Colorimetric Cyanide Chemosensor Based on 1′,3,3′,4-Tetrahydrospiro[chromene-2,2′-indole]

Article abstract of DOI:10.1002/open.201402117

A new class of chemosensors based on the 1′,3,3′,4-tetrahydrospiro[chromene-2,2′-indole] ring system, which detects cyanide with high specificity, is described. These chemosensors show a distinct color change when treated with cyanide in acetonitrile solution buffered with sodium phosphate, and this procedure is not affected by the presence of other common anions. The chemisensors exhibit high sensitivity to low concentrations of cyanide, meeting the European Union water quality control criterion of sensitivity below 0.05 mg L^-1, and show a very fast response within tens of seconds. The mechanism for detection is rationalized by the nucleophilic substitution of the phenolic oxygen atom at the indoline C-2 atom by the cyanide anion to form a stable indolylnitrile adduct and to generate the colored 4-nitrophenolate chromophore. These chemosensors can be synthesized by a simple procedure from commercially available starting materials.

Full text of DOI:10.1002/open.201402117

DOI: 10.1002/open.201402117
Colorimetric Cyanide Chemosensor Based on 1’,3,3’,4-
Tetrahydrospiro[chromene-2,2’-indole]
[a, b]
[b]
[a, b]
[b]
ˇˇ
Vytas Martynaitis, Vilija Krisciuniene,
ˇ
˙
Sonata Krikstolaityte, and
˙
˙
˙
Migle Dagiliene,
¯
[a, b]
ˇ
ˇ
Algirdas Sackus*
A new class of chemosensors based on the 1’,3,3’,4-tetrahydro-
spiro[chromene-2,2’-indole] ring system, which detects cyanide
with high specificity, is described. These chemosensors show
a distinct color change when treated with cyanide in acetoni-
trile solution buffered with sodium phosphate, and this proce-
dure is not affected by the presence of other common anions.
The chemisensors exhibit high sensitivity to low concentrations
of cyanide, meeting the European Union water quality control
criterion of sensitivity below 0.05 mgLÀ1, and show a very fast
response within tens of seconds. The mechanism for detection
is rationalized by the nucleophilic substitution of the phenolic
oxygen atom at the indoline C-2 atom by the cyanide anion to
form a stable indolylnitrile adduct and to generate the colored
4-nitrophenolate chromophore. These chemosensors can be
synthesized by a simple procedure from commercially available
starting materials.
Introduction
Cyanide (CNÀ) is a hazardous chemical that leads directly to
death in humans, even at low concentrations.[1,2] Chemical sub-
stances containing cyanide occur naturally[3,4] or can be pre-
pared artificially for use in various areas of industry, including
mining,[5] plastics manufacturing,[6] and electroplating.[7] Cassa-
va (Manihot esculenta) plants, which are consumed by millions
of people as a source of food,[8,9] contain cyanogenic gluco-
sides, such as linamarin.[10] Linamarin can be digested by lina-
marase, a naturally occurring enzyme, or hydrolyzed chemically
to liberate highly toxic hydrogen cyanide.[11] Therefore, the
presence of cyanide from raw carbohydrates containing cyano-
genic glucosides in food products has to be tightly con-
trolled.[12] However, much more concerning are the cyanides
used in industries, such as mining, where there is a high risk of
contamination of drinking water sources, especially in the pro-
cess of gold separation.[13] It is well documented that over the
last few decades, there have been many cases of transporta-
tion accidents and industrial dam failures, where large quanti-
ties of cyanide have entered river systems severely impacting
aquatic life.[5] In accordance with the the World Health Organi-
zation (WHO) recommendations, water containing more than
0.07 mgLÀ1 (27”10À7 m) cyanide should not be used as a do-
mestic supply,[14] while the European Union (EU) Drinking
Water Directive has set the maximum contaminant level for cy-
anide in drinking water at 0.05 mgLÀ1 (19”10À7 m).[15] These
low limits require extremely sensitive methods for cyanide de-
tection.
