Chiral amido-oxazolinate zinc complexes for asymmetric alternating copolymerization of CO2 and cyclohexene oxide

Article abstract of DOI:10.1021/om3006992

The synthesis and characterization of a series of chiral zinc complexes (L^1a-m)ZnN(SiMe₃)₂ (2a-m) with C ₁-symmetric amido-oxazolinate ligands (HL^1a-m = 2-(2′-R₁NH)phenyl-4-R₂-ox

Full text of DOI:10.1021/om3006992

Article
pubs.acs.org/Organometallics
Chiral Amido-Oxazolinate Zinc Complexes for Asymmetric
Alternating Copolymerization of CO2 and Cyclohexene Oxide
Srinivas Abbina and Guodong Du*
Department of Chemistry, University of North Dakota, 151 Cornell Street Stop 9024, Grand Forks, North Dakota 58202, United
States
S
* Supporting Information
ABSTRACT: The synthesis and characterization of a series of chiral zinc
complexes (L1a−m)ZnN(SiMe3)2 (2a−m) with C1-symmetric amido-oxazolinate
ligands (HL1a−m = 2-(2′-R1NH)phenyl-4-R2-oxazoline) have been described.
Single-crystal X-ray crystallographic studies confirm that 2a (R1 = 2,6-
dimethylphenyl, R2 = (S)-iPr) and 2d (R1 = 2,6-dimethylphenyl, R2 = (R)-iBu)
are three-coordinate, mononuclear complexes, and 2k (R1 = PhCO, R2 = (S)-iPr)
exists as an amide oxygen-bridged dimer in the solid state with zinc in a distorted tetrahedral geometry. These complexes are
viable initiators for alternating copolymerization of carbon dioxide (CO2) and cyclohexene oxide (CHO), yielding
poly(cyclohexene carbonate) (PCHC) with good to high carbonate linkage and moderate molecular weights and PDI values,
depending on the substituents. The PCHCs produced are typically isotactic, containing up to 72% m-centered tetrads by 2h (R1
= 2,6-diisopropylphenyl, R2 = (R)-iBu), and a rare case of syndiotactic PCHC (57% r-centered tetrads) is obtained with 2j (R1 =
(R)-1-(1-naphthyl)ethyl), R2 = (R)-iBu). The asymmetric induction is generally low, with up to 71% SS unit in the main chain of
the produced PCHCs.
Chart 1
INTRODUCTION
■
Utilization of carbon dioxide (CO2) as a renewable C1
feedstock has attracted increased interest due to its low cost,
nontoxicity, and availability in nature and from many industrial
processes.1 Given its thermodynamic and kinetic stability, one
approach is to couple CO2 with high-energy, ring-strained
heterocyclic molecules, leading to formation of alternating
copolymers.2 The most widely studied one is the alternating
copolymerization of CO2 with epoxides, in the presence of
catalysts/cocatalysts.3,4 The resultant aliphatic polycarbonates
possess attractive properties, such as biodegradability and low
oxygen permeability, and are regarded as promising new
generation materials as alternatives to conventional petrochem-
ical-derived polymers. Consequently, much effort has been
devoted toward the development of efficient catalysts for
alternating copolymerization of CO2 with epoxides. Beginning
with Inoue’s discovery of the ZnEt2/H2O catalyst in 1969,5 a
wide array of catalytic systems of metal complexes with various
ligands, such as phenoxides,6 salen and its derivatives,3a,7
porphyrins,8 and others,9 have been explored to promote the
transformation. In particular, zinc-β-diketiminate complexes
(Chart 1, A) developed by Coates10 have been prominent for
the copolymerization of CO2 with epoxides mainly due to their
high catalytic activity and precise control over molecular weight
and polydispersity. Furthermore, the system is consistent with a
living polymerization process.10,11
terpolymerization with two different epoxides or other types
of monomers,13 and polymer chain cross-linking.14 Inspired by
the success of chiral catalysts in asymmetric organic synthesis,
another strategy is to use them to impart control over the
absolute and relative stereochemistry of the resulting
polycarbonates. The first examples of asymmetric copolymer-
ization of CO2 and CHO were reported by Nozaki, with a 1:1
mixture of (S)-diphenyl(pyrrolidin-2-yl)methanol and ZnEt2 as
A number of strategies have been exploited to improve the
thermal and mechanical properties of polycarbonates and to
expand their applications.3 These include copolymerization
with epoxide momomers other than the commonly applied
cyclohexene oxide (CHO) and propylene oxide (PO),12
Received: July 24, 2012
Published: October 26, 2012
© 2012 American Chemical Society
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a chiral catalyst,15 and by Coates, with a series of well-defined,
Zn imine-oxazoline catalysts (Chart 1, B).16 The main-chain
chirality of the poly(cyclohexene carbonate) (PCHC) was
determined to be 70% ee and up to 72% ee, respectively, after
hydrolysis to 1,2-trans-cyclohexane diol. The former was
improved to 80% ee by judicious adjustment of reaction
conditions based on mechanistic understandings.17 Despite its
dinuclear nature, Trost’s zinc complex was shown to induce
rather low enantioselectivity (18% ee) in alternating copoly-
merization of CO2 and CHO.18 Along with the developments
of zinc-based catalysts, cobalt and chromium salen complexes
were also widely investigated for this asymmetric coupling as
well as with racemic epoxides.19 Remarkably, the highest
enantioselectivity to date (96% ee) for PCHC was achieved
with an unsymmetric enantiopure Co(III)-salen catalyst in
combination with a cocatalyst, bis(triphenylphosphine)iminium
chloride.20 Additionally, this approach may serve as a useful
method for enantioselective desymmetrization of meso-epoxides
and kinetic resolution of racemic epoxides.
Monoanionic and chelating β-diketiminate ligands have
found widespread use in coordination chemistry and catalysis.21
Modulation of substituents around the ligand skeleton can
strongly influence the steric and electronic behavior of the
ligands22 and enables them to be used in numerous catalytic
applications.23 Likewise, isoelectronic and structurally related
variants of β-diketiminate ligands, such as triazapentadienates,
formazanates, anilido-aldimines, and imine-oxazolinates, have
also been examined for different catalytic reactions.24
Particularly, the anilido-aldimine moiety has been incorporated
into dinucleating zinc complexes (Chart 1, C) that show high
activity toward CO2/CHO copolymerization.25 However, the
chiral version of these ligands is relatively less explored in
catalysis;26 the Coates’s Zn imine-oxazoline catalysts are an
eminent example.16
Scheme 1. Synthesis of Ligands and Zinc Amide Complexes
under higher temperature (80 °C) and with longer times (20 h
to days).10b In the H NMR spectra, the disappearance of the
1
free ligand NH signals around 10−11 ppm and the appearance
of new broad singlets in the upfield region at 0.1 to −0.05 ppm
are consistent with the incorporation of a single amido-
oxazolinate ligand in the products. The new broad signals,
assigned to the trimethylsilyl groups, usually appear as one
singlet at ambient temperature that integrates to 18 protons;
however, two broad singlets were observed in some cases for
N(SiMe3)2 protons. The splitting may indicate restricted
rotation of the N(SiMe3)2 group, resulting from its steric
t
interaction with the substituents on either side, e.g., Bu (2c,
2m) or DiPP (2g, 2h), or with another −N(SiMe3)2 group in a
dimeric arrangement (2k, 2l). In the 13C NMR spectra, the
peaks around 4.80−5.50 ppm were assigned to the carbon of
the silyl group, consistent with the formation of the amido-
oxazolinate zinc complexes.
Formation of homoleptic bis-ligated zinc complexes was not
observed, which may suggest that the steric bulk of the ligands
is sufficient to stabilize the unsaturated, three-coordinate metal
center and prevent the coordination of an additional ligand. In
fact, treatment of complex 2a with excess THF or pyridine
Recently, we have prepared a series of unsymmetrical β-
diketimine-type ligands incorporating a chiral 2-oxazoline
moiety (Chart 1, D).27 On the basis of the studies mentioned
above,16,25 their zinc complexes are expected to be effective
initiators for alternating copolymerization of CO2 and CHO.
Although being C1 symmetric, they could be advantageous, as
the dissymmetric environment is postulated to enhance the
asymmetric induction in the CO2/CHO copolymerization.20
Herein, we present the synthesis and characterization of zinc
complexes of several C1-symmetric chiral amido-oxazolinate
ligands and their catalytic applications in the alternating
copolymerization of CO2 and CHO.
