Meerwein-Pondorf-Verley-Oppenauer reaction

Wilk, B. K. Helom, J. L. CoughHn, C. W., Magnesium Meerwein-Pondorf-Verley-Oppenauer Reaction.The Origin of an Impurity in PDA-641 Batches. Org. Process Res. Dev. 1998,2,407. 

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. 

The Oppenauer oxidation. When a ketone in the presence of base is used as the oxidizing agent (it is reduced to a secondary alcohol), the reaction is known as the Oppenauer oxidation,95 This is the reverse of the Meerwein-Pondorf-Verley reaction (6-25), and the mechanism is also the reverse. The ketones most commonly used are acetone, butanone, and cyclohexanone. The most common base is aluminum f-butoxide. The chief advantage of the method is its high selectivity. Although the method is most often used for the preparation of ketones, it has also been used for aldehydes. 

Aluminum methoxide Al(OMe)3 is a solid which sublimes at 240 °C in vacuum. Aluminum isopropoxide melts in the range 120-140 °C to a viscous liquid which readily supercools. When first prepared, spectroscopic and X-ray evidence indicates a trimeric structure which slowly transforms to a tetramer in which the central Al is octahedrally coordinated and the three peripheral units are tetrahedral.162,153 Intramolecular exchange of terminal and bridging groups, which is rapid in the trimeric form, becomes very slow in the tetramer. There is MS and other evidence that the tetramer maintains its identity in the vapour phase.164 Al[OCH(CF3)2]3 is more volatile than Al[OCH(Me)2]3 and the vapour consists of monomers.165 Aluminum alkoxides, particularly Al(OPr )3, have useful catalytic applications in the synthetic chemistry of aldehydes, ketones and acetals, e.g. in the Tishchenko reaction of aldehydes, in Meerwein-Pondorf-Verley reduction and in Oppenauer oxidation. The mechanism is believed to involve hydride transfer between RjHCO ligands and coordinated R2C=0— A1 groups on the same Al atom.1... 

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]. 

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