Chemistry Reference
In-Depth Information
13.2.1.4.2. Alternating Copolymerization of Epoxides and Carbon Dioxide
The fi rst
alternating copolymerization of
meso
-epoxides and carbon dioxide using chiral
zinc-based catalysts was reported in 1999 from our group [77-79]. Polymerization of
cyclohexene oxide (
42
) with 30 atm of CO
2
in the presence of Et
2
Zn/(
S
) -
-
diphenylpyrrolidin - 2 - yl - methanol (
45
) at 40°C produced alternating copolymers (
43
)
in excellent yield (Scheme 13.9) [94,95]. The complete alternating nature of the
polymer was confi rmed by
1
H NMR analyses, which showed no signal corresponding
to ether linkages. The degree of the asymmetric induction of the cyclohexene 1,2-diol
unit in the main chain can be unambiguously determined to be 70% ee (
R
,
R
) by
degradation of
43
into the corresponding diols (
44
) using NaOH in MeOH/THF.
The intensive screening of catalysts and reaction conditions revealed that the highest
enantioselectivity (80% ee) could be obtained when the catalyst
46
was employed
for the polymerization [96]. Costes and coworkers also independently reported the
altenating polymerization of cyclohexene oxide and CO
2
using chiral
α
,
α
β
- diiminate zinc
complex (
47
) [97] .
Ph
Ph
i
-Pr
Ph
Ph
OEt
N
N
O
N
Ar
Zn
N
Et
2
Zn +
Zn
Zn
H
H
H
t
-Bu
N
O
OH
Et
Me
3
Si
SiMe
3
45
Ar = 2,6-(
i
-Pr)
2
C
6
H
3
Ph
Ph
46
47
O
Inversion at
R
Hydrolysis
S
O
S
HO
S
S
O
OH
n
R
(
S
,
S
)-
43
(
S
,
S
)-
44
+
CO
2
S
O
R
O
42
Hydrolysis
R
HO
R
R
O
Inversion at
S
OH
n
O
(
R
,
R
)-
44
(
R
,
R
)-
43
Scheme 13.9.
13.2.1.5. Condensation Polymerization
13.2.1.5.1. Diels - Alder Polymerization
Enantioselective Diels - Alder polymerization
between prochiral bisdienophiles (
48
,
49
) and bisdienes (
50
) using chiral Lewis acids (
51
,
52
) affords novel optically active polymers possessing main-chain confi gurational chiral-
ity [98]. For instance, the reactions of bismaleimide
48a
and bisdiene
50b
proceeded in
CH
2
Cl
2
in the presence of BINOL-derived chiral aluminum catalyst (
51
), leading to the
