Oppenauer oxidation, aldehydes from, with alcohols

A common side reaction during Oppenauer oxidations consists of the base-catalyzed condensation of the carbonyl compound, resulting from the oxidation, with the carbonyl compound used as oxidant. Sometimes, advantage is taken from this side reaction for synthetic purposes. For example, oxidation of primary alcohols with an aluminium alkoxide and acetone results in the formation of an intermediate aldehyde that condenses with acetone, resulting in a synthetically useful formation of an enone.59... 

The oxidation of cyclopropanecarbinol under Oppenauer conditions using ciiinamaldehyde as oxidant leads to the desired aldehyde contaminated with cyclobutanol, which probably arises from a ring expansion promoted by a complexation of the alcohol with the aluminium atom operating as a Lewis acid. 

Metal-catalyzed oxidation of alcohols to aldehydes and ketones is a subject that has received significant recent attention [21,56,57]. One such method that utilizes NHC ligands is an Oppenauer-type oxidation with an Ir or Ru catalyst [58-62]. These alcohol oxidation reactions consist of an equilibrium process involving hydrogen transfer from the alcohol substrate to a ketone, such as acetone (Eq. 5), or an alkene. Because these reactions avoid the use of a strong oxidant, the potential oxidative instability of NHC ligands is less problematic. Consequently, these reactions represent an important target for future research into the utility of NHCs. 

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. 

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