C O M M U N I C A T I O N S
Table 1. Enantioselective Polymerization of Racemic Epoxides
Catalyzed by 3/[PPN][OAc]: Measurement of sa
epoxide
subs. [epox]/ time conv. ee(SM) [mm] ee(P)
Mn
entry
(R)
[3]
(h)
(%)b (%)c (%)d (%)e s-factorf (kg/mol)g Mw/Mng
1
2
3
4
Me
Et
4000 0.25 34 51 (R) 98.6 99.1 370
1000 0.25 22 29 (R) 98.8 99.2 330
667 0.33 19 24 (R) 98.6 99.1 260
26.4
61.4
76.8
98.9
1.8
2.0
2.1
1.9
nBu
Ph
Figure 1. Isospecific polymerization of rac-propylene oxide catalyzed by
rac-3/[PPN][OAc].
1000 17.0
20 26 (R) 94.4 96.1
63
>99%) using 0.1 mol % rac-3 at 0 °C for 3.5 h. Therefore, rac-3
represents the first catalyst that exhibits high selectivity for the
isospecific polymerization of a range of epoxides.
In conclusion, we report the first highly selective polymerization
catalyst for the kinetic resolution of epoxides. The catalyst exhibits
exceptional levels of enantioselectivity and activity. The racemic
form of the catalyst polymerizes racemic epoxides to highly isotactic
polyethers in quantitative yield. Our future studies will focus on
the mechanism of action of 3, with intent to guide efforts to increase
stereoselectivity, activity, and substrate scope.
a General conditions: [PPN][OAc]/[3] ) 2:1, Trxn ) 0 °C; [epoxide]
)
2 M in toluene, except for styrene oxide, which was neat.
b Conversion of epoxide, determined gravimetrically or by 1H NMR
spectroscopy. c %ee of the remaining starting material, determined by
chiral gas chromatography for entries
1 and 4
and by 1H NMR
spectroscopy using a chiral shift reagent for entries 2 and 3.12 d Isotactic
[mm] triad content, determined by 13C NMR spectroscopy. e %ee of the
repeat units in the polymer, calculated using: ee(P) ) (2[mm] + [mr] +
[rm] - 1)1/2 f Calculated using s ) kS/kR ) ln(1 - c(1 + ee(P)))/ln(1 -
.
c(1 - ee(P))) where c is the conversion of epoxide. g Determined by
gel-permeation chromatography calibrated with polystyrene standards in
1,2,4-Cl3C6H3 at 140 °C.
Acknowledgment. We thank the NSF (CHE-0243605 and CHE-
0809778) and Sumitomo Chemicals for financial support.
Table 2. Enantioselective Polymerization of Racemic Epoxides
Catalyzed by 3/[PPN][OAc]: Preparative Reactionsa
epoxide
subs. (R)
Supporting Information Available: Experimental procedures for
catalyst synthesis and polymerizations, spectroscopic data for polymers,
and X-ray data for 2 (CIF). This material is available free of charge
entry
[epox]/[3]
time (h)
conv. (%)b
yield (%)c
ee(SM) (%)d
1
2
3
4
Me
Et
4000
1000
1000
500
1.5
52.0
15.0
15.0
51
51
52
55
49
49
48
45
>99 (R)
99 (R)
>99 (R)
>99 (R)
nBu
Ph
References
a General conditions: [PPN][OAc]/[3] ) 2:1 for all reactions; Trxn
)
(1) ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yama-
moto, H., Eds.; Springer: Berlin, 1999.
0 °C and [epoxide] ) 2 M in toluene for entries 1 and 2, Trxn ) 22 °C
and neat substrate for entries 3 and 4. b Determined gravimetrically after
removing residual monomer and solvent in Vacuo. c Yield of recovered
epoxide. d Determined by chiral gas chromatography for propylene and
styrene oxide and by 1H NMR spectroscopy using a chiral shift reagent
for butene and hexene oxide.12
(2) StereoselectiVe Polymerization with Single-Site Catalysts; Baugh, L. S.,
Canich, J. M., Eds.; CRC Press: Boca Raton, FL, 2007.
