Methyllithium

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Methyllithium
Skeletal formula of tetrameric methyllithium with all implicit hydrogens shown
Names
IUPAC name
Methyllithium
Other names
Lithium methanide
Identifiers
3D model (
JSmol
)
3587162
ChEBI
ChemSpider
ECHA InfoCard
 100.011.843 Edit this at Wikidata
EC Number
  • 213-026-4
288
  • InChI=1S/CH3.Li/h1H3; checkY
    Key: DVSDBMFJEQPWNO-UHFFFAOYSA-N checkY
  • [Li]C
Properties
CH3Li
Molar mass 21.98 g·mol−1
Reacts
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 pyrophoric
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 3: Liquids and solids that can be ignited under almost all ambient temperature conditions. Flash point between 23 and 38 °C (73 and 100 °F). E.g. gasolineInstability 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
3
3
2
W
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Methyllithium is the simplest

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

Synthesis

In the direct synthesis,

methyl bromide is treated with a suspension of lithium in diethyl ether
.

2 Li + MeBr → LiMe + LiBr

The

dioxane to precipitate LiBr(dioxane), which can be removed by filtration.[2] The use of halide-free vs LiBr-MeLi has a decisive effect on some syntheses.[3]

Reactivity

Methyllithium is both strongly

THF at room temperature, and solutions in ether are indefinitely stable. 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 and more reactive.

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 methylmagnesium halides, which are often equally effective, and less expensive or more easily prepared in situ.

It also reacts with carbon dioxide to give Lithium acetate:

CH3Li + CO2 → CH3CO2−Li+

alkyl halides and alkyl sulfonates, as well as for conjugate additions to α,β-unsaturated carbonyl compounds by methyl anion.[4] Many other transition metal methyl compounds have been prepared.[5]

ZrCl4 + 6 MeLi → Li2ZrMe6 + 4 LiCl

Structure

Two structures have been verified by single crystal

NMR spectroscopy. The tetrameric structure is a distorted cubane-type cluster, 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.[6]

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

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

Colour code: Li- purple 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[7] 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

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.[7]

References

  1. ^ Lusch, M. J.; Phillips, W. V.; Sieloff, R. F.; Nomura, G. S.; House, H. O. (1984). "Preparation of Low-Halide Methyllithium". Organic Syntheses. 62: 101; Collected Volumes, vol. 7, p. 346.
  2. doi:10.1021/ja971830o
    .
  3. S2CID 224930545
    .
  4. ISBN 9780471264187
    .
  5. doi:10.1021/ja00193a061
    .
  6. ISBN 978-3-527-29390-2
    .
  7. ^
    doi:10.1021/ja01565a024
    .
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