The effect of substituents on the response of 3,4-dihydro-3-(2-oxo-2- phenylethylidene)-quinoxalin-2(1H)-one derivatives toward binding of Cu 2+

Article abstract of DOI:10.1016/j.tet.2012.06.071

A new series of fluorogenic chelating reagents based on phenylethylidene-3,4-dihydro-1H-quinoxalin-2-one with different substituents (attached either to the quinoxaline-2-one (3) or phenyl ring (4)) have been investigated to examine the effect of the substituent (nature/position) on the spectral properties and response toward Cu²⁺ in the presence of other metal cations, in ethanol. It was found that all of the examined ligands exhibit a pronounced response to Cu²⁺ addition resulting in a red-shift in the UV-vis spectra and a strong quenching in the fluorescence spectra. Among the ligands examined, 3a exhibits the highest selectivity toward various metal cations. In general it appears that the best response selectivity of these ligands toward Cu²⁺ ion is obtained by either EDG in 3 or EWG in 4. For example, the fluorescence intensity of 3a (with OMe substituent) increases to about three times that of the unsubstituted derivative.

Full text of DOI:10.1016/j.tet.2012.06.071

Tetrahedron 68 (2012) 7450e7455
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
The effect of substituents on the response
of 3,4-dihydro-3-(2-oxo-2-phenylethylidene)-quinoxalin-2(1H)-one derivatives
toward binding of Cu2þ
,
b
b
,
a,
*
Efrat Korin a, Beny Cohen a , Yue-Xia Bai , Cheng-Chu Zeng , James Y. Becker
*
*
a Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel
b College of Life Science & Bioengineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 March 2012
Received in revised form 30 May 2012
Accepted 18 June 2012
Available online 26 June 2012
A new series of fluorogenic chelating reagents based on phenylethylidene-3,4-dihydro-1H-quinoxalin-2-
one with different substituents (attached either to the quinoxaline-2-one (3) or phenyl ring (4)) have
been investigated to examine the effect of the substituent (nature/position) on the spectral properties
and response toward Cu2þ in the presence of other metal cations, in ethanol. It was found that all of the
examined ligands exhibit a pronounced response to Cu2þ addition resulting in a red-shift in the UVevis
spectra and a strong quenching in the fluorescence spectra. Among the ligands examined, 3a exhibits the
highest selectivity toward various metal cations. In general it appears that the best response selectivity of
these ligands toward Cu2þ ion is obtained by either EDG in 3 or EWG in 4. For example, the fluorescence
intensity of 3a (with OMe substituent) increases to about three times that of the unsubstituted
derivative.
Keywords:
ꢀ
Quinoxalinone derivatives
Cu2þ recognition
Fluorescence
Chelating reagent
Chemosensors
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
considered of current major importance: Alzheimer, Parkinson,
Amyotrophic lateral sclerosis (ALS), Huntington, Prion, cancer, di-
The design of chelating ligands capable of selective complexa-
tion with a specific metal ion has been an important goal due to
their potential biological and pharmacological application. Over the
years, numerous chelating reagents for different metal ions have
been designed for various applications such as therapeutic reagents
for the treatment of metal intoxication,1e3 selective metal extrac-
tants,1,4 and chemosensors.5e8
abetes, cardiovascular diseases, and atherosclerosis.25,26
Previously we reported a series of organic fluorescent com-
pounds based on para- and meta-sulfonamido-3,4-dihydro-3-(2-
oxo-2-phenylethylidene)-quinoxalin-2(1H)-one derivatives (1) as
potential chemosensors for cations of biological interest such as,
Ca2þ, Mg2þ, Cu2þ, and Zn2þ, in both ethanol and acetonitrile.27 In
acetonitrile, derivatives 1 act as dosimeters as their oxidative
decomposition by Cu2þ resulted in a significant color change. It
should be noted that this particular behavior was exclusive for
Cu2þ allowing its detection even in the presence of other metal
cations. In ethanol, derivatives 1 show a selective binding toward
Cu2þ as was expressed by a red-shift in the UVevis absorption
spectra and a specific fluorescence quenching behavior. In this
In recent years, considerable effort has been devoted to the de-
velopment of selective chemosensors for essential trace metal such
as, Zn2þ 9e11 Cu2þ 12e15 Ni2þ 16e18 and Co2þ 18,19
, , . All of these metal
,
ions are important for certain biological processes but can become
toxic when consumed in uncontrolled amounts. For instance, ele-
vated levels of Zn2þ, Cu2þ or Co2þ are associated with neurode-
generative diseases (such as, Alzheimer and Parkinson).20e22 In
addition, elevated levels of Cu2þ, Co2þ or Ni2þ ions are suspected to
participate in the generation of cancer.22e24
Among the above-mentioned metal ions, chemosensors
designed for the detection and measurements of Cu2þ are of great
interest due to its involvement in various diseases that are
respect, no significant response to Zn2þ, Mg2þ, Co2þ, Ni2þ, Mn2þ
,
Ca2þ, and Agþ was observed. The changes in the UVevis and
fluorescence spectra that were observed in the presence of Cu2þ
were attributed to the reversible formation of a metaleligand
complex.