1
shows no sign of binding, as judged by H NMR. However,
when the steric bulk is reduced, e.g., in 2k and 2l, additional set
of signals appears in the 1H NMR spectra. For example, besides
the septet at 1.60 ppm, assigned to the methine proton of
−CH(CH3)2 in 2k, a smaller septet at 1.86 ppm is also
observed.29 This is attributed to the monomer/dimer
equilibrium in solution (Scheme 2), which is further supported
Scheme 2. Monomer/Dimer Equilibrium for 2k and 2l
RESULTS AND DISCUSSION
■
Synthesis of Zinc Complexes. The chiral, unsymmetrical
amido-oxazolinate ligands, a variation of β-diketimine type,
have been obtained via a palladium-catalyzed Buchwald−
Hartwig amination reaction (Scheme 1).27 The modular
approach allows independent variations of the two stereo-
directing groups (R1 and R2) in the ligand framework, and
various substituents are incorporated into the framework in
order to explore the steric and electronic effects of substituents
and their possible synergy in catalysis.
by dilution studies of 2k using 1H NMR. The relative intensity
of the aforementioned signals varies with dilution. As the
dilution of the solution was increased, 2k became a more
monomeric species, from which Keq = 75.4 M−1 (20 °C in
CDCl3) can be derived. The value is analogous to an acetate-
bridged β-diketiminate dimer (Keq = 207 M−1 at 20 °C in
C6D6).10d Addition of THF or pyridine has little effect on the
equilibrium. This may suggest that the side-arm amide oxygen
can coordinate intramolecularly or form a bridge between two
Following previous reports,10b the zinc amide complexes
(2a−m) were prepared by treatment of the free ligands with 1
equiv of Zn[N(SiMe3)2]2 in dry toluene at room temperature
(Scheme 1).28 The desired products were readily isolated in
high yields within a few hours. In comparison, similar reactions
with conventional β-diketiminate ligands were carried out
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metals. In the solid state, compound 2k exists as an amide
oxygen-bridged dimer, as confirmed by the X-ray structural
analysis. However, compound 2m, with a sulfinamido side arm,
shows no evidence of dimer/monomer equilibrium, and a
Nimino (1.968(9) Å) is considerably longer than that of Zn−
Namido (1.915(10) Å), presumably due to the stronger
interaction with the anionic amido nitrogen. The difference
(∼0.05 Å) is similar to those in other unsymmetrical NN
bidentate ligation with less resonance character,30 but longer
than those in more symmetrical, β-diketiminate complexes
(0.01−0.04 Å).10b,31 The bond distance of Zn−N3 (1.874(9)
Å) is on the shorter end of the Zn−Nsilylamide bonds
reported.31,32 The bond angle of N1−Zn−N3 (139.15(4)°) is
larger than that of N2 −Zn−N3 (125.35(4)°), due to higher
steric repulsion between the DMP group and the N(SiMe3)2
group. This leads to the N(SiMe3)2 group tilting toward the
imine side. On the other hand, due to the chiral center at the 4-
oxazoline position (S configuration), the plane defined by N3−
Si1−Si2 is not perpendicular to the N1−N2−N3 coordination
plane; instead, it is twisted 31.43(3)° from its regular
perpendicular position, with one of the silyl groups staying
1
single set of signals is observed in the H NMR spectrum. It is
possible that 2m exists as a monomeric species with
sulfinamido oxygen weakly coordinated to the zinc center.
Given the interest in bimetallic catalysis, the observation here
with 2k and 2l may suggest a different approach to constructing
bimetallic systems.
As zinc β-diketiminate complexes with various initiating
groups are known for the copolymerization reaction of
epoxides and CO2, we attempted to synthesize zinc complexes
with alkoxide and carboxylate groups, starting from different
zinc precursors, such as Zn(OAc)2 and ZnCl2, after
deprotonation of ligands with nBuLi.10a−d However, no desired
complex was isolated from these reactions. In an alternative
route, the zinc amide complex (2a) was allowed to react with a
stoichiometric amount of 2-propanol, and formation of alkoxide
was indicated by the new peaks at 3.42 and 1.60 ppm for
(L1a)Zn-OiPr. However, the reaction appeared to be
complicated by side reactions, and attempted purification led
to the decomposition of complex to free ligand. These
observations are in contrast with the conventional zinc β-
diketiminate complexes and may be a reflection of the inherent
basicity of the amido nitrogen in the present system.25
X-ray Structures. Single crystals of compound 2a were
obtained from concentrated solutions in toluene at −20 °C.
The solid-state structure was determined by single-crystal X-ray
diffraction techniques; the X-ray crystal data and data collection
and refinement parameters are summarized in Table S1 in the
Supporting Information. The structure of complex 2a is
depicted in Figure 1 with selected bond distances and bond
i
away from the Pr group on the same side. In turn, the DMP
group is 17.66(4)° away from perpendicular to the plane
defined by N(1)−N(2)−N(3) due to steric repulsions.
Moreover, the bite angle of N1−Zn−N2 was sharper than
the other bonds (N1−Zn−N3, N2−Zn−N3).
To further probe the influence of the chiral center on the
conformation of the zinc complexes, we determined the X-ray
crystal structure of 2d, containing an R configuration at the 4-
i
oxazoline position with a Bu group. The modification of the
chiral configuration on the oxazoline ring did not lead to
considerable changes in the coordination geometry around the
metal center. Complex 2d was isomorphic with 2a, featuring a
distorted trigonal-planar geometry and similar geometric
parameters (Figure 2). The smaller bond angle of N2−Zn−
Figure 2. ORTEP drawing of complex 2d with thermal ellipsoids
drawn at the 50% probability level. Selected bond distances (Å) and
bond angles (deg): Zn−N(1) 1.912(15), Zn−N(2) 1.975(16), Zn−
N(3) 1.877(15), N(1)−Zn−N(2) 95.84(6), N(1)−Zn−N(3)
142.72(7), N(2)−Zn− N(3) 121.43(7). Hydrogen atoms are omitted
for clarity.
Figure 1. ORTEP drawing of complex 2a with thermal ellipsoids
drawn at the 50% probability level. Selected bond distances (Å) and
bond angles (deg): Zn−N(1) 1.915(10), Zn−N(2) 1.968(9), Zn−
N(3) 1.874(9), N(1)−Zn−N(2) 94.98(4), N(1)−Zn−N(3)
139.15(4), N(2)−Zn−N(3) 125.35(4). Hydrogen atoms are omitted
for clarity.
angles. In agreement with the solution NMR data, 2a is a
monometallic, C1-symmetric, three-coordinate zinc complex,
and the geometry at zinc metal was best described as trigonal
planar, as the sum of the bond angles around the zinc atom is
359.48°. The chelating amido-oxazolinate ligand forms a
slightly puckered six-membered ring with the zinc metal
through its two nitrogen atoms. The bond distance of Zn−
N3 = 121.43(7)° in 2d compared to 2a (125.35(4)°) can be
attributed to the smaller steric bulk of iBu (2d) compared to iPr
(2a), which imposes less repulsion with the trimethylsilyl
group. In accord with this, the six-membered chelating ring is
almost planar; the Zn atom is displaced only by 0.0877(2) Å
from the plane through other five atoms. Because of the R
configuration at the chiral center, the silylamido groups twisted
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in the opposite orientation (compared with that in 2a), but to a
amido phenyl groups are arranged in an anti manner with
respect to the NN chelate.
i
lesser extent (8.54(7)° vs 31.43(3)° in 2a), again due to Bu
i
being less demanding than Pr. The DMP group is slightly
Copolymerization Studies. Our interest in the present
system has been twofold. First, we investigate the zinc
complexes as potential catalysts for alternating copolymeriza-
tion of CO2 and CHO. Second, we want to address the effect of
the ligand design on the asymmetric incorporation of CHO
monomer into the polymer chain. Compound 2a was used as a
catalyst for initial optimization. Reaction conditions such as
temperature, reaction time, pressure of the CO2, and additive
were varied. The selectivity, or the percentage of polycarbonate
linkage, was determined by measuring the relative intensity of
the methine proton signals of the carbonate linkage (δ = 4.6
ppm) and ether linkage (δ = 3.4 ppm). In no cases was the
generation of cyclic carbonate observed (δ = 4.0 ppm). Selected
results are listed in Table 1. It is observed that the temperature
and CO2 pressure have a strong influence on the outcome of
the copolymerization. The catalyst was inactive at room
temperature. As expected, the conversion and carbonate
linkages improved with the increasing CO2 pressure from 100
to 500 psi (entries 2−4). However, further increase in pressure
seemed to be detrimental (entry 5). When the initiating group
was changed from N(SiMe3)2 to an alkoxide OiPr by in situ
addition of 1 equiv of iPrOH, comparable results were observed
(entry 6 vs 4). Despite the reasonable conversion, only low
yields of polycarbonates were isolated, in part due to some
weight loss during the reaction (up to 30%). The cause is
unclear, but running the reaction without stirring seemed to
alleviate the loss, as isolated yields are significantly higher
(entry 7), at the expense of longer reaction time and slightly
lower carbonate linkage. Raising the temperature to 75 °C
increased the conversion and shortened the time to 20 h
without sacrificing yield and selectivity (entry 8).
tilted, by 12.81(6)°, from its perpendicular position.
