· (MW 140.12)
(reagent for the Barbier and the Reformatsky reactions and pinacol coupling)
Physical Data: mp 804 °C; bp ca. 3470 °C; d 6.77 g cm-3.
Form Supplied in: powder, chip, foil, and ingot are commercially available in 99.9% purity.
Handling, Storage, and Precautions: metallic cerium is sensitive to air and is stored in mineral oil or under argon. It readily burns in air from friction, and the preparation of cerium powder by scraping cerium ingots with a rasp should therefore be carried out in a glove bag or a dry box. Cerium is reputed to be of low toxicity.
Preparation of Organocerium Compounds.
Cerium powder or chips can be used for the Barbier reaction between alkyl halides and carbonyl compounds. Allylic and benzylic halides react most easily under mild conditions.1,2 The reactions are chemoselective and additions to carbonyl compounds containing functional groups such as nitriles, nitro compounds, esters, and vinylic iodides are possible (eq 1).
a-Halo esters undergo cerium-promoted Reformatsky reactions with carbonyl compounds. The reaction occurs even at -40 °C, and chemoselectivity is observed under these conditions (eq 2).1 b-Halo esters also react with ketones in the presence of cerium to give g-lactones together with pinacol coupling products (eq 3).3
Cerium in combination with Iodine is used for reductive coupling of carbonyl compounds (eq 4).4 Aromatic aldehydes and ketones (except benzophenone) are most easily dimerized to pinacols. On the other hand, the reactions of cyclic ketones depend on the ring size: 4-7 membered ketones undergo smooth dimerization, while cyclododecanone is subjected to reduction to give the corresponding secondary alcohol in 70% yield. Ester, nitrile, and vinyl halide moieties are not affected under these conditions, and over-reduction of the carbonyl substrate to generate alkenes is not observed with the cerium reagent. It is not clear whether this system has any advantages compared to the more widely used low-valent titanium reagents.
- 1. Imamoto, T.; Kusumoto, T.; Tawarayama, Y.; Sugiura, Y.; Mita, T.; Hatanaka, Y.; Yokoyama, M. JOC 1984, 49, 3904.
- 2. Fukuzawa, S.; Fujinami, T.; Sakai, S. JOM 1986, 299, 179.
- 3. Fukuzawa, S.; Sumimoto, N.; Fujinami, T.; Sakai, S. JOC 1990, 55, 1628.
- 4. Imamoto, T.; Kusumoto, T.; Hatanaka, Y.; Yokoyama, M. TL 1982, 23, 1353.
Chiba University, Japan
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