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 solutions in hydrocarbons (such as pentane); it is not usually prepared in the laboratory.
Preparation
tert-Butyllithium is produced commercially by treating tert-butyl chloride with lithium. Its synthesis was first reported by R. B. Woodward in 1941.[1]
Structure and bonding
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.[2] 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:[3]
Reactions
tert-Butyllithium is renowned for deprotonation of carbon acids (C-H bonds). One example is the double deprotonation of allyl alcohol.[4] Other examples are the deprotonation of vinyl ethers.[5][6][7]
In combination with n-butyllithiium, tert-butylllithium monolithiates ferrocene.[8] tert-Butyllithium deprotonates dichloromethane:[9]
Similar to n-butyllithium, tert-butyllithium can be used for lithium–halogen exchange reactions.[10][11]
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.
More so than other alkyllithium compounds, tert-butyllithium reacts with ethers.[2] 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.[12] In dimethoxyethane, the half-life is about 11 minutes at −70 °C[13]
In this example, the reaction of tert-butyllithium with (THF) is shown:
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.[14] Serious laboratory accidents involving tert-butyllithium have occurred. For example, in 2008 a staff research assistant, Sheharbano Sangji, in the lab of Patrick Harran[15] at the University of California, Los Angeles, died after being severely burned by a fire ignited by tert-butyllithium.[16][17][18]
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.
See also
References
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- ↑ Los Angeles Times, 2009-03-01