Methyllithium

Methyllithium is an organolithium reagent with the empirical formula CH₃Li. This s-block organometallic compound adopts an oligomeric structure both in solution and in the solid state. This highly reactive compound, invariably used as a solution in alkanes or ethers, is a reagent in organic synthesis as well as organometallic chemistry. Operations involving methyllithium require anhydrous conditions, because the compound is highly reactive toward water. Oxygen and carbon dioxide are also incompatible with MeLi. Methyllithium is usually not prepared, but purchased as a solution in various ethers.

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Synthesis

In the direct synthesis, methyl bromide is treated with a suspension of lithium in diethyl ether.

2 Li + MeBr → LiMe + LiBr

The lithium bromide forms a complex with the methyllithium. Most commercially available methyllithium consists of this complex. "Halide-free" methyllithium is prepared from methyl chloride.^[1] Lithium chloride precipitates from the diethyl ether since it does not form a strong complex with methyllithium. The filtrate consists of fairly pure methyllithium.

Reactivity

Methyllithium is both strongly basic and highly nucleophilic due to the partial negative charge on carbon and is therefore particularly reactive towards electron and proton donors. THF, usually a chemically inert solvent, is attacked by MeLi. Water and alcohols react violently. Most reactions involving methyllithium are conducted below room temperature. Although MeLi can be used for deprotonations, n-butyllithium is more commonly employed since it is less expensive, more reactive and less dangerous.

Methyllithium is mainly used as the synthetic equivalent of the methyl anion synthon. For example, ketones react to give tertiary alcohols in a two-step process:

Ph₂CO + MeLi → Ph₂C(Me)OLi

Ph₂C(Me)OLi + H⁺ → Ph₂C(Me)OH + Li⁺

Nonmetal halides are converted to methyl compounds with methyllithium:

PCl₃ + 3 MeLi → PMe₃ + 3 LiCl

Such reactions more commonly employ the Grignard reagents methyl magnesium halides, which are less dangerous than MeLi, equally effective, and more conveniently prepared in situ.

Transition metal methyl compounds are typically prepared from methyllithium:^[2]

ZrCl₄ + 6 MeLi → Li₂ZrMe₆ + 4 LiCl

Structure

Two structures have been verified by single crystal X-ray crystallography as well as by ⁶Li, ⁷Li, and ¹³C NMR spectroscopy. The tetrameric cluster consists of a distorted cubane, with carbon and lithium atoms at alternate corners. The Li---Li distances are 2.68 Å, almost identical with the Li-Li bond in gaseous dilithium. The C-Li distances are 2.31 Å. Carbon is bonded to three hydrogen atoms and three Li atoms. The nonvolatility of (MeLi)₄ and its insolubility in alkanes results from the fact that the clusters interact via further inter-cluster agostic interactions. In contrast the bulkier cluster (tertiary-butylLi)₄, where intercluster interactions are precluded by steric effects, is volatile as well as soluble in alkanes.^[3]

Colour code: Li- blue C- black H- white

The hexameric form features hexagonal prisms with Li and C atoms again at alternate corners.

Colour code: Li- blue C- black H- white

The degree of aggregation, "n" for (MeLi)_n, depends upon the solvent and the presence of additives (such as lithium bromide). Hydrocarbon solvents such as benzene^[4] favour formation of the hexamer, whereas ethereal solvents favour the tetramer.

Bonding

These clusters are considered "electron-deficient," that is, they do not follow the octet rule because the molecules lack sufficient electrons to form four 2-centered, 2-electron bonds around each carbon atom, in contrast to most organic compounds. The hexamer is a 30 electron compound (30 valence electrons.) If one allocates 18 electrons for the strong C-H bonds, 12 electrons remain for Li-C and Li-Li bonding. There are six electrons for six metal-metal bonds and one electron per methyl-η³ lithium interaction.

The strength of the C-Li bond has been estimated at around 57 kcal/mol from IR spectroscopic measurements.^[4]

References