Lithium Aluminum Hydride-Boron Trifluoride Etherate1


[16853-85-3]  · AlH4Li  · Lithium Aluminum Hydride-Boron Trifluoride Etherate  · (MW 37.96) (BF3.Et2O)

[109-63-7]  · C4H10BF3O  · Lithium Aluminum Hydride-Boron Trifluoride Etherate  · (MW 141.95)

(reducing agent for esters,2 thiol esters,3 acetals, and ketals;4 also a reagent for hydroboration of alkenes5)

Physical Data: see Lithium Aluminum Hydride and Boron Trifluoride Etherate

Solubility: most reactions are performed in ether or THF.

Preparative Methods: for reductions, a solution of the carbonyl compound and BF3.Et2O in ether is added to a cooled suspension of LiAlH4 in ether.2 For hydroborations, BF3.Et2O is added to a solution of the alkene in ether or THF followed by the addition of LiAlH4 in ether at 0 °C.5

Handling, Storage, and Precautions: see Lithium Aluminum Hydride and Boron Trifluoride Etherate.

Reduction of Esters.

The direct reduction of esters and lactones to ethers can be accomplished by employing a reagent prepared from lithium aluminum hydride and boron trifluoride etherate. When 30 mol of BF3.Et2O and 2 mol of LiAlH4 per mol of ester is used, a g-lactone is converted to the tetrahydrofuran in good yield (eq 1).6

Also, the yield of ether appears to increase as the alcohol portion of the ester goes from primary to tertiary, as seen in the three isomeric butyl esters (1a), (1b), and (1c) (eq 2).7 This reagent has also been used to generate crown ethers from crown lactones (eq 3).8

Reduction of Thiol Esters.

The combination of LiAlH4 and a large excess of BF3.Et2O will reduce thiol esters to sulfides. This reaction is similar to the reduction of esters to ethers. Thus cyclohexyl thiolacetate, when treated with LiAlH4-BF3.Et2O, gives the corresponding sulfide (eq 4).3

Reduction of Acetals.

Acetals are inert to LiAlH4, and are normally used as protecting groups for ketones and aldehydes. However, when the reagent prepared from LiAlH4-BF3.Et2O is employed, they are reduced to ethers. Lithium aluminum hydride and Aluminum Chloride also reduces acetals, but LiAlH4-BF3.Et2O is preferred because the yields are slightly better and BF3.Et2O is less troublesome during the workup. When treated with this reagent, acetal (3) gives the corresponding diether (eq 5).4

Hydroboration of Alkenes.

Diborane can be generated in situ by adding LiAlH4 to BF3.Et2O, which upon oxidative workup hydrates alkenes to alcohols. For example, when alkene (4) reacts with this reagent, it gives the corresponding alcohol (eq 6).9 Yet, when treated with diborane generated from Sodium Borohydride-BF3.Et2O, the alcohol is formed in a 55% yield and 40% of starting alkene is recovered. This method cannot be used in the presence of acetals and esters.

1. (a) Morand, P.: Lyall, J. CRV 1968, 85. (b) Bradshaw, J. S.; Jones, B. A. CRV 1984, 17.
2. Pettit, G. R.; Kasturi, T. R. JOC 1960, 25, 875.
3. Eliel, E. L.; Daignault, R. A. JOC 1964, 29, 1630.
4. Abdun-Nur, A-R.; Issidorides, C. H. JOC 1962, 27, 67.
5. Sondheimer, F.; Wolfe, S. CJC 1959, 37, 1870.
6. Pettit, G. R.; Ghatak, U. R.; Green, B.; Kasturi, T. K.; Piatak, D. M. JOC 1961, 26, 1685.
7. Pettit, G. R.; Piatak, D. M. JOC 1962, 27, 2127.
8. Ager, D. J.; Sutherland, I. O. CC 1982, 248.
9. Nussim, M.; Mazur, Y.; Sondheimer, F. JOC 1964, 29, 1120.

Victoria A. Moeller

The Ohio State University, Columbus, OH, USA

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