Carbon-lithium bond ionicity

Flowever, the electrons of a covalent bond are not necessarily shared equally by the bonded atoms, especially when the affinities of the atoms for electrons are very different. Thus, carbon-fluorine and carbon-lithium bonds, although they are not ionic, are polarized such that the electrons are associated more with the atom of higher electron affinity. This is usually the atom with the higher effective nuclear charge. 

Due to its high ionic character, the carbon-lithium bond is very reactive and adds under mild conditions to ethylene or dienes and under more severe conditions to other alkenes. Some functionalized alkenes can be used, and high regio- and stereo-selectivity is usually observed in these carbolithiation reactions, especially if a precoordination of the lithium organometallic with the alkene is possible. Intramolecular carbolithiations of alkenes proceed under mild conditions and allow the preparation of several stereochemically well defined mono- and bi-cyclic compounds. Alkynes are too reactive, and can lead, with organolithium derivatives, to several side reactions, and seldom afford the desired carbolithiated product in good yield. 

Furthermore, several of Worsfold s assessments seem to be open to question. The assertion that the association (between the allylic-lithium active centers) is between ionic species can be contrasted with the evidence provided by NMR spectroscopy 36,134 143) which has shown that the carbon-lithium bond of allylic-lithium species can possess considerable covalent character. Worsfold has also previously published 43 > concentrated solution viscosity results where the ratio of flow times, before and after termination, of a poly(isoprenyl)lithium solution was about 15. This finding is clearly incompatible with the conclusion that viscometry cannot detect the presence of aggregates greater than dimeric. 

The deep-red lithium cyclopropyl-diphenylmethanide (42b), which is stable in tetrahydrofuran solution, opens completely to the colourless lithium (3,3-diphenylprop-2-enyl)methanide in diethyl ether. The retro rearrangement to (42b) can be achieved simply by adding tetrahydrofuran to the solution of (42a) in diethyl ether [207]. Obviously, tetrahydrofuran, which is the better cation-solvating EPD solvent, makes the carbon-lithium bond in (42a) more ionic, thus favouring the ring closure to (42b), which has a more delocalized negative charge. 

These results have been attributed to an increased ionicity of the carbon-lithium bond in the case of 331 and 340 as compared to 342 and 343 . This conclusion is supported by electrochemical measurements and MNDO calculations A similar conclusion had been reached earlier by Boche and Martens for thermal cyclopropyl-allyl anion transformations. 

There is considerable controversy regarding the degree of covalent character in a carbon-lithium bond ". An uncritical comparison of electronegativities does indicate a high degree of ionic character as do extended Hiickel molecular orbital... 

Furthermore, several of Worsfold s assessments seem to be open to question. The assertion that the association (between the allylic-lithium active centers) is between ionic species can be contrasted with the evidence provided by NMR spectroscopy which has shown that the carbon-lithium bond of allylic-... 

The possibilities inherent in the anionic copolymerization of butadiene and styrene by means of organolithium initiators, as might have been expected, have led to many new developments. The first of these would naturally be the synthesis of a butadiene-styrene copolymer to match (or improve upon) emulsion-prepared SBR, in view of the superior molecular weight control possible in anionic polymerization. The copolymerization behavior of butadiene (or isoprene) and styrene is shown in Table 2.15 (Ohlinger and Bandermann, 1980 Morton and Huang, 1979 Ells, 1963 Hill et al., 1983 Spirin et al., 1962). As indicated earlier, unlike the free radical type of polymerization, these anionic systems show a marked sensitivity of the reactivity ratios to solvent type (a similar effect is noted for different alkali metal counterions). Thus, in nonpolar solvents, butadiene (or isoprene) is preferentially polymerized initially, to the virtual exclusion of the styrene, while the reverse is true in polar solvents. This has been ascribed (Morton, 1983) to the profound effect of solvation on the structure of the carbon-lithium bond, which becomes much more ionic in such media, as discussed previously. The resulting polymer formed by copolymerization in hydrocarbon media is described as a tapered block copolymer it consists of a block of polybutadiene with little incorporated styrene comonomer followed by a segment with both butadiene and styrene and then a block of polystyrene. The structure is schematically represented below ... 

The carbon-lithium bond is less ionic than that between carbon and other alkali metals and, in contrast to the alkyls of the latter, lithium alkyls are often soluble in hydrocarbons and also capable of association to form electron-deficient bonds. The alkyl lithiums, except the methyl, are hydrocarbon soluble whilst the lower members are capable of distillation. From these, and other considerations ethyl and butyl lithiums are of greatest interest, particularly since the actual organo-lithium catalyst used appears to have little influence on the polymer properties. 

The acceleration of the rates of organolithium reactions resulting from low concentrations of bases such as amines or ethers is very great (54, 60, 72-75), The effect no doubt results from complex formation which labilizes the carbon-lithium bond. Coordination of base also apparently results, as expected, in a more ionic transition state. The stereospecificity of olefin polymerization changes very markedly upon addition of base, and varies with base concentration in a manner which suggests that complex formation is responsible 31, 68, 73). 

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