Methyllithium

Last updated
Methyllithium
MeLi.png
Names
IUPAC name
Methyllithium
Other names
Lithium methanide
Identifiers
3D model (JSmol)
3587162
ChEBI
ChemSpider
ECHA InfoCard 100.011.843 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 213-026-4
288
PubChem CID
  • InChI=1S/CH3.Li/h1H3; Yes check.svgY
    Key: DVSDBMFJEQPWNO-UHFFFAOYSA-N Yes check.svgY
  • [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.svgHealth 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).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Methyllithium is the simplest 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 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.

Contents

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. Alternatively, commercial methyllithium can be treated with 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 basic and highly nucleophilic due to the partial negative charge on carbon and is therefore particularly reactive towards electron acceptors and proton donors. In contrast to n-BuLi, MeLi reacts only very slowly with 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 → CH3CO2Li+

Transition metal methyl compounds can be prepared by reaction of MeLi with metal halides. Especially important are the formation of organocopper compounds (Gilman reagents), of which the most useful is lithium dimethylcuprate. This reagent is widely used for nucleophilic substitutions of epoxides, 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 X-ray crystallography as well as by 6Li, 7Li, and 13C 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]

Tetramer1.jpg Methyllithium-tetramer-1-3D-balls.png Methyllithium-tetramer-2-3D-balls.png

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

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

Hexamer1.jpg Methyllithium-hexamer-2-3D-balls.png Methyllithium-hexamer-3-3D-balls.png

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 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. [7]

Related Research Articles

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The haloalkanes are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have also been shown to be serious pollutants and toxins. For example, the chlorofluorocarbons have been shown to lead to ozone depletion. Methyl bromide is a controversial fumigant. Only haloalkanes that contain chlorine, bromine, and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory may have activity as greenhouse gases. Methyl iodide, a naturally occurring substance, however, does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone layer depleter. For more information, see Halomethane. Haloalkane or alkyl halides are the compounds which have the general formula "RX" where R is an alkyl or substituted alkyl group and X is a halogen.

<span class="mw-page-title-main">Gilman reagent</span> Class of chemical compounds

A Gilman reagent is a diorganocopper compound with the formula Li[CuR2], where R is an alkyl or aryl. They are colorless solids.

In organic chemistry, a carbanion is an anion in which carbon is negatively charged.

<span class="mw-page-title-main">Organolithium reagent</span> Chemical compounds containing C–Li bonds

In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.

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<i>n</i>-Butyllithium Chemical compound

n-Butyllithium C4H9Li (abbreviated n-BuLi) is an organolithium reagent. It is widely used as a polymerization initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene (SBS). Also, it is broadly employed as a strong base (superbase) in the synthesis of organic compounds as in the pharmaceutical industry.

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<span class="mw-page-title-main">Phenyllithium</span> Chemical compound

Phenyllithium is an organometallic agent with the empirical formula C6H5Li. It is most commonly used as a metalating agent in organic syntheses and a substitute for Grignard reagents for introducing phenyl groups in organic syntheses. Crystalline phenyllithium is colorless; however, solutions of phenyllithium are various shades of brown or red depending on the solvent used and the impurities present in the solute.

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Organozinc chemistry is the study of the physical properties, synthesis, and reactions of organozinc compounds, which are organometallic compounds that contain carbon (C) to zinc (Zn) chemical bonds.

<span class="mw-page-title-main">Organocopper chemistry</span> Compound with carbon to copper bonds

Organocopper chemistry is the study of the physical properties, reactions, and synthesis of organocopper compounds, which are organometallic compounds containing a carbon to copper chemical bond. They are reagents in organic chemistry.

<span class="mw-page-title-main">Group 2 organometallic chemistry</span>

Group 2 organometallic chemistry refers to the chemistry of compounds containing carbon bonded to any group 2 element. By far the most common group 2 organometallic compounds are the magnesium-containing Grignard reagents which are widely used in organic chemistry. Other organometallic group 2 compounds are rare and are typically limited to academic interests.

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<span class="mw-page-title-main">Phenylsodium</span> Chemical compound

Phenylsodium C6H5Na is an organosodium compound. Solid phenylsodium was first isolated by Nef in 1903. Although the behavior of phenylsodium and phenyl magnesium bromide are similar, the organosodium compound is very rarely used.

In organometallic chemistry, metal–halogen exchange is a fundamental reaction that converts an organic halide into an organometallic product. The reaction commonly involves the use of electropositive metals and organochlorides, bromides, and iodides. Particularly well-developed is the use of metal–halogen exchange for the preparation of organolithium compounds.

<span class="mw-page-title-main">(Trimethylsilyl)methyllithium</span> Chemical compound

(Trimethylsilyl)methyllithium is classified both as an organolithium compound and an organosilicon compound. It has the empirical formula LiCH2Si(CH3)3, often abbreviated LiCH2tms. It crystallizes as the hexagonal prismatic hexamer [LiCH2tms]6, akin to some polymorphs of methyllithium. Many adducts have been characterized including the diethyl ether complexed cubane [Li43-CH2tms)4(Et2O)2] and [Li2(μ-CH2tms)2(tmeda)2].

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. Holland, Patrick L.; Smith, Michael E.; Andersen, Richard A.; Bergman, Robert G. (1997). "X-ray Crystal Structures of Cp*Ni(PEt3)X [X = Br, O(p-C6H4Me), NH(p-C6H4Me), S(p-C6H4Me), OCH3, CH2C6H5, Me, H, PEt3+]. Understanding Distortions and Trans Influences in Cyclopentadienyl Complexes". Journal of the American Chemical Society. 119 (52): 12815–12823. doi:10.1021/ja971830o.
  3. Göttker-Schnetmann, Inigo; Mecking, Stefan (2020). "A Practical Synthesis of (tmeda)Ni(CH3)2, Isotopically Labeled (tmeda)Ni(13CH3)2, and Neutral Chelated-Nickel Methyl Complexes". Organometallics. 39 (18): 3433–3440. doi:10.1021/acs.organomet.0c00500. S2CID   224930545.
  4. Lipshutz, B. H.; Sengupta, S. (1992). "Organocopper Reagents: Substitution, Carbo/Metallocupration, and Other Reactions". Organic Reactions . Vol. 41. pp. 135–631. doi:10.1002/0471264180.or041.02. ISBN   9780471264187.
  5. Morse, P. M.; Girolami, G. S. (1989). "Are d0 ML6 Complexes Always Octahedral? The X-ray Structure of Trigonal-Prismatic [Li(tmed)]2[ZrMe6]". Journal of the American Chemical Society. 111 (11): 4114–4116. doi:10.1021/ja00193a061.
  6. Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN   978-3-527-29390-2.
  7. 1 2 Brown, T. L.; Rogers, M. T. (1957). "The Preparation and Properties of Crystalline Lithium Alkyls". Journal of the American Chemical Society. 79 (8): 1859–1861. doi:10.1021/ja01565a024.