Signaling of chloramine: A fluorescent probe for trichloroisocyanuric acid based on deoximation of a coumarin oxime

Article abstract of DOI:10.1016/j.tetlet.2014.10.132

A new chloramine signaling probe, based on a coumarin oxime, was developed. The coumarin oxime 1 exhibited efficient off-on type fluorescent signaling behavior toward trichloroisocyanuric acid (TCCA) in an aqueous acetonitrile solution. The signaling is due to the TCCA-assisted transformation of the oxime function to its carbonyl analogue. The presence of common metal ions and anions did not interfere with the TCCA signaling of this probe. Probe 1 was found to be useful for the sensitive determination of the concentration of the practical oxidant TCCA in an aqueous environment, with a detection limit of 7.58 × 10^-7 M.

Full text of DOI:10.1016/j.tetlet.2014.10.132

Tetrahedron Letters 55 (2014) 7047–7050
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Signaling of chloramine: a fluorescent probe for trichloroisocyanuric
acid based on deoximation of a coumarin oxime
⇑
Seul Ki Lee, Myung Gil Choi, Suk-Kyu Chang
Department of Chemistry, Chung-Ang University, Seoul 156-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A new chloramine signaling probe, based on a coumarin oxime, was developed. The coumarin oxime 1
exhibited efficient off–on type fluorescent signaling behavior toward trichloroisocyanuric acid (TCCA)
in an aqueous acetonitrile solution. The signaling is due to the TCCA-assisted transformation of the oxime
function to its carbonyl analogue. The presence of common metal ions and anions did not interfere with
the TCCA signaling of this probe. Probe 1 was found to be useful for the sensitive determination of the
Received 21 August 2014
Revised 22 October 2014
Accepted 27 October 2014
Available online 31 October 2014
concentration of the practical oxidant TCCA in an aqueous environment, with a detection limit of
Keywords:
Chloramine
Trichloroisocyanuric acid
Fluorescent probe
Coumarin oxime
Deoximation
À7
7.58 Â 10 M.
Ó 2014 Elsevier Ltd. All rights reserved.
Chloramines are inorganic or organic nitrogen compounds,
which contain one or more chlorine atoms attached to the nitrogen
atoms. They are inexpensive, stable in aqueous solution, and
environmentally benign oxidants that are broadly used both in
daily life and for industrial applications.1 Because they are easier
to handle than chlorine gas or metal hypochlorites, they are widely
used in the purification of drinking water and as sanitizing agents
N,N-diethyl-p-phenylenediamine (DPD) is one of the more
commonly used techniques for the analysis of chlorine in water.9
However, we could not find any literature on the use of fluorescent
signaling approach for analyzing chloramines.
Reaction-based probes have attracted much research interest
recently due to their specificity and cumulative signaling
1
0
advantages. In particular, there are noticeable achievements in
the signaling or visualizing of biologically important reactive
oxygen or nitrogen species, such as hydrogen peroxide, peroxyni-
2
in swimming pools. Chloramines are also commonly used in
3
,4
synthetic reactions. The usefulness of chloramines arises from
the fact that they behave as a source of both a halonium ion and
various nitrogen-containing species.5 As a result, these reagents
can react with a wide range of functional groups, leading to a num-
ber of molecular transformations such as chlorination of alkenes
and aromatics, oxidation of ethers and alcohols, epoxidation of
alkenes, and nitrosation of amines.6 In organic syntheses, hypo-
chlorites could also be substituted with N-chloramines, especially
when anhydrous conditions are necessary. Among the organic
chloramines, in particular, trichloroisocyanuric acid (TCCA) and
sodium N-chloro-p-toluenesulfonamide (chloramine-T) have
become increasingly important.
1
1
trite, and hypochlorous acid. However, optical probes for practical
oxidants such as peracids and chloramines are not reported often in
spite of their importance in a variety of industrial applications. We
have developed a series of reaction-based optical probes for the
determination of these practical oxidants, using desulfurization of
thioamides for peracetic acid and oxone, thiocoumarin for mCPBA,
as well as oxidative hydrolysis of phenolic acetate for perborates.12
Protection of carbonyl functions by converting them to oximes
1
3
is one of the most useful tactics in organic synthesis. In this case,
deprotection of oximes to form carbonyls could be effected by a
1
4
variety of oxidation and reduction reactions. Recently, a broad
Chloramines have commonly been analyzed by titrimetry using
iodide, amperometry, and flow-injection analysis.7 Specific
analytical methods such as HPLC, GC, and mass spectrometry have
range of N-halosulfonamides has been reported for the regenera-
15
tion of carbonyl compounds from oximes (Scheme 1).
We
devised a new reaction-based probe for the convenient determina-
tion of practical oxidant chloramines by using the oxime-carbonyl
transformation. The oxime derivative of ethoxycoumarin exhibited
a pronouncedly sensitive fluorogenic signaling behavior toward
the industrially important chloramine TCCA in aqueous acetoni-
trile solution.
8
also been used for these oxidants. The colorimetric method using
⇑
Corresponding author. Tel.: +82 2 820 5199; fax: +82 2 825 4736.
E-mail address: skchang@cau.ac.kr (S.-K. Chang).
http://dx.doi.org/10.1016/j.tetlet.2014.10.132
040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
0

