New derivatives of quinoxaline - Syntheses, complex formation and their application as controlling ligands for zinc catalyzed epoxide-CO2-copolymerization

Article abstract of DOI:10.1515/znb-2002-0816

A series of amino-(type 3) as well as hydrazino-substituted quinoxalines (type 8) have been synthesized in order to study their ability to complex with iron(III) and zinc(II) ions. Cyclization of 2,3-dichloroquinoxaline (1) with a bis-amidine 9 leads to ring-fused quinoxalines of type 10. One of these compounds (10a) forms a unique macrocyclic hexameric complex 14 with zinc ions in the presence of 2,6-diisopropyl phenolate. In an analogous manner, the monomeric complexes 12 and 13 could be synthesized. All of these new zinc complexes catalyze the copolymerization of cyclohexene oxide and carbon dioxide with a high degree of selectivity in the resulting polymers.

Full text of DOI:10.1515/znb-2002-0816

New Derivatives of Quinoxaline Ð Syntheses, Complex Formation and
their Application as Controlling Ligands for Zinc Catalyzed
Epoxide-CO2ÐCopolymerization
O. Hampela, C. Rodeb, D. Waltherb, R. Beckerta and H. Görlsb
a
Institute of Organic and Macromolecular Chemistry, Friedrich Schiller University, D-07743
Jena, Humboldtstr. 10, Germany
b
Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University, D-07743
Jena, August-Bebelstr. 2, Germany
Reprint requests to Prof. R. Beckert. Fax: +493641948212, E-mail: c6bera@uni-jena.de
Z. Naturforsch, 57b, 946Ð956 (2002); recieved March 26, 2002
Copolymerization Epoxide-CO2, Quinoxalines, Zinc Complexes
A series of amino-(type 3) as well as hydrazino-substituted quinoxalines (type 8) have
been synthesized in order to study their ability to complex with iron(III) and zinc(II) ions.
Cyclization of 2,3-dichloroquinoxaline (1) with a bis-amidine 9 leads to ring-fused quinoxa-
lines of type 10. One of these compounds (10a) forms a unique macrocyclic hexameric com-
plex 14 with zinc ions in the presence of 2,6-diisopropyl phenolate. In an analogous manner,
the monomeric complexes 12 and 13 could be synthesized. All of these new zinc complexes
catalyze the copolymerization of cyclohexene oxide and carbon dioxide with a high degree
of selectivity in the resulting polymers.
1. Introduction
expect that these ligands will exhibit various coor-
dination modes in metal complexes and it is even
possible that they can function as controlling
ligands in catalytic reaction.
Ring-fused heterocycles which contain more
than one nitrogen atom are key structures in a large
variety of biochemical processes. For example,
purines, pteridines and flavines as well as their
metal complexes play an important role in many
enzymatic reactions. Molybdopterineenzymes are
2. Synthesis of Quinoxalines
There are basically two synthetic routes for
able to catalyze essential redox reactions, e.g. alde- obtaining 2,3-diamino-substituted quinoxalines:
hyde oxygenation [1], and the biotinesulfoxide substitution of the halogen atoms in 2,3-dichloro-
reduction [2], or represent co-factors of the active quinoxaline (1) and cyclization of o-phenylenedia-
centre of the nitrate reductase [3]. During the last mine with bis-imidoylchorides of type 2. Using
few years, quinoxalines have been of special in- these reactions, we have recently succeeded in syn-
terest due to their biological activity. This has led thesizing a series of 2,3-diaminoquinoxalines 3 and
to the development of a new class of structural have tested them with respect to their biological
elements for mycobacteriostatic drugs [4,5] based activity [4]. In this article we have performed an
on 2,3-diaminoquinoxalines. We describe here the aminolysis of 1 with amines of type 4 in order to
synthesis of new bi- and tricyclic quinoxalines obtain derivatized catechols after the deprotection
bearing two additional amines or imines in the of the OH groups. Heating the educts 4a,b with 1
positions 2 and 3 of the ring and discuss their use in the absence of solvent resulted in the quinoxa-
as new chelating ligands for Zn(II) and Fe(III). lines 3a and 3b which could be isolated and spec-
Furthermore, we report a successful copolymeriza- troscopically characterized. We also tested a path-
tion of carbon dioxide with cyclohexene oxide way starting from dopamine and compound 1.
which is catalyzed by several of our new zinc com- However, dopamine unexpectedly attacked both
plexes.
OH groups instead of the primary amino group
These quinoxaline-type ligands can act as either which resulted in the tetracycle 5.
neutral or anionic chelators and, in addition, could
It is essential to cleave the methyl ether groups
possibly act as bridging ligands. This leads one to in order to obtain ligands capable of complexing
0932Ð0776/2002/0800Ð0946 $06.00 ” 2002 Verlag der Zeitschrift für Naturforschung, Tübingen · www.znaturforsch.com
D
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O. Hampel et al. · Quinoxaline
947
Scheme 1. Synthesis of 2,3-Diaminoquin-
oxalines.
with iron(III). However, well-known cleaving
reagents (pyridinium chloride, trimethylsilyl
iodide, BX3, X= F, Br, I) failed to deprotect the
OMe functionalities in 3a and 3b. Even the highly
efficient Lewis acid/Lewis base combination of
boron tribromide and dimethylsulfide [6] did not
work. Neither did sodium ethanethiolate react, a
well-known mild reagent [7]. We were thus forced
to develop another strategy for the synthesis of
catechol derivatives.
The 2,3-dihydrazinoquinoxaline 6 has recently
been obtained by hydrazinolysis of 1 [8] and is a
neutral ambident N-donor ligand for different
transition metals [9]. Condensation of 6 with alde-
hydes (7aÐe) yields compounds 8aÐe.
Other nucleophiles capable of displacing both
chlorine atoms in 1 are the recently published
oxalamidines 9 [10]. After deprotonating them
with strong bases, these amidines react with dichlo-
roquinoxaline (1) to yield new, tricyclic com-
pounds 10 (Scheme 2). The 1H and 13C NMR data
indicated the absence of a structural symmetry
from the product. An X-ray crystal structural
analysis provided unambiguous proof of the con-
stitution (Fig. 1). In the solid state, 10a is stabilized
by the formation of a dimer via hydrogen bridges.
