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Chromate ester

Class of chemical compounds From Wikipedia, the free encyclopedia

Chromate ester
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A chromate ester is a chemical species that contains a chromium atom (symbol Cr) in a +6 oxidation state that is bonded to a oxide (O2-) and an alkoxide (OR-). Such compounds are common intermediates in [[Oxidation with chromium(VI) complexes|oxidation of organic compounds by chromates and related compounds.

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Structure of a chromate ester.

Synthesis

Chromate esters, which are mainly of academic interest, are few and often labile. They can be isolated with bulky organic substituents such as tert-butyl or triphenylmethyl. Key is the absence of an alpha C-H bond. Suitable sources of chromium are chromium trioxide (CrO3), chromyl chloride (CrO2Cl2), and similar Cr(VI) reagents. For example, the treatment of chromium trioxide with trityl chloride gives two chromate esters:[1]

CrO3 + Ph3CCl → CrClO2(OCPh3) (Ph = C6H5)
2 CrClO2(OCPh3) → CrO2(OCPh3)2 + CrCl2O2

Diesters

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 converts cyclohexene to 2-cyclohexenone.[2]

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Mechanistic significance

Chromate esters tend to react via redox 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:

formation of the chromate esterr: [CrO4]2− + HOCHR2 → CrO3(OCHR2) + OH
fragmentation of the chromate ester: CrO3(OCHR2) → CrO2 + R2C=O + OH

In this way, chromate esters are reactive intermediates in the Jones oxidation, the mechanistically related oxidations using pyridinium dichromate and pyridinium chlorochromate. Chromate esters of allyl alcohols may isomerize via formal [3,3]-sigmatropic shift to give rearranged enone products.[3]

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References

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