7
048
S. K. Lee et al. / Tetrahedron Letters 55 (2014) 7047–7050
20
18
16
14
1
1
+ TCCA
+ HOCl
Scheme 1. Deprotection of oximes with N-halosulfonamides.
12
1
0
8
6
4
2
0
Probe 1 for chloramine signaling was prepared by a two-step
reaction from 7-ethoxycoumarin (Scheme 2). Thionation of 7-eth-
1
only,
1 + H O , O -, TBHP,
1 + PAA
2
2
2
oxycoumarin with Lawesson’s reagent yielded thiocoumarin 2 (tol-
BO
3
-, percarbonate
1
2c
uene, 65%).
Reaction of thiocoumarin 2 with hydroxylamine
OH, EtOH,
0%). We tried to prepare more chromogenic coumarin deriva-
produced the desired 7-ethoxycoumarin oxime 1 (NH
2
350
400
450
Wavelength (nm)
500
550
1
6
8
tives having dialkylamino (coumarin 334) or benzothiazole sub-
stituents (coumarin 6), but the stability of the parent dyes under
the oxidative stress of the chloramine under consideration here
was inadequate.
Figure 1. Fluorescence spectra of 1 in the presence of practical oxidants. Inset:
fluorescence image of 1 in the absence and presence of TCCA. The spectra were
recorded in a mixture of CH
3
CN and phosphate buffer solution (pH 8.0, 10 mM),
À6
À5
(
1:9, v/v). [1] = 5.0 Â 10 M, [oxidant] = 5.0 Â 10 M. kex = 325 nm. PAA: peracetic
Probe 1 exhibited an absorption band at 325 nm in 90% aqueous
acetonitrile solution, phosphate buffered at pH 8.0, which is similar
to that of 7-ethoxycoumarin (kmax = 323 nm). On the other hand,
probe 1 showed weak fluorescence due to the quenching effect
of the oxime group under the same conditions.17 We surveyed
the preliminary fluorescent signaling behavior of probe 1 toward
a series of practical oxidants, such as hydrogen peroxide, tert-butyl
hydrogen peroxide (TBHP), perborate, peracetic acid (PAA), and the
chloramine TCCA (Fig. 1 and Fig. S1, Supplementary data). Among
the tested oxidants, only TCCA and HOCl showed prominent
signaling under the measurement conditions. The fluorescence
acid, TBHP: tert-butyl hydrogen peroxide.
Scheme 3. Signaling of chloramine by probe 1.
0
enhancements I/I observed at 392 nm were 105-fold for TCCA
and 102-fold for HOCl. The colorimetric or fluorescent signaling
of HOCl, due to the deoximation of oxime derivatives of various
substances (such as BODIPY and phenanthroimidazole dye) to their
carbonyl groups, have already been reported.18 Peracetic acid also
showed a considerable response (I/I
oxidants, such as hydrogen peroxide, TBHP, perborate, percarbon-
ate, and superoxide, showed negligible signaling: the ratio I/I
0
= 11.8). Other commonly used
0
varied between 0.79 for TBHP and 1.69 for perborate. The signifi-
cant response toward HOCl in the selective TCCA signaling by 1
might not be a practical problem, because TCCA is generally used
as a single component oxidant in many applications. We tried to
obtain a signaling condition that could differentiate TCCA from
HOCl, but failed. That can be ascribed to the fact that the TCCA in
1
9
aqueous solution in situ generates HOCl as an active oxidant.
Figure 2. Partial 1H NMR spectra of 1 in the absence and presence of TCCA, and
-ethoxycoumarin. [1] = [7-ethoxycoumarin] = 15 mM and [TCCA] = 6 mM in CDCl
Middle spectrum (1 + TCCA) was obtained after purification by column
Meanwhile, the UV–vis absorption behavior of compound 1 was
not useful for the determination of TCCA, because TCCA did not
induce any diagnostic changes in the spectral properties of 1
7
3
.
chromatography.
(Fig. S2, Supplementary data).
The signaling is due to the oxidative transformation of the
2
0
oxime moiety of 1 to a coumarin carbonyl function (Scheme 3).
160
1
The H NMR spectrum of the purified reaction product, obtained
by treatment of 1 with TCCA, was identical to those of ethoxy-
coumarin (Fig. 2). In the 1H NMR spectrum, the resonances for
20
140
120
100
1
+ TCCA
1
5
0
5
0
1
1 only,
1 + common
80
60
40
20
0
metal ions
350
400
450
500
550
Wavelength (nm)
Figure 3. Fluorescence intensity ratio I/I
presence of various metal ions. [1] = 5.0 Â 10 M, [TCCA] = [M ] = 5.0 Â 10 M. In
a mixture of CH CN and phosphate buffer solution (pH 8.0, 10 mM), (1:9, v/v).
ex = 325 nm.
0
at 392 nm of 1 + TCCA and 1 in the
À6
n+
À5
3
Scheme 2. Preparation of probe 1 for chloramine signaling.
k