Fig. 1. Dimeric unit of the ligand 10a.
˚
Selected bond distances (A): N1ÐC1 1.346(3), N2ÐC2
1.277(3), C1ÐC2 1.498(3), C1ÐN3 1.305(3), C2ÐN4
1.391(2), C3ÐN3 1.383(3), C3ÐN5 1.317(3), C3ÐC4
1.440(3), C4ÐN4 1.389(2), C4ÐN6 1.312(3), C5ÐN5
1.377(3), C10ÐN6 1.374(3), C5ÐC10 1.418(3), N1ÐN2A
3.097(3).
and ligands. It is clear that octahedral complexes
with an unknown arrangement of catechol ligands
and other, unknown ligands (water and/or chlo-
ride) are formed. We did not succeed in obtaining
crystals of these complexes, regardless of the con-
ditions selected. Further investigations on these
complexes as siderophore analogues will be re-
3. Complex Formation
Ligands 8bÐd form deep blue-green solutions in ported on in future.
the presence of Fe(III) ions. UV/IR spectroscopy
Selected quinoxalines were then reacted with a
and MS data indicate a 1:1 stoichiometry of iron mixture of ZnEt2 and HOR (R = 2.6-diisopropyl-
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948
O. Hampel et al. · Quinoxaline
Scheme 2. Synthesis of tricyclic
quinoxalines.
phenyl) to obtain new phenolate-zinc complexes in solution could be obtained. It is therefore un-
containing one or two phenolate ligands. It was clear which of the two possible tautomers 12a or
hoped that these complexes are capable of catalyz- 12b (Scheme 2) was formed. In addition, the 13C
ing the copolymerization of CO2 with oxiranes and NMR spectrum gives no further information about
that the catalytic reactivity can be controlled by the constitution of the complex.
one quinoxaline-type ligand. A literature search
showed that Zn complexes containing quinoxa-
lines as a backbone of a 1.2-diamine or -diimine
system are extremely rare. To the best of our
knowledge, only two bis-glyoxamato complexes
with quinoxaline substituents are known [12]. So
far, neither alkoxy- nor phenoxy-zinc complexes
containing these types of ligands have been re-
Scheme 3. Possible tautomers 12a and 12b.
ported.
Compound 11 reacted smoothly with a 1:2 mix-
ture of ZnEt2/HOR (R = 2.6-diisopropylphenyl)
X-ray diffraction analysis of single crystals of 12
to form a crystalline zinc complex 12 (Scheme 3). clearly shows that the tautomer 12a crystallizes
1
An elemental analysis and the H NMR spectrum with the 1.4-diazadiene form of the ligand. Fig. 2
indicates that complex 12 contains two phenolate displays the molecular structure and lists the most
ligands and one molecule of the quinoxaline which important bond lengths and angles. The complex
acts as a neutral ligand. For example, the signals is monomeric and the 1,4-diazadiene unit forms a
of the aliphatic protons show the expected ratio of five-membered chelate ring with the zinc atom.
aromatic protons to the two phenolato ligands (16 The nitrogen atoms of the quinoxaline ring are not
1
: 44). However, due to the complexity of the H coordinated. The Zn(II) center is tetrahedral and
NMR spectrum, no detailed structural information surrounded by the two imine nitrogen atoms and
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O. Hampel et al. · Quinoxaline
949
lectively with diethyl zinc (1.10 mmol in 10 ml of
toluene) and 2,6-diisopropylphenol (1 mmol in
10 ml of THF) to form orange crystals (complex
14). The 1H NMR spectrum of 14 showed that only
one phenolate group coordinates to the Zn atom,
thus indicating that the quinoxaline acts as a mo-
noanionic ligand. This was confirmed by X-ray
crystallography (Fig. 3). Rather surprisingly, the
complex exhibits a 36-membered macrocyclic ring
system composed of six monomeric units. All zinc
ions are tetrahedrally coordinated with the quin-
oxaline ligand bridging two zinc cations. Both the
imine nitrogen N2 und the amide nitrogen N1 in
the quinoxaline binds to one zinc center to form a
five-membered chelate ring. In addition, the
amine nitrogen N5 acts as a bridging donor to con-
nect two zinc centers. This results in the formation
of an unusual macrocyclic system in which six zinc
ions in a chair conformation are linked by six quin-
oxaline ligands (Fig. 4). The orientation of the li-
gands relative to the central zinc atom alternates.
The planar ligand is tilted by 77.7∞ and Ð77.7∞,
respectively.
Fig. 2. Molecular structure of complex 12.
˚
Selected bond distances (A) and bond angles (deg): ZnÐ
O1 1.880(3), ZnÐO2 1.891(3), ZnÐN1 2.063(3), ZnÐN2
2.066(3), N1ÐC1 1.304(5), N2ÐC2 1.284(5), C1ÐC2
1.509(6), C1ÐN3 1.331(5), C2ÐN4 1.355(6), C3ÐN3
1.463(6), C8ÐN4 1.447(6), C3ÐC8 1.510(6), O1ÐZnÐO2
118.1(1), O1ÐZnÐN1 103.2(1), O1ÐZnÐN2 122.3(1),
O2ÐZnÐN1 122.8(1), O2ÐZnÐN2 105.8(1), N1ÐZnÐ
N2 79.7(1).
This remarkably fast and selective self-assembly
of the macrocyclic complex 14 is somewhat unex-
pected since there are a great number of other
possibilities for stabilizing the monomeric unit of
the two oxygens of the phenolate groups. Bond
lengths and bond angles lie in the expected range
and are therefore not further discussed.
When ZnEt2/ROH was employed in a molar ra-
tio of 1:1 (instead of 1:2), complex 13 (Scheme 4)
could be isolated. According to its 1H NMR
spectrum, only one phenolate ligand per Zn ion is
coordinated and the quinoxaline is a monoanionic
ligand. In addition, one THF molecule is present
in the first coordination sphere of the metal.
Scheme 4. Structural formula of complex 13.
The tricyclic derivatives 10 contain an additional
quinoxaline ring which, in principle, could also co-
ordinate with the zinc ion. Indeed, 10a reacts se-
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950
O. Hampel et al. · Quinoxaline
Fig. 4: a) Molecular structure of the monomeric unit of
14 in the solid state. b) molecular structure of the six-
membered ring in 14.
