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 group. 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.

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, 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]