In order to control the presence of cyanide in food, feed-
stock, drinking water, and the environment, numerous studies
have focused on the development of methods for its detec-
tion, including the use of chemosensors in which a change in
color or fluorescence is monitored.[16] Many previous designs
for cyanide sensors relied on ditopic binding with crown ether
conjugates, hydrogen-bonding interactions, or copper–cyanide
affinity.[17,18] In recent years, there has been increasing interest
in the development of chemosensors based on the nucleophil-
ic addition reactions of the cyanide anion to an electrophilic
substrate to yield the covalent CÀCN bond and the formation
of a stable colored species. For example, the treatment of col-
orless [1,3]oxazine derivatives[19,20] with cyanide anions yields
adducts that absorb at approximately 430 nm, which is in the
visual part of the spectrum.[21,22] 2,8-Dinitroindolo[2,1-b]
[1,3]benzoxazine 1 was commercialized by Sigma–Aldrich as
a sensitive chemosensor for the detection of cyanide through
the formation of a stable colored adduct (2), possessing the
4-nitrophenolate chromophore (Scheme 1).[23]
ˇ
ˇ
˙
ˇˇ
˙
[a] M. Dagiliene, V. Krisciuniene, Prof. A. Sackus
¯
Institute of Synthetic Chemistry, Kaunas University of Technology
Radvile˙nu˛ pl. 19, 50254 Kaunas (Lithuania)
E-mail: algirdas.sackus@ktu.lt
˙
ˇˇ
˙
ˇ
˙
[b] M. Dagiliene, Prof. V. Martynaitis, V. Krisciu¯niene, Dr. S. Krikstolaityte,
It was recently shown that 6-nitro-1’,3’-dihydrospiro[chro-
mene-2,2’-indole] 3, which is well known for its photochromic
properties,[24] also behaves as a selective and sensitive cyanide
receptor in aqueous media.[25] This compound is converted to
the open-ring form upon UV irradiation, and when cyanide
anions are present in a solution adduct 4 is produced, resulting
in the formation of the colored 4-nitrophenolate chromophore
causing a new absorption band to appear. However, adduct 4
ˇ
ˇ
Prof. A. Sackus
Department of Organic Chemistry, Kaunas University of Technology
Radvile
˙nu˛ pl. 19, 50254 Kaunas (Lithuania)
ꢀ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
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followed by work-up of the intermediate salts (7a–h) with
base. The preparation of compounds 8a–c,f,g and their charac-
terization was previously described,[27] while compounds
8d,e,h are new 1’,3,3’,4-tetrahydrospiro[chromene-2,2’-indole]
derivatives. The preparation of the starting material, 1,2,3,3-tet-
ramethyl-5-nitroindolium iodide 5e, was described else-
where.[28]
Scheme 1. Mechanism for cyanide detection by the [1,3]oxazine-based che-
1’,3,3’,4-Tetrahydrospiro[chromene-2,2’-indoles] 8a–h are
chiral compounds with a single center of asymmetry. The
single-crystal X-ray analysis of compound 8a confirmed that
the molecule consists of indole and benzopyran moieties that
reside in near-perpendicular planes connected through the
chiral spiro C-2’ atom (Figure 1).[29]
mosensor.
Scheme 2. Mechanism for cyanide detection by the 1’,3’-dihydrospiro[chro-
mene-2,2’-indole]-based chemosensor.
is not stable and reverts back to its spirocyclic ground state
upon visible light irradiation (Scheme 2).
This study is focused on the design and investigation of
a new class of highly sensitive, selective and stable chemosen-
sors based on the 1’,3,3’,4-tetrahydrospiro[chromene-2,2’-
indole] ring system.[26] Recently, 1’,3,3’,4-tetrahydrospiro[chro-
mene-2,2’-indoles], which are notable for the single C3ÀC4
bond in their pyran ring, have been described as ultrafast
light-driven switches.[27]
Figure 1. ORTEP image of the (S)-enantiomer of compound 8a.
It was shown previously that 1’,3,3’,4-tetrahydrospiro[chro-
mene-2,2’-indole] 8a easily undergoes interconversion of the
(R)- and (S)-enantiomers when dissolved in organic solvent due
to the thermally induced C(2’)ÀO bond cleavage with formation
of the intermediate planar 3H-indolium cation and subsequent
ring-closure via the attack of the phenolate negatively charged
oxygen atom on either face of the aforementioned planar in-
termediates. In sodium-phosphate-buffered acetonitrile solu-
tion, compound 8a exists in an equilibrated mixture with the
open form.[27] The characteristic signal of compounds 8d,e,h in
Results and Discussion
Synthesis
The synthesis strategy for 1’,3,3’,4-tetrahydrospiro[chromene-
2,2’-indoles] 8a–h outlined in Scheme 3 is based on the alkyla-
tion of Fisher’s bases 6a–h with 2-chloromethyl-4-nitrophenol
1
the H NMR spectra taken in deuterated chloroform at room
temperature was a joint signal of 3,3-methyl groups in the
area of 1.24–1.37 ppm, which reflects the coalescence of the
separate singlets of the diastereotopic geminal methyl groups
due to the inversion at the chiral spiro carbon atom similar to
that of known compounds 8a–c,f,g.
