11008
J. Am. Chem. Soc. 1999, 121, 11008-11009
Table 1. Asymmetric Alternating Copolymerization of Epoxide 1
Optically Active Polycarbonates: Asymmetric
Alternating Copolymerization of Cyclohexene Oxide
and Carbon Dioxide
and Carbon Dioxidea
yield
yield
of 3d
%
epox- 4/Zn, temp, time, of 2,b
Mn
% ee
run ide mol/mol °C
h
%
(Mw/Mn)c
of 3e
Kyoko Nozaki,* Koji Nakano, and Tamejiro Hiyama
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0
0.60
0.80
1.10
1.20
1.00
1.00
60
60
60
60
60
40
40
60
60
60
60
60
60
60
24 >99 13000 (3.6) 85 51 (R,R)f
12
8
89 13000 (4.3) 95 57 (R,R)f
92 12000 (2.9) 87 58 (R,R)f
49 11000 (3.7) 98 59 (R,R)f
Department of Material Chemistry
Graduate School of Engineering, Kyoto UniVersity
Sakyo-ku, Kyoto 606-8501, Japan
4
2
37
9800 (2.7) 88 68 (R,R)f
12
4
48
48
48
85 13000 (1.9) 94 70 (R,R)f
20
0
8400 (2.2) 93 73 (R,R)f
3 (R,R)f
ReceiVed July 12, 1999
41 77000 (7.5) 79
Catalytic asymmetric synthesis polymerization is of much
interest as an efficient method to produce optically active polymers
from achiral monomers.1 Chiral catalysts create new chiral centers
in the main chain of the resulting polymers with control of the
absolute configuration. Precedents for such a methodology are
ring-opening polymerization of epoxide or episulfide,2 cyclopo-
lymerization of R,ω-dienes,3 polymerization of unsymmetrical
dienes4 or cyclic olefins,5 and alternating copolymerization of
R-olefins with carbon monoxide.6,7 Here, we report the first
example of asymmetric synthesis copolymerization of meso
epoxide 1 with CO2, initiated by a chiral Zn catalyst.8 Since the
ring-opening of epoxides involves configurational inversion at
one of the two chiral carbons,14 meso epoxides 1, achiral by
nature, produce copolymers 2, including chiral diol units -O-
CHR-CHR-O-. One intriguing feature of polycarbonates 2 is
their easy degradation into diols 3 and CO2 by alkali treatment,
which enables the unambiguous determination of the degree of
asymmetric induction.15 In this paper, we describe the synthesis
of completely alternating copolymer 2a from cyclohexene oxide
10
11
12
13 1b
14 1c
71 43000 (13.7) 76 11 (R,R)f
24 >99 18000 (1.7) 82 23 (R,R)f
48
68
48
98 13000 (1.7) 85 22 (R,R)f
66 12000 (3.9) 83 24 (R,R)g
8
20000 (2.0) 70 34 (R,R)f
a Meso epoxide (1, 10 mmol) was treated with carbon dioxide (30
atm) in the presence of a mixture of an amino alcohol 4 (0.50 mmol)
and Et2Zn (1.25 M in hexane, 0.50 mmol) in toluene (17 mL). After
aqueous workup, the resulting copolymer was precipitated with MeOH,
filtered, and eluted by CHCl3. b Calculated based on 1. c Estimated by
size-exclusion chromatography analysis using a polystyrene standard.
d Calculated based on 2. e Absolute configuration is shown in paren-
f
theses. Determined by GLC analysis with a chiral column (Chrompack,
CHIRASIL-DEX CB). g The product 3b was derivatized into its
dibenzoate, and the % ee was determined by HPLC analysis with a
chiral column (Daicel, CHIRALCEL OJ).
(1a) and CO2, using Et2Zn-chiral amino alcohol 4. Enantiomeric
excess of 70%, determined as 3a, has been achieved.
(1) Reviews: (a) Okamoto, Y.; Nakano, T. Chem. ReV. 1994, 94, 349. (b)
Tsuruta, T. J. Polym. Sci. Part D 1972, 179.
(2) Spassky, N.; Momtaz, A.; Kassamaly, A.; Sepulchre, M. Chirality 1992,
4, 295 and references cited there in.
(3) (a) Coates, G. W.; Waymouth, R. M. J. Am. Chem. Soc. 1991, 113,
6270. (b) Coates, G. W.; Waymouth, R. M. J. Am. Chem. Soc. 1993, 115, 91.
(4) (a) Pino, P. AdV. Polym. Sci. 1965, 4, 393. (b) Natta, G.; Farina, M.;
Donati, M. Makromol. Chem. 1961, 43, 251. (c) Tsunetsugu, T.; Fueno, T.;
Furukawa, J. Makromol. Chem. 1968, 112, 220.