˚
Selected bond distances (A) and bond angles (deg): ZnÐ
Fig. 3. a) Structural formula of the six membered-ring of
O1 1.921(4), ZnÐN1 2.024(5), ZnÐN2 2.012(5), ZnÐ
N5A 2.011(5), N1ÐC1 1.320(7), N2ÐC2 1.295(7), C1Ð
C2 1.504(7), C3ÐC4 1.449(8), C3ÐN3 1.335(7), C3ÐN5
1.340(7), C4ÐN4 1.391(7), C4ÐN6 1.306(7), C5ÐC10
1.421(8), C5ÐN5 1.378(7), C10ÐN6 1.370(8), O1ÐZnÐ
N1 112.2(2), O1ÐZnÐN2 106.7(2), O1ÐZnÐN5A
108.4(2), N1ÐZnÐN2 81.2(2), N1ÐZnÐN5A 118.2(2),
N2ÐZnÐN5A 127.7(2).
14; b) molecular structure of 14 in the solid state.
14 (formation of monomers stabilized by THF like
in complex 13 or formation of dimers or even
polymers). To the best of our knowledge, com-
pound 14 is the first macrocycle reported to con-
tain six zinc centers.
catalysts, the quinoxalines function as controlling
ligands.
4. Catalytic Copolymerization of CO2 with Cyclo-
hexene Oxide
Complexes 12, 13, and 14 are capable to act as
catalysts for the copolymerization of CO2 with
cyclohexene oxide according to Scheme 5. In these
Scheme 5. Copolymerization of CO2 with cyclohexene
oxide.
In order to elucidate the influence of the quin-
oxalines, we synthesized two binuclear complexes
15 and 16 (Scheme 6) which contain two zinc
atoms on opposite sites of the bridging oxalamidi-
nate ligands. Five-membered chelate rings similar
to the ring systems in 13 and 14 are present in both
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O. Hampel et al. · Quinoxaline
951
complexes; however, 15 and 16 do not contain and selectivity (run 7). In contrast to the mono-
quinoxaline rings.
meric systems, complex 14 shows a low catalytical
activity (TON = 32) in the copolymerization of
CO2 with cyclohexene oxide. Furthermore, the
CO2 content in the resulting polymer (41.5%) is
smaller than for copolymers obtained using mono-
meric complexes as catalysts. A possible explana-
tion might be that only a small portion of the
cyclohexamer dissociates in solution to yield the
catalytically active species. The fraction of com-
plex 14 which remains undissociated might be in-
active in the copolymerization reaction but, could
possibly be active in the epoxide ring opening step.
Scheme 6. Structural formula of the complexes 15 (R1 =
R2 = 4-tolyl) and 16 (R1 = 4-tolyl; R2 = mesityl).
The catalytic reaction between CO2 and cyclo- This could explain the low CO2 content (41.5%)
hexene oxide has been extensively investigated as in the resulting copolymer. For a comparision, the
a pathway for the utilization of CO2 as building complexes 15 and 16 both containing a bis-amidine
block for organic materials. Since the first report substructure were synthesized. They displayed
by Inoue [13], a variety of different catalysts have only a small activity (runs 4Ð6) which underlines
been investigated, most of them based on zinc as the importance of the quinoxaline ligand in these
metal center [reviews: 14Ð16]. Darensbourg has catalytically active complexes.
shown that bis(phenoxide) derivatives of zinc are
In comparison to 12, the cyclohexameric com-
very efficient catalysts for this copolymerization plex 14 coordinates one nitrogen group of the
[17, 18]. Coates explored ligand supported zinc neighbouring quinoxaline ring instead of a THF
catalysts with anionic ꢀ-diimine-type ligands bear- molecule. This results in a dramatic decrease in the
ing bulky N-substituents which are highly active catalytic activity and selectivity (run 3) due to the
[19]. NMR monitoring of the catalytic copolymer- stable coordination of this nitrogen.
ization of CO2 and cyclohexene oxide using cata-
It is obvious that the chelate ring as well as the
lysts based on diethyl zinc and phenols allowed a quinoxaline ligand has an important influence on
deeper insight into the mechanism of the initiation the catalytic activity of the zinc complexes investi-
and propagation reactions [20]. In addition, the re- gated here. Complex 12 is not as effective (50%;
action also works in supercritical carbon dioxide 740 g copolymer/g Zn) as the best catalysts de-
[21, 22].
scribed in the literature (1440 g copolymer/g Zn
We carried out the catalytic reactions reported complex, 90% carbonate [17]). However, its selec-
in this article at 80 ∞C and 80 bar using cyclohex- tivity is excellent (96,5% carbonate units were iden-
1
ene oxide as the solvent and employing complexes tified by H NMR spectroscopy). Complex 12 is
12Ð16 as homogenous catalysts. The results are probably able to split one molecule of ROH which
given in Table 1. The bis(phenoxide) zinc complex leads to a species with high catalytic activity due to
12 containing the quinoxaline 11 as a neutral li- only three coordination sites at the zinc center.
gand is one of the most active catalysts for the
One can thus conclude, that these new quinoxa-
copolymerization. An excellent selectivity is ob- line derivatives are very variable ligands for zinc
served (carbonate linkage calculated from the rel- and can be effectively employed in tuning selective
ative intensities of the 1H NMR signals of the catalytic copolymerization of CO2 with cyclohex-
methine protone next to the carbonate linkages ene oxide.
(δ = 4.61 ppm) and ether linkages (δ = 3.37 ppm,
96.5%) (run 1). The related complex 13 containing Experimental Section
only one phenoxide group and the same ligand in
All reagents were of commercial quality (Al-
the monoanionic form is less active. Nonetheless,
drich, Lancaster, Fluka, Merk). Solvents were
the selectivity of this catalyst is also high (run 2).
dried and purified using standard techniques. THF
was dried and deoxygenated by distillation in the
It is noteworthy that ligand 3a is also a powerful
controlling ligand yielding a high turnover number presence of sodium/benzophenone under argon.
