Solution, solid state structure and fluorescence studies of 2,3-functionalized quinoxalines: Evidence for a π-delocalized keto-enamine form with N-H···O intramolecular hydrogen bonds

Article abstract of DOI:10.1039/b009667i

Three quinoxaline derivatives 3, 4 and 5 were prepared by condensation of tetraones RC(=O)-CH₂-C(=O)-(=O)-CH₂-C(=O)R [1, R = Ph; 2, R = neo-Pen] with o-phenylenediamine or (R,R)-1,2-diaminocyclohexane. ¹H, ¹³C a

Full text of DOI:10.1039/b009667i

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Solution, solid state structure and Ñuorescence studies of
2,3-functionalized quinoxalines: evidence for a p-delocalized
keto-enamine form with N–HÆ Æ ÆO intramolecular hydrogen bonds
Rachid Touzani,a Taibi Ben-Hadda,*a Sghir Elkadiri,a Abdelkrim Ramdani,a Olivier Maury,b
Hubert Le Bozec,b Loic Toupetc and Pierre H. Dixneufb
a L aboratoire de Chimie Organique-Physique, Departement de Chimie, Faculte des Sciences
dÏOujda, Universite Mohammed 1er, 60000 Oujda, Morocco.
E-mail: benhadda=sciences.univ-oujda.ac.ma; Fax: ]212 6 50 06 03
b L aboratoire de Chimie Coordination et Catalyse (CNRS UMR 6509), Universite de Rennes 1,
Campus de Beaulieu, 35042 Rennes cedex, France. E-mail: dixneuf=univ-rennes1.fr
c Groupe de la Matiere Condensee (CNRS UMR 6626), Universite de Rennes 1,
Campus de Beaulieu, 35042 Rennes cedex, France
Received (in Strasbourg, France) 10th July 2000, Accepted 23rd November 2000
First published as an Advance Article on the web 15th February 2001
Three quinoxaline derivatives 3, 4 and 5 were prepared by condensation of tetraones RC(2O)ÈCH ÈC(2O)È
2
(2O)ÈCH ÈC(2O)R [1, R \ Ph; 2, R \ neo-Pen] with o-phenylenediamine or (R,R)-1,2-diaminocyclohexane. 1H,
2
13C and 15N NMR studies show that these derivatives are best described as their keto-enamine form with
NÈHÉ É ÉO intramolecular hydrogen bonds. This preferred tautomeric form is conÐrmed by X-ray structural studies
of 3 and 5. Compounds 3 and 4, containing an extended delocalized p-system, exhibit Ñuorescence properties. In
contrast, Ñuorescence is inhibited in 5, when the central aromatic part is replaced by a cyclohexyl group.
Metallo-organic complexes containing conjugated nitrogen
ligands represent an important class of nonlinear optical
chromophores.1 Efficient systems involving organometallic
and coordination compounds with pyridine,2 diimine,3 Salen-
type4 or Schi†-base5 ligands have recently been described. We
have also demonstrated the large dipolar and non-dipolar
optical non-linearities in metal complexes of functionalized
bipyridyl ligands.6 In connection with our research aimed at
developing new metallo-organic NLO-phores, we became
interested in the elaboration of highly conjugated bidentate
heterocyclic ligands. The 2,3-bis(phenylacyl)quinoxaline,
reported many years ago as a keto-imine form7 (A, Scheme 1),
was described a few years later as an enol-imine form (B);8
this latter tautomeric form is of particular interest since it pre-
sents an extended conjugation pathway with a potentially
bis(bidentate) N,O-chelating site. Furthermore, quinoxaline
derivatives have also been used as electron-withdrawing
groups in p-conjugated polymer chemistry,9 and are known to
be easily functionalized, giving rise to Ñuorescent materials.10
Thus, this type of structure should present all the require-
ments needed for the design of NLO-active molecules. In
order to get insight about the exact structure of A, which is of
crucial importance in terms of electronic delocalization, we
have carried out solution and solid-state structural studies,
and we have examined the optical properties of the 2,3-
bis(phenylacyl)quinoxaline. In addition, the modulation of the
conjugation pathway has been studied by the modiÐcation of
either the phenyl group of the phenylacyl moities or the
central aromatic part of the quinoxaline. We now report that
(i) the 2,3-bis(phenylacyl)quinoxalines contain two intramole-
cular NÈHÉ É ÉO, not NÉ É ÉHÈO, hydrogen bonds both in the
solid state and in solution, (ii) they possess a rigid delocalized
p-system and display Ñuorescence properties whatever the
nature of the acyl group, (iii) Ñuorescence is inhibited,
however, when the central aromatic part is replaced by a
cyclohexyl group.
