Azaindole-1,2,3-triazole conjugate as selective fluorometric sensor for dihydrogenphosphate

Article abstract of DOI:10.1039/c3ra42702a

A new receptor 1 has been designed and synthesized. The open cavity of 1 selectively recognizes H₂PO₄^- over a series of other anions in CH₃CN containing 0.01% DMSO by exhibiting a ratiometric change in emission.

Full text of DOI:10.1039/c3ra42702a

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Azaindole-1,2,3-triazole conjugate as selective
fluorometric sensor for dihydrogenphosphate3
Cite this: DOI: 10.1039/c3ra42702a
Kumaresh Ghosh,*a Debasis Kar,a Soumen Joardar,a Debashis Sahubc
and Bishwajit Ganguly*bc
2
A new receptor 1 has been designed and synthesized. The open cavity of 1 selectively recognizes H2PO4
over a series of other anions in CH3CN containing 0.01% DMSO by exhibiting a ratiometric change in
emission. In sensing, the cooperativity of the azaindoles in 1 has been proved using the model compound
2. Also the interaction behavior of the nitrogen in the azaindole has been proved by considering
compound 3, which did not show any discrimination towards the anions. Binding studies have been
carried out using fluorescence, UV-vis, 1H NMR and 31P NMR spectroscopic techniques.
Received 1st June 2013,
Accepted 1st July 2013
DOI: 10.1039/c3ra42702a
www.rsc.org/advances
However, the use of 7-azaindole in the construction of new
receptors for anions is unknown in the literature. Therefore,
Introduction
The development of new receptor structures for selective
sensing of anionic substrates is of enormous interest in the
field of molecular recognition research1 as anions play an
important role in a wide range of chemical and biological
processes.2 Among various important anions, phosphate
derivatives are considered to be crucial because of their
pivotal roles in biological systems. They take part in signal
transduction and energy storage in biological systems.3
Inorganic phosphates may remain in different forms such as
H2PO42, HPO42, PO432, P2O742, HP2O7
sensing of any one of these species is challenging. In
particular, dihydrogenphosphate (H2PO42) ion recognition
draws much attention.4 To date, a great number of receptors
for the construction of a new chemosensor for anions,
placement of this azaindole motif onto a suitable spacer via
1,2,3-triazole ring that provides a C–H bond as an amide
surrogate5i,10 to bind anions has been chosen as design
principle. Based on this, we introduce herein a new charge
neutral architecture 1 that is composed by an azaindole-1,2,3-
triazole conjugate. The open cleft of this receptor module
selectively recognizes H2PO42 through a ratiometric change in
emission in CH3CN containing 0.01% DMSO. In the recogni-
tion process, the cooperativity of the azaindole motifs has
been proved by considering the model compound 2.
Furthermore, the role of the nitrogen at the 7-position of the
azaindole in 1 was understood by considering the structure 3.
32
etc. The selective
2
that are capable of binding and sensing H2PO4 anion are
known in the literature.5 Suitable heterocyclic scaffolds with a
fluorescent probe under a semirigid or flexible spacer are
found to be beneficial for successful recognition of this
tetrahedral-shaped anion. In relation to this, different hetero-
cyclic motifs such as indole,6 carbazole,5l,7 pyrrole,8 and
pyridine5k,9 are well established building blocks in devising
sensors for anions.
aDepartment of Chemistry, University of Kalyani, Kalyani-741235, India.
