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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>
*[[Sarett oxidation|Sarett's reagent]]: a solution of CrO<sub>3</sub>(pyridine)<sub>2</sub> in pyridine. It was popularized for selective oxidation of primary and secondary alcohols to carbonyl compounds.
*[[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>).
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==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 name=OR/>
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 name=OR/>
{{center|[[File:ChroScope3.png]]}}
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==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>{{cite
Catalytic methods employing cheap, clean terminal oxidants in conjunction with catalytic amounts of chromium reagents produce only small amounts of metal byproducts.<ref name=OR/> However, undesired side reactions mediated by stoichiometric amounts of the terminal oxidant may occur.
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