Page 403 - Advanced Organic Chemistry Part B - Reactions & Synthesis

376 The Crabtree catalyst also exhibited superior stereoselectivity in comparison with other
catalysts in reduction of an exocyclic methylene group. 20
CHAPTER 5
Reduction of
Carbon-Carbon Multiple CH 3 OH OH CH 3 OH OH
Bonds, Carbonyl catalyst H
Groups, and Other H
Functional Groups O O H
H
CH
CH 2 3
] PF
[Ir(cod)(pyr)PR 3 6 > 99:1
Rh(nbd)(dppb) BF 4 90:10
Rh(Ph P) Cl 6:94
3
3
Pd/C 5:95
Presumably, the stereoselectivity in these cases is the result of coordination of iridium
by the functional group. The crucial property required for a catalyst to be stereodirective
is that it be able to coordinate with both the directive group and the double bond and
still accommodate the metal hydride bonds necessary for hydrogenation. In the iridium
catalyst illustrated above, the cyclooctadiene ligand (COD) in the catalysts is released
by hydrogenation, permitting coordination of the reactant and reaction with hydrogen.
Scheme 5.2 gives some examples of hydrogenations carried out with homoge-
neous catalysts. Entry 1 is an addition of deuterium that demonstrates net syn addition
with the Wilkinson catalyst. The reaction in Entry 2 proceeds with high stereoselec-
tivity and is directed by steric approach control, rather than a substituent-directing
effect. One potential advantage of homogeneous catalysts is the ability to achieve
a high degree of selectivity among different functional groups. Entries 3 and 4 are
examples that show selective reduction of the unconjugated double bond. Similarly
in Entry 5, reduction of the double bond occurs without reduction of the nitro group,
which is usually rapidly reduced by heterogeneous hydrogenation. Entries 6 and 7 are
cases of substituent-directed hydrogenation using the iridium (Crabtree) catalyst. The
catalyst used in Entry 8 is related to the Wilkinson catalyst, but on hydrogenation
of norbornadiene (NBD) has two open coordination positions. This catalyst exhibits
a strong hydroxy-directing effect. The Crabtree catalyst gave excellent results in the
hydrogenation of 3-methylpentadeca-4-enone to R-muscone. (Entry 9) A number of
heterogeneous catalysts led to 5–15% racemization (by allylic exchange).

5.1.3. Enantioselective Hydrogenation
The fundamental concepts of enantioselective hydrogenation were introduced in
Section 2.5.1 of Part A, and examples of reactions of acrylic acids and the important
case of -acetamido acrylate esters were discussed. The chirality of enantioselective
hydrogenation catalysts is usually derived from phosphine ligands. A number of chiral
phosphines have been explored in the development of enantioselective hydrogenation
21
catalysts, and it has been found that some of the most successful catalysts are derived
from chiral 1 1 -binaphthyldiphosphines, such as BINAP. 22

20
J. M. Bueno, J. M. Coteron, J. L. Chiara, A. Fernandez-Mayoralas, J. M. Fiandor, and N. Valle,
Tetrahedron Lett., 41, 4379 (2000).
21 B. Bosnich and M. D. Fryzuk, Top. Stereochem., 12, 119 (1981); W. S. Knowles, W. S. Chrisopfel,
K. E. Koenig, and C. F. Hobbs, Adv. Chem. Ser., 196, 325 (1982); W. S. Knowles, Acc. Chem. Res.,
16, 106 (1983).
22
R. Noyori and H. Takaya, Acc. Chem. Res., 23, 345 (1990).