Ethyllithium structure

A is approximately twice the observed covalent radius of lithium, and both the Li—C distance and the Li—C—Li angle are reminiscent of the trimethylaluminum structure previously described. In the two tetra-mers for which structural data are available, the Li—Li distances are substantially shorter in the range of 2.4 to 2.6 A with Li—C distances in the 2.2-2.5 A range. Crystallization effects may play a substantial role in the ethyllithium system since this compound is hexameric in noninteracting solvents but crystallizes in the tetrameric form. 

In solution lithium alkyls are extensively associated especially in non-polar solvents. Ethyllithium in benzene solution exists largely as a hexamer (9, 43) in the concentration range down to 0.1 molar and there is no evidence for a trend with concentration so presumably the hexamers persist to even lower concentrations. Indeed even in the gas phase at high dilution it exists as hexamer and tetramer in almost equal amounts (3). In a similar way n-butyllithium in benzene or cyclohexane is predominantly hexameric (62, 122). t-Butyl-lithium however is mostly tetrameric in benzene or hexane (115). In ether solution both lithium phenyl and lithium benzyl exist as dimers (122) and it has been suggested that butyllithium behaves similarly in ether (15) although this does not agree with earlier cryoscopic measurements (122). It is however certain that more strongly basic ethers cause extensive breakdown of the structure. 

Richardson and Sacher (41) showed that the anionic polymerization of butadiene with ethyllithium in THF produces mainly 1.2 structure in the polybutadiene. Roha (2) reviewed the typical anionic polymerization of butadiene to polymers containing 1.2 structure by catalysts such as alkyl sodium and alkyl potassium. Sodium naphthalene with THF produces 88% 1,2 polybutadiene (42). 

Fig. 2. Crystal structure of ethyllithium (Reprinted with permission from Ref. 8>, Copyright 1981...

Hall U3>, Hsieh 106>, Roovers and Bywater107), Tanlak and co->workers114), and Bordeianu and co-workersI1S) followed the initiation of styrene under polymerization conditions in aromatic or alkane solvents using ethyllithium, z-propyllithium, or isomers of butyllithium. Without exception, these authors found a first power dependency of initiation rate on total active center concentration. Hsieh s results106) and those of Roovers and Bywater 107, also indicate that the first order character for initiation is independent of the degree of association (4 or 6) of the alkyllithium. The first order dependence of the initiation step on total active center concentration is also maintained over the period where cross-aggregated structures, PSLi (RLi)x, are present. 

Figure 11.23 Presumed structure of chain-end groups of polybutadiene and polyisoprene initiated with ethyllithium...

These experimental measurements have prompted ab initio MO calculations of model additions of lithium hydride (Houk et al., 1985). With ethylene, monomeric lithium hydride initially forms a stable rc-complex [7], which passes through a four-centre cyclic transition structure [8] to yield the ethyllithium [9], with energies relative to reactants of-50.0, 28.5, and... 

Even if the solid-state structures known so far exhibit separated ions, the nature of the different species in solution is not yet certain. The H and Li NMR studies of methyllithium mixed with dimethylzinc (and dimethyhnagnesium) in ether solutions have shown that complex species such as Li2Zn(CH3)4 and Li3Zn(CH3)s are formed, and no 1 1 complex was observed. Furthermore, other studies (conductivity, cryoscopy, NMR and UV spectroscopy) of ethyllithium solutions in diethylzinc and MZnEt4 solutions (M = Ca, Ba, Sr) have shown the existence of comphcated exchange equilibria between various species. 

There has been extensive computational study of the structure of organolithium compounds. The structures of the simple monolithium compounds are very similar to the corresponding hydrocarbons. The gas phase structure of monomeric methyllithium has been determined to be tetrahedral with an H-C-H bond angle of 106°. These structural parameters are close to those calculated at the MP2/6-31IG level of theory Ethyllithium, and vinyllithium are also structurally similar to the corresponding... 

Methyllithium and ethyllithium are tetrameric in the solid state,203,205 and most organolithium reagents are highly associated in solution as well. - The degree of association is related to the solvent and structure of the organolithium, but tends to be higher for straight-chain than for branched (secondary and tertiary)... 

Regiochemistry is ordinarily not a concern in the addition of nucleophiles to structures having carbon-heteroatom multiple bonds that are not conjugated with carbon-carbon double or triple bonds, because only 1,2-addition is expected. If the addition creates a new chiral center, however, then stereoisomeric addition products can be formed. Considerable interest in this area was sparked by a report by Cram that nucleophilic addition to ketones having chiral a carbon atoms gave unequal yields of the possible diastereomeric adducts. For example, addition of ethyllithium to the methyl ketone (+ )-92 gave primarily the erythro product (+ )-93. 

The structure of the hexamer species remains conjectural to some extent, since a hexameric species has not yet been observed in X-ray work. The hexamer and tetramer species must, however, be closely similar in energy the vapor of ethyllithium contains both species, hydrocarbon solutions contain predominantly hexamers, and the solid state consists of loosely associated tetramers. Further, the infrared spectra of ethyllithium in the solid state and in hydrocarbon solution are closely similar. A model for the hexamer which involves the same bonding formalism just presented for the... 

Alkylalkali compounds readily eliminate metal hydride to give the alkene this reaction is especially pronounced with the higher alkali metals. The microscopic reverse process has been modeled by the addition of LiH to ethylene. As shown in Figure 6, the reaction first involves a r-complex between ethylene and lithium hydride in which the structures of ethylene and of LiH are changed but little from the separated reactants. In the cyclic transition structure the C-Li bond has significantly shortened towards its value in the product ethyllithium. The C-H bond is still quite long in the transition structure but is on its way to a normal C-H bond. The same type of... 

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