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'''Oxidation with chromium(VI)
==Inventory of Cr(VI)-pyridine and pyridinium reagents==
Cr(VI)-pyridine and pyridinium reagents have the advantage that they are soluble in organic solvents as are the alcohol substrates.
One family of reagents employs the complex CrO<sub>3</sub>(pyridine)<sub>2</sub>.<ref>{{cite book |doi=10.1007/0-387-25725-X_1|chapter=Chromium-based Reagents|title=Oxidation of Alcohols to Aldehydes and Ketones|series=Basic Reactions in Organic Synthesis|year=2006|pages=1–95|isbn=0-387-23607-4}}</ref>
*
*[[Collins reagent]] is a solution of the same CrO<sub>3</sub>(pyridine)<sub>2</sub> but in dichloromethane. The Ratcliffe variant of Collins reagent relates to details of the preparation of this solution, i.e., the addition of chromium trioxide to a solution of pyridine in methylene chloride.<ref name=JCC>{{cite journal | author = J. C. Collins, W.W. Hess | title = Aldehydes from Primary Alcohols by Oxidation with Chromium Trioxide: Heptanal | journal = Organic Syntheses | volume = 52 | pages = 5 | doi = 10.15227/orgsyn.052.0005 | year = 1972| doi-access = free }}</ref>
The second family of reagents are ''salts'', featuring the pyridinium cation (C<sub>5</sub>H<sub>5</sub>NH<sup>+</sup>).
*[[pyridinium dichromate]] (PDC) is the pyridium salt of dichromate, [
*[[pyridinium chlorochromate]] (PCC) is the pyridinium salt of [CrO<sub>3</sub>Cl]<sup>
These salts are less reactive, 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 [[Babler oxidation|allylic transposition]] to afford enones from [[allyl]]ic alcohols.
The above reagents represent improvements over the [[Jones reagent]], a solution of [[chromium trioxide]] in aqueous [[sulfuric acid]].
==Mechanism and stereochemistry==
[[Chromate ester]]s are implicated in these reactions. The chromate ester
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
An important process mediated by chromium(VI)-amines is the oxidative transposition of tertiary allylic alcohols to give enones.<ref
Oxidative cyclizations of olefinic alcohols to cyclic ethers may occur via [3+2], [2+2],<ref
==Scope and limitations==
Buffering agents may be used to prevent acid-labile protecting groups from being removed during chromium(VI)-amine oxidations. However, buffers will also slow down oxidative cyclizations, leading to selective oxidation of alcohols over any other sort of oxidative transformation. Citronellol, for instance, which cyclizes to pugellols in the presence of PCC, does not undergo cyclization when buffers are used.<ref>Fieser, L. F.; Fieser, M. ''Reagents for Organic Synthesis''; Wiley-Interscience, New York, 1979, '''7''', 309.</ref><ref name=whatup>
|author=James H. Babler, Michael J. Coghlan|pages= 469–474|year= 2007|doi=10.1080/00397917608082626|journal=Synthetic Communications}}</ref>
{{center|[[File:ChroScope1.png]]}}
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
{{center|[[File:ChroScope2.png]]}}
Enones can be synthesized from tertiary allylic alcohols through the action of a variety of chromium(VI)-amine reagents, in a reaction known as the [[Babler oxidation]]. 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
{{center|[[File:ChroScope3.png]]}}
Suitably substituted olefinic alcohols undergo oxidative cyclization to give tetrahydrofurans. Further oxidation of these compounds to give tetrahydropyranyl carbonyl compounds then occurs.<ref
{{center|[[File:ChroScope4.png]]}}
In addition to the limitations described above, chromium(VI) reagents are often unsuccessful in the oxidation of substrates containing heteroatoms (particularly nitrogen). Coordination of the heteroatoms to chromium (with displacements of the amine ligand originally attached to the metal) leads to deactivation and eventual decomposition of the oxidizing agent.
==Comparison with other methods==
Methods employing dimethyl sulfoxide (the [[Swern oxidation|Swern]] and [[Moffatt oxidation]]s) are superior to chromium(VI)-amines for oxidations of substrates with heteroatom functionality that may coordinate to chromium.<ref>Tidwell, T. ''Org. React.'' '''1990''', ''39'', 297.</ref> [[Dess-Martin periodinane]] (DMP) offers the advantages of operational simplicity, a lack of heavy metal byproducts, and selective oxidation of complex, late-stage synthetic intermediates.<ref>
Catalytic methods employing cheap, clean terminal oxidants in conjunction with catalytic amounts of chromium reagents produce only small amounts of metal byproducts.<ref
==Historic references==
*Poos, G. I.; Arth, G. E.; Beyler, R. E.; Sarrett, L. H. ''J. Am. Chem. Soc.'', '''1953''', ''75'', 422.
*{{cite journal | title = Improved Procedure for Oxidations with the Chromium Trioxide-Pyridine Complex| author = Ronald Ratcliffe and Ronald Rodehorst | journal = [[J. Org. Chem.]] | year = 1970 | volume = 35 | issue = 11 | pages = 4000–4001 | doi = 10.1021/jo00836a108}}
==References==
{{reflist|30em}}
[[Category:Organic oxidation reactions]]
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