Page 405 - Advanced Organic Chemistry Part B - Reactions & Synthesis

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CHAPTER 5 PPh 2
Reduction of PPh 2
Carbon-Carbon Multiple
Bonds, Carbonyl
Groups, and Other
Functional Groups BINAP

Ruthenium complexes containing this ligand are able to reduce a variety of double
bonds with e.e. above 95%. In order to achieve high enantioselectivity, the reactant
must show a strong preference for a specific orientation when complexed with the
catalyst. This ordinarily requires the presence of a functional group that can coordinate
with the metal. The ruthenium-BINAP catalyst has been used successfully with unsat-
24
23
urated amides, allylic and homoallylic alcohols, and unsaturated carboxylic acids. 25
CH 3 CH 3 Ru(S-BINAP)(OAc) 2 CH 3 CH 3

CH 3 OH CH 3 OH
99% e.e. Ref. 12

The mechanism of such reactions using unsaturated carboxylic acids and
Ru BINAP O CCH is consistent with the idea that coordination of the carboxy
3 2
2
26
group establishes the geometry at the metal ion. The configuration of the new stereo-
center is then established by the hydride transfer. In this particular mechanism, the
second hydrogen is introduced by protonolysis, but in other cases a second hydride
transfer step occurs.
R R –
H +
O H 2 O
P O P O
* Ru * Ru
P O P O O
O H
H
CO 2
*
CO H
2
R R –
O O
P O P O
* Ru * Ru
P O P O
O H +
O
* *

23
R. Noyori, M. Ohta, Y. Hsiao, M. Kitamura, T. Ohta, and H. Takaya, J. Am. Chem. Soc., 108, 7117
(1986).
24 H. Takaya, T. Ohta, N. Sayo, H. Kumobayashi, S. Akutagawa, S. Inoue, I. Kasahara, and R. Noyori,
J. Am. Chem. Soc., 109, 1596 (1987).
25 T. Ohta, H. Takaya, M. Kitamura, K. Nagai, and R. Noyori, J. Org. Chem., 52, 3174 (1987).
26
M. T. Ashby and J. T. Halpern, J. Am. Chem. Soc., 113, 589 (1991).