Alkyllithium compounds aggregation

Abdul-Sada, A.K. Greenway, A.M. Sed-don, K.R. The Extent of Aggregation of Air-Sensitive Alkyllithium Compounds As Determined by Fast-Atom-Bombardment-MS. J. Organomet. Chem. 1989, 375, C17-C19. 

Plavsic, Srzic and Klasinc performed a detailed electron impact mass spectrometric investigation of alkyllithium compounds . They have concluded that MeLi, /-PrLi, 5-BuLi and r-BuLi consist of tetrameric clusters only, while n-PrLi, n-BuLi and i-BuLi form mixtures of tetramers and hexamers, in the gas phase. Fast atom bombardment (FAB) mass spectrometry was used by Abdul-Sada, Greenway and Seddon to show that the extent of aggregation of f-BuLi is tetrameric while n-BuLi is hexameric (Table 1). Nevertheless, for both alkyllithium compounds the ion corresponding to (RLi)Li+ is the most abundant in the spectrum, as also shown in other studies of alkyllithium vapor. 

Simple alkyllithium compounds arc aggregated in solution, in the solid slate, and even in the gas phase. The important differences between the v arious alkyllithium compounds arc their degrees of aggregation in solution and their relative reactivity as initiators for anionic polymerization of... 

Alkyllithium compounds have solubility and stability because of their ability to associate to form aggregates of definite structure. Such aggregation confers stability but is not extensive enough to cause insolubility. Methyllithium and n-butyllithium, for instance, exist in a highly associated form, typical of electron-deficient bonding, e.g., (MeLi) and (BuLi)4. 

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]. 

In solution [7] and solid state (-)-sparteine-coordinated i-PrLi exists as an unsymmetric aggregate containing two lithium alkyls [5] this dissociates during the reaction under formation of a precoordinated complex. Apart from j-BuLi, j-PrLl is the only known simple alkyllithium compound which can, in combination with (-)-sparteine, efiSciently be used for asymmetric deprotonation of iV-Boc-pyrrolidine [2]. 

Although the aggregation of alkyllithium compounds in etheral and other solvents has been known for quite a few years 1J5l> I i-i44)i detailed information about the kinds of aggregates and the rearrangements within a certain aggregate or between different aggregates became available only in recent years 145-U7). Because of their importance in mechanistic and synthetic carbanion chemistry 143 148), it therefore seems appropriate to include some of the pertinent results in this review article. 

Recently, Hilmersson and co-workers have observed that in heterogeneous aggregates involving lithium amides and alkyllithium compounds, a Juu coupling can be measured between nonequivalent lithium nuclei belonging to a quadrilateral only if the two complementary atoms are carbons. A similar... 

Thus, it is assumed that only the unassociated alkyllithium compound (formed by dissociation of the aggregate, Eq. (2.70)) reacts with monomer in the initiation step (Eq. (2.71)) so the rate of initiation can be expressed by Eq. (2.72). 

A good example of this kinetic behavior was found in the study of the n-butyllithium-styrene system in benzene, in which a kinetic order dependency on n-butyllithium concentration was observed, consistent with the predominantly hexameric degree of association of n-butyllithium (Worsfold and By water, 1960). However, this expected correspondence between the degree of association of the alkyllithium compound and the fractional kinetic order dependence of the initiation reaction on alkyllithium concentration was not always observed (Young et al., 1984). One source of this discrepancy is the assumption that only the unassociated alkyllithium molecule can initiate polymerization. With certain reactive initiators, such as 5 c-butyllithium in hexane solution, the initial rate of initiation exhibits approximately a first-order dependence on alkyllithium concentration, suggesting that the aggregate can react directly with monomer to initiate polymerization (Bywater and Worsfold, 1967a). A further... 

The ABA triblocks which have been most exploited commercially are of the styrene-diene-styrene type, prepared by sequential polymerization initiated by alkyllithium compounds as shown in Eqs. (99-101) [263, 286]. The behavior of these block copolymers illustrates the special characteristics of block (and graft) copolymers, which are based on the general incompatibility of the different blocks [287]. Thus for a typical thermoplastic elastomer (263), the polystyrene end blocks (-15,000-20,000 MW) aggregate into a separate phase, which forms a microdispersion within the matrix composed of the polydiene chains (50,000-70,000 MW) [288-290]. A schematic representation of this morphology is shown in Fig. 3. This phase separation, which occurs in the melt (or swollen) state, results, at ambient temperatures, in a network of... 

Higher aggregates are known for some alkyllithium compounds these have been modeled by computations of various structures for methyllithium hexamers the results can be rationalized by electrostatic considerations. 

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. 

The extensive use of alkyllithium initiators is due to their solubility in hydrocarbon solvents. A common example is n-butyllithium which is usually available as a solution in n-hexane. The C-Li bond is not ionic in hydrocarbon media where the initiator molecules exist as aggregates. Initiation is thus fairly slow in hydrocarbon media. Addition of tetrahydrofuran to this solvent increases the concentration of unaggregated initiator (which is more active for initiation) by forming a 1 1 complex with this compound. Alkyls and aryls of the heavier alkali metals, such as Na and K, are poorly soluble in hydrocarbons because of the greater ionic character of the Na—C and K-C bonds. 

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