Results and discussion
Synthesis
Quinoxaline derivatives were prepared in a two-step reaction.
Tetraketones 111 and 2 (Scheme 2) were Ðrst prepared on a
large preparative scale in 80 and 60% yields, respectively, by a
Claisen condensation of a methyl ketone (acetophenone or
neo-pentyl methyl ketone) with diethyl oxalate. These tetra-
ketones 1, 2 are present in solution as their bis-enol tauto-
meric forms as shown by the strongly deshielded hydroxy
(14.511 and 15.0 ppm) and vinylic (6.4511 and 6.30 ppm)
protons. Classical condensation of 1 and 2 with cyclic
diamines [o-phenylenediamine or (R,R)-1,2-diaminocyclo-
hexane] in reÑuxing ethanol12 yielded the corresponding
quinoxaline-like derivatives 3,7b,8 4 and 5 (Scheme 2). These
compounds were characterized by means of elemental
analysis, 1H, 13C and 15N NMR studies and optical measure-
ments and for 3 and 5 by X-ray di†raction analysis.
Solution and solid state structure of 3
The 1H NMR data of 3 reveals the presence of a vinylic
proton at 6.5 ppm, clearly excluding the keto-imine form (A),8
and also that of an acidic proton, strongly deshielded at 15.0
ppm (Table 1). It is worth noting that the signals are not sig-
niÐcantly a†ected upon lowering the temperature to 223 K. In
order to determine the position of the acidic protonÈon the
nitrogen or oxygen atomÈmultiple bond C/H coupling
experiments were performed. Selective 1H irradiation in a
non-decoupled 13C NMR spectrum reveals that the acidic
proton H(1) (Fig. 1) is weakly coupled to C(7) suggesting the
predominance of the keto-enamine form (C). This result is
DOI: 10.1039/b009667i
New J. Chem., 2001, 25, 391È395
391
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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intramolecular NÈHÉ É ÉO hydrogen bond,13 rather than the
reported NÉ É ÉHÈO one.8
In order to get more information about the dynamic equi-
libria between the di†erent tautomeric forms, an H/D
exchange experiment with D O in puriÐed CDCl was per-
2
3
formed (Scheme 3). Not surprisingly, the acidic H(1) hydrogen
atoms were immediately replaced by deuterium atoms,
whereas the H/D exchange of the vinylic protons H(9) was
very slow. This experiment clearly indicates that the equi-
librium between the tautomeric forms C and A is very slow
(Scheme 1), conÐrming the predominance and the high stabil-
ity of the keto-enamine form (C) in solution.14
The solid state structure (Fig. 1) is fully in agreement with
the structure proposed in solution for 3. Crystal data and
selected bond lengths and angles are reported in Tables 2 and
3, respectively. The molecule is quasi-planar with a C sym-
2
metry axis: the external phenyl rings are twisted by 25¡. The
nitrogen atom N(1) is trivalent with an N(1)ÈH(1) bond length
of about 0.88 A, conÐrming the position of the acidic proton.
In addition, N(1), C(2), C(8a) and H(1) are perfectly coplanar,
resulting from the sp2 hybridization of the nitrogen atom due
to the conjugation with the delocalized p-system. The C(10)È
O(1) distance [1.250(2) A] is in the range for those of carbonyl
Scheme 1 Di†erent tautomeric forms of quinoxaline derivatives.
conÐrmed by the non-decoupled 15N NMR spectrum (Table
1), which shows a doublet (1J
\ 87 Hz) at [250.3 ppm.
