Preparation of Organometallic Compounds

Organometallic compounds vary widely in their properties and reactivity, just as do the elements from which they are produced. These compounds may be lithium alkyls, Grignard reagents, or organotin compounds. Accordingly, there is no universal method for preparing the compounds, but we will present here some of the types of reactions that have been widely employed. 

This technique is most appropriate when the metal is highly reactive. It should be kept in mind that even though a formula may be written as if the species is a monomer, several types of organometallic compounds are associated. Examples of this type of reaction are 

The most important reaction of this type is that in which Grignard reagents are produced  

Before its use as a fuel additive to reduce engine knock was banned in the United States, tetraethyllead was produced in enormous quantities. One method of producing the compound was the reaction between lead and ethyl chloride, in which the reactivity of lead was enhanced by its amalgamation with sodium. 

Mercury alkyls can be prepared by an analogous reaction in which an amalgam of sodium and mercury is used. 

Percent Ionic Character of Carbon-Metal Bonds3 

All of these compounds must be handled with great care because some are dangerously reactive and others are very toxic. They seldom are isolated from the solutions in which they are prepared but are used immediately in other reactions. 

The reactivity order of the halides is I Br Cl F. Whereas magnesium and lithium react well with chlorides, bromides, and iodides, zinc is satisfactory 

Alkenyl, alkynyl, and aryl halides, like alkyl halides, can be converted to the corresponding magnesium and lithium compounds. However, the reaction conditions, such as choice of solvent, can be critical. Bromoethene, for instance, can be converted to ethenylmagnesium bromide in good yield if the solvent is oxacyclopentane [tetrahydrofuran, (CH2)40]  

The more reactive allylic and benzylic halides present a problem—not so much in forming the organometallic derivative as in keeping it from reacting further with the starting halide. An often unwanted side reaction in the preparation of organometallic compounds is a displacement reaction, probably of the Sn2 type  

Because of the wide range of organometallic compounds, a large number of reactions have been employed in their synthesis. A few of the preparative methods for compounds of the Group IA and HA metals were shown in Chapter 7. In this section, some of the types of synthetic reactions will be shown with an emphasis on methods that have general applicability. 

The reaction of a metal with an alkyl halide. This technique is widely used when the metal is an active one. The equations will be written as if monomeric species are formed, although, as has been discussed, some of the metal alkyls are associated. Examples of this type of reaction are the following  

A similar reaction can be carried out in which mercury is amalgamated with sodium  

The driving force for these reactions is the formation of an ionic sodium salt. 

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Ge, P, As, Sb, and Bi, can also be prepared in this manner." Except for alkali metal alkyls and Grignard reagents, the reaction between RM and M X is the most conunon method for the preparation of organometallic compounds." ... 

Synthesis with sacrificial electrodes is employed as a direct method in several other preparations of organometallic compounds and complexes. 3-Hydroxy-2-methyl-4-pyrone derivatives of Sn 1 (and of Zn, Cu, In and Cd as well) were prepared using the metal as an anode. The low oxidation state Sn(II) compound is obtained by direct electrolysis134. 

Ultrasound is widely used for the preparation of organometallic compounds from reactive metals [67]. Here the mode of action of ultrasound is twofold ... 

TABLE 1. A survey of the standard methods applied to the preparation of organometallic compounds three crosses mean generally applicable , two restricted, though important scope of appU-cabiUty and one narrow scope ... 

The first report of the preparation of organometallic compounds by direct reaction between metals atoms and organic ligands came in 1969 (727). Research into synthesis using metal atoms has expanded steadily since then and now seems likely to increase very rapidly as the necessary equipment becomes commercially available.1 The field has been reviewed in part before (55, 67, 78a, 84a, 129), but no comprehensive review of the organometallic aspects has yet been published. 

The most widely used method for the preparation of organometallic compounds of the heavier alkali metals is from the corresponding organo-lithium reagent and an alkoxide. 

Preparation of Organometallic Compounds. Metals with Organic Halides... 

The Grignard reaction is often one of the first reactions encountered for the preparation of organometallic compounds. As such it provides a method for the conversion of an alkyl bromide to an alkane. From the example shown below it is seen that the overall oxidation level change from the organic reactants to the products is from 0 to —2, so a reduction has occurred. Magnesium is the reductant and is itself oxidized from 0 to +2 oxidation state. The actual reduction takes place in the first step of the process in which the C-Br bond is converted to a C-Mg-Br bond. The reaction with water is merely a hydrolysis that does not change the oxidation state of carbon. 

R. D. Rieke, Preparation of Organometallic Compounds from Highly Reactive Metal Powders, Science 1989, 246, 1260-1264. 

Transesterification reactions provide an applicable synthetic route to the preparation of organometallic compounds with unusual structures. Tricyclic organometallic compounds, which have the general formula... 

P. Timms, The direct preparation of organometallic compounds from atoms or small molecules. 

Yamazaki, Y., Hosono, K.. Matsuda, H., Minami, N., Asai, M., Nakanishi, H. Enzymic preparation of organometallic compounds with second-order optical nonlinearities. Biotechnol. Bioeng. 38, 1218-1222 (1991)... 

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