Meerwein-Verley-Ponndorf reaction

One of the chemoselective and mild reactions for the reduction of aldehydes and ketones to primary and secondary alcohols, respectively, is the Meerwein-Ponndorf-Verley (MPV) reduction. The lifeblood reagent in this reaction is aluminum isopropoxide in isopropyl alcohol. In MPV reaction mechanism, after coordination of carbonyl oxygen to the aluminum center, the critical step is the hydride transfer from the a-position of the isopropoxide ligand to the carbonyl carbon atom through a six-mem-bered ring transition state, 37. Then in the next step, an aluminum adduct is formed by the coordination of reduced carbonyl and oxidized alcohol (supplied from the reaction solvent) to aluminum atom. The last step is the exchange of produced alcohol with solvent and detachment of oxidized alcohol which is drastically slow. This requires nearly stoichiometric quantities of aluminum alkoxide as catalyst to prevent reverse Oppenauer oxidation reaction and also to increase the time of reaction to reach complete conversion. Therefore, accelerating this reaction with the use of similar catalysts is always the subject of interest for some researchers. 

Recently, Flack et al. have reported using aluminum t-butoxide as accelerator for MPV reduction [46]. This reaction could be carry on only in 2-propanol, because t-butoxide do not bear an a-proton and the exchange of at least, one isopropoxide instead of t-butoxide, is vital for the reaction 

But there are some difficulties for industrial use of aluminum alkox-ides. One is their moisture sensitivity that Berkessel et al. overcame it with 

In a similar manner, Zhu et al. have been grafted zirconium 1 - propox-ide on SBA-15, MCM-41, MCM-48[49] and silica gel [50] by refluxing both reactants in n-hexane. After earful drying of the product, they used them in reduction of citral in 2-propanol at 82 C. They chose citral because in one hand, it has C=C double bond in addition to C=0 and the experiment could show chemoselectivity of MPV reduction for carbonyl group. In addition, citral occurs as cis- and trans-isomers known as neral 43, and geranial 44, respectively, and the reduced alcohols, nerol and geraniol, are used in the fragrance industry for their pleasant rose-like odor. 

They also investigated the effect of pyridine 52, and benzoic acid 53, as poisoning substrate and found that although the catalyst did not poison with 52 but 53 decreased catalytic activity rigorously and several washing with 2-propanol did not regenerate more than one third of original activity. 

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Sandmeyer reaction, 943 saponification. 809-810 SN1 reaction, 373-375 Sn2 reaction, 363-364 Stork enamine reaction, 897-898 transamination, 1167 Williamson ether synthesis, 655 Wittig reaction, 720-721 Wolff-Kishner reaction, 715-716 Meerwein-Ponndorf-Verley reaction, 746... 

Smis assisted version of this reduction has been reported. The Meerwein-Ponndorf-Verley reaction usually ... 

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. This is the reverse of the Meerwein-Ponndorf-Verley reaction (16-23), and the mechanism is also the reverse. The ketones most commonly used are acetone, butanone, and cyclohexanone. The most common base is aluminum r r/-butoxide. The chief advantage of the method is its high selectivity. Although the method is most often used for the... 

The Meerwein-Ponndorf-Verley reaction is a classic method for ketone/ aldehyde carbonyl group reduction, which involves at least 1 equivalent of aluminum alkoxide as a promoter. In this reaction, the hydrogen is transferred from isopropanol to the ketone/aldehyde substrate, so the reaction can also be referred to as a transfer hydrogenation reaction. 

The Meerwein-Ponndorf-Verley reaction usually321 involves a cyclic transition state 322... 

It was noticed as early as 1925 that alkoxides of calcium, magnesium and particularly aluminum could catalyze the reduction of aldehydes by ethanol as shown in equation (65).242,243 Removal of very volatile acetaldehyde is easily achieved to drive the reaction to the right. In 1926, Ponndorf devised a method in which both aldehydes and ketones could be reduced to alcohols by adding excess alcohol and aluminum isopropoxide.244 Such reductions are today referred to as Meerwein-Ponndorf-Verley reactions. Although alkoxides of a number of metals, e.g. sodium, boron, tin, titanium and zirconium, have been used for these reactions, those of aluminum are by far the best. 

The Meerwein-Ponndorf-Verley reaction is a useful method for the reduction of carbonyl groups to alcohols. Most typically, aluminium isopropoxide is used as a reducing agent. The acetone produced can be easily removed by distillation, thus driving the equilibrium reaction in the desired direction. When the carbonyl compound was refluxed... 

Aramendia, M. A., Borau, V., Jimenez, C., Marinas, J. M., Ruiz, J. R. and Urbano, F. J. Activity of basic catalysts in the Meerwein-Ponndorf-Verley reaction of benzaldehyde with ethanol, J. Colloid Interface Sci., 2001, 238, 385-389. 

While the use of zeolites as catalysts for C=0 reduction with alcohols as a reductant goes back to the eighties, stereoselective versions of this reaction have been developed more recently. In this so-called MPV (Meerwein-Ponndorf-Verley) reaction, the reactant carbonyl compound and the reducing alcohol coordinate simultaneously on a Lewis acid centre. In the reduction of 4-t-butylcyclohexanone, the trans product would normally be thermodynamically favored. However, the cis alcohol isomer is economically much more interesting. Van Bekkum and coworkers discovered that in the constrained pores of an Al- or Ti-Beta zeolite, more than 95 % of the product has the cis configuration (59) ... 

Both the Meerwein-Ponndorf-Verley reaction and the Cannizzaro reaction are hydride transfers in which a carbonyl group is reduced by an alkoxide group, which is oxidized. Note that each aluminum triisopropoxide molecule is capable of reducing three ketone molecules. 

Lanthanide isopropoxides, usually written Ln(OPr )3, but more likely to be oxo-centred clusters Ln50(0Pr )i3, are used, not just as starting materials for the synthesis of catalysts such as the naphthoxides but also as catalysts in their own right. They have been used in the Meerwein-Ponndorf-Verley reaction, where carbonyl compounds are reduced to alcohols, recent studies having shown that the reaction takes place exclusively by a carbon-to-carbon hydrogen transfer. 

The reduction of a carbonyl compound containing an additional functional group is a common practice. If the other group is easily reduced, best results are frequently obtained by the Meerwein-Ponndorf-Verley reaction (method 80). The following paragraphs, however, describe certain useful selective reductions. 

Figure 2.14 Solvent free Cannizzaro, Tischenko and Meerwein-Ponndorf-Verley reactions.

The Meerwein-Ponndorf-Verley reaction involves reduction of a ketone by treat-ment with an excess of aluminum triisopropoxide. The mechanism of the process is closely related to the Cannizzaro reaction in that a hydride ion acts as a leaving group. Propose a mechanism. 

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