et), fully or almost fully reversible oxidationwithout cleavagewas observed, making these species promising candidates for battery applications. Finally, in the case of R = Ph where NO cleavage occurred, a phenoxy cation and an aminyl radical were generated. Based on these results, TEMPO-based alkoxyamines can provide a variety of electrochemically generated carbon-centered radicals and carbocations for use in synthesis, polymerization and surface modification.

Bench-and air-stable 1-methoxy-2,2,6,6-tetramethylpiperidine (TEMPO-Me) is relatively unreactive at ambient temperature in the absence of an electrochemical stimulus. In this report, we demonstrate that the one-electron electrochemical oxidation of TEMPO-Me produces a powerful electrophilic methylating agent in situ. Our computational and experimental studies are consistent with methylation proceeding via a SN2 mechanism, with a strength comparable to the trimethyloxonium cation. A protocol is developed for the electrochemical methylation of aromatic acids using TEMPO-Me.

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 (CV) experiments and quantum chemistry is used to study the oxidation behavior of TEMPO-i-Pr under different conditions. In dichloromethane, using a non-coordinating electrolyte (TBAPF6), TEMPO-i-Pr undergoes reversible oxidation, which indicates that the intermediate radical-cation is stable towards 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 SN2 pathways, and indicate that oxidative alkoxyamine cleavage can be 'activated' by introducing coordinating solvents or electrolytes or 'inhibited' through the use of non-coordinating solvents and electrolytes. Scheme 1. Electrochemical cleavage of alkoxyamines and their subsequent fragmentation pathways.

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