Lithium Carbonate-Lithium Bromide

Li2CO3-LiBr
(Li2CO3)

[554-13-2]  · CLi2O3  · Lithium Carbonate-Lithium Bromide  · (MW 73.89) (LiBr)

[7550-38-8]  · BrLi  · Lithium Carbonate-Lithium Bromide  · (MW 86.85)

(reagent for dehydrohalogenation of a-halo ketones)

Physical Data: see Lithium Carbonate and Lithium Bromide.

Dehydrohalogenations.

The combination of lithium carbonate and lithium bromide has been found to be a powerful reagent for effecting the dehydrohalogenations of a-halo ketones.1 A typical procedure involves heating of the halo ketone with a 1:1 mixture of lithium carbonate-lithium bromide in refluxing DMF. Frequently this procedure is used for the introduction of a conjugated alkene. This method has been shown in certain cases to be superior to other dehydrohalogenation methods. An interesting example is given by the dehydrobromination of dibromoacetoxyetiocholane (eq 1).1

Excellent yields (96%) of the 1,4-dienone were obtained using the carbonate-bromide pair. However, when Lithium Chloride in DMF was used as the base, a mixture of the 1,4- and 1,6-dienones was obtained. The same investigators have also observed that the use of shorter reaction times allows for the isolation of the monodebrominated compound in lower yield without contamination from the didebrominated product (eq 2).2

This procedure was utilized by Grieco to prepare a key dienophile for a synthesis of quassin (eq 3).3 The octalone is selectively brominated at the more substituted position and then dehydrobrominated to furnish the enone in excellent overall yield. A similar dehalogenation procedure was also attempted using collidine as the base, producing the enone in 40% yield.

This reaction has been employed in the synthesis of tuberiferin.4 Selective bromination of the less substituted a-position, followed by treatment of the crude bromide with lithium carbonate-lithium bromide, produced the enone in good overall yield (eq 4).

This combination of reagents also proved to be valuable for the synthesis of dienones from enone precursors. A two-step sequence of halogenation-dehydrohalogenation was engaged to produce substituted cyclohexadienones (eq 5).5 Bromination of the enone through its enol acetate, followed by base-induced bromide elimination, yields the dienone in 85% isolated yield. When the dehydrobromination was attempted using collidine, the dienone was produced in an attenuated 40% yield.

Paquette has recently applied this methodology towards an intermediate found useful for the synthesis of 18-oxo-3-virgenene (eq 6).6 The overall yield for the sequential bromination-dehydrobromination was 71%.

Certain dienones can be prepared directly from ketones: dibromination of the ketone followed by bis-dehydrohalogenation yields the dienone. This methodology proved valuable in a synthesis of substituted homotropones (eq 7).7

A key dienone for the construction of linear tetraquinane intermediates used in the synthesis of fusiccosin and ophiobolin natural products was prepared by a similar protocol. Bromination of the enone with elemental bromide followed by exposure to the carbonate-bromide combination produced the dienone in acceptable yield (eq 8).8

This dehalogenation procedure has been documented to work effectively on a-chloro ketones as well. The protocol was employed in a synthesis of dihydrojasmone (eq 9).9

Related Reagents.

2,4,6-Collidine; 1,5-Diazabicyclo[4.3.0]non-5-ene; 1,8-Diazabicyclo[5.4.0]undec-7-ene; Lithium Carbonate.


1. Joly, R.; Warnant, J.; Nomine, G.; Bertin, D. BSF 1958, 366.
2. Joly, R.; Warnant, J. BSF 1958, 367.
3. Vidari, G.; Ferrino, S.; Grieco, P. A. JACS 1984, 106, 3539.
4. Ando, M.; Wada, T.; Kusaka, H.; Takase, K.; Hirata, N.; Yanagi, Y. JOC 1987, 52, 4792.
5. Plieninger, H.; Ege, G.; Grasshoff, H. J.; Keilich, G.; Hoffmann, W. CB 1961, 94, 2115.
6. Wang, X.; Paquette, L. A. TL 1993, 34, 4579.
7. Noyori, R.; Hayakawa, Y.; Makino, S.; Takaya, H. CL 1973, 3.
8. Mehta, G.; Krishnamurthy, N. CC 1986, 1319.
9. (a) Ho, T.-L. SC 1981, 11, 7. (b) Ho, T.-L. SC 1974, 4, 265.

Dennis Wright & Mark C. McMills

Ohio University, Athens, OH, USA



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