Alternating copolymerization of cyclohexene oxide and CO2 catalyzed by zinc complexes with new 3-amino-2-cyanoimidoacrylate ligands

Article abstract of DOI:10.1002/adsc.200505113

New 3-amino-2-cyanoimidoacrylate ligands with varying steric demands have been synthesized. Zinc acetate complexes of these ligands catalyze the copolymerisation of CO₂ and cyclohexene oxide, showing high activities (TOF up to over 200 h ^-1).

Full text of DOI:10.1002/adsc.200505113

COMMUNICATIONS
Alternating Copolymerization of Cyclohexene Oxide
and CO2 Catalyzed by Zinc Complexes with New
3-Amino-2-cyanoimidoacrylate Ligands
Mario Krçger, Cristina Folli, Olaf Walter, Manfred Dçring*
Forschungszentrum Karlsruhe, Institute for Technical Chemistry (ITC-CPV), Hermann-von-Helmholtz Platz 1,
76344 Eggenstein-Leopoldshafen, Germany
Fax: (þ49)-7247-822-244, e-mail: manfred.doering@itc-cpv.fzk.de
Received: March 15, 2005; Accepted: May 6, 2005
Abstract: New 3-amino-2-cyanoimidoacrylate li-
gands with varying steric demands have been synthe-
sized. Zinc acetate complexes of these ligands cata-
lyze the copolymerisation of CO2 and cyclohexene
oxide, showing high activities (TOF up to over
200 hÀ1).
a new synthetic route towards chelating imidoester li-
gands with functional groups on the ligand backbone.
Zinc acetate complexes thereof were made and success-
fully tested as catalysts for the copolymerization of cy-
clohexene oxide and CO2.
The 3-amino-2-cyanoimidoacrylate ligands were built
up gradually in four steps (Scheme 2). First, cyanoacetic
acid was condensed to the corresponding acetamide 1,
preferably with ortho-substituted anilines, via chlorina-
tion of the acid.[17] Treatment of the acetamide 1 with
an oxonium salt lead to the related imidoester 2.[18]
The ligand was completed by a Claisen orthoester con-
densation at the nucleophilic carbon of 2 to give 2-cya-
no-3-ethoxyimidoacrylate,[19] and subsequent substitu-
Keywords: carbon dioxide fixation; copolymeriza-
tion; green chemistry; homogeneous catalysis; ligand
design
The use of CO2 as a C1 building block is one of the most tion of the ethoxy group by an ortho-substituted aniline
important goals in green and synthetic chemistry be- of choice yielded the desired 3-amino-2-cyanoimido-
cause it is abundance and its non-toxic, non-flammable acrylates 3.[20,21] The two final steps can be carried out
and inexpensive properties.[1–4] The formation of ali- in a one-pot procedure without purification of the inter-
phatic polycarbonates by catalytic copolymerization of mediate 2-cyano-3-ethoxyimidoacrylate. By this route
CO2 with epoxides (Scheme 1) is a very promising way we have prepared five ligands with varying steric de-
to reach this goal.[5–7] The reaction was first discovered mands (3a–e). Generally, the procedure should allow
by Inoue and co-workers[8] and is of major interest due for a wide range of substituents on the ligand. Thus we
to the promising material properties and the biodegrad- are able to get a broad picture of structure-activity rela-
ability of the resulting polycarbonates. Over the past tionships with this type of ligands.
decade the field has gained new momentum by the dis-
covery of several homogeneous, highly active catalysts
like zinc phenoxides,[9] chromium porphyrins,[10] and
chromium salen complexes.[11] The growing interest in
copolymerization gave rise to a number of recent re-
views.[12–14] Especially zinc complexes with b-diimine li-
gands have set new benchmarks in the homogenous cat-
alysis of polycarbonate formation.[15] Coates et al. have
shown that electron-withdrawing groups on the diimine
backbone enhance catalytic activities.[16] We developed
Scheme 1. Copolymerisation of cyclohexene oxide and car-
bon dioxide.
Scheme 2. Ligand synthesis.
Adv. Synth. Catal. 2005, 347, 1325–1328
DOI: 10.1002/adsc.200505113
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Mario Krçger et al.
COMMUNICATIONS
Scheme 3. Complex synthesis.