Suitable crystals of complex 2k were obtained from
recrystallization in dry toluene at room temperature, and
single-crystal X-ray determinations confirm its dimeric form in
the solid state (Figure 3). The two zinc centers are bridged by
Figure 3. ORTEP drawing of complex 2k with thermal ellipsoids
drawn at the 50% probability level. Selected bond distances (Å) and
bond angles (deg): Zn(1)−N(1) 2.032(2), Zn(1)−N(5) 2.075(2),
Zn(1)−N(6) 1.921(19), Zn(1)−O(2) 2.025(16), N(1)−C(1)-
1.321(3), O(1)−C(1)1.271(3), N(1) −Zn(1)−N(5) 88.42(8),
N(1)−Zn(1)−N(6) 132.52(8), N(5)−Zn(1)−N(6) 115.02(8),
N(1)−Zn(1)−O(2) 100.22(7), N(5)−Zn(1)−O(2) 99.32(7). Hydro-
gen atoms are omitted for clarity.
To investigate the effect of ligand architecture on catalytic
performance, complexes 2a−m with systematic modifications in
substituents were employed for CHO and CO2 copolymeriza-
tion under optimized conditions (500 psi of CO2 pressure, 75
°C, without stirring, 20 h). The results are summarized in Table
2. Although the reaction rate is slow, with a TOF generally
around 3−4 h−1, all the compounds showed appreciable
activity, and a white powdery polymer was isolated after
workup. It appeared that the conversions were capped at ∼80%,
as increased reaction time showed little improvement, probably
due to the high viscosity at the end of the reaction.34 Little or
no cyclic carbonate was noted; up to 95% polycarbonate
linkage (2b) was obtained in the polymers. This level of
selectivity is lower than the optimized β-diketiminato zinc
the amide oxygen of the other ligand to form a tub-like eight-
membered ring (Zn1−N1−C1−O1−Zn2−N2−C2−O2). Each
zinc metal is ligated with the two nitrogens of the ligand, one
nitrogen of the silylamide, and one oxygen atom from
benzamide in a distorted tetrahedral geometry. The bond
distances of Zn−N are considerally longer than those in
monomeric zinc complexes 2a and 2d, but are typical of a
tetrahedron geometry due to its more crowded environment
than the trigonal-planar geometry.33 The relative distances
follow a similar pattern to that in monomeric zinc complexes, as
Zn−Nimino (2.075(2) Å) is ∼0.043 Å longer than Zn−Namido
(2.032(2) Å) and Zn−Nsilylamide (1.921(19) Å) is the shortest.
The two bis(trimethylsilyl)amido groups are oriented in a syn
fashion. The isopropyl groups on the oxazoline ring and the
a
Table 1. Screening of the Conditions for Copolymerization of CO2 and CHO
b
c
d
entry
temp (°C)
RT
pressure (PSI)
100
t/h
stirring (rpm)
1000
conversion (%)
yield (%)
carbonate linkages (%)
1
2
3
4
5
24
24
24
24
24
24
48
20
0
42
59
60
27
60
62
80
50
50
50
50
50
50
75
100
250
500
750
500
500
500
1000
1000
1000
1000
1000
n. s.
12
7
75
85
93
75
96
81
85
15
11
13
38
36
e
6
f
7
f
8
n. s.
a
b
Reaction conditions: Copolymerization reactions were run in neat cyclohexene oxide (CHO) using 1 mol % catalyst 2a. Determined by 1H NMR
c
d
spectroscopy on crude reaction mixture. Isolated yield assuming 100% polycarbonates. Calculated by the integration of methine resonances in 1H
NMR spectra of polymers. With added PrOH (1 equiv to 2a). Not stirred.
e
f
i
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a
Table 2. Copolymerization of CHO and CO2 Using Catalysts 2a−m
b
c
d
e
f
f
g
entry
cat.
conversion (%)
carbonate linkages (%)
m-centered tetrad (%)
yield (%)
Mn (kg/mol)
PDI
1.3
SS/RR
1
2a
2b
2c
2d
2e
2f
80
77
69
62
66
83
82
81
58
59
78
81
18
85
95
90
72
85
6
69
67
69
66
62
36
18
32
18
23
42
32
43
20
13
17
16
4
6.5
13.4
3.1
42/58
i
h
2
4.2
1.4
2.0
7.0
1.9
1.7
1.6
7.0
1.2
1.5
1.2
1.1
3
48/52
56/44
52/48
4
9.1
h
5
11.6
26.1
6.1
6
7
2g
2h
2i
56
72
90
60
69
83
49
70
72
65
43
63
70
70
41/59
71/29
8
11.3
14.1
2.9
h
9
53/47
i
10
11
12
13
2j
2k
2l
4.1
48/52
52/48
44/56
2.6
2m
7.0
a
b
Copolymerization reactions were carried out in neat CHO at 75 °C with [CHO]:[catalyst] = 100:1 at 500 psi of CO2. Determined by 1H NMR
c
d
spectroscopy. Calculated by the integration of methine resonances in H NMR spectra of polymers. Determined by 13C NMR spectroscopy.
1
e
f
Isolated yields, assuming 100% polycarbonates. Determined by gel permeation chromatography calibrated with polystyrene standards in
g
h
i
tetrahydrofuran. Determined by chiral GC on diols after hydrolysis. Bimodel distribution of polymer. Racemic
Figure 4. 13C NMR spectrum of the poly(cyclohexene carbonate) generated by catalyst 2c: (a) carbonyl region; (b and c) methylene region.
catalysts,10 but is comparable with the more closely related
anilido-aldimine zinc catalysts.25 Electron-withdrawing groups
are known to increase the Lewis acidity and catalytic activity of
the metal.10,16 However, introduction of CF3 groups at the meta
positions of the phenyl group significantly raised the catalytic
activity of 2f toward homopolymerization of CHO, as only a
small amount of polycarbonate linkage (6%) was incorporated,
although the molecular weight is high (26.2 kg/mol).
Introduction of a second alkyl chiral group at the amido side
(2i, 2j) led to lower conversion, presumably because the free
rotation of the less bulky substituents on amido nitrogen may
hinder CHO entrance to the active site.4c,10b,35 Restricted
rotation of the aniline has been observed to be important in
facilitating catalytic activity in olefin polymerization.36 Despite
the less bulky substituent in 2k and 2l, both showed high
conversion, which may be indicative of bimetallic action.
However, compound 2m with sulfinamide was very sluggish,
and it afforded a polymer with high polyether linkage and
narrow PDI value.
The molecular weight of the polycarbonate was determined
by GPC against a polystyrene standard. The polycarbonate
produced by the zinc catalysts showed similar or lower
molecular weight compared with calculated values based on
conversion, indicative of the presence of a chain transfer
process during the reaction, and the molecular weight
distribution is somewhat broad (PDI 1.1−2.0). In a few cases
(2b, 2e, and 2i), the PDI values were inflated by the presence
of an additional peak at higher molecular weights.29 The
bimodal distributions of polycarbonates have been noted in
several zinc-catalyzed copolymerizations and may arise from the
presence of multiple active sites or monomer/dimer equili-
brium of the catalytic species.8a,37
distinct resonances were observed at 154.03, 153.52, and
153.37 ppm in the carbonyl regions of PCHCs (Figure 4a).
