Copolymerization of cyclohexene oxide with CO2 by using intramolecular dinuclear zinc catalysts

Article abstract of DOI:10.1002/chem.200401159

The intramolecular dinuclear zinc complexes generated in situ from the reaction of multidentate semi-azacrown ether ligands with Et₂Zn, followed by treatment with an alcohol additive, were found to promote the copolymerization of CO₂

Full text of DOI:10.1002/chem.200401159

Copolymerization of Cyclohexene Oxide with CO2 by Using Intramolecular
Dinuclear Zinc Catalysts
Youli Xiao, Zheng Wang, and Kuiling Ding*[a]
Abstract: The intramolecular dinuclear
zinc complexes generated in situ from
the reaction of multidentate semi-aza-
crown ether ligands with Et2Zn, fol-
lowed by treatment with an alcohol ad-
ditive, were found to promote the co-
polymerization of CO2 and cyclohex-
ene oxide (CHO) with completely al-
ternating polycarbonate selectivity and
high efficiency. With this type of novel
initiator, the copolymerization could be
accomplished under mild conditions at
1 atm pressure of CO2, which repre-
sents a significant advantage over most
catalytic systems developed for this re-
action so far. The copolymerization re-
action was demonstrated to be a living
process as a result of the narrow poly-
dispersities and the linear increase in
the molecular weight with conversion
of CHO. In addition, the solid-state
structure of the dinuclear zinc complex
was characterized by X-ray crystal
structural analysis and can be consid-
ered as a model of the active catalyst.
On the basis of the various efforts
made to understand the mechanisms of
ization is initiated by the insertion of
À
CO2 into the Zn OEt bond to afford a
carbonate–ester-bridged complex. The
dinuclear zinc structure of the catalyst
remains intact throughout the copoly-
merization. The bridged zinc centers
may have a synergistic effect on the co-
polymerization reaction; one zinc
center could activate the epoxide
through its coordination and the
second zinc atom may be responsible
for carbonate propagation by nucleo-
philic attack by the carbonate ester on
the back side of the cis-epoxide ring to
afford the carbonate. The mechanistic
implication of this is particularly impor-
tant for future research into the design
of efficient and practical catalysts for
the copolymerization of epoxides with
CO2.
the
catalytic
reaction,
including
MALDI-TOF mass analysis of the co-
polymersꢀ end-groups, the effect of al-
cohol additives on the catalysis and
CO2 pressure on the conversion of
CHO, as well as the kinetic data gained
from in situ IR spectroscopy, a plausi-
ble catalytic cycle for the present reac-
tion system is outlined. The copolymer-
Keywords: carbon dioxide · cooper-
ative phenomena · copolymeriza-
tion · epoxides · polycarbonates ·
zinc
Introduction
ways to effectively utilize CO2, the synthesis of poly-
carbonates through metal-catalyzed coupling reactions of
CO2 and epoxides, was first reported by Inoue et al. in
1969[2] and has attracted much attention over the past few
decades.[3,4] A wide variety of catalytic systems, including
both heterogeneous catalyst mixtures[5] and homogeneous
discrete metal complex catalysts,[6–14] have been developed
over the past decade with the latter being the focus of most
current research owing not only to their high activities, but
also to their well-defined structures that allow mechanistic
investigations.
Among the metal complex catalysts developed so far for
CO2/epoxide copolymerization, zinc complexes have been of
considerable interest.[9,11,12,14] Mechanistic studies of the cat-
alyzed copolymerization reactions involving these complexes
suggest that various dimeric zinc complexes formed by the
bridging of two zinc monomers are the effective catalysts
and that a bimetallic epoxide enchainment mechanism often
CO2 is not only an inexpensive and abundant one-carbon
chemical feedstock, but also an atmospheric greenhouse gas
that contributes to global warming. Thus, the development
of efficient catalytic processes that employ this nontoxic
carbon source with the simultaneous reduction of its envi-
ronmental impact is of great interest and has been a long-
standing goal for chemists.[1] One of the most promising
[a] Y. Xiao, Dr. Z. Wang, Prof. Dr. K. Ding
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Road, Shanghai 200032 (China)
Fax : (+86)21-6416-6128
E-mail: kding@mail.sioc.ac.cn
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
3668
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200401159
Chem. Eur. J. 2005, 11, 3668 – 3678

FULL PAPER
dominates the polymerization process.[9d,11c,11e,14b] For exam-
ple, Darensbourg et al. showed that dimeric zinc–phenolate
complex 1 could promote CO2/cyclohexene oxide (CHO)
copolymerization efficiently.[9d] An elegant kinetic study per-
bring the two metal centers sufficiently close together to
allow for a synergistic effect of the two metallic centers in
CO2/CHO copolymerization. In this report, we describe our
results of a novel use of intramolecular dinuclear zinc com-
plexes in the catalysis of CO2/CHO copolymerization under
mild conditions with good activity and excellent chemoselec-
tivity.
Results and Discussion
This research was initiated by testing the potential catalytic
activity of Trostꢀs zinc complex 5a in the copolymerization
of CO2 and CHO (Scheme 1). Complex 5a was prepared by
formed by Coates and co-workers on the copolymerization
of CO2 with CHO catalyzed by a zinc b-diiminate complex
revealed that dimeric zinc complex 2 is involved in the tran-
sition state of the epoxide ring-opening event.[11e] In 1999,
Nozaki et al. reported the first example of the asymmetric
copolymerization of CO2/CHO using a mixture of ZnEt2
and (S)-a,a-diphenyl(pyrrolidin-2-yl)methanol, which af-
fords the polycarbonate in excellent yields and with moder-
ate enantioselectivity.[14a] Further mechanistic studies dem-
onstrated that a dimeric zinc complex 3 might be the active
species and that the copolymerization was initiated by CO2
Scheme 1. Copolymerization of cyclohexene oxide with CO2 in the pres-
ence of a dinuclear zinc complex as catalyst.
the in situ reaction of two equivalents of Et2Zn with one
equivalent of ligand 4a. Although the exact structure of this
complex has not been confirmed yet owing to the difficulties
associated with its isolation and structural characterization,
we prefer the structure of 5a proposed by Trost et al., since
it is well supported by the stoichiometric evolution of
ethane in the complex formation,[15a,c,g] ESI-MS spectromet-
ric studies,[15b] as well as MM2 force field calculations.[15a] In
the presence of complex 5a (5 mol%), the copolymerization
reaction was carried out under 30 atm of CO2 in toluene at
608C for 24 h. It was found that this catalytic system exhibit-
ed good activity in the CO2/CHO copolymerization reaction,
affording poly(cyclohexene carbonate) (PCHC) in a quanti-
[14b]
À
insertion into the Zn OEt bond of the complex.
All
these studies have demonstrated that it is highly likely that
catalysts that contain intramolecular dinuclear zinc com-
plexes with multidentate ligands could be designed for the
copolymerization of olefin epoxide with CO2.
Trostꢀs dinuclear zinc complex 5a, easily prepared from
the multidentate semi-azacrown ether ligand 4a and Et2Zn,
has been successfully applied to the catalysis of a variety of
enantioselective reactions.[15] On the basis of the mechanistic
understanding described above, we envisioned that this com-
plex may serve as a promising catalyst for the copolymeriza-
tion of CO2 and epoxide owing to its intramolecular dinu-
clear zinc structure. It is reasonable to believe that 4a can
tative yield (>99%) and with
a high Mn value of
44100 gmolÀ1 and a relatively narrow polydispersity (Mw/Mn
ratio) of 1.82. The chemoselectivity was excellent as shown
1
by the H NMR spectroscopic analysis. The completely al-
ternating nature (>99% carbonate linkage) of the copoly-
merization reaction was reflected in the presence of the me-
thine proton peak at d=4.63 ppm (for carbonate) and the
absence of a methine proton peak at around d=3.45 ppm
(which can be attributed to the formation of polyether in
the homopolymerization of CHO; Figure 1). The enantiose-
lectivity of the copolymerization reaction was evaluated by
following the procedure described by Nozaki et al.[14a] After
Chem. Eur. J. 2005, 11, 3668 – 3678
www.chemeurj.org
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3669

K. Ding et al.
[142.2n (repeating unit)+99.1 (cyclohexyloxy group)+1.0
(H)+23.0 (Na+ ion)]. Therefore, the set of stronger signals
can be assigned to copolymer I and the set of weaker signals
to copolymer II (Figure 3). It can be deduced from this mass
Figure 1. 1H NMR spectrum of poly(cyclohexene carbonate).
Figure 3. Structures of copolymers I and II with the ethoxy or cyclohex-
hydrolysis of the polycarbonate with aqueous NaOH, the
enantiomeric excess (ee) of the resulting cyclohexane-1,2-
diol was determined by chiral GC to be only 18% with an
S,S configuration.
yloxy moiety as the initiating group.
spectrum (Figure 2a) that the copolymerization reaction was
À
The completely alternating structure of the poly(cyclohex-
ene carbonate) obtained rendered MALDI-TOF mass spec-
trometry the ideal tool for end-group analysis to probe the
active species responsible for the initiation of the polymeri-
zation reaction.[16] As shown in Figure 2a, at least two series
initiated by insertion of CO2 into a Zn alkoxide bond of the
catalytic species, either an ethoxy or a cyclohexyloxy group
attached to a zinc center, and terminated by protolysis to
give the corresponding copolymers I or II, respectively. The
1
ethoxy initiation hypothesis was also supported by the H
NMR spectrum of the copolymers (Figure 1). The signal at
d=4.18 ppm, which can be assigned to methylene protons,
clearly indicates the presence of an ethoxy group in the
structure of the copolymer obtained.[14c] Thus, the in situ
generated dinuclear zinc complex 5a probably acts as a cata-
lyst precursor. The alkoxy-containing active species, such as
6a, was probably produced in the reaction of ligand 4a with
an impurity present in the Et2Zn reagent, probably EtZ-
nOEt.[14b,17] The ligand-exchange reaction of the labile ethyl
group of complex 5a with a trace amount of cyclohexanol,
presumably formed by the reduction of cyclohexene oxide
with unreacted diethylzinc, gave the minor cyclohexyloxy-
zinc active species.
