Simple Organolithium Compounds

However, the available examples, 1-4, have stabilized carbanion moieties and the lithiums also are stabilized by coordination with electron pair donor atoms. Two compounds in the literature involve an sp -hybridized carbon atom cr-bonded to a lithium atom [1 (2) and 3 (3)]. Compound 2 (4) also involves a terminally bonded lithium atom but the more or less planar benzylic carbon can perhaps be described as being sp hybridized. 

Compounds 1 and 2 are monomeric species, probably owing to the large amount of steric bulk in the neighborhood of the lithium atom coordinating solvents [N,M, N, A/ -pentamethyldiethylenetriamine (PMDE-TA) in 1 and TMEDA and THE in 2] complex with lithium to complete its coordination sphere. The geometry about the lithium atom is nearly tetrahedral in both of these compounds. 

Compound 3 is a dimeric structure, but the interactions of the lithium atom are analogous to those in 1 and 2. A sulfur atom from a second 

Complex 4 (6), (MeC2BioHio)Li(pmdeta) [pmdeta = (CH3)2N(CH2 2-N(CH3)(CH2)2N(CH3)2], a lithiated carborane, also is terminally bonded and represents a simple example of a carbon-lithium o -bond. The C—Li distance is 2.18 A. The structural features at the lithium atom are analogous to those for compounds 1-3. 

Organolithium Compounds with Bridging Lithium Atoms 

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Several of the simple organolithium compounds, such as methyl, n-butyl, /-butyl, and phenyl derivatives, are commercially available as hydrocarbon or ether solutions. There are other preparative methods, but we will defer consideration of these until Part B, Chapter 5. 

Grignard reagents) are of primary importance. Because of the very weak acidity of most hydrocarbons, the simple organolithium compounds, e.g., methyllithium, butyllithium, phenyllithium, are usually not prepared by proton-transfer reactions. Instead, the most general preparative methods starts with the corresponding halogen compound ... 

It is generally considered that polymeric and simple organolithium compounds react quantitatively and relatively rapidly with 1,1-diphenylethylene (DPE) to produce the corresponding 1,1-diphenylalkyllithium species, as shown in Eq. (1) [19] ... 

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. ... 

Lewis bases effect dramatic changes in microstructure, initiation rates, propagation rates, and monomer reactivity ratios for alkyllithium—initiated polymerizations of vinyl monomers (1-6). Some insight into the molecular basis for these observations has been provided by a variety of NMR, colligative property, and light-scattering measurements of simple and polymeric organolithium compounds in hydrocarbon and basic solvents... 

In general, simple alkyllithiums exist predominantly as either hexamers (for sterically unhindered RLi) or tetramers (for sterically hindered RLi) in hydrocarbon solvents and as tetramers in basic solvents (9-12). Polymeric organolithium compounds such as poly(styryl)lithium exist as dimers in hydrocarbon solution and are unassociated in basic solvents such as tetrahydrofuran (13-15). The state of association of poly-(dienyl)lithiums in hydrocarbon solution is a subject of current... 

Lewis bases exert dramatic effects on the rate, stereochemistry, and reaction pathway in organolithium chemistry 4). A partial explanation for these observations can be deduced from the effects of Lewis bases on the degree of association of organolithium compounds as shown in Table 3. In general, the presence of basic molecules tends to decrease the average degree of association of organolithium compounds. Thus, simple alkyllithiums which are hexameric in hydrocarbon solution... 

To the best of our knowledge, X-ray structural data of complexes with simple dihapto interactions between a lithium atom and the n system of an alkene or alkyne ligand are unknown, but there is some spectroscopic evidence for weak it interactions in solutions of 3-alkenyllithium compounds from 7Li-and H-NMR data (4). Interactions of this sort are presumably important in addition (polymerization) reactions between organolithium compounds and alkenes or alkynes. 

The first vinyllithium carbolithiation reaction was reported by Chamberlin and Bloom15, who showed that Shapiro-derived organolithium 10 cyclized onto a terminal alkene giving stereoselectively (>50 1) bicyclic compounds 11, after treatment with electrophiles (Scheme 4). The intermediate alkyllithiums 12 are generated via a 5-exo-trig cyclization reaction from 10, which undergo the carbolithiation reaction at approximately the same rate as reported by Bailey for the simple parent compound 5-hexenylIithium, i.e. with a half-life of a few minutes at 0 °C. 

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