Page 401 - Advanced Organic Chemistry Part B - Reactions & Synthesis

374 X H X H X
H
O
Rh-Al 2 3
CHAPTER 5 +
H
Reduction of 2
Carbon-Carbon Multiple 8 H H
Bonds, Carbonyl
Groups, and Other X cis,cis-9 cis,trans-9
Functional Groups
OH 67 33
OH
CH 2 52 48
NH 2 1.5 98.5
CH 3 64 36
OCH 3 88 12
CO CH 3 85 15
2
CONH 2 81 19

5.1.2. Hydrogenation Using Homogeneous Catalysts
In addition to solid transition metals, numerous soluble transition metal
9
complexes are active hydrogenation catalysts. One of the first to be used was
tris-(triphenylphosphine)rhodium chloride, known as Wilkinson’s catalyst. 10 Hydro-
genation by homogeneous catalysts is believed to take place by initial formation of a
complex. The addition of hydrogen to the metal occurs by oxidative addition and
increases the formal oxidation state of the metal by two. This is followed by transfer of
hydrogen from rhodium to carbon to form an alkylrhodium intermediate. The final step
is a second migration of hydrogen to carbon, leading to elimination of the saturated
product (reductive elimination) and regeneration of active catalyst.

H
Rh H Rh H Rh H
Rh + RCH CHR 2 RCH CH R
2 2
RCH CHR RCH CHR RCH CHR
2
In some cases an alternative sequence involving addition of hydrogen at rhodium
prior to complexation of the alkene may operate. 11 The phosphine ligands serve both
to provide a stable soluble complex and to adjust the reactivity at the metal center.
The -bonded intermediates have been observed for Wilkinson’s catalyst 12 and for
several other related catalysts. 13 For example, a partially hydrogenated structure has
been isolated from methyl -acetamidocinnamate. 14

9
A. J. Birch and D. H. Williamson, Org. React., 24, 1 (1976); B. R. Jones, Homogeneous Hydrogenation,
Wiley, New York, 1973.
10 J. A. Osborn, F. H. Jardine, J. F. Young, and G. Wilkinson, J. Chem. Soc. A, 1711 (1966).
11 I. D. Gridnev and T. Imamoto, Acc. Chem. Res., 37, 633 (2004).
12
D. Evans, J . A. Osborn and G. Wilkinson, J. Chem. Soc. A, 3133 (1968); V. S. Petrosyan, A. B. Permin,
V. I. Bogdaskina, and D. P. Krutko, J. Orgmet. Chem., 292, 303 (1985).
13 H. Heinrich, R. Giernoth, J. Bargon, and J. M. Brown, Chem. Commun., 1296 (2001); I. D. Gridnev,
N. Higashi and T. Imamoto, Organometallics, 20, 4542, (2001).
14
J. A. Ramsden, T. D. Claridge and J. M. Brown, J. Chem. Soc., Chem. Commun., 2469 (1995).