HhN
The position of the signal is characteristic of an sp2 hybridized
nitrogen and the large coupling constant is explained by an
Scheme 3 H/D exchange reaction.
Scheme 2 Synthesis of the quinoxaline-like derivatives 3È5
Table 1 Selected NMR data
Acidic
proton H(1)
d
Vinylic
proton
d
15N
Compound
d(1J /Hz)
HhN
3
4
5
15.0
14.6
11.3
6.5
5.6
6.3
[250.3 (87)
[252.3 (87)
[270.7 (94)
Fig. 1 ORTEP drawing of compound 3.
392
New J. Chem., 2001, 25, 391È395

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Table 2 Crystallographic data and reÐnement details for compounds
3 and 5
3
5
Formula
M
Crystal system
Space group
C
366.40
H
N O
C
372.45
Monoclinic
H
N O
24 18
2
2
24 24
2
2
Orthorhombic
Pbcn
23.255(3)
11.484(2)
6.936(9)
90
90
90
1852(2)
4
0.710 73
0.85
P
21
a/A
b/A
c/A
a/¡
11.008(3)
18.305(3)
10.524(6)
90
109.91(3)
90
1994(1)
4
0.710 73
0.79
b/¡
c/¡
U/A
Ž
3
Fig. 2 UV-vis (bold line) and Ñuorescence spectra of compound 3.
Z
j(Mo-Ka)/A
k/cm~1
T /K
293
1777(0.000)
1005
293
4483(0.414)
1794
ture (C). Representative absorption and emission spectra for 3
are reported in Fig. 2. All the optical data, measured in
dichloromethane, are summarized in Table 4. The two com-
pounds exhibit the same absorption proÐle: intense and struc-
tured bands between 400 and 500 nm.16 The molecular
absorption coefficients are quite large (ca. 25 000 dm3 mol~1
cm~1) and the bathochromic shift (3 [ 4) is consistent with a
better delocalization in the p-conjugated system of 3 as com-
pared to 4. Luminescence is observed for both compounds;
modiÐcation of the substituents (neopentyl vs. phenyl) only
induces a modulation of the quantum yield efficiency. Thus,
the two quinoxaline derivatives 3 and 4, better described by
their keto-enamine tautomeric form (C), bear similar extended
p-conjugated systems as they present very similar absorption
and Ñuorescence properties.
Meas. reÑections (R
Indep. reÑections
[I [ 2p(I)]
)
int
R
[I [ 2p(I)], all data
0.0408, 0.1056
0.0957, 0.1156
0.0511, 0.2081
0.1040, 0.1464
1
wR [I [ 2p(I)], all data
2
compounds and the C(2)ÈC(9) bond [1.371(2) A] is slightly
elongated as compared to classical carbonÈcarbon double
bonds (Table 3). The C(9)ÈC(10) and C(2)ÈC(3) distances
[1.431(2) and 1.482(3) A, respectively] lie in between single
and double bond lengths, conÐrming the high electronic delo-
calization. The H(1)É É ÉO(1) distance of about 1.8 A and the
N(1), H(1), O(1) angle of ca. 154¡, suggests an intra-molecular
N(1)ÈH(1)É É ÉO(1) hydrogen bond in accordance with the
strong NMR deshielding of H(1) [15.0 ppm].15 Thus, in con-
trast to previously reported studies,7b,8 it appears that com-
pound 3 is best described both in the solid state and in
solution by the keto-enamine form (C).
