Page 421 - Advanced Organic Chemistry Part B - Reactions & Synthesis

394 Several other versions of these catalysts have been developed. Arene complexes
of monotosyl-1,2-diphenylethylenediamine ruthenium chloride give good results with
CHAPTER 5 , -ynones. The active catalysts are generated by KOH. These catalysts also function
55
Reduction of by hydrogen transfer, with isopropanol serving as the hydrogen source. Entries 6 to 8
Carbon-Carbon Multiple
Bonds, Carbonyl in Scheme 5.3 are examples.
Groups, and Other
Functional Groups
R
Ts R
Ph N Ts
KOH Ph N
Ru
Ru
N Cl
Ph N
H 2 Ph H Cl
Catalyst D: Arene = mesitylene
Catalyst E: Arene = p-cymene

Scheme 5.5 gives some examples of the application of these Ru(II)-diphosphine
and diamine catalysts. Entries 1 and 2 are examples of the hydrogenation
of -dicarbonyl compounds with Ru BINAP Cl . Excellent enantioselectivity is
2
observed, although elevated hydrogen pressure is required. Entry 3 proceeds in fair
yield and enantioselectivity, and without reduction of the conjugated carbon-carbon
double bond. Entry 4 uses the cymene complex catalyst E under hydrogen transfer
conditions. Entry 5 involves tandem 1,4- and 1,2-reduction and was done under
hydrogen transfer conditions, using formic acid as the hydride donor. Entries 6 to
8 show good yields and enantioselectivity for several alkynyl ketones of increasing
structural complexity. In the latter two cases, only a single stereoisomer was observed.
Certain functional groups can be entirely removed and replaced by hydrogen,
a reaction known as hydrogenolysis. For example, aromatic halogen substituents
are frequently removed by hydrogenation over transition metal catalysts. Aliphatic
halogens are somewhat less reactive but hydrogenolysis is promoted by base. 56 The
most useful type of hydrogenolysis reaction involves removal of oxygen functional
groups at benzylic and allylic positions. 57

H , Pd
2
CH OR CH 3 + HOR
2
Hydrogenolysis of halides and benzylic groups presumably involves intermediates
formed by oxidative addition to the active metal catalyst to generate intermediates
similar to those involved in hydrogenation. The hydrogenolysis is completed by
58
reductive elimination. Many other examples of this pattern of reactivity are discussed
in Chapter 8.

55
K. Matsumura, S. Hashiguchi, T. Ikariya, and R. Noyori, J. Am. Chem. Soc., 119, 8738 (1997).
56 A. R. Pinder, Synthesis, 425 (1980).
57 W. H. Hartung and R. Simonoff, Org. React., 7, 263 (1953); P. N. Rylander, Catalytic Hydro-
genation over Platinum Metals, Academic Press, New York, 1967, Chap. 25; P. N. Rylander, Catalytic
Hydrogenation in Organic Synthesis, Academic Press, New York, 1979, Chap. 15; P. N. Rylander,
Hydrogenation Methods, Academic Press, Orlando, FL, 1985, Chap. 13.
58
The mechanism of benzylic hydrogenolysis has not been definitively established. For other possibilities,
see R. B. Grossman, The Art of Writing Reasonable Organic Mechanisms, 2nd Edition, Springer,
New York, 2003, pp. 309–310.