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952
O. Hampel et al. · Quinoxaline
Table 1. Catalytic formation of the copolymer from cyclohexene oxide and CO2.
Run
Catalyst
(mmol)
TONa
TOFb
g
Mn
Mw
D
%
Polymer
g molÐ1
g molÐ1
Carbonate
1
2
3
4
5
6
7
12 (0.152)
13 (0.096)
14 (0.082)
15 (0.105)
16 (0.106)
161 (0.075)
A2 (0.158)
740
538
32
268
54.7
351
761
10.7
7.8
0.5
3.9
0.8
5.1
11
7.35
3.37
0.17
3.67
0.74
3.45
7.88
13500
15300
21900
13700
20600
10000
20000
62000
81000
62000
78000
97500
30800
314000
4.59
5.25
2.85
5.70
4.74
3.11
15.7
96.5
95.5
41.5
91.5
80.7
93.7
96.3
Reaction conditions: 80 ∞C, 80 bar CO2, reaction time: 69 h; solvens cyclohexene oxide (10Ð13 g). aTON: g polymer/
g Zn;.bTOF: TON/h; D = Mw/Mn; 1100 ∞C ; 2A: product from the reaction of 3a/ZnEt2/ROH (1:1:1) (R = 2.6-diisopro-
pylphenyl).
All other reactions in THF were carried out in an fined by full-matrix least squares techniques
argon atmosphäre using standard Schlenk tech- against F2o (SHELX-97 [26]). For the compound 10a
nique. Cyclohexene oxide was distilled over CaH2. without the methyl group C24 and only for the
2,6-Diisopropylphenol was distilled over molecu- amine groups of 12 and 14 the hydrogen atoms
˚
lar sieve 3A. ZnEt2 (purchased from Aldrich) was were located by difference Fourier synthesis and
used as received. Commercial high purity carbon refined isotropically. The other hydrogen atoms of
dioxide gas was used without further purification. structures and for the THF-molecule of 14 were
Reactions were monitored by thin layer chro- included at calculated positions with fixed thermal
matography (TLC), on plastic plates coated with parameters. All non-hydrogen atoms were refined
neutral alumina with fluorescence indicator (Poly- anisotropically [24]. XP (SIEMENS Analytical X-
gram ALOX/UV254 from Machery Nagel). Col- ray Instruments, Inc.) was used for structure repre-
umn chromatography was carried out on neutral sentations.
alumina (Merck, aluminium oxide 90 activ neutral,
Crystal data for 10a [27]: C24H20N6, Mr = 392.46
activity I, particle size 0.063Ð0.2 mm, 70Ð230 g molÐ1, yellow prism, size 0.12 ¥ 0.12 ¥ 0.04 mm3,
¯
ASTM) which was tuned to activity V with 15% triclinic, space group P1, a = 8.4060(10), b =
water. Melting points were measured with a Galen 9.9780(10), c = 13.195(2) A, α = 83.668(9), ꢀ =
˚
3
˚
III (Boetius system) from Cambridge Instruments, 74.189(9), γ = 67.285(8) ∞, V = 982.2(2) A , T = Ð
and left uncorrected. UVÐVIS spectra were ob- 90 ∞C, Z = 2, ρcalcd. = 1.327 gcmÐ3, µ(MoÐKα) =
tained using a Perkin Elmer Lambda 19 spectro- 0.83 cmÐ1, F(000) = 412, 6506 reflections in h(Ð
1
photometer. The H- and 13C NMR spectra were 10/10), k(Ð12/12), l(Ð15/16), measured in the
obtained with a Bruker DRX 400 (400 MHz) or range 2.71∞ Յ Θ Յ 27.43∞, completeness at
a Bruker AC 250 (250 MHz) spectrometer. Mass Θmax = 94.5%, 4229 independent reflections,
spectra were taken from measurements with a Fin- Rint = 0.045, 2904 reflections with Fo > 4σ(Fo), 339
nigan MAT SAQ 710 mass spectrometer. Elemen- parameters, 0 restraints, R1obs = 0.061, wR2
=
obs
tal analysis were recorded on a LECO CHNS- 932. 0.151, R1all = 0.096, wR2 = 0.179, GOOF = 1.049,
Gel permeation chromatography was performed largest difference peak aallnd hole: 0.252/Ð0.274 e
Ð3
˚
A
with CHCl3 as eluent at a flow rate of 1,0 ml/min
on a JASCO system equipped with pump 980, UV-
.
Crystal data for 12 [27]: C46H60N4O2Zn¥1/2
detector 975 (254 nm) and refractometer 930. Pol- CH3OH, Mr = 782.37 g molÐ1, colourless prism, size
yststyrene standards were used for calibration. All 0.12 ¥ 0.10 ¥ 0.10 mm3, monoclinic, space group
new compounds gave satisfactory elemental analy- P21/c, a = 10.2637(4), b = 24.865(1), c = 19.5811(6)
3
˚
˚
ses or HRMS.
A, ꢀ = 101.147(2)∞, V = 4903.0(3) A , T = Ð90 ∞C,
Crystal structure determination: The X-ray dif- Z = 4, ρcalcd. = 1.060 g cmÐ3, µ (MoÐKα) = 5.38 cmÐ
fraction data for the compounds were collected on 1, F(000) = 1676, 15440 reflections in h(Ð13/13), k(Ð
a Nonius KappaCCD diffractometer, using graph- 32/29), l(Ð25/25), measured in the range 2.02∞ Յ Θ
ite-monochromated Mo-Kα radiation. Data were Յ 27.48∞, completeness at Θmax = 87.3%, 9826 in-
corrected for Lorentz and polarization effects, but dependent reflections, Rint = 0.042, 5492 reflections
not for absorption [23, 24]. The structures were with Fo > 4σ(Fo), 493 parameters, 1 restraint,
solved by direct methods (SHELXS [25]) and re- R1obs = 0.078, wR2obs = 0.230, R1all = 0.120, wR2
=
all
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O. Hampel et al. · Quinoxaline
953
0.251, GOOF = 0.971, largest difference peak and 125.8 (CHarom.), 131.8, 138.9, 142.5, 147.9, 149.1
Ð3
˚
hole: 1.118/Ð0.444 e A
.
