Metal-Free 2,2,6,6-Tetramethylpiperidin-1-yloxy Radical (TEMPO) Catalyzed Aerobic Oxidation of Hydroxylamines and Alkoxyamines to Oximes and Oxime Ethers

TEMPO-Mediated oxidation of hydroxylamines (=hydroxyamines) and alkoxyamines to the corresponding oxime derivatives is reported (TEMPO=2,2,6,6-tetramethylpiperidin-1-yloxy radical; Scheme 2). These environmentally benign oxidations proceed in good to excellent yields (Table 1). For alkoxyamines, oxidation to the corresponding oxime ethers can be performed by using dioxygen as a terminal oxidant in the presence of 510 mol-% of TEMPO or 4-substituted derivatives thereof as a catalyst (Scheme 3 and Table 2). Importantly, benzyl bromides can directly be transformed to oxime ethers via in situ alkoxyamine formation by a nucleophilic substitution followed by TEMPO-mediated oxidation (Scheme 4 and Table 3).