E-mail: ghosh_k2003@yahoo.co.in; Fax: +913325828282; Tel: +913325828750
bComputation and Simulation Unit (Analytical Discipline and Centralized
Instrument Facility), CSIR-Central Salt and Marine Chemicals Research Institute,
Bhavnagar, Gujarat, 364002, India. E-mail: ganguly@csmcri.org
Results and discussion
Synthesis
cAcademy of Scientific and Innovative Research; CSIR-Central Salt and Marine
Chemicals Research Institute, Bhavnagar, Gujarat, 364002, India
Scheme 1 summarizes the synthesis of 1, 2 and 3. For 1, 1,3-
bis(azidomethyl)benzene 4 was subjected to click reaction with
3 equiv. of 2-alkynyl-7-azaindole 8a, which was obtained from
7-azaindole after performing a series of reactions as men-
tioned in Scheme 1b. In Scheme 1b, the intermediate
compound 6a was synthesized according to a literature
Electronic supplementary information (ESI) available: Characterization spectra
3
for 1, 2 and 3, figures showing the change in absorption and fluorescence
spectra of 1 with anions, photographs of color change of 1 in the presence of
HP2O732, figures showing emission titration spectra for 2 and 3 with anions,1H
NMR of 1 in the presence of H2PO42 in d6-DMSO, PM6 optimized structures. See
http://dx.doi.org/10.1039/c3ra42702a
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containing 0.01% DMSO. Solvatochromic effects on com-
pound 3 were also realized in the same solvents in UV and
fluorescence (ESI ) and the results were very similar to those
3
obtained for compound 1. Thus the change in absorption and
emission of both 1 and 3 in DMSO and CH3OH is attributed to
the hydrogen bonding interaction of the solvents with the
binding sites of the receptors. The interference of these two
solvents in the binding sites was further realized from
fluorescence upon gradual addition into the solutions of 1
and 3 in CH CN containing 0.01% DMSO (ESI ). In each case a
3
3
measurable change in emission corroborated the considerable
interaction of both CH3OH and DMSO 1 via hydrogen bond
formation and therefore we thought that the anion binding
studies in these two solvents would not be fruitful. To confirm
this, fluorescence titration of 1 with the anions (F2, Cl2, Br2,
I2, AcO2, NO32, HSO42, H2PO4 and HP2O7
tetrabutylammonium salts) was carried out in DMSO. No
measurable change in emission was noticed (ESI ). This was
as their
Scheme 1 (a). (i) Acetone-water (4 : 1, v/v) NaN3, reflux, 3 h; (ii) 8a/8b (3
equiv.), sodium ascorbate, CuSO4, EtOH–H2O (1 : 1, v/v); (iii) 8a (1.2 equiv.),
sodium ascorbate, CuSO4, EtOH–H2O (1 : 1, v/v); (iv) 1-Heptyne, sodium
ascorbate, CuSO4, EtOH–H2O (1 : 1, v/v); (b). (v) PhSO2Cl, K2CO3, dry acetone,
reflux, 16 h; (vi) LDA, TMEDA, I2, THF, 230 uC; (vii) NaOtBu, dioxane, sealed tube,
80 uC, 3 h; (viii) TMS–acetylene, Pd(PPh3)2Cl2.,CuI, Et3N, THF, rt; (ix) TBAF, THF, rt.
2
32
3
also true for compound 3 (ESI ).
3
Thus the binding and recognition ability of 1 toward the
mentioned anions was assessed in CH3CN containing 0.01%
DMSO. The emission spectrum of 1 (c = 2.25 6 1025 M; lexc
=
procedure.11 Model compound 2 was synthesized following
the same reaction protocol as maintained for the synthesis of
1. The use of 1.2 equiv. of 8a in the click reaction with 4
afforded compound 9 in appreciable yield. Pursuance of
further click reaction on 9 with 1-heptyne gave the unsymme-
trical triazole receptor 2 in good yield. Compound 3 was
obtained by click reaction of 4 with compound 8b, which was
synthesized from indole according to Scheme 1b. Like 6a, the
intermediate compound 6b for the synthesis of 3 was obtained
from a literature procedure.11 All the compounds were fully
characterized spectroscopically.
310 nm) in CH3CN containing 0.01% DMSO displayed the
typical emission of an azaindole at 363 nm. Stepwise addition
of different anions to the solution of 1 (c = 2.25 6 1025 M) in
CH3CN containing 0.01% DMSO perturbed the emission to
different extents. Fig. 2, in this aspect, demonstrates the
change in the fluorescence ratio of 1 in the presence of 15
2
equiv. of a particular anion. Among the anions, only H2PO4
induced
a large quenching of the emission of 1. On
progression of titration with H2PO42, the monomer emission
at 363 nm decreased followed by an increase in a weakly
intense peak at 480 nm (Fig. 3).