(Cqarom.). Ð MS (DCI, H2O): m/z (%) = 489 (100)
Crystal data for 14 [27]: C216H216N36O6Zn6 ¥3 [M++1], 475 (20), 324 (30), 173 (40).
Ð
C4H8O, Mr = 4020.78 g molÐ1, orange-red prism, C28H32N4O4 (488.58): calcd. C 68.83, H 6.60 N
size 0.10 ¥ 0.10 ¥ 0.08 mm3, hexagonal, space 11.47; found C 68.73, H 6.57, N 11.55.
¯
group R3, a = 32.148(2), b = 32.148(2), c =
Synthesis of 2,3-Bis-(benzylidenehydrazino)-
3
˚
˚
23.610(1) A, V = 21132(2) A , T = Ð90 ∞C, Z = 3, quinoxalines of type 8 (general procedure):
ρcalcd. = 0.948 g cmÐ3, µ (MoÐKα) = 5.53 <cmÐ1
,
0.20 g (1 mmol) of 2,3-bis(hydrazino)-quinoxa-
F(000) = 6336, 9258 reflections in h(0/41), k(Ð36/ line 6 were dissolved in 70 ml of ethanol. The mix-
0), l(Ð30/30), measured in the range 5.34∞ Յ Θ ture was heated and then 2 mmol of the corre-
Յ 27.87∞, completeness at Θmax = 82.7%, 9258 sponding aldehyde 7 and 0.5 ml of hydrochloric
independent reflections, 5985 reflections with Fo > acid were added. After cooling to room temper-
4σ(Fo), 416 parameters, 0 restraints, R1obs = 0.091, ature, the hydrazone thus formed was filtered off
wR2
= 0.239, R1all = 0.154, wR2 = 0.296, and dried.
obs
GOOF = 1.120, largest difference peaakll and hole:
2,3-Bis-(4-methoxybenzylidene-hydrazino)-
quinoxaline (8a): Yield: 64%; orange crystals. Ð
M. p. 166 ∞C. Ð UV/vis (EtOH) λmax (lg εmax) =
235 (4.4), 358 nm (4.5). Ð IR (ATR): ν = 3349
(NH), 3044 (CH.), 2929, 2841 (CH/CH3), 1657,
1594, 1509 (C=C/NH/C=N), 1492, 1420 (CÐN),
Ð3
˚
1.056/Ð0.769 e A
.
Synthesis of 2,3-bis-substituted quinoxalines of type
3 (general procedure):
A mixture of 1.00 g (5 mmol) of 2,3-dichloroqui- 1251, 1165, 1025 (CÐOÐC) cmÐ1. Ð 1H NMR (250
noxaline (1) and the corresponding amount of the MHz, DMSOÐd6): δ = 3.85 (s, 6H, CH3), 7.01Ð
amine (0.01 mol) was heated until a clear red solu- 7.10 (m, 4H, Harom.), 7.21Ð7.27 (m, 2H, Harom.),
tion has occured. The mixture was then extracted 7.77Ð7.85 (m, 2H, Harom.), 8.05 (d, 3J = 8.8 Hz, 4H,
with ethanol and treated with anhydrous hydrogen
H
arom.), 8.76 (s, 2H, NH). Ð 13C NMR (62 MHz,
chloride. The precipitated product was further DMSOÐd6): δ = 53.6 (CH3), 112.5, 115.3, 122.6,
purified by recrystallization from ethanol.
128.0, 128.9, 154.8 (CHarom.), 123.5, 124.2, 139.8,
N,NЈ-Bis-(3,4-dimethoxyphenyl)-quinoxaline-
158.5 (Cqarom.), 160.2 (CH=N). Ð MS (DCI, H2O):
2,3-diamine (3a): Yield: 22%; yellow crystals. Ð M. m/z (%) = 427 (100) [M++1], 294 (10), 269 (30),
p. 214 ∞C. Ð IR (ATR): ν = 3282, 3222, 3143 (NH), 205 (10). Ð C24H22N6O2 (426.47): calcd. C 67.59,
3068, 3000 (CH.), 2937, 2829 (CH3), 1623, 1568, H 5.20, N 19.71; found C 67.51, H 5.23, N 19.78.
1511 (C=C/NH/C=N), 1456 (CÐN), 1219 (CÐOÐ
2,3-Bis-(2-hydroxybenzylidene-hydrazino)-
C) cmÐ1. Ð H NMR (250 MHz, DMSOÐd6): δ = quinoxaline (8b): Yield: 61%; red crystals. Ð M. p.
3.71 (s, 12H, CH3), 4.13 (sbr., 2H, NH), 6.73Ð6.78 172 ∞C. Ð UV/vis (MeOH) λmax nm (lg εmax) = 235
(m, 2H, Harom.), 7.03Ð7.07 (m, 2H, Harom.), 7.25Ð (4.5), 276 (4.3), 369 (4.5), 404 (4.3), 427 nm (4.2). Ð
7.30 (m, 2H, Harom.), 7.49Ð7.53 (m, 4H, Harom.). Ð IR (ATR): ν = 3429, 3346 (OH/NH), 3060 (CH.),
13C NMR (62 MHz, DMSOÐd6): δ = 55.8, 65.5 2933, 2829, 2726 (CH), 1654, 1627, 1604 (C=C/C=
1
(CH3), 110.0, 113.1, 125.6, 126.7, 128.0 (CHarom.), N), 1550, 1455 (CÐN), 1250 (CÐOH) cmÐ1. Ð H
1
129.2, 132.0, 141.7, 145.8, 153.6 (Cqarom.). Ð MS NMR (400 MHz, DMSOÐd6): δ = 4.07 (sbr., NH/
(DCI, H2O): m/z (%) = 433 (80) [M++1], 401 (10), OH), 6.95 (t, 3J = 7.5 Hz, 2H, Harom.), 7.04 (d, 3J =
266 (100). Ð C24H24N4O4 (432.47): calcd. C 66.65, 8.2 Hz, 2H, Harom.), 7.23Ð7.27 (m, 2H, Harom.),
H 5.59, N 12.95; found C 66.59, H 5.62, N 12.87.