The role of the sulfonamide moiety and the presence of two
potential binding sites in 1 have not been clear in the above-
mentioned results. Therefore, the present work has been focused
on the synthesis and chemical modifications of 1 to simpler de-
* Corresponding authors. Tel.: þ972 8 646 1639; fax: þ972-8-6472943 (B.C.); tel.:
þ86 10 67396211; fax: þ86 10 67392001 (C.-C.Z.); tel.: þ972 8 6461197; fax: þ972 8
6472943 (J.Y.B.); e-mail addresses: bcohen@bgu.ac.il (B. Cohen), zengcc@bjut.
edu.cn (C.-C. Zeng), becker@bgu.ac.il (J.Y. Becker).
rivatives such as, the unsubstituted building block
2 and
substituted derivatives of 3 and 4 (Fig. 1).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.06.071

E. Korin et al. / Tetrahedron 68 (2012) 7450e7455
7451
O
S N
O
X
O
O
H
HN
O
HN
H
NH
H
NH
1
O
2
X =H, Cl, CH3, OCH3, NO2
O
X
O
HN
O
Y
HN
O
4a: Y = OCH3
4b: Y = Cl
4c: Y = NO2
3a: X = OCH3
3b: X = Cl
3c: X = CF3
H
NH
H
NH
4a-c
3a-c
Fig. 1. Chemical structures of 3,4-dihydro-1H-quinoxalin-2-ones 1e4.
Our main goal has been to examine the effect of different sub-
stituents on the quinoxalin-2-one moiety 3, and at the para-posi-
tion of the phenyl ring in 4, on their spectral properties and
response toward Cu2þ in ethanol, and in the presence of other
metal cations.
2. Results and discussion
2.1. Synthesis of 2e4
The unsubstituted 3,4-dihydro-2-quinoxalin-2-one derivative
(2) and the substituted series 3 and 4 were synthesized directly by
condensation of the corresponding diketo acid and o-phenyl-
enediamine. However, when the reaction was performed in water,
as the synthesis of 1, a complex mixture of products, including the
starting diketo acid, was obtained. Thereby, a mixture of diketo acid
and o-phenylenediamine was refluxed in DMF/H2O to give the
desired 3,4-dihydro-2-quinoxalin-2-one derivatives 2, 3, and 4 in
good to excellent yields (Scheme 1). High quality samples could be
obtained after recrystallization from DMF.
Fig. 2. Absorption spectra of 24 mMof2derivatives in ethanol (I) 2, 3a, 3b, 3c, (II) 2, 4a, 4b, 4c.
Table 1
Absorption and fluorescence properties of free 2 and its substituted derivatives (3
and 4) and the Dlmax of the red-shift observed in the UVevis spectra due to addition
of 100 equiv of either Cu2þ, Ni2þ or Zn2þ, in ethanol
R2
H2N
H2N
R2
O
OH
COOH
a,b
O HN
H
‘Host’ lmax (nm)
lem (nm) Frel Dlmax
Dlmax
Dlmax
‘host’þCu2þ ‘host’þNi2þ ‘host’þZn2þ
NH
DMF/H2O, reflux
H
R1
O
R1
2
w393, 415, 438 471, 491 1.00 25
w410, 434, 458 503 3.16 17
w393, 415, 437 471, 490 1.19 29
w383, 405,426 459, 480 0.53 34
w393, 417, 438 471, 491 1.40 30
32
d
36
42
34
d
9
29
4
2, 3, 4
3a
3b
3c
4a
4b
4c
31
37
30
d
6
Scheme 1. Synthesis of 3,4-dihydro-1H-quinoxalin-2-ones 2, 3, and 4.
393, 418, 441
441
476, 491 0.27 26
530 0.07 22
2.2. UVevis and fluorescence spectra of 3,4-dihydro-1H-
quinoxalin-2-one derivatives 2e4
a
Maximum absorption wavelength of free host (24 mM) at room temperature in
ethanol; (lmax ꢂ0.5 nm).
b
Underlined wavelengths are the peaks with the strongest absorption.
