Chemistry Reference
In-Depth Information
10.4.2. Asymmetric CO /Olefi ns Terpolymerization
The CO/alkene copolymer chains are packed in orderly fashion, which makes them
highly crystalline but also very fragile. One way to avoid this problem is to introduce
another alkene to somehow disturb the crystal packing. The strictly alternating CO/
alkene polyketones obtained contains the two types of units: CO/alkene
1
and CO/
alkene
2
(Scheme 10.15 ).
R
2
O
[Pd]
R
1
R
2
+
m
n
CO
O
R
1
Scheme 10.15.
Similar to the copolymerization catalysts, the terpolymerization of CO with aliphatic
olefi ns is usually performed by using complexes bearing phosphorus-based ligands [108],
while the only reported example of terpolymerization of CO with two vinylarenes was
promoted by Pd(phenanthroline) complexes [109,110]. The terpolymerization of CO
with propene and ethene using chiral diphosphine ligands
108
derived from carbohy-
drate (Fig. 10.26) was recently reported and showed that the presence of methoxy
substituents on the phenyl rings bonded to the phosphorus atoms increased the degree
of propene incorporation [111]. The terpolymerization of CO with one aliphatic and one
aromatic olefi ns is somehow more versatile, and various types of ligand systems, such as
P - P [112] , P - OP [113] , P - N [109,114] and N - N [94,95d,112,115 - 118] , have been success-
fully used.
Brookhart and Consiglio reported the use of chiral ligands (
108
and
109,
Fig. 10.26 )
in this reaction to produce optically active terpolymers. In contrast with the results
obtained with dinitrogen ligands, the terpolymers produced by using the P- OP ligand
(
R
,
S
)-BINAPHOS were shown to contain a much higher incorporation of aliphatic
alkenes than vinylarenes.
More recently, the use of the cationic palladium systems [PdMe(NCMe)(N-N′ )]
[BArF] containing pyridine-imidazoline ligands
cis
-
107a
,
cis
-
107b
, and
cis
and
trans
-
107d
was reported in the terpolymerization of CO/TBS/ethane, and circular dichroism analy-
sis of the terpolymer obtained with the enantiomerically pure ligand
107d
shows that it
is optically active [119] .
ArP
PAr
Ph
O
O
O
O
O
N
N
N
N
CH
2
OMe
O
Me
Me
1
1
Ar = Ph, 2-OMe-Ph
108
Figure 10.26.
P-P and N-N ligands used for the Pd-catalyzed terpolymerization of CO with olefi ns.
