Page 121 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

4.1 SILYL PROTECTING GROUPS 101

Bz
H 2 O
Si (BzEtSiO) 3
Cl
Et
Cl “Benzylethylsilicone”
Bz = PhCH 2

Based on your mechanistic knowledge, provide an explanation of this chemistry that
Kipping found so baffling.

As mentioned, silicon-based protecting group chemistry hinges on silicon’s well-known
oxophilicity. That said, although Si–O bonds are strong, Si–F bonds are much stronger.
Thermodynamic considerations thus explain why sand and glass dissolve in HF and why
you shouldn’t use glass vessels to handle HF. What this means for protection/deprotection
chemistry is that fluoride may be used to deprotect silyl ethers. Typically, a milder surro-
gate of HF such as pyridinium fluoride (py⋅HF) or tetrabutylammonium fluoride (TBAF)
in THF/H O is used for this purpose:
2
py HF
or
Me SiOR ROH + Me SiF (4.7)
3
3
TBAF, THF/H 2 O
Note that there must be a source of protons for generating the alcohol, so TBAF alone would
not do the job:

Me Me
R + + R
Si H Si O
Me O Me
Me Me H
(4.8)
R
− Me Me Me R
F + R − O +
Si F Si Si + O
O H Me
Me Me F Me H
Me
H Me

REVIEW PROBLEM 4.4

Carbon tetrafluoride and tetrachloride do not react with water under ordinary condi-
tions. Silicon tetrachloride, by contrast, is instantly hydrolyzed by water.
+ −
SiCl + 4H O → Si(OH) + 4H + 4Cl
4 2 4