2.2.1. UVevis measurements. The absorption spectra of the build-
ing block 2 and its substituted derivatives (3 and 4) in ethanol so-
lutions were investigated and their spectra are shown in Fig. 2(I)
and (II), respectively. The absorption spectrum of 2 in ethanol
displays a shoulder band and two intense absorption bands cen-
groups (EWG) (e.g., 3c with, CF3) shift the spectrum to shorter
wavelengths (w10 nm hypschromic blue-shift) relative to the
unsubstituted compound 2, whereas derivatives of 3 substituted
with electron-donating groups (EDG) (e.g., 3a with OCH3) shift the
spectrum to longer wavelengths (w19 nm red-shift) in comparison
to 2. On the other hand, examining the effect of the substituent in
derivatives of 4 on their spectra reveals that all types of sub-
stituents (either EWG or EDG) caused a red-shift compared to the
unsubstituted derivative (2), and in particular, the NO2 substituted
derivative 4c, which caused the most significant red-shift of
w26 nm (Table 1). Furthermore, in the derivatives of 4, the nature
of the substituent also seems to have an effect on the intensity of
the absorption, where the derivative substituted with Cl had the
tered at about 393 nm
(
3
¼2.2ꢀ104 Mꢁ1 cmꢁ1), 417 nm
(
3
¼3.4ꢀ104 Mꢁ1 cmꢁ1), and 438 nm (
3
¼3.0ꢀ104 Mꢁ1 cmꢁ1),28 re-
spectively. These intense absorptions in the visible region can be
assigned to a
pep transition (K band) that occurs in this conju-
*
gated system. Similar absorption spectral patterns were also ob-
served for all other derivatives of 3 and 4, except for 4c, which
showed only one broad absorption band around 441 nm. It can be
seen from the spectral data listed in Table 1 that the absorption
values are influenced by the substituent.
In the case of derivatives of 3, it appears that the nature of the
substituent affects mainly the spectral band position with no sig-
nificant change in the absorbance intensity. As can be seen from
Fig. 2(I) and Table 1, derivatives of 3 with electron-withdrawing
lowest absorbance intensity (
comparison to 4a and 4c derivatives (with
3
¼0.38ꢀ104 Mꢁ1 cmꢁ1 at 418 nm) in
3
w3ꢀ104 Mꢁ1 cmꢁ1).

7452
E. Korin et al. / Tetrahedron 68 (2012) 7450e7455
2.2.2. fluorescence measurements. The effect of substituents in the 3
and 4 series on the fluorescence spectra in ethanol was also exam-
2.3. Spectral response of 3,4-dihydro-1H-quinoxalin-2-one
derivatives 3 and 4 toward different metal ions
ined. The emission spectra of unsubstituted derivative
2
(
lex¼415 nm) have two bands around 471 and 491 nm (Fig. 3). As
2.3.1. UVevis measurements. The ability of 3 and 4 derivatives to
can be seen from Fig. 3 and Table 1 the fluorescence spectral prop-
erties of substituted 2 are also affected by the nature of the sub-
stituent. In both 3 and 4 series, the highest fluorescence intensity
was observed for the derivative that was substituted with a strong
EDG (OCH3), while the derivative that was substituted with strong
EWG (such as CF3 and NO2) had the lowest fluorescence intensity.
form complexes in ethanol with different metal ions such as Zn2þ
,
Mg2þ, Co2þ, Ni2þ, Mn2þ, Ca2þ, Agþ or Cu2þ was first studied by using
UVevis spectroscopy. The spectra obtained for 2 in the absence and
presence of the above cations showed that addition of 100 equiv of
either Cu2þ, Zn2þ or Ni2þ induced a spectral response. The addition
of Zn2þ or Ni2þ resulted in the appearance of new peaks around 465
and 470 nm, respectively. However the addition of Cu2þ caused the
spectra of 2 to shift to longer wavelengths (w29 nm red-shift), (see
Fig. 4(I) and Table 1). It is important to note that the new band
around 463 nm created by the shift after Cu2þ addition was 6 and 4
times more intense than the new peaks that appeared on the ad-
dition of Ni2þ or Zn2þ. Therefore among the cations examined, the
most prominent response of the spectrum of 2 in ethanol was cre-
ated by the addition of Cu2þ ions. It should be noted that spectral
changes observed upon addition of Cu2þ, Ni2þ or Zn2þ were fully
reversible, since the addition of either EDTA or HClO4 (see
Supplementary data, Figs. S1eS3) led to an instant return of the
original absorption of the free complexing agent. Thus the UVevis
spectral response observed between 2 and Cu2þ/Ni2þ/Zn2þ could be
assigned to the formation of a reversible complex in which the or-
ganic ligand undergoes deprotonation upon complexation.
