Cerium compounds are widely used in organic synthesis. This section covers some of the important processes.
4.1.1 Cerium (Ce)
Cerium powder is air sensitive and is stored in mineral oil or under argon. It is a reagent for the Barbier reaction of alkyl halides with carbonyl compounds. The addition can be performed chemoselectively in the presence of functional groups such as nitriles, nitro compounds and esters (Scheme 1).
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Scheme 1
The reaction conditions are also effective for the Reformatsky reaction of a -haloesters with ketones (Scheme 2).
Scheme 2
The reductive coupling of carbonyl compounds can also be accomplished using the combination of cerium and I 2 at ambient temperature (Scheme 3).
Scheme 3
4.1.2 Cerium(IV) Ammonium Nitrate ((NH4)2 Ce(NO3 )6)
Cerium(IV) ammonium nitrate (CAN) is an orange colour strong oxidizing agent and soluble in water and to smaller extent in polar solvent. Thus, the reaction is often performed in a mixed solvent such as aqueous CH3CN. During the reaction, the consumption of the CAN can be judged by the fading of an orange colour to pale yellow, if the substrate or products are not strongly coloured. Some of the important processes of the CAN oxidations follow.
T.-L. Ho, Encyclopedia of Reagents for Organic Synthesis, John Wiley and Sons, Inc., L. A. Paquette, Ed., New York, 1995, 2 , 1029. |