Chemosensing mechanism
Steady state absorbance spectra of compounds 8a–h, mea-
sured for solutions in the acetonitrile/phosphate buffer, re-
vealed absorption in the UV region of the electronic spectra
(Table 1). However, when a sodium cyanide solution, buffered
with sodium phosphate (pH 7.6), was added to the aforemen-
tioned solutions of compounds 8a–h (0.1 mm of 8a–h, 1 mm
NaCN in the cell), a new absorption band was observed in the
visible area at approximately 420 nm (Table 1, Figure 2).
The appearance of this band in the visible region can be ra-
tionalized by spirochromene ring opening and formation of
4-nitrophenolate chromophores 9a–h due to the nucleophilic
substitution of the phenolic oxygen by a cyanide group. In this
Scheme 3. Reagents and conditions: a) H2O, Na2CO3, rt, 1 min; b) CH3CN, rt,
6 h; c) H2O, NH4OH, rt, 1 min.
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Table 1. UV-vis spectral data for compounds 8a–h in CH3CN/phosphate
buffer.
Compd
lmax of 8
[nm]
e”103
[dm3 molÀ1 cmÀ1
lmax of 9
[nm]
e”103
[dm3 molÀ1 cmÀ1
]
]
8a
205
243
326
51.4
16.3
14.6
250
282
422
17.2
7.7
29.0
8b
8c
8d
8e
8 f
205
246
326
42.3
14.4
12.5
253
286
420
12.2
9.1
20.7
206
245
313
46.3
15.7
13.6
255
300
422
41.9
23.3
24.8
210
246
326
37.2
10.7
11.2
253
293
422
10.4
4.2
21.8
Scheme 4. Formation of 4-nitrophenolate chromophore 9.
229
321
375
11.8
12.7
19.0
254
422
13.8
30.0
230
330
380
14.1
14.6
22.1
257
424
14.5
32.3
8g
8h
205
244
313
46.1
16.7
12.4
254
298
419
42.3
23.4
23.4
Scheme 5. Formation of 1H-indole-2-carbonitrile 10. Reagents and condi-
tions: a) THF, NaCN, H2O, rt, 1 h.
205
242
312
54.3
17.9
12.9
252
298
418
31.3
17.2
17.6
nitriles 9a–h. In such cases, the ring-opening reaction of 8a–h
becomes irreversible (Scheme 4).
When compound 8a was treated with sodium cyanide in
tetrahydrofuran (THF) containing a small amount of water, the
reaction afforded indole-2-carbonitrile 10 (Scheme 5), the
structure of which was confirmed by spectroscopic methods
and elemental analysis. The IR spectrum of 10 contains an ab-
sorption band at 2222 cmÀ1, characteristic for nitriles.[30] The
heteronuclear multiple bond coherence (HMBC) spectrum re-
vealed three bonds range coupling between the methylidene
protons at 2.15 and 2.30 ppm and the nitrile carbon at
118.49 ppm.[31,32] These protons also interact with the quater-
nary carbon at 76.75 ppm, separated by two bonds, and with
the quaternary carbons at 46.76 and 128.16 ppm, separated by
three bonds. The full assignments presented in Figure 3 were
Figure 2. Absorption (A) spectra of 8a (0.1 mm, 298 K) in a mixture of
CH3CN/phosphate buffer (Na2HPO4/NaH2PO4, 7.5 mm, pH 7.6) (19:1, v/v)
without (spectrum A) and with (spectrum B) NaCN (10 equiv).
case, the limiting step of the reaction is the C(spiro)ÀO covalent
bond cleavage and the formation of intermediates B, which
are in an equilibrated mixture with the starting 1’,3,3’,4-tetra-
hydrospiro[chromene-2,2’-indoles] (8a–h). However, intermedi-
ates A are quickly consumed when the nucleophilic addition
of the cyanide anion to the C-2 carbon occurs to form stable
Figure 3. 1H (blue) and 13C (red) NMR chemical shifts [ppm] for 10 in
[D6]DMSO.
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based on the combined application of standard NMR tech-
niques such as COSY, NOESY, ROESY, APT, DEPT, HSQC and
HMBC spectra.
atom on the formation of adducts 9g,h after the addition of
sodium cyanide to a solution of compounds 8g,h, respectively.