This ultimately led to a ratiometric change in emission
which was convenient in the present study for identifying
H2PO42 from other anions examined. The change in emission
at the two different wavelengths is displayed in Fig. 4a. In
reality, it is worth mentioning that ratiometric chemosensors
Anion binding studies
Prior to studying the anion binding behaviour, the absorption
and emission spectra of 1 were recorded in different solvents
considering its solubility. In this regard, while Fig. 1a shows
the absorption spectra, Fig. 1b represents the emission spectra
of 1 in different solvents. In the UV, the absorption intensity of
1 is considerably greater in DMSO compared to CH3OH and
CH3CN containing 0.01% DMSO. Compound 1 was not fully
soluble in CH3CN and therefore a small amount of DMSO was
added to make the solution homogeneous. Similar to the UV,
in fluorescence a significant perturbation in the emission of 1
was observed in DMSO and CH3OH with respect to CH3CN
2
for H2PO4 are rare in the literature.5c,12 Ratiometric chemo-
sensors offer advantages over the conventional monitoring of
fluorescence intensity at a single wavelength. A dual emission
system can minimize the measurement errors because of
factors such as phototransformation, receptor concentrations,
and environmental effects.13
Fig. 2 Change in the fluorescence ratio of 1 (c = 2.25 6 1025 M) at 363 nm
upon addition of 15 equiv. of different guests in CH3CN containing 0.01%
DMSO.
Fig. 1 (a) Absorption and (b) emission spectra of 1 (c = 3.27 6 1025 M) in
different solvents.
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Fig. 3 Emission titration spectra of 1 (c = 2.25 6 1025 M) in CH3CN containing
0.01% DMSO upon addition of 15 equiv. of H2PO42; Inset: color change of 1 in
the titration under UV illumination.
Fig. 5 Change in the fluorescence ratio of 1 (c = 2.18 6 1025 M) upon addition
2
of 15 equiv. of H2PO4 to the receptor solution containing other anions in
2
We believe that strong complexation of H2PO4 into the
CH3CN containing 0.01% DMSO.
cleft of 1 perturbes the np* state of the azaindole moiety in a
stabilizing manner for which this lowest energy singlet excited
state (np*) comes closer to the ground singlet state and results
in quenching of the emission.14 Furthermore, as the hydrogen
bonds are set in into the cleft, azaindole moieties become
more conjugated with the 1,2,3-triazole motifs, as a result of
which a broad emission at 480 nm begins to appear with lower
intensity.
2
H2PO4 to the solution of 2 in CH3CN containing 0.01%
DMSO brought about only a small change in emission (ESI ).
3
2
The selectivity in fluorescence sensing of H2PO4 by
chemosensor 1 was understood by recording the emission
spectra of 1 in the presence and absence of other anions in
CH3CN containing 0.01% DMSO. As shown in Fig. 5, the
2
The stoichiometry of the complex of 1 with H2PO4 was
anions chosen in the present study show negligible inter-
determined to be 1 : 1 as confirmed by the Job plot15 (Fig. 4b).
2
ference in the binding of H2PO4
.
The non linear curve fitting16 of the emission data (ESI ) gave a
3
The ground state interaction of 1 with the anions was also
binding constant value of (1.97 ¡ 0.59) 6 104
M
21. Such
investigated by UV-vis study. In all cases except H2PO42, the
2
strong binding of H2PO4 into the cleft of 1 induced a color
change (bluish green to blue) of the receptor solution under
illumination of UV light. The inset of Fig. 3 displays this color
absorption centered at 306 nm in 1 decreased to a small extent
2
upon complexation (ESI ). During interaction with H PO
,
3
2
4
the absorption at 306 nm gradually decreased in a ratiometric
manner with a red shift of 12 nm (Fig. 6), which suggested a
32
change attribute. In contrast, HP2O7 did not result in any
2
change in the color of the original receptor solution (ESI ). The
3
hydrogen bond mediated strong interaction of H2PO4 into
32
binding constant value for HP2O7
was also difficult to
the cleft of 1 in the ground state.
determine due to such a minor change in emission. It should
To test the hypothesis that the recognition event is mainly
1
due to hydrogen bonding effects between 1 and H2PO42, a H
be further noted that the greater change in emission of 1 in the
2
NMR spectrum of 1 was recorded in the presence of H2PO42 in
CD3CN containing 2% d6-DMSO (used to make the solution
homogeneous). In the presence of H2PO42, the signal for NH
at 11.5 ppm became invisible presumably due to broadening
caused by hydrogen bond interactions (Fig. 7). Proton Hb of
the triazole ring and Hd of the m-xylene spacer moved
presence of H2PO4 anion is due to the collaborative binding
effect of two azaindole moieties with the triazole rings. This
was proved by considering the model compound 2 with one
azaindole motif. Under similar condition, gradual addition of
significantly downfield (Dd for Hb = 0.86 ppm and for Hd
=
Fig. 4 (a) Change in emission intensity of 1 at 486 and 363 nm with the guest
concentration in CH3CN containing 0.01% DMSO. (b) UV-vis Job plot for 1 with
tetrabutylammonium dihydrogenphosphate in CH3CN containing 0.01% DMSO
([H] = [G] = 3.25 6 1025 M).