Based on the previous assignments,16,38 the largest peak at
154.03 ppm was correlated to m-centered tetrads ([mmm] and
[mmr]), and the remaining tetrad upfield resonances (δ =
153.52 and 153.37 ppm) were assigned to the r-centered
tetrads ([mrm] and [mrr]). The other common tetrads in
isotactic polymers [rmr] and [rrr] were not observed. In accord
with this, two series of methylene triad resonances were
observed corresponding to two nonequivalent methylene
carbons in the 13C NMR spectra (Figure 4b and c). The two
downfield resonances (δ = 29.90 (b), 23.28 (c)) correspond to
[mm] triads, and the intensity is relatively higher than the other
triads [mr] (δ = 29.66 (b), 23.06 (c)) and [rr] (δ = 29.05 (b),
22.54 (c)). The other peaks in this region correspond to
polyether linkage.15,38 On the basis of these assignments, most
of the catalysts yield isoenriched PCHC with high m-centered
tetrads. Increased steric bulk in the catalysts seems to favor
higher microstructure regularity in the resultant polycarbonates,
as the highest isotacticity (72% m-centered tetrads) was
obtained with catalyst 2h, which bears 2,6-DiPP and iBu groups
on its ligand backbone. Curiously, catalyst 2j produces a slightly
syndioenriched PCHC with 57% r-centered tetrads, which is a
rather rare case.38 The two different configurations of chiral
centers might have influence on this reverse selectivity.
To further investigate the microstructure assignment and
reaction mechanisms, statistical methods were applied to
simulate the tetrad distributions observed in PCHC.16,38
When two models based on chain-end control and
enantiomorphic site control mechanisms are compared, the
latter seems to give better agreement between calculated and
observed tetrad intensities.29,39 The enantioselectivity parame-
ter α, the probability of an R monomer unit adding at the R site
or an S monomer unit adding at the S site, has been calculated,
The microstructure (tacticity) of the resulting polymers was
characterized by 13C NMR spectroscopy. Typically three
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with the highest value being 0.85 with 2h.29 The numbers are
comparable with those obtained with zinc imine-oxazoline
catalysts.16 On this basis we hypothesize the present system
follows an enantiomorphic site control mechanism.11,16
However, this process may be competitive with a chain-end
control mechanism.38
EXPERIMENTAL SECTION
■
General Procedures. All reactions that involved compounds
sensitive to air and/or moisture were carried out under a dry nitrogen
atmosphere using freshly dried solvents and standard Schlenk line and
glovebox techniques. All chemicals were purchased from Aldrich
except where noted. Toluene was distilled under nitrogen from Na/
benzophenone. CDCl3 and C6D6 were dried over CaH2 and Na/
benzophenone, respectively, and distilled and degassed prior to use.
Carbon dioxide (Airgas, high purity, 99.995%) was used as received.
Cyclohexene oxide was distilled from CaH2 following three freeze−
pump−thaw cycles and stored in a glovebox prior to use. Zinc
bis(trimethylsilyl)amide was prepared according to the literature.41
NMR spectra were recorded on a Bruker AVANCE-500 NMR
spectrometer (1H and 13C). Chiral GC analysis was carried out on an
Agilent 7890 with FID detector using a chiral column (cyclodex-B, 30
m × 0.250 mm × 0.25 μm). The temperature program was as follows:
injector temperature 250 °C, detector 300 °C, oven initial temperature
120 °C, hold for 30 min, ramp at 30 °C/min to 200 °C, hold for 10
min. Inlet flow: 85 mL/min (split mode, 68:1). Gel permeation
chromatography (GPC) analysis was performed on a Varian Prostar,
using a PLgel 5 μm Mixed-D column, a Prostar 355 RI detector, and
THF as eluent at a flow rate of 1 mL/min (20 °C). Polystyrene
standards were used for calibration.
The hydrolysis of resulting copolymers, by an alkali
treatment, followed by neutralization with 1 M HCl, gave
1,2-trans-cyclohexanediol in 70−80% isolated yields. Chiral GC
analysis of the diol indicates that these chiral catalysts can
induce asymmetry during the copolymerization, although the
enantiomeric excess was generally low (Table 2). Among them,
catalyst 2h exhibited the highest enantiomeric excess of 71:29
(SS:RR) (Table 2, entry 8). It should be noted that catalyst 2h
also afforded the highest isotacticity in the series. When
compared with related zinc imine-oxazoline catalysts (see Chart
1, B) with an RR:SS ratio up to 86:14,16 the stereoselectivity
exhibited by the present catalysts is low, especially considering
the resemblance in their structural features and the similarity in
the enantioselectivity parameter α. The exact reason is not
understood at the moment, although it is suspected that the
low selectivity of the present system might be due to some
electronic effect resulting from attenuated resonance in the
ligand backbone.40 Nevertheless, they have the same sense of
chiral induction, and the chirality of the diol is mostly
influenced by the configuration of the oxazoline substituents,
as an R configuration at the 4-oxazoline position results in
enrichment of opposite configurations in the diol unit.16 The
combination of a bulky group at the amide side and a long,
bulky group at the imine side seems to work best. The second
chiral group at the amide side may also play a significant role;
however, a synergistic interaction between the two is not
observed.
Synthesis of (4S)-4,5-Dihydro-2-[2′-(2,6-dimethylanilino)phenyl]-
4-tert-butyloxazole (HL1c). Synthesis of this new ligand was
performed analogously following the literature.27 Yield: 98%. 1H
NMR (500.1 MHz; CDCl3; 298 K): 0.95 (9H, s, C(CH3)3), 2.22 (6H,
s, ArMe), 4.27 (2H, m, NCH(R)CH2O), 4.30 (1H, m, NCH(R)-
CH2O), 6.23 (1H, d, J = 8.80, m-PhHN), 6.65 (1H, t, J = 7.50, p-
PhH(CH3)2), 7.10−7.15 (4H, br, ArH), 7.79 (1H, d, J = 7.90, o-
PhHN), 10.01 (1H, br, NH). 13C NMR (125.8 MHz; CDCl3; 298 K):
18.62 (ArMe), 26.07 (C(CH3)3), 34.10 (C(CH3)3), 67.02 (NCH(R)-
CH2O), 76.42 (NCH(R)CH2O), 108.66, 112.72, 115.35, 126.22,
129.90, 132.29 (CHarom), 136.47, 137.03, 138.23, 147.84, 164.05
(Cquart). GC/MS: m/z 322[M]+, 307, 291, 222, 208, 194. HRMS
(ESI): m/z calcd for C21H26N2O [M]+ 322.20451; found 322.21233.
Synthesis of (4R)-4,5-Dihydro-2-[2′-(3,5-bis(trifluoromethyl)-
anilino)phenyl]-4-phenyloxazole (HL1f). Synthesis of this new ligand
was performed analogously following the literature.27 Yield: 66%. H
1
CONCLUSIONS
NMR (500.1 MHz; CDCl3; 298 K): 3.84 (1H, t, J = 8.12,
NCH(R)CH2O), 4.39 (1H, t, J = 8.80, NCH(R)CH2O), 5.51 (1H,
m, t, J = 8.70, NCH(R)CH2O), 6.46 (1H, d, J = 7.8 m-PhHN), 6.96
(3H, m, ArH), 7.02−7.04 (4H, m, ArH), 7.05−7.09 (3H, m, ArH),
7.60 (1H, dr, J = 8.2, o-PhHN), 10.51 (1H, br, NH). 13C NMR (125.8
MHz; CDCl3; 298 K): 70.32 (NCH(R)CH2O), 73.62 (NCH(R)
CH2O), 112.27 (Ar-CF3), 114.35 (Ar-CF3), 115.29, 119.67, 119.80,
■
A family of new chiral zinc complexes has been synthesized via
the reaction of Zn[N(SiMe3)2]2 and the corresponding C1-
symmetric, monoanionic amido-oxazolinate ligands (HL1a−m).
While most of them exist as mononuclear complexes, as
confirmed by single-crystal X-ray structural analysis of 2a and
2d, complexes 2k and 2l, with less bulky substituents, are
dimeric in the solid state and are in equilibrium between
monomer and dimer in solution. All of them are shown to be
viable initiators for copolymerization of CO2 and CHO,
yielding polymers with up to 95% polycarbonate linkage and
moderate molecular weights and polydispersity values.
Modifications around the ligand architecture have a significant
influence on the polymerization process. The resultant
polymers are isotactic with enriched m-centered tetrads, except
for 2j, which produces a syndioenriched PCHC. Induction of
main chain chirality is feasible, with up to 42% ee obtained with
2h, and can be correlated roughly with the chiral centers in
ligands, although enantioselectivity is usually very low. In
general, catalysts with one chiral center on the oxazoline moiety
(R2 position) and a sufficiently bulky group on the amido
nitrogen (R1 position) seem to provide better structural
requirements for activity and selectivity. Current efforts are
under way for a better understanding of the effect of ligand
architecture in the copolymerization process and improving the
activity and selectivity of the catalysts.
