Aluminum alkoxides, reduction with mechanism

The Meerwein-Ponndorf-Verley (MPV) reaction is an important route in the reduction of ketones with aluminum alkoxides (111). The mechanism has been... 

Evans, D.A. Nelson, S.G. Gagne, M.R. Mud, A.R. J. Am. Chem. Soc., 1993,115,9800. It has been that shown in some cases reduction with metal alkoxides, including aluminum isopropoxide, involves free-radical intermediates (SET mechanism) Screttas, C.G. Cazianis, C.T. Tetrahedron, 1978, 34, 933 Nasipuri, D. Gupta, M.D. Baneijee, S. Tetrahedron Lett., 1984, 25, 5551 Ashby, E.C. Argyropoulos, J.N. Tetrahedron Lett.,... 

The mechanism for reduction by LiAlH4 is very similar. However, because LiAlH4 reacts very rapidly with protic solvents to form molecular hydrogen, reductions with this reagent must be carried out in aprotic solvents, usually ether or THF. The products are liberated by hydrolysis of the aluminum alkoxide at the end of the reaction. 

The most clearly established mechanism for a hydrogen-transfer reaction is that in which an aluminum alkoxide is used as the catalyst for the reduction of a carbonylic group with an alcohol. By using deuterium as a marker... 

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. 

The mechanism of reduction begins with hydride addition, which gives a tetrahedral intermediate. Elimination of an aluminum alkoxide leads to an iminium ion. Addition of a second hydride gives the final amine product. 

Reduction of an Ester (Section 18.10/ Reduction of an ester by lithium aluminum hydride gives two alcohols. The mechanism involves initial nucleophilic attack by a hydride ion onto the carbonyl carbon to give a tetrahedral addition intermediate, which collapses through the loss of alkoxide to give an aldehyde, which reacts with a second hydride to give the product alcohol. 

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