NMR studies, X-ray structure and optical data for 5
In order to get more insight into the structureÈproperties
relationship, we envisaged replacing the central aromatic part
of the quinoxaline group by a cyclohexyl fragment. It is worth
noting that the resulting compound 5 (Scheme 2) is no longer
a quinoxaline but a 4a,5,6,7,8,8a-hexahydroquinoxaline. In
spite of this modiÐcation of the molecular structure, the keto-
enamine form of type C is always predominant, as suggested
by the 15N NMR data (Table 1), but the 15N chemical shift is
displaced to lower Ðeld (*d \ 20 vs. 3) suggesting a lesser sp2
character of the nitrogen atoms. This tendency is also con-
Ðrmed by the shielding of the acidic proton (*d \ 3.7 vs. 3).
The X-ray study shows that the solid state structure of 5 is
composed of two independent molecules that are very similar.
For clarity only one, presented in the ORTEP drawing (Fig.
3), will be discussed. The cyclohexyl ring adopts a ““chairÏÏ
conÐguration, inducing an important torsion in the vicinal
heterocycle with a dihedral angle [N(1), C(8a), C(4a), N(4)] of
ca. 53¡. The nitrogen atoms N(1) and N(4) are trivalent with
N(1)ÈH(1) and N(4)ÈH(4) bonds of 0.80(6) and 0.90(7) A,
respectively; they remain planar, indicating an sp2 hybrid-
ization. ConÐrmation of the keto-enamine structure C is also
given by the strong carbonyl character of the C(10)ÈO(1) and
C(10a)ÈO(1a) bonds [1.245(6) and 1.239(6) A, respectively]
and by the double bonds C(2)ÈC(9) and C(3)ÈC(9a) [1.381(7)
and 1.391(8) A, respectively]. Good electronic delocalization is
NMR studies of 4 and optical data of 3 and 4
Replacing the external phenyl ring by neopentyl moieties, as
in compound 4, does not signiÐcantly a†ect the structure of
the molecule. 1H and 15N NMR data (Table 1) are similar to
those of 3 and are in agreement with the keto-enamine struc-
Table 3 Selected bond lengths (A) and angles (¡) for compounds 3
and 5
3
5
N(1)ÈC(2)
N(1)ÈC(8a)
N(1)ÈH(1)
1.349(2)
1.388(2)
0.878(19)
N(1)ÈC(2)
1.345(7)
1.335(7)
1.459(7)
1.477(7)
0.80(6)
N(4)ÈC(3)
N(1)ÈC(8a)
N(4)ÈC(4a)
N(1)ÈH(1)
N(4)ÈH(4)
0.90(7)
C(2)ÈC(3)
C(4a)ÈC(8a)
C(2)ÈC(9)
1.482(3)
1.393(4)
1.371(2)
C(2)ÈC(3)
1.482(8)
1.471(7)
1.381(7)
1.391(8)
1.432(8)
1.425(8)
1.245(6)
1.239(6)
123.1(5)
121.8(5)
121(4)
118(4)
116(4)
120(4)
115.9(5)
116.8(5)
107.9(5)
107.4(5)
C(4a)ÈC(8a)
C(2)ÈC(9)
C(3)ÈC(9a)
C(9)ÈC(10)
C(9a)ÈC(10a)
C(10)ÈO(1)
C(9)ÈC(10)
1.431(2)
C(10)ÈO(1)
1.250(2)
C(10a)ÈO(1a)
C(8a)ÈN(1)ÈC(2)
C(4a)ÈN(4)ÈC(3)
C(8a)ÈN(1)ÈH(1)
C(4a)ÈN(4)ÈH(4)
C(2)ÈN(1)ÈH(1)
C(3)ÈN(4)ÈH(4)
N(1)ÈC(2)ÈC(3)
N(4)ÈC(3)ÈC(2)
N(1)ÈC(8a)ÈC(4a)
N(4)ÈC(4a)ÈC(8a)
C(8a)ÈN(1)ÈC(2)
C(8a)ÈN(1)ÈH(1)
C(2)ÈN(1)ÈH(1)
N(1)ÈC(2)ÈC(3)
N(1)ÈC(8a)ÈC(4a)
125.20(15)
122.50(12)
112.20(12)
166.77(10)
118.01(10)
Table 4 UV-vis and Ñuorescence data for compounds 3È5
Absorption
/nm (e/dm3 mol~1 cm~1)
Emission
/nm [/(%)]
Compound
j
j
max
em
3
4
5
486 (26 000)
453 (31 000)
465 (18 000)
435 (21 000)
425 (28 000)
499, 523 [0.14]
488, 512 [0.06]
None
New J. Chem., 2001, 25, 391È395
393

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data are listed in ppm and are reported relative to tetra-
methylsilane (1H, 13C), residual solvent peaks being used as
internal standard or relative to nitromethane (15N) with exter-
nal calibration. Complete assignments of the 13C spectra
required non-decoupled 13C NMR spectra with selective 1H
decoupling. Non-decoupled 15N NMR spectra were measured
in 15 mm diameter NMR tubes. UV-vis spectra were recorded
on a Kontron Uvikon 941 spectrophotometer in dichloro-
methane solution. Infra-red spectra were recorded in KBr
pellets using a Brucker IFS28 FTIR spectrometer. Elemental
analysis were performed by the Service Central dÏAnalyse du
CNRS (Solaize, France).
