Meerwein-Ponndorf-Verley-Oppenauer

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). 

Scheme 20.6 Mechanism of the Meerwein-Ponndorf-Verley-Oppenauer reaction.

Meerwein-Ponndorf-Verley-Oppenauer catalysts typically are aluminum alkox-ides or lanthanide alkoxides (see above). The application of catalysts based on metals such as ytterbium (see Table 20.7, entries 6 and 20) and zirconium [85, 86] has been reported. 

Based on the catalytic activity of aluminum alkoxides in the Meerwein-Ponndorf-Verley-Oppenauer reaction, Berkessel et al. envisioned that aluminum complexes can act as alcohol racemization catalysts [32]. Aluminum alkoxide complexes generated from a 1 1 mixture of AlMes and a bidentate ligand such as binol or 2,2 -biphenol were effective catalysts for alcohol racemization. At room temperature, 10mol% of the aluminum catalyst racemized 1-phenylethanol completely within 3h in the presence of 0.5 equiv. of acetophenone. The aluminum catalysts were... 

The racemization mechanism of sec-alcohols has been widely studied [40, 41]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), e.g., aluminum complexes in the Meerwein-Ponndorf-Verley/Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via a hydrogen transfer process through metal hydrides or metal dihydrides as intermediates (Scheme 5.21) [42]. 

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. 

Namy, J. L., Souppe, J., Collin, J., Kagan, H. B. New preparations of lanthanide aikoxides and their catalytical activity in Meerwein-Ponndorf-Verley-Oppenauer reactions. J. Org. Chem. 1984,49, 2045-2049. 

Kow, R., Nygren, R., Rathke, M. W. Rate enhancement of the Meerwein-Ponndorf-Verley-Oppenauer reaction in the presence of proton... 

Otvos, L., Gruber, L., Meisel-Agoston, J. The Meerwein-Ponndorf-Verley-Oppenauer reaction. I. Investigation of the reaction mechanism with radiocarbon. Racemization of secondary alcohols. Acta Chim. Acad. Sci. Hung. 1965,43, 149-153. 

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. 

Klomp, D., Maschmeyer, T., Hanefeld, U., Peters Jeep, A. Mechanism of homogeneously and heterogeneously catalysed meerwein-ponndorf-verley-oppenauer reactions for the racemisation of secondary alcohols. Chemistry (Weinheim an der Bergstrasse, Germany)... 

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... 

This reaction was first reported concurrently by Meerwein and Schmidt and Verley in 1925, and by Ponndorf in 1926, respectively. It is an aluminum alkoxide-catalyzed reduction of carbonyl compounds (ketones and aldehydes) to corresponding alcohols using another alcohol (e.g isopropanol) as the reducing agent or hydride source. Therefore, it is generally known as the Meerwein-Ponndorf-Verley reduction (MPV) or Meerwein-Ponndorf-Verley reaction. Occasionally, it is also referred to as the Meerwein-Ponndorf reduction, Meerwein-Ponndorf reaction, or Meerwein-Schmidt-Ponndorf-Verley reaction. About 12 years later, Oppenauer reported the reversion of this reaction in which alcohols were reversely oxidized into carbonyl compounds. Since then, the interchanges between carbonyl compounds and alcohols in the presence of aluminum alkoxide are generally called the Meerwein-Ponndorf-Oppenauer-Verley reduction or Meerwein-Ponndorf-Verley-Oppenauer reaction." ... 

Oxidations and Meerwein-Ponndorf-Verley/Oppenauer Conversions... 

In addition, transition-metal-free dehydrogenative a-alkylation of ketones with primary alcohols involving the Meerwein-Ponndorf-Verley-Oppenauer redox cycle has recently been developed by using LiOrBu or NaOH as base [237, 238]. 

Propose a mechanism that accounts for the second and third equivalents of hydride in these aldehyde reductions. Hint. Remember the Meerwein-Ponndorf-Verley-Oppenauer equilibration. 

Meerwein-Ponndorf-Verley-Oppenauer (MPVO) equilibration... 

The Meerwein-Ponndorf-Verley/Oppenauer reaction [1] induced by air-oxidised samarium(II) iodide has been found to be a means to directly isomerise aldohexopyranoses into ketohexopyranoses. ladonisi and co-workers [2] report-... 

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... 

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