Page 140 - 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

488 Catalysis, Homogeneous

FIGURE 75 Cyclopropanation yielding pyrethroid insecticides.
FIGURE 73 Ring-closing metathesis.

in this reaction. Schematically the reaction mechanism
acid to a substituted amide Cilastatin, a pharmaceutical is
can be imagined as a sequence involving alkene complex-
obtained.
ation, oxidative addition of the silane breaking the Si−H
Cyclopropanation of dienes leads to an important class
bond, and insertion and reductive elimination, as shown in
of compound used as insecticides, the pyrethroid-type
Fig. 78. It is referred to as the Chalk-Harrod mechanism
compounds related to chrysanthemic acid, a natural insec-
and it follows exactly the elementary steps described in the
ticide obtained from East African chrysanthemums. Cat-
introduction. The precise nature of the platinum catalyst
alysts used comprise rhodium (II) dimers of carboxylates
for this process, however, has not been delineated. The cat-
and related compounds. Chiral complexes can lead to high
alysts are extremely sensitive to the conditions applied and
enantioselectivities. This chemistry has been successfully
in practice one encounters incubation times, capricious ki-
developed by Doyle. In Fig. 75 the synthesis of the ba-
netics, alkene isomerization, formation of unsaturated side
sic skeleton has been depicted, which requires replace-
products, etc.
ment of the ethyl group by other groups. Interestingly,
A large variety of metal complexes catalyze the hy-
the “carbene” fragment can also be inserted into N−H
drosilylation reaction. Other double bonds can serve as
or C−H bonds. The former reaction is used for making
the silane acceptor, such as alkynes, ketones, and imines.
β-lactams.
Forthelattertwosubstratesasymmetriccatalystsalsohave
The activity of ruthenium complexes for this reaction
been developed.
affords a bridge between the metathesis reaction in the
previous section and cyclopropanation, an area that has
been explored by Noels. A common intermediate can be N. Polyesters
imagined for the two reactions leading either to metathesis
One of the largest applications of homogeneous catal-
or cyclopropanation, as shown schematically in Fig. 76.
ysis involves the manufacture of polyesters. In a sim-
ple condensation process diacids and diols are poly-
M. Hydrosilylation merized forming the polymers. The simplest one is
polyethyleneterephthalate (PET), which is made from
The catalytic addition of a silicon compound contain-
ethylene glycol and terephthalic acid as discussed above.
ing an Si−H bond to a C=C double bond is the most
common way for the synthesis of silane or silicon com-
pounds. The silicon compounds are used to make poly-
mers (silicone rubbers) or a large variety of agents used
for the modification of surfaces. For making polymers the
reagent used is, for instance, methyldichlorosilane, which
is added to vinyl compounds using a catalyst (Fig. 77). The
dichlorosilane compounds are hydrolyzed to give silicone
polymers.
For industrial processes the oldest and preferred cata-
lyst for the hydrosilylation reaction is Speier’s catalyst,
H 2 PtCl 6 , which can be used in parts per million quanti-
ties. The catalyst is reduced in situ and probably alcohols
and/or other oxygen-containing compounds are involved

FIGURE 74 Cyclopropanation using diazocompounds as “car- FIGURE 76 Relation between metathesis and cyclopropanation
bene” precursor. mechanism.