Tetramers methyllithium

Methyllithium. MethyUithium [917-54 ] CH Li, crystallizes from benzene or hexane solution giving cubic crystals that have a salt-hke constitution (128). Crystalline methyllithium molecules exist as tetrahedral tetramers (129). Solutions of methyllithium are less reactive than those of its higher homologues. Methyllithium is stable for at least six months in diethyl ether at room temperature. A one-molar solution of methyllithium in tetrahydrofuran (14 wt %) and cumene (83 wt %) containing 0.08 M dimethyknagnesium as stabilizer loses only 0.008% of its activity per day at 15°C and is nonpyrophoric (117). 

As documented in detail for organolithium species, ligand and donor play a key role in determining the degree of aggregation. Methyllithium adopts a hexameric structure in hydrocarbon solvents.13,15 In the presence of monodentate, donors such as THF or diethyl ether tetramers are observed, while the increase in donor denticity to 2 (1,1-Dimethoxyethane (DME), N,N,N, N -Tetramethylethylenediamine (TMEDA)) affords monomeric structures. Further documenting the differences between solution and solid states, [CH3Li]4 adopts a tetrameric structure in the latter.15,15a-15c... 

FIGU RE 12.1 The structure of the tetramer of methyllithium. Only two of the four methyl groups are shown. 

Moore and Moser [44] obtained 38 and the tetramers 39 of 6 as products of the reaction of 35 with methyllithium in diethyl ether. The best yield of 38 (55%) resulted at 35 °C, whereas only a trace of 38 was observed at -80 °C, at which temper-... 

Tetrameric [MeLi]4 (1), [EtLi]4 (2) and [t-BuLi]4 (3) are white pyrophoric powders. While methyllithium is soluble only in polar solvents like diethyl ether, the two others are soluble even in non-polar hydrocarbons like hexane. In non-donating solvent the tetrameric aggregation is retained. Each of the four U3 triangles is /ra-capped by a Ca atom above the center of the equilateral metal triangle. Even in the solid-state none of the three tetramers adopts ideal symmetry (Figure 6). 

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. 

However, a detailed reexamination within the framework of Kohn-Sham MO theory leads us to a view that differs in a number of ways.51 In the following, we show that the C—Li bond in CH3Li (25) may very well be envisaged as an electron pair bond (24b), although a rather polar one, of course. But our point is not just the shift of the bonding picture back to the more covalent side of the 24a-24b spectrum. In particular, if we go to the oligomers (e.g., the methyllithium tetramer 27), a fundamentally new phenomenon occurs in the C—Li bonding mechanism. This phenomenon emphasizes the presence of dis-... 

Molecular structure of (a) methyllithium tetramer (Li4Me4) (H atoms have been omitted) and (b) vinyllithium-THF tetramer (LiHC=CH2-THF)4 (THF ligands have been omitted). 

The X-ray crystal structures of lithiated sulfoximines were reported in 1986/87 by Gais. Lithiated (S)-lV,S-dimethyl-S-phenylsulfoximine crystallized as its tetra-methylethylenediamine (tmeda) complex as a chiral tetramer of structure [(S)-N-methyl-5-phenylsulfonimidoyl)methyllithium]4-2(tmeda) with approximately C2 symmetry.41 Two of the lithium cations of the tetramer were coordinated to a tmeda molecule and the O atoms of two different carbanionic species. The other two lithium cations were found to be coordinated to the N atoms of three different sulfonimidoyl carbanionic species and to one C atom (the a-carbon) of each of these carbanionic species. These lithium cations were thus found to form four-mem-bered chelate rings involving the atoms, Li-Ca-S-N. A later study was successful... 

The NMR spectra of organolithium derivatives have received considerable attention. At — 60°C, the Li NMR spectrum of 57% C-enriched methyllithium shows a pattern due to lithium-carbon coupling which can only readily be explained in terms of a tetramer [LiMe]4 with three methyl groups attached to each lithium. The chemical shift of the methyl group determined by INDOR is —16 ppm in tetrahydrofuran. It was concluded that the shift is due to 0.1 electrons on each methyl group and the bond is predominantly covalent (151,152). A similar result is found for [(Mea C)Li]4 and — Li) was... 

Structurally similar tetramers are maintained in solutions with electronic donors (THF or TMEDA). For example, the same core structure exists for the methyllithium TMEDA adduct. Ab initio methods have been used to study the aggregation of various methyl lithium oligomers in the presence of various donor ligands. The heavier alkali metal (M = K, Rb, Cs) methyl compounds have more ionic... 

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. 

Another example of a solvated, cubic tetramer is methyllithium— TMEDA (46). In this example an aggregate of composition [(MeLi)4-2TMEDA]n with almost ideal Td symmetry crystallized from an ethereal solution of methyllithium and TMEDA at room temperature. Hiis material consists of infinitely long chains of cubic tetramer linked by TMEDA molecules. Since TMEDA usually has a strong preference for formation of a chelate ring with a single lithium atom, it is somewhat unusual that such an intramolecular chelate is not observed here. 

This discussion of aliphatic carbanion structures has included mainly organolithium compounds simply because the structures of most aliphatic caibanions incorporate lithium as the counterion and also because this alkali metal cation is the most widely used by synthetic organic chemists. For comparison the entire series of Group la methyl carbanion structures, i.e. MeNa, MeK, MeRb and MeCs, have been determined. Methylsodium was prepared by reaction of methyllithium with sodium r-butoxide. Depending upon the reaction conditions, the products obtained by this procedure contain variable amounts of methyllithium and methylsodium (Na Li atom ratios from 36 1 to 3 1). Hie crystal structure of these methylsodium preparations resembles the cubic tetramer (38) obtained for methyllithium with the Na— Na distances of 3.12 and 3.19 A and Na—C distances of 2.58 and 2.64 A. 

The first reported polylithium compound, dilithiomethane, is obtained in 6% yield by the reaction of dibromethane with lithium. Higher yields are obtained by pyrolyzing methyllithium tetramer in the solid state. This procedure is improved by conducting the reaction under vacuum with rapid stirring . The temperature must be carefully monitored because heating above 250°C leads to rearrangement to secondary products. Furthermore, better results are obtained with methyllithium that is not complexed with LiBr. 

Both 6Li and 7Li possess magnetic moments and electric quadrupole moments. Carbon-13 coupling to the most abundant isotope, 7Li (92-58%, / = 3/2), has been observed only in the methyllithium tetramer. (21) In (LiCH3)4, /(C-7Li) = +15 Hz, much larger than the value of 0-77 Hz calculated by Cowley and White. (22)... 

Cyclohexa-1,2-diene (76), generated by debromination of 6,6-dibromobicyclo[3.1.0]hexane (75) with methyllithium in the absence of a trapping agent, gave the analogous dimer 77 and a tetramer. ... 

Many organolithium compounds are soluble in hydrocarbons exceptions are methyllithium and phenyllithium w hich are associated in these solvents. Butyllithium is mostly hexameric and /c/Y-butyllithium is tetrameric in cyclohexane. A Lewis basic solvent can interact with an organolithium oligomer, thereby decreasing the degree of association. Thus, methyllithium. which is tetrameric in the solid phase, becomes a solvated tetramer in ether, and BuLi. hexameric in hydrocarbons, becomes tetrameric in ether. In the more basic THE BuLi has a degree of association between dimeric and tetrameric at -108 °C, and phenyllithium is between monomeric and dimeric [3]. 

TMED produces a crystalline complex with methyllithium tetramer, but even a large excess of TMED does not break down the tetramer to dimer or monomer (8). This result shows that methyl bridges are stronger than tertiary amine complexation to lithium. 

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