Chromium reagents oxidative rearrangements

Trifluoro-l-hydroxy-1-methylethyl)cyclohexene is monooxidized at both allylic positions in the ring with different reagents to give a sole product 66 or a product mixture of 67 and 68. The selectivity of the reaction depends on the oxidizing agent used. The rearranged product 68 is the major one when oxidation is accomplished with chromium(VI) oxide in dichloromethane.103... 

In a similar fiashion to the Collins reagent, PCC will also induce oxidative rearrangement of tertiary allylic alcohols (Table S). PCC, and several other chromium oxidants, will also cause tertiary cyclopropyl alcohols to rearrange to give 3,y-unsaturated carbonyl compounds (equation 8). ... 

Tertiary allylic alcohols form a chromate ester that, as it lacks a hydrogen on a to the alcohol, instead of suffering a normal oxidation to ketone rearranges to an enone. This transformation, which can be brought about by other chromium-based reagents, is normally carried out with PCC when it is purposefully sought at (see page 55). 

Two independent syntheses of quadrone eiiq>loyed an allylic oxidation with rearrangement, as shown in equation (3), where the chromium trioxide-(3,5-dimethylpyrazole) reagent (CrOs DMP) was used. In some cases, the success of the reaction strongly depends on the nature of the oxidant, as shown in an iq>proach to (-)-upial (equation 4). Here the chi ium trioxide-heterocycle reagents, which are weaker oxidants, are quite inferior compared to the Fieser reagent ... 

An attempted allylic oxidation of (68 equation 29) was found by Paquette to be difficult to achieve using a range of reagents due to problems with polymerization and rearrangement. The chromium trioxide-pyridine complex was the only reagent combination found to be successful, albeit in low yield. 

The invariance of the rate on aldehyde concentration is consistent with only Cr v acting as an oxidant for this substrate. Evidence has been presented for the reaction of this reagent v/ith isopropyl alcohoE as an essential step in the Cr oxidation of this substrate in acetic acid. A stable Cr alkoxide of a secondary alcohol has been prepared. The complex tetrakis-(3,3-dimethyl-2-butoxy)chromium(iv) is, however, sensitive to moisture and oxygen. The difference in its stability with respect to primary and secondary alcohols results from the presence of the bulky t-butyl groups which probably prevent the rearrangement of the molecule to the correct conformation required for hydrogen transfer in the oxidation step. 

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