Page 454 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Aryl ketones are also reduced with triethylsilane and TiCl . This method can be used 427
4
to prepare -arylaminoacids. 177
SECTION 5.4
O 1) TMSCl
Et N Group IV Hydride
3
2
2
ArCCH 2 CHNHCO 2 CH 3 ArCH CH CHNHCO CH 3 Donors
2
2) (C H ) SiH,
2 5 3
CO H TiCl 4 CO H
2
2
Aliphatic ketones can be reduced to hydrocarbons by triethylsilane and gaseous BF . 178
3
The BF is a sufficiently strong Lewis acid to promote formation of a carbocation
3
from the intermediate alcohol.
–
BF 3 –
OBF 3 R R
+ O Et SiH + Et SiH
3
R C R C 3 RCH R
2
RCR
H H
A combination of Friedel-Crafts alkylation and reduction can be achieved using
InCl and chlorodimethylsilane. The Lewis acid presumably promotes both the Friedel-
3
Craft reaction and the subsequent reduction. 179
5 mol% InCl 3 CH 2 Ph
Br + PhCH O Br
(CH ) SiHCl 93%
3 2
34:6:60 o:m:p
There are several procedures for reductive condensation of silyl ethers with
carbonyl compounds to form ethers. One method uses TMSOTf as the catalyst. 180
O
TMSOTf
PhCCH 3 + PhCH 2 OTMS PhCHOCH Ph
2
Et 3 SiH
CH 3 100%
A number of related procedures have been developed. For example, TMSI can be
used. 181
TMSI
O + TMSO O
Et SiH
3
75%
The trimethylsilyl group can be replaced by a dialkylsilyloxy group, in which case the
silyl ether serves as the hydride donor.
TMSI
PhCH CHCH O + (CH ) HSiO(CH ) CH 3 PhCH CHCH 2 O(CH ) CH 3
2 3
3 2
2 3
88%
Ref. 182
177
M. Yato, K. Homma, and A. Ishida, Heterocycles, 49, 233 (1998).
178 J. L. Frey, M. Orfanopoulos, M. G. Adlington, W. R. Dittman, Jr., and S. B. Silverman, J. Org. Chem.,
43, 374 (1978).
179
T. Miyai, Y. Onishi, and A. Baba, Tetrahedron Lett., 39, 6291 (1998).
180
S. Hatakeyama, H. Mori, K. Kitano, H. Yamada, and M. Nishizawa, Tetrahedron Lett., 35, 4367 (1994).
181 M. B. Sassaman, K. D. Kotian, G. K. S. Prakash, and G. Olah, J. Org. Chem., 52, 4314 (1987).
182
K. Miura, K. Ootsuka, S. Suda, H. Nishikori, and A. Hosomi, Synlett, 313 (2002).