N,NЈ-Bis-[2-(3,4-dimethoxyphenyl)-ethyl]-
7.36 (t, 3J = 7.0 Hz, 2H, Harom.), 7.74Ð7.78 (m, 2H,
3
Harom.), 8.19 (d, J = 6.8 Hz, 2H, Harom.), 9.12 (s,
quinoxaline-2,3-diamine (3b): Yield: 31%; pale 2H, CH=N). Ð 13C NMR (100 MHz, DMSOÐd6):
yellow crystals. Ð M. p. 120 ∞C. Ð IR (ATR): ν = δ = 116.5, 117.3, 119.5, 124.7, 128.5, 133.1
3365, 3272 (NH), 3004 (CH), 2944, 2837 (CH2/ (CHarom.), 119.52, 125.6, 141.6, 158.0 (Cqarom.),
CH3), 1653 (C=N), 1572, 1515, 1464 (C=C/NH/CÐ 154.5 (CH=N). Ð MS (DCI, H2O): m/z (%) = 399
N), 1261, 1141 (CÐOÐC) cmÐ1. Ð H NMR (400 (30) [M++1], 241 (30), 123 (25), 80 (100). Ð
1
3
MHz, DMSOÐd6): δ = 2.99 (t, J = 7.3 Hz, 4H, C22H18N6O2 (398.42): calcd. C66.32, H 4.55, N
CH2ÐPh), 3.67 (s, 6H, OÐCH3), 3.75 (s, 6H, OÐ 21.09; found C 66.25, H 4.57, N 21.00.
CH3), 3.92 (m, 4H, CH2), 6.83 (sbr., 4H, Harom.),
2,3-Bis-(2,3-dihydroxybenzylidene-hydrazino)-
7.01 (s, 2H, Harom.), 7.27Ð7.34 (m, 2H, Harom.), quinoxaline (8c): Yield: 70%; orange crystals. Ð M.
7.67 (sbr., 2H, Harom.), 9.57 (sbr., 2H, NH). Ð 13C p. 235 ∞C (dec.). Ð UV/vis (EtOH) λmax (lg εmax) =
NMR (100 MHz, DMSOÐd6): δ = 33.6, 44.4 362 (4.5), 406 (4.3), 430 nm (4.1). Ð IR (ATR): ν =
(CH2), 55.9, 65.0 (CH3), 112.4, 113.3, 121.1, 125.77, 3058 (OH/NH/CH), 1674, 1619, 1600 (C=C/NH/
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954
O. Hampel et al. · Quinoxaline
C=N), 1473 (CÐN), 1236, 1211, 1161 (CÐOH) cmÐ (CHarom.), 132.1, 134.7, 135.7, 139.0, 139.7, 140.3,
1. Ð H NMR (250 MHz, DMSOÐd6): δ = 3.90 140.4, 143.0, 147.2, 150.3 (Cqarom.). Ð MS (DCI,
1
(sbr., 6H, OH/NH), 6.77Ð6.83 (m, 2H, Harom.), 7.00 H2O): m/z (%) = 393 (100) [M++1], 267 (20), 93
3
(d, J = 7.5 Hz, 2H, Harom.), 7.24Ð7.28 (m, 2H, (80). Ð C24H20N6 (392.46): calcd. C 73.45, H 5.14,
3
H
arom.), 7.56 (d, J = 7.7 Hz, 2H, Harom.), 7.70Ð N 21.41; found C 73.39, H5.19, N 21.31.
7.74 (m, 2H, Harom.), 9.11 (s, 2H, CH). Ð 13C NMR
(4-tert-Butylphenyl)-[4-(4-tert-butyl-phenyl)-3-
(62 MHz, DMSOÐd6): δ = 118.7, 119.3, 119.8, imino-3,4-dihydro-pyrazino[2,3-b]-quinoxalin-2-
121.5, 125.0 (CHarom.), 120.4, 141.8, 146.2, 147.1, yl]-amine (10b): Yield: 38%; yellow crystals. Ð M.
147.7 (Cqarom.), 163.6 (CH). Ð MS (DCI, H2O): m/ p. 278 ∞C. UV/vis (CHCl3) λmax (lg εmax) = 394
z (%) = 431 (65) [M++1], 273 (100), 257 (40), 133 (4.3), 411 nm (4.3). Ð IR (ATR): ν = 3321 (NH),
(20). Ð C22H18N6O4 (430.42): calcd. C 61.39, H 3060 (CH), 2959, 2901, 2867 (CH3), 1581, 1229 (C=
4.22, N 19.53; found C 61.32, H 4.19, N 19.63.
N/CÐN) cmÐ1. Ð 1H NMR (250 MHz, CDCl3): δ =
1.24Ð1.28 (m, 18H, CH3), 5.54 (sbr., NH2), 6.84 (d,
2,3-Bis-(3,4-dihydroxybenzylidene-hydrazino)-
quinoxaline (8d): Yield: 39%; red crystals. Ð M. p. 3J = 8.5 Hz, 2H, Harom.), 7.12Ð7.20 (m, 2H, Harom.),
230 ∞C (dec.). Ð UV/vis (EtOH) λmax (lg εmax) = 7.29 (d, 3J = 6.6 Hz, 2H, Harom.), 7.36Ð7.43 (m, 2H,
383 nm (4.5). Ð IR (ATR): ν = 3006 (OH/NH),
H
arom.), 7.57Ð7.62 (m, 2H, Harom.), 7.88 (d, 3J = 8.8
2972 (CH), 1671, 1654, 1639, 1597 (C=C/NH/C= Hz, 2H, Harom.), 9.31 (s, 1H, NH). Ð 13C NMR (62
N), 1517 (CÐN), 1283, 805 (CÐOH) cmÐ1. Ð H MHz, CDCl3): δ = 30.3, 30.5 (CH3), 33.3, 34.0 (Cq),
1
NMR (250 MHz, DMSOÐd6): δ = 4.17 (sbr., 6H, 119.8, 125.0, 125.4, 126.4, 126.96, 126.97, 127.1,
3
OH/NH), 6.71 (d, J = 8.2 Hz, 2H, Harom.), 7.00Ð 127.7 (CHarom.), 130.6, 134.1, 137.6, 138.4, 145.3,
7.12 (m, 4H, Harom.), 7.35 (s, 2H, Harom.), 7.50Ð 145.8, 16.6, 148.4, 149.1, 151.9 (Cqarom.). Ð MS
7.54 (m, 2H, Harom.), 8.41 (s, 2H, CH). Ð 13C NMR (DCI, H2O): m/z (%) = 447 (30) [M++1], 351 (80),
(62 MHz, DMSOÐd6): δ = 115.7, 116.1, 117.3, 73 (100). Ð C30H32N6 (476.62): calcd. C 75.60, H
122.9, 124.8 (CHarom.), 125.3, 125.6, 141.9, 146.1, 6.77, N 17.63; found C 75.53, H 6.80, N 17.70.
