Methallylmagnesium Chloride1

(X = Cl)

[5674-01-1]  · C4H7ClMg  · Methallylmagnesium Chloride  · (MW 114.87) (X = Br)

[33324-92-4]  · C4H7BrMg  · Methallylmagnesium Bromide  · (MW 159.32)

(structural unit in organic synthesis; reagent for aromatic annulation and for propenyl ketone synthesis; acetone enolate equivalent)

Alternate Name: 2-methyl-2-propenylmagnesium chloride.

Solubility: sol ether, THF.

Analysis of Reagent Purity: contains 2,5-dimethyl-1,5-hexadiene, the symmetrical dimer. The amount of dimer present depends upon the method of preparation.

Handling, Storage, and Precautions: air- and moisture-sensitive and is best stored at -20 °C under argon.

Grignard Synthesis.

The preparation of allylic and benzylic organomagnesium halides often takes place irreproducibly or in poor yield due to competing dimerization of the reactive halide. The most common solution to the general problem is to use excess magnesium.1a Alternatively, a more reactive form of the metal can be utilized. Magnesium turnings can be activated by treatment with Iodine or 1,2-Dibromoethane.1b A highly reactive form of magnesium can be prepared by reducing anhydrous magnesium chloride with Lithium in THF, using naphthalene as an electron carrier.2 The metallic powder which results (Rieke magnesium) can be converted to Grignard reagents in high yield by addition of an alkyl halide. Metallic magnesium can also be evaporated and co-condensed with THF at low temperature in a metal-atom reactor.3 This leads to the formation of a reactive magnesium slurry which is also very effective for the Grignard synthesis.4 When magnesium powder is irradiated in an ultrasonic bath in the presence of anthracene in THF, a magnesium complex of anthracene is formed which is in equilibrium with free anthracene and a reactive form of magnesium metal. Slow addition of halides at -65 °C led to the Grignard reagents.5 A disadvantage of this method is the requirement that the reaction product(s) be separated from anthracene.

Preactivation of magnesium turnings by mechanical stirring in the absence of solvent for 2 d under dry nitrogen in a Schlenk tube constitutes a simple method for the clean synthesis of diverse allylic and benzylic Grignard reagents.6 Fragmentation of the metal surface to form microcrystalline particles effectively increases the surface area of the metal. Yields of (1)-(5) were high in all cases. The simplicity of this method, the absence of alkali metal salts or anthracene in the final product, and especially the high yields recommend its use.

Propenyl Ketone Synthesis.7

Addition of excess methallylmagnesium chloride to an ester leads to the tertiary alcohol (eq 1). Exposure of the alcohol to Potassium Hydride in HMPA at 40 °C results in cleavage of one allylic C-C bond with loss of methallyl anion, and irreversible proton transfer to form the ketone enolate. Workup with aq NH4Cl produces the unsaturated ketone as a mixture of a,b- and b,g-isomers. This is an unconventional but effective method for converting esters to ketones.

Aromatic Annulation.

Nucleophilic addition of methallylmagnesium chloride to the a-oxoketene dithioacetal derived from menthone (eq 2) forms a tertiary allylic alcohol.8 Exposure of this material to Boron Trifluoride Etherate in nitromethane initiates a series of reactions which results in the annulation of an aromatic ring. Similar chemistry has been reported for vinylogous silyl esters.9 An advantage of this approach to aromatic ring synthesis is the control over the alkyl substitution pattern. A methyl, or other cation-stabilizing group, in the Grignard reagent is essential for high yields of annulated products.10

Acetone Enolate Equivalent.

Methallylmagnesium chloride serves as a convenient masked form of acetone enolate. Oxidative cleavage of the methylene group unmasks the carbonyl group.11

Related Reagents.

Allylic barium halides offer certain advantages over the corresponding Grignard reagents. For example (eq 3), reduction of BaI2 with lithium biphenylide in THF, followed by addition at -78 °C of a slight deficiency of 1-chloro-2-methyl-2-butene, produces an organobarium chloride which was treated with cyclohexanone.12 Workup leads to the tertiary alcohol in 92% yield with excellent selectivity.

See also Methallyllithium.

1. (a) Benkeser, R. A. S 1971, 347. (b) Lai, Y.-H. S 1981, 585. (c) Nützel, K. MOC 1973, 13/2a, 88. (d) Fürstner, A. AG(E) 1993, 32, 164.
2. (a) Riecke, R. D.; Li, P. T.-J.; Burns, T. P.; Uhm, S. T. JOC 1981, 46, 4323. (b) Rieke, R. D.; Bales, S. E. JACS 1974, 96, 1775.
3. (a) Kündig, E. P.; Perret, C. HCA 1981, 64, 2606. (b) Klabunde, K. J.; Efner, H. F.; Satek, L.; Donley, W. JOM 1974, 71, 309.
4. Oppolzer, W.; Kündig, E. P.; Bishop, P. M.; Perret, C. TL 1982, 23, 3901.
5. (a) Oppolzer, W.; Schneider, P. TL 1984, 25, 3305. (b) Harvey, S.; Junk, P. C.; Raston, C. L.; Salem, G. JOC 1988, 53, 3134. (c) Bogdanović, B.; Janke, N.; Kinzelmann, H.-G. CB 1990, 123, 1507.
6. Baker, K. V.; Brown, J. M.; Hughes, N.; Skarnulis, A. J.; Sexton, A. JOC 1991, 56, 698.
7. Snowden, R. L.; Linder, S. M.; Muller, B. L.; Schulte-Elte, K. H. HCA 1987, 70, 1858.
8. Dieter, R. K.; Lin, Y. J. TL 1985, 26, 39.
9. (a) Tius, M. A.; Thurkauf, A. TL 1986, 27, 4541. (b) Tius, M. A.; Kannangara, G. S. K. OS 1992, 71, 158.
10. Tius, M. A.; Savariar, S. S 1983, 467.
11. Corey, E. J.; Nicolaou, K. C.; Toru, T. JACS 1975, 97, 2287.
12. Yanagisawa, A.; Habaue, S.; Yamamoto, H. JACS 1991, 113, 8955.

Marcus A. Tius

University of Hawaii, Honolulu, HI, USA

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