Chemistry Reference
In-Depth Information
Scheme 8.9
Various water-soluble ligands used for palladium-catalyzed carbon monoxide-
ethene copolymerization.
Previous experiments carried out with ethene using
38
(M=K) [66] had revealed
a low catalytic activity, possibly due the rather impure ligand [67]. In the presence
of Brønsted acids, such as trifluoroacetic or
p
-toluenesulfonic acid
38
(M=Na)
showed turnover frequencies up to 7.6
10
3
mol (mol h)
-1
at 70
C [68]. The anisyl
systems
39
and
40
, the former with turnover frequencies up to 2
10
5
mol
(mol h)
-1
, were even more active [69]. The productivity of
41
is somewhat lower
(
1.4
10
4
mol (mol h)
-1
at 85
C). Depending on the reaction conditions, the ac-
tivities of systems
42
and
43
may be even higher than that of
38
[44].
The substitution of nickel for palladium catalyst precursors has, in principle,
two advantages: the lower price of the metal and the lower tendency to plate. The
effectiveness of nickel in the carbonylation of ethene to low molecular weight
polyketones has been known for a long time [4]. Only recently, however, have
nickel catalyst precursors with a reasonably high catalytic activity been discovered.
The newest compounds are nickel complexes, which are modified by semicorrine-
type ligands (Scheme 8.10).
Turnover numbers close to 1.2
10
4
mol mol
-1
at 60
C were obtained with
44
,
R=C
3
F
7
or C
7
F
15
[70]. These catalysts are much more efficient than the previously
reported nickel-phenylpyrazolylborate complex
45
[71], nickel-pyridine carboxylate
46
[72], and nickel-dithiophenol derivatives
47
[73]. Catalyst precursors
48
[74],
49
[74], and
50
[75] have been reported in the patent literature, the most active being
48
, with a turnover frequency of 8.7
10
3
mol (mol h)
-1
C. Model studies
using [Ni(dppp)(S)(CH
3
)]X and [Ni(
o
-MeOdppe)(S)(CH
3
)]X (
48
, Scheme 8.10) have
revealed significant mechanistic differences with the analogous palladium system,
the most important being the involvement of low-energy five-coordinated inter-
mediates for the nickel catalysis [76].
Copper catalysts have also been mentioned as active for the copolymerization of
ethene [77].
at 80
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