(5) (a) Okamoto, Y.; Nakano, T.; Kobayashi, H.; Hatada, K. Polym. Bull.
1991, 25, 5. (b) Onimura, K.; Tsutsumi, H.; Oishi, T. Chem. Lett. 1998, 791.
(6) Nozaki, K.; Hiyama, T. J. Organomet. Chem. 1999, 576, 248 and
references cited therein.
(7) For helix-sense selective polymerization of achiral monomers, see ref
1a. Enantiomer selective polymerization is also known, for example: Spassky,
N.; Pluta, C.; Simic, V.; Thiam, M.; Wisniewski, M. Macromol. Symp. 1998,
128, 39. See also refs 1a and 2.
Copolymers 2 were obtained by treatment of meso epoxides 1
with 30 atm of CO2 in the presence of a mixture of Et2Zn and
chiral amino alcohol 4.16 The representative results are sum-
marized in Table 1. Using a 1:1 mixture of Et2Zn and (S)-R,R-
diphenylpyrrolidine-2-yl-methanol (4),17 copolymer 2a was given
in a quantitative yield from cyclohexene oxide (1a) (run 1). The
completely alternating nature of 2a was manifested by 1H NMR,
as shown in Figure 1. The peak assigned to the methine proton
is observed at δ 4.60 (for carbonate, -CH-OCO2CH-);18 no
peak was observed at δ 3.45 (for ether, -CH-OCH-) attributable
to a homopolymer of 1a.
(8) Since the pioneering work by Inoue and Tsuruta using a mixture of
Et2Zn and H2O,9 mixtures of Et2Zn with alcohols, carboxylic acids, or phenols
were applied as an achiral catalyst to the copolymerization of epoxide and
CO2.10 Recent elegant works by Darensbourg11 and Coates12 provided new
types of catalysts with well-defined structures, with which much higher
catalytic activities and smaller molecular-weight distribution were achieved.
Reaction in super-critical CO2 also improved the catalytic activity.13
(9) Inoue, S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci., Part B 1969, 7,
287.
(10) Review articles, for example: (a) Rokicki, A.; Kuran, W. J. Macromol.
Sci., ReV. Macromol. Chem. 1981, 21, 135. (b) Inoue, S. Carbon Dioxide as
a Source of Carbon; Aresta, M., Forti, G., Eds.; Reidel Publishing Co.:
Dordrecht, 1987; p 331. (c) Darensbourg, D. J.; Holtcamp, M. W. Coord.
Chem. ReV. 1996, 153, 155. (d) Beckman, E. J. Science 1999, 283, 946.
(11) (a) Darensbourg, D. J.; Holtcamp, M. W. Macromolecules 1995, 28,
7577. (b) Darensbourg, D. J.; Niezgoda, S. A.; Draper, J. D.; Reibenspies, J.
H. J. Am. Chem. Soc. 1998, 120, 4690. (c) Darensbourg, D. J.; Holtcamp, M.
W.; Struck, G. E.; Zimmer, M. S.; Niezgoda, S. A.; Rainey, P.; Robertson, J.
B.; Draper, J. D.; Reinbenspies, J. H. J. Am. Chem. Soc. 1999, 121, 107.
(12) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998,
120, 11018.
Hydrolysis of 2a with aqueous NaOH gave (1R,2R)-cyclohex-
ane-1,2-diol {(R,R)-3a} of 51% ee in 85% isolated yield.15 With
shorter reaction time, the percent ee of (R,R)-3a was slightly
improved (runs 1-5), and at lower reaction temperature, 40 °C,
the enantiomeric excess of (R,R)-3a was raised to 70% (run 6).
In all runs, meso-3a {(1R*,2S*)-3a} was not obtained, which
confirms the completely SN2-type ring-opening of 1a during the
copolymerization.14
The 13C NMR of the sample of run 6 (Figure 2 (i)) raises a
question regarding the reported assignment of syndiotactic and
(13) (a) Super, M.; Berluche, E.; Costello, C.; Beckman, E. Macromolecules
1997, 30, 368. (b) Super, M.; Beckman, E. J. Macromol. Symp. 1998, 127,
89.
(16) The experimental procedure is submitted as Supporting Information.
(17) The structure of the active species is not clear at this moment. Addition
of amino alcohol 4 to Et2Zn (in hexane-toluene-d8) at 20 °C was followed
(14) Using achiral Zn catalysts, the ring-opening has been reported to
proceed in a completely SN2 fashion.15
by H NMR which implies that both of the ethyl groups of Et2Zn might be
1
(15) Inoue, S.; Koinuma, H.; Yokoo, Y.; Tsuruta, T. Makromol. Chem.
1971, 143, 97.
protonated. The charts are available in the Supporting Information.
(18) Koinuma, H.; Hirai, H. Makromol. Chem. 1977, 178, 1283.
10.1021/ja992433b CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/12/1999