The response time of the reference compound, 2,8-dinitroindo-
lo[2,1-b][1,3]benzoxazine 1, was evaluated in analogous condi-
tions, and the formation of colored adduct 2 proceeds more
slowly in comparison with compounds 8a–d. The full process
takes approximately 20 min (Figure 5), similar to that of com-
pounds 8g,h.
Response time
For various types of chemosensors, the response time is very
important for the practical detection of analytes. In our case,
we observed that the response time depends on the structure
of the compound. After the addition of sodium cyanide to the
solutions of compounds 8a–d, a strong new absorption band
at 422 nm appeared within 1–3 min (Figure 4), while for com-
pounds 8e,f, this process took up to 1.5 h (Figure 5). We con-
Selectivity and sensitivity
To test the selectivity of chemosensor 8a, parallel investiga-
tions were carried out with a series of other anions (FÀ, ClÀ,
À
BrÀ, IÀ, CH3COOÀ, C2O42À, HCO3À, HSO3À, HSO4À, NO2À, NO3
,
SCNÀ, SO32À, SO42À, S2O32À). The addition of excess amounts of
these anions did not result in significant absorbance spectral
changes during UV/vis titration (Figure 6), indicating that this
Figure 4. Absorbance changes at 422 nm for 8a–d (0.1 mm, 298 K) in a mix-
ture of CH3CN/phosphate buffer (Na2HPO4/NaH2PO4, 7.5 mm, pH 7.6) (19:1
v/v) after addition of NaCN (10 equiv).
Figure 6. Absorbance at 422 nm of 8a (0.1 mm, 298 K) in a mixture of
CH3CN/phosphate buffer (Na2HPO4/NaH2PO4, 7.5 mm, pH 7.6) (19:1 v/v) in
the presence of CNÀ or other common anions (10 equiv) or ion mixtures
(10 equiv of CNÀ, HSO4À, ClÀ, BrÀ, and FÀ); A0 is the absorbance of 8a at
422 nm in the absence of CNÀ.
potential chemosensor demonstrates excellent selectivity for
cyanide over other common anions. The high selectivity of
chemosensing can be explained by the fact that only the addi-
tion of a cyanide anion on the indoline nucleus forms a strong
enough covalent bond with the indole C-2 atom to afford
stable nitrile 9, while the addition of the other aforementioned
anions is a reversible process.
To evaluate sensitivity, the calibration curve of cyanide con-
centration versus absorption at 422 nm for 8a was plotted
(Figure 7), showing that this chemosensor is sensitive to rela-
tively low concentrations of CNÀ and meets the European
Union Drinking Water Directive criterion for water quality, with
sensitivity below 0.05 mgLÀ1 (19”10À7 m).
Figure 5. Absorbance changes at 422 nm for 8e–h and at 411 nm for
1 (0.1 mm, 298 K) in a mixture of CH3CN/phosphate buffer (Na2HPO4/
NaH2PO4, 7.5 mm, pH 7.6) (19:1 v/v) after addition of NaCN (10 equiv).
cluded that the presence of a nitro group at C-5 of the indole
nucleus stabilizes the closed form of the molecule and slows
down the formation of the final adduct 9e,f. Both electronic
and steric effects can be considered for explaining the influ-
ence of the allyl and benzyl groups at the indole nitrogen
Conclusion
The derivatives of 1’,3,3’,4-tetrahydrospiro[chromene-2,2’-
indole], synthesized by a simple procedure from commercially
available starting materials, undergo transformations to the
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Reagents and solvents were purchased from Sigma–Aldrich and
used without further purification. Water was doubly distilled. The
anionic solutions for selectivity testing were prepared from sodium
salts of various anions.
1,2,3,3,7-Pentamethyl-3H-indolium iodide (5d): 2,3,3,7-Tetrameth-
yl-3H-indole (0.865 g, 5 mmol) was mixed with iodomethane
(1.42 g, 10 mmol), and the mixture was heated at reflux for 24 h.