On the basis of the investigation described above, it has
been recognized that the dinuclear zinc complex 6a might
be the major active species in the catalytic reaction. An in-
crease in the concentration of the ethoxide-containing zinc
species was expected to have a favorable impact on its cata-
lytic performance. Accordingly, the effect of ethanol addi-
tive on the catalysis of the copolymerization reaction was
examined. The addition of 0.2–0.4 equivalents (relative to
ligand 4a) of ethanol to the in situ prepared complex 5a im-
proved the yields of the copolymers and led to a simulta-
neous decrease in the molecular weight (Figure 4). Addition
of 0.4 equivalents of ethanol was found to be optimal, af-
fording the copolymer with a molecular weight of 2.32ꢂ
Figure 2. MALDI-TOF mass spectra of the copolymers obtained by using
in situ prepared complex 5a in combination with a) 0, b) 0.4, and c) 1.0
equivalent of ethanol additive.
of signals with regular repeat intervals (repeating units of
142.2 mass units) were observed in the mass spectra. The
mass values within the series of the stronger signals match
[142.2n (repeating unit)+45.1 (ethoxy group)+1.0 (H)+
23.0 (Na+ ion)], while the values for the weaker ones match
3670
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 3668 – 3678

Copolymerization
FULL PAPER
1
tion (>99% polycarbonate by H NMR spectroscopy). This
is a very interesting result, since most copolymerization sys-
tems developed so far employ much higher CO2 pres-
sure.[7–14] Only very recently the first example of ambient-
pressure CHO/CO2 copolymerization using a porphyrin/
MnOAc catalytic system was reported by Sugimoto et al.,
although the turnover frequency (TOF) was somewhat
modest (ca. 3 hÀ1).[6] Mechanistically, the independence of
CHO conversion on the CO2 pressure indicates that the in-
À
sertion of CO2 into the Zn alkoxide bond is not the rate-de-
termining step of the reaction.
Figure 4. The effect of ethanol additive on the yield (dashed line) and
molecular weight (solid line) of copolymers (the data were obtained for
the reactions in 6 h).
To gain some preliminary kinetic information about the
copolymerization reaction performed under 1 atm pressure
of CO2, the reaction was monitored under the conditions
specified above by in situ IR spectroscopy, by analysis of the
emerging carbonyl stretch at 1752 cmÀ1 (n(CO) for the poly-
carbonate).[8d,e,11e,18] The spectra and the plot of absorbance
versus time are shown in Figure 5. As shown in the bottom
104 gmolÀ1 (Mw/Mn =1.43) in 95% yield. MALDI-TOF mass
spectrometric analysis of the resulting copolymers at this
stage showed that the relative intensities of the signals cor-
responding to copolymer I (Figure 2b) had been dramatical-
ly enhanced with respect to those in Figure 2a. This is in
good agreement with the expectation that an increase in the
concentration of ethanol would result in a higher portion of
polymer I with the simultaneous suppression of the compet-
ing pathway. However, addition of further ethanol resulted
in a decrease in catalytic activity, while the molecular weight
distribution (polydispersity) was found to remain in the
range of 1.18–1.28, and addition of two equivalents of etha-
nol completely inhibited the reaction. MALDI-TOF mass
spectrometric analysis of the copolymers attained upon ad-
dition of 1 equivalent of ethanol (Figure 2c) suggests an
overwhelming predominance of Zn–ethoxide as the initia-
tor.
An investigation of the impact of CO2 pressure on its co-
polymerization with epoxide might provide some useful in-
formation about the catalytic mechanism. To examine the
effect of CO2 pressure on the reaction, the copolymerization
reactions of CHO and CO2 at CO2 pressures of 1–20 atm
were performed by using in situ prepared 5a (5 mol%) in
the presence of 2 mol% ethanol at 608C. All the reactions
tested were complete within 6 h and the results are summa-
rized in Table 1. Remarkably, the conversion was found to
be independent of CO2 pressure and the copolymerization
proceeds smoothly even under 1 atm of CO2 without signifi-
cant loss of conversion (97%) or degree of CO2 incorpora-
Figure 5. Top: Three-dimensional stack plot of the IR spectra collected
every 2 min for the reaction of cyclohexene oxide and CO2 under 1 atm
pressure. Bottom: Time profile of the absorbance at 1752 cmÀ1 (corre-
sponding to the formation of poly(cyclohexene carbonate)).
Table 1. The effect of CO2 pressure on the conversion of cyclohexene
oxide and the molecular weight of the copolymer.[a]
half of Figure 5, apart from the small lag within the initial
0.5 h, the carbonyl absorbance increased steadily in a nearly
linear fashion over the next 5 h and remained constant after
6 h. This result indicates that the rate of polycarbonate for-
mation was a little slow in the initiation stage, but then in-
creased somewhat and remained almost constant in the fol-
lowing hours until completion, suggesting a constant concen-
tration of propagating species leading to a smooth overall
reaction under 1 atm of CO2. The initiation period shown in
Figure 5 (bottom) may be the time required to activate the
catalyst; presumably not all of the catalyst molecules are ini-
[c]
p(CO2)
[atm]
Conv. of
Carbonate
Mn
Mw/Mn
CHO [%][b]
linkages [%][b]
[kgmolÀ1
]
[c]
1
2
3
4
20
10
5
>99
97
98
>99
>99
>99
>99
16.9
19.3
20.9
19.2
1.35
1.55
1.51
1.56
1
97
[a] All the copolymerization reactions of CHO (1m in toluene) and CO2
were performed using in situ prepared 5a (5 mol%) in the presence of
2 mol% of ethanol additive at 608C for 6 h. [b] Determined by H NMR
spectroscopy. [c] Determined by gel-permeation chromatography (GPC)
and calibrated with polystyrene standards in tetrahydrofuran.
1
Chem. Eur. J. 2005, 11, 3668 – 3678
www.chemeurj.org
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3671

K. Ding et al.
Table 2. Stereoelectronic effect of ligands on Zn-catalyzed CHO/CO2 co-
polymerization.[a]
tiated simultaneously in the early stages leading to a small
lag in the initial reaction rate. This phenomenon has also
been reported for the salen/Cr system by Darensbourg
et al.[8e] Since the reaction was performed under 1 atm of
pressure, the time needed for CO2 saturation to reach equi-
librium in solution[11e] might be responsible for the catalyst
activation period.
Tuning the steric and electronic features of ligands in
metal-catalyzed epoxide/CO2 copolymerization reactions
often has a profound effect on the catalytic efficiencies.[9,11]
Thus, to probe the substituent effects of the dinucleating
ligand, a series of phenol-bridged bis(a,a-diarylprolinol) li-
gands (4a–4j) with different stereoelectronic features were
synthesized following a modified version of Trostꢀs proce-
dure, in which the corresponding dichlorides were used as
the starting materials instead of the dibromides.[15f] The cata-
Ligand 4 Conv. of
[5 mol%] CHO
[%][b]
Yield Carbonate
[%][c] linkages
[%][b]
Mn
Mw/
Mn
[d]
[d]
[kgmolÀ1
]
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4 f
4g
4h
4i
>99
92
77
87
93
94
>99
94
89
95
>99 >99
79 >99
19 >99
71 >99
63 >99
70 >99
24.9
24.8
6.33
16.3
7.73
11.5
18.4
21.1
5.85
48.5
1.63
1.58
1.54
1.49
1.32
1.34
1.75
1.72
1.94
1.84
>99
22
70 >99
41 >99
88 >99
10 4j
[a] All the copolymerization reactions of CHO (1.0m in toluene) and
CO2 (1 atm) were carried out by using in situ prepared catalyst precursor
(ligand 4 (5 mol%)+ Et2Zn (10 mol%) in toluene) in the presence of
1
ethanol additive (2 mol%) at 808C for 48 h. [b] Determined by H NMR
spectroscopy. [c] Isolated yield. [d] Determined by GPC and calibrated
with polystyrene standards in tetrahydrofuran.
backbone (ligand 4i and 4j) led to an unsatisfactory yield
(entry 9) and molecular weight distribution (entries 9 and
10). In summary, ligand 4a turns out to be the best choice
for this catalytic system.
From an environmental viewpoint, solvent-free processes
are ideal.[19] Moreover, the concentrations of both catalyst
and substrate can be significantly increased under solvent-
free conditions, which allows a further decrease in the cata-
lyst loading and improvements in the efficiency of the catal-
ysis. Thus, CHO/CO2 copolymerization was carried out in
neat CHO with a lower catalyst loading (0.5 mol%) under
20 atm of CO2 pressure at 808C for 2 h. As shown in
Table 3, the polymerization proceeded smoothly to give
poly(cyclohexene carbonate) with good carbonate selectivity
(94%) and molecular weight distribution (Mw/Mn =1.30) in
moderate yield (entry 1). Since the ethanol additive has a
significant impact on the catalysis of the polymerization, as
disclosed above, a variety of protonic additives, such as alco-
hols, phenols, and acetic acid, were then screened (Table 3,
entries 2–13). Under the experimental conditions, alcohol
additives were found to be superior to the phenolic and
acidic additives in terms of both activity and chemoselectivi-
ty, as well as the molecular weight control (entries 1–3
versus 4–13). Of the three alcohols tested, ethanol appears
to be the best additive for copolymerization.
