Structures and Reactivities of Organolithium Compounds

The characteristic species present in organolithium compounds under various conditions are reviewed. The effects of environment on the kinetics and energetics of exchange reactions are discussed. A summary is given of various pathways by which organolithium compounds might react. Kinetic and spectroscopic evidence for these processes under various conditions are reviewed. A discussion is presented of the significance of certain results obtained by chemically induced dynamic nuclear polarization (CIDNP). T extensive researches on organolithium compounds of the past decade provide a basis for understanding in detail the chemistry of these compounds. The purpose of this contribution is to review the various lines of evidence regarding the constitution of organolithium reagents, and to relate these to various possible pathways by which they might undergo chemical reaction. The x-ray structural work of Dietrich1 (ethyllithium), Weiss and Lucken2 (methyllithium) and Stucky and Patterman3 (benzyllithium) provide a basis for evaluation of models for the structures of the organolithium compounds in solutions. The order of volatilities, CH3Li C2H5Li < (CH3)2CHLi < (CH3)3CLi, suggests that there is significant association between the structural units in the solid, provided that steric interactions do not interfere. From this point of view, the structure of tert-butyllithium would provide the ideal example of bonding within the characteristic oligomeric unit, in this case, a tetramer. Unfortunately, the solid appears to have an enormous unit cell4. In addition, it is extremely reactive with oxygen, and alkoxide purity is a serious problem. In solution the characteristic structural units observed are the hexamer, tetramer, dimer and monomer, depending on the nature of the organic group and the nature of the solvent5. The prevailing structures in hydrocarbon solvents are hexamer and tetramer when the organic group is saturated alipha tic. Hexameric association is found when the alkyl group is primary; tert-butyllithium, on the other hand, is tetrameric. For intermediate degrees of steric interactions between alkyl groups it may happen that the free energy difference between hexamer and tetramer is small, so that an equilibrium between the two forms is seen, as with trimethylsilylmethyllithium, in benzene6. The associations of organolithium compounds in basic solvents such as ethers,