Chemistry Reference
In-Depth Information
+
Pd
H
O
n-1
O
O
Diketone
MeOH
O
+
Pd
H
n
C
2
H
4
H
n
O
Ketoester
O
+
H
MeOH
Pd
+
n
OMe
Pd-OMe
CO
n
n CO
n-1 C
2
H
4
O
MeOH
H
+
O
P
+
O
Pd-H
+
+
Pd
OMe
Pd
OMe
n
C
2
H
4
O
n C
2
H
4
n-1 CO
+
CO
Pd
OMe
MeOH
O
n
O
OMe
MeO
n
O
O
Diester
+
Pd
H
O
n-1
O
Scheme 10.13.
In the CO/vinylarenes copolymerization, the main features of the catalytic cycle are
comparable to those of the CO/ethene copolymerization. In particular, the propagation
step is identical with alternating insertions of CO into Pd-alkyl bonds and of styrene
into Pd-l bonds. However, the initiation and termination steps are distinct and depend
on the nature of the olefi nic substrate [84]. The termination pathway that prevails in
methanol consists in fast
-hydrogen elimination from alkyl species and thus yields a
Pd-H complex at the end of the cycle.
β
10.4.1.3. Asymmetric Copolymerization of CO with Propene and Aliphatic
1-Alkenes
Unlike the reaction with ethene, the CO/propene copolymerization opens
the possibility of stereoregular copolymers. In this process, the regio- and stereochem-
istry are of critical importance. The mode of insertion of the
- olefi n into the Pd-acyl
or Pd-carbomethoxy bond in a 1,2 or 2,1 fashion governs the regiochemistry of the reac-
tion (Scheme 10.14 ).
Assuming that a regioregular copolymer is formed, the stereochemistry of the reac-
tion can lead to the production of isotactic, syndiotactic, or atactic structures (Fig. 10.22).
In the copolymerization of CO and aliphatic alkenes, the best results are obtained
by using catalyst-containing bidentate phosphine ligands. The electronic and steric prop-
erties of these ligands were shown to control the activity and productivity of the catalyst
and the regio- and stereoselectivities of the reaction.
α