Fig. 6 UV-vis titration spectra for 1 (c = 2.25 6 1025 M) with H2PO4 (c = 1 6
2
1023 M) in CH3CN containing 0.01% DMSO.
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Fig. 7 Partial 1H NMR (400 MHz) of 1 (c = 5.14 6 1023 M) in the absence (A)
and presence of 1 equiv. (B) and 2 equiv. (C) of tetrabutylammonium
dihydrogenphosphate in CD3CN containing 2% d6-DMSO (for identification of
protons see the adjacent labeled structure).
0.59 ppm) thus proving their profound participation in the
complexation. The indole ring protons Ha and Hc showed
slight downfield shifting on complexation. However, H NMR
Fig. 9 B3LYP/6-31G(d)//PM6 calculated optimized geometries for 1 complexed
2
with (a) solvent CH3CN [DBE = 28.8 kcal mol21], (b) both CH3CN and HSO4
1
2
[DBE = 29.3 kcal mol21], and (c) both CH3CN and H2PO4 [DBE = 233.8 kcal
mol21]. BE refers to the binding energy and the numerical values in the
structures are the hydrogen bond distances in Å.
experiments conducted in pure d6-DMSO did not show any
chemical shift change of the interacting protons in the
2
presence of H2PO4
(ESI ) and thereby suggested the
3
interference of DMSO into the cleft of 1. The strong interaction
2
of H2PO4 into the cleft of 1 was further confirmed with 31P
CH3CN is 28.8 kcal mol21, which is similar to the binding
energy of HSO42 with 1. Such strong binding of H2PO42 in the
presence of CH3CN in the cleft of 1 can perturb the np* state of
the azaindole moiety, which results in the quenching of
emission.14 In our opinion, the absence of the pyridine ring
nitrogen atoms in 3 will give the opposite result. Therefore, the
influence of the solvent seems to be important for the selective
binding of the receptor 1 toward the anions. Interestingly, the
parent molecule 1 prefers to orient its arms in opposite
directions, however, they show cooperation while interacting
NMR experiments. The signal for the P-atom in H2PO42 at 2.21
ppm suffered an upfield shift by 1.26 ppm in the presence of
chemosensor 1 (Fig. 8). As an explanation for the upfield shift,
we believe that the azaindole rings in 1 associate with the
2
H2PO4 anion in a slightly sandwich manner, resulting in the
P-atom falling in the shielding region.
To propose a structure for the hydrogen bonded complex of
1 with H2PO42, we carried out computational studies. In the
2
study, we considered two anions, H2PO4 and HSO42, for
with the analytes (ESI ).18
binding with the receptor molecule 1. The structural features
of 1 reveal that strong hydrogen bonding between the pyridyl
nitrogen atom of the 7-azaindole and the polar solvent is
possible according to the mode as shown in Fig. 9a. Indeed,
the B3LYP/6-31G(d)//PM6 calculated results17 in the gas phase
3
The effect of the nitrogen at the 7-position of the azaindole
2
on increasing the binding strength for H2PO4 was under-
stood with the model compound 3. The absence of nitrogen at
the 7-position of the indole in 3 was reflected while we carried
out a similar study with the same anions. Under similar
experimental conditions, compound 3 did not exhibit selectiv-
ity in fluorescence. Fig. 10, in this regard, represents the
2
(Fig. 9) indicate that 1 can bind H2PO4 more strongly than
2
HSO4 in the presence of an explicit CH3CN molecule. The
2
binding energy of H2PO4 with 1 is 233.8 kcal mol21, while
2
that of HSO4 is 29.3 kcal mol21 in the presence of a CH3CN
molecule. The binding energy of 1 with the explicit solvent
Fig. 10 Change in the fluorescence ratio of 3 (c = 5.01 6 1025 M) at 366 nm
upon addition of 15 equiv. of different guests in CH3CN containing 0.01%
DMSO.
