Oxidation with chromium(VI) complexes: Difference between revisions

Revision as of 12:46, 20 June 2011

Revision as of 13:52, 22 December 2011

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{{Orphan|date=December 2011}}

'''Oxidation with chromium(VI)-amine complexes''' involves the conversion of alcohols to carbonyl compounds or more highly oxidized products through the action of chromium(VI) oxide-amine adducts and salts. Representative members of this family of reagents include [[Collins reagent]], [[pyridinium chlorochromate]] (PCC), and [[pyridinium dichromate]] (PDC).<ref>Luzzio, F. A. ''[[Org. React.]]'' '''1998''', ''53'', 1. doi: ([http://dx.doi.org/10.1002/0471264180.or053.01 10.1002/0471264180.or053.01])</ref>

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Sarrett identified the adduct of pyridine and chromium(VI) oxide ([[Collins reagent]]) as a selective compound for the oxidation of primary and secondary alcohols to carbonyl compounds.<ref>Poos, G. I.; Arth, G. E.; Beyler, R. E.; Sarrett, L. H. ''J. Am. Chem. Soc.'', '''1953''', ''75'', 422.</ref> Despite its selectivity, Collins reagent suffers from difficulties associated with its preparation, stability, and efficiency. The less reactive adducts pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC) are more easily handled and more selective than Collins reagent in oxidations of alcohols. These reagents, as well as other, more exotic adducts of nitrogen heterocycles with chromium(VI), facilitate a number of oxidative transformations of organic compounds, including cyclization to form [[tetrahydrofuran]] derivatives and allylic transposition to afford enones from [[allyl]]ic alcohols.

Oxidation with chromium(VI) amines has two primary limitations. Operationally, the tarry byproducts of chromium oxidations cause reduced yields and product sequestration.<ref>Ratcliffe, R.; Rodehorst, R. ''J. Org. Chem.'', '''1970''', ''35'', 4000.</ref> In addition, Cr(VI)-amines (particularly PCC) may react with [[acid]]-labile functionality. Thus, these agents have been employed in oxidations of relatively simple substrates, often in excess to account for reagent trapping and decomposition. The use of adsorbents such as Celite® or silica gel facilitates the removal of chromium byproducts and eliminates many of the operational difficulties associated with chromium-mediated oxidations.

<span style="float:right;padding-right:50px;padding-top:30px;">'''''(1)'''''</span><center>[[File:ChroGen.png]]</center>

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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. The exact mechanism has been debated, although a recent structure-reactivity study provided evidence for 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>

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Oxidative cyclization can be used to prepare substituted tetrahydrofurans. Cyclization of dienols leads to the formation of two tetrahydrofuran rings in a ''syn'' fashion.<ref>McDonald, F. E.; Towne, T. B. ''J. Am. Chem. Soc.'', '''1994''', ''116'', 7921.</ref>

<span style="float:right;padding-right:50px;padding-top:30px;">'''''(7)'''''</span><center>[[File:ChroScope2.png]]</center>

Enones can be synthesized from tertiary allylic alcohols through the action of a variety of chromium(VI)-amine reagents. The reaction is driven by the formation of a more substituted double bond. (''E'')-Enones form in greater amounts than (''Z'') isomers because of chromium-mediated geometric isomerization.<ref name=whatup /><ref>Majetich, G.; Condon, S.; Hull, K.; Ahmad, S. ''Tetrahedron Lett.'', '''1989''', ''30'', 1033.</ref><ref name=whatup></ref>

<span style="float:right;padding-right:50px;padding-top:10px;">'''''(8)'''''</span><center>[[File:ChroScope3.png]]</center>

Suitably substituted olefinic alcohols undergo oxidative cyclization to give tetrahydrofurans. Further oxidation of these compounds to give tetrahydropyranyl carbonyl compounds then occurs.<ref>Schlecht, M. F.; Kim, H.-J. ''Tetrahedron Lett.'', '''1986''', ''27'', 4889.</ref>

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==Experimental conditions and procedure==

===Typical conditions===

Reagent-grade pyridine is usually sufficient for the preparation of PDC and PCC. Although these reagents may darken over time, their loss in activity is minimal.{{Clarify|date=June 2011}} Isolated reagents should be stored in a desiccator in the dark. Care should be taken when adding chromium trioxide to pyridine, as ignition of pyridine has been known to occur. Celite® or silica gel may be used to facilitate the removal of polymeric chromium byproducts from the reaction mixture; these adsorbents also serve as convenient buffer systems and [[desiccant]]s.

Reduced chromium residues can be removed from glassware with concentrated HCl or 10–15% aqueous HF. Solid chromium waste should never be thrown away, as residual CrO₃ may ignite. Chromium(VI) reagents are toxic and should be handled with care in a well-ventilated fume hood.