Methyl lithium dimers

An attempted synthesis, Scheme 65, of a heterozincate from dimethylzinc and bis (2-pyridyl)methyl lithium gave instead the dimeric [bis (2-pyridyl)methyl methylzinc] complex 84, shown in Figure 43.143 The intermolecular zinc-carbon bond to the bridging carbon atoms is remarkably short (2.269(3) A), while the zinc-methyl bond is slightly elongated (1.974(3) A). 

That the allene route should always be kept in mind, though, is demonstrated for example by 6,6-dibromobicyclo[3.1. OJoctane and 8,8-dibromo-bicyclo[5.1.0]-octane, respectively. When these bicyclic dibromides are reacted with methyl-lithium at room temperature they are evidently converted into the corresponding cyclic allenes since these intermediates may be either trapped by reagents like styrene or dimerize, [2+2]adducts being formed in both cases. 

Summaiy Deprotonation of l-hydroxyalkyltris(tnmethylsilyl)silanes, (Me3Si)3Si-C(OH)r R (1) with methyl lithium in ether at low temperature leads to transient silenes, (Me3Si)2Si=CR R (2a R = R = Me 2b R = H, R = u 2c R = H R = Mes), which dimerize in absence of trapping agents to give l-isopropenyl-2-isopropyl-l,l,2,2-tetrakis-(trimethylsilyl)disilane 3, ( )-3,4-di-tbutyl-1,1,2,2-tetrakis(trimethylsilyl)-1,2-disilacyclo-... 

The synthesis, structures, and reactions of silicon-substituted alkyl derivatives of metals from groups 1, 2, and 3 have been recently reviewed. A diverse range of structures has been discovered with the alkali metals. More recently, some additional structures and an important discussion of structural trends observed among the silicon-substituted alkyl derivatives of the alkali metals have also been published. " Tris(trimethylsilyl)methyl lithium forms a solvent-free dimer, as well as several ate complexes in the presence of bases (THF or TMEDA). In the alkyl-bridged dimer of tris(trimethylsilyl)methyl lithium (2), there are interactions between the C-H bonds of the silyl ligand and the lithium atoms (3) (Li to C distances of 254.1(7) and 246.6(6) pm). ... 

The difficulties of direct oxidative insertion with metals other than Mg or Li mean that o-complexes are often made from organo-lithium or Grignard reagents by metal exchange. This reaction amounts to a nucleophilic substitution at the metal without a change of oxidation state so the metal is used in whatever oxidation state is finally needed. Attack of methyl lithium on a Cu(I) halide gives methyl copper 50, a o-complex of Me- and Cu(I). If an excess of MeLi is present an ate complex is formed, lithium dimethylcuprate 51. This is formally a compound of a copper anion 51a, just as BF4 is a borate. The term ate complex refers to such formally anionic complex in which the metal has one extra anionic ligand. Its true structure is dimeric 51b and it exists as an equilibrium with 52 in solution.20... 

In an attempt to perform an addition of an allyl anion to a reactive alkene, syn-1-allyloxynorbornene was treated with methyl-lithium. After hydrolysis (413) and (414) were obtained in the ratio 3 7. It is concluded that (413) does not arise by a concerted [2 + 4] cycloaddition of (410) followed by cleavage of the initial alkoxy carbanion (41IX but rather a stepwise process via (412) leads to the two products. Treatment of cycloheptatriene with potassium amide gives dimers (417) (70%) and (418) (14%). To determine whether the reaction involves addition of cycloheptatriene to cycloheptatrienyl anion (415) or to the adduct ion (416) the experiment was performed with 1,2,3,4,5,6-hexadeuteriocycloheptatriene. The isotope distri-... 

Cyclonona-l,2,5-triene, its 7-methoxy derivative, and cyclonona-l,2,5,7-tetraene, have been prepared by treatment of 9,9-dibromobicyclo[6,l,0]non-3-ene, its 5-methoxy derivative, and 9,9-dibromobicyclo[6,l,0]nona-3,5-diene, respectively, with methyl-lithium. In contrast, 9,9-dibromobicyclo[6,l,0]nona-2,6-diene undergoes transannular C—H carbene insertion however, cycIodeca-l,2,5,6-tetraene was prepared as a mixture of diastereoisomers from the bisdibromocarbene adduct of cyclo-octa-1,4-diene. Cyclonona-l,2,5,7-tetraene was found to dimerize rapidly. ... 

Convenient new routes to tricyclo[4,1,0,0 ]hept-3-ene and its derivatives have been disclosed. Acetone-sensitized irradiation of bicyclo[3,2,0]hept-6-en-2-one affords the ketone (624), whose enol phosphate is reduced by lithium-ammonia to give the parent alkene (625). " A second route is also described, starting from the 7,7-di-bromonorcarane derivatives (626) and (627). Reaction of (626) with methyl-lithium in ether at 0°C afforded a 3 1 mixture of the tricycloheptenes (628) and (629) similar reaction of (627) gave (630 40%), but reactions of the parent dibromide were unsuccessful. Catalytic Ag ion causes the rearrangement of (628) and (629) to, respectively, 3-methyl- and 1-methyl-cycloheptatriene. Initial Ag" attack at the least hindered edge bond is implicated. Attempted preparation of the tetrahedrane dimer (631) by the addition of dibromocarbene to homobenzvalene followed by treatment of the adduct so obtained with excess methyl-lithium in ether at 0°C afforded instead 5-ethynyl-cyclohexa-1,3-diene. 