150.0 (Cqarom.), 1560 (CH). Ð MS (DCI, H2O) m/
Complex 12: ZnEt2 (2.20 mmol, 1.1m solution in
z (%): 431 (10) [M++1], 311 (20), 296 (30), 207 toluene) were dissolved in THF (10 ml) and
(30), 153 (100). Ð C22H18N6O4 (430.42): calcd. treated with an excess of 2,6-diisopropylphenol
C61.39, H 4.22, N 19.53; found C 61.31, H 4.25, (570.4 mg, 3.2 mmol). After stirring for 30 min-
N 19.53.
2,3-Bis-(2-pyridylmethylidene-hydrazino)-
utes, a solution of 2,3-bis(4-tolylamino)-hexahy-
dro-quinoxaline 11 (762.2 mg, 2.20 mmol) in THF
quinoxaline (8e): Yield: 65%; red crystals. Ð M. p. was added. After stirring for 20 h at 20 ∞C the
245 ∞C (dec.). Ð UV/vis (MeOH) λmax (lg εmax) = solvent was removed and the powder was washed
377 nm (4.5). Ð IR (ATR): ν = 3371 (NH), 3016 with pentane. The resulting yellow complex (70Ð
(CH), 2921 (CH), 1652, 1602, 1535, 1453 (C=N/CÐ 80% yield) was dissolved in diethyl ether and kept
1
N) cmÐ1. Ð H NMR (250 MHz, DMSO-d6): δ = at Ð20 ∞C. After standing overnight, yellow single
5.19 (sbr., NH), 7.29Ð7.41 (m, 3H, Harom.), 7.91Ð crystals were isolated that were suitable for an X-
1
8.04 (m, 2H, CH), 8.17Ð8.21 (m, 3H, Harom.), ray structural analysis. Ð H NMR (THF-d8): δ =
8.58Ð8.64 (m, 3H, Harom.), 8.89Ð8.95 (m, 3H, 0.77Ð1.35 (m, 28H, CH3 + CH2), 1.62 (m, 2H,
H
arom.). Ð MS (DCI, H2O): m/z (%) = 423 (20) CH2), 2.00Ð2.03 (m, 2H, CH2), 2.31 (s, 6H, CH3Ð
[M++3 H2O], 369 (55) [M+1], 319 (40), 265 (70), tolyl), 3.05Ð3.37 (m, 5H, CH), 3.58 (m, 1H, CH),
237 (50), 176 (50), 146 (100). Ð C20H16N8 (368.40): 6.29Ð6.36 (m, 2H, CH), 6.65Ð6.78 (m, 4H, CH),
calcd. C 65.21, H 4.38, N 30.42; found C 65.17, H 6.90Ð7.30 (m, 10H, CHÐtolyl + NH). Ð 13C NMR
4.35, N 30.47.
(THFÐd8): δ = 20.9, 23.3, 23.5, 23.7 (CH3), 27.3,
(3-Imino-4-p-tolyl-3,4-dihydro-pyrazino[2,3-b]-
27.4, 29.5, 29.8 (CH2), 51.1, 56.6 (CH), 114.1, 114.3,
quinoxalin-2-yl)-p-tolyl-amine (10a): Yield: 15%; 122.1, 123.6, 123.8, 124.2, 128.8, 129.6, 130.8, 135.5,
yellow crystals. Ð M. p. 261 ∞C. Ð IR (ATR): ν = 135.6, 137.6, 137.7, 137.9, 141.7, 141.8, 150.1, 150.5,
3379, 3219 (NH), 3063, 3028 (CH), 2917, 2865 162.8, 162.9 (CHarom + Cqarom.). Ð C46H60N4O2Zn
(CH3), 1580, 1533, 1508, 1437 (C=N/C-N) cmÐ1. Ð (766.36): calcd. C 72.10, H 7.89, N 7.31; found C
1H NMR (250 MHz, CDCl3): δ = 2.38 (s, 3H, 70.63, H 8.03, N 7.29.
CH3), 2.55 (s, 3H, CH3), 7.20Ð7.28 (m, 5H,
Complex 13: ZnEt2 (1.10 mmol, 1.1m solution in
H
arom.), 7.48Ð7.65 (m, 6H, Harom. + 2 NH), 7.97 (d, toluene) were dissolved in THF (10 ml) and
3
3J = 8.4 Hz, 2H, Harom.), 8.08 (d, J = 8.0 Hz, 1H, treated with an equivalent amount of 2,6-diisopro-
H
arom.), 9.42 (s, 1H, NH). Ð 13C NMR (62 MHz, pylphenol (196.1 mg, 1.10 mmol) and stirred for
CDCl3): δ = 21.4, 21.8 (CH3), 121.2, 127.8, 127.9, 30 minutes followed by treatment with 11
128.5, 129.0, 129.3, 129.4, 130.1, 130.14, 131.3, 131.8 (381.1 mg, 1.10 mmol) in THF. After 20 hours the
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O. Hampel et al. · Quinoxaline
955
solvent was removed and the powder was washed the complex a 13C NMR spectrum could not be
with pentane. The resulting yellow complex was recorded. Ð C62H78N4O4Zn2 (1074.05): calcd. C
obtained in 70Ð80% yield. Ð 1H NMR (THFÐd8): 69.33, H 7.32, N 5.22; found C 67.90, H 7.55, N
δ = 0.82Ð0.90 (m, 3H, CH2), 1.16Ð1.19 (d, 12H, 4.86; C62H78N4O4Zn2¥1 THF: calcd. C 69.16, H
CH3), 1.30 (m, 3H, CH2), 1.73 (m, 2H, CH2), 2.21Ð 7.56, N 4.89.