Fig. 3. (I) The emission spectra of 24 mM 2 (ex 415 nm), 3a (ex 434 nm), 3b
(ex 415 nm), and 3c (ex 405 nm) in ethanol. Inset: the relative fluorescence intensity of
each of the investigated compounds compared to 2. From left to right 2 (ex 415 nm), 3a
(ex 434 nm), 3b (ex 415 nm) and 3c (ex 405 nm) in ethanol. (II) The emission spectra of
24 mM 2 (ex 415 nm), 4a (ex 417 nm), 4b (ex 418 nm), and 4c (ex 441 nm) in ethanol.
Inset: the relative fluorescence intensity of each of the investigated compounds
compared to 2. From left to right 2 (ex 415 nm), 4a (ex 417 nm), 4b (ex 418 nm), and 4c
(ex 441 nm) in ethanol.
Examining the effect of substituent on the position of lem reveals
that different trends appear for the 3 and 4 derivatives. When 3
substituted with EWG (CF3) caused a slight blue-shift compared to 2,
while EDG caused a red-shift. On the other hand the emission spectra
of all 4 derivatives exhibited red-shifts compared to 2 and the red-
shift increased upon changing the substituent from EDG to EWG.
Therefore it appears that the spectral properties of 2 analogs can
be tuned by modifying the nature of the substituent that is attached
on either the quinoxaline-2-one or phenyl ring. This information
may be used to tailor-design the spectral properties of the 2 analogs
in terms of absorbance and emission maxima including fluores-
cence and/or absorbance intensity.
Fig. 4. UVevis absorption spectra of the host in the absence and presence of 100 equiv
of metal ions in ethanol: (I) 24 mM of 2, (II) 24 mM of 3a.
Furthermore, competition experiments of added Cu2þ to 2 so-
lutions in the presence of one another metal ions (Mg2þ, Mn2þ
Co2þ, Ni2þ, Ca2þ, Zn2þ or Agþ) indicated that the recognition of
,

E. Korin et al. / Tetrahedron 68 (2012) 7450e7455
7453
Cu2þ by 2 was not significantly affected by either of these co-
existing cations, because no change was observed in the red-
shifted absorption band (except for a slight change in its in-
tensity, see Supplementary data, Fig. S4).
paramagnetic cations are known to cause a fluorescent quenching
while diamagnetic transition metal ions (Zn2þ or Agþ) can cause
fluorescence enhancement.29e31 It should be noted that the rec-
ognition of either Zn2þ, Ni2þ, Cu2þ or Agþ was found to be re-
versible as the addition of HClO4 reverses the fluorescence spectra
back to that of free 2. Acid addition should cause the formed
complexes to dissociate back to the free ligand and the metal ion,
which in turn expressed by the retrieval of the fluorescence signal
of the free 2 (see Supplementary data, Fig. S5).
The effect of co-existing cation on the response of 2 toward
copper was also investigated by measuring the emission spectra
collected with Cu2þ in the presence 100 equiv of either of the fol-
lowing cations: Mg2þ, Mn2þ, Co2þ, Ni2þ, Ca2þ, Zn2þ or Agþ. As can
be seen from Fig. 6, the presence of Cu2þ and another metal ion had
no significant interference with the quenching created by Cu2þ
ions. Therefore the effect of Cu2þ ions prevails on that of other co-
existing cations.
A similar spectral response to that of 2 was also observed for 3b,
3c, 4a absorbance spectra in the presence of 100 equiv of either:
Zn2þ, Mg2þ, Co2þ, Ni2þ, Mn2þ, Ca2þ or Agþ. Unlike the latter men-
tioned compounds, the response of 3a toward these cations was
more selective, and only the addition of 100 equiv of Cu2þ ions
caused a significant change in the shape and position of the spectra
(Fig. 4(II)). Comparing the response of the different derivatives of 3
to Cu2þ suggests that the nature of substituent on the quinoxaline-
2-one effects the selective response toward Cu2þ, as switching the
substituent from OCH3 to CF3 increases the effect of other cations
(such as Zn2þ, Ni2þ, and Agþ) on the spectra (in respect of in-
tensities red-shift position and shape of the spectra).
Moreover, comparing the effect of substituent on the response
toward Cu2þ addition reveal that for compounds from 3 series, an
EWG substituent increased the red-shift while an EDG substituent
decreased the red-shift observed upon Cu2þ addition.
Examining the response of all compounds in the 4 series to Cu2þ
addition reveals that for all of them the spectra of the free organic
compound was red-shifted upon treatment with Cu2þ (See Table 1).
