Polymer-Supported Chromic Acid

Polyvinylpyridinium chlorochromate has been prepared by reacting cross-linked polyvinylpyridine with CrOg in acid solution (Frechetet al., 1978). It has been shown to be similar in oxidizing properties to the polymer-supported chromic acid polymer previously mentioned (Cainelli et al., 1976a). It could be readily regenerated. 

A compound prepared and first described as nicotinium dichromate (NDC) by Palomo et al.,379 was later shown by X-ray-crystal analysis380 to be a betainic mixed anhydride of nicotinic and chromic acid (NACAA). Because of its unique structure, it deserves a close scrutiny of its oxidative properties.381 Replacement of the chloride anion in the quaternary ammonium resin, Dowex 1-X8, for the dichromate anion, leads to a polymer supported dichromate, which is able to make selective benzylic oxidations.382 Finally, poly[vinyl(pyridinium dichromate)] (PVPDC), a polymeric analogue of PDC, must be mentioned whose use in the oxidation of alcohols allows for a very easy work-up.383... 

Y,7-Dimethylallyl bromide and geranyl bromide are converted into the corresponding aldehydes in 75 and 82% yields, respectively, by heating at 100 °C with sodium dichromate in hexamethylphosphoramide in the presence of dicyclohexyl-18-crown-6 ether [614]. Similar results are achieved with polymer-supported chromic acid [540] (equation 192). 

Cainelli, G., Cardillo, G., Orena, M., Sandri, S. Polymer supported reagents. Chromic acid on anion exchange resins. A simple and practical oxidation of alcohols to aldehydes and ketones. J. Am. Chem. Soc. 1976, 98, 6737-6738. 

Scheme 4.6 Application of polymer-supported chromic acid in the synthesis of rosiglitazone (27).

The polymer supported chromic acid (Amberlyst A-26, HCrO" form is commercially available) and has been used to oxidise primary and secondary alcohol to carbonyl compounds and also oxidizes allylic and benzylic halides to aldehydes and ketones. ... 

Polymer-supported chromic acid (Amberlyst A-26, HCr04 -form) has been used to oxidize primary and secondary alcohols to carbonyl compounds (Cainelli et al., 1976a). The same reagent was used to prepare... 

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