Fig. 8 31P NMR of (a) tetrabutylammonium dihydrogenphosphate (c = 6.18 6
1023 M) and with (b) with equiv. amount of receptor 1 (c = 6.18 6 1023 M) in
CD3CN containing 2% d6-DMSO.
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ratiometric host by exhibiting large change in emission in
CH3CN containing 0.01% DMSO. The interplay of the
7-azaindole and 1,2,3-triazole moieties of 1 in hydrogen
bonding and the dimension of the flexible cleft introduce
selectivity in the recognition process. Experimentation on the
model structure 3 gave no meaningful result in emission and
proved unequivocally the role of the ring nitrogens at the
7-position of the azaindole motifs of 1 in exerting hydrogen
bonding effects through the involvement of CH3CN for
2
selection of H2PO4 over a series of other anions.
Fig. 11 (a) Change in emission of 3 (c = 5.01 6 1025 M) upon addition of
H2PO42 in CH3CN containing 0.01% DMSO; (b) Change in the fluorescence ratio
of 1 (c = 3.10 6 1025 M) at 374 nm upon addition of 15 equiv. of different
phosphate containing guests in CH3CN containing 0.01% DMSO: H2O (4 : 1 v/
v).
Experimental
2-((trimethylsilyl)ethynyl)-1H-pyrrolo[2,3-b]pyridine (7a)
To a stirred solution of compound 6a11 (0.5 g, 2.05 mmol) in
THF (10 mL) were added Et3N (0.85 mL, 6.15 mmol), TMS-
acetylene (0.87 mL, 6.15 mmol) and the reaction mixture was
degassed for 10 min. Then CuI (26 mg, 0.106 mmol) and Pd
(PPh3)2Cl2 (71 mg, 0.05 mmol) were added to the reaction
mixture and the mixture was allowed to stir at room
temparature for next 4 h. After completion of reaction, the
reaction mixture was filtered through celite pad. The filtrate
was extracted with ethyl acetaste, washed with water, brine
solution and dried over Na2SO4. Solvent was evaporated off
under reduced pressure and the crude product was purified by
column chromatography using 50% ethyl acetate in hexane to
afford the pure compound 7a (388 mg).
fluorescence ratio of 3 in the presence of 15 equiv. of the
anions. During the interaction of 3 with the anions, the peak at
366 nm was intensified keeping the emission at 440 nm
almost unaltered (ESI ). No defined ratiometric change in
3
emission (e.g., Fig. 11a for 3 with H2PO42) was noticed as
2
observed for 1 with H2PO4 (see Fig. 3). In the interaction
process, the change in emission intensity for 3 at 366 nm (see
32
Fig. 10) is almost the same for H2PO42, HP2O7 and F2. The
binding constant values (Ka) for H2PO42, HP2O732 and F2 ions
were ascertained from emission titration data and they were
found to be (2.04 ¡ 0.3) 6 103 M21, (4.95 ¡ 1.0) 6 103 M21
and (1.56 ¡ 0.02) 6 103 M21, respectively. Careful scrutiny of
the binding constant values revealed that the receptor 1 binds
1H NMR (400 MHz, CDCl3) d 10.14 (1H, s), 8.34 (1H, d, J = 4
Hz), 7.88 (1H, d, J = 8 Hz), 7.08 (1H, dd, J1 = 8 Hz, J2 = 4 Hz),
6.70 (1H, s), 0.28 (9H, s); 13C NMR (100 MHz,CDCl3) d 148.3,
143.2, 129.4, 120.6, 120.0, 116.2, 106.5, 99.1, 97.0, 20.09; Mass
(LCMS): 215.0 (M + 1)+.
2
H2PO4 y9 times more strongly than receptor 3. This
increment in the Ka value is due to the azaindole ring
nitrogens, which exert hydrogen bonding effects on the guest
through the involvement of CH3CN in the cavity of 1 (see
Fig. 9c).
2-Ethynyl-1H-pyrrolo [2, 3-b] pyridine (8a)
Moreover, it should be noted that compound 3 gave no
To a stirred solution of compound 7a (220 mg, 1 mmol) in THF
(10 mL) was added tetrabutylammonium fluoride (TBAF) [0.3
mL, 1.3 mmol; (1 M solution in THF)] at 0 uC and the reaction
mixture was stirred at room temperature for next 1 h. After
completion of the reaction, the reaction mixture was extracted
with ethyl acetaste, washed with water and dried over Na2SO4.