S. K. Lee et al. / Tetrahedron Letters 55 (2014) 7047–7050
7049
1
1
1
1
60
40
20
00
To obtain an insight into the practical applicability of this
method, the selective signaling of 1 for TCCA was tested under
competitive conditions. In the presence of commonly encountered
metal ions or anions as background species, the TCCA-selective sig-
naling of 1 was found not to be significantly affected (Fig. 4 for
metal ions and Fig. S5, Supplementary data, for anions). This
observation implies that the coumarin oxime 1 could be used as
a TCCA-selective fluorescent probe in the presence of commonly
encountered metal ions and anions in environmental samples.
The stability of the probe was satisfactory between pH 3 and 10,
and the fluorescence was not affected by pH changes. However,
the TCCA signaling increased as the pH of the solution increased,
but was not significantly changed in alkaline media at pH 8–10
8
6
4
2
0
0
0
0
0
(
1
Fig. S6, Supplementary data). In addition, the TCCA signaling by
was fast and completed within 1 min after sample preparation
+
1 + TCCA
Figure 4. Changes in the fluorescence intensity ratio of 1-TCCA system at 392 nm in
(Fig. S7, Supplementary data).
À6
n+
the presence of metal ions as background. [1] = 5.0 Â 10 M, [TCCA] = [M ] = 5.0 -
The quantitative signaling behavior of 1 was studied by
fluorescence titration with TCCA (Fig. 5). As the concentration of
TCCA increased, the emission intensity at 392 nm increased stea-
dily, as shown in the inset of Figure 5. Plotting of the fluorescence
intensity at 392 nm, as a function of TCCA concentration, showed a
À5
 10 M. In a mixture of CH
3
CN and phosphate buffer solution (pH 8.0, 10 mM),
(
1:9, v/v). kex = 325 nm.
20
18
16
14
12
10
8
6
4
2
0
À6
20
15
10
5
0
useful calibration plot up to 3.0 Â 10 M. From the concentration-
dependent signaling behavior, the detection limit of 1 for the
[TCCA]
determination of TCCA, in 90% aqueous acetonitrile, was estimated
À7
21
to be 7.58 Â 10 M.
Finally, application of the probe 1 as a practically useful test
2
2
strip was attempted (Fig. 6). Probe 1 in acetonitrile solution
(5 mM) was applied on a TLC plate (Silica gel). Addition of TCCA
solution of varying concentrations on the plate resulted in the
enhancements of blue fluorescence. The obtained intensity profile
implies that the probe could readily signal TCCA down to 10
range without any sophisticated instruments.
0
0.2
0.4
0.6
0.8
Equiv. of TCCA
À5
M
In summary, a new trichloroisocyanuric acid (TCCA)-selective
probe, based on the coumarin oxime, was developed. The oxime
derivative of ethoxycoumarin showed prominent fluorescence-on
type signaling behavior toward industrially important chloramine
TCCA, due to the oxime-carbonyl transformation. The selective
signaling was also achieved in the presence of common metal ions
and anions as background species. In addition, the TCCA-selective
signaling was possible in the presence of other commonly used
practical oxidants such as peracids and peroxides.
3
50
400
450
500
550
Wavelength (nm)
Figure 5. Fluorescence titration of 1 with TCCA. Inset shows the changes in
À6
fluorescence intensity at 392 nm of 1 as a function of TCCA. [1] = 5.0 Â 10 M,
À6
[
TCCA] = from 0 to 4.0 Â 10 M. In 90% aqueous acetonitrile (pH 8.0 phosphate
buffer, 10 mM). kex = 325 nm.
the 3-H and 4-H protons that are adjacent to the oxime function of
1
(7.10 and 6.84 ppm) were shifted significantly downfield (7.63
Acknowledgment
and 7.36 ppm). The transformation of the oxime of 1 to its carbonyl
form was also confirmed by
Supplementary data).
The coumarin oxime 1 showed TCCA-selective signaling behav-
ior in the presence of common metal ions. For the alkali, alkaline
earth, and representative transition metal ions that were studied,
13
C NMR spectroscopy (Fig. S3,
This research was supported by the Korea Research Foundation
of the Korean Government (NRF-2013R1A1A2A10004615).
Supplementary data
0
the fluorescence intensity ratio I/I at 392 nm varied in a narrow
range between 0.93 for Mg2 and 1.44 for Ag ions (Fig. 3). Oxime
1
+
+
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.10.
132. These data include MOL files and InChiKeys of the most
important compounds described in this article.
also showed negligible responses toward commonly encountered
anions. The ratio I/I
0
fluctuated in a limited range between 1.02 for
anions (Fig. S4, Supplementary data).
À
À
F
and 4.43 for HCO
3
23
Figure 6. Pictures of changes in (a) fluorescence and (b) blue channel intensities of 1 in the presence of varying amounts of TCCA under a UV lamp. Intensities of blue
À4
channel were obtained by recording blue channel values (0–255) of image in Adobe Photoshop. [TCCA] = from 0 to 1.0 Â 10 M in acetonitrile solution.

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