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 (O^2-) and an alkoxide (OR-). Such compounds are common intermediates in [[Oxidation with chromium(VI) complexes|oxidation of organic compounds by chromates and related compounds.

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]

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]