Chromyl chloride, reaction with alkenes

The name Etard reaction is often applied to any oxidation with chromyl chloride, for example, oxidation of glycols (19-7) and Alkenes (19-10). 

Oxidation with chromium(VI) oxide, t-butyl chromate or chromyl chloride may also take place at the allylic position with the formation of unsaturated ketones. The alkene may enhance the rate of displacement of an allylic halide, and the reaction may even take place with the participation of the double bond, so leading to an allylic rearrangement (Scheme 3.25). 

The reaction of chromyl chloride with alkenes gives epoxides, chlo-rohydrins, chloroketones, and ketones [668, 667, 668, 669, 670] or aldehydes (in the presence of zinc) [671, 672]. Benzene homologues are oxidized to aldehydes [667, 668] or ketones [666, 668, 673], Primary alcohols are converted into aldehydes [674, 675], and trimethylsilyl ethers of enols are transformed into a-hydroxy ketones [676]. 

This reagent can also be used for the oxidation of methylated aryl derivatives to the corresponding aryl aldehyde, in what has become known as the Etard reaction. Tillotson and Houston found that the Etard reaction is catalyzed by small amounts of alkene, added to or present in the reaction medium. The reaction involves addition of chromyl chloride to a carbon disulfide or carbon tetrachloride solution of the arene. A dark brown, insoluble, and explosive intermediate usually precipitates. Dilute sulfurous acid is added to decompose the precipitate to the aldehyde. Toluene is converted to benzaldehyde and ethylbenzene was oxidized to phenylacetaldehyde with this reagent. 

Chromyl chloride can also produce ketones, but a common side reaction is formation of an a-chloro ketone, as in the conversion of cyclododecene to a-chlorocyclododecanone (401) in 79% yield. In a subsequent reaction, the chloride moiety in 401 was reduced to give cyclododecanone in 95% yield with zinc and acetic acid. Chromyl chloride also reacts with alkenes to give a trans-chlorohydrin rather than a carbonyl compound (see sec. 2.10.C for the preparation of chlorohydrins), as in the conversion of cyclohexene to tran5-2-chlorocyclohexanol. 5 It is interesting that in this particular case (the temperature was maintained at -78°C in dichloromethane), the cis-chlorohydrin was formed. ... 

Electrophilic Addition.—The hydroxylation of alkenes, including several steroids, with osmium tetroxide has been reviewed. Addition of HOBr to the 19-functionalized-5a-cholest-6-enes (13) gave mixtures of the corresponding bromohydrins (14) and the 6,19-epoxides (15) whereas the analogous B-homo-compounds (16) gave only the 6,19-epoxides (17). Similar effects were noted for the related HBr- and HC104-catalysed opening of the 6a,7a-epoxides and it was observed that the predominant attack by the 19-0 atom [5(0)" attack] in the B-homo-series lay in the possibility of its linear approach with the C-6—Br or C-6—O bond. The reactions of chromyl chloride and chromyl fluoride with steroidal alkenes and dienes have been reported " and it was observed that the... 

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