Samarium(III) Chloride


[10361-82-7]  · Cl3Sm  · Samarium(III) Chloride  · (MW 256.71)

(promotes electrochemical coupling reactions of aromatic diketones and aromatic esters to yield the 1,2-diol or 1,2-diketone, respectively;1,2 catalyzes aldol condensations and facilitates cyanohydrin formation;3 forms an organosamarium species with stannanes that adds 1,2 to conjugated ketones;5 catalyzes the ring opening of epoxides and chemoselective removal of acetal protecting groups;6,7 promotes acetylation of aldehydes8)

Physical Data: mp 678 °C; d 4.46 g cm-3.

Solubility: sol 6.4 g/100mL pyridine; 99.9 g/100 mL hot water; 92.4 g/100 mL cold water.

Form Supplied in: white-yellowish powder, ampouled under argon.

Handling, Storage, and Precautions: somewhat air and water sensitive. Reactions are typically performed under an inert atmosphere of argon or nitrogen.

Formation of Cyclopropane Diols and 1,2-Diketones.

Samarium(III) chloride can be utilized in the formation of cyclopropane diols via the coupling of carbonyl compounds (eq 1). In this process an active SmII species is generated by the electrochemical reduction of a catalytic amount of SmIII.1 The use of this method, however, is restricted to carbonyls containing aromatic functionality.

Diaryl 1,2-diketones are obtained in moderate to good yields in a similar method by electrochemical coupling of aromatic esters (eq 2).2 This transformation utilizes the in situ preparation and regeneration of the SmII species but is limited to substrates with aromatic functionality on the ester. Halides on the aromatic moiety undergo reduction under these conditions.

Aldol Reactions and Cyanohydrin Formation.

SmCl3 catalyzes the aldol addition of silyl enol ethers to aldehydes wherein the product is isolated as a mixture of the alcohol and TMS ether (eq 3).3

Additionally, samarium(III) chloride facilitates the formation of cyanohydrins from aryl or alkyl aldehydes and Cyanotrimethylsilane (eq 4).3 Epoxides undergo regioselective ring opening when treated with SmCl3 and Me3SiCN to afford the corresponding cyanohydrin (eq 5).3

Ether Formation.

Ethers and thioethers are formed in good yield upon treatment of allylic alcohols with catalytic SmCl3 (eqs 6 and 7).4

1,2-Addition to Conjugated Ketones.

Treatment of a,b-unsaturated ketones with samarium(III) chloride, n-Butyllithium, and Tetraallylstannane results in excellent yields of product resulting from 1,2-addition of the organosamarium reagent generated under the reaction conditions (eq 8).5

Epoxide Ring Opening and Protecting Group Removal.

SmCl3 catalyzes the stereoselective and regioselective ring opening of epoxides (eqs 9 and 10) by thiol nucleophiles to generate the resulting thioalcohol in high yield.6 The samarium(III) chloride protocol has advantages over other Lewis acid catalysts in that shorter reaction times are required.

Samarium(III) chloride is also effective in the removal of protecting groups. Cyclic acetals undergo cleavage faster than acyclic acetals, providing an efficient means for selective removal of protecting group functionality (eqs 11 and 12).7 Additionally, transformation of acetals to aldehydes is slower than that to ketones, providing excellent chemoselectivity in protecting group removal. Acid-sensitive functionalities such as t-butyldimethylsilyl ethers and methoxymethyl ethers are inert to these reaction conditions, as are methyl and benzyl ethers.

Chemoselective Acetalization of Aldehydes.

Samarium(III) chloride possesses the ability to chemoselectively acetalize aldehydes in the presence of the carboxylic acid functionality, i.e. no esterification occurs under these reaction conditions (eq 13).8

1. Léonard, E.; Duñach, E.; Périchon, J. CC 1989, 276.
2. Hébri, H.; Duñach, E.; Heintz, M.; Troupel, M.; Périchon, J. SL 1991, 901.
3. Vougioukas, A. E.; Kagan, H. B. TL 1987, 28, 5513.
4. Ouertani, M.; Collin, J.; Kagan, H. B. T 1985, 41, 3689.
5. Fukuzawa, S.; Sato, K.; Fujinami, T.; Sakai, S. CC 1990, 939.
6. Vougioukas, A. E.; Kagan, H. B. TL 1987, 28, 6065.
7. Ukaji, Y.; Koumoto, N.; Fujisawa, T. CL 1989, 1623.
8. Lawson, K. R.; McDonald, B. P.; Mills, O. S.; Steele, R. W.; Sutherland, J. K.; Wear, T. J.; Brewster, A.; Marsham, P. R. JCS(P1) 1988, 663.

Gary A. Molander & Christina R. Harris

University of Colorado, Boulder, CO, USA

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.