{{short description|Class of chemical compounds}} thumb|Structure of a chromate ester.|130px 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<sup>2-</sup>) and an alkoxide (OR-). Such compounds are common intermediates in oxidation of organic compounds by chromates and related compounds.
==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 (CrO<sub>3</sub>), chromyl chloride (CrO<sub>2</sub>Cl<sub>2</sub>), and similar Cr(VI) reagents. For example, the treatment of chromium trioxide with trityl chloride gives two chromate esters:<ref>{{cite journal |doi=10.1021/ic00335a009 |title=Chromyl Complexes with aryloxy and siloxy ligands |date=1990 |last1=Stavropoulos |first1=Pericles |last2=Bryson |first2=Nathan |last3=Youinou |first3=Marie Therese |last4=Osborn |first4=John A. |journal=Inorganic Chemistry |volume=29 |issue=10 |pages=1807–1811 }}</ref> :{{chem2|CrO3 + Ph3CCl -> CrClO2(OCPh3)}} (Ph = C<sub>6</sub>H<sub>5</sub>) :{{chem2|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 formula {{chem2|CrO2(OR)2}}. When R lacks an alpha hydrogen, these diesters are isolable. One chromate diester has attracted significant attention: ((CH<sub>3</sub>)<sub>3</sub>CO)<sub>2</sub>CrO<sub>2</sub>. It is not an intermediate, rather a reagent. For example, it converts cyclohexene to 2-cyclohexenone.<ref>Fillmore Freeman, "Di-tert-butyl Chromate" Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons, Ltd. {{doi|10.1002/047084289X.rd059m}}</ref>
==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: {{chem2|[CrO4](2-) + HOCHR2 -> CrO3(OCHR2)- + OH-}} :fragmentation of the chromate ester: {{chem2|CrO3(OCHR2)- -> CrO2 + R2C\dO + 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 reaction|[3,3]-sigmatropic shift]] to give rearranged enone products.<ref>{{cite book |title= Modern Methods of Organic Synthesis |url= https://archive.org/details/modernmethodsorg00carr |url-access= limited |edition= 4th |author= Carruthers, W. |author2= Coldham, I. |year= 2004 |isbn= 978-0-521-77097-2 |page= [https://archive.org/details/modernmethodsorg00carr/page/n394 381] }}</ref>
== References == {{reflist}}
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Category:Chromates Category:Esters