Solvent was evaporated off under reduced pressure. The crude
product was purified by column chromatography using 80%
ethyl acetate in hexane to afford the pure compound 8a (420
distinguishable marked change in absorbance (ESI ). Thus,
these findings necessitate the presence of azaindole in the
design for selective sensing of the anion.
3
While compound 1 was established as a good fluorometric
sensor for H2PO42, for the application of the sensor in
aqueous systems we further performed the complexation study
of 1 in CH3CN containing 0.01% DMSO : H2O (4 : 1, v/v) with
different phosphate salts as well as phosphate group contain-
ing biomolecules such as ATP, ADP and AMP. In this context,
the change in emission of 1 was found to be considerable in
the presence of sodium phosphate, ATP, ADP and AMP
(Fig. 11b) with no meaningful selectivity. Other inorganic
phosphate salts induced minor changes in the emission. Thus,
in semi aqueous systems the charge neutral chemosensor 1 is
not a promising candidate for selective sensing of any
particular phosphate – based anionic substrate.
1
mg, yield: 86%, mp 62 uC). H NMR (400 MHz, CDCl3) d 10.2
(1H, s), 8.37 (1H, d, J = 4 Hz), 7.90 (1H, d, J = 8 Hz), 7.09 (1H,
dd, J1 = 8 Hz, J2 = 4 Hz), 6.75 (1H, s), 3.38 (1H, s); FT-IR: n cm21
(KBr): 3436, 3286, 2897, 2817, 2355, 1694, 1583; Mass (LCMS):
144.2 (M + 2)+, 142.8 (M)+.
2-Iodo-7-azaindole (6a)
Compound 6a11a,b was prepared according to the reported
1
procedure. H NMR (400 MHz, d6DMSO) d 12.20 (1H, s), 8.13
(1H, d, J = 4 Hz), 7.85 (1H, d, J = 8 Hz), 7.01 (1H, dd, J1 = 8 Hz, J2
= 4 Hz), 6.69 (1H, s); Mass (LCMS): 245.0 (M + 2)+.
Conclusion
In conclusion, the 7-azaindole-1,2,3-triazole conjugate 1 with a
2
m-xylene spacer has proven to be effective as H2PO4 binding
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organic part was washed with brine solution, and dried over
Na2SO4. The solvent was removed under vacuum and the pure
compound 9 (288 mg; yield: 82%) was isolated after purifica-
tion done through column chromatography using 1% MeOH
in CH2Cl2 as eluent.
2-((trimethylsilyl)ethynyl)-1H-indole (7b)
To a stirred solution of compound 6b11 (0.5 g, 2.06 mmol) in
THF (10 mL) were added Et3N (0.85 mL, 6.17 mmol), TMS-
acetylene (0.88 mL, 6.17 mmol) and the reaction mixture was
degassed for 10 min. Then CuI (39 mg, 0.2 mmol) and Pd
(PPh3)2Cl2 (72 mg, 0.1 mmol) were added to the reaction
mixture and the reaction mixture was allowed to stir at room
temparature for next 4 h. After completion of reaction, the
reaction mixture was filtered through celite pad. The filtrate
was extracted with ethyl acetaste, washed with water, brine
solution and dried over Na2SO4. Solvent was evaporated off
under reduced pressure. The crude product was purified by
column chromatography using 35% ethyl acetate in hexane to
afford the pure compound 7b (375 mg).
1H NMR (400 MHz, CDCl3) d 10.23 (1H, s), 8.35 (1H, dd, J1 =
4.4 Hz, J2 = 1.6 Hz), 7.87 (1H, dd, J1 = 8 Hz, J2 = 1.2 Hz), 7.78
(1H, s), 7.43 (1H, t, J = 8 Hz), 7.35–7.28 (3H, m), 7.08–7.05 (1H,
m), 6.62 (1H, s), 5.61 (2H, s), 4.36 (2H, s); 13C NMR (100 MHz,
d6-DMSO) d 149.4, 143.2, 140.9, 136.8, 136.7, 130.3, 129.7,
128.6, 128.3, 128.2, 122.2, 121.0, 116.4, 97.3, 53.7, 53.3 (one
carbon in the aromatic region unresolved); Anal Calcd for
C
17H14N8: C, 61.81; H, 4.27; N, 33.92; Found: C, 61.84; H, 4.30;
N, 33.96.
