Grignard Reagent

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  • Topic: Diethyl ether, Grignard reaction, Alkyl
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1598

Organometallics 2009, 28, 1598–1605

CoVer Essay
The Grignard Reagents
Dietmar Seyferth
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed February 4, 2009 During the past 100 years the Grignard reagents probably have been the most widely used organometallic reagents. Most of them are easily prepared in ethereal solution (usually diethyl ether or, since the early 1950s, THF) by the reaction of an organic halide with metallic magnesium (eq 1). Table 1. Composition of Diethyl Ether Solutions of Various Grignard Reagents at Equilibrium (in mol %), 2RMgX h R2Mg + MgX2a RX in RX + Mg reacn CH3I C2H5I C2H5Br C2H5Cl n-C3H7I n-C3H7Br n-C3H7Cl C6H5I C6H5Br a

RMgX 87.0 43.0 41.0 15.0 24.0 24.0 17.0 38.0 30.0

R2Mg ) MgX2 6.5 28.5 29.5 42.5 38.0 38.0 41.5 31.0 35.0

RX + Mg f RMgX (X ) Cl, Br, I)

(1)

Most of them are stable in ethereal solution (although atmospheric moisture and oxygen should be excluded) and in general are quite reactive. Discovered by Victor Grignard at the University of Lyon in France in 1900,1 their ease of preparation and their broad applications in organic and organometallic synthesis made these new organomagnesium reagents an instant success. The importance of this contribution to synthetic chemistry was recognized very early, and for his discovery Grignard was awarded a Nobel Prize in Chemistry in 1912. Our cover molecule is the monomeric ethylmagnesium bromide bis(diethyl etherate) (1), whose solid-state molecular structure was determined by an X-ray diffraction study by Lloyd Guggenberger and Robert Rundle in 1964 using crystals isolated from a diethyl ether solution of a C2H5Br/Mg reaction mixture by slow cooling with a stream of cold gaseous nitrogen.2-4

Adapted from: Schlenk, W., Jr. Ber. Dtsch. Chem. Ges. 1931, 64,

734.

Wilhelm Schlenk and his son discovered 80 years ago, more than one magnesium-containing species exists in the diethyl ether solution of a Grignard reagent.5 A redistribution of the substituents on magnesium takes place, and the RMgX species ends up in equilibrium with the two symmetrical species, the diorganomagnesium and the magnesium dihalide: the “Schlenk Equilibrium” (eq 2).5

2RMgX h R2Mg + MgX2

(2)

Generally written as “RMgX” in textbooks, monographs and research papers, the Grignard reagents in ethereal solution are more complicated than this simple formula indicates. As (1) (a) Grignard, V. Compt. rend. Hebd. Seances Acad. Sci. 1900, 130, ´ 1322. (b) Grignard, V. Dissertation “Theses sur les combinaisons organo` magnesienes mixtes et leur application a des syntheses”, University of Lyon, ` ` Lyon, France, 1901. (2) (a) Guggenberger, L. J.; Rundle, R. E. J. Am. Chem. Soc. 1964, 86, 5344. (b) Guggenberger, L. J.; Rundle, R. E. J. Am. Chem. Soc. 1968, 90, 5375. (3) A crystalline solid, CH3MgI · (n-C5H11)2O, was isolated and identified as such by elemental analysis (Mg and I) in 1908: Zerewitinoff, Th. Ber. Dtsch. Chem. Ges. 1908, 41, 2244. The oxonium structure

The species that contain Mg-halogen bonds can be precipitated from Grignard reagent solutions in diethyl ether by the addition of 1,4-dioxane. An insoluble, polymeric 1,4-dioxane adduct is formed, leaving behind a solution of R2Mg5sa useful preparation of dialkyl- and diarylmagnesium reagents.6 Wilhelm Schlenk, Jr. analyzed the 1,4-dioxane precipitations from a number of Grignard reagent solutions.7 Assuming that the precipitation is essentially instantaneous, i.e., that the calculated R2Mg, MgX2, and RMgX percentages reflect the actual composition of the Grignard reagent solution at equilibrium, Schlenk reported the compositions collected in Table 1. Direct evidence (5) Schlenk, W.; Schlenk, W., Jr. Ber. Dtsch. Chem. Ges. 1929, 62, 920. (6) (a) Cope, A. C. J. Am. Chem. Soc. 1935, 57, 2238. (b) As Erwin Weiss found, evaporation of diethyl ether solutions of methyl- and ethylmagnesium bromide and chloride at reduced pressure...
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