Unfortunately the absorbance spectra of 4b and 4c from the 4 series
present some difficulty in the assessment of the effect of different
metal ions on their absorbance spectra as 4b has very low absor-
bance while the spectrum of 4c overlaps with the free cation (such
as, Ni2þ or Co2þ). Therefore substantial interference of free cations
absorbencies is expected for these two derivatives.
In contrast to the behavior of 4b and 4c, the response of 4a to-
ward metal ions could be readily examined. The spectra of 4a were
affected by the addition of either Cu2þ, Ni2þ or Zn2þ in a similar
manner as 2. This shows that switching the location of OCH3 sub-
stituent from the quinoxaline-2-one to the benzyl ring decreases
the selectivity of the response.
Fig. 6. Relative fluorescence intensity (F/F0) of 2 in the presence of 100 equiv Cu2þ and
100 equiv of one more metal ion (either Cu2þ, Zn2þ, Mg2þ, Co2þ, Ni2þ, Mn2þ, Ca2þ or
Agþ) in ethanol.
2.3.2. Fluorescence measurements. Fluorescence spectroscopic
measurements were also employed to study the cation recognition
properties of 2 and its derivatives toward different metal ions. As
shown in Fig. 5, the emission spectra of 2 (lex¼463 nm) in the
presence of 100 equiv of metal ions such Mn2þ, Ca2þ, and Mg2þ did
not cause any change in either position or intensity, while the ad-
dition of either Zn2þ or Agþ results in a red-shift accompanied by
significant increase in fluorescence intensity (that was doubled or
quadrupled, respectively, from that of the free 2). On the other
hand, the addition of cations such as Co2þ, Ni2þ, and Cu2þ results in
a strong fluorescence quenching. These changes in fluorescence
intensity can be attributed to the nature of the cation where
The fluorescence emission intensity ratios (F/F0) of different 3
and 4 derivatives in the presence and absence of 100 equiv of either
Zn2þ, Mg2þ, Co2þ, Ni2þ, Mn2þ, Ca2þ, Agþ or Cu2þ are presented in
Fig. 7(I) and (II), respectively. As can be seen from Fig. 7(I) in the 3
series the two derivatives substituted with an EWG (3b and 3c)
behave in a similar manner toward these metal ions. For the de-
rivative 3c, substituted with a strong EWG (CF3) each one of the
metal ions that was added appears to affect the emission spectra
(even Mg2þ, Mn2þ, Ca2þ induced a decrease in intensity). On the
other hand 3a with EDG, OCH3 substituent, exhibits a more selec-
tive recognition where only the addition of 100 equiv of Co2þ, Ni2þ
or Cu2þ induced changes on the fluorescence intensity ratio. Nev-
ertheless it is important to note that the fluorescent enhancement
observed for 3c upon addition of 100 equiv of Zn2þ or Agþ was the
highest compared to the other complexing agents. Therefore
among the 3 series, derivatives substituted with EWG groups ap-
pear to enhance the response toward the different metal ions while
EDG decrease their effect resulting in a more selective compound
for Cu2þ. Quite the opposite effect was observed for derivatives in
the 4 series (Fig. 7(II)), where the fluorescence response was less
selective to Cu2þ for compounds substituted with EDG then those
with EWG.
Based on the UVevis and more specifically the fluorescence
spectra, it can be concluded that the response of 2 toward metal
ions is influenced both by the nature of the substituent (EDG/EWG)
and the position of substituent on the ring. This observation
Fig. 5. Fluorescent emission spectra of 2 (24 m ,
M) in the absence and presence of Cu2þ
Zn2þ, Mg2þ, Co2þ, Ni2þ, Mn2þ, Ca2þ or Agþ (100 equiv) in ethanol (lex¼463 nm).

7454
E. Korin et al. / Tetrahedron 68 (2012) 7450e7455
indifferent to the presence of Mg2þ and Ca2þ ions but appear to
bind metal ions with neurological importance (Zn2þ, Cu2þ, Co2þ
,
and Ni2þ). Therefore, they could be good building blocks for the
design of a new ligand for chelating therapy such as Clioquinol (a
ligand that is able to bind the same metal ions used clinically for the
treatment of Alzheimer’s disease and Huntington’s disease).
4. Experimental section
4.1. General
Absorption spectra were acquired with an Agilent (HP) 8453
UVeVis Diode Array spectrophotometer (single beam) with
wavelength accuracy <ꢂ0.5 nm and photometric accuracy
<ꢂ0.005 AU. Fluorescence excitation spectra were acquired with
a JASCO spectrofluorometer (Model FP-6500, Jasco Intern. Co., Ja-
pan) equipped with a 150 W xenon lamp. Both excitation and
emission slit widths were 3 nm. HRMS were performed by two
instruments: GCT-MS Micromass (UK) for EI-TOF, and LTQ Orbitrap
XL mass spectrophotometer (Thermo Scientific, Germany and USA).