Fluorescence
Fluorescence experiments were performed in dilute dichloro-
methane solution (ca. 10~5 mol l~1) using a PTI spectrometer.
Fluorescence quantum yields were measured on non-degassed
samples at room temperature. A solution of quinine sulfate in
H SO was used as the standard for the quantum yield mea-
2
4
surement (/ \ 0.546 for j \ 365 nm). Refraction index cor-
ex
rection has been performed.17
Crystallography
The samples were studied on an automatic CAD4 Nonius dif-
fractometer with graphite monochromated Mo-Ka radi-
ation.18 The cell parameters were obtained by Ðtting a set of
25 high-theta reÑections. After Lorentz and polarization cor-
rections,19 the structure was solved with SIR-97,20 which
reveals the non-hydrogen atoms of the structure. After aniso-
tropic reÐnement, all the hydrogen atoms are found with a
Fourier di†erence map. The entire structure was reÐned with
SHELXL9721 by the full-matrix least-square techniques (using
Fig. 3 ORTEP drawing of compound 5.
also suggested by the short C(2)ÈC(3), C(9)ÈC(10) and C(9a)È
C(10a) single bonds [1.482(8), 1.432(8) and 1.425(8) A,
respectively]. It is worth noting that the phenyl rings lie per-
fectly in the plane of the conjugated p-system, in contrast to
those of 3. The distances H(1)É É ÉO(1) and H(4)É É ÉO(1a) (1.8 A)
compare well with that of compound 3, and conÐrm the intra-
molecular hydrogen bond. Replacing the central aromatic
part of the 2,3-bis(phenylacyl)quinoxaline, 3, by a cyclohexyl
fragment does not signiÐcantly a†ect the structure of the mol-
ecule. Of course the heterocyclic ring is distorted, but the com-
pound is already present as the keto-enamime tautomer (C)
with a highly conjugated p-system. The absorption spectrum
of 5 (Table 4) presents an hypsochromic shift compared to 3
(*j \ 60 nm), consistent with the breaking of the quinoxaline
conjugation pathway. The most striking di†erence between 3
and 5 lies in the fact that 5 shows no Ñuorescence. This result
points out the role of the molecular structure: replacement of
the fused aromatic ring by a cyclohexyl fragment inhibits the
Ñuorescence properties.
F magnitude; x, y, z, U for C, N and O atoms, x, y, z in
ij
riding mode for H atoms). ORTEP views were realized with
PLATON98.22 All the calculations were performed on a
Silicon Graphics Indy computer. Crystal data and reÐnement
details for compounds 3 and 5 are presented in Table 2.
CCDC reference number 440/247. See http://www.rsc.org/
suppdata/nj/b0/b009667i/ for crystallographic Ðles in .cif
format.