2.32 (2 ¥ s, 6H, CH3Ðtolyl), 3.22Ð3.36 (m, 4H,
Complex 16: ZnEt2 (2.20 mmol, 1.1m solution in
CH), 6.33Ð7.16 (m, 11H, CHarom.), 8.00 (sbr, 1H, toluene) was dissolved in THF (10 ml), treated
NH). Ð 13C NMR (THFÐD8): δ = 20.8, 23.3 with an excess of 2,6-diisopropylphenol (713.1 mg,
(CH3), 27.5 (CH2), 51.1, 56.6 (CH), 120.6, 123.6, 4.0 mmol) and stirred for 30 minutes. A solution
123.8, 135.5, 135.6, 151.8 (CHarom.), 114.3, 122.1, of bis(mesityl)-bis(tolyl)-oxalamidine (552.9 mg,
124.2, 128.8, 129.6, 130.8, 141.7, 141.8, 150.1, 150.5 1.10 mmol) in THF was then added. After 20 h the
(CHarom. + Cqarom.) C38H50N4O2Zn (660.20): calcd. solvent was removed and the powder was washed
C 69.13, H 7.63, N 8.49; found C 70.35, H 7.83, with diethyl ether. The resulting white complex
N 8.26.
was obtained in 70Ð80% yield. Ð 1H NMR (THF-
Cyclic hexamer (complex 14): ZnEt2 (0.60 mmol, d8): δ = 0.66Ð1.27 (6 ¥ d, 24H, CH3), 1.58Ð2.26
1.1m solution in toluene) were dissolved in 10 ml (m, 24H, CH3-tolyl, mesityl, + THF), 2.90 (m, 2H,
THF and treated with an excess of 2,6-diisopropyl- CH), 3.23 (m, 2H, CH), 6.23Ð6.79 (m, 16H,
phenol (178.3 mg, 1.0 mmol) and stirred for CHarom.), 6.89Ð6.96 (m, 2H, CH). Ð 13C NMR
30 minutes.
0.60 mmol) in THF was then added. The suspen- (CH3), 27.4, 27.6 (CH), 122.3, 123.7, 128.3, 129.1,
sion was stirred until all of the product was dis- 131.2, 137.6 (CHarom.). C66H86N4O4Zn2
A
solution of 10a (235.7 mg, (THFÐd8): δ = 1.2, 19.4, 19.7, 20.5, 23.2, 23.5, 26.3
Ð
solved (about 3Ð4). From this solution, a small (1130.15): calcd. C 70.14, H 7.67, N 4.96; found C
amount (10%) of the complex was isolated as or- 69.64, H 7.81, N 4.83.
ange-red crystals. The main mass of the product
Copolymerisation of cyclohexene oxide and car-
1
was obtained as a orange-red powder. Ð H NMR bon dioxide: 80Ð120 mg of the corresponding zinc
(THFÐd8): δ = 1.19, 1.21 (d, 12H, CH3), 2.51, 2.58 complex was either dissolved or suspended (14, 15
(2 ¥ s, 6H, CH3Ðtolyl), 3.24Ð3.34 (h, 2H, CH), and 16) in the 900 fold amount of cyclohexene ox-
6.73Ð6.95 (t, 1H, CH), 6.95Ð7.54 (m, 12H, ide and put in an 300 ml autoclave which had been
CHarom.), 7.84Ð8.01 (2 ¥ d, 3H, CHarom. + NH). Ð dried under vacuum. The autoclave was cooled to
13C NMR (THFÐd8): δ = 20.9, 21.2, 23.2 (CH3), 0 ∞C and placed under 50 bar of carbon dioxide
27.5 (CH), 120.6, 124.2, 125.3, 126.7, 128.0, 128.1, and then heated to 80 ∞C. After the reaction time
128.4, 128.8, 129.5, 129.5, 131.9, 132.9, 133.2, 135.5, of 69 hours the autoclave was cooled to 0 ∞C and
138.1, 138.6, 140.5, 142.8, 145.0, 145.3, 151.8, 152.9, the CO2 was removed. The polymer was dissolved
155.2 (CHarom + Cqarom.). Ð MS (ESI, MeOH): m/ in dichloromethane and reprecipitated with meth-
z = 1714 (trimer Ð 2 tolyl, C94H94N18O3Zn3), 1264 anol/HCl (450/50 ml).
(dimer, C72H72N12O2Zn2). Ð C216H216N36O6Zn6
(3804.47): calcd. C 68.19, H 5.72, N 13.25%; found spectroscopy (CDCl3) for percentages of ether
C 69.23, H 6.10, N: 12.60. = 3.37 and
and carbonate linkages (1H:
Polymer samples were analysed by NMR
δ
Complex 15: ZnEt2 (1.98 mmol, 1.1m solution in 4.61 ppm). 13C NMR was utilized for determining
toluene) were dissolved in 10 ml THF and treated the tacticity of the polymer. There are two 13C res-
with an excess of 2,6-diisopropylphenol (534.8 mg, onances at δ = 153.7 and 153.1 ppm which are as-
3.0 mmol) and stirred for 30 minutes. A solution signed to syndiotactic and isotactic copolymer.
of tetra-p-tolyloxalamidine (442.2 mg, 0.99 mmol) The molecular weights (Mn and Mw) of the metha-
in THF was then added. After 20 hours the solvent nol-insoluble copolymers were measured by gel
was removed and the powder was washed with di- permeation chromatography (GPC) by compari-
ethyl ether. The resulting yellow complex was ob- son with polystyrene standards.
1
tained in 70Ð80% yield. Ð H NMR (toluene-d8):
Acknowledgements
δ = 1.09, 1.12 (d, 24 H, CH3), 1.34Ð1.40 (m, 12H,
CH3-tolyl), 1.85 (m, 9H, CH2ÐTHF), 2.91 (sbr, 4H,
CH), 3.46Ð3.53 (m, 16H, CH2ÐTHF), 6.42Ð7.04 einschaft (SFB 436) and the Fonds der Chemischen
(m, 24 H, CHarom.). Ð Due to the low solubility of Industrie are gratefully acknowledged.
The supports of the Deutsche Forschungsgem-
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956
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