Page 606 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

588 6.2. Carbanion Character of Organometallic Compounds

CHAPTER 6
The organometallic derivatives of lithium, magnesium, and other strongly
Carbanions and Other
Carbon Nucleophiles electropositive metals have some of the properties expected for salts of carbanions.
Owing to the low acidity of most hydrocarbons, organometallic compounds usually
cannot be prepared by proton transfer reactions. Instead, the most general preparative
methods start with the corresponding halogen compound.

CH I + 2Li CH Li + LiI
3
3
CH (CH ) Br + Mg CH (CH ) MgBr
2 3
3
3
2 3
PhBr + 2Li PhLi + LiBr
There are other preparative methods, which are considered in Chapter 7 of Part B.
Organolithium compounds derived from saturated hydrocarbons are extremely
strong bases and react rapidly with any molecule having an −OH, −NH, or −SH group
by proton transfer to form the hydrocarbon. Accurate pK values are not known, but
range upward from the estimate of ∼50 for methane. The order of basicity CH Li <
3
CH CH Li < CH CLi is due to the electron-releasing effect of alkyl substituents
3 3
3
2 3
and is consistent with increasing reactivity in proton abstraction reactions in the order
CH Li < CH CH Li < CH CLi. Phenyl- , methyl, n-butyl- , and t-butyllithium
3
3
3 3
2 3
are all stronger bases than the anions of the hydrocarbons listed in Table 6.2. Unlike
proton transfers from oxygen, nitrogen, or sulfur, proton removal from carbon atoms
is usually not a fast reaction. Thus, even though t-butyllithium is thermodynamically
capable of deprotonating toluene, the reaction is quite slow. In part, the reason is that
the organolithium compounds exist as tetramers, hexamers, and higher aggregates in
hydrocarbon and ether solvents. 28
In solution, organolithium compounds exist as aggregates, with the degree of
aggregation depending on the structure of the organic group and the solvent. The
nature of the species present in solution can be studied by low-temperature NMR.
n-Butyllithium in THF, for example, is present as a tetramer-dimer mixture. 29 The
tetrameric species is dominant.
[(BuLi)4·(THF)4] + 4 THF 2 [(BuLi)2·(THF)4]

Tetrameric structures based on distorted cubic structures are also found for
CH Li and C H Li 4 30 and they can be represented as tetrahedral of lithium ions
5
4
3
2
with each face occupied by a carbanion ligand.
R Li R
Li Li
R Li R

28
G. Fraenkel, M. Henrichs, J. M. Hewitt, B. M. Su, and M. J. Geckle, J. Am. Chem. Soc., 102, 3345
(1980); G. Fraenkel, M. Henrichs, M. Hewitt, and B. M. Su, J. Am. Chem. Soc., 106, 255 (1984).
29 D. Seebach, R. Hassig, and J. Gabriel, Helv. Chim. Acta, 66, 308 (1983); J. F. McGarrity and C. A. Ogle,
J. Am. Chem. Soc., 107, 1805 (1984).
30
E. Weiss and E. A. C. Lucken, J. Organomet. Chem., 2, 197 (1964); E. Weiss and G. Hencken,
J. Organomet. Chem., 21, 265 (1970); H. Koester, D. Thoennes, and E. Weiss, J. Organomet. Chem.,
160, 1 (1978); H. Dietrich, Acta Crystallogr., 16, 681 (1963); H. Dietrich, J. Organomet. Chem., 205,
291 (1981).