3-Methyl-2-butenylmagnesium Chloride1

(X = Cl)

[32916-51-1]  · C5H9ClMg  · 3-Methyl-2-butenylmagnesium Chloride  · (MW 128.90) (X = Br)

[63847-45-0]  · C5H9BrMg  · 3-Methyl-2-butenylmagnesium Bromide  · (MW 173.35)

(terpene synthesis)

Alternate Names: chloro(3-methyl-2-butenyl)magnesium; prenylmagnesium chloride.

Physical Data: 13C NMR solution data have been reported.2

Solubility: sol ether, THF.

Purification: solutions should be titrated before use by one of the standard methods.

Preparative Methods: prepared in situ under an inert atmosphere from prenyl chloride and an excess of magnesium in ether3 or in THF.4

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

Grignard Synthesis.

The reagent is prepared by slow addition at 5 °C of 1.0 equiv of prenyl chloride in THF to 1.2 equiv of Magnesium turnings in the same solvent (350 ml per mol of the chloride). The reaction is initiated at 25 °C. The yield is estimated to be 90%.4 The reagent appears to be a s-bonded structure with the metal attached to the primary carbon. The internal isomer (metal bonded to the tertiary carbon) is present in low concentration. Structural changes to the reagent immediately prior to the reaction cannot be ruled out.2

Prenylation of Allylic Functional Groups.5-7

The copper(I)-catalyzed addition of prenylmagnesium chloride (1) to allylic chlorides leads in good yield and in high selectivity to the product of g-attack (eq 1).5

Similar reactivity is observed for the copper(I)-catalyzed displacement of allylic phosphonates by (1) (eq 2).6

In the absence of copper(I), SN2 displacement of allylic phosphonates by (1) takes place at the tertiary carbon of the nucleophile (eq 3). Bidentate complexation of two of the phosphate oxygen atoms with the metal is postulated to take place.7

Addition Reactions.

Addition of (1) to aldehydes8 and ketones9 also takes place at the tertiary carbon with excellent selectivity. The trend is reversed in acylsilanes, particularly those with bulky isopropyl groups on silicon.8,10 This tendency is reinforced by substituting (1) for prenylzinc bromide. In general, the copper(I)-catalyzed addition reactions of (1) occur at the primary carbon. For example, the copper(I)-catalyzed ring opening of Epichlorohydrin by (1) takes place at the primary carbon of the nucleophile.11

The addition of (1) to 3,3-dimethylcyclopropene, followed by trapping of the intermediate cyclopropyl organometal by Carbon Dioxide, constitutes an effective approach to chrysanthemic acid analogs.12 The Dichlorobis(cyclopentadienyl)titanium-catalyzed addition of (1) to Isoprene also takes place at the tertiary carbon, and is regioselective with respect to both nucleophile and isoprene. Trapping of the intermediate organometal with carbon dioxide produces carboxylic acid (2) in excellent yield (eq 4).13

Related Reagents.

Allylmagnesium Bromide; Crotylmagnesium Bromide; Methallylmagnesium Chloride.

1. Kwart, H.; Miller, R. K. JACS 1954, 76, 5403.
2. Schlosser, M.; Stähle, M. AG(E) 1980, 19, 487.
3. Ohno, M.; Matsuoka, S.; Eguchi, S. JOC 1986, 51, 4553.
4. Fehr, C.; Galindo, J.; Perret, R. HCA 1987, 70, 1745.
5. Yanagisawa, A.; Noritake, Y.; Nomura, N.; Yamamoto, H. SL 1991, 251.
6. Yanagisawa, A.; Nomura, N.; Noritake, Y.; Yamamoto, H. S 1991, 1130.
7. Yanagisawa, A.; Hibino, H.; Nomura, N.; Yamamoto, H. JACS 1993, 115, 5879.
8. Yanagisawa, A.; Habaue, S.; Yamamoto, H. T 1992, 48, 1969.
9. Barbot, F.; Miginiac, P. BSF(2) 1977, 113.
10. Yanagisawa, A.; Habaue, S.; Yamamoto, H. JOC 1989, 54, 5198.
11. Takano, S.; Yanase, M.; Takahashi, M.; Ogasawara, K. CL 1987, 2017.
12. Lehmkuhl, H.; Mehler, K. LA 1978, 1841.
13. Akutagawa, S.; Otsuka, S. JACS 1975, 97, 6870.

Marcus A. Tius

University of Hawaii, Honolulu, HI, USA

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