Solvation, alkyllithium compounds

In THF, the alkyllithium compounds are aggregated [157] and the situation is reminiscent of the conditions in hydrocarbon solutions. At high concentrations, the association number (i. e. the number of molecules in the aggregate) decreases. This anomaly is explained by the existence of aggregate—solvent complexes, for example (MeLi)4 8THF Benzyllithium and its polymeric analogue polystyryllithium are not associated. Phenyllithium is mostly present as a dimer or monomer. Both forms are in equilibrium and are solvated. Only the monomeric form of the initiator is active. In practice, benzyllithium reacts only in the form of an ion pair. The fraction of the free benzyl anion must be very small [151c]. 

Solvation of the organolithium polymeric species, with possibly some disruption of the polymers to less-associated species, leads to adducts which may function as a source of carbanions in this sense the adducts behave as bases in furnishing a nucleophilic moiety. It is important to realize, however, that such basic behavior can occur only in the presence of a basic solvent, or it requires a strong Lewis add. Examples of the latter situation are found in the reaction of alkyllithium compounds vrith other organometals, as described in the following section. 

Among unsolvated organolithium compounds only the alkyllithiums are soluble in noncoordinating solvents such as alkanes and arenes. Their states of aggregation depend on the structure close to lithium. Thus primary, tertiary and secondary alkyllithiums, all unsolvated, assemble into respectively hexamers, tetramers and equilibrium mixtures of hexamers and tetramers. Most organolithium compounds dissolve in and coordinate with donor compounds such as ethers and tertiary amines. The actual structures depend critically on the nature of the donor. Thus, diethyl ether solvates tend to be mainly cubic tetramers (with some dimers) while THF favors mixtures of monomers and dimers. Tertiary vicinal diamines such as TMEDA and 1,2-di-Af-piperidinoethane, DPE, favor bidentated coordinated dimers. Finally, in the presence of triamines such as pentamethyl-triethylenediamine PMDTA and l,4,7-trimethyl-l,4,7-triazacyclononane TMTAN, many organolithium compounds form tridentately complexed monomers. 

Closely related to the solvated alkyllithiums are mixed complexes of the organoalkali compounds with alkali alkox-ides, models for the so-called super-bases .Complexes of methyllithium with alkali methoxides and their reactions with methane have been studied theoretically... 

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