120.01, 122.44, 124.60, 126.70, 128.02, 129.07, 130.78 (CHarom
)
132.20, 132.77, 133.27, 142.31, 143.60, 143.98, 163.40 (Cquart). GC/
MS: m/z 450[M]+, 331, 331, 304. HRMS (ESI): m/z calcd for
C23H17F6N2O [M + H]+ 451.12451; found 451.12450.
Synthesis of [(L1a)ZnN(SiMe3)2] (2a). A solution of ligand HL1a
(414 mg, 1.342 mmol) in toluene (10 mL) was added into zinc
bis(trimethylsilyl)amide (543 mg, 1.406 mmol) in toluene (5 mL) at
room temperature. After being stirred for 8 h, the pale yellow solution
was dried in vacuo, giving the desired compound as a yellow powder
(98%, 704 mg). Recrystallization from toluene at −20 °C gave yellow-
colored crystals (90%, 643 mg) suitable for single-crystal X-ray
1
diffraction analysis. H NMR (500.1 MHz; CDCl3; 298 K): −0.15
(18H, s, N(SiMe3)2), 0.90 (3H, d, J = 7.14, (CH3)2CH), 1.02 (3H, d, J
= 7.10, (CH3)2CH), 2.01 (3H, s, ArMe), 2.24 (3H, s, ArMe), 2.41 (1H,
m, (CH3)2CH), 4.39 (1H, t, J = 7.10, NCH(R)CH2O), 4.47 (1H, t, J =
8.0, NCH(R)CH2O), 4.51 (1H, m, NCH(R)CH2O), 6.22 (1H, d, J
=8.70, m-PhHN), 6.45 (1H, t, J = 8.10, p-PhH(CH3)2), 7.01 (1H, t, J =
7.94, m-PhHN), 7.08 (2H, br, m-PhH(CH3)2), 7.16 (1H, t, J = 7.84, p-
PhHN), 7.82 (1H, d, J = 8.06 o-PhHN). 13C NMR (125.8 MHz;
CDCl3; 298 K): 4.87 (SiMe3), 15.10 ((CH)Me2), 19.10 (ArMe), 19.16
(ArMe), 31.38 ((CH)Me2), 66.94 (NCH(R)CH2O), 69.24 (NCH-
(R)CH2O), 103.86, 113.05, 116.24, 124.41, 129.99, 131.60 (CHarom),
134.25, 134.51, 147.2, 157.18, 169.50 (Cquart). Anal. Calcd for
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C26H41N3OSi2Zn: C, 58.57; H, 7.75; N, 7.88. Found: C, 57.18; H,
7.91; N, 7.58.
NCH(R)CH2O), 4.90 (1H, t, J = 8.90, NCH(R)CH2O), 5.53 (1H, m,
NCH(R)CH2O), 6.27 (1H, d, J = 8.85, m-PhHN), 6.50 (1H, t, J =
7.57, p-PhH(CH3)2), 6.99 (1H, t, J = 7.81, p-PhHN), 7.06 (1H, m, m-
PhHN), 7.12 (2H, m, m-PhH(CH3)2), 7.35−7.40 (5H, m,
C3H3NOPhH), 7.93 (1H, d, J = 8.37, o-PhHN). 13C NMR (125.8
MHz; CDCl3; 298 K): 4.87 (SiMe3), 18.67 (ArMe), 18.98 (ArMe),
68.11 (NCH(R)CH2O), 74.0 (NCH(R)CH2O), 103.86, 116.40,
118.24, 121.33, 124.39, 125.54, 127.09, 128.46, 128.92, 129.27,
129.50, 131.67 (CHarom), 134.00, 134.09, 134.70, 141.34, 147.10,
157.46, 170.19 (Cquart). Anal. Calcd for C29H39N3OSi2Zn·0.5CH2Cl2:
C, 58.11; H, 6.61; N, 6.89. Found: C, 58.25; H, 6.79; N, 6.43.
Synthesis of [(L1f)ZnN(SiMe3)2] (2f). To a solution of ligand HL1f
(79 mg, 0.175 mmol) in toluene (4 mL), was added zinc
bis(trimethylsilyl)amide (68 mg, 0.175 mmol) at room temperature.
After being stirred for 8 h, the pale yellow colored solution was dried
in vacuo, giving the desired compound as a yellow powder (68%, 82
Synthesis of [(L1b)ZnN(SiMe3)2] (2b). A solution of ligand HL1b
(237 mg, 0.735 mmol) in toluene (5 mL) was added into zinc
bis(trimethylsilyl)amide (283 mg, 0.735 mmol) in toluene (5 mL) at
room temperature. After being stirred for 8 h, the pale yellow colored
solution was dried in vacuo, giving the desired compound as a yellow
powder (89%, 357 mg). 1H NMR (500.1 MHz; CDCl3; 298 K): −0.24
(18H, s, CH3), 0.84 (3H, m, CH3CHCH2CH3), 1.02 (3H, m,
CH3CHCH2CH3), 1.27 (1H, m, CH3CHCH2 CH3), 1.34 (1H, m,
CH3CHCH2CH3), 1.43 (1H, m, CH3CHCH2CH3), 2.00 (3H, s,
ArMe), 2.18 (3H, s, ArMe), 4.49 (1H, t, J = 8.54, NCH(R)CH2O),
4.55 (1H, t, J = 9.67, NCH(R)CH2O), 4.61 (1H, t, J = 8.55,
NCH(R)CH2O), 6.23 (1H, d, J = 9.30, m-PhHN), 6.47 (1H, t, J =
7.90, p-PhH(CH3)2), 7.01 (1H, t, J = 7.67, p-PhHN), 7.07 (1H, br, m-
PhHN), 7.23−7.48 (2H, br, m-PhH(CH3)2), 7.81 (1H, d, J = 8.29, o-
PhHN). 13C NMR (125.8 MHz; CDCl3; 298 K): 4.80 (SiMe3), 12.10
(CHCH2(CH3)2), 12.21 (CHCH2(CH3)2), 18.69 (ArMe), 18.98
(ArMe), 26.85 (CHCH2(CH3)2), 37.98 (CHCH2(CH3)2), 66.61
(NCH(R)CH2O), 68.17 (NCH(R)CH2O), 103.96, 113.03, 116.25,
124.36, 125.53, 128.47 (CHarom), 131.02, 131.20, 134.41, 147.22,
157.11, 169.45 (Cquart). Anal. Calcd for C27H43N3OSi2Zn·0.2C7H8: C,
60.30; H, 7.95; N, 7.43. Found: C, 60.39; H, 7.61; N, 6.67.
1
mg). H NMR (500.1 MHz; CDCl3; 298 K): −0.32 (18H, s, CH3),
4.56 (1H, m, NCH(R)CH2O), 4.93 (1H, t, J = 7.80, NCH(R)CH2O),
5.51 (1H, m, NCH(R)CH2O), 6.46 (2H, m, ArH), 7.06 (1H, m,
ArH), 7.19 (2H, m, ArH), 7.33−7.44 (3H, m, ArH), 7.53 (2H, m,
ArH), 7.58 (1H, br, ArH), 8.02 (1H, d, J = 9.35, o-PhHN). 13C NMR
(125.8 MHz; CDCl3): 5.52 (SiMe3), 68.45 (NCH(R)CH2O), 74.39
(NCH(R)CH2O),115.30 (CF3), 117.50 (CF3), 122.50, 124.70, 125.55,
126.84, 127.33, 128.47, 128.62, 129.30, 129.74, 132.14 (CHarom),
133.14, 135.22, 140.41, 152.67, 156.69, 157.38, 170.50 (Cquart).
Synthesis of [(L1g)ZnN(SiMe3)2] (2g). To a solution of ligand HL1g
(400 mg, 1.097 mmol) in toluene (15 mL) was added zinc
bis(trimethylsilyl)amide (424 mg, 1.098 mmol) at room temperature.
