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Achromate ester is a chemical structure that contains achromium atom (symbol Cr) in a +6oxidation state that is connected via anoxygen (O) linkage toalkyl or aryl group. The Cr itself is in itschromate form, with several oxygens attached, and the Cr–O–C attachment makes this chemical group structurally similar to otherester functional groups.
Chromate esters, which are mainly of academic interest, are few and often labile. They can be isolated with bulky organic substituents such astert-butyl ortriphenylmethyl. Key is the absence of an alpha C-H bond. Suitable sources of chromium arechromium trioxide (CrO3),chromyl chloride (CrO2Cl2), and similar Cr(VI) reagents. For example, treatment of chromium trioxide withtrityl chloride gives two chromate esters:[1]
Usually chromate ester refers to monoesters, which are transient intermediates in the oxidation of alcohols by chromium(VI) oxides. Dialkychromates have the formulaCrO2(OR)2. When R lacks an alpha hydrogen, these diesters are isolable. One chromate diester has attracted significant attenion:((CH3)3CO)2CrO2. It is not an intermediate, rather a reagent. For example, it convertscyclohexene to 2-cyclohexenone.[2]
Chromate esters tend to react viaredox reactions to liberate chromium(IV). This conversion is rapid when an alpha C-H bond is present (primary and secondary alcohols). Idealized equations for the oxidation of an alcohol are:
In this way, chromate esters arereactive intermediates in theJones oxidation, the mechanistically related oxidations usingpyridinium dichromate andpyridinium chlorochromate. Chromate esters ofallyl alcohols may isomerize via formal[3,3]-sigmatropic shift to give rearrangedenone products.[3]
