Chromate ester

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Structure of a chromate ester. CrO2Cl(OCPh3).svg
Structure of a chromate ester.

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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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]

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]

References

  1. ↑ Stavropoulos, Pericles; Bryson, Nathan; Youinou, Marie Therese; Osborn, John A. (1990). "Chromyl Complexes with aryloxy and siloxy ligands". Inorganic Chemistry. 29 (10): 1807–1811. doi:10.1021/ic00335a009.
  2. ↑ Fillmore Freeman, "Di-tert-butyl Chromate" Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons, Ltd. doi : 10.1002/047084289X.rd059m
  3. ↑ Carruthers, W.; Coldham, I. (2004). Modern Methods of Organic Synthesis (4th ed.). p.  381. ISBN   978-0-521-77097-2.
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