Further optimization of the catalyst loading showed that,
in conjunction with ethanol as additive, the copolymeriza-
tion proceeded smoothly even when the catalyst loading was
as low as 0.1 mol% of 5a and that the activity remained es-
sentially unchanged (Table 4). In neat CHO, the catalytic
system exhibited good activity with a TOF of 142 hÀ1 at a
catalyst loading level of 0.2% (entry 2), and thus is one of
the most efficient catalysts reported so far.[3d,e,7d,11]
To probe the nature of the copolymerization reactions,
polymerization experiments were performed in neat CHO
at 808C under 20 atm of CO2 pressure in the presence of
0.5 mol% of 6a generated from the reaction of 5a with
lytic performance of these ligands in zinc-catalyzed CHO/
CO2 copolymerization was then investigated under an ambi-
ent pressure of CO2 by using in situ prepared catalyst pre-
cursors [ligand 4 (5 mol%)+Et2Zn (10 mol%) in toluene]
in the presence of ethanol additive (2 mol%) at 808C. The
results of the polymerization are summarized in Table 2. It
was found that the presence of electron-donating groups (li-
gands 4b and 4d), as well as a phenyl group (ligand 4e) or
chlorine (ligand 4h) at the para-positions of the a,a-diaryl
moieties of the prolinol led to slightly decreased CHO con-
versions and considerably lower yields (entries 2, 4, 5, and
8) than those obtained with ligand 4a (entry 1). On the
other hand, the introduction of strongly electron-withdraw-
ing groups into the a,a-diaryl moieties of the prolinol li-
gands (ligands 4 f and 4g) either reduced the yield (entry 6)
or had a disastrous effect on the chemoselectivity (entry 7).
The high percentage of polyether linkages obtained when
utilizing catalyst 4g is probably partly due to the homopoly-
merization of the epoxide prior to the addition of carbon di-
oxide. The 1H NMR spectra indicate that 6% CHO was
converted to polyether within five minutes prior to the in-
troduction of CO2. The meta-substituted methyl group
(ligand 4c) resulted in a significantly lower yield (entry 3).
Modifications to the bridging phenolic part of the ligand
3672
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 3668 – 3678

Copolymerization
FULL PAPER
Table 3. The catalysis of CHO/CO2 copolymerization using 5a in the presence of various additives under sol-
CHO/CO2 copolymerization is
a living process when catalyzed
by the dinuclear zinc complex
6a. However, for the copoly-
merization reactions carried out
in toluene, considerably higher
Mn values were observed based
on the amount of monomer
consumed per complex 5a
(Table 1, Figure 4, and the data
obtained in the absence of alco-
hol). This seemingly abnormal
phenomenon can be rational-
ized in terms of the incomplete
transformation of 5a to the real
active species 6a owing to the
substoichiometric amount of
ethanol used (0.4 equiv based
on ligand 4a, or none) and (or)
the partial decomposition of
the active species 6a caused by
contaminants, such as trace
amounts of water, in these sys-
tems.[8d]
vent-free conditions.[a]
[d]
Additive
Conv. of
Yield
[%][c]
Carbonate
Mn
Mw/Mn
[d]
CHO [%][b]
linkages [%][b]
[kgmolÀ1
]
1
2
3
4
5
6
7
8
EtOH
MeOH
nBuOH
HOAc
79
69
69
59
66
44
39
3.4
19
64
20
24
11
55
43
45
30
39
16
16
1.1
4.2
42
4.3
5.2
1.8
94
93
88
76
86
68
64
35
49
90
66
40
34
16.9
13.0
10.3
12.5
11.9
3.6
4.8
N.D.
4.0
2.0
7.0
4.8
5.5
1.30
1.23
1.21
1.18
1.24
1.63
1.27
N.D.
1.98
1.37
1.22
1.73
1.40
PhOH
4-O2NPhOH
4-tBuPhOH
4-MeOPhOH
2,4-(tBu)2PhOH
2,6-(tBu)2PhOH
2,6-(iPr)2PhOH
2,6-Me2PhOH
4-BrPhOH
9
10
11
12
13
[a] All the copolymerization reactions were performed in neat CHO (1.0 mL, 10.0 mmol) under 20 atm of CO2
pressure at 808C for 2 h in the presence of 5a (0.5 mol%) and various additives (0.5 mol%). [b] Determined
1
by H NMR spectroscopy. [c] Isolated yield. [d] Determined by GPC and calibrated with polystyrene standards
in tetrahydrofuran.
Table 4. Effect of catalyst loading on CHO/CO2 copolymerization under solvent-free conditions.[a]
[e]
5a
[mol%]
t
Conv. of
TON[c]
TOF
Carbonate
Mn
Mw/Mn
[d]
[e]
[h]
CHO [%][b]
[hÀ1
]
linkages [%][b]
[kgmolÀ1
]
1
2
3
4
0.5
0.2
0.1
0.1
2
2
2
3
80
57
26
36
160
285
260
360
80
94
92
91
93
16.9
21.0
19.2
22.4
1.30
1.28
1.29
1.28
142
130
120
To examine the effect of the
bridging
phenolic
hydroxy
group in the ligand on the cata-
lytic activity of the zinc com-
plex, two new bis(diarylproli-
nol) ligands 7 and 8, in which
the hydroxy group was either
[a] All the copolymerization reactions were performed in neat CHO under 20 atm of CO2 pressure at 808C
using 5a and an equimolar amount of EtOH. [b] Determined by H NMR spectroscopy. [c] Turnover number;
moles of CHO consumed per mole of ligand. [d] Turnover frequency; moles of CHO consumed per mole of
ligand per hour. [e] Determined by GPC and calibrated with polystyrene standards in tetrahydrofuran.
1
removed or blocked with
a
EtOH. Samples were withdrawn periodically from the reac-
tion mixture and analyzed by 1H NMR spectroscopy and
GPC. The data obtained were used to construct a plot of
molecular weight (Mn) and polydispersity (Mw/Mn) versus
conversion of cyclohexene oxide (Figure 6). A linear in-
crease in the molecular weight of the copolymer with the
conversion of CHO was observed within a reaction time of
2 h. The linear nature of the Mn plot combined with the
narrow polydispersities (Mw/Mn <1.30) suggest that the
methyl group, were designed and synthesized. However, we
did not isolate any polymers by using the zinc catalysts de-
rived from 7 and 8 under the same experimental conditions.
The dramatic difference in catalytic activity caused by the
subtle modification of the structure of these ligands prompt-
ed us to investigate the underlying structural reasons for this
1
behavior. The H NMR spectra of the zinc complexes were
recorded after mixing pure Et2Zn and the ligands 4, 7, or 8
(ligand/Et2Zn=0.5:1) in [D6]benzene at room temperature
(see Figure S4 of the Supporting Information). For the 4a/
Et2Zn mixture, a well-resolved spectrum was obtained, sug-
gesting that the resulting complex 5a has a well-defined
structure, although at this moment it is still difficult to give
an unambiguous assignment of the peaks. In contrast, exten-
1
sive broadening of the H NMR signals was observed for
Figure 6. Plot of Mn (versus polystyrene standards) and polydispersity
(Mw/Mn) for poly(cyclohexene carbonate) as a function of the conversion
of cyclohexane with a mixture of 5a (0.5 mol%) and EtOH (0.5 mol%).
Chem. Eur. J. 2005, 11, 3668 – 3678
www.chemeurj.org
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3673

K. Ding et al.
the two complexes formed from 7/Et2Zn and 8/Et2Zn, indi-
cating the intermolecular polymeric or oligomeric nature of
these complexes. The importance of the hydroxy group in
the linker of the bis-prolinol ligand 4a in the copolymeriza-
tion reaction, combined with the structural features dis-
1
closed by the H NMR spectra of the zinc complexes gener-
ated from ligands 4a, 7, and 8, demonstrate that it is critical
for the zinc complex to have an intramolecular dinuclear
structure for the activation of the substrates probably due to
the cooperative effect of the two zinc cations during the cat-
alysis.
Numerous attempts to isolate 6a from the as-synthesized
1:1 mixture of 5a and EtOH in toluene or other solvents
proved to be unsuccessful. Fortunately, the structure of a
closely related complex 9, which may serve as a structural
model of 6a, could be elucidated by single-crystal X-ray
analysis. Treatment of the in situ prepared 5a with one
equivalent of p-nitrophenol in toluene/THF resulted in a
pale yellow solution of complex 9. The resulting solution
was left to stand for several days in order for the dinuclear
zinc complex 9 to form pale yellow crystals in 50% yield
(Scheme 2). A single crystal was selected for X-ray analy-
Figure 7. ORTEP drawing of complex 9 with thermal ellipsoids shown at
the 50% probability level. Hydrogen atoms have been omitted for clarity.