The resultant crystalline material was isolated by filtration and re-
crystallized from EtOH to afford 5d as a brownish crystalline solid
1
(1.12 g, 71%): m.p. 248–2508C; H NMR (300 MHz, TFA-d): d=1.65
(s, 6H, 2”3-CH3), 2.86 (s, 6H, 2-CH3, 7-CH3), 4.34 (s, 3H, NCH3),
7.41–7.59 ppm (m, 3H, Ar-H); 13C NMR (75 MHz, TFA-d): d=15.6,
20.8, 24.2 (2”CH3), 40.8, 56.3, 123.3, 129.7, 132.8, 135.9, 142.4,
144.8, 198.1 ppm; IR (KBr): 3024, 2966, 1625 cmÀ1 (C[dbond]N+);
MS (ESI): m/z (%): 188 [MÀI]+ (100); Anal. calcd for C13H18IN:
C 49.54, H 5.76, N 4.44; found: C 49.61, H 5.37, N 4.24.
Figure 7. Absorbance at 422 nm of 8a (0.1 mm, 298 K) in a mixture of
CH3CN/phosphate buffer (Na2HPO4/NaH2PO4, 7.5 mm, pH 7.6) (19:1 v/v) in
the presence of different concentrations of CNÀ, where A0 is the absorbance
of 8a at 422 nm in the absence of CNÀ (SD=0.0002).
[2-(2-Hydroxy-5-nitrophenyl)ethyl-1]-1,3,3,7-tetramethyl-3H-in-
dolium chloride (7d): A stirred solution of iodide 5d (1.575 g,
5 mmol) in distilled water (15 mL) was treated with Na2CO3 (1.06 g,
10 mmol) at rt. The mixture became turbid immediately and was
then extracted with Et2O (3”20 mL). The combined organic phase
was dried over anhydrous Na2SO4, filtered, and concentrated in va-
cuo to afford crude 1,3,3,7-tetramethyl-2-methylidene-2,3-dihydro-
1H-indole (6d) as an oily material.
colored ring-open form possessing the 4-nitrophenolate chro-
mophore when treated with cyanide in acetonitrile solution
buffered with sodium phosphate and show a distinct color
change, which can be measured colorimetrically or detected
by the naked eye. Furthermore, these compounds, in particu-
lar, exhibit a high selectivity and are not affected by the pres-
ence of halides or other anions, which are common interfer-
ents in the conventional sensing schemes for cyanide. This
new chemosensor exhibits high sensitivity to low concentra-
tions of cyanide, meeting the water quality control criterion of
sensitivity below 0.05 mgLÀ1 and also shows a very fast re-
sponse up to tens of seconds.
Crude 6d was dissolved in CH3CN (3 mL), and 2-chloromethyl-4-ni-
trophenol (0.938 g, 5 mmol) was added to the solution. The mix-
ture was stirred at rt for 6 h. The resultant crystalline material was
isolated by filtration, washed with cold CH3CN (1 mL), and dried in
vacuo to afford 7d as an off-white crystalline solid (0.90 g, 48%):
1
m.p. 200–2038C; H NMR (300 MHz, [D6]DMSO): d=1.58 (s, 6H, 2”
3’-CH3), 2.78 (s, 3H, 7-CH3), 2.95–3.01 (m, 2H, CH2), 3.32–3.38 (m,
2H, CH2), 4.31 (s, 3H, NCH3), 7.31 (d, J=8.9 Hz, 1H, Ar-H), 7.39 (d,
J=7.5 Hz, 1H, Ar-H), 7.5 (t, J=7.5 Hz, 1H, Ar-H), 7.67 (d, J=7.5 Hz,
1H, Ar-H), 8.07 (dd, J=8.9, 2.5 Hz, 1H, Ar-H), 8.35 (d, J=2.5 Hz, 1H,
Ar-H), 12.1 ppm (s, 1H, OH); 13C NMR (75 MHz, [D6]DMSO): d=19.1,
21.5 (2”CH3), 26.2, 38.8, 39.0, 53.5, 115.3, 121.1, 124.6, 126.3, 126.6,
127.2, 129.3, 132.5, 139.2, 140.3, 142.8, 162.3, 195.3 ppm; IR (KBr):
3624 (OH), 3050, 2923, 1520 (NO2, asymm), 1345 cmÀ1 (NO2,
symm); MS (ESI): m/z (%): 339 [M]+ (100); Anal. calcd for
C20H23ClN2O3: C 64.08, H 6.18, N 7.47; found: C 64.45, H 6.25,
N 7.71.
