Sec-Butyllithium Explained

sec-Butyllithium is an organometallic compound with the formula CH₃CHLiCH₂CH₃, abbreviated sec-BuLi or s-BuLi. This chiral organolithium reagent is used as a source of sec-butyl carbanion in organic synthesis.^[1]

Synthesis

sec-BuLi can be prepared by the reaction of sec-butyl halides with lithium metal:^[2]

Properties

Physical properties

sec-Butyllithium is a colorless viscous liquid.^[1] Using mass spectrometry, it was determined that the pure compound has a tetrameric structure.^[3] It also exists as tetramers when dissolved in organic solvents such as benzene, cyclohexane or cyclopentane. The cyclopentane solution has been detected with ⁶Li-NMR spectroscopy to have a hexameric structure at temperatures below −41 °C.^[4] In electron-donating solvents such as tetrahydrofuran, there exists an equilibrium between monomeric and dimeric forms.^[5]

Chemical properties

The carbon-lithium bond is highly polar, rendering the carbon basic, as in other organolithium reagents. Sec-butyllithium is more basic than the primary organolithium reagent, n-butyllithium. It is also more sterically hindered. sec-BuLi is employed for deprotonations of particularly weak carbon acids where the more conventional reagent n-BuLi is unsatisfactory. It is, however, so basic that its use requires greater care than for n-BuLi. For example diethyl ether is attacked by sec-BuLi at room temperature in minutes, whereas ether solutions of n-BuLi are stable.^[1]

The compound decomposes slowly at room temperature and more rapidly at higher temperatures, giving lithium hydride and a mixture of butenes.^{[6] [7]}

Applications

Many transformations involving sec-butyllithium are similar to those involving other organolithium reagents.

In combination with sparteine as a chiral auxiliary, sec-butyllithium is useful in enantioselective deprototonations.^[8] It is also effective for lithiation of arenes.^[9]

Notes and References

1. Encyclopedia: Ovaska, T. V.. s-Butyllithium. Encyclopedia of Reagents for Organic Synthesis. 2001. John Wiley & Sons. New York. 10.1002/047084289X.rb397. 0471936235 . .
2. Hay, D. R. . Song, Z. . Smith, S. G. . Beak, P. . Complex-induced proximity effects and dipole-stabilized carbanions: kinetic evidence for the role of complexes in the α-lithiations of carboxamides . . 1988 . 110 . 8145–8153 . 10.1021/ja00232a029 . 24.
3. Plavsic . D. . Srzic . D. . Klasinc . Leo . 1986 . Mass spectrometric investigations of alkyllithium compounds in the gas phase . The Journal of Physical Chemistry . en . 90 . 10 . 2075–2080 . 10.1021/j100401a020 . 0022-3654.
4. Fraenkel . Gideon . Henrichs . Mark . Hewitt . Michael . Su . Biing Ming . 1984 . Structure and dynamic behavior of a chiral alkyllithium compound: carbon-13 and lithium-6 NMR of sec-butyllithium . Journal of the American Chemical Society . en . 106 . 1 . 255–256 . 10.1021/ja00313a052 . 0002-7863.
5. Bauer . Walter. . Winchester . William R. . Schleyer . Paul von R. . 1987-11-01 . Monomeric organolithium compounds in tetrahydrofuran: tert-butyllithium, sec-butyllithium, supermesityllithium, mesityllithium, and phenyllithium. Carbon-lithium coupling constants and the nature of carbon-lithium bonding . Organometallics . en . 6 . 11 . 2371–2379 . 10.1021/om00154a017 . 0276-7333.
6. Glaze . William H. . Lin . Jacob . Felton . E. G. . 1965 . The Thermal Decomposition of sec-Butyllithium . The Journal of Organic Chemistry . en . 30 . 4 . 1258–1259 . 10.1021/jo01015a514 . 0022-3263.
7. Glaze . William H. . Lin . Jacob . Felton . E. G. . 1966 . The Pyrolysis of Unsolvated Alkyllithium Compounds . The Journal of Organic Chemistry . en . 31 . 8 . 2643–2645 . 10.1021/jo01346a044 . 0022-3263.
8. 10.15227/orgsyn.082.0022. Preparation of (S,S)-1,2-bis(tert-Butylmethylphosphino)ethane ((S,S)-t-bu-bisp*) as a Rhodium Complex . Organic Syntheses . 2005 . 82 . 22. Karen V. L.. Crépy. Tsuneo. Imamoto . free.
9. 10.15227/orgsyn.072.0163. X.. Wang. S. O.. de Silva. J. N.. Reed. R.. Billadeau. E. J.. Griffen. A.. Chan. V.. Snieckus. 7-Methoxyphthalide. Org. Synth.. 1995. 72. 163.