Magnesium Oppenauer oxidation

Chemically modified AI2O3,32 and an aluminium and magnesium carbonate33 have been studied in Oppenauer oxidations employing oxidants other than chloral. 

In this paper, the published yields were modest and the full versatility of the procedure was not checked. However, this paper established the conceptual principle that magnesium alkoxides could be efficiently oxidized in the presence of good hydride abstractors, such as l,l -(azodicarbonyl)di-piperidine (ADD), via a hydride transfer resembling the mechanism of the Oppenauer oxidation. 

Carboxamidation of RLi or RMgX.1 This reaction can be effected by reaction of RLi (or RMgX) with DMF to give a hemiaminal a followed by an Oppenauer oxidation. The second step requires the presence of a magnesium alkoxide such as magnesium 2-ethoxyethoxide 1, Mg(OCH2CH2OC2H5)2, either as a catalyst for the oxidation or for stabilization of a, possibly as a mixed cluster. 

The Oppenauer oxidation can also afford a convenient alternative to more traditionally used oxidants. Oxidation of quinine (13) using benzophenone and KO/Bu gave the ketone in excellent yield (Figure 3.14) other oxidants were less effective [27]. This reaction can be viewed as an Oppenauer oxidation or a Meerwein-Pondorf-Verley reduction, depending on whether one considers the oxidation or the reduction to be the primary reaction. A magnesium-catalyzed Meerwein-Pondorf-Verley reduction was determined to form significant amounts of impurities in a Grignard reaction [28]. 

Diaryl ketones are formed in the Grignard reaction of ArCHO with AriMgX LiCl, when PhCHO is added during workup. A redox process (Mg-Oppenauer oxidation of the halo-magnesium diarylmethoxides) is involved. 

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