Phenyllithium monomer

Figure 5 Deuterium-induced isotopic fingerprints in Li NMR spectra of partially deuteriated organolithium aggregates (a) phenyllithium monomer (THF/penta-methyldiethylenetriamine — 122°Q (b) phenyllithium dimer (Et20/tetramethylethyl-enediamine -111°C) (c) methylhthium tetramer (R=CHj EtjO, — 92 Q (d) fluxional phenyllithium tetramer (Et20, — 102 C). The measu H/ H isotope shifts for 5( Li) are 19.2, 10.4, 15.6, and 7.0 ppb, respectively. All systems were Li labelled and 50% of the organic ligands were perdeuterated v C Li) = 58.88 MHz...

Figure 13 Typical spin multiplets of the lithiated carbon from simple alkyllithium aggregates showing C, Li coupling (a) phenyllithium monomer (—100°C) (b) n-butyllithium dimer (—100°C) (c) /ert-butyllithium tetramer (—88°C) the coupling constants are 14.8, 7.9, and S.4Hz, respectively...

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. 

In the solid state NMR study, uncomplexed phenyllithium, assumed to be a tetramer, as well as the TMEDA complexed dimer and the PMDTA complexed monomer were investigated. Both Li and Li isotopes were used in the preparations. The C spectra of the complexes are presented in Figure 12. It is evident that the substitution of Li with Li has profound effects on the Unewidths, especially of the ipso-carbon at ca 180 ppm in the aggregated uncomplexed system (Figure 12a and 12b, respectively). This is in accordance with the previously mentioned study of methyllithium. However, even the other positions are affected by the dipolar couplings to the four quadrupolar lithium cations, but to a lesser extent due to the larger C-Li distances. 

With the aim to obtain additional benchmark values, a larger number of substituted phenyllithium complexes with known solid state structures were included in this study . They range from monomers of different solvation, over dimers and one trimer to different tetramers. The investigated aryl systems are shown in Scheme 2 and the obtained x values are reported in Table 7. 

Pure monomeric forms can be produced if one adds 1-3 equivalents of HMPA (Figure 2.17) to the organolithium compound in diethyl ether or better still in THF solution, since the basic oxygen of this additive is an excellent electron pair donor, as illustrated in Figure 10.3 using the formation of [phenyllithium(HMPA)(THF)2] monomer B as an example. 

Part, 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... 

TABLE 5. Thermodynamic (°) and Eyring activation (J) parameters for the interconversion of dimers, D, and monomers, M, of phenyllithium in THF-dg25 ... 

In THF, the alkyllithium compounds are aggregated [157] and the situation is reminiscent of the conditions in hydrocarbon solutions. At high concentrations, the association number (i. e. the number of molecules in the aggregate) decreases. This anomaly is explained by the existence of aggregate—solvent complexes, for example (MeLi)4 8THF Benzyllithium and its polymeric analogue polystyryllithium are not associated. Phenyllithium is mostly present as a dimer or monomer. Both forms are in equilibrium and are solvated. Only the monomeric form of the initiator is active. In practice, benzyllithium reacts only in the form of an ion pair. The fraction of the free benzyl anion must be very small [151c]. 

There is again an obvious correlation between reaction order and degree of association. The one quarter order for methyllithium could be explained satisfactorily by the usual assumption that the reactive species is methyllithium monomer in equilibrium with its tetramer. For phenyllithium a partial dissociation would lead to an order between one half and unity, as observed, if the dissociated product only were active. Other schemes involving some reactivity of both species would be equally plausible [101]. The first order behaviour with benzyllithium would require that the major reactive species is the ion-pair and not the free benzyl anion which must be present in small concentration. 

Phenyllithium dissolves in hexane by addition of TMEDA. Hie phenyllithium TMEDA adduct subsequently crystallizes out of solution as the dimer (113) corresponding to general structural type (16)." With diethyl ether solvation, phenyllithium exists as a solid tetramer (114). In ether solution PhLi is known to be either dimeric or tetrameric. Monomeric phenyllithium was successfully crystallized with PMDETA as the ligand. This monomer is depicted as (115). Note the difference in the coordination number of the carbanionic center in the monomer (115), the dimer (113), and the tetramer (114), i.e. one, two and three, respectively. 

