Oppenauer oxidation sensitivity

The Oppenauer oxidation presents two important limitations on one side it is unable to oxidize certain alcohols because of unfavourable thermodynamics, and on the other side, base-induced reactions between the oxidant and the product may become dominant. That is why, it is seldom employed for the obtention of aldehydes because these compounds react readily under basic conditions. On the other hand, although aluminium alkoxides promote aldol condensations, many base-sensitive functional groups such as most esters but not formates—25 resist its action. 

Functional Group and Protecting Group Sensitivity to Oppenauer Oxidation... 

Oxidations under Oppenauer conditions are highly selective for alcohols, normally resulting other functionalities sensitive to oxidation unchanged. This happens because the Oppenauer oxidation operates via a mechanism involving a hydride transfer from a metallic alkoxide, which is very specific for alcohols. Over-oxidations have been described only for situations in which very reactive oxidants, such as p-quinone, are employed.14... 

Although aluminium alkoxides are able to promote base-induced reactions, the basic conditions involved are not extremely strong and many base-sensitive functional groups remain unaffected during Oppenauer oxidations, including alkyl halides,51 epoxides52 and most esters.53 On the other hand, the very sensitive formate esters are hydrolyzed under Oppenauer conditions and the resulting alcohols are oxidized in situ.25... 

Although the Mukaiyama oxidation is not in the top list of the most frequently used alcohol oxidants, the authors of this book have decided to pay full attention to this procedure because it succeeds in very sensitive organometallic compounds, where most other oxidants fail. The Mukaiyama oxidation operates via a somehow unique mechanism involving a hydride transfer from a metal alkoxide to a very good hydride acceptor, which resembles the Oppenauer oxidation. In variance with the Oppenauer oxidation, the Mukaiyama protocol involves much milder conditions and it does not promote as easily base-induced side reactions. 

Typical for the Oppenauer reaction is high chemoselectivity, even in the presence of other oxidation-sensitive functional groups such as double bonds. Stable anchoring is not a problem with these Si02-based Zr catalysts, which is evident since the surface chemistry of Zr is reminiscent of that of Ti. 

Among the non-steroidal alcohols applied to the Oppenauer oxidation are the cis and trans a-decalols (14), which give excellent yields of the corresponding a-decalones (IS).22 Oxidation of phenolic compounds bearing pendent aliphatic secondary alcohols can be readily performed without prior protection of the phenolic alcohol functionality.23 Acid-sensitive acetal 16 is smoothly converted to the corresponding ketone 17, by exploiting a modified experimental procedure.3,24... 

The Oppenauer oxidation is a preferred method for oxidizing sensitive alcohols such as the sterols. Modifications have been reported which permit the reaction to be carried out at room temperature. 

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