Alkyllithium aggregate

Most other studies have indicated considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium or n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and by reaction with dissociated alkyllithium. Evidence for the initial formation of a complex can be observed in the form of a shift in the carbonyl absorption band in the IR spectrum. Complex formation presumably involves a Lewis acid-Lewis base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

This lithiation reaction is carried out in the presence of the chelating diamine TMEDA 19 because, in hydrocarbon solvents, alkyllithiums are thought to react as aggregates such as 20 or mixtures of aggregates and dissociated species.7 Donor solvents like THF or even better diamines like 19 effectively break down alkyllithium aggregates, forming monomers and dimers 21 in solution, and thereby significantly increase their basicity and reactivity towards deprotonation.7 The use of MTBE as the solvent avoids the competitive attack of n-butyllithium on THF at the required reaction temperature of < 5 °C.7c... 

Figure 13 Typical spin multiplets of the lithiated carbon from simple alkyllithium aggregates showing C, Li coupling (a) phenyllithium monomer (—100°C) (b) n-butyllithium dimer (—100°C) (c) /ert-butyllithium tetramer (—88°C) the coupling constants are 14.8, 7.9, and S.4Hz, respectively...

The observed inverse correlation between reaction order dependence for alkyllithium and degree of alkyllithium aggregation is not observed in aliphatic solvents. The use of aliphatic solvents leads to decreased rates of initiation and pronounced induction periods. In fact, a different reaction mechanism involving the direct addition of monomer with aggregated organolithium species has been proposed for aliphatic solvents [3, 56],... 

It is prudent to note that Brown [31] has pointed out that the mechanism shown in Scheme 2 is not consistent with available experimental evidence regarding the kinetics and energetics of dissociation of alkyllithium aggregates. Thus, there is no direct experimental evidence which demonstrates the existence of species such as monomeric n-butyllithium as a distinct molecular species in hydrocarbon solvents [8]. In addition, ah initio calculations predict that the enthalpy of dissociation of tetramers into dimers is 144-152 kj/mol... 

For kinetics in aliphatic solvents, the inverse correspondence between reacdon order dependence for alkyllithium initiator concentration and the degree of alkyllithium aggregation is not observed. In addition, the rates of initiation in aliphatic solvents are several orders of magnitude less than that in aromatic... 

In general terms, it appears likely that alkyllithium reagents have the possibility of reacting through any of several aggregated and dissociated forms. 

It must be noted that the cyclic model fails to account for the role of the additional alkyllithium and diethylaluminum that are required in precise amounts to achieve high selectivity. A simple model that recognizes a possible role for the additional reagents suggests the intermediacy of an extended, noncyclic transition state G with the aldchydic oxygen coordinated to some undefined Lewis acidic species derived from the additional components of the reaction mixture26,44. Aggregates composed of enolate, alkyllithium and dialkylaluminum species are also possible. 

Due to the high interest in metalation reactions with lithium amide or alkyllithiums, an indicator scale of lithium ion pairs in THF has been developed119. Aggregation studies have indicated that organolithium species exist predominantly, if not exclusively, as monomers in the 10-3-10-4 M concentration range. Particular attention has been devoted to the lithium and caesium ion-pair acidities of diphenylamine in THF120 that, at 25 °C, have been found to be 19.05 and 24.20, respectively. 

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. 

The situation becomes even more complex when other lithium salts are present in the solution. For example, when a lithium halide such as LiBr is added to solutions of alkyllithiums, mixed aggregates are formed where a bromide anion takes the... 

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... 

The use of aliphatic solvents causes profound changes in the observed kinetic behavior for the alkyllithium initiation reactions with styrene, butadiene, and isoprenc. i.e.. Ihe inverse correspondence between the reaction order dependence for alkyllithium and degree of organolithium aggregation is generally not observed. Also, initial rales of initiation in aliphatic solvents are several orders of magnitude less lhan those observed, under equivalent conditions, in aromatic solvents. Furthermore, pronounced induction periods are observed in aliphatic hydrocarbon solvents,... 

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