Lithium Aluminum Hydride-Cerium(III) Chloride


[16853-85-3]  · AlH4Li  · Lithium Aluminum Hydride-Cerium(III) Chloride  · (MW 37.96) (CeCl3)

[7790-86-5]  · CeCl3  · Lithium Aluminum Hydride-Cerium(III) Chloride  · (MW 246.47) (CeCl3.7H2O)

[18618-55-8]  · CeCl3H14O7  · Lithium Aluminum Hydride-Cerium(III) Chloride  · (MW 372.61)

(reducing agent for alkyl halides and phosphine oxides;1 in combination with NaBH4 is a system used to prepare phosphine boranes;2 catalyst for the isomerization and dimerization of alkenes3,4)

Physical Data: see Lithium Aluminum Hydride and Cerium(III) Chloride.

Solubility: see Lithium Aluminum Hydride and Cerium(III) Chloride; reactions using this reagent system typically use THF or DME as solvent.

Form Supplied in: formed in situ from widely available reagents.

Preparative Method: the organic substrate (1 equiv) and lithium aluminum hydride (4.5 equiv) are added to a suspension of dried cerium(III) chloride (1.5 equiv) in THF.1

Handling, Storage, and Precautions: see Lithium Aluminum Hydride and Cerium(III) Chloride.

Reduction of Fluorides and Chlorides.

Alkyl fluorides and chlorides are reduced to their alkane counterparts upon treatment with lithium aluminum hydride-cerium(III) chloride. Despite the relatively inert nature of alkyl (eq 1) and aryl fluorides, they are reduced as readily as chlorides (eqs 2 and 3).1 The hard Lewis acid character of the cerium cation is believed to perturb the halogen to an extent such that reduction is easily effected by lithium aluminum hydride. Furthermore, a radical mediated process has been postulated in which hydrogen abstraction from the solvent (THF or DME) is operational.1

Reduction of Phosphine Oxides.

Worthy of increased attention is the use of this reagent system in the reduction of phosphine oxides (eq 4).1 The results from these reductions are general, and sterically hindered phosphine oxides are no exception (eq 5).1 Unfortunately, racemization is observed in cases where the phosphine oxide is optically active.

Formation of Phosphine Boranes.

The scope of the lithium aluminum hydride-cerium(III) chloride reduction of phosphine oxides is broadened when the reagents are used in conjunction with Sodium Borohydride. Tertiary and secondary phosphine oxides are transformed into their respective phosphine boranes with great facility (eqs 6 and 7).2 Further manipulations of these phosphine boranes provide access to a variety of useful phosphine and tethered phosphine derivatives. The borane protecting/activating group is easily removed with retention of configuration when assisted by Triethylamine. The utility of this reagent has been demonstrated through the synthesis of optically pure ligands for use in catalytic asymmetric hydrogenation such as (S,S)-1,2-ethanediylbis[(o-methoxyphenyl)phenylphosphine].2

Isomerization and Dimerization of Alkenes.

The title reagent system is also known to promote the isomerization of alkenes. Bicyclo[3.2.1]oct-2-ene was synthesized in a single maneuver from 4-vinylcyclohexene in 48% yield.3 Similar activity of the reagent system was realized in the dimerization of 2-alkenes.4 It should be noted, however, that the use of the reagent in this manner is of limited synthetic value due to the harsh conditions necessary to effect the transformations.

1. Imamoto, T.; Takeyama, T.; Kusumoto, T. CL 1985, 1491.
2. Imamoto, T.; Kusumoto, T.; Suzuki, N.; Sato, K. JACS 1985, 107, 5301.
3. Stapp, P. R. JOC 1966, 31, 4258.
4. Stapp, P. R. U.S. Patent 3 414 633, 1968 (CA 1969 70, 46 798q).

Jeffrey N. Johnston

The Ohio State University, Columbus, OH, USA

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