Experimental Section
Synthesis procedures
General: Reactions were monitored by thin layer chromatography
(TLC) on precoated silica gel plates (Kieselgel 60F254, Merck). Com-
pounds were visualized with UV light and charring after treatment
with a 1% KMnO4 solution. Column chromatography was per-
formed using silica gel SI 60 (43–60 mm, Merck). Melting points
(m.p.) were determined in open capillary tubes with a Büchi B-540
melting point apparatus and are uncorrected. Infrared (IR) spectra
were recorded on a PerkinElmer Spectrum One spectrometer using
1’,3’,3’,7’-Tetramethyl-6-nitro-1’,3,3’,4-tetrahydrospiro[chromene-
2,2’-indole] (8d): A solution of 7d (1.124 g, 3 mmol) in EtOH
(5 mL) was diluted with water (15 mL). 10% aq NH3 solution (~
0.5 mL) was added dropwise to the solution while stirring until the
mixture became turbid. The separated product was extracted with
Et2O (3”20 mL), and the combined extracts were dried over anhy-
drous Na2SO4, filtered, and concentrated in vacuo. The solid residue
was recrystallized from CH3CN to give 8d as a yellowish crystalline
solid (0.518 g, 51%): m.p. 206–2098C (CH3CN); 1H NMR (300 MHz,
CDCl3): d=1.24 (s, 6H, 2”3’-CH3), 2.31–2.36 (m, 2H, CH2), 2.50 (s,
3H, 7’-CH3), 3.05–3.10 (m, 5H, CH2, NCH3), 6.76–6.82 (m, 2H, 5’-H,
8-H), 6.91–6.96 (m, 2H, 4’-H, 6’-H), 7.99 (dd, J=9.0, 2.7 Hz, 1H, 7-H),
8.04 ppm (d, J=2.7 Hz, 1H, 5-H); 13C NMR (75 MHz, CDCl3): d=
20.1, 23.4, 24.0, 24.1, 25.6, 32.0, 48.8, 104.7, 116.7, 119.1, 119.2,
119.8, 121.5, 124.1, 125.1, 131.7, 137.6, 140.5, 146.5, 161.8 ppm; IR
(KBr): 3084, 2975, 1513 (NO2, asymm), 1330 cmÀ1 (NO2, symm);
HRMS (ESI): m/z [M+H]+ calcd for C20H23N2O3: 339.1703; found:
339.1706; Anal. calcd for C20H22N2O3: C 70.99, H 6.55, N 8.28;
found: C 70.76, H 6.61, N 8.54.
1
KBr pellets. H NMR spectra were recorded at 300 MHz on a Varian
Unity Inova spectrometer and at 700 MHz on a Bruker Avance III
spectrometer. 13C NMR spectra were recorded using the same in-
struments at 75 and 176 MHz, respectively. NMR experiments were
performed at 258C. Chemical shifts (d) are expressed in parts per
million (ppm) downfield relative to tetramethylsilane (TMS), and
coupling constants (J) referring to apparent peak multiplicity are
reported in Hertz (Hz) using standard abbreviations. UV/vis spectra
were determined on a PerkinElmer Lambda 35 spectrometer using
quartz cells with a light path length of 0.5 cm. Mass spectra (MS)
were measured using a Waters ZQ ion spray instrument and the
Shimadzu LCMS-2020. High-resolution ESI-TOF mass spectra
(HRMS) were measured on a Bruker maXis spectrometer. Elemental
analyses were conducted using the Elemental Analyser CE-440
(Exeter Analytical Inc.) in the Microanalytical Laboratory, Depart-
ment of Organic Chemistry, Kaunas University of Technology (Lith-
uania).
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1’,3’,3’-Trimethyl-5’,6-dinitro-1’,3,3’,4-tetrahydrospiro[chromene-
2,2’-indole] (8e): A stirred solution of iodide 5e (1.69 g, 5 mmol) in
distilled water (15 mL) was treated with Na2CO3 (1.06 g, 10 mmol)
at rt. The mixture became turbid immediately and was extracted
with Et2O (3”20 mL). The combined organic phase was dried over
anhydrous Na2SO4, filtered, and concentrated in vacuo to afford
crude intermediate enamine 6e as a brownish oil.
2966; 1516 (NO2, asymm), 1337 cmÀ1 (NO2, symm); HRMS (ESI): m/z
[M+H]+ calcd for C25H25N2O3: 401.1860; found: 401.1863; Anal.
calcd for C25H24N2O3: C 74.98, H 6.04, N 7.0; found: C 74.84, H 5.59,
N 6.65.
