Chemistry Reference
In-Depth Information
9.1
Introduction
Ever since the catalytic Pauson-Khand reaction (PKR) was discovered,
1
most studies have
focused on its homogeneous reactions.
2
Knowledge about such reactions through extensive
studies has provided significant help in developing new homogeneous catalysts. In contrast,
the development of heterogeneous catalysts for PKR lags far behind, presumably due to
difficulties in studying the reaction process. For a PKR, only limited types of heterogeneous
reactions have been reported that include reactions between a liquid and a solid, which is
the most common type of a heterogeneous PKR, and a handful of PKRs between an organic
solvent and water (in biphasic conditions).
3
Reactions between a gas and a solid have not
been reported.
While heterogeneous solid catalysts have inherent operational advantages that include
ease of handling or separation for recycling, both of which are difficult in their homogeneous
counterpart, the complexity of the reaction that arises mainly from the various irregular
interactions between the stabilizer, solvent or substrate and the metal surface often hampers
the design of a reaction. Thus, tailoring the heterogeneous catalysts to reach the desired
selectivity is not easy, and further understanding regarding the reaction mechanisms of
sufficient precision is still required although it is extremely difficult.
Distinguishing heterogeneous catalysis from homogeneous is an important issue and
yet is not always obvious. For example, problems regarding the true homogeneity or real
heterogeneity of catalytic systems used in the Pd(0)-catalyzed C-C cross-coupling reactions
have often been a subject of debate
4
because they seem to be essential for understanding
the reaction mechanism and function of Pd catalysts. For this reason, many useful methods
for assessing heterogeneity and homogeneity in catalytic reactions have been proposed.
5
In relation to PKRs, however, this kind of problem has not arised yet and thus we will
directly prompt discussion about the variety of heterogeneous catalysis of the PK and
related reactions instead of starting with the general aspects of heterogeneity of them. For
a general discussion of homogeneous/heterogeneous catalytic systems, please see other
topics and reviews.
6
9.2 Development of Heterogeneous Catalysts for PKR
Endeavors to develop heterogeneous catalysts of the PKR have been channeled into two
different approaches: one is the immobilization of homogeneous catalysts (e.g., cobalt
carbonyls) on solid supports such as insoluble polymers
7
and dendrons
8
and the other is the
use of metallic cobalt itself as a catalyst. The development of heterogenized homogeneous
catalysts, which are intended to combine the advantages of homogeneous catalysts with
those of heterogeneous ones, seems to be highly desirable. However, they suffer from many
difficulties that include tedious and/or expensive syntheses of polymeric ligands, reduced
catalytic activities and selectivities relative to the homogeneous catalyst, and excessive
leaching of the catalytic species from the solid support. A catalytic system that overcomes
