Organolithium oligomers

Many organolithium compounds are soluble in hydrocarbons exceptions are methyllithium and phenyllithium w hich are associated in these solvents. Butyllithium is mostly hexameric and /c/Y-butyllithium is tetrameric in cyclohexane. A Lewis basic solvent can interact with an organolithium oligomer, thereby decreasing the degree of association. Thus, methyllithium. which is tetrameric in the solid phase, becomes a solvated tetramer in ether, and BuLi. hexameric in hydrocarbons, becomes tetrameric in ether. In the more basic THE BuLi has a degree of association between dimeric and tetrameric at -108 °C, and phenyllithium is between monomeric and dimeric [3]. 

Organolithium compounds tend to associate into dimers, higher oligomers and polymers of two types Complexes where the Li atoms are linked to each other by a chain of one or more atoms of other elements (C, N, O etc.), and complexes where the Li and other metallic atoms are close to each other, forming clusters. Section V presents examples of application of instrumental methods—mainly NMR and XRD—to structural elucidation of these associated species. 

In 1969, Bulten and Noltes210 synthesized the perethyloligogermanes Et(Et2Ge) Et (n = 2-6) by the organolithium method. The oligomer with n = 6 was thermally stable and heating at 250 °C for 8 hours resulted in only 20% decomposition. 

In covalent organolithium compounds and covalent Grignard reagents neither the lithium nor the magnesium possesses a valence electron octet. This is energetically disadvantageous. In principle, the same mechanism can be used to stabilize these metals that monomeric boranes BH3 n Rb use to attain a valence electron octet at the boron atom (Section 3.3.3) the formation either of oligomers or, with suitable electron pair donors, of Lewis acid/Lewis base complexes. 

When steric hindrance is minimal, organolithium compounds often form aggregates (oligomers) tetramers are frequently observed. Their structures may be described as tetrahedral arrays of lithium atoms with the substituents occupying each face of the tetrahedron or, alternatively, as cubic arrays with alternating lithium and carbon atoms (15). The Li—Li distances are shorter than the C—C separations, however. 

The reactions observed when (l-chlorocyclopropyl)cyclopropylacetylene was treated with an organolithium reagent are sensitive to the nature of the reagent. When butyllithium was used, 1 -(2-cyclopropylethynyl)cyclopropyllithium was essentially the only product formed. However, when the same compound was exposed to phenyllithium, cyclopropyl(l-phenylcyclopro-pyl)acetylene (6) was obtained in 49% yield together with four oligomers in a total yield of 33%." 55... 

These anionic ring opening polymerizations are usually carried out either in bulk or in solution. A host of catalyst types are active. For synthetic references using specific catalysts, the reader is referred to several excellent sources (4,7,31,32). Representative catalysts include hydroxides, alcoholates, phenolates, silanolates, siloxanolates, mercaptides of the alkali metals, organolithium and potassium compounds, and quaternary ammonium and phosphonium bases and their silanolates and siloxanolates. Some physical characteristics of linear oligomers are given in Table 5 (10). 

Since then, Ullman coupling has been used, e.g., in preparing some pyrrole oligomers. Copper compounds have also been used in coupling of organolithium compounds e.g., some thiophene oligomers have been prepared in this way. 

Organolithium reagents are oligomers (i.e., dimers, trimers, and higher species) in nondonor solvents such as alkanes LiMe is a tetramer with a cubane structure 14.1, for example. RLi forms solvates with THE Addition of the chelating ligand Me2NCH2CH2NMe2 (TMEDA) leads to formation of a monomer, and this increases the reactivity. n-BuLi can deprotonate toluene... 

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