Chromate ester

In this article, we will explore in depth the topic of Chromate ester and its impact on our contemporary society. From its origins to its relevance today, we will analyze how Chromate ester has shaped our lives and influenced various aspects of our daily lives. Through a multidisciplinary approach, we will examine different perspectives and relevant studies that will allow us to comprehensively understand the importance of Chromate ester in our modern world. Likewise, we will examine possible future scenarios and their potential evolution, with the purpose of offering a complete and updated vision of this topic of global relevance.

Structure of a chromate ester.

A chromate ester is a chemical structure that contains a chromium atom (symbol Cr) in a +6 oxidation state that is connected via an oxygen (O) linkage to alkyl or aryl grousp. The Cr itself is in its chromate form, with several oxygens attached, and the Cr–O–C attachment makes this chemical group structurally similar to other ester functional groups.

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, 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 -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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