3.3 Organolithium Reagents

Organolithium reagents are strong bases and nucleophiles. Due to their thermal instability and extremely high reactivity they require elaborate precautions during use. Many organolithium reagents are commercially available as dilute solution in hydrocarbon solvents. In such solvents they are polymeric species with n = 4 to 6. In ethers, however, they are mostly tetrameric in nature. In the presence of strong chelating ligands such as HMPA and DMPU, the degree of association decreases and they exist as monomeric species. This leads to an enhancement in their reactivity. Tetrameric structures are based on distorted cubic structures where the lithium atoms occupy alternate corners of the cube and the alkyl groups occupy a face of the cube.

3.3.1 Preparation

•  Simple organolithium reagents are prepared by the reaction of organic halides with lithium metal (Scheme 1). The order of reactivity of the organic halides decreases in the following order RI > RBr > RCl. In case of alkyl halides, loss of stereochemical integrity at the site of the reaction is generally observed. However, with vinyl halides, the stereochemistry of the double bond is usually preserved (Scheme 2).

Scheme 1

Scheme 2

•  Use of metal-halogen exchange process involving an organic halide and an alkyl lithium can also be followed for the preparation of organolithium reagents. In these reactions the equilibrium lies to the right if the organic group is able to accommodate the electron density than the organic species on the left (Scheme 3).

Scheme 3