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8.1.1 Mechanistic Understanding

DOI: 10.1055/sos-SD-225-00271

Dornan, L. M.; Hughes, N. L.; Muldoon, M. J.Science of Synthesis: Catalytic Oxidation in Organic Synthesis, (20171530.

Nitroxyl radicals can be used in a number of different ways for alcohol oxidation, and the mechanism depends on the nature of any cocatalyst and the terminal oxidant being used. When hypochlorite/bromide, nitrogen dioxide, or hypervalent iodine reagents [e.g., (diacetoxyiodo)benzene] are used, these oxidize the radical (and its reduced form) to the corresponding oxoammonium salt (e.g., 6), as illustrated in Scheme 2 for the case of TEMPO (5). The oxoammonium cation can then carry out a two-electron oxidation of alcohols to carbonyl compounds. The mechanism of this has recently been studied in detail using both experimental results and density functional theory (DFT).[‌6‌,‌7‌] In Scheme 3, proposed transition states under neutral and basic conditions are illustrated. Under neutral conditions, it is suggested that hydride transfer from the alcohol to the electrophilic oxygen occurs, followed by proton transfer to the nitrogen. These studies found that using pyridine as a base lowered the activation energy and increased the rate of alcohol oxidation, although, as we will see in this chapter, reactions are not always run under basic conditions.

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