1H NMR (400 MHz, CDCl3) d 8.16 (1H, s), 7.56 (1H, d, J = 8
Hz), 7.28 (1H, d, J = 8 Hz), 7.21 (1H, t, J = 8 Hz), 7.10 (1H, t, J = 8
Hz), 6.76 (1H, s), 0.26 (9H, s); 13C NMR (100 MHz,CDCl3) d
135.9, 127.5, 123.7, 121.0, 120.5, 118.6, 110.7, 109.3, 98.5, 97.0,
20.09; Mass (LCMS): 213.0 (M)+.
Compound 1
To a solution of 4 (200 mg, 1.06 mmol) in ethanol-water (1 : 1
v/v, 15 mL), alkyne 8a (453 mg, 3.19 mmol) was added. Then a
fresh solution of sodium ascorbate (42 mg, 0.2 mmol) and
CuSO4?5H2O (26 mg, 0.1 mmol) in distilled water (3 mL) was
added and the mixture was stirred under air for 16 h. After
completion of reaction, ethanol was evaporated, water was
added and extracted with 10% MeOH in CH2Cl2. The organic
layer was washed with brine solution, and dried over Na2SO4.
The solvent was removed under vacuum and the product 1
(410 mg, yield: 82%, mp 270 uC) was isolated after by column
chromatography using 8% MeOH in CH2Cl2 as eluent.
1H NMR (400 MHz, d6-DMSO) d 12.12 (2H, s), 8.52 (2H, s),
8.19 (2H, d, J = 4 Hz), 7.92 (2H, d, J = 4 Hz), 7.48–7.42 (2H, m),
7.35 (2H, d, J = 8 Hz), 7.07–7.03 (2H, m), 6.82 (2H, s), 5.73 (4H,
s); 13C NMR (100 MHz, d6-DMSO) d 149.4, 143.2, 140.9, 136.9,
130.3, 129.9, 128.4, 128.3, 128.1, 122.3, 120.9, 116.5, 97.4, 53.2;
HRMS (TOF MS ES+): calcd. for C26H20N10 (M + H)+ 473.1951:
found 473.1967; FT-IR: n cm21 (KBr): 1599, 1580, 1431, 1411,
1282; Anal Calcd for C26H20N10: C, 66.09; H, 4.27; N, 29.64;
Found: C, 66.06; H, 4.28; N, 29.62.
2-Ethynyl-1H-indole (8b)
To a stirred solution of compound 7b (210 mg, 1 mmol) in
THF (10 mL) was added TBAF [0.3 mL, 1.3 mmol; (1 M solution
in THF)] at 0 uC and the reaction mixture was stirred at room
temperature for 1 h. The reaction mixture was extracted with
ethyl acetaste, washed with water and dried over Na2SO4.
Solvent was evaporated off under reduced pressure. The crude
product was purified by column chromatography using 60%
ethyl acetate in hexane to have pure compound 8b (450 mg,
yield: 86%, mp 48 uC). 1H NMR (400 MHz, d6-DMSO) d 11.6
(1H, s), 7.51 (1H, d, J = 8 Hz), 7.31 (1H, d, J = 8 Hz), 7.15 (1H, t, J
= 8 Hz), 7.02 (1H, t, J = 8 Hz), 6.73 (1H, s), 4.46 (1H, s); FT-IR: n
cm21 (KBr): 3387, 3268, 2923, 2857, 2365, 1607; Mass (LCMS):
141.0 (M)+, 140.0 (M 2 1)+.
2-Iodoindole (6b)
Compound 6b11c was prepared according to the reported
procedure. 1H NMR (400 MHz, CDCl3) d 8.06 (1H, s), 7.52 (1H,
d, J = 8 Hz), 7.32 (1H, d, J = 8 Hz), 7.14–7.05 (2H, m), 6.71 (1H,
s); Mass (LCMS): 243.0 (M)+.
