Surface-mediated reactions. 9. Selective oxidation of primary and secondary amines to hydroxylamines

OXONE over silicagel or, in some cases, alumina has been found to oxidize the primary and secondary amines 3 selectively to the corresponding hydroxylamines 4, in either the presence or absence of a solvent. Treatment of Boc-protected L-lysine (6) under the latter conditions afforded hydroxylamine 7 in excellent yield. The trialkylamine la and pyridine (Ib), in which selectivity is not an issue, were readily oxidized to the corresponding oxides 2 by OXONE over silica gel or alumina, as well as by (CH3)(3)COOH over silica gel. Solvent-free oxidation assisted by microwave irradiation was more forcing, while still affording the hydroxylamines 4 selectively, and is the synthetic method of choice. The mechanistic aspects of these reactions are discussed.