Many ferrocenylsilanes can be polymerized by anionic initiators such as n-butyllithium, phenyllithium or ferrocenyllithium (Fig. 8.21). The reaction occurs at ambient temperature and affords living polymers. The utility of anionic polymerization is that the molecular weights can be controlled and also that block copolymers can be prepared. The main disadvantage of the anionic polymerization is that the monomer and the solvent should be rigorously purified and should be free of acidic impurities including water. Even traces of impurities can be detrimental [53]. 

Anionic polymerization of o-divinylbenzene was examined by Aso et al. [294]. The authors used n-BuLi, phenyllithium, and naphthalene/alkali metal in THF, ether, dioxane, and toluene at temperatures between —78 and 20 °C. Generally, it was found that as with radical and cationic initiators, a competition between cyclopolymerization and conventional 1,2-polymerization occurs, with the tendency for cyclization to be lower than with the other mechanisms. The polymerization initiated with the lithium organic compounds resulted in polymers with up to 92% double bonds per monomer unit (THF, 20 °C). Polymerization with lithium, potassium, and sodium naphthalene also showed a rather weak tendency for cyclization. In THF at 0°C and 20 °C the cyclization tendency increased with decreasing ionic radii of the counter cation, while in dioxane the reverse effect was observed, and in ether still another dependence was found (K > Li > Na). Nitadori and Tsuruta [299] used lithium diisopropyl amide in THF at 20 °C to polymerize m- and p-divinylbenzene. The authors obtained soluble products with molecular weight up to 100 000 g/mol (GPC) and showed the polymers to contain pendant double bonds by IR and NMR spectra. It seemed to be important that a rather large excess of free amine (the initiator was formed by reaction of -BuLi with excess diisopropylamine) was present in the polymerization mixture. In later studies [300,301] a closer view was taken on polymerization kinetics and the steric course of the polymerization reaction. 

Lithium [749,750,760-762] and sodium [750,760] organic compounds, lithium alcoholates [752,757,760-762], sodiomalonic diesters [755], complex bases from alkali imides and alcohols or alcoholates [756], phosphines [758,759], and others [751,753,754] have been used as initiators. It was found that with THF as solvent and fluorenyllithium or phenyllithium as initiator, molar mass is independent of initiator and monomer concentration. Relatively low masses of 2600 to 4200 were found. With DMF as solvent, the molecular mass increases with the monomer concentration at low (1.5mmol/L) initiator levels. With cyclopentadienyllithium or cyclopentadienyl sodium at high concentrations (68 mmol/L) and DMF as solvent, the molecular mass increases strongly with the monomer concentration. This is explained on the basis of a polyfunctionality of cyclopentadienyllithium and cyclopentadienyl sodium initiators. This view is supported by ozonolysis of the incorporated initiator, which leads to a decrease in the molar masses only of those polymers that were initiated by cyclopentadienyllithium or cyclopentadienyl sodium [750]. 

Phenyllithium is known to form, in equilibrium with the monomer, a dimer 25 (Scheme 1-21) held together by electron-deficient partial bonds. However, phenyllithium can also adopt the structure of a lithium lithiate ion pair 26 (Scheme 1-21). What makes the difference The solvent plays a capital role. As long as the solvation forces remain moderate as in diethyl ether, the doubly carbon-lithium-carbon bridged dimer 25 is energetically most favorable. In a 2 1 mixture of cylohexane and diethyl ether, phenyllithium even assembles a tetrameric cluster. " " in neat THF, the four-centered dimer 25 is found in equilibrium with the monomer. " However, in the presence of the powerful electron-donor hexamethylphosphoric triamide (HMPA), it is the ion pair 26 that coexists with the monomer. Analogous lithiate complexes have been identified with... 

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