4.2. Reagents and general procedure
The solutions of the metal ions were prepared from Cu2þ, Zn2þ
,
Mn2þ, Naþ as perchlorate salts and Mg2þ, Co2þ, Ni2þ, Ca2þ, Agþ as
nitrate salts (Caution: perchlorate salts are potentially explosive).
All of the reagents were purchased from commercial suppliers
(Aldrich; Merck). For testing the effect of pH, 70% HClO4 (Mal-
linckrodt chemicals) was used and base solution was made from
NaOH pellets (Frutarom). The solvent ethanol was obtained from
BioLab, Israel. All of the chemicals mentioned in this work were of
analytical grade and used without further purification.
4.2.1. A general procedure for the synthesis of 3,4-dihydroquinoxalin-
2-ones 2, 3, and 4. A mixture of diketo acid (1 mmol), o-phenyl-
enediamine (1.2 mmol), and 60 mL mixed solvent (vDMF/vH2O¼1:1)
was refluxed under stirring for 2e4 h while monitoring the
reaction by TLC. After the completion of the reaction, the mixture
was cooled to room temperature and a large amount of water
was added. The desired products were collected by filtration,
washed with water and then with ether to remove residual di-
amine. High quality of products was obtained after recrystallization
from DMF.
Fig. 7. Relative fluorescence intensities of 2, 3, and 4 derivatives (24 mM) in the ab-
sence and presence of 100 equiv Cu2þ, Zn2þ, Mg2þ, Co2þ, Ni2þ, Mn2þ, Ca2þ or Agþ in
ethanol (I) 2 (ex 463 nm), 3a (ex 475 nm), 3b (ex 466 nm), and 3c (ex 460 nm), (II) 2
(ex 463 nm), 4a (ex 468 nm), 4b (ex 467 nm), and 4c (ex 466 nm).
suggests that the selective response of 2 to Cu2þ can be optimized
by simply tuning the EDG ability attached to the quinoxaline-2-one
ring or the EWG ability attached to the phenyl ring.
4.2.1.1. (Z)-3-(2-Oxo-2-phenylethylidene)-3,4-dihydroquinoxalin-
2-one (2). Yield: 75%; mp: 277e278 ꢃC; 1H NMR (400 MHz, DMSO-
3. Summary and conclusions
d6):
d
¼6.82 (s, 1H, CH]C), 7.12e7.15 (m, 3H, ArquinoxaloneeH), 7.53
(s, 1H, ArquinoxaloneeH), 7.50e7.60 (m, 3H, AreH), 7.97e7.99 (m, 2H,
In conclusion, 3,4-dihydro-2-quinoxalin-2-one derivatives hav-
ing EDG and EWG on either the quinoxaline-2-one (3) or phenyl
ring (4) were synthesized and their effect on their spectral prop-
erties and response toward various metal ions was examined in
ethanol. All of the investigated ligands derivatives posses the ability
to reversibly bind to Cu2þ causing a red-shift in their absorption
spectra and ‘ONeOFF’ fluorescence response. Using their fluores-
cence spectra it was shown that the recognition selectivity to Cu2þ
ion can be finely tuned by the position and nature of the sub-
stituent, where either EDG in 3 or EWG in 4 enhanced the response
selectivity toward Cu2þ. Furthermore, since the fluorescence in-
tensity of these compounds was increased by using EDG (on either
3 or 4) it seems that for chemosensors applications it will be better
to design derivatives where the selectivity would be adjusted via
EDG substituent on the quinoxaline-2-one. Here 3a had the highest
fluorescence intensity and exhibits the best selectivity to Cu2þ, as
its fluorescence response to different metal ions was mainly af-
fected by Cu2þ, Ni2, and Co2þ. Finally, compounds 2, 4a, and 3b were
AreH), 12.04 (s, 1H, NeH), 13.67 (s, 1H, NeH); 13C NMR (100 MHz,
DMSO-d6):
d
¼89.6, 115.8, 117.1, 124.2, 124.5, 124.6, 127.2, 127.5,
129.2, 1, 139.1, 146.1, 156.2, 188.9; IR (KBr):
n
¼3437, 1628, 1376,
1264 cmꢁ1
;
ESI-MS: m/z, 262.5 (Mþꢁ1). HRMS (EI-TOF) for
C16H12N2O2: found (M)¼264.0902 (calcd 264.0899).
