Santanu Bhattacharya et al.
FULL PAPER
same solution for approximately 4 h, a completely new set
of peaks emerged, which again supported the formation of
new species.
tigated by various means. The time-course studies in the
presence of an excess amount of CNꢀ showed that the reac-
tion follows an apparent first-order kinetic pathway. The ac-
tivation entropy calculated by using the Eyring–Polanyi
equation suggested the associative nature of the reaction.
1
A comparison between the H NMR spectra of the unoxi-
dized diester precursor of 1, that is, 3, and that of the oxi-
dized diester 2 (Scheme 1) revealed that CNꢀ ion formed an
adduct with 1 under these conditions. Each proton of 3
showed 1H NMR spectroscopic signals upfield relative to
that of the oxidized 2. It further indicates that the added cy-
anide ion attacks the C=C bond that maintains the conjuga-
tion of the 4-substituted aromatic ring with the bis-indolyl
moiety (Scheme 2). Thus when the reaction is over, it com-
pletely cuts off the conjugation, which causes a pronounced
change in color from red to colorless.
1
Furthermore, H NMR spectroscopic titration and 13C NMR
spectroscopic studies confirmed the mode of reaction be-
tween probe 1 and CNꢀ ion. This is further supported by the
DEPT-135 spectrum and mass spectral analysis. Taken to-
gether, a plausible mechanism of the reaction is presented
that is shown to operate by means of a Michael-type adduct
formation under ambient conditions of pH and temperature
in water.
Experimental Section
General
All solvents and reagents were purified and dried by means of the usual
methods. All starting materials were obtained from the best known com-
mercial suppliers and used as received. IR spectra were recorded using
1
a Perkin–Elmer FTIR spectrum BX. H and 13C NMR spectra were mea-
sured using either a 300/75 MHz or a 400/100 MHz spectrometer. HRMS
analyses were performed using an electron spray ionization (ESI) mass
spectrometer (Q-TOF YA263 high-resolution instrument). UV/Vis ab-
sorption spectra were obtained using a Shimadzu UV-2100 spectropho-
tometer. Fluorescence spectra were recorded using a Fluorolog Horiba
Jobin Yvon spectrofluorometer. The stock solution of the compound
1 was made in DMSO and the final concentration of DMSO in all the
studies were less than 0.5%
Scheme 2. Plausible mechanism of the cyanide adduct formation with 1.
The actual mechanism of the reaction was probed by
13C NMR spectroscopy, DEPT-135 (DEPT=distortionless
enhancement by polarization transfer), and mass spectrome-
try. After recording the 13C NMR spectra following the CNꢀ
addition reaction, the peak that corresponded to the forma-
tion of a tetrahedral carbon appeared at d=43.8 ppm (see
the Supporting Information). The peak at d=43.8 ppm was
also not observed in the DEPT spectrum, thereby confirm-
ing the reaction of cyanide to that of the a,b-unsaturated
double bond (see the Supporting Information).
Synthesis of 5
Ethyl bromoacetate (4.67 g, 28.0 mmol) was added slowly to a mixture of
aniline (0.931 g, 10.0 mmol) and K2HPO4 (5.22 g, 30.0 mmol) in CH3CN
(10 mL) at 08C and then allowed to stir at RT for 30 min. The reaction
mixture was then heated to reflux for 24 h.[26] After evaporation of the
excess amount of solvent under vacuum, water (5 mL) was added to it. A
crude material was obtained after separation and extraction with EtOAc
followed by evaporation of the organic solvent. The compound 5 was ob-
tained after purification of the crude oil by column chromatography over
silica gel (eluted at 5% EtOAc/n-hexane). Yield 99%, 2.62 g (pale
The formation of the 1-CNꢀ adduct was also evident from
the detection of a mass spectral peak at 477.155 (m/z calcd
477.156), which might be assigned due to the formation of
the cyanide addition product of 1 (see the Supporting Infor-
mation).
1
yellow oil). H NMR (300 MHz, CDCl3): d=1.29 (t, J=5.4 Hz, 6H), 4.18
(s, 4H), 4.19–4.24 (q, 4H), 6.63 (d, J=8.1 Hz, 2H), 6.80 (t, J=7.35 Hz,
1H), 7.21–7.26 ppm (m, 2H); IR (neat): n˜2981.2, 1745.2, 1733.6, 1600.6,
1508.0, 1386.5, 1172.5, 1024.0, 748.2 cmꢀ1; MS (ESI): m/z: 266 [M+H]+,
288 [M+Na]+; HRMS: m/z: calcd for C14H19NO4Na [M+Na]+: 288.1212;
found: 288.1213.
Synthesis of 4
Conclusion
POCl3 (3.54 g, 23.0 mmol) was added dropwise over a period of 10 min to
cold N,N-dimethylformamide (DMF; 5.6 mL, 72.4 mmol), and then the
mixture was allowed to mix under stirring for 30 min.[27] Compound 5
(5.52 g, 20.8 mmol) was added to this slowly under cold conditions and
the solution was heated at 908C for 2 h. After completion of the reaction,
the reaction mixture was cooled and poured over ice-cold water. This
mixture was then neutralized to pH 7 by addition of solid sodium acetate.
It was then extracted with EtOAc, washed successively with water and
brine, and dried over anhydrous Na2SO4. Crude oily material was ob-
tained after evaporation of the solvent. Compound 4 was obtained from
this after purification by silica gel column (eluent: 20% EtOAc/n-
hexane) as a white solid. Yield 92%, 5.6 g. M.p. 57–598C; 1H NMR
(300 MHz, CDCl3): d=1.26 (t, J=7.0 Hz, 6H), 4.17–4.25 (m, 8H), 6.63
(d, J=8.4 Hz, 2H), 7.72 (d, J=8.4 Hz, 2H), 9.76 ppm (s, 1H); 13C NMR
(75 MHz, CDCl3): d=14.1, 53.3, 61.5, 111.7, 127.3, 131.9, 152.5, 169.6,
190.4 ppm; IR (neat): n˜ =3478.9, 2919.2, 2829.0, 2749.7, 1742.3, 1672.0,
In conclusion, we have synthesized a new colorimetric probe
for the effective detection of cyanide ions in water. The de-
tection of CNꢀ by using probe 1 was found to be totally free
of interference from any other anions. The probe gives an
immediate response to the cyanide ion both by visible color
change as well as by spectroscopic means (UV/Vis and emis-
sion spectra). The cyanide ion can be detected below ppm
level by using fluorescence emission spectroscopy. In con-
trast to other reported reaction-based probes for cyanide,
the present system does not require any preoptimized condi-
tions such as mixing of solvents, phase-transfer reagents, or
high temperature. The mechanism of the reaction was inves-
2810
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 2805 – 2812