Chromate ester

Short description: Class of chemical compounds

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.

## Contents

* 1 Synthesis
* 1.1 Diesters
* 2 Mechanistic significance
* 3 References

## 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]

CrO
3 \+ Ph
3CCl → CrClO
2(OCPh
3) (Ph = C6H5)
2 CrClO
2(OCPh
3) → CrO
2(OCPh

2 \+ CrCl

### Diesters

Usually chromate ester refers to monoesters, which are transient intermediates in the oxidation of alcohols by chromium(VI) oxides. Dialkychromates have the formulaCrO
2(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: [CrO
4]2− \+ HOCHR
2 → CrO
3(OCHR
2)−
\+ OH−

fragmentation of the chromate ester: CrO
3(OCHR
2)−
→ CrO
2 \+ R
2C=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. https://archive.org/details/modernmethodsorg00carr.

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