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'''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|10.1002/0471264180.or053.01}}</ref>
==Inventory of Cr(VI)-pyridine reagents==
All of these reagents consist of 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>
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.▼
*Sarrett'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.<ref>Poos, G. I.; Arth, G. E.; Beyler, R. E.; Sarrett, L. H. ''J. Am. Chem. Soc.'', '''1953''', ''75'', 422.</ref
showed that pyridine reacts with CrO<sub>3</sub> in
*[[Collins reagent]] is a solution of CrO<sub>3</sub>(pyridine)<sub>2</sub> 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.
▲
==Practical considerations==
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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