Compound 2
To a solution of 9 (250 mg, 1.06 mmol) in ethanol-water (1 : 1
v/v, 15 mL), 1-heptyne (87 mg, 0.908 mmol) was added. A fresh
solution of sodium ascorbate (42 mg, 0.2 mmol) and
CuSO4?5H2O (26 mg, 0.1 mmol) in distilled water (3 mL) was
added and the mixture was stirred under air for 16 h. After
completion of reaction, ethanol was evaporated, water was
added, extracted with 10% MeOH in CH2Cl2, organic part was
washed with brine solution, and dried over Na2SO4. The
solvent was removed under vacuum and the product 2 (260
mg, yield: 80%, mp 185 uC) was isolated by column
chromatography using 2% MeOH in CH2Cl2 as eluent. 1H
NMR (400 MHz, d6-DMSO) d 12.11 (1H, s), 8.49 (1H, s), 8.18
(1H, br s), 7.93 (1H, d, J = 8 Hz), 7.85 (1H, s), 7.41 (1H, t, J = 8
Hz), 7.32–7.24 (3H, m), 7.07–7.04 (1H, m), 6.83 (1H, s), 5.70
(2H, s), 5.54 (2H, s), 2.54 (2H, t, J = 8 Hz), 1.55–1.51 (2H, m),
1.24–1.22 (4H, m), 0.83–0.82 (3H, t, J = 4 Hz); 13C NMR (100
MHz, d6-DMSO) d 148.8, 147.1, 142.6, 140.3, 136.9, 136.2,
129.7, 129.2, 127.7, 127.5, 127.2, 121.8, 121.7, 121.0, 116.0,
96.8, 52.6, 52.2, 30.6, 28.4, 24.8, 21.6, 13.7 (one carbons in the
aromatic region unresolved); HRMS (TOF MS ES+): calcd. for
1,3-bis(azidomethyl)benzene (4)
A mixture of 1,3-bis(bromomethyl)benzene (1.32 g, 5 mmol)
and NaN3 (0.81 g, 12.5 mmol) in acetone-water (4 : 1 v/v, 30
mL) was refluxed for 3 h. Acetone was evaporated and the mass
was extracted with ethyl acetate (20 mL 6 3). Evaporation of
ethyl acetate afforded compound 4 as colorless oil (0.850 g,
yield: 90%). 1H NMR (400 MHz, CDCl3) d 7.40 (1H, t, J = 8 Hz),
7.29–7.27 (3H, m), 4.36 (4H, s).
2-[1-(3-Azidomethyl-benzyl)-1H-[1,2,3]triazol-4-yl]-
1H-pyrrolo[2,3-b]pyridine (9)
Compound 4 (220 mg, 1.06 mmol) and alkyne 8a (181 mg, 1.28
mmol) were taken in aq. ethanol (20 mL, 1 : 1 v/v). To this
solution, a fresh solution of sodium ascorbate (21 mg, 0.10
mmol) and CuSO4?5H2O (13 mg, 0.05 mmol) in distilled water
(3 mL) was added and the mixture was stirred under air for 16
h. After completion of reaction ethanol was evaporated, water
was added and extracted with 10% MeOH in CH2Cl2. The
C
24H26N8 (M + H)+ 427.2359: found 427.2389; FT-IR: n cm21
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(KBr): 2955, 2925, 2857, 1614, 1430, 1289, 1216; Anal Calcd for
C24H26N8: C, 67.58; H, 6.14; N, 26.27; Found: C, 67.60; H, 6.13;
N, 26.28.
constant (Ka) and correlation coefficients (R) were obtained
from a non-linear least-square analysis of I versus CH and CG.
Compound 3
Acknowledgements
Following the same procedure as mentioned for compound 1,
compound
3 was prepared. The amounts taken were:
We thank DST New Delhi, India for financial support. D. K.
thanks CSIR, New Delhi, India for a fellowship. One of the
authors’ DS thanks UGC (India) for Junior research fellowship.
compound 4 (200 mg, 1.06 mmol), alkyne 8b (450 mg, 3.19
mmol), sodium ascorbate (42 mg, 0.2 mmol), CuSO4?5H2O (26
mg, 0.1 mmol). After completion of reaction, ethanol was
evaporated, water was added, extracted with 10% MeOH in
CH2Cl2, organic part was washed with brine solution, and
dried over Na2SO4. The solvent was removed under vacuum
and the product 3 (430 mg, yield: 86%, mp 165 uC) was isolated
by column chromatography using 3% MeOH in CH2Cl2 as
eluent.
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Binding constant determination16
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(1)
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