Meerwein-Ponndorf-Verley reduction Oppenauer oxidation

Oppenauer oxidation) (Meerwein-Ponndorf-Verley Reduction)... 

Secondary alcohols may be oxidised to the corresponding ketones with aluminium /erl.-butoxide (or tsopropoxide) in the presence of a large excess of acetone. This reaction is known as the Oppenauer oxidation and is the reverse of the Meerwein - Ponndorf - Verley reduction (previous Section) it may be expressed ... 

Meerwein-Ponndorf-Verley Reduction and Oppenauer Oxidation... 

The most common catalysts for the Meerwein-Ponndorf-Verley reduction and Oppenauer oxidation are Alm and Lnm isopropoxides, often in combination with 2-propanol as hydride donor and solvent. These alkoxide ligands are readily exchanged under formation of 2-propanol and the metal complexes of the substrate (Scheme 20.5). Therefore, the catalytic species is in fact a mixture of metal alkoxides. 

Apart from aluminium, many other metals were tested in Meerwein-Ponndorf-Verley reductions and Oppenauer oxidations during the early years of research on hydride transfer from alkoxides.26 A consensus was... 

Meerwein-Ponndorf-Verley Reduction opposite of Oppenauer oxidation Synthesis 1994,1007 Organic Reactions 1944, 2,178... 

The aluminium-catalyzed hydride shift from the a-carbon of an alcohol component to the carbonyl carbon of a second component, which proceeds via a six-membered transition state, is referred to as the Meerwein-Ponndorf-Verley Reduction (MPV) or the Oppenauer Oxidation, depending on which component is the desired product. If the alcohol is the desired product, the reaction is viewed as the Meerwein-Ponndorf-Verley Reduction. 

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. 

Catalytic Oppenauer oxidations (Eq. 28) and Meerwein-Ponndorf-Verley reductions (Eq. 29) were studied in detail [232,234]. The gadolinium derivative, employed in situ without elimination of LiCl, was reported to be ten times more reactive in the MPV reduction of cyclohexanone as the standard reagent Al(OiPr)3 [235]. 

Meerwein-Ponndorf-Verley reduction was efficiently and selectively achieved by use of l-(4-dimethylaminophenyl)ethanol as the reducing alcohol (2-4 equiv.) and Zr(0-/-Bu)4 (0.2 equiv.) as the catalyst [32b]. Oppenauer oxidation was selectively achieved by using chloral (1.2-3 equiv.) as the hydrogen acceptor and Zr(0-t-Bu)4 (0.2 equiv.) as the catalyst [32c]. 

Isopropyl Alcohol and Aluminum Isopropoxide. This is called the Meerwein-Ponndorf-Verley reduction It is reversible, and the reverse reaction is known as the Oppenauer oxidation (see 19-3) ... 

Meerwein-Ponndorf-Verley reduction Oppenauer oxidation... 

Related reactions Meerwein-Ponndorf-Verley reduction, Oppenauer oxidation, Tishchenko reaction ... 

Oppenauer oxidation. The bidentate aluminum species and pivalaldehyde constitute an oxidizing system for secondary alcohols. The bis(diisopropoxyaluminum) analog is a highly efficient catalyst for the Meerwein-Ponndorf-Verley reduction. 

In fact, a variation of this reaction has been utilized in the well-known Meerwein-Ponndorf-Verley reduction of carbonyl compounds (reverse of Oppenauer oxidation of alcohols) by aluminum isopropoxide The reaction involves a six-centered transition state, wherein the P-hydride is delivered into an incoming carbonyl group [Eq. (6.86)]. The stereochemistry of this reaction has been studied in detail. ... 

Zeolite titanium beta has been tested in the liquid- and gas-phase Meerwein-Ponndorf-Verley reduction of cyclohexanones and the Oppenauer oxidation of cyclohexanols. A high selectivity towards the thermodynamically unfavourable cis-alcohol was observed, which has been ascribed to transition-state selectivity in the pores of the zeolite. Under gas-phase conditions the dehydration of alcohols to cycloalkenes is observed as a side reaction. The catalyst was found to be active even in the presence of water and ammonia. 

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]. 

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. 

Aluminum alkoxides, particularly those formed from secondary alcohols, have been of interest to synthetic chemists since the mid-1920s due to their catalytic activity. Examples of these trialkoxides include aluminum isopropoxide (AIP) and aluminum sec-butoxide (ASB). They are easily prepared at lab or plant scale and provide highly selective reductions and oxidations under mild conditions. These reductions are termed Meerwein-Ponndorf-Verley (MPV) reactions after the chemists (1-3) who first investigated their utility. Because a MPV reaction are accuratelybe described as an equilibrium process, the reverse reaction (oxidation) can also be exploited. These associated reactions are termed Oppenauer oxidations (4). Meerwein-Ponndorf-Verley reductions and Oppenauer oxidations as well as other reaction types and applications will be discussed, but first some background is provided concerning structure, preparation, and characterization of aluminum isopropoxide and related compounds. 

Aluminium isopropoxide is a Lewis acid and it is also a good catalyst for the Oppenauer oxidation and Meerwein-Ponndorf-Verley reduction reactions. In the presence of a ketone, it will oxidise d-isomenthol to d-isomenthone (Oppenauer oxidation). The hydrogen atom on C-4 is now enolisable and therefore epimerisation can occur, catalysed by the aluminium isopropoxide acting as a Lewis acid. This will give /-menthone. This can now be reduced (Meerwein-Ponndorf-Verley reduction) to /-menthol by an alcohol and aluminium isopropoxide. The ketone and alcohol for the redox reactions could be the menthols/ menthones themselves or traces of acetone/isopropanol in the aluminium isopropoxide. Obviously, the reactions shown in Figure 4.28 are all reversible. The equilibrium will eventually be driven over completely to /-menthol since the latter is the most thermodynamically favoured of all of the isomeric components in the system. 

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