Dibromomethane-Zinc/Copper Couple1

CH2Br2-Zn/Cu
(CH2Br2)

[74-95-3]  · CH2Br2  · Dibromomethane-Zinc/Copper Couple  · (MW 173.83) (Zn)

[7440-66-6]  · Zn  · Dibromomethane-Zinc/Copper Couple  · (MW 65.39)

(reagent combination for cyclopropanations)

Alternate Name: bromomethylzinc bromide.

Physical Data: CH2Br2: mp -52 °C; bp 96-98 °C; d 2.477 g cm-3. Zn: mp 419.5 °C; d 7.140 g cm-3.

Solubility: sol Et2O, THF.

Preparative Methods: the original method for the preparation of a Zinc/Copper Couple that will react with Dibromomethane calls for the formal preparation of the Zn/Cu couple from 30-mesh granular Zinc instead of zinc dust.2 In this procedure, 35 g of granular zinc (30 mesh) is added to a hot solution of 50 mL of acetic acid containing 0.5 g of CuOAc.H2O (see Copper(I) Acetate). The mixture is kept hot and shaken for 1-3 min. The acetic acid is carefully decanted from the couple. The couple is then washed with 50 mL of acetic acid and three 50 mL portions of Et2O. The couple is shaken for 1 min with each washing. The Et2O moistened couple is ready for use or can be dried with a stream of nitrogen. More recently a number of different methods for activation of the Zn/Cu couple have appeared. These newer modifications include the use of ultrasound3 or catalytic amounts of Titanium(IV) Chloride4 or Acetyl Chloride.5 They require no prior preparation of the couple. The couple is prepared in situ by reaction of zinc dust and Copper(I) Chloride.6 The method of choice appears to be the acetyl chloride method.

Handling, Storage, and Precautions: CH2Br2 is toxic. Zn is a moisture-sensitive, flammable solid. When the reactions are complete, the reaction mixture is filtered from the zinc residue. It is recommended that the residue be wetted with water prior to disposal.

Cyclopropanations.

The use of CH2Br2 in place of the more commonly employed Diiodomethane in the cyclopropanation of an alkene1 has the advantages that it is less expensive and more stable. It has not been used as extensively because of the lower reactivity of the dibromide with the various forms of the Zn/Cu couple. The first reported form of a Zn/Cu couple that would react effectively with the dibromide was made from granular zinc.2 It suffered from long reaction times and lower yields in comparison with the diiodomethane derived reagent. It was found that sonication could be used to effect cyclopropanations of alkenes with dibromomethane3 and an in situ generated Zn/Cu couple prepared from Zn dust and CuCl.6 The ultrasound method, however, suffered from irreproducibility. A more convenient method for activation involves the use of a catalytic amount (2 mol %) of TiCl4. The use of more than 2% TiCl4 results in unmanageable reactions. Activation of the couple with TiCl4 is also effective when using diiodomethane. In either the dibromo- or diiodomethane reactions there is a competing formation of ethylene. This problem is greater for dibromomethane than for diiodomethane. The only drawback in the titanium mediated procedure is that it is not always tolerant of Lewis acid sensitive groups.4 Acetyl chloride has been found to activate the Zn/Cu couple as well as TiCl4 and has the advantage that Lewis acid sensitive groups are tolerated. This now appears to be the method of choice.5 Table 1 shows the results3-11 of some representative alkenes under some of the different cyclopropanation conditions. When the difference in yield between the diiodo and dibromo conditions are minimal and the alkene is not the limiting reagent, the dibromomethane conditions offer an inexpensive alternative. The somewhat lower yields obtained are offset by the cost savings.

The regioselectivity of the reagent in its reactions with limonene (eq 1) and 4-vinylcyclohexene (eq 2) has been studied. Using a ratio of diene:CH2Br2:Zn of 1:1:1.5, the major product of the reaction was recovered diene (47%). The monocyclopropanated products were isolated in ~28% yield with a 2.5:1.0 ratio of exocyclic cyclopropane:ring cyclopropane. There was little stereoselectivity observed in ring cyclopropanated products, with a 1:1.1 ratio of cis and trans isomers being produced. A small (4%) amount of biscyclopropanation was obtained under these conditions. If the diene:CH2Br2:Zn ratio was changed to 1:2:3, the amount of recovered diene decreases but the amount of biscyclopropane increased. For 4-vinylcyclohexene with the 1:1:1.5 ratio, the ring cyclopropane was favored over the exocyclic cyclopropane by 1.75:1. The combined yield of these two products is again low (~7%). The major product was recovered diene (53%).12

Miscellaneous.

Reaction of Triethylsilane with dibromomethane and Zn/Cu couple results in an insertion of methylene into the Si-H bond. The reaction with triethylsilane (eq 3) produces triethylmethylsilane. The reaction with diiodomethane produces the insertion product in 64% yield.13

The 1H and 13C NMR data for bromomethylzinc bromide and bis(bromomethyl)zinc have been reported.14


1. Simmons, H. E.; Cairns, T. L.; Vladuchik, S. A.; Hoines, C. M. OR 1973, 20, 1.
2. LeGoff, E. JOC 1964, 29, 2048.
3. Friedrich, E. C.; Domek, J. M.; Pong, R. Y. JOC 1985, 50, 4640.
4. Friedrich, E. C.; Lunetta, S. E.; Lewis, E. J. JOC 1989, 54, 2388.
5. Friedrich, E. C.; Lewis, E. J. JOC 1990, 55, 2491.
6. Rawson, R. J.; Harrison, I. T. JOC 1970, 35, 2057.
7. Simmons, H. E.; Smith, R. D. JACS 1959, 81, 4256.
8. Bergman, R. G. JACS 1969, 91, 7405.
9. Murai, S.; Aya, T.; Renge, T.; Rhu, I.; Sonoda, N. JOC 1974, 39, 858.
10. Blanchard, E. P.; Simmons, H. E. JOC 1965, 30, 4321.
11. Friedrich, E. C.; Biresaw, G. JOC 1982, 47, 1615.
12. Friedrich, E. C.; Niyati-Shirkhodaee, F. JOC 1991, 56, 2202.
13. Seyferth, D.; Dertouzos, H.; Todd, L. J. JOM 1965, 4, 18.
14. Fabisch, B.; Mitchell, T. N. JOM 1984, 269, 219.

Michael J. Taschner

The University of Akron, OH, USA



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