n-Butyllithium-Potassium t-Butoxide1


[109-72-8]  · C4H9Li  · n-Butyllithium-Potassium t-Butoxide  · (MW 64.06) (t-BuOK)

[865-47-4]  · C4H9KO  · n-Butyllithium-Potassium t-Butoxide  · (MW 112.22)

(superbase for metal-hydrogen exchange of a carbon acid R-H;2 can generate organopotassium compounds3)

Physical Data: see n-Butyllithium and Potassium t-Butoxide; NMR4 and ESR5 studies of the n-BuLi/t-BuOK mixture have been reported.

Solubility: see n-Butyllithium and Potassium t-Butoxide.

Preparative Method: prepared in situ under nitrogen by addition of equimolar potassium t-butoxide to a solution of n-butyllithium at 0 °C.

Handling, Storage, and Precautions: use in a fume hood; must be prepared and transferred under inert gas (Ar or N2) to exclude oxygen and moisture. Solutions in THF decompose above -40 °C.

Metal-Hydrogen Exchange.

Freshly prepared n-BuLi/t-BuOK is more basic than n-BuLi and n-BuK.6 When treated with this superbase, hydrocarbons in the low acidity range of pKa 35-50 undergo a clean metal-hydrogen exchange: benzene,2 cyclopropane derivatives,7,8 cumene,7 a variety of 1- or 2-alkenes having allylic C-H bonds,2,9-13 cyclohexene,7,14 a-pinene,15 conjugated dienes,11,16-18 ene sulfides,19 4H-pyrans,20 and 1,4-dihydropyridines.20 n-BuLi/t-BuOK did not succeed in metalating saturated hydrocarbons such as octane and cyclohexane.

Benzene and substituted benzene, in the presence of n-BuLi/t-BuOK, undergo site selective metal-hydrogen exchange (eq 1). Best results are obtained in THF at -75 °C or -50 °C. Upon quenching with carbon dioxide, the corresponding acids are obtained in good to excellent yield.21,22

When alkenes are treated with n-BuLi/t-BuOK, the metalation occurs regioselectively: allylic methyl groups are much more readily attacked than methylene groups, which are less reactive than methine centers.9,12 Furthermore, cis-2-alkenes generally react faster than their trans counterparts. This holds even for 4,4-dimethyl-2-pentene (eq 2).23

Above -60 °C, ethereal solvents are rapidly attacked by n-BuLi in the presence of t-BuOK.6b Nevertheless, acetal functions are suitable protecting groups for superbase metalations of alkenols.

The n-BuLi/t-BuOK mixture can convert 2-alkynyl ethers into allenic ethers and some a-substituted derivatives (eq 3).24

In the cases where metal-hydrogen exchange leads to a resonance-stabilized organometallic species, the intermediate organopotassium compounds precipitate.3 Benzyl-, allyl-, pentadienyl-, and cyclooctadienylpotassium can be prepared in this way.9,13,16 The accompanying lithium t-butoxide can be completely removed by washing with benzene or toluene. The resulting organopotassium can serve as a reagent of general applicability in preparing transition metal complexes through transmetalation (eq 4).25

1. Schlosser, M. PAC 1988, 60, 1627.
2. Schlosser, M. JOM 1967, 8, 9.
3. Lochmann, L.; Pospíšil, J.; Lím, D. TL 1966, 257.
4. Boche, G.; Etzrodt, H. TL 1983, 24, 5477.
5. Wilhelm, D.; Clark, T.; Schleyer, P. v. R.; Courtneidge, J. L.; Davies, A. G. JOM 1984, 273, C1.
6. (a) Schlosser, M.; Strunk, S. TL 1984, 25, 741. (b) Lehmann, R.; Schlosser, M. TL 1984, 25, 745.
7. Hartmann, J.; Schlosser, M. HCA 1976, 59, 453.
8. Schlosser, M.; Schneider, P. HCA 1980, 63, 2404.
9. Schlosser, M.; Hartmann, J. AG(E) 1973, 12, 508.
10. Schlosser, M.; Hartmann, J.; David, V. HCA 1974, 57, 1567.
11. Hartmann, J.; Muthukrishnan, R.; Schlosser, M. HCA 1974, 57, 2261.
12. Schlosser, M. AG(E) 1974, 13, 701.
13. Schlosser, M.; Hartmann, J. JACS 1976, 98, 4674.
14. Hartmann, J.; Stähle, M.; Schlosser, M. S 1974, 888.
15. Rauchschwalbe, G.; Schlosser, M. HCA 1975, 58, 1094.
16. Schlosser, M.; Rauchschwalbe, G. JACS 1978, 100, 3258.
17. Bosshardt, H.; Schlosser, M. HCA 1980, 63, 2393.
18. Schlosser, M.; Bosshardt, H.; Walde, A.; Stähle, M. AG(E) 1980, 19, 303.
19. Muthukrishnan, R.; Schlosser, M. HCA 1976, 59, 13.
20. Schlosser, M.; Schneider, P. AG(E) 1979, 18, 489.
21. Schlosser, M.; Katsoulos, G.; Takagishi, S. SL 1990, 747.
22. Schlosser, M.; Choi, J. H.; Takagishi, S. T 1990, 46, 5633.
23. Stähle, M.; Hartmann, J.; Schlosser, M. HCA 1977, 60, 1730.
24. Verkruijsse, H. D.; Verboom, W.; Van Rijn, P. E.; Brandsma, L. JOM 1982, 232, C1.
25. Longoni, G.; Chini, P.; Canziani, F.; Fantucci, P. G 1974, 104, 249.

Xiaoyang Xia

Emory University, Atlanta, GA, USA

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