Page 111 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

Catalysis, Homogeneous 459

of the diene increases. As a result the reaction becomes
faster than the uncatalyzed one. Accidentally it becomes
also more regioselective. In this instance the catalyzed
reaction is very similar to the uncatalyzed reaction. Often
this is not the case. In particular when the reagents are
very unreactive toward one another (ethene versus ethene,
methanol versus CO, see above) the simple picture of a
catalyst that lowers the reaction barrier is beyond the truth.
The following description is more general. A cata-
lyst provides a more attractive reaction pathway for the
reagents in order to arrive at the products. This new path-
way may involve many steps and may be rather compli-
cated. Summarizing, a catalyst provides a new reaction
pathway with a low barrier of activation, which may in-
volve many intermediates and many steps. The sequence
of steps is called the mechanism of the reaction. Mech-
FIGURE 2 Selectivity in nickel catalyzed butadiene reactions.
anism also refers to the more detailed description of a
reaction at the molecular bonding level. During the pro-
of the catalyst. The success of organometallic catalysts
cess, the catalytic cycle, the catalyst participates in many
lies in the relative ease of catalyst modification by chang-
“complexes” all of which might be named “the” catalyst.
ing the ligand environment. Crucial properties to be in-
It cycles continuously from one species to another. In this
fluenced are the acceleration of the reaction and the se-
sense the catalyst itself remains unchanged during the cat-
alytic conversion (Ostwald, first paragraph). The catalyst lectivity to certain products. One metal can give a variety
may spend most of its time in the form of a certain species, of products from one single substrate simply by changing
which is often the species undergoing the slowest reaction the ligands around the metal center. Figure 2 shows the
of all in the catalytic cycle. This species may be observed products that can be obtained from butadiene with nickel
by in situ spectroscopy and it is referred to as the resting catalysts.
state of the catalyst. Several types of selectivity can be distinguished in a
chemical reaction: chemoselectivity; regioselectivity; di-
astereoselectivity; and enantioselectivity. See Fig. 3 for an
B. Scope of Homogeneous Catalysis explanation. Note the difference between diastereoselec-
Homogeneous catalysis, by definition, refers to a catalytic tivity and enantioselectivity; the former starts with a chiral
system in which the substrates for a reaction and the cata- substrate and an achiral catalyst to produce diastereomers,
lyst components are brought together in one phase, most while the latter starts with an achiral (often called prochi-
often the liquid phase. More recently a narrower defini- ral) substrate and the optical activity is induced by a chiral
tion has become fashionable according to which homoge- catalyst.
neous catalysis involves organometallic complexes as the
catalysts (strictly speaking an organometallic compound II. ELEMENTARY STEPS FOR
should contain a bond between a carbon atom and the HOMOGENEOUS CATALYSIS
metal, but this is not true for all catalysts to be discussed, as
we will include coordination complexes in our definition).
A. Creation of a “Vacant Site” and
As a shorthand notation it will also be used for the present Coordination of the Substrate
contribution, but it should be borne in mind that there
are many interesting and important reactions employing To bring the reactants together a metal center must have a
homogeneous catalysts that are not organometallic or co- vacant site. Metal catalysis begins, we could say, with the
ordination complexes. Examples of such systems or cat- creation of a vacant site. In the condensed phase solvent
alysts are general acid and base catalysis (ester hydroly- molecules will always be coordinated to the metal ion and
sis), organic catalysts (thiazolium ions in Cannizzarro re- “vacant site” is an inaccurate description. Substrates are
actions), enzymatic processes, nitrogen oxides, chloride present in excess and so may be the ligands. Therefore, a
atoms, etc. competition in complex formation exists between the de-
Organometallic catalysts consist of a central metal sur- siredsubstrateandotherpotentialligandspresentintheso-
roundedbyorganic(andinorganic)ligands.Boththemetal lution. Often a negative order in one of the concentrations
and the large variety of ligands determine the properties of the “ligands” present can be found in the expression for