Oxidation with chromium(VI) complexes: Difference between revisions

Revision as of 15:07, 17 November 2024

Revision as of 15:09, 17 November 2024

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==Mechanism and stereochemistry==

[[Chromate ester]]s are implicated in these reactions. The chromate ester decomposes to the aldehyde or carbonyl by transfer of an alpha proton. Large [[kinetic isotope effectseffect]]s (k_H/k_D) are observed.<ref name=OR/>

<span style="float:right;padding-right:50px;padding-top:30px;">'''''(2)'''''</span>{{center|[[File:ChroMech1.png]]}}

Oxidative annulation of alkenols to form six-membered rings may be accomplished with PCC. This process is postulated to occur via initial oxidation of the alcohol, attack of the alkene on the new carbonyl, then re-oxidation to a ketone. Double-bond isomerization may occur upon treatment with base as shown below.<ref name=OR/>

{{center|[[File:ChromeScopeCyc.png]]}}

An important process mediated by chromium(VI)-amines is the oxidative transposition of tertiary allylic alcohols to give enones.<ref>Luzzio, F. A.; Moore, W. J. ''J. Org. Chem.'', '''1993''', ''58'', 2966.<name=OR/ref> The mechanism of this process likely depends on the acidity of the chromium reagent. Acidic reagents such as PCC may cause ionization and recombination of the chromate ester (path A), while the basic reagents (Collins) likely undergo direct allylic transposition via sigmatropic rearrangement (path B).

{{center|[[File:ChroMech2.png]]}}

Oxidative cyclizations of olefinic alcohols to cyclic ethers may occur via [3+2], [2+2],<ref>Piccialli, V. ''Synthesis'' '''2007''', 2585.</ref> or [[epoxidation]] mechanisms. Insights into the mechanism is provided by structure-reactivity, implicating direct epoxidation by the chromate ester.<ref>Beihoffer, L.A; Craven, R.A.; Knight, K.S; Cisson, C.R.; Waddell, T.G. ''Trans. Met. Chem.'' '''2005''', ''30'', 582.</ref> Subsequent epoxide opening and release of chromium leads to the observed products.