Zinc acetate complexes of the ligands were made via
the reaction with diethylzinc to produce an ethylzinc
complex as intermediate. Successive treatment with ace-
tic acid yielded the desired complexes (Scheme 2).[15,22]
Acetate serves as an initiating group which mimics car-
bon dioxide and mechanistic studies have revealed
that acetate is attacked by cyclohexene oxide in the ini-
tial step of the copolymerisation.[23] Complexes with alk-
oxides[15] and sulfinates[24] as initiating groups exhibit
Figure 1. Structure of 6a. Selected bond distances (Š) and an-
gles (deg): Zn(1) N(1) 1.989(7), Zn(1) N(2) 1.999(7),
À
À
À
À
Zn(1) O(2)
1.937(8),
Zn(1) O(6)
1.918(7),
À
À
À
À
N(1) Zn(1) N(2) 95.5(3), O(2) Zn(1) O(6) 120.1(3),
À
À
O(2) Zn(1) N(2) 113.1(4).
similar activities, but for reasons ofconsistencyand com- and isopropyl groups (4d, e) on the anilines seem to be
parability, acetate was chosen as initiating group slightly more active than the ones combining ethyl and
throughout.
isopropyl groups (4b, c). On the other hand no signifi-
In the solid state, the zinc acetate complexes exist in cant influence of the position of the larger and smaller
the dimeric form, which is confirmed by the X-ray struc- substituents can be found. Ligands having the methyl
tures of 4a, c and e. Figure 1 shows complex 4a as an ex- or ethylgroup on the sideof the imido ester (4c, e) exhib-
ample. The two tetrahedral zinc centers are bridged by it nearly the same activity than the ones having the iso-
two acetates, the zinc-zinc distance being 4.161 Š. The propyl substituent in this positions (4b, d).
bulkier sides of the ligand carrying the imidoester crys-
tallize anti to each other, presumably to minimize steric obtain even more active catalysts by systematic changes
repulsions. in the ligand architecture, exploiting the broad structur-
Starting from these promising first results we hope to
All (3-amino-2-cyanoimidoacrylate) zinc acetates (4) al variety that can be synthesized based on the presented
catalyzed the copolymerization of cyclohexene oxide new synthetic route. In addition we will try to improve
and CO2. Copolymerizations were carried out in neat the insight into the character of the copolymerization
monomer mixtures with catalyst loadings of 0.1% at by mechanistic studies.
temperatures from 90–1008C and a pressure of
Overall we have developed a flexible synthetic route
4 MPa. Complexes having an ethyl or methyl group as towards a new class of ligands, 3-amino-2-cyanoacryli-
ortho-substituent on one benzene ring (4b–e) were midic acid esters, and demonstrated their ability to
more active and selective than doubly isopropyl substi- form zinc acetate complexes which are highly active in
tuted 4a (Table 1). The resulting poly(cyclohexene car- the copolymerization of cyclohexene oxide and CO2.
bonate) had molecular weights above 15,000 g/mol and
incorporated over 72% carbonate linkages. Homopoly-
merization of cyclohexene oxide occurred as a compet-
Experimental Section
ing side reaction, but at 908C only about 10% polyether
bonds were formed (Table 1). The copolymers were
atactic, as 13C NMR analysis revealed a random distribu-
tion of isotactic and syndiotactic carbonate. Narrow pol-
ydispersities (<1.26) denote the living character of the
copolymerization.
General Remarks
All reactions with air-or water-sensitive compounds were car-
ried out under dry argon using standard Schlenk line tech-
niques. NMR spectra were recorded on a Bruker spectrometer
In a recent review, catalytic activities>200 TOFin the
epoxide/CO2 copolymerization have been defined as
high (moderate: 5–200 TOF, low:<5 TOF).[14] Thus, ac-
tivities of our 3-amino-2-cyanoacrylimidic acid ester
zinc acetate systems (4e) are high, surpassing the 200
1
(250 MHz for H, 62.9 MHz for 13C) at 293 K. Mass spectra
were obtained using electron ionization (EI), electron spray
ionization (ESI) or field ionization (FI). FI and EI spectra
were recorded with Micromass GCT and Finnigan MAT
GCQ spectrometers, ESI spectra were recorded with a Hew-
TO hÀ1 mark. Ligands with a combination of methyl lett-Packard 1100 MSD spectrometer. Melting points were ei-
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Adv. Synth. Catal. 2005, 347, 1325–1328

Alternating Copolymerization of Cyclohexene Oxide and CO2
COMMUNICATIONS
Table 1. Performance of catalysts in the cyclohexene oxide/CO2 copolymerization.
