Tris(cyclopentadienyl)lanthanum-Sodium Hydride1


[1272-23-7]  · C15H15La  · Tris(cyclopentadienyl)lanthanum-Sodium Hydride  · (MW 334.21) (NaH)

[7646-69-7]  · HNa  · Tris(cyclopentadienyl)lanthanum-Sodium Hydride  · (MW 24.00)

(reducing agent for alkenes, organohalogen compounds, and organoheteroatom oxides; catalyst for hydrogenation and isomerization of alkenes)

Physical Data: Cp3La: mp 395 °C (slight dec.). NaH: mp 800 °C (dec.).

Solubility: Cp3La: sol THF; reacts with H2O and protic solvents. NaH: slightly sol THF.

Form Supplied in: Cp3La: white crystalline solid. NaH used as 80% dispersion in mineral oil.

Preparative Methods: because Cp3La is sensitive to air and moisture, the preparative procedure and all subsequent operations on the compound must be carried out with Schlenk techniques under purified argon. The following procedure has proven efficacious: to a suspension of anhydrous Lanthanum(III) Chloride in THF was added 4-5 equiv of sodium cyclopentadienide in THF; the mixture was refluxed with stirring for 4 h; the solvent was removed under reduced pressure, and the dry residue was transferred to a sublimation apparatus; on heating at 250-260 °C/10-3-10-4 mmHg, Cp3La sublimed as a white crystalline solid. Alternatively, the residue can be recrystallized from THF to afford Cp3La(THF) as colorless crystals.

Handling, Storage, and Precautions: tris(cyclopentadienyl)lanthanum and Sodium Hydride must be used as prepared or purified, and sealed in glass ampules with argon or nitrogen for storage.

Reduction of Alkenes.

Tris(cyclopentadienyl)lanthanum and its analogs, combined with Sodium Hydride, are effective reducing agents for C=C bonds,2,3 aryl halides, vinyl halides4,5 and certain organoheteroatom oxides.6 Reduction of 1-hexene with Cp3Ln/NaH (Ln = rare earth metal) in THF at 45 °C gives hexane (after hydrolysis). The reducing activity depends strongly upon the ionic radius of the trivalent ion. Generally, the activity decreases in the order: early rare earths > mid rare earths > heavy rare earths. The reduction rate of internal C=C bonds is much slower than that of terminal ones. As a consequence, the Cp3Ln/NaH system can be used to reduce terminal C=C bonds regioselectively (eq 1).

Reduction of Organohalogen Compounds.

Aryl and vinyl halides are stoichiometrically or catalytically reduced by Cp3La/NaH systems and analogs in THF to afford the corresponding aromatics and alkenes, respectively, in excellent yields (eq 2).4,5 The reductive activity of the light rare earth reagents is higher than that of heavy ones. The order of ease of reductive dehalogenation of aryl halides by the Cp3La/NaH system is in agreement with that of the usual reductive dehalogenation of organic halides, i.e. I > Br > Cl > F. As a result, chemoselective reduction of p-bromochlorobenzene and p-chloroiodobenzene to chlorobenzene can be achieved. However, surprisingly, the reactivity pattern of alkyl halides with the Cp3La/NaH system and analogs is quite different from that of aryl and vinyl halides. Treatment of alkyl halides with Cp3La/NaH followed by hydrolysis does not generate the corresponding alkanes, but rather produces alkylated cyclopentadienes (eq 3).5

Other Reductions.

Some organoheteroatom oxides such as triphenylarsine oxide, diphenyl sulfoxide and pyridine N-oxide are smoothly deoxygenated by the Cp3La/NaH system and analogs in THF in good yields. The suitable molar ratio of reactants is 2:8:1 (Cp3La/NaH/oxide). Although triphenylphosphine oxide is essentially inert to Cp3Sm/NaH, the combination of Cp3Sm and Lithium Aluminum Hydride facilitates its reduction to phosphine.6

Isomerization and Hydrogenation of Alkenes.

The Cp3La/NaH system and analogs can catalyze isomerization of 1-alkenes to cis- and trans-2-alkenes in almost quantitative yields (eq 4).2,7 The catalytic activity seems to be related to the ionic radius of the rare earth metal. In contrast to their reducing activity, the isomerization activity of late rare earth reagents is better than that of early ones. It is worth noting that these systems can also catalyze the isomerization of sulfur-containing alkenes such as PhSCH2CH=CH2,8 while most transition metal catalysts would be deactivated by sulfur compounds. These systems are also effective for the catalytic hydrogenation of alkenes. 1-Hexene was completely converted into hexane under 4 atm pressure of hydrogen in the course of 2 days.2

1. Molander, G. A. CRV 1992, 92, 29.
2. Qian, C.; Ge, Y.; Deng, D.; Gu, Y.; Zhang, C. JOM 1988, 344, 175.
3. Qian, C.; Deng, D.; Ye, C.; Xie, Z.; Ge, Y.; Li, Y.; Gu, Y.; ICA 1987, 140, 21.
4. Qian, C.; Zhu, D.; Gu, Y. J. Mol. Catal. 1990, 63, L1.
5. Qian, C.; Zhu, D.; Gu, Y. JOM 1991, 401, 23.
6. Qian, C.; Zhu, D. SL 1990, 417.
7. Qian, C.; Zhu, D.; Li, D. JOM 1992, 430, 175.
8. Qian, C.; Zhu, D. Unpublished results.

Changtao Qian & Dunming Zhu

Shanghai Institute of Organic Chemistry, China

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