_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Catalysis of reactions of alkenes by transition metal complexes
butene or add a second ethane molecule leading to chain growth. The relative
importance of these two paths can be controlled by changing the phosphine
ligands. In the system [ (17 3 -C 3 H 5 )NiCl],/EtAlC1 2 /L at - 20°C/ 1 atm. for instance,
only 1-butene is formed when L = Me 3 P. Bulky phosphines hinder /3-hydrogen
transfer more than alkene insertion. When L = P( cyclohexyl) 3 , 70% bute ne, 2 5%
hexene and 5% higher oligomers are formed. In the extreme case of L = PBu~.
olefin elimination is essentially repressed, so that polyethene is produced.
Selective codimerization of ethene and 1, 3-butadiene is also possible because
the diene first forms an 17 3 -allyl complex with the metal centre (Fig. 12.4). An
industrial process along these lines has been developed in the USA by Du Pont.
A rhodium catalyst is employed; rhodium trichloride in ethanol itself gives 80%
selectivity towards the desired product, trans-1, 4-hexadiene, which is used in the
manufacture of an ethene-propene-hexadiene synthetic rubber. The diene
introduces some double bonds into the polymer chains which are required for
vulcanization.
12.3.5 Oligomerization of butadiene
Wilke and his coworkers at the Max Planck Institut fiir Kohlenforschung at
Miilheim, Ruhr, ha ve extensively studied oligomerization reactions of butadiene
using a wide range of transition metal catalysts. Those based on nickel and on
palladium are the most useful. Nickel systems generally afford cyclic products,
whereas linear materials are obtained using palladium.
The nickel catalyst is obtained by reduction of a nickel(II) compound such as
the acetylacetonate in the presence of butadiene, or simply by exchange of
butadiene and bis(17 3 -allyl)nickel or bis(cycloocta-1. 5-diene)nickel. In the absence
of added ligands, butadiene is rapidly and catalytically converted at low
temperatures into 1, 5, 9-cyclododecatriene, mainly the trans, trans. trans isomer.
The trimerization bears the mark of a 'template' process, three butadiene
molecules linking very specifically around a 'naked nickel' centre. Detailed
studies show that the process is not synchronous but occurs in a stepwise fashion
(Fig. 12.5 ).
The C 12 1J 2 • 17 3 • 17 3 -intermediate has been isolated from the reaction mixture at
- 40°C. The 1 H n.m.r. spectrum shows that in solution two isomers are present in
which the allyl groups bear anti substituents. With PMe 3 an adduct is formed in
which isomerization to syn substituents has occurred, presumably via an 17 1 -
intermediate. This isomerization explains why the trans, trans. trans isomer of
cyclododecatriene is the major product. Cyclododecatriene is used as a precursor
to Nylon 12.
When a tertiary phosphine or phosphite L is added in the ratio l: 1 (Ni:L),
coordination of a third butadiene molecule is prevented, so that dimers rather
than trimers result. The first key intermediate is an 17 1 ,17 3 -octadienyl complex.
This gives rise to vinylcyclohexene (VCH). If Lis weakly basic. X rearranges to an
17 3 , 17 3 -C 8 H 12 species Y which leads to cis-1. 5-cyclooctadiene (COD) (Fig. 12.6).
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