À
Selected bond distances (ꢃ) and angles (degrees): Zn1 O1 2.005(6),
À
À
À
À
Zn1 O2 2.081(5), Zn1 O3 1.902(5), Zn1 N1 2.190(6), Zn1 O5 2.096(6),
À
À
À
À
Zn2 O1 2.000(6), Zn2 O2 2.091(5), Zn2 O4 1.896(5), Zn2 O6 2.096(6),
À
Zn2 N2 2.203(6); O1-Zn1-O2 77.9(2), O1-Zn2-O2 77.8(2), Zn1-O1-Zn2
105.1(3), Zn1-O2-Zn2 99.3(2).
bridged intramolecular homodinuclear zinc complexes[21]
suggesting that the propensity of the zinc atoms to form this
core locks the complex in the conformation described
herein. This may in part explain the marked difference in
the catalytic activities exhibited by bis-prolinol ligands 4a, 7,
and 8. The active dinuclear species 6a, with a structure simi-
lar to that of 9, is easily formed upon addition of Et2Zn and
EtOH to ligand 4a owing to the presence of the phenolic
hydroxy group. However, in the case of ligands 7 and 8, the
absence of this bridging hydroxy group hindered the forma-
tion of similar zinc complexes.
Based on these observations and the mechanistic informa-
tion reported previously by others,[9,11,14] we have proposed a
plausible mechanism for the catalytic system described in
this work (Scheme 3). Upon the addition of ethanol to the
zinc complex generated in situ by the reaction of ligand 4
with diethylzinc, the ethoxy-bridged dinuclear zinc species 6
can be formed as a result of ligand exchange with EtOH.
Complex 6 may adopt a structure similar to that of 9 and
serve as the catalytically active species in the system de-
scribed herein. The copolymerization reaction is initiated by
Scheme 2. Synthesis of complex 9.
sis.[20] As shown in Figure 7, the structure is indeed a dinu-
clear one as expected. The coordination sphere around each
zinc atom is approximately square pyramidal with the two
bridging phenoxo oxygen atoms (O1 and O2) and the nitro-
gen and oxygen atoms of the chelating prolinol moieties
forming the base and a THF molecule at the apex. The ge-
ometries around the Zn1 and Zn2 atoms are very similar to
each other with the THF rings pointing in opposite direc-
tions. The four-membered Zn2O2 core is almost planar
(e.s.d. 0.002 ꢃ), with the nonbonding Zn–Zn distance fixed
at 3.1776(12) ꢃ. A similar Zn2O2 core with similar bond pa-
rameters can be found in other m-hydroxo- or m-alkoxo-
À
the insertion of CO2 into the Zn OEt bond to afford the
carbonate–ester-bridged complex 10.[8f,11a,14b] The two zinc
centers should be situated sufficiently close to each other in
complex 10 to allow a synergistic effect in the copolymeriza-
tion, that is, one zinc center may activate the epoxide
through its coordination, like that of the THF molecule in
complex 9, and the second zinc atom might be responsible
3674
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 3668 – 3678

Copolymerization
FULL PAPER
Experimental Section
General considerations: All air- and/or water-sensitive compounds were
manipulated under dry nitrogen using a Braun Labmaster glove box or
standard Schlenk line techniques. NMR spectra were recorded in deuter-
iochloroform with a Varian Mercury 300 (1H: 300 MHz; 13C: 75 MHz;
19F: 282 MHz) spectrometer. Chemical shifts are reported in ppm relative
to an internal standard: tetramethylsilane (d=0 ppm) for 1H NMR and
deuteriochloroform (d=77.0 ppm) for 13C NMR spectroscopy. Mass spec-
tra (ESI) were recorded with a Mariner LC-TOF spectrometer. MALDI-
TOF mass spectra were recorded with a PerSeptive Biosystems Voyager
DE-STR spectrometer equipped with a 337 nm nitrogen laser, with a-
cyano-4-hydroxycinnamic acid (a-CHCA) as the matrix and sodium tri-
fluoroacetate as the ionizing agent. Elemental analyses were performed
with an Elemental VARIO EL apparatus. Optical rotations were mea-
sured with a Perkin-Elmer 341 automatic polarimeter; [a]D values are
given in units of 10À1 degcm2 gÀ1. Infrared spectra were obtained with a
BIO-RAD FTS-185 Fourier transform spectrometer in KBr pellets. The
three-dimensional stack plot of the IR spectra was collected on a Met-
tler-Toledo ReactIR 4000 Reaction Analysis System. The molecular
weights and molecular distributions were measured with a Waters 150C
gel-permeation chromatograph equipped with microstyragel columns
(HR1, HR3, and HR4) and an RI detector at 358C using polystyrene as
the calibration standard and THF as the eluent (flow rate: 1.0 mLminÀ1).
All the solvents were dried and purified before use following standard
procedures under argon. Cyclohexene oxide was distilled over CaH2
before use. Diethylzinc was purchased from Aldrich and used as re-
ceived. The purity of carbon dioxide was 99.999% and used as received.
Scheme 3. Proposed mechanism for the Zn-catalyzed copolymerization of
cyclohexene oxide and CO2 using ligand 4.
Ligand synthesis: Ligands 4a–j were prepared by following a modified
version of Trostꢀs procedure.[15f]
for carbonate propagation by nucleophilic attack by the car-
bonate ester on the back side of the cis-epoxide ring in a co-
operative manner (11) to afford 12. It is evident that the di-
nuclear zinc structure of the catalyst remains intact through-
out the catalysis of the copolymerization reaction. Subse-
quent alternating enchainment of CO2 and CHO completes
the catalytic cycle.
(S,S)-2,6-Bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-methyl-
phenol (4a): 2,6-Bis(chloromethyl)-4-methylphenol (0.615 g, 3.00 mmol)
was added in one portion to a stirred solution of (S)-diphenyl(pyrrolidin-
2-yl)methanol (1.672 g, 6.60 mmol) and K2CO3 (3.649 g, 26.40 mmol) in
dry DMF (20 mL) at 08C. The resulting solution was stirred at room tem-
perature for 12 h before being diluted with water (70 mL) and Et2O
(70 mL). The organic phase was separated and the aqueous phase was ex-
tracted with Et2O (3ꢂ50 mL). The combined organic phases were
washed with water (2ꢂ50 mL) and brine and then dried with Na2SO4.
After removal of the solvent, the residue was purified by column chroma-
tography on silica gel using petroleum ether/EtOAc (20:1 to 5:1) as the
Conclusions
eluent to yield 4a (1.793 g, 94%) as a white amorphous solid. [a]D20
=
+46.3 (c=1.10 in CHCl3) [Lit.: [15f]] [a]D25 =+49.8 (c=3.0 in CHCl3)]; 1H
NMR (300 MHz, CDCl3): d=1.43–1.63 (m, 4H), 1.75–1.82 (m, 2H),
1.97–2.04 (m, 2H), 2.14 (s, 3H), 2.33–2.41 (m, 2H), 2.76–2.83 (m, 2H),
3.21 (d, J=12.3 Hz, 2H), 3.36 (d, J=12.6 Hz, 2H), 3.94 (dd, J=4.5,
9.3 Hz, 2H), 6.58 (s, 2H), 7.10–7.32 (m, 12H), 7.55 (d, J=8.4 Hz, 4H),
7.68 ppm (d, J=8.4 Hz, 4H); 13C NMR (75.5 MHz, CDCl3): d=20.38,
24.01, 29.61, 54.99, 57.77, 71.36, 78.88, 123.98, 125.93, 125.98, 126.38,
126.60, 127.12, 127.95, 128.19, 128.82, 146.40, 146.98, 152.61 ppm; FTIR
(KBr): n˜ =3350, 3058, 2960, 2922, 1598, 1480, 1449, 1033 cmÀ1; ESI-MS:
m/z: 639.3 [M++H].
In summary, we have demonstrated a novel use of intramo-
lecular dinuclear zinc complexes of multidentate ligands 4a–
j as catalysts in the copolymerization of cyclohexene oxide
and CO2 to afford completely alternating polycarbonates
with good activity. With this type of novel initiator, copoly-
merization can be accomplished under mild conditions at
1 atm pressure of CO2, which represents a significant ad-
vantage over most catalytic systems developed for this reac-
tion so far. In addition, the solid-state structure of a dinu-
clear zinc complex with ligand 4a was characterized, provid-
ing valuable information about the mechanism of the cata-
lytic polymerization reaction. This strategy allows consider-
able scope for ligand modification, and thus might lead to
the design of new, efficient, and practical catalysts for the
copolymerization of epoxides with CO2. Future work will be
directed towards further mechanistic studies of these intra-
molecular CHO/CO2 copolymerization reactions catalyzed
by dinuclear zinc complexes and the simplification of the
multidentate ligands, as well as the extension of this strategy
to a broad range of metal-catalyzed epoxide/CO2 copoly-
merization reactions.
(S,S)-2,6-Bis[2-(hydroxydi-4-tolylmethyl)pyrrolidin-1-ylmethyl]-4-methyl-
phenol (4b): Following a similar procedure to that described for the
preparation of 4a, the reaction of 2,6-bis(chloromethyl)-4-methylphenol
with (S)-di-4-tolyl(pyrrolidin-2-yl)methanol afforded 4b in 66% yield.