After being stirred for 8 h, the yellow-colored solution was dried in
vacuo, giving the desired compound as a yellow powder (89%, 575
mg). 1H NMR (500.1 MHz; CDCl3; 298 K): −0.30 (9H, s, CH3), 0.03
(9H, s, CH3), 0.92 (3H, d, J = 6.81, CH(CH3)2), 1.07 (6H, d, J = 7.03,
ArCH(CH3)2), 1.17 (3H, d, J = 6.91, CH(CH3)2), 1.29 (6H, d, J =
6.75, ArCH(CH3)2), 1.57 (1H, m, CH(CH3)2), 3.02 (1H, m,
ArCH(CH3)2), 3.28 (1H, m, ArCH(CH3)2), 4.40 (1H, t, J = 7.75,
NCH(R)CH2O), 4.47 (1H, t, J = 8.49, NCH(R)CH2O), 4.54 (1H, m,
NCH(R)CH2O), 6.25 (1H, d, J = 9.25, m-PhHN), 6.44 (1H, t, J =
8.49, p-PhH(CH3)2), 7.19 (1H, t, J = 7.13, m-PhHN), 7.20−7.24 (3H,
m, ArH), 7.81 (1H, d, J = 8.41, o-PhHN). 13C NMR (125.8 MHz;
CDCl3; 298 K): 5.10 (SiMe3), 5.19 (SiMe3), 21.20 (CH(CH3)2), 22.40
(CH(CH3)2), 24.25 (CH(CH3)2), 25.68 (ArCH(CH3)2), 26.54
(ArCH(CH3)2), 27.92 (ArCH(CH3)2), 28.92 (ArCH(CH3)2), 39.25
(ArCH(CH3)2), 41.01 (ArCH(CH3)2), 63.01 (NCH(R)CH2O), 73.55
(NCH(R)CH2O), 105.05, 112.12, 116.60, 122.26, 124.53, 127.46,
128.27 (CHarom), 132.42, 136.49, 138.48, 144.89, 155.88, 168.54
(Cquart).
Synthesis of [(L1c)ZnN(SiMe3)2] (2c). To a solution of ligand HL1c
(113 mg, 0.350 mmol) in toluene (5 mL) was added zinc
bis(trimethylsilyl)amide (135 mg, 0.351 mmol) at room temperature.
After being stirred for 8 h, 1H NMR confirmed the incomplete
conversion of the ligand into a zinc complex. An additional amount of
zinc bis(trimethylsilyl)amide (8 mg, 0.021 mmol) was added. After 4 h
of stirring, the yellow-colored solution was dried in vacuo, giving the
desired compound as a yellow powder (92%, 177 mg). 1H NMR
(500.1 MHz; CDCl3; 298 K): −0.36 (9H, s, CH3), 0.04 (9H, s, CH3),
1.01 (9H, s, C(CH3)3), 2.23 (3H, s, ArMe), 2.27 (3H, s, ArMe), 4.21
(1H, m, NCH(R)CH2O), 4.40 (1H, t, J = 9.70, NCH(R)CH2O), 4.56
(1H, m, NCH(R)CH2O), 6.20 (1H, d, J = 9.13, m-PhHN), 6.42 (1H,
m, p-PhH(CH3)2), 7.02−7.18 (3H, m, ArH), 7.20 (1H, m, ArH), 7.83
(1H, d, J = 8.28, o-PhHN). 13C NMR (125.8 MHz; CDCl3; 298 K):
5.19 (SiMe3), 5.28 (SiMe3), 18.46 (ArMe), 18.84 (ArMe), 25.27
(C(CH3)3), 34.95 (C(CH3)3), 68.41 (NCH(R)CH2O), 73.34 (NCH-
(R)CH2O), 103.37, 112.85, 115.82, 124.50, 128.99, 129.28, 131.81
(CHarom), 134.20, 134.62, 134.78, 147.09, 157.37, 170.54 (Cquart).
Synthesis of [(L1d)ZnN(SiMe3)2] (2d). A solution of ligand HL1d
(316 mg, 0.980 mmol) in toluene (10 mL) was added into zinc
bis(trimethylsilyl)amide (378 mg, 0.980 mmol) in toluene (5 mL) at
room temperature. After being stirred for 8 h, the pale yellow colored
solution was dried in vacuo, giving the desired compound as a yellow
powder (552 mg). The light yellow colored solid was recrystallized
from toluene at −20 °C to give yellow-colored crystals (93%, 502 mg).
1H NMR (500.1 MHz; CDCl3; 298 K): −0.01 (18H, s, CH3), 1.21
(6H, m, CH2CH(CH3)2), 1.74 (1H, m, CH2CH(CH3)2), 1.93 (1H, m,
CH2CH(CH3)2), 2.22 (3H, s, ArMe), 2.39 (3H, s, ArMe), 2.45 (1H,
m, CH2CH(CH3)2), 4.39 (1H, t, J = 8.23, NCH(R)CH2O), 4.74 (1H,
m, NCH(R)CH2O), 4.84 (1H, t, J = 8.90, NCH(R)CH2O), 6.42 (1H,
d, J = 8.90, m-PhHN), 6.63 (1H, m, p-PhH(CH3)2, 7.19 (1H, br, p-
PhHN), 7.33 (2H, m, m-PhH(CH3)2), 7.45 (1H, m, m-PhH(CH3)2),
8.03 (1H, d, J = 8.50, o-PhHN). 13C NMR (125.8 MHz; CDCl3; 298
K): 5.04 (SiMe3), 18.71 (CHCH2(CH3)2), 19.07 (CHCH2(CH3)2),
22.23 (CHCH2(CH3)2), 23.96 (ArMe), 25.83 (ArMe), 45.68
(CH2CH(CH3)2), 63.60 (NCH(R)CH2O), 71.80 (NCH(R)CH2O),
104.03, 113.10, 116.26, 124.39, 125.47, 128.56, 129.03 (CHarom),
134.04, 134.17, 134.47, 147.27, 157.05, 169.50 (Cquart). Anal. Calcd for
C27H43N3OSi2Zn: C, 59.26; H, 7.92; N, 7.68. Found: C, 59.04; H,
7.84; N, 7.44.
Synthesis of [(L1h)ZnN(SiMe3)2] (2h). To a solution of ligand HL1h
(137 mg, 0.362 mmol) in toluene (6 mL) was added zinc
bis(trimethylsilyl)amide (140 mg, 0.362 mmol) at room temperature,
and the mixture was stirred for 8 h. The pale yellow colored solution
1
was dried in vacuo, to give a yellow compound (89%, 195 mg). H
NMR (500.1 MHz; CDCl3; 298 K): −0.22 (9H, s, CH3), 0.03 (9H, s,
CH3), 0.92 (3H, d, J = 6.81, CH2CH(CH3)2), 1.07 (6H, d, J = 7.03,
ArCH(CH3)2), 1.17 (3H, d, J = 6.91, CH2CH(CH3)2), 1.29 (6H, d, J
= 6.75, ArCH(CH3)2), 1.57 (1H, m, CH2CH(CH3)2), 1.76 (1H, m,
CH2CH(CH3)2), 2.22 (1H, m, CH2CH(CH3)2), 3.07 (1H, m,
ArCH(CH3)2), 3.28 (1H, m, ArCH(CH3)2), 4.26 (1H, t, J = 7.75,
NCH(R)CH2O), 4.60 (1H, m, NCH(R)CH2O), 4.66 (1H, t, J = 8.49,
NCH(R)CH2O), 6.26 (1H, d, J = 9.25, m-PhHN), 6.44 (1H, t, J =
8.49, p-PhH(CH3)2), 7.05 (1H, t, J = 7.13, m-PhHN), 7.20−7.28 (3H,
m, ArH), 7.83 (1H, d, J = 8.41, o-PhHN). 13C NMR (125.8 MHz;
CDCl3; 298 K): 5.21 (SiMe3), 5.30 (SiMe3), 22.30 (CHCH2(CH3)2),
23.86 (CHCH2(CH3)2), 24.25 (CHCH2(CH3)2), 24.55
(CHCH2(CH3)2), 25.33 (ArCH(CH3)2), 25.54 (ArCH(CH3)2),
25.92 (ArCH(CH3)2), 25.92 (ArCH(CH3)2), 28.25 (ArCH(CH3)2),
45.01 (ArCH(CH3)2), 63.61 (NCH(R)CH2O), 71.55 (NCH(R)-
CH2O), 104.05, 113.12, 118.60, 124.26, 125.53, 128.46, 131.25
(CHarom), 133.61, 138.10, 144.61, 144.90, 158.59, 169.50 (Cquart).
Synthesis of [(L1i)ZnN(SiMe3)2] (2i). To a solution of ligand HL1i
(56 mg, 0.173 mmol) in toluene (10 mL) was added zinc
Synthesis of [(L1e)ZnN(SiMe3)2] (2e). To a solution of the ligand
HL1e (265 mg, 0.774 mmol) in toluene (10 mL) was added zinc
bis(trimethylsilyl)amide (301 mg, 0.780 mmol) at room temperature.