Syntheses
1,6-Bis(neopentyl)hexan-1,3,4,6-tetraone (2). A suspension of
methylate NaOMe (27 g, 0.5 mol) was stirred in dry diethyl
ether (300 ml) at 0 ¡C. A mixture of 4,4-dimethyl-2-pentanone
(57 g, 0.5 mol) and diethyl oxalate (36.5 g, 0.25 mol) was
added, drop by drop, over 25 min. The mixture was stirred for
12 h until TLC indicated that all the 4,4-dimethyl-2-pentan-
one had been consumed. Compound 2 was precipitated and
collected by simple Ðltration, dissolved in aqueous solution
and neutralized with glacial acetic acid to yield 42.3 g (60%)
as a whiteÈyellow precipitate. Mp \ 96È98 ¡C. 1H NMR
(CDCl ): d 15.0 (br s, w \ 70 Hz, 2H, OH), 6.30 (s, 2H,
Conclusion
We have shown that 2,3-substituted quinoxaline or hexa-
hydroquinoxaline, bearing alkyl- or phenylacetyl moieties,
exist as the keto-enamine form (C) in both solution and the
solid state. The structure of these molecules has a dramatic
impact on their Ñuorescence properties: the latter being
restricted to the quinoxaline derivatives. The excellent rigid
delocalization system observed in compound 3 is of major
interest for the design of new metallo-organic NLO-phores.
Further studies are currently in progress in order to increase
the intra-molecular charge transfer by functionalization with
electron-donating and -withdrawing groups, and to examine
the coordination of these bis-bidentate N,O-ligands to tran-
sition metals.
3
1@2
2CH), 2.30 (s, 4H, CH ), 1.0 (s, 18H, CH ). 13C NMR
2
3
(CDCl ): d 201.5 (C2O), 172.9 (2CÈOH), 100.6 (CÈH), 53.9
3
(CH ÈtBu), 32.2 (CMe ), 29.9 (CH ). IR (KBr/cm~1) 1604
2
3
3
(l ). LRMS (EI): m/z 282 (C
H
O , M`, 100%).
CJO
16 26
4
3. o-Phenylenediamine (108 mg, 1 mmol) and tetraketone
(1; 290 mg, 1 mmol) were stirred in reÑuxing ethanol (50 ml)
for 30 min. After cooling the mixture to room temperature,
the desired compound crystallized as analytically pure orange
crystals from dichloromethane (293 mg, 80%). Mp \ 218È
220 ¡C. 1H NMR (CDCl ): d 15.0 (br s, w \ 8.2 Hz, 2H,
Experimental
3
1@2
H ), 8.0È7.8 (m, 4H, m-H), 7.6È7.4 (m, 6H, o-H, p-H), 7.10 (m,
General procedures
1
4H, H ), 6.5 (s, 2H, CH-CO). 13C NMR (CDCl ): d 188.9
(C2O), 146.4 (C , C ), 139.3 (i-C), 131.7 (p-C), 128.6 (m-C),
127.2 (o-C), 126.3 (C , C ), 125.1 (C , C ), 116.9 (C , C ),
87.5 (CHÈCO). 15N NMR (CDCl ): d [250.3 (d, 1J
Hz). IR (KBr cm~1): 1591 (s, l ). UV [CH Cl , j/nm
5h7
3
NMR spectra (1H, 13C) were recorded on a Bruker DPX 200
(operating at 200.12 MHz for 1H, 50.32 MHz for 13C) or on a
Bruker AM 300 (operating at 300.13 MHz for 1H, at 75.47
MHz for 13C and at 30.42 MHz for 15N) spectrometer. NMR
2
3
4a
8a
6
7
5
8
\ 87
3
NhH
CJO
2 2
394
New J. Chem., 2001, 25, 391È395

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3
4
S. D. Cummings, L.-T. Cheng and R. Eisenberg, Chem. Mater.,
1997, 9, 440.
(e/dm3 mol~1 cm~1)]: 292 (21 000), 313 (21 000), 427 (31 000),
453 (31 000), 486 (26 000). Anal calc. (found) for C
C 78.67 (77.71), H 4.95 (4.80), N 7.65 (7.51%).
