Meerwein-Ponndorf-Verley-Oppenauer MPVO reaction

Very recently the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction has been exploited for the racemization of alcohols using inexpensive aluminum-based catalysts. Combination of these complexes with a lipase (CALB) results in an efficient DKR of sec-alcohols at ambient temperature. To increase the reactivity of the aluminum complexes, a bidentate ligand, such as binol, is required. Also, specific acyl donors need to be used for each substrate [31] (Eigure 4.9). 

Among the hydrogen transfer reactions, the Meerwein-Ponndorf-Verley reduction and its counterpart, the Oppenauer oxidation, are undoubtedly the most popular. These are well-established selective and mild redox reactions and they have been studied extensively [4, 5]. Nevertheless, traditional Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reactions have some drawbacks, as they usually suffer from poor reactivity of the traditional Al(OiPr)3/iPrOH system, for which continuous removal of the produced acetone is necessary in order to shift the equilibrium between reduction of the ketone and oxidation of the donor alcohol. 

Yager, B. J., Hancock, C. K. Equilibrium and kinetic studies of the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction. J. Org. Chem. 1965, 30, 1174-1179. 

Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reactions are usually mediated by metal alkoxides such as Al(0/-Pr)3. The activity of these catalysts is related to their Lewis-acidic character in combination with ligand exchangeability. The mechanism of these homogeneous MPVO reactions proceeds via a cyclic six-membered transition state in which both the reductant and the oxidant are co-ordinated to the metal center of the metal alkoxide catalyst (Scheme 1). The alcohol reactant is co-ordinated as alkoxide. Activation of the carbonyl by co-ordination to Al(III)-alkoxide initiates the hydride-transfer reaction from the alcoho-late to the carbonyl. The alkoxide formed leaves the catalyst via an alcoholysis reaction with another alcohol molecule, usually present in excess [Ij. 

CLASSIC MEERWEIN-PONNDORF-VERLEY-OPPENAUER (MPVO) REACTIONS... 

Berkessel and coworkers reported the DKR of secondary alcohols based on the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction (Scheme 5.24) [40]. Trimethylaluminum (AlMej) was employed for the in situ generation of aluminum... 

More recently, Lnm alkoxides were shown to have much higher catalytic activity in this reaction, which allowed their use in only catalytic amounts [6, 7]. Later, however, much higher reactivities for Alln-catalyzed Meerwein-Ponndorf-Verley and Oppenauer (MPVO) reactions have also been achieved with dinuc-lear Alnl complexes [8,9] and with Alln alkoxides generated in situ [10]. Several reviews on the MPVO reactions have been published [11-14],... 

The Meerwein-Ponndorf-Verley reduction of carbonyl compounds and the Oppenauer oxidation of alcohols, together denoted as MPVO reactions, are considered to be highly selective reactions. For instance, C=C double bonds are not attacked. In MPV reductions a secondary alcohol is the reductant whereas in Oppenauer oxidations a ketone is the oxidant. It is generally accepted that MPVO reactions proceed via a complex in which both the carbonyl and the alcohol are coordinated to a Lewis acid metal ion after which a hydride transfer from the alcohol to the carbonyl group occurs (Fig. 1) [1]. Usually, metal ec-alkoxides are used as homogeneous catalysts in reductions and metal t-butoxides in oxidations [1]. 

Summary Meerwein-Ponndorf-Verley and Oppenauer reactions (MPVO) are catalysed by metal oxides which possess surface basicity or Lewis acidity. Recent developments include the application of basic alkali or alkaline earth exchanged X-type zeolites and the Lewis-acid zeolites BEA and [Ti]-BEA. The BEA catalysts show high stereoselectivity, as a result of restricted transition state selectivity, in the MPV reduction of substituted alkylcyclohexanones with i-PrOH. 

The Meerwein-Ponndorf-Verley reduction of aldehydes and ketones and its reverse, the Oppenauer oxidation of alcohols, are hydrogen-transfer reactions that can be performed under mild conditions and without the risk of reducing or oxidizing other functional groups [1]. The hydrogen donors are easily oxidizable secondary alcohols (e. g. i-PrOH) and the oxidants are simple ketones (e. g. cyclohexanone). Industrial applications of the MPVO reactions are found in the fragrance and pharmaceutical industries, for example. 

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