Page 143 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

4.10 SILYL CATIONS 123

Ph
Ph +
Mes Mes Ph
Ph +
Si H Si + H × 2 (4.57)
Mes Me Mes Ph
Me Ph

+
Now, to produce an Mes Si cation, we need to carry out an Mes/Me ligand exchange
3
+
between the MeMes Si cation produced above and the third molecule of MeMes SiH that
2
2
has not reacted so far. A mesityl-bridged intermediate appears plausible for the process:
Mes
+ Mes
Si
Mes
Me Mes Si
Si H Si
H Me
Me
Mes
Mes Me
(4.58)
H
Si +
Mes
Me
Me +
Si
Mes
Mes
+
We have thus produced the first (of two) Mes Si cation and a new silane Me MesSiH. The
3
2
latter can now engage in a second Mes/Me ligand exchange reaction with an MeMes Si +
2
+
cation to produce the second Mes Si cation as well as Me SiH as the final products. The
3
3
mechanism of this step is analogous to that shown above and is left as an exercise.
Use of extremely weakly coordinating anions (including several based on “carboranes,”
whose structures we will not discuss for reasons of space), hereafter simply referred to
−
as [WCA] , allows the synthesis of even relatively unhindered silyl cations, such as the
triethylsilyl cation:
− −
[WCA] [WCA]
− Ph 3 CH
Si H + Ph + Ph +
C Si (4.59)
Ph