Catalyst[a]
t [h]
T [ 8C]
p (CO2) [MPa]
TON[b]
TOF[b]
gpoly gM
Carbonate
Mn
Mw/Mn
À1
[d]
[hÀ1
]
[hÀ1
]
linkages [%][c]
[kg molÀ1
]
4a
4b
4c
4d
4e
4e
2
2
2
2
2
1
100
100
90
90
100
90
4
4
4
4
4
4
170
327
315
367
396
210
135
164
158
183
198
210
293
356
343
399
430
456
58
72
86
88
75
88
12.6
20.0
17.5
24.5
15.8
10.0
1.72
1.26
1.16
1.15
1.21
1.16
[a]
A [CHO]:[Zn] ratio of 1000: 1 was used throughout.
[b]
[c]
[d]
Mole of CHO consumed per mole of zinc (per hour for TOF).
1
Determined by H NMR.
Determined by GPC in THF.
ther determined by using capillaries in a melting point appara-
tus (Büchi), or by differential scanning calorimetry (DSC) with
a Mettler Toledo DSC822e. IR spectra were recorded with a
Perkin Elmer System 2000 FT-IR.
collected, dried over Na2SO4, and the solvent was removed un-
der reduced pressure. Distillation of the residue (13.7 g) gave
the pure product; yield: 7.52 g (28 mmol, 46%); bp 1038C. 1H
NMR (DMSO-d6): d¼7.18–6.97 (m, 3H, Haryl), 4.35 (q, 2H,
OCH2), 3.40 (s, 2H, CH2), 2.72 (m7, 2H, CH), 1.34 (t, 3H,
CH3), 1.14 (d, 6H, CH3), 1.06 (d, 6H, CH3); 13C NMR
(DMSO-d6): d¼151.84, 142.00, 137.54, 124.24, 123.40, 115.21,
62.87, 27.98, 23.52, 22.78, 19.57, 14.30; EIþ-MS: m/z¼273
(Mþ).
Materials
2,6-Diisopropylaniline was distilled under vacuum and cyclo-
hexene oxide was distilled from calcium hydride under vac-
uum. All other reagents were purchased commercially and
used as received.
Ethyl 2-cyano-3-[(2,6-diisopropylphenyl)amino]-N-(2,6-
diisopropylphenyl)prop-2-enimidoate (3a): A mixture of eth-
yl 2-cyano-N-(2,6-diisopropylphenyl)ethanimidoate (2a;
1.18 g, 4.2 mmol), triethyl orthoformate (0.61 g, 4.2 mmol),
and acetic anhydride (5.47 g, 8.4 mmol) was refluxed for 5 h.
The solvent was removed under reduced pressure. The result-
ing thick oil was dissolved in methanol (5 mL), 2,6-diisopropyl-
aniline (0.93 g, 4.7 mmol) was added and the solution was re-
fluxed for 30 min. The reaction mixture was then stored over-
night at 58C and the product crystallized. Recrystallization
from methanol afforded colorless crystals of the product; yield:
0.77 g (1.68 mmol, 42%); mp 1578C. 1H NMR (CDCl3): d¼7.53
General Synthetic Procedures
2-Cyano-N-(2,6-diisopropylphenyl)acetamide (1a): Cyano-
acetic acid (12.76 g, 0.15 mol) was added to a stirred suspension
of phosphorus pentachloride (31.27 g, 0.15 mol) in dichloro-
methane (450 mL) at ambient temperature. The reaction mix-
ture was heated at reflux for 30 min. After cooling, 2,6-diiso-
propylaniline (17.73 g, 0.10 mol) was added in 10 min and the
reaction mixture was heated for 2 h under reflux. It was then
cooled in an ice/water bath, and water (200 mL) was added. Af-
ter stirring for 30 min, the reaction mixture was neutralized by
addition of sodium carbonate solution. The water phase was
extracted twice with dichloromethane; the organic phases
were then collected and dried over Na2SO4. The solvent was re-
moved under reduced pressure and the obtained white solid
was recrystallized from toluene and dried in vacuum; yield:
¼
(d, 1H, C CH), 7.28–7.02 (m, 6H, Haryl), 4.37 (d, 1H, NH), 3.72
(q, 2H, CH2), 3.04 (m7, 2H, CH), 2.89 (m7, 2H, CH), 1.18 (t, 3H,
CH3), 1.13 (d, 6H, CH3), 1.10 (d, 6H, CH3); 13C NMR (CDCl3):
d¼156.8, 145.4, 145.0, 139.0, 135.9, 128.7, 124.4, 124.2, 123.3,
29.1, 28.8, 28.7, 24.3, 24.0, 23.9, 23.3, 22.9, 15.6; IR (KBr): n¼
À1
À
¼
¼
¼
3185 (N H), 2198 (C N), 1647 (C N), 1587 cm (C C);
EIþ-HR-MS: m/z¼459.3255, calcd. for C30H41N3O: 459.3250;
d¼0.5 mDa.
