tert-Butyllithium

''tert''-Butyllithium is a chemical compound with the formula (CH₃)₃CLi. As an organolithium compound, it has applications in organic synthesis since it is a strong base, capable of deprotonating many carbon molecules, including benzene. ''tert''-Butyllithium is available commercially as hydrocarbon solutions; it is not usually prepared in the laboratory.

Preparation

''tert''-Butyllithium is produced commercially by treating ''tert''-butyl chloride with lithium metal. Its synthesis was first reported by R. B. Woodward in 1941.

Structure and bonding

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Like other organolithium compounds, ''tert''-butyllithium is a cluster compound. Whereas ''n''-butyllithium exists both as a hexamer and a tetramer, ''tert''-butyllithium exists exclusively as a tetramer with a cubane structure. Bonding in organolithium clusters involves sigma delocalization and significant Li−Li bonding. Despite its complicated structure, ''tert''-butyllithium is usually depicted in equations as a monomer.

The lithium–carbon bond in ''tert''-butyllithium is highly polarized, having about 40 percent ionic character. The molecule reacts like a carbanion, as is represented by these two resonance structures.

Reactions

''tert''-Butyllithium is renown for deprotonation of carbon acids (C-H bonds). One example is the double deprotonation of allyl alcohol. Other examples are the deprotonation of vinyl ethers.

In combination with ''n''-butyllithiium, ''tert''-butylllithium monolithiates ferrocene. ''tert''-Butyllithium deprotonates dichloromethane:
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Similar to ''n''-butyllithium, ''tert''-butyllithium can be used for lithium-halogen exchange reactions.

Solvent compatibility

To minimize degradation by solvents, reactions involving ''tert''-butyllithium are often conducted at very low temperatures in special solvents, such as the Trapp solvent mixture.

Moreso than other alkyllithium compounds, ''tert''-butyllithium reacts with ethers. In diethyl ether, the half-life of ''tert''-butyllithium is about 60 minutes at 0 °C. It is even more reactive toward tetrahydrofuran (THF), the half-life in THF solutions is about 40 minutes at −20 °C. In dimethoxyethane, the half-life is about 11 minutes at −70 °C
In this example, the reaction of ''tert''-butyllithium with (THF) is shown:

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Safety

''tert''-butyllithium is a pyrophoric substance, meaning that it spontaneously ignites on exposure to air. Air-free techniques are important so as to prevent this compound from reacting violently with oxygen and moisture:

:''t''-BuLi + O₂ → ''t''-BuOOLi
:''t''-BuLi + H₂O → ''t''-BuH + LiOH

The solvents used in common commercial preparations are themselves flammable. While it is possible to work with this compound using cannula transfer, traces of ''tert''-butyllithium at the tip of the needle or cannula may ignite and clog the cannula with lithium salts. While some researchers take this "pilot light" effect as a sign that the product is "fresh" and has not degraded due to time or improper storage/handling, others prefer to enclose the needle tip or cannula in a short glass tube, which is flushed with an inert gas and sealed at each end with septa. Serious laboratory accidents involving ''tert''-butyllithium have occurred. For example, in 2008 a staff research assistant, Sheharbano Sangji, in the lab of Patrick Harran at the University of California, Los Angeles, died after being severely burned by a fire ignited by ''tert''-butyllithium.''Los Angeles Times'', 2009-03-01
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Large-scale reactions may lead to runaway reactions, fires, and explosions when ''tert''-butyllithium is mixed with ethers such as diethyl ether, and tetrahydrofuran. The use of hydrocarbon solvents may be preferred.

References

{{DEFAULTSORT:Butyllithium, t-
Lithium compounds
Organolithium compounds
Reagents for organic chemistry
Superbases
Tert-butyl compounds