H
N O :
24 18
2 2
(a) G. Lenoble, P. G. Lacroix, J. C. Daran, S. Di Bella and K.
Nakatani, Inorg. Chem., 1998, 37, 2158; (b) P. G. Lacroix, S. Di
Bella and I. Ledoux, Chem. Mater., 1996, 8, 541; (c) W. Chiang,
D. Vanengen and M. E. Thompson, Polyhedron, 1996, 15, 2369.
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Zyss, Organometallics, 1998, 17, 1750.
4. o-Phenylenediamine (100 mg, 1 mmol) and (280 mg, 1
mmol) of tetraketone 2 were stirred in reÑuxing ethanol (50
ml) for 30 min. After cooling, the yellow precipitate was Ðl-
tered o† and recrystallized from dichloromethane (210 mg,
5
6
(a) A. Hilton, T. Renouard, O. Maury, H. Le Bozec, I. Ledoux
and J. Zyss, Chem. Commun., 1999, 2521; (b) T. Renouard, H. Le
Bozec, S. Brasselet, I. Ledoux and J. Zyss, Chem. Commun., 1999,
871; (c) M. Bourgault, K. Baum, H. Le Bozec, G. Pucetti, I.
Ledoux and J. Zyss, New J. Chem., 1998, 22, 517; (d) C. Dhenaut,
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Bozec, Nature (L ondon), 1995, 374, 339.
The reaction was Ðrst described by (a) I. L. Finar, J. Chem. Soc.,
1958, 4094 but the condensation product obtained in hot acetic
acid was described as a diazepine rather than a quinoxaline on
the basis of IR data; (b) J. F. Wolfe, D. E. Portlock and D. J.
Feuerbach, J. Org. Chem., 1974, 39, 2006.
60%). Mp \ 220È222 ¡C. 1H NMR (CDCl ): d 14.6 (br s, 2H,
3
H ), 7.0 (s, 4H, H ), 5.6 (s, 2H, CHÈCO), 2.3 (s, 4H,
1
5h7
CH ÈtBu), 1.0 (s, 18H, Me). 13C NMR (CDCl ): d 198.8
2
3
(C2O), 144.5 (C , C ), 126.1 (C , C ), 124.5 (C , C ), 116.5
4a 8a
(C , C ), 92.0 (CHÈCO), 56.1 (CH ÈtBu), 31.9 [CH ÈC(CH )],
2
3
6
7
5
8
2
2
3
30.1 [CH ÈC(CH )]. 15N NMR (CDCl ): d [254.3 (d,
7
2
3
3
1J
\ 87 Hz). IR (KBr cm~1): 1595 (s, l ). UV [CH Cl ,
NhH
CJO
2 2
j/nm (e/dm3 mol~1 cm~1)]: 389 (19 000), 409 (22 000),
435 (21 000), 465 (18 000). Anal. calc. (found) for
C
H
N O É H O: C 71.13 (71.87), H 8.41 (8.37), N 7.54
22 30
2
2
2
8
9
E. M. Kaiser and J. D. Petty, J. Organomet. Chem., 1976, 108,
139.
B. L. Lee and T. Yamamoto, Macromolecules, 1999, 32, 1375.
(7.63%).
10 (a) A. Katoh, T. Yoshida and J. Ohkanda, Heterocycles, 2000, 52,
911; (b) A. R. Ahmad, L. K. Mehta and J. Parrick, J. Chem. Soc.,
Perkin T rans. 1, 1996, 2443; (c) A. R. Ahmad, L. K. Mehta and J.
Parrick, T etrahedron, 1995, 51, 12899.
