Organolithiums in solution

Organolithiums (with the exceptions of methyllithium and phenyllithium) are remarkably soluble even in hydrocarbon solvents,12 and simple organolithium starting materials are available as stable hydrocarbon solutions (Table 1.2.1). Methyllithium and phenyllithium are indefinitely stable at ambient temperatures in the presence of ethers, and are solubilised by the addition of ether or THF. 

Hydrocarbon solutions of n-, 5- and r-BuLi re the ultimate source of most organolithiums, but a number of other bases are widely used to generate organolithiums from more acidic substrates. Among these are LDA, LiTMP and other more hindered lithium amide bases, and hindered aryllithiums such as mesityllithium and triisopropylphenyllithium. 

but not all, of the effect of these solvents is due to deaggregation of the organolithium. The importance of aggregation in determining reactivity is illustrated by methyllithium, which, as a monomer, should be more basic than phenyllithium by about 10 / Ka units. However, a 0.01 M solution of MeLi in THF is only three times more reactive than PhLi, and at 0.5 M concentration in THF PhLi is more reactive than MeLi.13 

Wakefield, B. J. The Chemistry of Organolithium Compounds Pergamon Oxford, 1974. 

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General points about organolithiums in solution are considered briefly first, followed by chapters addressing the synthesis of functionalised organolithiums, and in particular the... 

This complex should be used when the organolithium is in solution in a hydrocarbon solvent. For organolithium reagents prepared in ether (see Note 4), the same complex may be used or, more conveniently, copper iodide (Cull can be used. The Cul purchased from Prolabo or Merck 4 Company, Inc. may be used directly. Other commercial sources of Cul (Fluka, Aldrich Chemical Company, Inc., Alfa Products, Morton/Thiokol, Inc.) furnish a salt which affords better results when purified. 1 mol of Cul is stirred for 12 hr with 500 ml of anhydrous tetrahydrofuran, then filtered on a sintered glass funnel ( 3), washed twice with 50 ml of anhydrous tetrahydrofuran, once with 50 ml of anhydrous ether and finally dried under reduced pressure (0.1 imO for 4 hr. 

The combination of equimolar amounts of tris(trimethylsilyl)methyllithium and zinc bromide in a THF/diethyl ether mixture, Scheme 27, furnished tris(trimethylsilyl)methylzinc bromide, as a lithium bromide/ether adduct.43 The compound, which may also be formulated as a lithium alkyldibromozincate, showed no ligand redistribution reactions. It is monomeric in solution and can be treated with 1 equiv. of an organolithium reagent to afford heteroleptic diorganozinc compounds. 

In solution, dipole-dipole interactions constitute a relaxation mechanism, and the dipolar relaxation which is the basis for the well-known nuclear Overhauser effect (NOE), mostly used in the homonuclear H, H case. The 2D HOESY method between H and Li has been used to obtain structural information of many organolithium systems in solution and this field was reviewed in 1995. Li is commonly used as the relaxation is dominated by the dipole-dipole mechanism and the relaxation time is relatively long. Knowledge of the proximity of the lithium cation relative to protons in the substrate is used to derive information about the structure and aggregation of organolithium systems in solution. In a few cases quantitative investigations have been made °. An average error of the lithium position of ca 0.2 A was reported. 

The general conclusion from this investigation is that even aggregated organolithium species can be studied by solid state C NMR spectroscopy, and that these complexes seem to retain the solid state structure in solution. 

Therefore, the appearance of the C—Li bands at unexpected low wavenumbers and their behaviour upon isotopic substitution demonstrate that these bands represent complex modes of vibration in polymeric molecules rather than simple C—Li stretching motions (Figure 1). It is well known that organolithium compounds are strongly associated in solution Furthermore, the C—Li bands occurred in the mulls and solution spectra of ethyllithium at similar positions to those in the vapour spectra, namely in the region from 570 to 340 cm (Table 1) . In benzene solution the bands were found at 560 and 398 cm for CiHs Li and at 538 and 382 cm for CiHs Li. This seems to confirm the previous finding of Berkowitz and coworkers that ethyllithium is polymeric even in the vapour phase. ... 

In their papers Rodionov and coworkers described the polymerization of organolithium compounds in terms of the formation of lithium bonds (Scheme 1), analogous to hydrogen bonds, which brought about cyclic or linear association of these compounds in solution . However, the strong association of alkyllithium compounds, persisting even in the vapour phase, indicates that their association takes place through the formation of... 

A sample of organolithium compound solution is added to an excess of standardized iodine solution in Et20 the excess iodine is extracted with aqueous KI and titrated with standard thiosulfate solution. Evaluation of the organometallic compound is made according to equation. A possible interference can be expected from the couphng reaction in equation 20. This was shown to be neghgible, as demonstrated spectrofluorometrically for phenyllithium , whereas a low titer was found for n-BuLi, attributable to this side... 

Part of the NMR spectroscopy techniques mentioned in Section IV.B can be used for quantitative determination of organolithium compounds in solution. 

The structure of the organolithium compound 89 in solution, obtained on metaUation of a cyclic aldonitrone (88), according to equation 29, seems to have the resonant structure (89), as determined by C, N and Li NMR spectroscopy. Based on the spectroscopic evidence and ab initio calculations (MP2/6-31 + G ) on a simplified model compound (H... 

Fractional kinetic orders of homogenous reactions in solution may point to association of a particular reagent. The kinetics of the initiation step of styrene polymerization in the presence of n-BuLi (equation 33) is in accordance with the assumption that this organolithium compound in a nonbonding solvent forms aggregates of six molecules on the average" . 

A planar arrangement (297) for a cluster of four Li atoms, consisting of two equilateral triangles, is found by XRD for the solid complex 298. Each Li atom is coordinated to methylene groups of both types (from n-BuLi and the metallated carbosilane) and to N and O atoms of the substituent in the carbosilane. Further characterization of the solid 298 can be made by Li and CP/MAS NMR spectroscopies, and in solution by H, Li, and Si NMR spectroscopies. The combination of both organolithium compounds in 298 is found to form a more effective reagent than each of them alone °. 

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