[a]2D0 =+58.8 (c=1.00 in CH2Cl2); 1H NMR (300 MHz, CDCl3): d=1.48–
1.80 (m, 6H), 1.94–2.01 (m, 2H), 2.14 (s, 3H), 2.22 (s, 3H), 2.28 (s, 3H),
2.32–2.40 (m, 2H), 2.76–2.82 (m, 2H), 3.22 (d, J=12.9 Hz, 2H), 3.39 (d,
J=13.2 Hz, 2H), 3.90 (dd, J=4.2, 9.3 Hz, 2H), 6.58 (s, 2H), 7.06–7.11
(m, 8H), 7.42 (d, J=8.4 Hz, 4H), 7.53 ppm (d, J=8.1 Hz, 4H); 13C NMR
(75.5 MHz, CDCl3): d=20.37, 20.91, 20.96, 23.95, 29.60, 54.97, 57.50,
71.20, 78.56, 124.10, 125.67, 127.08, 128.66, 128.81, 128.90, 135.71, 135.88,
143.79, 144.32, 152.65 ppm; FTIR (KBr): n˜ =3325, 3023, 2920, 2869, 1614,
1509, 1479, 1447, 1098 cmÀ1; ESI-MS: m/z: 695.4 [M++H]; elemental
analysis calcd (%) for C47H54N2O3: C 81.23, H 7.83, N 4.03; found: C
80.80, H 8.10, N 3.80.
Chem. Eur. J. 2005, 11, 3668 – 3678
www.chemeurj.org
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3675

K. Ding et al.
(S,S)-2,6-Bis[2-(hydroxydi-3-tolylmethyl)pyrrolidin-1-ylmethyl]-4-methyl-
phenol (4c): Following a similar procedure to that described for the prep-
aration of 4a, the reaction of 2,6-bis(chloromethyl)-4-methylphenol with
CH2Cl2); 1H NMR (300 MHz, CDCl3): d=1.62–1.77 (m, 6H), 1.90–1.99
(m, 2H), 2.11 (s, 3H), 2.44–2.52 (m, 2H), 2.79–2.84 (m, 2H), 3.16 (d, J=
12.6 Hz, 2H), 3.24 (d, J=12.6 Hz, 2H), 4.00 (dd, J=3.6, 9.4 Hz, 2H),
6.55 (s, 2H), 7.76 (s, 2H), 7.79 (s, 2H), 8.07 (s, 4H), 8.18 ppm (s, 4H); 13C
NMR (75.5 MHz, CDCl3): d=20.10, 23.26, 29.26, 54.90, 57.91, 71.39,
77.87, 117.78, 121.30, 121.39, 121.43, 121.54, 123.18, 125.01, 125.05, 125.68,
126.10, 128.07, 128.63, 128.67, 129.63, 131.18, 131.26, 131.62, 131.70,
132.06, 132.14, 132.50, 132.59, 147.63, 148.85, 152.78 ppm; 19F NMR
(282 MHz, CDCl3): d=À63.23, À63.20 ppm; FTIR (KBr): n˜ =3320, 3103,
2979, 2882, 1626, 1484, 1371, 1279, 1172, 1134 cmÀ1; ESI-MS: m/z: 1183.3
[M++H]; elemental analysis calcd (%) for C51H38F24N2O3: C 51.79, H
3.24, N 2.37; found: C 51.95, H 3.29, N 2.18.
(S)-di-3-tolyl(pyrrolidin-2-yl)methanol afforded 4c in 48% yield. [a]D20
=
+86.4 (c=1.03 in CH2Cl2); 1H NMR (300 MHz, CDCl3): d=1.56–1.83
(m, 6H), 1.96–2.03 (m, 2H), 2.14 (s, 3H), 2.28 (s, 3H), 2.33 (s, 3H), 2.35–
2.42 (m, 2H), 2.80–2.86 (m, 2H), 3.19 (s, 4H), 3.89 (dd, J=4.5, 9.6 Hz,
2H), 6.56 (s, 2H), 6.90 (d, J=7.2 Hz, 2H), 7.00 (d, J=7.2 Hz, 2H), 7.12–
7.22 (m, 4H), 7.33–7.41 (m, 6H), 7.52 ppm (s, 2H); 13C NMR (75.5 MHz,
CDCl3): d=20.42, 21.68, 21.72, 24.10, 29.66, 55.13, 57.27, 71.50, 78.69,
122.82, 122.97, 123.96, 126.38, 126.59, 127.03, 127.12, 127.26, 127.79,
127.98, 128.73, 137.59, 137.65, 146.33, 146.95, 152.49 ppm; FTIR (KBr):
n˜ =3305, 2948, 2919, 2868, 1604, 1304, 1149, 1091, 1000 cmÀ1; ESI-MS:
m/z: 695.5 [M++H]; elemental analysis calcd (%) for C47H54N2O3: C
81.23, H 7.83, N 4.03; found: C 80.89, H 8.15, N 3.72.
(S,S)-2,6-Bis{2-[hydroxybis(4-chlorophenyl)methyl]pyrrolidin-1-ylmeth-
yl}-4-methylphenol (4h): Following a similar procedure to that described
for the preparation of 4a, the reaction of 2,6-bis(chloromethyl)-4-methyl-
phenol with (S)-bis(4-chlorophenyl)(pyrrolidin-2-yl)methanol afforded
4h in 90% yield. [a]2D0 =+50.4 (c=1.03 in CH2Cl2); 1H NMR (300 MHz,
CDCl3): d=1.43–1.75 (m, 6H), 1.94–2.01 (m, 2H), 2.15 (s, 3H), 2.33–2.42
(m, 2H), 2.76–2.82 (m, 2H), 3.20 (d, J=12.6 Hz, 2H), 3.51 (d, J=
12.6 Hz, 2H), 3.87 (dd, J=4.2, 9.0 Hz, 2H), 6.60 (s, 2H), 7.26–7.29 (m,
8H), 7.45 (d, J=8.4 Hz, 4H), 7.59 ppm (d, J=8.4 Hz, 4H); 13C NMR
(75.5 MHz, CDCl3): d=20.36, 23.82, 29.52, 54.96, 58.07, 70.97, 78.30,
123.76, 127.29, 127.42, 128.25, 128.43, 128.96, 132.47, 132.68, 144.61,
145.24, 152.68 ppm; FTIR (KBr): n˜ =3326, 2952, 2870, 1488, 1403, 1094,
1013 cmÀ1; ESI-MS: m/z: 777.2 [M++H]; elemental analysis calcd (%)
for C43H42Cl4N2O3: C 66.50, H 5.45, N 3.61; found: C 66.43, H 5.62, N
3.24.
(S,S)-2,6-Bis{2-[hydroxybis(4-methoxyphenyl)methyl]pyrrolidin-1-ylmeth-
yl}-4-methylphenol (4d): Following a similar procedure to that described
for the preparation of 4a, the reaction of 2,6-bis(chloromethyl)-4-methyl-
phenol with (S)-bis(4-methoxyphenyl)(pyrrolidin-2-yl)methanol afforded
4d in 63% yield. [a]2D0 =+36.0 (c=1.87 in CH2Cl2); 1H NMR (300 MHz,
CDCl3): d=1.27–1.66 (m, 6H), 1.80–1.87 (m, 2H), 2.01 (s, 3H), 2.17–2.25
(m, 2H), 2.61–2.67 (m, 2H), 3.08 (d, J=13.8 Hz, 2H), 3.36 (d, J=
12.9 Hz, 2H), 3.58 (s, 6H), 3.62 (s, 6H), 3.71 (dd, J=4.5, 9.3 Hz, 2H),
6.47 (s, 2H), 6.66–6.72 (m, 8H), 7.42 (d, J=8.7 Hz, 4H), 7.55 ppm (d, J=
8.7 Hz, 4H); 13C NMR (75.5 MHz, CDCl3): d=20.38, 23.96, 29.55, 54.97,
55.05, 55.12, 57.90, 71.27, 78.38, 113.22, 113.44, 124.11, 127.07, 128.86,
139.00, 139.50, 152.75, 157.84, 158.00 ppm; ESI-MS: m/z: 759.4 [M++H];
FTIR (KBr): n˜ =3398, 2952, 2835, 1608, 1510, 1248, 1177, 1035 cmÀ1; ele-
mental analysis calcd (%) for C47H54N2O3: C 74.38, H 7.17, N 3.69;
found: C 74.26, H 7.10, N 3.29.
(S,S)-2,6-Bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-nitro-
phenol (4i): Following a similar procedure to that described for the prep-
aration of 4a, the reaction of 2,6-bis(bromomethyl)-4-nitrophenol with
(S,S)-2,6-Bis{2-[hydroxy(biphenyl-4-yl)methyl]pyrrolidin-1-ylmethyl}-4-
methylphenol (4e): Following a similar procedure to that described for
the preparation of 4a, the reaction of 2,6-bis(chloromethyl)-4-methylphe-
nol with (S)-biphenyl-4-yl(pyrrolidin-2-yl)methanol afforded 4e in 80%
yield. [a]2D0 =+93.3 (c=0.88 in CH2Cl2) [Lit.: [15f]] [a]2D5 =+86.5 (c=1.58
in CH2Cl2)]; 1H NMR (300 MHz, CDCl3): d=1.50–1.64 (m, 4H), 1.83–
1.86 (m, 2H), 2.00–2.05 (m, 2H), 2.13 (s, 3H), 2.38–2.43 (m, 2H), 2.79–
2.84 (m, 2H), 3.24 (d, J=12.6 Hz, 2H), 3.58 (d, J=12.6 Hz, 2H), 4.02
(dd, J=4.5, 9.6 Hz, 2H), 6.60 (s, 2H), 7.24–7.43 (m, 14H), 7.52–7.59 (m,
14H), 7.67 (d, J=8.4 Hz, 4H), 7.82 ppm (d, J=8.7 Hz, 4H); 13C NMR
(75.5 MHz, CDCl3): d=20.38, 23.84, 29.62, 54.97, 57.86, 71.19, 78.68,
124.02, 126.34, 126.42, 126.76, 126.99, 127.07, 127.20, 128.60, 128.67,
129.05, 139.15, 139.32, 140.59, 140.74, 145.50, 146.17, 152.79 ppm; FTIR
(KBr): n˜ =3330, 3028, 2962, 2870, 1600, 1485, 1403, 1007 cmÀ1; ESI-MS:
m/z: 943.5 [M++H].
