Page 455 - Advanced Organic Chemistry Part B - Reactions & Synthesis

428 Ph(CH ) OSiH[CH(CH ) ] + PhCH CHCH O Ph(CH ) OCH CH CHPh
3 2 2
2
2 2
2 2
89%
CHAPTER 5
Reduction of Ref. 183
Carbon-Carbon Multiple
Bonds, Carbonyl
Groups, and Other These reactions presumably proceed by catalytic cycles in which the carbonyl
Functional Groups
component is silylated. The silyl ether can then act as a nucleophile, and an oxonium
ion is generated by elimination of a disilyl ether. The reduction of the oxonium ion
regenerates the silyl cation, which can continue the catalytic cycle.
+
RCH O + + SiR′′ 3 RCH O SiR′′ 3
RCH OSiR′′ 3 RCH O SiR′′
+
+
+
RCH O SiR′′ 3 +R′OSiR′′ 3 3 RCH O R′
O + O SiR′′ 3
R′ SiR′′ 3 R′
+
RCH O R′ + H SiR′′ 3 RCH OR′ +R′′ Si +
2
3
Various other kinds of Lewis acids can also promote the reaction. For example,
Cu OTf and Et SiH have been used to prepare a number of benzyl and alkyl ethers. 184
3
2
10% Cu(OTf) 2
O + C H OTMS O C H
8 17
8 17
Et SiH
3
72%
The reductive condensation can also be carried out using BiBr and Et SiH. The active
3 3
catalyst under these conditions is Et SiBr, which is generated in situ. 185
3
BiBr 3
OTBDMS + CH CH CH O OCH 2 CH 2 CH 3
2
3
Et 3 SiH
Reduction of ketones to triphenylsilyl ethers is effected by the unique Lewis acid
perfluorotriphenylborane. Mechanistic and kinetic studies have provided considerable
insight into the mechanism of this reaction. 186 The salient conclusion is that the hydride
is delivered from a borohydride ion, not directly from the silane. Although the borane
forms a Lewis acid-base complex with the ketone, its key function is in delivery of
the hydride.
F )
Ph SiH + B(C 6 5 3 Ph 3 Si H B(C F )
6 5 3
3
O +
O SiPh 3
Ph Si HBC F ) + ArCR ArCR [BH(C F ) ] –
6 5 3
6 5 3
3
+
O SiPh 3 Ar
[BH(C F ) ] – H OSiPh + B(C F )
ArCR 6 5 3 3 6 5 3
R
183
X. Jiang, J. S. Bajwa, J. Slade, K. Prasad, O. Repic, and T. J. Blacklock, Tetrahedron Lett., 43, 9225
(2002).
184 W.-C. Yang, X.-A. Lu, S. S. Kulkarni, and S.-C. Huang, Tetrahedron Lett., 44, 7837 (2003).
185 N. Komatsu, J. Ishida, and H. Suzuki, Tetrahedron Lett., 38, 7219 (1997).
186
D. J. Parks, J. M. Blackwell, and W. E. Piers, J. Org. Chem., 65, 3090 (2000).