5. (R,R)-1,2-diaminocyclohexane (114 mg, 1 mmol) and the
tetraketone 1 (280 mg, 1 mmol) were stirred in reÑuxing
ethanol (50 ml) for 30 min. After cooling to room temperature,
the yellow precipitate was Ðltered and recrystallized in
dichloromethane (260 mg, 70%). Mp \ 254È256 ¡C. 1H NMR
(CDCl ): d 11.30 (br s, 2H, H ), 8.0È7.8 (m, 4H, m-H), 7.6È7.4
11 R. W. Saalfrank, N. Low, B. Demleitner, D. Stalke and M. Tei-
chert, Chem. Eur. J., 1998, 4, 1305.
12 For the condensation of 1,6-p-chlorobenzyl-1,3,4,6,tetraone on o-
phenylendiamine, see: M. Poje and K. Balenovic, J. Heterocycl.
Chem., 1979, 16, 417.
13 (a) W. von Philipsborn and R. Muller, Angew. Chem., Int. Ed.
Engl., 1986, 25, 383; (b) J. Mason, Chem. Rev., 1981, 81, 205.
14 Acid traces always present in untreated chloroform catalyzed the
H/D exchange reaction and complete exchange in both positions
H(1) and H(9) is observe in a few minutes.8 PuriÐcation of chlo-
roform by simple Ðltration over alumina is required to observe
the described phenomena.
15 For a statistical study of NÈHÉÉÉO2C bonds, see: (a) R. Taylor,
O. Kennard and W. Versichel, J. Am. Chem. Soc., 1983, 105,
5761; (b) For a recent example, see: F. H. Beijer, R. P. Sijbsma,
H. Kooijman, A. K. Spek and E. W. Meijer, J. Am. Chem. Soc.,
1998, 120, 6761.
16 Small negative solvatochromism (212 cm~1 on the most red-
shifted band) is observed when changing from toluene to
dichloromethane. This result suggests the n-p* origin of this
band.
3
1
(m, 6H, o-H, p-H), 6.3 (s, 2H, CHÈCO), 3.20 (m, 2H, H
4a, 8a
1.8 (m, 4H, H ), 1.2 (m, 4H, H ). 13C NMR (CDCl ): d
5, 8 6, 7
190.2 (C2O), 153.2 (C , C ), 140.0 (i-C), 131.4 (p-C), 128.5 (m-
),
3
2
3
C), 127.1 (o-C), 89.0 (CHÈCO), 54.7 (C , C ), 29.7 (C , C ),
4a 8a
5
NhH
8
24.0 (C , C ). 15N NMR (CDCl ): d [270.7 (d, 1J
\ 94
6
7
3
Hz). IR (KBr/cm~1): 1600 (s, l ). UV [CH Cl , j/nm,
CJO
2 2
(e/dm3 mol~1 cm~1)]: 425 (28 000), 370 (20 000). Anal. calc.
(found) for C
N 7.17 (6.54%).
H
N O É H O: C 73.82 (73.07), H 6.65 (6.08),
24 24
2
2
2
Acknowledgements
The authors are grateful to the ““Action Integree France-
MarocÏÏ No. 160/SM/98 for Ðnancial support and to Dr S.
Sinbandith (CRMPO, University of Rennes) for the NMR
studies.
17 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
18 C. K. Fair, MolEN, An Interactive Intelligent System for Crystal
Structure Analysis, User Manual, EnrafÈNonius, Delft, The
Netherlands, 1990.
19 A. L. Spek, HELENA. Program for the Handling of CAD4-
Di†ractometer Output SHELX(S/L), Utrecht University,
Utrecht, The Netherlands, 1997.
20 A. Altomare, M. C. Burla, M. Camalli, G. Carscarano, C. Giaco-
vazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori and R.
Spagna, J. Appl. Crystallogr., 1998, 31, 74.
21 G. M. Sheldrick, SHELX97. Program for the ReÐnement of
Crystal Structures, University of Gottigen, Gottingen, Germany,
1997.
22 A. L. Spek, PLATON, A Multipurpose Crystallographic Tool,
Utrecht University, Utrecht, The Netherlands, 1998.
References and notes
1
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(a) B. J. Coe, S. Houbrechts, I. Asselberghs and A. Persoons,
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2
New J. Chem., 2001, 25, 391È395
395