Phenyl-1-naphthylsulphine reacted with methyl-lithium to give an 80% yield of the sulphoxide (93) together with 10% of the olefin, a dimer of... 

For a more detailed look at this reaction, iodomethane reacts with lithium metal, which is assumed to exist as a simple dimer (Li-Li). The products are lithium iodide and CHgLi (methyllithium, 33). When the lithium dimer comes close to the C-I bond of iodomethane, the polarized C-I bond induces a polarized Li-Li structure (an induced dipole) and the transition state of the reaction is taken to be 31. Rather than transferring two electrons, the Li-Li bond breaks with transfer of only one electron (homolytic cleavage remember that Li is in group 1), which leads to formation of a methyl radical ( 0113) and a lithium radical ( Li), as well as a lithium cation and an iodine anion (see 32). Transition state 31 represents the transfer of single electrons to generate radicals. When the methyl radical and the lithium radical combine, each donates... 

Recendy, the potential for beryllium insertion into C—N bonds of NHCs has been investigated by Arrowsmith et al. [42]. Reaction of 2 equivalents of methyl lithium with 1 equivalent of both ber)dlium chloride and the NHC (IPr) resulted in the formation of the methylberjdliumhydride-IPr dimer (Figure 15.12), similar to that of the diphenylber)dlium-NHC complex mentioned earlier. 

Several research groups are exploring synthetic routes to tetra-t-butylethylene, regarded as the ultimate sterically-hindered olefin, and a number of new tied back relatives have been described. Warner and Jacobson and their co-workers have shown that the gem-dibromocyclopropane (8) undergoes carbene dimerization on treatment with methyl-lithium to give, albeit in low yield, the olefin (9). 

An alternative interpretation of these fractional kinetic orders in alkyl-lithium concentration as proposed by Wakefield [19] is that the rate-determining step involves coordination of a DPE molecule to one face of the polyhedral organolithium aggregate. As suggested previously [8], incomplete or stepwise dissociation equilibria such as those shown in Scheme 3 would be expected to require less energy as predicted by theoretical calculations [32]. It is important to note that Brown and coworkers [34, 35] have reported that dissociation energies for tetramer-dimer equilibria are 46.1 kj/mol and lOOkJ/mol for methyl-lithium in ether [34] and ferf-butyllithium in cyclopentane [35], respectively. 

Similar treatments of Cl3(0)Mo, (EtO)3MoCl2, and (EtO)2MoCl3 with methyl-lithium afford the corresponding molybdenum-methylidenes 56b, 56c, and S6d, which also methylenate aldehydes [122]. As regards the structure of 56b, later investigations indicated it to be the dimeric l,3-dioxo-l,3-dimolybdenum(V) cydo-butane complex, similar to S6a [124]. The reactivity order of these reagents for... 

As reported last year, 6,6-dibromobicyclo [3,1,0] hexane (603) with methyl-lithium gives cyclohexa-1,2-diene, which has but a transient existence before it either dimerizes or is trapped intermolecularly. The dimers, prepared from mono- and di-deuterio-(603) have been made and investigated by Fourier-transform n.m.r. spectroscopy. The vinyl lallyl deuterium ratio in the dimer from [ H]-(603) was found to be 1.04 and this corresponds to for the formation of the doubly allylic C—C bond in (604). Although the communication illustrates an interesting technique, it does not lead to a firm mechanistic conclusion about the nature of the allene dimerization process. 

Fig. 7.3. Crystal structures of some lithium etiolates of ketones. (A) Unsolvated hexameric enolate of methyl t-butyl ketone (B) tetrahydrofuran solvate of tetramer of enolate of methyl r-butyl ketone (C) tetrahydrofuran solvate of tetramer of enolate of cyclopentanone (D) dimeric enolate of 3,3-dimethyl-4-(r-butyldimethylsiloxy)-2-pentanone. (Structural diagrams are reproduced from Refs. 66-69.) by permission of the American Chemical Society and Verlag Helvetica Chimica Acta AG.

Fig. 1.6. Crystal structure of dimer of lithium salt of N-phenylimine of methyl -butyl ketone. Two molecules of diethyl ether are present. Reproduced from J. Am. Chem. Soc., 108, 2462 (1986), by permission of the American Chemical Society. 

Spectroscopic investigations of the lithium derivatives of cyclohexanone (V-phenylimine indicate that it exists as a dimer in toluene and that as a better donor solvent, THF, is added, equilibrium with a monomeric structure is established. The monomer is favored at high THF concentrations.110 A crystal structure determination was done on the lithiated A-phenylimine of methyl r-butyl ketone, and it was found to be a dimeric structure with the lithium cation positioned above the nitrogen and closer to the phenyl ring than to the (3-carbon of the imine anion.111 The structure, which indicates substantial ionic character, is shown in Figure 1.6. 

---