Oxidation with chromium(VI) complexes: Difference between revisions

Revision as of 13:49, 24 June 2020

Revision as of 14:27, 24 June 2020

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==Mechanism and stereochemistry==

[[Chromate ester]]s are implicated in these reactions. The chromate ester decomposedecomposes to the aldehyde or carbonyl by transfer of a proton alpha to the hydroxy groupproton. Large kinetic isotope effects are observed.<ref>Banerji, K. K. ''J. Org. Chem.'', '''1988''', ''53'', 2154.</ref>

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

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 in equation (3) below.<ref>Corey, E. J.; Boger, D. ''Tetrahedron Lett.'', '''1978''', ''19'', 2461.</ref>

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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.</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).

<span style="float:right;padding-right:50px;padding-top:30px;">'''''(4)'''''</span><center>[[File:ChroMech2.png]]</center>

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. TheInsights exactinto the mechanism hasis beenprovided debated, although a recentby structure-reactivity, study provided evidence forimplicating 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.

<span style="float:right;padding-right:50px;padding-top:30px;">'''''(5)'''''</span><center>[[File:ChroMech3.png]]</center>