After being stirred for 8 h, the pale yellow colored solution was dried
in vacuo, giving the desired compound as a yellow powder (96%, 421
mg). 1H NMR (500.1 MHz; CDCl3; 298 K): −0.35 (18H, s,
N(SiMe3)2), 1.98 (3H, s, ArMe), 2.23 (3H, s, ArMe), 4.53 (1H, m,
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bis(trimethylsilyl)amide (67 mg, 0.173 mmol) at room temperature,
and the mixture was stirred for 8 h. The pale yellow colored solution
was dried in vacuo, to afford the desired compound as a yellow powder
species are only partially identified due to overlap): Major: −0.04 (9H,
s, CH3), 0.08 (9H, s, CH3), 0.62 (3H, d, J = 7.15, CH2CH(CH3)2),
0.97 (3H, d, J = 6.80, CH2CH(CH3)2), 1.27 (1H, m, CH2CH(CH3)2),
1.50 (1H, m, CH2CH(CH3)2), 1.73 (1H, m, CH2CH(CH3)2), 3.75
(1H, t, J = 9.10, NCH(R)CH2O), 4.50 (2H, t, J = 8.90,
NCH(R)CH2O), 6.84 (2H, m, ArH), 7.04 (2H, m, ArH), 7.16 (1H,
m, ArH), 7.52 (1H, m, ArH), 7.63 (2H, m, ArH), 7.91 (1H, d, J = 8.10,
o-PhHN). Minor: 0.52 (3H, d, J = 6.50, CH2CH(CH3)2), 1.04 (3H, d,
J = 6.80, CH2CH(CH3)2), 1.24 (1H, m, CH2CH(CH3)2), 1.44 (1H,
m, CH2CH(CH3)2), 1.93 (1H, m, CH2CH(CH3)2), 4.00 (1H, m,
NCH(R)CH2O), 4.14 (1H, m, NCH(R)CH2O), 4.22 (1H, m,
NCH(R)CH2O), 6.94 (2H, m, ArH), 7.36 (1H, m, ArH), 8.11 (1H,
d, J = 7.50, o-PhHN). 13C NMR (125.8 MHz; CDCl3; 298 K): Major:
5.85 (SiMe3), 21.12 (CHCH2(CH3)2), 22.36 (CHCH2(CH3)2), 24.01
(CHCH2(CH3)2), 25.02 (CHCH2(CH3)2), 67.61 (NCH(R)CH2O),
74.55 (NCH(R)CH2O), 104.05, 113.12, 119.60, 124.26, 125.53,
128.46, 133.61, 138.10, 145.49 (CHarom), 158.19, 165.09, 169.50,
176.18 (Cquart). Minor: 5.01 (SiMe3), 20.01 (CHCH2(CH3)2), 23.01
(CHCH2(CH3)2), 24.18 (CHCH2(CH3)2), 26.12 (CHCH2(CH3)2),
69.76 (NCH(R)CH2O), 77.85 (NCH(R)CH2O), 104.35, 113.32,
119.01, 124.89, 126.01, 129.06, 133.89, 139.08, 145.01(CHarom),
158.59, 165.89, 170.04, 176.30 (Cquart).
1
(68%, 65 mg). H NMR (500.1 MHz; CDCl3; 298 K): 0.21 (18H, s,
CH3), 1.12 (6H, m, CH2CH(CH3)2), 1.32 (1H, m, CH2CH(CH3)2),
1.56 (1H, m, CH2CH(CH3)2), 1.74 (1H, m, CH2CH(CH3)2), 2.22
(3H, d, J = 8.35, ArCHCH3), 4.18 (1H, m, ArCHCH3), 4.58 (2H, m,
NCH(R)CH2O), 5.05 (1H, m, NCH(R)CH2O), 6.50 (1H, t, J = 9.35,
m-PhHN), 6.83 (1H, d, J = 9.35, m-PhHN), 7.12 (2H, t, J = 8.13, o-
PhH), 7.20−7.28 (4H, m, ArH), 7.97 (1H, d, J = 6.45, o-PhHN). 13C
NMR (125.8 MHz; CDCl3; 298 K): 5.52 (SiMe3), 21.68
(CHCH2(CH3)2), 24.14 (CHCH2(CH3)2), 26.25 (ArCHCH3), 44.68
(CHCH2(CH3)2), 58.08 (ArCHCH3), 63.85 (NCH(R)CH2O), 71.20
(NCH(R)CH2O), 104.97, 112.56, 116.92, 126.34, 128.44, 128.62,
129.87, 131.85, 134.26 (CHarom), 147.21, 158.03, 169.51 (Cquart). Anal.
Calcd for C27H43N3OSi2Zn: C, 59.26; H, 7.92; N, 7.68. Found: C,
59.35; H, 7.68; N, 7.73.
Synthesis of [(L1j)ZnN(SiMe3)2] (2j). To a solution of ligand HL1j
(283 mg, 0.790 mmol) in toluene (10 mL) was added zinc
bis(trimethylsilyl)amide (305 mg, 0.790 mmol) at room temperature.
After being stirred for 8 h, the pale yellow colored solution was dried
in vacuo, giving the desired compound as a yellow powder (88%, 405
mg). 1H NMR (500.1 MHz; CDCl3; 298 K): 0.04 (18H, s, CH3), 0.89
(3H, d, J = 7.60, CH(CH3)2), 1.04 (3H, d, J = 8.01, CH(CH3)2), 1.98
(3H, d, J = 8.22, ArCHCH3), 2.67 (1H, m, CH(CH3)2), 4.42 (2H, m,
NCH(R)CH2O), 4.61 (1H, m, NCH(R)CH2O), 5.50 (1H, br,
C12H11), 6.40 (1H, t, J = 7.80, p-PhHN), 6.45 (1H, d, J = 9.35, m-
PhHN), 6.97 (1H, t, J = 7.80, m-PhHN), 7.20 (1H, d, J = 7.80, ArH),
7.33 (1H, t, J = 9.35, ArH), 7.55 (1H, m, ArH), 7.61 (1H, t, J = 9.35,
ArH), 7.71 (1H, d, J = 9.35, o-PhHN), 7.85 (1H, d, J = 9.35, ArH),
7.92 (1H, d, J = 9.35, ArH), 8.25 (1H, d, J = 9.35, ArH). 13C NMR
(125.8 MHz; CDCl3; 298 K): 5.50 (SiMe3), 14.35 (CH(CH3)2), 19.37
(CH(CH3)2), 28.31 (ArCHCH3), 30.44 (CH(CH3)2), 55.50 (ArCH-
(CH3)), 66.13 (NCH(R)CH2O), 69.27 (NCH(R)CH2O), 105.20,
112.70, 117.22, 121.78, 123.06, 125.52, 126.13, 127.0, 128.40, 129.30,
131.04 (CHarom), 134.43, 142.20, 147.95, 159.62, 160.73, 169.53
(Cquart). Anal. Calcd for C30H43N3OSi2Zn·0.2C7H8: C, 62.68; H, 7.47;
N, 6.98. Found: C, 62.59; H, 7.25; N, 6.32.
Synthesis of [(L13−) ZnN(SiMe3)2] (2m). To a solution of ligand
HL1m (364 mg, 1.129 mmol) in toluene (15 mL) was added zinc
bis(trimethylsilyl)amide (436 mg, 1.129 mmol) at room temperature.
After being stirred for 8 h, the pale red colored solution was dried in
vacuo, giving the desired compound as a yellow powder (86%, 531
mg). 1H NMR (500.1 MHz; CDCl3; 298 K): −0.28 (9H, s, CH3), 0.07
(9H, s, CH3), 0.92 (6H, br, CH2CH(CH3)2), 1.11 (2H, m,
CH2CH(CH3)2), 1.31 (1H, m, CH2CH(CH3)2), 1.54 (9H, s,
C(CH3)3), 2.81 (1H, t, J = 8.83, NCH(R)CH2O), 3.69 (1H, t, J =
6.91, NCH(R)CH2O), 4.06 (1H, t, J = 8.49, NCH(R)CH2O), 6.86
(1H, t, J = 7.63, p-PhHN), 7.04 (1H, d, J = 8.23, m-PhHN), 7.32 (1H,
t, J = 7.93, m-PhHN), 7.69 (1H, d, J = 7.93, o-PhHN). 13C NMR
(125.8 MHz; CDCl3; 298 K): 5.20 (SiMe3), 5.75 (SiMe3), 21.28
(CHCH2(CH3)2), 23.68 (CHCH2(CH3)2), 25.08 (CHCH2(CH3)2),
25.93 (C(CH3)3), 43.17 (CHCH2(CH3)2), 59.56 (C(CH3)3), 65.30
(NCH(R)CH2O),70.51 (NCH(R)CH2O), 115.72, 119.02, 123.49,
131.09 (CHarom), 145.95, 154.52, 165.72 (Cquart). Anal. Calcd for
C23H43N3O2SSi2Zn·0.3C7H8: C, 52.51; H, 7.99; N, 7.29. Found: C,
52.69; H, 7.66; N, 7.32.
