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- The Stahl oxidation is a copper-catalyzed aerobic oxidation of primary and secondary alcohols to aldehydes and ketones. Known for its high selectivity and mild reaction conditions, the Stahl oxidation offers several advantages over classical alcohol oxidations. Key features of the Stahl oxidation are the use of a 2,2'-bipyridyl-ligated copper(I) species in the presence of a nitroxyl radical and N-methyl imidazole in polar aprotic solvent, most commonly acetonitrile or acetone. Copper(I) sources can vary, though sources with non-coordinating anions like triflate, tetrafluoroborate, and hexafluorophosphate are preferred, with copper(I) bromide and copper(I) iodide salts demonstrating utility in select applications. Frequently, tetrakis(acetonitrile)copper(I) salts are used. For most applications, reactions can be run at room temperature and ambient air contains sufficiently high enough oxygen concentrations to be used as the terminal oxidant. Compared to chromium-, DMSO-, or periodinane-mediated oxidations, this proves safe, environmentally-friendly, practical, and highly economical. In general, the Stahl oxidation is selective for oxidizing primary alcohols over secondary alcohols (both aliphatic and benzylic), and favors the oxidation of primary benzylic alcohols over primary aliphatic alcohols when TEMPO is used as the nitroxyl radical. This is in contrast to the Oppenauer oxidation, which favors the oxidation of secondary alcohols over primary and several other specialty oxidations. Over-oxidation of primary alcohols to carboxylic acids is rare, though lactones can form in certain diol-containing substrates. The use of less hindered nitroxyl radicals like ABNO or AZADO allow for the oxidation of both primary and secondary alcohols. (en)
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