Page 192 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

172 Scheme 2.6. (Continued)

CHAPTER 2
a. C. A. Brown, J. Am. Chem. Soc., 91, 5901 (1969).
Stereochemistry, b. M. C. Dart and H. B. Henbest, J. Chem. Soc., 3563 (1960).
Conformation, c. S. Siegel and G. V. Smith, J. Am. Chem. Soc., 82, 6082, 6087 (1960).
and Stereoselectivity d. H, C. Brown and W. C. Dickason, J. Am. Chem. Soc., 92, 709 (1970).
e. S. Krishnamurthy and H. C. Brown, J. Am. Chem. Soc., 98, 3383 (1976)
f. H. C. Brown and J. Muzzio, J. Am. Chem. Soc., 88, 2811 (1966)..
g. O. Arjona, R. Perez-Ossorio, A. Perez-Rubalcaba, and M. L. Quiroga, J. Chem. Soc., Perkin Trans. 2, 597 (1981);
C. Alvarez-Ibarra, O. Arjona P. Perez-Ossorio, A. Perez-Rubalcaba, M. L. Quiroga, and M. J. Santesmases, J. Chem.
Soc, Perkin Trans. 2, 1645 (1983).
h. G. J. McGarvey and M. Kimura, J. Org. Chem., 47, 5420 (1982).
The mechanistic description of heterogeneous hydrogenation is somewhat vague,
partlybecausethereactivesitesonthemetalsurfacearenotaspreciselydescribedassmall
molecule reagents in solution. As understanding of the chemistry of homogeneous hydro-
genation catalysts has developed, it has become possible to extrapolate those mechanistic
concepts to heterogeneous catalysts. It is known that hydrogen is adsorbed onto the metal
surface, forming metal-hydrogen bonds similar to those found in transition metal hydride
complexes. Alkenes are also adsorbed on the catalyst surface and at least three types of
intermediates have been implicated in hydrogenation. The initially formed intermediate
is pictured as attached at both carbon atoms of the double bond by -type bonding, as
∗
shown in A. The bonding involves the alkene and orbitals interacting with acceptor
and donor orbitals of the metal. A hydrogen can be added to the adsorbed group, leading
to B, which involves a carbon-metal bond. This species can react with another hydrogen
to give the alkane. Alkanes have little affinity for the catalyst surface, so this reaction is
effectively irreversible. A third intermediate species, shown as C, accounts for double-
bond isomerization and the exchange of hydrogen that sometimes accompanies hydro-
genation. This intermediate is equivalent to an allyl group bound to the metal surface by
bonds. It can be formed from adsorbed alkene by abstraction of an allylic hydrogen atom
by the metal. Formation of the allyl species is reversible and can lead to alkene isomer-
ization. Analogous reactions take place at single metal ions in homogeneous catalysis. A
major uncertainty in heterogeneous catalysis is whether there are cooperative interactions
involving several metal centers.

R CH 2 R R
R R R R R
R C C CH R R C H R C H
2
C C C
H H HH H H H H H HH
M M M M M M M M M
A π-complex B σ-bond C π-allyl complex

In most cases, both hydrogen atoms are added to the same side of the reactant (syn
addition). If hydrogenation occurs by addition of hydrogen in two steps, as implied
by the mechanism above, the intermediate must remain bonded to the metal surface
in such a way that the stereochemical relationship is maintained. Adsorption to the
catalyst surface normally involves the less sterically congested face of the double
bond, and as a result, hydrogen is added from the less hindered face of the molecule.
There are many cases of hydrogenations where hydrogen addition is not entirely syn
and independent corroboration of the stereochemistry is normally necessary.
The facial stereoselectivity of hydrogenation is affected by the presence of polar
functional groups that can influence the mode of adsorption to the catalyst surface. For