Content deleted Content added
No edit summary |
|||
Line 1:
'''Chromium(VI)-amine oxidation reactions''' involve 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.</ref>.
==Introduction==
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. Collins reagent suffers from difficulties associated with its preparation, stability, and efficiency, however. The milder 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 give enones from
There are two primary limitations of oxidation by chromium(VI) amines. Operationally, the tarry byproducts of chromium oxidations cause reduced yields and reagent sequestration. In addition, Cr(VI)-amines (particularly PCC) may react with [[acid]]-labile functionality. Thus, they have been employed in oxidations of relatively simple substrates, often in excess to account for reagent trapping and decomposition.
<center>[[File:ChroGen.png]]</center>
Line 15:
An important process mediated by chromium(VI)-amines is oxidative transposition of tertiary allylic alcohols to give enones. 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, while the basic reagents (Collins) likely undergo direct allylic transposition via sigmatropic rearrangement.
<center>[[File:ChroMech2.png]]</center>
Oxidative cyclizations of olefinic alcohols to cyclic ethers may occur via [3+2], [2+2], or [[epoxidation]] mechanisms. The exact mechanism has been debated, although there is evidence for a direct epoxidation by the chromate ester. Subsequent epoxide opening and release of chromium(IV) leads to the observed products.
<center>[[File:ChroMech3.png]]</center>
==Scope and Limitations==
Buffering agents may be used to protect acid-labile protecting groups from removal 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.
<center>[[File:ChroScope1.png]]</center>
Oxidative cyclization can be used to prepared substituted tetrahydrofurans. Cyclization of dienols leads to the formation of two tetrahydrofuran rings in a ''syn'' fashion.
| |||