Page 129 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

4.5 SILYL ANIONS 109

Me 3 Si

Me Me

Me TiCl 4
O O
Me
Me
Me
78%

4.5 SILYL ANIONS

Silyl anions are not commonly mentioned in introductory inorganic courses, but they
should be! Many silyl anions are considerably more stable than their carbanion analogs and
many can be generated with surprising ease. Thus a tertiary amine suffices to deprotonate
trichlorosilane:

Cl Me Cl
Me + −
H Si N H + Si
N (4.29)
Cl Me Cl
Me Cl Me Cl
Me
The trichlorosilyl anion so produced can be put to a variety of uses. With a simple alkyl
halide, it carries out an S 2 displacement, producing an alkyltrichlorosilane:
N
Cl − Cl
−
− X
Si R X Si R (4.30)
Cl Cl
Cl Cl
−
With hexachloroethane, the Cl Si anion does an E2 elimination to produce tetra-
3
chloroethylene, as shown below:
Cl
−
Si Cl Cl − Cl Cl Cl
Cl
Cl C C − Cl (4.31)
Cl Si Cl + C C
Cl
Cl Cl Cl
Cl Cl Cl
Let’s put the matter in an energetics perspective. The gas-phase acidity of silane (i.e.,
∘
+
the ΔH for the reaction SiH → SiH 3 − + H ) is about 372 ± 2 kcal/mol, which is much
4
“higher” than that of methane (416 kcal/mol). Note that a lower number implies a higher
acidity. Methyl groups decrease the acidity of silanes, but trimethylsilyl groups have a
dramatic acidity-enhancing effect. Tris(trimethylsilyl)silane thus has a gas-phase acidity