Page 849 - Advanced Organic Chemistry Part B - Reactions & Synthesis

p-Tol-BINAP-AgF effects enantioselective additions with trimethoxysilanes. 119 825
These reactions give anti products, regardless of the configuration of the allylic silane.
SECTION 9.2
Organosilicon
OH Compounds
ArCH O + CH 3 Si(OCH ) 6 mol % p -tol-BINAP
3 3
10 mol % AgF Ar
CH 3
96% e.e.

The combination BINAP-Ag O-KF with 18-crown-6 also leads to high enantioselec-
2
tivity. 120

9.2.4. Reactions with Iminium Ions

Iminium ions are reactive electrophiles toward both alkenyl and allylic silanes.
Useful techniques for closing nitrogen-containing rings are based on in situ generation
of iminium ions from amines and formaldehyde. 121

CH 2
CF CO H +
3
2
PhCH NCH CH CCH Si(CH ) PhCH NCH CH CCH Si(CH ) PhCH N CH 2
2
3 3
2
2
2
2
2
3 3
2
2
2
H 2 C O
CH CH 73%
H 2 2
When primary amines are employed, the initially formed 3-butenylamine undergoes a
further reaction forming 4-piperidinols. 122
+
PhCH NH + CH 2 CHCH Si(CH ) + CH 2 O PhCH N OH
3
2
2
2
3 3
Reactions of this type can also be observed with 4-(trimethylsilyl)-3-alkenylamines. 123
R 3
1
R NCH CH CH CR 3 CH 2 O
2
2
H H +
) N
Si(CH 3 3
R 1
Mechanistic investigation in this case has shown that there is an equilibrium between an
alkenyl silane and an allylic silane by a rapid 3,3-sigmatropic process. The cyclization
occurs through the more reactive allylic silane.
119 A. Yanagisawa, H. Kageyama, Y. Nakatsuka, K. Asakawa, Y. Matsumoto, and H. Yamamoto, Angew.
Chem. Int. Ed. Engl., 38, 3701 (1999).
120
M. Wadamoto, N. Ozasa, A. Yanagisawa, and H. Yamamoto, J. Org. Chem., 68, 5593 (2003).
121
P. A. Grieco and W. F. Fobare, Tetrahedron Lett., 27, 5067 (1986).
122 S. D. Larsen, P. A. Grieco, and W. F. Fobare, J. Am. Chem. Soc., 108, 3512 (1986).
123
C. Flann, T. C. Malone, and L. E. Overman, J. Am. Chem. Soc., 109, 6097 (1987).