Erbium(III) Chloride1


[10138-41-7]  · Cl3Er  · Erbium(III) Chloride  · (MW 381.73) (.6H2O)


(mild Lewis acid for selective acetalization;2 used with NaBH4 as a selective reducing agent;3 catalyst for Friedel-Crafts-type reactions;4 forms enolates from a-bromo ketones5 and oxyallyl cations6 from a,a-dibromo ketones)

Physical Data: mp 774 °C; bp 1500 °C; d 4.1 g cm-3.

Solubility: sol H2O, methanol, ethanol; slightly sol 2-propanol, THF.

Form Supplied in: pink crystals, deliquescent. Drying: anhydrous erbium chloride is prepared from the hydrate by reflux in Thionyl Chloride, or heating to 400 °C in the presence of a two-fold excess of ammonium chloride in a vacuum with intense mixing.

Handling, Storage, and Precautions: anhydrous ErCl3 should be used after drying without delay. Erbium exhibits an oral toxicity of LD50 in rats 6.7 g kg-1.

Selective Acetalization.

Erbium trichloride is an efficient catalyst for acetalization.2 In methanol solution, ErCl3 rapidly promotes an equilibrium between aldehydes and their acetals which is shifted to completion by the addition of trimethyl orthoformate as a water scavenger. High yields are obtained after short reaction times at room temperature with most substrates (eqs 1-3).

Acetalization of ketones is more difficult.3 As a rule, conjugated ketones (i.e. aromatic and a,b-unsaturated) do not react (eq 4), but cycloalkanones give excellent yields (eq 5). The reaction is sensitive to steric hindrance, permitting selective reactions with dicarbonyl substrates (eq 6).

Selective Reductions of Carbonyl Compounds.

ErCl3 induces regiospecific reduction of a,b-unsaturated ketones to allylic alcohols in the presence of Sodium Borohydride in a fashion similar to Cerium(III) Chloride. The major use of ErCl3 involves the selective reduction of dicarbonyl compounds which is based on the in situ acetalization of the more reactive carbonyl group by methanol.3,7 The unprotected, less reactive group is thus reduced. The acetal need not necessarily be isolated, and the overall process can thus be run in one pot (eqs 7 and 8).

In aqueous ethanol the hydrate or hemiacetal rather than the acetal is formed, and these derivatives function as transient protecting groups. The deprotection step can thus be avoided, making the overall process shorter.8,9

Friedel-Crafts Alkylation Catalyst.

Benzyl halides react with benzene in the presence of catalytic amounts (0.3 equiv) of a series of lanthanide (including erbium) chlorides.4 Yields of ~60% are obtained after 18 h at 75 °C. In this procedure the catalyst is easily recovered and reused (eq 9). Double alkylation is observed in the reaction of acetyl chloride and benzaldehyde in benzene or toluene (eq 10).10 Chloromethylation has been reported using chloromethyl methyl ether.10

Erbium Enolates from a-Bromo ketones.

a-Bromo ketones, treated with lanthanide chlorides and Sodium Iodide or Tin(II) Chloride, yield the corresponding enolates which undergo cross-aldol reaction upon addition of a carbonyl compound.5 ErCl3-SnCl2 gives a 76% yield of the aldol product (eq 11), lower than that obtained with La or Ce, but the stereoselectivity is higher.

Oxyallyl Cations from a,a-Dibromo Ketones.

With lanthanide chlorides, including erbium, and SnCl2, a,a-dibromo ketones furnish oxyallyl cations, which can be trapped with various dienes.6 The corresponding [3 + 2] cycloadducts are isolated in good yield (eq 12) although comparable or better yields are obtained with the corresponding cerium salt.

1. (a) Molander, G. A. CRV 1992, 92, 29. (b) Kagan, H. B.; Namy, J. L. T 1986, 42, 6573.
2. Luche, J. L.; Gemal, A. L. CC 1978, 976.
3. Gemal, A. L.; Luche, J. L. JOC 1979, 44, 4187.
4. Mine, N.; Fujiwara, Y.; Taniguchi, H. CL 1986, 357.
5. Fukuzawa, S.; Tsuruta, T.; Fujinami, T.; Sakai, S. JCS(P1) 1987, 1473.
6. Fukuzawa, S.; Fukushima, M.; Fujinami, T.; Sakai, S. BCJ 1989, 62, 2348.
7. Brandes, S. J.; Katzenellenbogen, J. A. Mol. Pharmacol. 1987, 32, 381.
8. Gemal, A. L.; Luche, J. L. TL 1981, 22, 4077.
9. Luche, J. L.; Gemal, A. L. JACS 1979, 101, 5848.
10. Davydov, D. V.; Vinogradov, S. A.; Beletskaya, I. P. IZV 1990 708 (CA 1990, 113, 77 763k); IZV 1990, 706 (CA 1990, 113, 58 572f).

Jean-Louis Luche

Université Paul Sabatier, Toulouse, France

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