Chemistry Reference
In-Depth Information
substituted ethene at the stage of incorporation during the polymerization process
(Scheme 13.6). The representative ligands for palladium-catalyzed enantioselective
alternating copolymerization of monosubstituted ethenes and CO are shown below.
PPh
2
O
P
O
O
PBu
2
PBu
2
PCy
2
PCy
2
P
O
O
P
25
26
27
28
(
R
,
S
)-BINAPHOS
(
S
)-BICHEP
(
S
,
S
)-Me-DUPHOS
O
O
O
PCy
2
N
∗
PPh
2
N
N
N
Fe
Ph
2
P
N
Ph
29
30
31
32
(
R
)(
S
p
)-JOSIPHOS
R
cat. Pd
−
L*
R
+
CO
∗
O
n
Scheme 13.6.
The fi rst synthesis of optically active copolymers of monosubstituted ethenes and CO
([
α
]
D
+10.4 in (CF
3
)
2
CHOH (HFIP)) was achieved in 1990 by using Pd(OAc)
2
/
(
) - 4,5 - bis(dibutylphosphinomethyl) - 2,2 - dimethyl - 1,3 - dioxolane (
25
) as a catalyst [68].
In 1992, Consiglio and coworkers developed enantioselective alternating copolymeriza-
tion catalyzed by palladium-(
S
) - BICHEP (
26
) complexes [69 - 72] . The polymers obtained
with
26
have essentially complete head-to-tail regioselectivity and high
like
- diad stere-
oselectivity (>93%). However, the enatioselectivity has been either low or not deter-
mined except in a few cases. In 1995, a more effective catalyst, [Pd((
S
,
S
) -
27
)(MeCN)
2
]
(BF
4
)
2
, was developed for the asymmetric alternating copolymerization of monosubsti-
tuted ethenes with carbon monoxide to form optically active, isotactic polymers [73,74].
The polymers synthesized in MeNO
2
/MeOH have both 1,4-ketone structures and spiro-
ketal structures in the main chain. The pure propylene/CO poly(1,4-ketone) was pre-
pared by reprecipitation of the poly(spiroketal/1,4-ketone) copolymers, showing optical
rotations of up to [
−
]
D
+22° (in HFIP). The optical purity of the stereogenic centers in
the main chain is over 90%, as estimated by NMR analysis using a chiral Eu shift reagent.
The copolymerization of functionalized terminal alkenes with CO has also been inves-
tigated by using [Pd((
S
,
S
) -
27
)(MeCN)
2
](BF
4
)
2
as a catalyst [74]. Recently, a modifi ed
(
R
)(
S
p
) - JOSIPHOS system (
29
) [75,76] was reported to show the highest catalytic activ-
ity (1797 g g(Pd)
− 1
· h
− 1
) for the copolymerization of propene/CO to give a polymer with
α
