Page 839 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Most reactions of alkenyl and allylic silanes require strong carbon electrophiles and 815
Lewis acid catalysts are often involved. The most useful electrophiles from a synthetic
standpoint are carbonyl compounds, iminium ions, and electrophilic alkenes. SECTION 9.2
There are also some reactions of allylic silanes that proceed through anionic Organosilicon
Compounds
silicate species. These reactions usually involve activation by fluoride and result in
transfer of an allylic anion.

F O
–
CHCH SiR + – CH CHCH Si R CH CHCH C O –
CH 2 2 3 F 2 2 3 2 2
Trichloro- and trifluorosilanes introduce another dimension into the reactivity of allylic
silanes. The silicon in these compounds is electrophilic and can expand to pentaco-
ordinate and hexacoordinate structures. These reactions can occur through a cyclic or
chelated TS.

SiX 3
SiX 3
C O C O

9.2.3. Addition Reactions with Aldehydes and Ketones
A variety of electrophilic catalysts promote the addition of allylic silanes to
carbonyl compounds. 86 The original catalysts included typical Lewis acids such as
TiCl or BF . 87 This reaction is often referred to as the Sakurai reaction.
4 3
OH
TiCl 4
CH 2 CHCH SiR + R C O or R CCH CH CH 2
2
2
3
2
2
BF 3
These reactions involve activation of the carbonyl group by the Lewis acid. A nucleo-
phile, either a ligand from the Lewis acid or the solvent, assists in the desilylation step.
R C O
2
MX n R 2 C OMX n–1 + R SiNu + X
–
3
C CH CH CH
H 2 2 2
C CH 2 SiR
H 3
Nu
Various other Lewis acids have been explored as catalysts, and the combination
InCl - CH SiCl has been found to be effective. 88 The catalysis requires both compo-
3 3 3
nents and is attributed to assistance from O-silylation of the carbonyl compound.
86
A. Hosomi, Acc. Chem. Res., 21, 200 (1988); I. Fleming, J. Dunoques, and R. Smithers, Org. React.,
37, 57 (1989).
87 A. Hosomi and H. Sakurai, Tetrahedron Lett., 1295 (1976).
88
Y. Onishi, T. Ito, M. Yasuda, and A. Baba, Eur. J. Org. Chem., 1578 (2002); Y. Onishi, T. Ito,
M. Yasuda, and A. Baba, Tetrahedron, 58, 8227 (2002).