3.1 Classification of Organometallic Compounds
Organometallic compounds may be classified in different ways. They may be classified according to the type of metal-carbon bond or according to the metal center involved in these compounds. For our convenience we will follow the latter classification throughout this text.
3.1.1 Grignard Reagents
Grignard reagents are organometallic compounds having Mg-C bond. Magnesium can form two covalent bonds with carbon. Of the various organomagnesium compounds possible, organomagnesium halides and to a lesser extent dialkyl magnesium compounds are widely used for synthesis. Although the Grignard reagents are usually formulated as RMgX, but in reality they are a mixture of a variety of species. The ratio of the species in solution varies with the organic group, the halogen, the solvent, the concentration and the temperature. It is believed that in case of organomagnesium chlorides (RMgCl) in diethyl ether, the predominant species over a wide range of concentrations is a solvated, halogen-bridged dimer 1 . The degree of association varies as the halide is changed to Br or I.
However, in case of THF, due to the highly coordinating nature of the solvent, there is a lesser degree of association. Therefore, monomeric species dominate in THF, however, there are significant concentrations of R2Mg, MgX2 and RMgX in equilibirium.
3.1.1.1 Addition of Grignard Reagents to Carbonyl Groups
The addition of Grignard reagents to carbonyl group is one of the most important methods for carbon-carbon bond formation. Though the overall reaction is quite simple but it is highly susceptible to a number of side reactions. The reactivity of Grignard reagents towards different carbonyl group containing compounds also varies thus giving rise to different end products depending on reactants (Scheme 1).
Scheme 1