Synthesis of [(L1k)ZnN(SiMe3)2] (2k). To a solution of ligand HL1k
(333 mg, 1.081 mmol) in toluene (6 mL) was added zinc
bis(trimethylsilyl)amide (418 mg, 1.082 mmol) at room temperature.
After being stirred for 8 h, the pale yellow colored solution was dried
in vacuo, and the residue was recrystallized from toluene at −30 °C to
Copolymerization of Cyclohexene Oxide/CO2. In a glovebox, a
60 mL Teflon-lined Parr high-pressure reactor vessel that was
previously dried in an oven was charged with a zinc catalyst (1 mol
%) and CHO (1 equiv). The vessel was sealed, taken out of the
glovebox, and brought to desired temperature and CO2 pressure. After
the mixture was stirred for the allotted time, it was cooled and a small
1
afford 2k (84%, 483 mg) as light pinkish crystals. H NMR (500.1
MHz; CDCl3; 298 K; peaks for the minor species are only partially
identified due to overlap): Major: 0.06 (9H, s, CH3), 0.09 (9H, s,
CH3), 0.64 (3H, d, J = 7.12,CH(CH3)2), 0.68 (3H, d, J =
7.12,CH(CH3)2), 1.60 (1H, m, CH(CH3)2), 4.09 (1H, m, NCH(R)-
CH2O), 4.28 (1H, t, J = 7.91, NCH(R)CH2O), 4.35 (1H, t, J = 7.09,
NCH(R)CH2O), 6.87 (1H, d, J = 9.210, m-PhHN), 7.10 (4H, m,
ArH), 7.20−7.25 (6H, m, ArH), 7.45 (2H, d, J = 9.41, COPh-H), 7.73
(1H, d, J = 9.41, o-PhHN). Minor: 0.59 (3H, d, J = 7.09, CH(CH3)2),
0.84 (3H, d, J = 7.06, CH(CH3)2), 1.86 (1H, m, CH(CH3)2), 6.96
(1H, t, J = 9.49, m-PhHN), 7.62 (2H, d, J = 8.81, COPh-H), 7.69 (1H,
d, J = 9.50, o-PhHN). 13C NMR (125.8 MHz; CDCl3; 298 K): Major:
5.95 (SiMe3), 14.86 ((CH)Me2), 18.93 ((CH)Me2), 30.24 ((CH)Me2),
67.56 (NCH(R)CH2O), 69.61 (NCH(R)CH2O), 121.32, 127.32,
128.28, 128.43, 129.22, 129.66, 130.66, 133.56, 138.05 (CHarom),
140.77, 151.41, 168.82, 176.34 (Cquart). Minor: 5.69 (SiMe3), 15.58
((CH)Me2), 19.26 ((CH)Me2), 31.13 ((CH)Me2), 68.03 (NCH(R)-
CH2O), 69.98 (NCH(R)CH2O), 120.31, 127.63, 127.90, 128.53, 128.
79, 129.06, 130.06, 132.98, 137.45 (CHarom), 139.58, 150.87, 168.40,
176.14 (Cquart). Anal. Calcd for C50H74N6O4Si4Zn2·0.4C7H8: C, 57.49;
H, 7.05; N, 7.62. Found: C, 57.88; H, 6.56; N, 7.65.
1
aliquot of reaction mixture was taken for H NMR spectroscopy to
determine the conversion. When no further conversion was noted, the
polymerization mixture was transferred into a round-bottom flask with
CH2Cl2 (3−5 mL), and the polymer was precipitated from addition of
methanol (18−30 mL). After separation, the polymer was dried in
vacuo to constant weight to determine the yield. Molecular weight
(Mn) and PDIs were determined by GPC using polystyrene standards.
Hydrolysis of Polymers and Chiral GC Analysis. In a typical
procedure, a small round-bottom flask was loaded with polycarbonate
(20 mg, 0.141 mmol) and NaOH (11 mg, 0.281 mmol) in MeOH (4
mL). The mixture was refluxed for 3 h and then neutralized with
HCl(aq) (1 M). The crude mixture was then extracted with ether.
After drying over anhydrous MgSO4, a small aliquot was injected into a
GC equipped with a Cyclodex-B column to determine the
enantiomeric excess of the 1,2-trans-cyclohexanediol (tR = 14.32 min
for (S,S)-1,2-trans-cyclohexanediol, tR = 14.75 min for (R,R)-1,2-trans-
cyclohexanediol).
Synthesis of [(L1l)ZnN(SiMe3)2] (2l). To a solution of ligand HL1l
(68 mg, 0.211 mmol) in toluene (6 mL) was added zinc
bis(trimethylsilyl)amide (84 mg, 0.211 mmol) at room temperature.
After being stirred for 8 h, the pale yellow colored solution was dried
in vacuo, giving the desired compound as a yellow powder (98%, 113
X-ray Crystallography. All data for compounds 2a, 2d, and 2k
were collected on a Bruker APEX-II CCD diffractometer. The
intensity data were corrected for absorption and decay (SADABS).42
The data were integrated with SAINT,43 and the structure was solved
and refined using SHELXTL.44 The factors for the determination of
the absolute structure were refined according to established
1
mg). H NMR (500.1 MHz; CDCl3; 298 K; peaks for the minor
7401
dx.doi.org/10.1021/om3006992 | Organometallics 2012, 31, 7394−7403

Organometallics
Article
procedures.45 Residue electrons were noted in the lattice of 2k, which
were found with the SQUEEZE routine from the PLATON package46
to be the toluene solvate. X-ray crystal data, data collection parameters,
and refinement parameters are summarized in Table 1, and further
crystallographic details can be found in the Supporting Information.
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B. Y. Inorg. Chem. 2009, 48, 10455−10465. (d) Darensbourg, D. J.;
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C. K. Chem. Sci. 2012, 3, 1245−1255. (b) Nakano, K.; Kobayashi, K.;
Nozaki, K. J. Am. Chem. Soc. 2011, 133, 10720−10723. (c) Buchard,
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
Figures of representative H NMR spectra of zinc complexes,
GPC of polycarbonates, and a table showing observed and
calculated tetrad distributions of PCHC in pdf format and X-
ray crystal data files for 2a, 2d, and 2k in cif format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*Tel: +1-701-777-2241. Fax: +1-701-777-2331. E-mail: gdu@
chem.und.edu.
Notes
The authors declare no competing financial interest.
(10) (a) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem.
Soc. 1998, 120, 11018−11019. (b) Cheng, M.; Moore, D. R.; Reczek, J.
J.; Chamberlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem.
Soc. 2001, 123, 8738−8749. (c) Allen, S. D.; Moore, D. R.; Lobkovsky,
E. B.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 14284−14285.
(d) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew.
Chem., Int. Ed. 2002, 41, 2599−2602. (e) Moore, D. R.; Cheng, M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2003, 125, 11911−
11924. (f) Byrne, C. M.; Allen, S. D.; Lobkovsky, E. B.; Coates, G. W.
J. Am. Chem. Soc. 2004, 126, 11404−11405.
ACKNOWLEDGMENTS
■
Financial support from ND EPSCoR through NSF grant EPS-
0814442 and the Office of the Vice President for Research at
UND through an FRSM award is greatly appreciated. We thank
H. Wang and Dr. E. Kolodka for GPC measurements, Dr. A.
Ugrinov in the Chemistry and Biochemistry department at
North Dakota State University (NSF-CRIF CHE-0946990),
and Dr. V. G. Young, Jr. in the X-ray Crystallographic
Laboratory at University of Minnesota, Twin Cities, for X-ray
structure determinations.
(11) Brule, E.; Guo, J.; Coates, G. W.; Thomas, C. M. Macromol.
Rapid Commun. 2011, 32, 169−185.
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18610−18613. (b) Wu, G.-P.; Wei, S.-H.; Ren, W.-M.; Lu, X.-B.;
Darensbourg, D. J. J. Am. Chem. Soc. 2011, 133, 15191−15199.
(c) Wu., G.-P.; Wei, S.-H.; Lu, X.-B.; Ren, W.-M.; Darensbourg, D. J.
Macromolecules 2010, 43, 9202.
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