4.2.1.2. (Z)-3-(2-Oxo-2-phenylethylidene)-6-methoxy-3,4-dihydro-
quinoxalin-2-one and (Z)-3-(2-oxo-2-phenylethylidene)-7-methoxy-
3,4-dihydroquinoxalin-2-one (3a). Yield: 71%; mp: 225e227 ꢃC; 1H
NMR (400 MHz, DMSO-d6):
6.72e7.99 (m, 9H, AreH), 11.90 and 12.03 (s, 1H, NeH), 13.06 and
14.01 (s, 1H, NH); 13C NMR (100 MHz, DMSO-d6):
d
¼3.77 and 3.78 (s, 3H, OCH3),
d
¼55.9, 56.1, 88.7,
89.8, 100.4, 101.5, 110.9, 111.8, 116.7, 119.0, 120.9, 125.3, 127.2, 127.4,
128.7, 128.7, 129.1, 129.2, 132.0, 132.4, 138.9, 139.2, 146.1, 146.3,
155.6, 156.2, 156.3, 156.9, 186.2, 188.8; IR (KBr):
n
¼3437, 1630, 1535,
1465, 1375, 1269 cmꢁ1; ESI-MS: m/z, 292.6 (Mþꢁ1). HRMS for
C17H14N2O3: found (MþH)þ¼295.10788 (calcd 295.10772); found
(MþNa)þ¼317.08978 (calcd 317.08966).

E. Korin et al. / Tetrahedron 68 (2012) 7450e7455
7455
4.2.1.3. (Z)-3-(2-Oxo-2-phenylethylidene)-6-chloro-3,4-dihydro-
quinoxalin-2-one and (Z)-3-(2-oxo-2-phenylethylidene)-7-chloro-
3,4-dihydroquinoxalin-2-one (3b). Yield: 69%; mp: 281e285 ꢃC; 1H
samples were prepared by adding 80
m
L of one of the mentioned
metal stock to 4 mL of a host stock solution. For the competition
experiment test solutions were prepared by adding 80 L of the
competing cations and 80
L of Cu2þ to 4 mL of the 2 stock solution.
m
NMR (400 MHz, DMSO-d6):
7.12e7.99 (m, 8H, AreH), 12.07 (s, 1H, NeH), 13.38 and 13.53 (s, 1H,
NH); 13C NMR (100 MHz, DMSO-d6):
d
¼6.83 and 6.85 (s, 1H, CH]C),
m
d
¼90.1, 90.5, 115.1, 116.7, 117.1,
4.4. Evaluation of the effect of EDTA/acid ratio on the
reversibility of the spectral response of 2 and its complexes
118.6, 123.7, 123.8, 123.9, 125.9, 126.1, 127.5, 127.6, 127.8, 127.9,
128.4, 129.2, 132.5, 132.6, 138.9, 145.4, 145.5, 156.1, 156.2, 188.9,
¼3436, 1690, 1620, 1540, 1462, 1400, 1368 cmꢁ1
;
Stock solutions of 2.4ꢀ10ꢁ5 M of 2 in ethanol and 1.2ꢀ10ꢁ1 M of
189.2; IR (KBr):
n
ESI-MS: m/z, 296.6 (Mþꢁ1). HRMS for C16H11ClN2O2: found
EDTA in tri-distilled water were prepared. To test the effect of EDTA
(MþH)þ¼299.05801
(calcd
299.05818);
found
on copper/nickel/zinc complexes with 2, 80 mL of EDTA was added
(MþNa)þ¼321.03964 (calcd 321.04013).
to 4 mL of 2 and 80
To test the acidity effect on these complexes 80 mL of 70% HClO4 was
m
L of one of these metallic cations (1.2ꢀ10ꢁ1 M).
4.2.1.4. (Z)-3-(2-Oxo-2-phenylethylidene)-6-(trifluoromethyl)-
3,4-dihydroquinoxalin-2-one and (Z)-3-(2-oxo-2-phenylethylidene)-
7-(trifluoromethyl)-3,4-dihydroquinoxalin-2-one (3c). Yield: 73%;
added to a solution that contain 4 mL of 2 and 80 mL of one of these
metallic cation (1.2ꢀ10ꢁ1 M) (Caution: HClO4 is a strong oxidant).
mp: 285e287 ꢃC; 1H NMR (400 MHz, DMSO-d6):
d
¼6.88 and 6.90
(s, 1H, CH]C), 7.26e8.10 (m, 8H, AreH), 12.14 and 12.24 (s, 1H,
NeH), 13.39 (s, 1H, NH); 13C NMR (100 MHz, DMSO-d6):
¼90.7,
91.2, 112.4, 116.3, 117.6, 120.7, 127.3, 127.6, 127.7, 127.9, 129.3, 132.8,
Acknowledgements
d
C.-C.Z. thanks the National Basic Research Program of China (No.
