Question

Reactivity of organometalic compounds

Answer 1

Organic compounds incorporating carbon-metal bonds are called organometallic compounds. Such compounds have been known and studied for nearly 200 years, and their unique properties have been widely used to effect synthetic transformations. Depending on the reduction potential of the metal, the reactivity of organometallic compounds varies markedly, the most reactive requiring low to moderate temperatures and inert conditions (atmosphere and solvents) for preparation and use. In general, the reactivity parallels the ionic character of the carbon-metal bond, which may be estimated from the proton and carbon chemical shifts of methyl derivatives.

% Ionic Character of H3C – Metal   (CH3)2Hg < (CH3)2Cd < (CH3)2Zn <(CH3)2Mg < CH3Li

The first reported organometallic compounds were prepared by the reductive substitution of alkyl halides, as shown in the following three equations. All these metals have strong or moderate negative reduction potentials, with lithium and magnesium being the most reactive. Halide reactivity increases in the order: Cl < Br < I. Alkylsodium and potassium compounds are not made in this way because Wurtz coupling of the alkyl moiety ( giving R_R ) tends to predominate. This can also be a problem when allyl or benzyl halides are converted to Grignard or lithium reagents. 

R-X   +   Zn   –––>   R-Zn-X     An Alkyl Zinc Reagent     1850 E. Frankland

R-X   +   Mg   –––>   R-Mg-X     A Grignard Reagent     1900 V. Grignard

R-X   +   2Li   –––>   R-Li   +   LiX     An Alkyl Lithium Reagent     1917 W. Schlenk (1930 K. Ziegler)

Simple alkyl derivatives of all three kinds are pyrophoric (burn spontaneously on exposure to air) and react with water to generate the corresponding alkane (RH); however, the zinc compounds are distinctly less reactive in other respects. Diethylzinc may be prepared and distilled (b.p. 117 ºC) under a protective atmosphere of CO2. Grignard and alkyl lithium reagents are not distillable liquids, and react rapidly with CO2 to give carboxylic acid salts.

Although the formulas drawn here for the alkyl lithium and Grignard reagents reflect the stoichiometry of the reactions and are widely used in the chemical literature, they do not accurately depict the structural nature of these remarkable substances. Mixtures of polymeric and other associated and complexed species are in equilibrium under the conditions normally used for their preparation. For example, simple alkyl lithiums are largely hexameric clusters in hydrocarbon solvents, but change to tetrameric and dimeric forms in various ether solvents. Grignard reagents require an ether solvent for their formation, and have been crystallized as monomeric and dimeric ether complexes. The following equilibrating species, called the Schlenk equilibrium, have been identified in ether solution.



Since magnesium halides are moderate Lewis acids, their presence in solution may influence the outcome of certain chemical reactions. One example of this perturbation is the reaction of cyclohexene oxide with methylmagnesium bromide, as shown on the right in the following equation. Magnesium bromide rearranges the epoxide to cyclopentanecarbaldehyde, which then adds the Grignard reagent in the expected manner. Dimethylmagnesium, on the other hand, simply adds to the epoxide by opening the strained ring. Methylithium adds in a similar fashion. 
Pure dialkylmagnesium reagents may be prepared by alternative routes (vida supra), or by removing the magnesium halide by precipitation (dioxane is added).