[(3a)Zn(OAc)]2 (4a): Zinc acetate complexes of the ligand
were prepared via the corresponding ethylzinc complex by lit-
erature procedures. X-ray quality crystals were obtained by
covering a methylene chloride solution with pentane. 1H
1
23.3 g (0.095 mol, 95%); mp 160–1628C. H NMR (CDCl3):
d¼7.73 (s, 1H, NH), 7.28–7.07 (m, 3H, Haryl), 3.33 (s, CH2),
2.87 (m7, 2H, CH), 1.07 (d, 12H, CH3). EIþ-MS: m/z¼244
(Mþ).
¼
NMR (CDCl3): d¼7.46 (d, 1H, C CH), 7.30–6.90 (m, 6H,
Ethyl 2-cyano-N-(2,6-diisopropylphenyl)ethanimidoate
(2a): A mixture of 2-cyano-N-(2,6-diisopropylphenyl)aceta-
mide (1a; 14.70 g, 60 mmol) and a 1.0 M solution of triethy-
loxonium tetrafluoroborate in dichloromethane (66 mL,
66 mmol) was stirred for 5 days at ambient temperature. The
solvent was removed under vacuum and the residue was wash-
ed twice with absolute diethyl ether. Then the residue was tak-
en up in 60 mL of absolute diethyl ether and cooled to 08C.
Triethylamine (9.2 mL, 66 mmol) was slowly added, and the
mixture was stirred at room temperature for 2 h. The organic
phase was separated and the residue was washed three times
with absolute diethyl ether. The diethyl ether solutions were
Haryl), 4.09 (q, 2H, CH2), 2.98 (m7, 4H, CH), 1.86 (bs, 3H,
OC(O)CH3), 1.36 (t, 3H, CH3), 1.03 (m, 12H, CH3). MS
(ESI): m/z¼582 (Mþ þ1).
General Procedure for Copolymerisation Reactions
An autoclave (Parr) was heated to 958C under vacuum for 16 h
and cooled under vacuum. [(4a)Zn(OAc)] (6a; 48 mg,
0,082 mmol) and cyclohexene oxide (7.87 g, 82 mmol) were
brought into the autoclave, which was then heated to reaction
Adv. Synth. Catal. 2005, 347, 1325–1328
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Mario Krçger et al.
COMMUNICATIONS
temperature and pressurized with CO2 to the appropriate pres-
sure (40 bar). After 2 h the reactor was cooled, vented and a
small sample taken for analysis. The resulting polycarbonate
was dissolved in CH2Cl2 (5 mL), precipitated from MeOH
(20 mL), collected and dried under vacuum to constant weight.
[9] D. J. Darensbourg, M. W. Holtcamp, G. E. Struck, M. S.
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[10] S. Mang, A. I. Cooper, M. E. Colclough, N. Chauhan,
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[11] D. J. Darensbourg, J. C. Yarbrough, J. Am. Chem. Soc.
2002, 124, 6335.
Crystallographic Data on [(3a)Zn(OAc)]2 (4a)
[12] D. J. Darensbourg, R. M. Mackiewicz, D. R. Billodeaux,
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[13] H. Sugimoto, S. Inoue, J. Poly. Sci. A: Chem. 2004, 42,
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Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-266169. Copies of the data can be
obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [Fax: int. code
þ44(1223)336–033; E-mail: deposit@ccdc.cam.ac.uk].
[16] S. D. Allen, D. R. Moore, E. B. Lobkovsky, G. W. Coates,
J. Am. Chem. Soc. 2002, 124, 14284.
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