2009CB930200), Beijing Natural Science Foundation (No. 7112008)
and Beijing City Education Committee (KM201010005009). The
authors are thankful to Mrs. E. Solomon for technical assistance.
J.Y.B. is grateful to Mrs. Irene Evens for supporting this research.
138.9, 145.2, 156.3, 156.6, 189.3, 189.9; IR (KBr):
n
¼3438, 1627, 1465,
1350, 1328, 1233 cmꢁ1; ESI-MS: m/z, 330.6 (Mþꢁ1). HRMS for
C17H11F3N2O2: found (MþH)þ¼333.08453 (calcd 333.08454);
found (MþNa)þ¼355.06647 (calcd 355.06648).
Supplementary data
4.2.1.5. (Z)-3-[2-Oxo-2-(4-methoxyphenylethylidene)]-3,4-dihydro-
quinoxalin-2-one (4a). Yield: 70%; mp: 238e240 ꢃC; 1H NMR
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.06.071.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
(400 MHz, DMSO-d6):
d
¼3.85 (s, 3H, OCH3), 6.79 (s, 1H, CH]C), 7.06
(d, 2H, J¼8.8 Hz, AreH), 7.11e7.14 (m, 3H, ArquinoxaloneeH), 7.46e7.48
(m, 1H, ArquinoxaloneeH), 7.97 (d, 2H, J¼8.8 Hz, AreH), 11.98 (s, 1H,
NeH), 13.59 (s, 1H, NH); 13C NMR (100 MHz, DMSO-d6):
d¼55.9, 89.4,
114.5, 115.8, 116.7, 124.1, 124.7, 127.0, 129.6, 131.8, 145.5, 156.3, 162.8,
References and notes
188.2; IR (KBr):
n
¼3437, 1626, 1416, 1368, 1260 cmꢁ1; ESI-MS: m/z,
292.6 (Mþꢁ1). HRMS (EI-TOF) for C17H14N2O3: found (M)¼294.1009
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4.2.1.6. (Z)-3-[2-Oxo-2-(4-chlorophenylethylidene)]-3,4-dihydro-
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(400 MHz, DMSO-d6):
d
¼6.80 (s, 1H, CH]C), 7.16 (s, 3H,
ArquinoxaloneeH), 7.55e7.71 (m, 1H, ArquinoxaloneeH), 7.58 (d, 1H,
J¼8.4 Hz, AreH), 8.01 (d, 2H, J¼8.4 Hz, AreH), 12.09 (s, 1H, NeH),
13.65 (s, 1H, NH); 13C NMR (100 MHz, DMSO-d6):
d
¼89.4, 115.9,
117.2, 124.2, 124.7, 127.3, 129.1, 129.3, 132.0, 137.2, 137.8, 146.4, 156.1,
167.4, 187.4; IR (KBr):
n
¼3433, 1630, 1413 cmꢁ1; ESI-MS: m/z, 297.2
(Mþꢁ1). HRMS (EI-TOF) for C16H11N2O2Cl: found (M)¼298.0513
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4.2.1.7. (Z)-3-[2-Oxo-2-(4-nitrophenylethylidene)]-3,4-
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dihydroquinoxalin-2-one (4c). Yield: 74%; mp: 312e313 ꢃC; 1H NMR
(400 MHz, DMSO-d6):
d
¼6.86 (s, 1H, CH]C), 7.17e7.18 (m, 3H,
ArquinoxaloneeH), 7.58 (d, 1H, J¼7.6 Hz, AreH), 8.21 (d, 2H, J¼8.4 Hz,
AreH), 8.34 (d, 2H, J¼8.4 Hz, AreH), 12.16 (s, 1H, NeH), 13.81 (s, 1H,
NH); 13C NMR (100 MHz, DMSO-d6):
d
¼89.4, 115.9, 117.6, 124.2,
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22. Mates, J. M.; Segura, J. A.; Alonso, F. J.; Marquez, J. Free Radical Biol. Med. 2010,
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49, 1328e1341.
(EI-TOF) for C16H11N3O4: found (M)¼309.0753 (calcd 309.0750).
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Stock solutions of ligands 2, 3a, 3b, 3c, 4a, 4b, and 4c
(2.4ꢀ10ꢁ5 M) in ethanol were prepared. Their UVevis and fluo-
rescence spectra were collected. To evaluate selectivity to specific
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,
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Co2þ, Ni2þ, Mn2þ, Ca2þ or Agþ were prepared in ethanol. Test