Meerwein-Pondorf-Verley reaction

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

The Meerwein-Pondorf-Verley reaction involves transfer of a hydride from the oxygen-substituted carbon atom of an isopropoxide group to a carbonyl group, thus effecting the reduction of the carbonyl compound to an alcohol. 

Figure B.4. Intermediate in the Meerwein-Pondorf-Verley reaction.

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. 

A reaction mechanism quite similar to the Claisen-Tishchenko and Meerwein-Pondorf-Verley reactions assumes tetravalent aluminum in an anionic complex and is more likely to correspond to the requirements. 

Manganese pentacarbonyl (Mn(CO)5), 177 Markovnikov s rule, 277 McLafferty rearrangement, 65 MCPBA, see zn-Chloroperbenzoic acid Meerwein-Pondorf-Verley reaction, 84, 308 Metal cations... 

In modem terminology, the core of Marckwald s definition is the conversion of an achiral substance into a chiral, nonracemic one by the action of a chiral reagent. By this criterion, the chiron approach falls outside the realm of asymmetric synthesis. Marckwald s point of reference of course, was biochemical processes, so it follows that modern enzymatic processes [30-32] are included by this definition. Marckwald also asserted that the nature of the reaction was irrelevant, so a self-immolative reaction or sequence such as an intermolecular chirality transfer in a Meerwein-Pondorf-Verley reaction would also be included ... 

The reaction of a chiral alkene with borane in the proper stoichiometry may afford alkyl boranes R BH2 or dialkyl boranes R BH, where R is a chiral ligand. Attempts to achieve highly selective reductions of ketones using such reagents have met with little success, however. Trialkyl boranes R3B were first reported to reduce aldehydes and ketones (under forcing conditions) in 1966 by Mikhailov [50]. Mechanistic studies (summarized in ref. [46]) showed that there are two limiting mechanisms for the reduction of a carbonyl compound by a trialkylborane, as shown in Scheme 7.4 a pericyclic process reminiscent of the Meerwein-Pondorf-Verley reaction (Scheme 7.4a), and a two step process that involves dehydro-... 

Lion between C and the incoming acid. Carried to the extreme, a substitution reaction ensues in which the configuration at C may be retained. If the center is H rather than C, the corresponding reaction is a hydride transfer (abstraction), as in the Cannizzaro or Meerwein-Pondorf-Verley reactions. If the interaction falls short of abstraction, a hydride bridge may be fonned. Both aspects are discussed further in Chapter 10. 

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