(S)-diphenyl(pyrrolidin-2-yl)methanol afforded 4i in 74% yield. [a]D20
=
+68.6 (c=1.02 in CH2Cl2); 1H NMR (300 MHz, CDCl3): d=1.60–1.71
(m, 4H), 1.82–1.89 (m, 2H), 2.01–2.12 (m, 2H), 2.34–2.42 (m, 2H), 2.82–
2.88 (m, 2H), 3.31 (d, J=13.5 Hz, 2H), 3.39 (d, J=13.5 Hz, 2H), 4.00
(dd, J=4.8, 9.6 Hz, 2H), 7.10–7.36 (m, 12H), 7.56 (d, J=7.5 Hz, 4H),
7.67 (d, J=7.8 Hz, 4H), 7.70 ppm (s, 2H); 13C NMR (75.5 MHz, CDCl3):
d=24.10, 29.56, 55.24, 57.18, 71.21, 79.00, 123.86, 124.59, 125.91, 125.95,
126.68, 126.73, 128.17, 128.22, 139.01, 146.26, 146.42, 162.02 ppm; FTIR
(KBr): n˜ =3380, 3058, 2958, 2872, 1595, 1492, 1466, 1449, 1334,
1290 cmÀ1; ESI-MS: m/z: 670.3 [M++H]; elemental analysis calcd (%)
for C42H43N3O5: C 75.31, H 6.47, N 6.27; found: C 75.09, H 6.74, N 5.82.
(S,S)-2,6-Bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-tert-bu-
tylphenol (4j): Following a similar procedure to that described for the
preparation of 4a, the reaction of 2,6-bis(bromomethyl)-4-tert-butylphe-
nol with (S)-diphenyl(pyrrolidin-2-yl)methanol afforded 4j in 74% yield.
[a]2D0 =+56.6 (c=1.03 in CH2Cl2); 1H NMR (300 MHz, CDCl3): d=1.22
(s, 9H), 1.48–1.64 (m, 4H), 1.75–1.82 (m, 2H), 1.97–2.04 (m, 2H), 2.35–
2.44 (m, 2H), 2.76–2.81 (m, 2H), 3.25 (d, J=12.9 Hz, 2H), 3.40 (d, J=
12.6 Hz, 2H), 3.97 (dd, J=4.8, 9.3 Hz, 2H), 6.00 (s, 2H), 7.09–7.21 (m,
4H), 7.26–7.32 (m, 8H), 7.55 (d, J=8.4 Hz, 4H), 7.69 ppm (d, J=7.2 Hz,
4H); 13C NMR (75.5 MHz, CDCl3): d=24.10, 29.62, 31.56, 33.78, 55.03,
57.99, 71.33, 78.88, 123.47, 125.33, 125.90, 125.93, 126.38, 126.61, 127.98,
128.23, 140.80, 146.37, 147.04, 152.48 ppm; FTIR (KBr): n˜ =3350, 3058,
(S,S)-2,6-Bis{2-[hydroxybis(4-trifluoromethylphenyl)methyl]pyrrolidin-1-
ylmethyl}-4-methylphenol (4 f): Following a similar procedure to that de-
scribed for the preparation of 4a, the reaction of 2,6-bis(chloromethyl)-4-
methylphenol with (S)-bis(4-trifluomethylphenyl)(pyrrolidin-2-yl)metha-
nol afforded 4 f in 77% yield. [a]2D0 =+35.3 (c=1.02 in CH2Cl2); 1H
NMR (300 MHz, CDCl3): d=1.54–1.73 (m, 6H), 1.95–2.02 (m, 2H), 2.14
(s, 3H), 2.39–2.42 (m, 2H), 2.79–2.80 (m, 2H), 3.18 (d, J=12.6 Hz, 2H),
3.45 (d, J=12.6 Hz, 2H), 4.01 (dd, J=3.6, 9.4 Hz, 2H), 6.57 (s, 2H),
7.58–7.60 (m, 8H), 7.71 (d, J=8.1 Hz, 4H), 7.87 ppm (d, J=7.8 Hz, 4H);
13C NMR (75.5 MHz, CDCl3): d=14.12, 20.28, 22.69, 23.65, 29.36, 29.54,
29.66, 29.70, 30.27, 31.92, 54.92, 57.96, 70.85, 78.51, 122.21, 122.26, 123.58,
125.21, 125.26, 125.34, 125.39, 125.54, 125.82, 125.86, 126.10, 126.37,
127.59, 128.77, 128.95, 129.09, 129.20, 129.39, 129.63, 129.82, 149.67,
150.45, 152.70 ppm; 19F NMR (282 MHz, CDCl3): d=À62.87,
À62.81 ppm; FTIR (KBr): n˜ =3340, 2973, 2877, 2828, 1617, 1482, 1416,
1326, 1166, 1125, 1070, 1017 cmÀ1; ESI-MS: m/z: 911.3 [M++H]; elemen-
tal analysis calcd (%) for C47H42F12N2O3: C 61.98, H 4.65, N 3.08; found:
C 62.02, H 4.74, N 2.83.
2961, 2870, 1589, 1485, 1449, 1303, 1214 cmÀ1
; ESI-MS: m/z: 681.4
[M++H]; elemental analysis calcd (%) for C46H52N2O3: C 81.14, H 7.70,
N 4.11, found: C 80.98, H 7.72, N 3.90.
(S,S)-1,3-Bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-5-methyl-
benzene (7): Following a similar procedure to that described for the prep-
aration of 4a, the reaction of 1,3-bis(bromomethyl)-5-methylbenzene
with (S)-diphenyl(pyrrolidin-2-yl)methanol afforded
7 in 83% yield.
[a]2D0 =+115.3 (c=1.02 in CH2Cl2); 1H NMR (300 MHz, CDCl3): d=
1.63–1.80 (m, 6H), 1.94–2.01 (m, 2H), 2.23 (s, 3H), 2.30–2.35 (m, 2H),
2.86–2.91 (m, 2H), 2.95 (d, J=12.3 Hz, 2H), 3.17 (d, J=12.9 Hz, 2H),
3.98 (dd, J=4.5, 9.3 Hz, 2H), 5.00 (s, 2H), 6.60–6.64 (d, J=12.0 Hz 2H),
7.08–7.34 (m, 12H), 7.61 (d, J=7.5 Hz, 4H), 7.77 ppm (d, J=8.4 Hz,
4H); 13C NMR (75.5 MHz, CDCl3): d=21.25, 24.15, 29.79, 55.48, 60.41,
70.58, 77.86, 125.53, 125.63, 125.76, 126.20, 126.34, 127.90, 128.06, 128.14,
137.36, 139.40, 146.68, 148.07 ppm; FTIR (KBr): n˜ =3280, 3059, 2942,
(S,S)-2,6-Bis(2-{hydroxybis[3,5-bis(trifluoromethyl)phenyl]methyl}pyrroli-
din-1-ylmethyl)-4-methylphenol (4g): Following a similar procedure to
that described for the preparation of 4a, the reaction of 2,6-bis(chloro-
methyl)-4-methylphenol with (S)-bis[3,5-bis(trifluoromethyl)phenyl](pyr-
rolidin-2-yl)methanol afforded 4g in 81% yield. [a]2D0 =+43.6 (c=1.04 in
3676
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 3668 – 3678

Copolymerization
FULL PAPER
2793, 1606, 1489, 1449, 1391, 1106 cmÀ1; ESI-MS: m/z: 639.3 [M++H]; el-
emental analysis calcd (%) for C43H46N2O2: C 82.92, H 7.44, N 4.50;
found: C 82.83, H 7.38, N 4.30.
under reduced pressure, filtered, and left to stand at room temperature
for several days before the zinc complex 9 was obtained as pale yellow
block crystals. The crystals were vacuum-dried before characterization.
