_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

ArCN
i
Ar COR
ClCN
RCOCt/
AlCt3
Tetra-alkyls and -aryls of Group IV
ArH
H+
ct2 /Fe
ArCt
Br2
!--____::'--- ArBr
12
ArN02
HN0 3
Fig. 4.1 Some reactions of arylsilanes.
Ari
4.2.2 Reactions of tetra-organosilicon compounds
The inertness of tetraalkylsilanes to thermal decomposition and to chemical
attack was alluded to in the introduction. Electrophiles cleave alkyl-silicon
bonds, but only in the presence of a Lewis acid:
~ICI,
Me 4 Si + HCI -------> Me,SiCI + CH 4
1 2 . Ali, 1 2 Ali,
Et 4 Si --~ Et,Sii --~ Et,Sii,
Et1
- Etl
Aryl-silicon bonds are more reactive. Acid cleavage of Et,SiPh proceeds 10000
times as fast as proton exchange in benzene. R,Si behaves as an electronreleasing
group which stabilizes the positively charged Wheland intermediate.
HA
The relative rates of acid cleavage of R,M-Ph in crease dramatically down the
Group as follows: M =Si, 1: Ge, 36: Sn, 3.5 x 10': Pb, 2 x 10 8 • R,Si substituents
on an arene can similarly be replaced by a wide range of groups by reaction with
suitable electrophiles (Fig. 4.1 ).
Vinyl-silicon bonds are also rather easily cleaved by electrophiles. It appears
that the R,Si group is able to stabilize /)-carbonium ions. Allylsilanes are prone to
attack even by fairly weak electrophiles. Acetals. aldehydes and ketones react in
the presence of Lewis acids to form unsaturated alcohols. The reaction is
accompanied by an allylic transposition:
Vinyl sila ne:
/H
H2C=C~
SiMe 3
101