N-Butyllithium

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The chemical compound n-butyllithium (abbreviated BuLi) is the most prominent organolithium reagent. It enjoys wide use as a polymerisation 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 organic synthesis, both industrially and in the laboratory.

Butyllithium is commercially available as solutions (15%, 25%, 2 M, 2.5 M, 10 M, etc.) in alkanes such as pentane, hexanes, or heptanes. Annual worldwide production and consumption of butyllithium and other organolithium compounds is estimated at 1800 metric tonnes.

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Handling properties

  Although it is a colourless liquid, n-butyllithium is usually encountered as a pale yellow solution in alkanes. Such solutions are stable indefinitely if properly stored,^[1] but in practice, they degrade upon aging. Fine white precipitate (lithium hydride) is deposited and the color changes to orange.

BuLi reacts violently with water:

C₄H₉Li + H₂O → C₄H₁₀ + LiOH

BuLi also reacts with CO₂ to give lithium pentanoate:

C₄H₉Li + CO₂ → C₄H₉CO₂Li

Structure and bonding

Main article: Organolithium reagent

n-BuLi exists as a cluster both in the solid state and in a solution in most solvents. The tendency to aggregate is common for organolithium compounds. The aggregates are held together by delocalized covalent bonds between lithium and the terminal carbon of the butyl chain.^[2] In the case of n-BuLi, the clusters are tetrameric (in ether) or hexameric (in cyclohexane). The tetrahedral cluster is a distorted cubane structure with Li and CH₂R groups at alternating vertices. An equivalent description describes the tetramer as a Li₄ tetrahedron interpenetrated with a tetrahedron [CH₂R]₄. Bonding within the cluster is related to that used to describe diborane, but more complex since eight atoms are involved. Reflecting its "electron-deficient character," n-butyllithium is highly reactive toward Lewis bases.

Due to the large difference between the electronegativities of carbon (2.55) and lithium (0.98), the C-Li bond is highly polarized. The charge separation has been estimated to be 55-95%. For practical purposes, n-BuLi can often be considered to react as the butyl anion, n-Bu⁻, and a lithium cation, Li⁺ even though this model is incorrect: n-BuLi is not ionic.

Preparation

The standard preparation for n-BuLi is reaction of butyl bromide or butyl chloride with Li metal:^[1]

2 Li + C₄H₉X → C₄H₉Li + LiX

where X = Cl, Br

The lithium for this reaction contains 1-3% sodium. Solvents used for this preparation include benzene, cyclohexane, and diethyl ether. When BuBr is the precursor, the product is a homogeneous solution, consisting of a mixed cluster containing both LiBr and LiBu. BuLi forms a weaker complex with LiCl, so that the reaction of BuCl with Li produces a precipitate of LiCl.

Reactions

Butyllithium is a strong base, but it is also a powerful nucleophile and reductant, depending on the other reactants. Furthermore, in addition to being a strong nucleophile, n-BuLi binds to aprotic Lewis bases, such as ethers and tertiary amines, which disagregate the clusters by binding to the lithium centers. Its use as a strong base is referred to as metalation. Reactions are typically conducted in tetrahydrofuran and diethyl ether, which are good solvents for the resulting organolithium derivatives (see below).

Metalation

Main article: Organolithium reagent

One of the most useful chemical properties of n-BuLi is its ability to deprotonate a wide range of weak Bronsted acids. t-Butyllithium and s-butyllithium are more basic still. n-BuLi can deprotonate (that is, metalate) many types of C-H bonds, especially where the conjugate base is stabilized by electron delocalization or one or more heteroatoms (non carbon atoms). Examples include acetylenes (H-CC-R), methyl sulfides (H-CH₂SR), thioacetals (H-CH(SR)₂, e.g. dithiane), methylphosphines (H-CH₂PR₂), and ferrocene (Fe(H-C₅H₄)(C₅H₅).^[3] The stability and volatility of the butane resulting from such deprotonation reactions is convenient.

LiC₄H₉ + R-H C₄H₁₀ + R-Li

The kinetic basicity of n-BuLi is affected by the reaction solvent or cosolvent. Ligands that complex Li⁺ such as tetramethylethylenediamine (TMEDA), hexamethylphosphoramide (HMPA), and 1,4-diazabicyclo[2.2.2]octane (DABCO) further polarize the Li-C bond and accelerate the metalation. Such additives can also aid in the isolation of the lithiated product, a famous example of which is dilithioferrocene.

Fe(C₅H₅)₂ + 2 LiC₄H₉ + 2 TMEDA → 2 C₄H₁₀ + Fe(C₅H₄Li)₂(TMEDA)₂

Schlosser's base is a superbase produced by treating butyllithium with potassium tert-butoxide. It is kinetically more reactive than butyllithium and is often used to accomplish difficult metalations. The butoxide anion complexes the lithium and effectively produces butylpotassium, which is more reactive than the corresponding lithium reagent.