Yield: 50%. 1H NMR (300 MHz, CDCl3): d=1.04–1.54 (m, 6H), 1.70–
1.90 (m, 10H), 2.11 (s, 3H), 2.36 (s, 3H, toluene-CH3), 2.40–2.70 (m,
4H), 3.66–3.91 (m, 14H), 6.67 (s, 2H), 7.01–7.08 (m, 4H), 7.11–7.28 (m,
13H, of which 5H can be assigned to the aromatic protons of toluene),
7.81 (s, J=6.3 Hz, 4H), 7.99 (s, J=7.5 Hz, 4H), 8.34–8.37 (m, 2H),
8.78 ppm (s, 2H); 13C NMR (75.5 MHz, CDCl3): d=19.71, 20.02, 21.44
(toluene-CH3), 25.25, 28.72, 55.65, 60.38, 69.20, 72.34, 76.57, 120.41,
124.48, 125.15, 125.28, 125.52, 125.59, 125.81, 126.50, 127.54, 127.58,
128.21, 129.02, 131.11, 137.79, 153.10, 153.30, 156.50, 173.49 ppm; FTIR
(KBr): n˜ =2976, 2876, 1584, 1493, 1475, 1446, 1298, 1166, 1109, 1063, 864,
(S,S)-2-Methoxy-1,3-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmeth-
yl]-5-methylbenzene (8): Following a similar procedure to that described
for the preparation of 4a, the reaction of 1-methoxy-2,6-bis(bromometh-
yl)-4-methylbenzene with (S)-diphenyl(pyrrolidin-2-yl)methanol afforded
8 in 81% yield. [a]2D0 =+9.0 (c=1.00 in CH2Cl2); 1H NMR (300 MHz,
CDCl3): d=1.50–1.72 (m, 6H), 1.84–1.91 (m, 2H), 2.26 (s, 3H), 2.27–2.36
(m, 2H), 2.80–2.87 (m, 2H), 3.15 (s, 3H), 3.18 (s, 4H), 3.94 (dd, J=3.3,
8.7 Hz, 2H), 5.22 (s, 2H), 6.82 (s, 2H), 7.03–7.29 (m, 12H), 7.54 (d, J=
8.7 Hz, 4H), 7.70 ppm (d, J=8.4 Hz, 4H); 13C NMR (75.5 MHz, CDCl3):
d=20.99, 23.57, 29.68, 54.45, 55.09, 61.23, 70.51, 77.47, 125.53, 125.67,
126.08, 126.26, 127.89, 128.03, 129.14, 131.89, 132.96, 146.83, 148.09,
154.06 ppm; FTIR (KBr): n˜ =3343, 3058, 2945, 2872, 1597, 1491, 1449,
1381, 1212 cmÀ1; ESI-MS: m/z: 653.4 [M++H]; elemental analysis calcd
(%) for C44H48N2O3: C 80.95, H 7.41, N 4.29; found: C 80.91, H 7.85, N
3.98.
707, 698 cmÀ1
;
elemental analysis calcd (%) for 9·toluene
(C64H71N3O8Zn2): C 67.37, H 6.27, N 3.68; found: C 66.62, H 6.61, N 3.68.
A single crystal suitable for X-ray analysis was picked out from the
mother liquor and sealed in a capillary tube. The X-ray diffraction data
were collected with a Bruker Smart APEX diffractometer at 208C by
using MoKa radiation (l=0.71069 ꢃ) with a w–2q scan mode. Crystal
data: C69H87N3O11Zn2, Mr =1265.16, trigonal, space group P32, a=
17.6424(6), b=17.6424(6), c=19.4741(10) ꢃ, a=90, b=90, g=1208, V=
A typical procedure for the copolymerization of cyclohexene oxide and
CO2 in toluene: In a glove box, a solution of diethylzinc (0.10 mL, 1.0m
in hexane, 0.10 mmol) was added to a solution of 4a (0.032 g, 0.05 mmol)
in toluene (1.0 mL). The mixture was stirred at room temperature for
30 min. A solution of ethanol (0.02 mL, 1.0m in toluene, 0.02 mmol) was
then added to the mixture. The solution was stirred at room temperature
for an additional 15 min. This solution was transferred into an autoclave
and then cyclohexene oxide (0.1 mL, 1.0 mmol) was introduced. The
vessel was pressurized to 30 atm with carbon dioxide. After stirring at
608C for the necessary reaction time, the mixture was cooled to ambient
temperature and the CO2 pressure was slowly released. The mixture was
diluted with CH2Cl2 (30 mL) and washed with aqueous HCl (1m, 2ꢂ
10 mL) and brine (2ꢂ10 mL). The organic layer was dried with Na2SO4
and concentrated to 3 mL by vacuum evaporation. The copolymer was
precipitated by adding MeOH (30 mL), collected by filtration, and dried
in vacuo to a constant weight.
5249.3(4) ꢃ3, Z=3, 1calcd =1.201 gcmÀ3
,
F(000)=2010, m(MoKa)=
0.742 mmÀ1. Data collection and refinement: Diffraction data measured
to 2qmax =2.66–53.988. A total of 12664 unique reflections with positive
intensities were recorded. The final refinement, based on F2, converged
at R=0.0697 (wR2 =0.1814) for 3284 observations with I0 >2s(I0) and
R=0.1059 (wR2 =0.1983) for all data. At convergence, S=0.734 and
D1=À0.323 eꢃÀ3. A detailed list of the crystal data and data collection
parameters is given in the Supporting Information.
Acknowledgements
Financial support from the National Natural Science Foundation of
China, the Chinese Academy of Sciences, the Major Basic Research De-
velopment Program of China (Grant no. G2000077506), and the Commis-
sion of Science and Technology, Shanghai Municipality is gratefully ac-
knowledged.
A typical procedure for the copolymerization of cyclohexene oxide and
CO2 at atmospheric pressure: In a Schlenk tube, a solution of diethylzinc
(0.10 mL, 1.0m in hexane, 0.10 mmol) was added to a solution of 4a
(0.032 g, 0.05 mmol) in toluene (1.0 mL) under argon. The mixture was
stirred at room temperature for 30 min. Then a solution of ethanol
(0.02 mL, 1.0m in toluene, 0.02 mmol) was added to the mixture. The so-
lution was stirred at room temperature for an additional 15 min and then
cyclohexene oxide (0.1 mL, 1.0 mmol) was introduced. Carbon dioxide
was bubbled into the solution whilst stirring at 608C for the necessary re-
action time. Copolymer work up was carried out following the same pro-
cedure as that described above.
[1] For comprehensive reviews on the uses of CO2, see: a) H. Arakawa,
M. Areata, J. N. Armor, M. A. Barteau, E. J. Beckman, A. T. Bell,
J. E. Bercaw, C. Creutz, E. Dinjus, D. A. Dixon, K. Domen, D. L.
DuBois, J. Eckert, E. Fujita, D. H. Gibson, W. A. Goddard, D. W.
Goodman, J. Keller, G. J. Kubas, H. H. Kung, J. E. Lyons, L. E.
Manzer, T. J. Marks, K. Morokuma, K. M. Nicholas, R. Periana, L.
Que, J. Rostrup-Nielson, W. M. H. Sachtler, L. D. Schmidt, A. Sen,
G. A. Somorjai, P. C. Stair, B. R. Stults, W. Tumas, Chem. Rev. 2001,
101, 953–996; b) W. Leitner, Angew. Chem. 1995, 107, 2391–2405;
Angew. Chem. Int. Ed. Engl. 1995, 34, 2207–2221; c) G. Musie, M.
Wei, B. Subramaniam, D. H. Busch, Coord. Chem. Rev. 2001, 219,
789–820; d) P. G. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 1995, 95,
259–272; e) D. H. Gibson, Chem. Rev. 1996, 96, 2063–2095; f) A. I.
Cooper, J. Mater. Chem. 2000, 10, 207–234.
A typical procedure for the copolymerization of cyclohexene oxide and
CO2 in neat CHO: In a glove box, a solution of diethylzinc (0.10 mL,
1.0m in hexane, 0.10 mmol) was added to a solution of 4a (0.032 g,
0.05 mmol) in CH2Cl2 (1.0 mL). The mixture was stirred at room temper-
ature for 30 min. Following the addition of ethanol (0.05 mL, 1.0m in tol-
uene, 0.05 mmol), the solution was stirred at room temperature for an ad-
ditional 15 min and then the solvent was removed in vacuo. The resulting
white residue was dissolved in cyclohexene oxide (1.0 mL, 10.0 mmol).
This solution was transferred into an autoclave. The vessel was pressur-
ized to 20 atm with carbon dioxide. After stirring at the desired tempera-
ture for the specified reaction time, the mixture was cooled to ambient
temperature and the CO2 pressure was slowly released. A small sample
of the crude material was removed for characterization, the product was
dissolved in methylene chloride (5 mL), and the polymer was precipitat-
ed from methanol (50 mL). The product was then dried in vacuo to con-
stant weight.
[2] S. Inoue, H. Koinuma, T. Tsuruta, J. Polym. Sci. Part B: Polym. Lett
1969, 7, 287–292.
[3] For reviews on epoxide/CO2 copolymerization reactions, see: a) D. J.
Darensbourg, M. W. Holtcamp, Coord. Chem. Rev. 1996, 153, 155–
174; b) M. S. Super, E. J. Beckman, Trends Polym. Sci. 1997, 5, 236–
240; c) A. Rokicki, W. Kuran, J. Macromol. Sci. Rev. Macromol.
Chem. Phys. 1981, C21, 135–186; d) G. W. Coates, D. R. Moore,
Angew. Chem. 2004, 116, 6784–6806; Angew. Chem. Int. Ed. 2004,
43, 6618–6639; e) H. Sugimoto, S. Inoue, J. Polym. Sci. Part A:
Polym. Chem. 2004, 42, 5561–5573.
[4] For the properties of these polymers that make them attractive as
materials, see: a) M. Okada, Prog. Polym. Sci. 2002, 27, 87–133;
b) S. Y. Yang, X. G. Fang, L. B. Chen, Polym. Adv. Technol. 1996, 7,
605–608.
Preparation and X-ray crystal structural analysis of complex 9: In a glove
box, a solution of diethylzinc (1.0 mL, 1.0m in hexane, 1.0 mmol) was
added to a solution of 4a (0.320 g, 0.5 mmol) in toluene (5.0 mL). The
mixture was stirred at room temperature for 30 min. p-Nitrophenol
(0.0696 g, 0.5 mmol) was added to this solution and some solid was pre-
cipitated. THF (5.0 mL) was added and the resulting homogeneous so-
lution was stirred for an additional 15 min. The solvent was concentrated
Chem. Eur. J. 2005, 11, 3668 – 3678
www.chemeurj.org
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3677

K. Ding et al.
[5] For examples of heterogeneous catalysis of epoxide/CO2 copolymer-
ization reactions, see: a) J. H. Jung, M. Ree, T. Chang, J. Polym. Sci.