2-[2-(2-Hydroxy-5-nitrophenyl)ethyl-1]-1,3,3-trimethyl-2,3-dihy-
dro-1H-indole-2-carbonitrile (10): A stirred solution of 8a (0.15 g,
0.462 mmol) in THF (5 mL) was treated with NaCN (0.045 g,
0.924 mmol) and a drop of distilled water was added. The mixture
was stirred for 1 h at rt. The solvent was evaporated in vacuo, and
the residue was purified by flash chromatography on silica gel
(hexane/acetone, 6:1 v/v) to afford 10 as yellow solid (0.079 g,
Crude 6e was dissolved in CH3CN (3 mL), 2-chloromethyl-4-nitro-
phenol (0.938 g, 5 mmol) was added, and the mixture was stirred
for 6 h at rt. Then, the reaction mixture was poured into water
(50 mL), and 10% aq NH3 solution (~0.75 mL) was added dropwise
while stirring until the mixture became turbid. The separated prod-
uct was extracted with Et2O (3”20 mL), and the combined extracts
were dried over anhydrous Na2SO4, filtered, and concentrated in
vacuo. The residue was subjected to flash chromatography on
silica gel (hexane/acetone, 4:1 v/v), and the obtained solid was re-
crystallized from acetonitrile to give 8e as a yellowish crystalline
solid (0.739 g, 40%): Rf =0.17 (hexane/acetone, 4:1 v/v); m.p. 232–
2348C (CH3CN); 1H NMR (300 MHz, CDCl3): d=1.28 (s, 6H, 2”3’-
CH3), 2.29–2.45 (m, 2H, CH2), 2.96 (s, 3H, NCH3), 3.03–3.17 (m, 2H,
CH2), 6.56 (d, J=8.7 Hz, 1H, 7’-H), 6.79 (d, J=9.0 Hz, 1H, 8-H), 7.91
(d, J=2.4 Hz, 1H, 4’-H), 7.99 (dd, J=9.0, 2.7 Hz, 1H, 7-H), 8.05 (d,
J=2.7 Hz, 1H, 5-H), 8.17 ppm (dd, J=8.7, 2.4 Hz, 1H, 6’-H);
13C NMR (75 MHz, CDCl3): d=21.9, 23.3, 24.1, 25.7, 28.6, 49.3, 104.1,
106.0, 117.0, 118.2, 121.3, 124.5, 125.4, 126.5, 137.9, 140.8, 141.3,
154.0, 161.0 ppm; IR (KBr): 3068, 2967, 1510 (NO2, asymm),
1322 cmÀ1 (NO2, symm); HRMS (ESI): m/z [M+H]+ calcd for
C19H20N3O5: 370.1397; found: 370.1399; Anal. calcd for C19H19N3O5:
C 61.78, H 5.18, N 11.38; found: C 61.63, H 5.19, N 11.60.
1
49%): Rf =0.12 (hexane/acetone, 6:1 v/v); m.p. 176–1788C; H NMR
(700 MHz, [D6]DMSO): d=1.23 (s, 3H), 1.63 (s, 3H), 2.15 (td, J=
13.3, 4.8 Hz, 1H), 2.30 (td, J=13.5, 4.1 Hz, 1H), 2.80 (s, 3H), 2.83
(td, J=13.0, 4.1 Hz, 1H), 2.96 (td, J=13.1, 4.9 Hz, 1H), 6.67 (d, J=
7.8 Hz, 1H), 6.82 (t, J=7.4 Hz, 1H), 7.00 (d, J=8.9 Hz, 1H), 7.12 (d,
J=7.6 Hz, 1H), 7.15 (d, J=7.3 Hz, 1H), 8.04 (dd, J=8.9, 2.8 Hz, 1H),
8.18 (d, J=2.8 Hz, 1H), 11.26 ppm (s, 1H); 13C NMR (176 MHz,
[D6]DMSO): d=23.3, 23.9, 25.3, 30.7, 31.1, 46.8, 76.8, 108.7, 115.1,
118.5, 120.0, 121.5, 124.2, 126.0, 128.0, 128.2, 136.8, 139.5, 148.5,
161.2 ppm; IR (KBr): 3381 (OH), 3081, 2970, 2222 (C[tbond]N), 1525
(NO2, asymm), 1340 cmÀ1 (NO2, symm); MS (ESI): m/z (%): 352
[M+H]+ (100); Anal. calcd for C20H21N3O3: C 68.36, H 6.02, N 11.96;
found: C 68.56, H 6.07, N 11.61.