(3) (a) Okamoto, Y.; Nakano, T. Chem. ReV. 1994, 94, 349–372. (b) Coates,
G. W.“Polymerization Reactions” in ref 1, Vol. 3, pp 1329-1349.
(4) For some leading lactone references, see: (a) Spassky, N.; Wisniewski,
M.; Pluta, C.; Le Borgne, A. Macromol. Chem. Phys. 1996, 197, 2627–
2637. (b) van Buijtenen, J.; van As, B. A. C.; Verbruggen, M.; Roumen,
L.; Vekemans, J. A. J. M.; Pieterse, K.; Hilbers, P. A. J.; Hulshof, L. A.;
Palmans, A. R. A.; Meijer, E. W. J. Am. Chem. Soc. 2007, 129, 7393–
7398. For a leading alkene reference, see: (c) Baar, C.; Levy, C. J.; Min,
E. Y.-J.; Henling, L. M.; Day, M. W.; Bercaw, J. E. J. Am. Chem. Soc.
2004, 126, 8216–8231. For a thiirane reference using a binol/Zn complex,
see: (d) Se´pulchre, M. Makromol. Chem. 1987, 188, 1583–1596.
(5) Nielsen, L. P. C.; Jacobsen, E. N. Catalytic, Asymmetric Epoxide Ring-
Opening Chemistry, in Aziridines and Epoxides in Organic Synthesis;
Yudin, A., Ed.; Wiley: New York, 2006; Chapter 10 and references therein.
(6) For a review, see: Ajiro, H.; Allen, S. D.; Coates, G. W. Discrete Catalysts
for Stereoselective Epoxide Polymerization in ref. 2, Chapter 24, pp 627-
644.
based on the % ee of the starting material, we examined the
enantiomeric purity of units in the resultant polymer as a function
of conversion, which gives significantly higher accuracy (Table 1).12
As can be seen in entries 1-4, 3/[PPN][OAc] exhibits an un-
precedented level of enantioselectivity for the polymerization of
epoxides. The catalyst system is highly active for the polymerization
of aliphatic epoxides and exhibits s-factors 260-370. The system
is also active for styrene oxide. Although the rate and enantiose-
lectivity are significantly slower than those for aliphatic epoxides,
the s-factor (63) is well above the threshold for preparative utility.
To fully explore the ability of 3/[PPN][OAc] to prepare enan-
tiopure epoxides, we optimized the kinetic resolution of various
substrates (Table 2). Highly active propylene oxide was cleanly
resolved (>99% ee; 98% of maximum theoretical yield) in 4 h
using 0.025 mol% 3 at 0 °C. Butene and hexene oxides required
0.1 mol% catalyst and longer reaction times but were also resolved
in high yield (96-98% of maximum theoretical yield). Due to a
lower s-factor, styrene oxide was resolved giving enantiopure
epoxide in 90% yield.
(7) To date, polymerization catalysts for the enantioselective polymerization
of epoxides exhibit low s-factors (s ) kfast/kslow) and activities; see ref 6
and references therein. The kinetic resolution copolymerization of propylene
oxide with CO2 has been reported with s ) 9.7. Cohen, C. T.; Coates,
G. W. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 5182–5191.
(8) A highly active catalyst system for the isoselective polymerization of
racemic epoxides to form isotactic poly(propylene oxide) has been
reported: Peretti, K. L.; Ajiro, H.; Cohen, C. T.; Lobkovsky, E. B.; Coates,
G. W. J. Am. Chem. Soc. 2005, 127, 11566–11567.
(9) (a) Braune, W.; Okuda, J. Angew. Chem., Int. Ed. 2003, 42, 64–68. (b)
Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem.
Soc. 2003, 125, 11911–11924. (c) Lee, B. Y.; Kwon, H. Y.; Lee, S. Y.;
Na, S. J.; Han, S. I.; Yun, H. S.; Lee, H.; Park, Y. W. J. Am. Chem. Soc.
2005, 127, 3031–3037.
(10) Ajiro, H.; Peretti, K. L.; Coates, G. W. Manuscript in preparation.
(11) For an example of a bimetallic catalyst utilizing a chiral binaphtol dialdehyde
linker, see: Guo, Q.-X.; Wu, Z.-J.; Luo, Z.-B.; Liu, Q.-Z.; Ye, J.-L.; Luo,
S.-W.; Cun, L.-F.; Gong, L.-Z. J. Am. Chem. Soc. 2007, 129, 13927–13938.
(12) See Supporting Information.
(13) Cohen, C. T.; Chu, T.; Coates, G. W. J. Am. Chem. Soc. 2005, 127, 10869–
10878.
(14) Thomas, R. M.; Jeske, R. C.; Hirahata, W.; Coates, G. W. Manuscript in
preparation.
Using rac-3 (equimolar mixture of (R,R,S,R,R)-3 and (S,S,R,S,S)-
3), we investigated the isoselective polymerization of racemic
epoxides. A wide range of terminal epoxides (alkyl, aryl, alkoxy
methyl, fluoroalkyl substituents) undergo rapid polymerization to
highly isotactic polyether in >99% yield.14 Specifically, propylene
oxide was quantitatively polymerized to isotactic polymer ([mm]
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