Part A: Polym. Chem. 1999, 37, 1863–1876; b) J. H. Jung, M. Ree, T.
Chang, J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 3329–3336;
c) C. S. Tan, T. J. Hsu, Macromolecules 1997, 30, 3147–3150; d) B. Y.
Liu, X. J. Zhao, X. H. Wang, F. S. Wang, J. Polym. Sci. Part A:
Polym. Chem. 2001, 39, 2751–2754.
[12] For zinc b-diiminate catalyzed copolymerization of CO2 and epox-
ides with ethyl sulfinate as the initiating group, see: a) R. Eberhardt,
M. Allmendinger, G. A. Luinstra, B. Rieger, Organometallics 2003,
22, 211–214; b) R. Eberhardt, M. Allmendinger, M. Zintl, C. Troll,
G. A. Luinstra, B. Rieger, Macromol. Chem. Phys. 2004, 205, 42–47.
[13] a) H. S. Kim, J. J. Kim, B. G. Lee, O. S. Jung, H. G. Jang, S. O. Kang,
Angew. Chem. 2000, 112, 4262–4264; Angew. Chem. Int. Ed. 2000,
39, 4096–4098; b) H. S. Kim, J. J. Kim, S. D. Lee, M. S. Lah, D.
Moon, H. G. Jang, Chem. Eur. J. 2003, 9, 678–686.
[6] H. Sugimoto, H. Ohshima, S. Inoue, J. Polym. Sci. Part A: Polym.
Chem. 2003, 41, 3549–3555.
[14] a) K. Nozaki, K. Nakano, T. Hiyama, J. Am. Chem. Soc. 1999, 121,
11008–11009; b) K. Nakano, K. Nozaki, T. Hiyama, J. Am. Chem.
Soc. 2003, 125, 5501–5510; c) K. Nakano, K. Nozaki, T. Hiyama,
Macromolecules 2001, 34, 6325–6332.
[7] For use of supercritical CO2 as both reactant and solvent in metal-
complex-catalyzed copolymerization of CO2 with epoxides, see:
a) M. S. Super, E. Berluche, C. Costello, E. J. Beckman, Macromole-
cules 1997, 30, 368–372; b) T. Sarbu, J. Beckman, Macromolecules
1999, 32, 6904–6912; c) T. Sarbu, T. Styranec, E. J. Beckman, Nature
2000, 405, 165–168; d) S. Mang, A. I. Cooper, M. E. Colclough, N.
Chauhan, A. B. Holmes, Macromolecules 2000, 33, 303–308;
e) L. M. Stamp, S. A. Mang, A. B. Holmes, K. A. Knights, Y. R. de
Minuel, I. F. MaConvey, Chem. Commun. 2001, 2502–2503.
[8] For examples of CO2/epoxide copolymerization with homogeneous
metal complexes other than zinc, see: a) Z. Qin, C. M. Thomas, S.
Lee, G. W. Coates, Angew. Chem. 2003, 115, 5642–5645; Angew.
Chem. Int. Ed. 2003, 42, 5484–5487 and references therein; b) X.-B.
Lu, Y. Wang, Angew. Chem. 2004, 116, 3658–3661; Angew. Chem.
Int. Ed. 2004, 43, 3574–3577; c) W. J. Kruper, D. V. Dellar, J. Org.
Chem. 1995, 60, 725–727; d) D. J. Darensbourg, J. C. Yarbrough, J.
Am. Chem. Soc. 2002, 124, 6335–6342; e) D. J. Darensbourg, J. C.
Yarbrough, C. Ortiz, C. C. Fang, J. Am. Chem. Soc. 2003, 125, 7586–
7591; f) D. J. Darensbourg, S. A. Niezgoda, J. D. Draper, J. H. Rei-
benspies, J. Am. Chem. Soc. 1998, 120, 4690–4698; g) D, J. Dare-
nsbourg, J. R. Wildeson, S. J. Lewis, J. C. Yarbrough, J. Am. Chem.
Soc. 2002, 124, 7075–7083; h) M. H. Chisholm, Z. Zhou, J. Am.
Chem. Soc. 2004, 126, 11030–11039; see also references [7d,e].
[9] a) D. J. Darensbourg, M. W. Holtcamp, Macromolecules 1995, 28,
7577–7579; b) D. J. Darensbourg, M. W. Holtcamp, G. E. Struck,
M. S. Zimmer, S. A. Niezgoda, P. Rainey, J. B. Robertson, J. D.
Draper, J. H. Reibenspies, J. Am. Chem. Soc. 1999, 121, 107–116;
c) D. J. Darensbourg, M. S. Zimmer, P. Rainey, D. L. Larkins, Inorg.
Chem. 2000, 39, 1578–1585; d) D. J. Darensbourg, J. R. Wildeson,
J. C. Yarbrough, J. H. Reibenspies, J. Am. Chem. Soc. 2000, 122,
12487–12496.
[15] a) B. M. Trost, H. Ito, J. Am. Chem. Soc. 2000, 122, 12003–12004;
b) B. M. Trost, H. Ito, E. R. Silcoff, J. Am. Chem. Soc. 2001, 123,
3367–3368; c) B. M. Trost, E. R. Silcoff, H. Ito, Org. Lett. 2001, 3,
2497–2500; d) B. M. Trost, V. S. C. Yeh, Org. Lett. 2002, 4, 3513–
3516; e) B. M. Trost, V. S. C. Yeh, Angew. Chem. 2002, 114, 889–
891; Angew. Chem. Int. Ed. 2002, 41, 861–863; f) B. M. Trost,
V. S. C. Yeh, H. Ito, N. Bremeyer, Org. Lett. 2002, 4, 2621–2623;
g) B. M. Trost, L. R. Terrell, J. Am. Chem. Soc. 2003, 125, 338–339;
h) B. M. Trost, T. Mino, J. Am. Chem. Soc. 2003, 125, 2410–2411;
i) B. M. Trost, A. Fettes, B. T. Shireman, J. Am. Chem. Soc. 2004,
126, 2660–2661.
[16] S. D. Hanton, Chem. Rev. 2001, 101, 527–570.
[17] Trace amounts of oxygen in the reaction system could oxidize the
ethyl zinc species to ethyl peroxide, which might be reduced to
ethoxy zinc species, see: a) J. Lewinski, Z. Ochal, E. Bojarski, E.
Tratkiewicz, I. Justyniak, J. Lipkowski, Angew. Chem. 2003, 115,
4791–4794; Angew. Chem. Int. Ed. 2003, 42, 4643–4646; b) D.
Enders, J. Zhu, L. Kramps, Liebigs Ann./Recl. 1997, 1101–1113.
[18] D. J. Darensbourg, S. J. Lewis, J. L. Rodgers, J. C. Yarbrough, Inorg.
Chem. 2003, 42, 581–589.
[19] For examples of solvent-free catalysis, see: a) M. Tokunaga, J. F.
Larrow, F. Kakiuchi, E. N. Jacobsen, Science 1997, 277, 936–938;
b) N. K. Anand, E. M. Carreira, J. Am. Chem. Soc. 2001, 123, 9687–
9688; c) T. Shibata, N. Toshida, K. Takagi, J. Org. Chem. 2002, 67,
7446–7450; d) J. Long, J. Hu, X. Shen, B. Ji, K. Ding, J. Am. Chem.
Soc. 2002, 124, 10–11; e) Y. Yuan, X. Zhang, K. Ding. Angew.
Chem. 2003, 115, 5636–5638; Angew. Chem. Int. Ed. 2003, 42, 5478–
5480.
[20] CCDC-255984 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[21] For examples of intramolecular homodinuclear zinc complexes with
two alkoxo bridges, see: a) H. Adams, N. A. Bailey, P. Bertrand,
C. O. R. de Barbarin, D. E. Fenton, S. Gou, J. Chem. Soc. Dalton
Trans. 1995, 275–279; b) C. Bazzicalupi, A. Bencini, A. Bianchi, V.
Fusi, L. Mazzanti, P. Paoletti, B. Valtancoli, Inorg. Chem. 1995, 34,
3003–3010.
[10] D. J. Darensbourg, P. Rainey, J. Yarbrough, Inorg. Chem. 2001, 40,
986–993.
[11] a) M. Cheng, E. B. Lobkovsky, G. W. Coates, J. Am. Chem. Soc.
1998, 120, 11018–11019; b) M. Cheng, D. R. Moor, J. J. Reczek,
B. M. Chamberlain, E. B. Lobkovsky, G. W. Coates, J. Am. Chem.
Soc. 2001, 123, 8738–8749; c) D. R. Moor, M. Cheng, E. B. Lobkov-
sky, G. W. Coates, Angew. Chem. 2002, 114, 2711–2714; Angew.
Chem. Int. Ed. 2002, 41, 2599–2602; d) S. D. Allen, D. R. Moore,
E. B. Lobkovsky, G. W. Coates, J. Am. Chem. Soc. 2002, 124, 14284–
14285; e) D. R. Moor, M. Cheng, E. B. Lobkovsky, G. W. Coates, J.
Am. Chem. Soc. 2003, 125, 11911–11924; f) C. M. Byrne, S. D.
Allen, E. B. Lobkovsky, G. W. Coates, J. Am. Chem. Soc. 2004, 126,
11404–11405.
Received: November 17, 2004
Revised: February 2, 2005
Published online: April 13, 2005
3678
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 3668 – 3678