Oxidative Transposition

R2C=CHCH2R — R2 XH=CHR (PhSe)2, MgS04, MeOH, electrolysis JACS 103 4606 (1981) 

KN(SiMe3)2 / camphorsulfonyl oxaziridine Me,SO, (MesSihNH/m-aCtHtCCbH/n-BaiNF 

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Scheme 9 Intramolecular sulfone alkylation and oxidative transposition...

Sometimes, tertiary allylic alcohols interfere with the oxidation of primary and secondary alcohols with PDC, causing low-yielding transformations into the desired aldehydes and ketones.161 Secondary allylic alcohols occasionally suffer oxidative transposition to enones rather than a direct oxidation.162... 

Rather than a direct oxidation to dienone, the secondary alcohol suffers an oxidative transposition to give a mixture of enone and enal. 

PDC has a lesser tendency to effect oxidative transposition of allylic alcohols than other chromium-based reagents.163... 

This oxidative transposition of tertiary allylic alcohols into enones or enals is carried out under mild conditions and has ample application in organic synthesis. Although, it can be carried out with other chromium-based reagents (see pages 16 and 35), PCC is the reagent of choice.272... 

Although the PCC-mediated oxidative transposition of tertiary allylic alcohols is carried out under very mild conditions, normally it is possible to selectively oxidize a primary or secondary alcohol to aldehyde or ketone with PCC, without affecting a tertiary allylic alcohol present in the same molecule.273... 

Nevertheless, transposed enones can be formed as minor compounds,274 and a few times the oxidative transposition can predominate over the normal oxidation of primary or secondary alcohols.275... 

Of course, using excess of PCC allows the operation of both, an oxidative transposition of a tertiary allylic alcohol and a normal oxidation of a primary or a secondary alcohol.276... 

The oxidation with PCC causes both, a normal oxidation of the primary alcohol and an oxidative transposition of the tertiary allylic alcohol. 

Although secondary allylic alcohols can suffer an oxidative transposition via the corresponding allylic chromate ester, in the same manner that the tertiary allylic alcohols normally, a direct oxidation to the corresponding enone with no transposition predominates.277 Nevertheless, minor amounts of enone, resulting from an oxidative transposition, can be formed.278 The formation of transposed enone may be minimized using the less transposing-prone PDC, instead of PCC.279... 

When the oxidative transposition of secondary allylic alcohols is purposefully looked after, it can be fostered by the addition of p-toluenesulfonic acid.280 Most probably, the added acid catalyzes the equilibration of the intermediate allylic chromate esters, allowing the major formation of transposed enone when the corresponding chromate ester is less hindered. This means that an oxidative transposition of a secondary allylic alcohol can only dominate when the thermodynamics of the equilibrating allylic chromate esters are favourable. 

Thanks to the addition of p-TsOH that catalyzes the equilibration of the intermediate allylic chromate ester the major product is the desired enal, resulting from an oxidative transposition. Failure to add p-TsOH leads to the major formation of the untransposed enone, ... 

Very hindered secondary allylic alcohols may have a great tendency to suffer oxidative transpositions, even without the help of added acid a fact undoubtedly due to the release of steric tension, resulting from the transposition of the initially formed chromate ester.281... 

The authors of this book are not aware of any case, in which a primary allylic alcohol suffers an oxidative transposition with PCC. Such case would be most unlikely, because it would involve an equilibrating pair of allylic chromate ester, in which the less stable minor one would evolve to a carbonyl compound. 

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