Solvation, alkyllithium

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... 

The work by Morton and Ells (60) showed that this difference in reactivity was due to differences in the rate with which the different monomers reacted with the different alkyllithiums (styryl or butenyl). Styryllithium ends reacted rapidly with butadiene, but a butenyl-lithium end reacted quite slowly with styrene. Butadiene was polymerized to near exclusion of styrene during the initial part of the reaction. Special solvation of the catalyst by the polymerizing butadiene was not the cause of this copolymerization. 

However, for hexameric alkyllithiums in hydrocarbon solution, bases can coordinate to form either a solvated hexamer (Eq. (9)) ... 

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. 

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]. 

A DPT proton affinity study of vinyl and allyl anions of cyclic vinyl ethers and cycloalkenes has revealed the acidifying effect of oxygen a- to a vinyl anion. However, deprotonation by alkyllithium reagents in solution is further complicated by solvation, aggregation, and transition-state factors. 

Most simple organolithium reagents can also be prepared by reaction of the appropriate halide with lithium metal. The simple alkyllithium reagents exist mainly as hexamers in hydrocarbon solvents. In ethers the evidence indicates that tetramers are dominant. The tetramers, in turn, are solvated with ether molecules. Certain highly hindered alkyllithiums have been observed to be more reactive than simpler alkyl systems. This has been attributed to steric hindrance, which prevents the formation of the tetramer. Higher reactivity is generally associated with the less-aggregated species. ... 

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. 

Propene is appreciably more acidic than propane because of the relative stability of the conjugated carbanion that results from deprotonation (Section 14-1). Therefore, allylic lithium reagents can be made from propene derivatives by proton abstraction by an alkyllithium. The process is facilitated by A,A,A, Al -tetramethylethane-l,2-diamine (telramethylethylenediamine, TMEDA), a good solvating agent. 

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