Mg-Oppenauer oxidation

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. 

Meerwein-Pondorf-Verley reduction, discovered in the 1920s, is the transfer hydrogenation of carbonyl compounds by alcohols, catalyzed by basic metal compounds (e.g., alkoxides) [56-58]. The same reaction viewed as oxidation of alcohols [59] is called Oppenauer oxidation. Suitable catalysts include homogeneous as well as heterogeneous systems, containing a wide variety of metals like Li, Mg, Ca, Al, Ti, 2r and lanthanides. The subject has been reviewed recently [22]. In this review we will concentrate on homogeneous catalysis by aluminium. Most aluminium alkoxides will catalyze MPV reduction. 

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. 

A soln. of ( -methoxyphenyl)lithium (prepared in THF from anisole and 1.7 M -butyllithium in cyclohexane) stirred at room temp, overnight, treated with 0.5 eqs. Mg-2-ethoxyethoxide at 5°, stirred at room temp, for 1 h, cooled in ice, 1 eq. di-methylformamide added, the mixture stirred at bath temp, for 1 h, then at room temp, for 2 h, benzophenone (as oxidant) added, and stirring continued overnight - N,N-dimethyl-o-methoxybenzamide. Y 71%. The Mg-alkoxide functions by stabilizing the intermediate 1-aminoalkoxide and as a catalyst in the Oppenauer oxidation. F.e., incl. reaction of -BuLi, 2-lithiofuran, 2-lithiomethylpyridine, and benzyllithium, also from diorganomagnesium compds., s. C.G. Screttas, B.R. Steel, J. Org. Chem. 53, 5151-3 (1988). 

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