Analytical procedures
For the measurement of UV/vis absorption, compounds 8a–h were
dissolved in a mixture of CH3CN/phosphate buffer (Na2HPO4/
NaH2PO4, 7.5 mm, pH 7.6) (19:1, v/v, 298 K). Each solution (0.1 mm)
was transferred to a spectrophotometer quartz cell (0.5 cm light
path length) and 0.025 mL 72 mm aq NaCN solution was added.
This volume was negligible compared with the initial volume of
the solution in the cell (1.8 mL). The mixtures were shaken, and the
absorption was measured from 200 to 600 nm against a blank of
CH3CN/phosphate buffer (19:1, v/v, 298 K).
1’-Benzyl-3’,3’-dimethyl-6-nitro-1’,3,3’,4-tetrahydrospiro[chro-
mene-2,2’-indole] (8h): A solution of 2,3,3-trimethyl-3H-indole
(1.59 g, 10 mmol) in CH3CN (10 mL) was treated with benzyl iodide
(2.39 g, 11 mmol), and the mixture was heated at reflux for 24 h.
After cooling to rt, the solvent was removed in vacuo, and the re-
action mixture was kept under high vacuum for 20 min to give
crude 5h as a brown amorphous solid, which was used for further
reaction without purification.
To construct a calibration curve of cyanide concentration versus
the most sensitive absorption at 422 nm for 8a, different cyanide
solutions (25, 50, 75, 125, 250, 500 mL of 0.72 mm and 25, 125, 250,
375 mL of 3.6 mm) were added to 100 mL of 0.1 mm 8a solution in
CH3CN/phosphate buffer. A 72 mm NaCN stock solution was pre-
pared from NaCN and diluted to 36 mm, 3.6 mm and 0.72 mm. All
of the added volumes of cyanide were negligible, with 0.875 mL as
the highest volume, compared with the initial volume of the 8a
solution.
A stirred solution of crude 5h (1.955 g, 5 mmol) in CH2Cl2 (15 mL)
was treated at rt with Et3N (1.012 g, 1.39 mL, 10 mmol). The sepa-
rated product was extracted with Et2O (3”20 mL), and the com-
bined extracts were washed with water (3”30 mL), dried over an-
hydrous Na2SO4, filtered, and concentrated in vacuo to afford
crude intermediate enamine 6h as a brownish oil.
Crude 6h was dissolved in CH3CN (3 mL), 2-chloromethyl-4-nitro-
phenol (0.938 g, 5 mmol) was added, and the mixture was stirred
for 6 h at rt. Then, the reaction mixture was poured into water
(50 mL), and 10% aq NH3 solution (~0.75 mL) was added dropwise
while stirring until the mixture became turbid. The separated prod-
uct was extracted with Et2O (3”20 mL), the combined extracts
were dried over anhydrous Na2SO4, filtered, and concentrated in
vacuo. The residue was subjected to flash chromatography on
silica gel (hexane/acetone, 6:1 v/v). The powder-like solid was
dried under high vacuum to give 8h as a yellowish solid (0.581 g,
Acknowledgements
This research was funded by a grant (no. MIP-022/2013) from the
Research Council of Lithuania.
1
29%): Rf =0.15 (hexane/acetone, 6:1 v/v ); m.p. 62–648C; H NMR
(300 MHz, CDCl3): d=1.37 (s, 6H, 2”3’-CH3), 2.35–2.42 (m, 2H,
CH2), 3.02–3.06 (m, 2H, CH2), 4.55 (s, 2H, NCH2), 6.51–6.53 (m, 1H,
7’-H), 6.83–6.93 (m, 2H, 5’-H, 8-H), 7.28–7.32 (m, 2H, 6’-H, 4’-H),
7.34–7.39 (m, 5H, Ar-H), 7.98–8.02 ppm (m, 2H, 7-H, 5-H); 13C NMR
(75 MHz, CDCl3): d=23.6, 23.7, 24.5, 25.2, 47.2, 50.1, 107.9, 116.7,
118.0, 120.1, 121.4, 122.5, 124.2, 125.1, 126.1, 126.5, 127.1, 127.5,
128.0, 128.7, 137.0, 138.7, 140.5, 147.6, 161.7 ppm; IR (KBr): 3061,
Keywords: 1’,3,3’,4-tetrahydrospiro[chromene-2,2’-indoles] · 2-
chloromethyl-4-nitrophenols
· 2-methylidene-2,3-dihydro-1H-
indoles · chemosensors · cyanide · water quality
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Published online on February 19, 2015
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www.chemistryopen.org
369
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim