Lithium Compounds Phenyllithium

The effect of a 3-substituent on the orientation of the addition of alkyl- and aryl-lithium compounds is interesting and salient results appear in Table 23. Attack at C-2 is favoured over that at C-6 unless either, or both, the C-3 substituent and the attacking alkyl group are very large. Reaction of isopropyllithium with 3-methylpyridine does not follow this trend. Benzyllithium is anomalous in that it attacks preferentially at C-4. A study of the relative rates of these alkylations revealed the remarkable fact that a 3-methyl or 3-ethyl group activates the 2-position (but not the 6-) towards attack by phenyllithium but not methyllithium. However, a 3-isopropyl and 3-cyclohexyl group deactivates C-2 relative to... 

This type of metallic exchange is used much less often than 12-35 and 12-36. It is an equilibrium reaction and is useful only if the equilibrium lies in the desired direction. Usually the goal is to prepare a lithium compound that is not prepared easily in other ways, for example, a vinylic or an allylic lithium, most commonly from an organotin substrate. Examples are the preparation of vinyllithium from phenyllithium and tetravinyltin and the formation of a-dialkylamino organolithium compounds from the corresponding organotin compounds ... 

Fig. 13. The reaction of 1,1-diphenylethylene with various lithium compounds in tetrahydrofuran. Variation of rate with formal concentration of lithium alkyl or aryl. (A) n-Butyllithium ( ) benzyllithium (- ) allyllithium (o) methyllithium ( ) vinyl-lithium (0) phenyllithium. Solvent tetrahydrofuran [101].

Activation of organolithium compounds. n-Butyllithium and phenyllithium react very slowly with diphenylacetylene. However, the 1 1 complex of either lithium compound and TMEDA reacts with diphenylacetylene at room temperature. For example, the reaction of /-butyllithium under these conditions followed by carbonation gives cis-4,4-dimethyl-2,3-diphenyl-2-pentenoic acid (1) and a trace of 2-phenyl-3-f-butylindone (2). Thus addition takes place as well as metallation.1... 

Optimum Conditions for Preparing Phenyllithium from Benzene. The optimum procedure for metalation of an aromatic compound in high yield was to use the aromatic compound itself as the solvent. The alkyl-lithium compound used mainly in this study was n-butyllithium because it was convenient and economical. 

Ethylenation of n-butyllithium, phenyllithium, and benzylic lithium compounds does not occur at low temperature and ordinary pressure (9). Under more rigorous conditions, telomerization of ethylene in aromatic hydrocarbons proceeds vigorously in the presence of an organolithium compound and an amine. Although n-butyllithium is introduced initially, rapid transmetalation occurs to the more acidic aromatic hydrocarbon (telogen) which subsequently adds to ethylene (taxogen) and initiates the carbanionic polymerization of ethylene. This polymerization proceeds to modest molecular weight, but it is terminated by transmetalation back to the aromatic hydrocarbon which initiates another chain to complete the catalytic cycle. 

The rate of RLi additions to methyl isothiocyanate seems to be strongly dependent upon the basicity of RLi [9]. Whereas phenyllithium reacts rapidly at — 90 °C, the addition of the much more weakly basic 2-thiazolyllithium (pK of 1,3-thiazole is about 29) has to be carried out at —30 to —50 °C and the reaction with lithium acetylides RC=CLi (pK acetylenes is 26 or lower) requires 0 to 20 °C (all reactions were carried out at the same 0.5 to 1 mol/1 concentrations). These temperatures indicate the conditions for other organolithium compounds and also the chance of success for other reactions. A rough estimation of the pK values of RH has to be made first, taking into account stabilizing effects of substituents. Side reactions have not yet been observed in the conversions of isothiocyanates with lithium compounds. 

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

All vapourisation processes of solutions made of unstable substances are dangerous because the concentration of the unstable substance increases. In this category the heterogeneous reactions can be grouped together they lead to accidents because of compounds with too thin a particle size distribution. So it is possible to control the reaction of phenyllithium by using thick pieces of lithium. 

The crystal structures of many organolithium compounds have been determined.44 Phenyllithium has been crystallized as an ether solvate. The structure is tetrameric with lithium and carbon atoms at alternating corners of a highly distorted cube. The lithium atoms form a tetrahedron and the carbons are associated with the faces of the tetrahedron. Each carbon is 2.33 A from the three neighboring lithium atoms and an ether molecule is coordinated to each lithium atom. Figures 7.2a and b show, respectively, the Li-C cluster and the complete array of atoms, except for hydrogen 45 Section 6.2 of Part A provides additional information on the structure of organolithium compounds. 

So far, only cuprates with a 1 1 copper/lithium ratio have been considered. Treatment of phenyllithium with various substoichiometric quantities of copper bromide in DMS as solvent afforded so-called higher order cuprates, of which two were characterizable by X-ray crystallography. These have the overall stoichiometries Cu2Li3Ph5(DMS)4 and Cu4Li5Ph9(DMS)4 [114, 115). The structure of the former compound in the solid state is shown in Fig. 1.26. 

R,R )-4,5-Bis(diphenylphosphinomethyl)-2,2-diphenyl-l,3-dioxolane (Compound 23). A solution of 12.8 mmol of lithium diphenylphosphide in 65 mL of THF was prepared using 2-chloropropane to destroy the phenyllithium. This mixture was stirred for 16 h at 25°C with the above bis-tosylate. Evaporation of the solvent followed by hydrolysis and extraction gave a product which crystallized from ethanol mp 135-137°C, [a] ° = -40.4°C (1, C6H6). 

There is no doubt that in these. reactions the nitrogen atom of the pyridine ring is complexed with either the lithium alkyl or aryl or with the lithium bromide which is usually present in many preparations of organolithium compounds. It has been established that, either in the presence of an excess of lithium bromide or in the total absence of this salt, phenyllithium still gives the same ortho .para ratio on reaction with 3-picoline.229 To account for the predominant formation of the 2,3-isomer in the reaction of CH3Li with 3-alkylpyridines, it was suggested261 that the transition states for these reactions were similar... 

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