Lithium Aluminum Hydride-Cobalt(II) Chloride


[16853-85-3]  · AlH4Li  · Lithium Aluminum Hydride-Cobalt(II) Chloride  · (MW 37.96) (CoCl2)

[7646-79-9]  · Cl2Co  · Lithium Aluminum Hydride-Cobalt(II) Chloride  · (MW 129.83)

(reducing agent for alkenes,1,2 alkynes,1,2 halides,1,3,4 and tosylates;3 catalyst for the reduction of arenes5)

Physical Data: see Lithium Aluminum Hydride and Cobalt(II) Chloride

Solubility: see Lithium Aluminum Hydride and Cobalt(II) Chloride; reactions using this reagent system typically utilize THF as solvent.

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

Preparative Method: the organic substrate (1 equiv) in THF is transferred via syringe into a cold (-40 °C) flask containing dry cobalt(II) chloride (1 equiv for stoichiometric use, 0.1 equiv for catalytic use). To this mixture is slowly added the calculated amount of lithium aluminum hydride (1 equiv) in THF solution, which is accompanied by a color change to black, concomitant with the evolution of gas.1

Handling, Storage, and Precautions: see Lithium Aluminum Hydride and Cobalt(II) Chloride.

Reduction of Alkenes and Alkynes.

Reduction of alkenes by lithium aluminum hydride alone has historically proven to be an inefficient manipulation. Addition of CoCl2 to the system dramatically overcomes most obstacles to this process. 1-Octene is reduced by LiAlH4-CoCl2 when using the transition metal chloride in either stoichiometric or catalytic amounts (eq 1).1 Yields in excess of 90% are not atypical for the reduction of mono-, di-, and trisubstituted carbon-carbon double bonds. Eq 2 is illustrative of the efficiency of this reagent.2 Benzene is reported to have been fully reduced in 76% yield in the presence of hydrogen.5

Alkynes may be reduced to alkenes in similar fashion. Terminal alkynes are reduced to the alkene oxidation state without event (eq 3).1 However, reduction of aryl-substituted or internal alkynes is typically a low yield process.1,2 Reduction of these p-systems is postulated to proceed through a metal-hydride reducing agent.2

Reduction of Halides and Tosylates.

The title reagent system is perhaps of most use in the reduction of halides and tosylates. Alkyl halides are reduced in high yield irrespective of the halogen (eq 4).1,3 Furthermore, n-octyl tosylate was reduced in quantitative yield using the catalytic variation of this system. Secondary and tertiary bromides are reduced easily when stoichiometric quantities of the transition metal chloride are used (eq 5).1,3

Replacement of bromine by hydrogen in triarylvinyl bromides, with concomitant loss of stereochemical integrity where applicable, has been achieved in high yield.4 Similarly, phenyl bromide and iodide are reduced to benzene in 74% and 98% yield, respectively, when stoichiometric amounts of CoCl2 are utilized.3

The use of Nickel(II) Chloride as the transition metal chloride provides a reagent that behaves similarly to the cobalt(II) chloride system, often giving higher yields.3

1. Ashby, E. C.; Lin, J. J. TL 1977, 18, 4481.
2. Ashby, E. C.; Lin, J. J. JOC 1978, 43, 2567.
3. Ashby, E. C.; Lin, J. J. JOC 1978, 43, 1263.
4. Obafemi, C. A.; Lee, C. C. CJC 1990, 68, 1998.
5. Murugesan, N.; Sarkar, S. IJC(A) 1976, 14A, 107.

Jeffrey N. Johnston

The Ohio State University, Columbus, OH, USA

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