Aluminum isopropoxide, catalyst Oppenauer oxidation

Oppenauer oxidation, Eastham and TeranishP prepared cholestenone by oxidation of cholesterol in toluene solution with aluminum isopropoxide as catalyst... 

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 ninth item in Table 8.5 is of particular interest as it involves the catalytic conversion of an alcohol to the corresponding carbonyl compound while another carbonyl compound is being converted to an alcohol. Indeed, the Oppenauer oxidation, which uses, for example, aluminum isopropoxide Al[i-OPr]3, Al[OCH(CH3)2]3 as a catalyst, is the microscopic reverse of the Meerwein-Poimdort-Verley reduc-... 

In a different vein and as already pointed out in Chapter 8 (Scheme 8.6), the Meerwein-Ponndorf-Verley reduction is the reverse of the Oppenauer oxidation of aldehydes and ketones, and it is only a change of solvent that dictates whether the reaction that occurs is an oxidation or a reduction.The same catalyst is used. Scheme 9.19 is the reverse of Scheme 8.6. Thus, it is now suggested that the carbonyl oxygen of cyclohexen-3-one displaces an isopropoxy group (2-propoxy [ OCH(CH3)2]) from the catalyst, aluminum isopropoxide [Al(0-iPr)3].Then, after intramolecular hydride transfer, propanone (acetone, CH3COCH3) is lost by displacement from aluminum by the solvent, 2-propanol (isopropanol [CH3CH(OH)CH3]), and finally, the cyclo-... 

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