An example of the use of n-butyllithium as a base is the addition of methyl carbonate to an amide, where n-butyllithium serves to deprotonate the amine:

n-BuLi + R₂NH + (MeO)₂CO → R₂N-CO₂Me + LiOMe + BuH

Halogen-lithium exchange

Butyllithium reacts with some organic bromides and iodides in an exchange reaction to form the corresponding organolithium derivative. The reaction usually fails with organic chlorides and fluorides:

C₄H₉Li + RX → C₄H₉X + RLi (X = Br, I)

This reaction is a useful method for preparation of several types of RLi compounds, particularly aryllithium and some vinyllithium reagents. The utility of this method is significantly limited, however, by the presence in the reaction mixture of n-BuBr or n-BuI, which can react with the RLi reagent formed, and by competing dehydrohalogenation reactions, in which n-BuLi serves as a base:

2C₄H₉Br + RLi → C₄H₉R + LiBr

2C₄H₉Li + R'CH=CHBr → C₄H₁₀ + R'C≡CLi + LiBr

These side reaction are significantly less important for RI than for RBr, since the iodine-lithium exchange is several orders of magnitude faster than the bromine-lithium exchange. For these reasons, aryl, vinyl and primary alkyl iodides are the preferred substrates, and t-BuLi rather than n-BuLi is usually used, since the formed t-BuI is immediately destroyed by the t-BuLi in a dehydrohalogenation reaction (thus requiring 2 equiv of t-BuLi). Alternatively, vinyl lithium reagents can be generated by direct reaction of the vinyl halide (e.g. cyclohexenyl chloride) with lithium or by tin-lithium exchange (see next section).^[1]

Transmetalations

A related family of reactions are the transmetalations, wherein two organometallic compounds exchange their metals. Many examples of such reactions involve Li exchange with Sn:

C₄H₉Li + Me₃SnAr → C₄H₉SnMe₃ + LiAr

where Ar is aryl and Me is methyl

The tin-lithium exchange reactions have one major advantage over the halogen-lithium exchanges for the preparation of organolithium reagents, in that the product tin compounds (C₄H₉SnMe₃ in the example above) are much less reactive towards lithium reagents than are the halide products of the corresponding halogen-lithium exchanges (C₄H₉Br or C₄H₉I). Other metals and metaloids which undergo such exchange reactions are organic compounds of mercury, selenium, and tellurium.

Carbonyl Additions

Organolithium reagents, including n-BuLi are used in synthesis of specific aldehydes and ketones. One such synthetic pathway is the reaction of an organolithium reagent with disubstituted amides:

R¹Li + R²CONMe₂ → LiNMe₂ + R²C(O)R¹

Carbolithiations

Butyllithium will add to certain activated terminal alkenes, such as styrene, butadiene or even ethylene itself to form a new organolithium reagent. This reaction is the basis for the commercially important use of butyllithium for the production of polystyrene and polybutadiene.

C₄H₉Li + CH₂=CH-C₆H₅ → C₄H₉-CH₂-CH(Li)-C₆H₅

Degradation of THF

THF is deprotonated by butyllithium, especially in the presence of TMEDA, by loss of one of four protons adjacent to oxygen. This process, which consumes butyllithium to generate butane, induces a reverse cycloaddition to give enolate of acetaldehyde and ethylene. Therefore, reactions of BuLi in THF are typically conducted at low temperatures, such as –78 °C, as is conveniently produced by dry ice in acetone. Higher temperatures (-25 °C or even -15 °C) are also used.

Thermal decomposition

When heated, n-BuLi, analogously to other alkyllithium reagents with "β-hydrogens", undergoes β-hydride elimination to produce an 1-butene and LiH:

C₄H₉Li + Δ LiH + CH₃CH₂CH=CH₂

Safety precautions

It is chemically sensible to store and handle all alkyl-lithiums in sealed systems under inert gas to prevent loss of activity and for reasons of safety. In particular tertiary-butyllithium is extremely reactive towards air and moisture, its hydrolysis being significantly exothermic enough to ignite the solvent (commercial sources typically using tetrahydrofuran, diethyl ether, or hexanes), and thus often inflaming upon exposure to the atmosphere. In some instances it may self-seal, e.g. in needles, preventing further access of oxygen due to hydroxide and oxide barriers being formed. This is analogous to oxide surface layers on the perceived stable metallic form of aluminum. While t-BuLi is classified as being spontaneously pyrophoric in an air atmosphere, n-BuLi is not, but to prevent degradation is still handled under a dry nitrogen atmosphere.

References

1. ^ ^{a b c} Brandsma, L.; Verkraijsse, H. D. "Preparative Polar Organometallic Chemistry I"; Springer-Verlag: Berlin, 1987.ISBN 3-540-16916-4.
2. ^ Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-29390-6
3. ^ Sanders, R.; Mueller-Westerhoff, U. T. (1996). "The Lithiation of Ferrocene and Ruthenocene - A Retraction and an Improvement". Journal of Organometallic Chemistry 512: 219-224. doi:10.1016/0022-328X(95)05914-B.

Further reading