The indium(iii) chloride catalyzed synthesis of sulfur incorporated 3-acylcoumarins; Their photochromic and acetate sensing properties

Article abstract of DOI:10.1039/c4ra09228g

The synthesis and evaluation of the photochromic properties of 3-acylcoumarins are very important as they exhibit selective sensing properties. We found that InCl₃ efficiently catalyzes the condensation of 2-hydroxybenzaldehydes and β-keto este

Full text of DOI:10.1039/c4ra09228g

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The indium(III) chloride catalyzed synthesis of sulfur incorporated 3-acylcoumarins; their
photochromic and acetate sensing properties.
H. Surya Prakash Rao*, Avinash Desai
Department of Chemistry, Pondicherry University, Pondicherry – 605 014 India.
E.mail: hspr.che@pondiuni.edu.in; Telephone: +91413 2654411; Fax: +91413 2656230
Abstract: Synthesis and evaluation of photochromic properties of 3ꢀacylcoumarins is very
important as they exhibit selective sensing properties. We found that InCl efficiently catalyzes
3
condensation of 2ꢀhydroxybenzaldehydes and βꢀketo esters to provide near quantitative yield of
3ꢀacylcoumarins. The 3ꢀ(phenylsulfanyl/phenylsulfinyl/phenylsufonyl)propanoyl coumarins with
electronꢀdonating NMe group at C7 position were prepared to evaluate the influence of sulfur
2
on emission and acetate ion sensing properties. Our studies revealed that 7ꢀ(diethylamino)ꢀ3ꢀ(3ꢀ(
phenylsulfanyl/phenylsulfinyl/phenylsufonyl)propanoyl)ꢀ2Hꢀchromenꢀ2ꢀone exhibits excellent
fluorescence emission in hexane and ethyl acetate (Φ ≈ 80%). The 3ꢀacylcoumarin with sulfone
in the C3ꢀacyl sideꢀchain detects acetate anions selectively and this property can be conveniently
followed by fluorescence emission spectroscopy.
Keywords: Coumarin, indium(III) chloride, fluorescence, acetate sensor.
Introduction
Coumarins (2Hꢀlꢀbenzopyranꢀ2ꢀones; benzoꢀannulated oxygen heterocycles) are a
1
distinct class of heterocyclic compounds because of their natural occurrence, beneficial
2
3
medicinal and lasing properties. Among them, the 3ꢀacylcoumarins having electron

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withdrawing acyl group at C3 position and electron donating dialkylamino group at C7 position
4
exhibit high intensity fluorescence emission, a property useful for lasing applications.
5
Furthermore, suitably substituted 3ꢀacylcoumarins act as acetate anions sensors. Thus, there is
continuing interest in synthesis, characterization and evaluation of photochromic properties of 3ꢀ
acylcoumarins. Knoevenagel condensation of a 2ꢀhydroxybenzaldehyde (salicylaldehyde)
derivative and an acetoacetic acid ester under basic conditions is a general method for synthesis
6
7
of 3ꢀacylcoumarins. Since fluorescence emission is highly susceptible to microenvironment, we
became interested in synthesis and evaluation of fluorescence properties of hitherto unknown 3ꢀ
acylcoumarins with a sulfur atom in C3ꢀacyl sideꢀchain, e.g. 3c (Scheme 1). We attempted
synthesis of 3ꢀacylcoumarin 3c from 2ꢀhdyroxybenzldehyde 1a and the βꢀketo ester 2c in
8
9
presence of catalytic or stoichiometric amounts of bases like piperidine, piperidinium acetate,
9
1
,8ꢀdiazabicycloundecꢀ7ꢀene (DBU) which have been previously used for coumarin synthesis.
Instead of formation of the coumarin 3c, above reactions lead to disintegration of the βꢀketo ester
c into thiophenol and a mixture of unidentified products. Alternately, we employed Lewis acid
2
1
0
11
12
4,
catalysts like ZnCl2, SnCl , and TiCl but the reactions were unsuccessful. Finally, when
4
we employed catalytic amounts of InCl the condensation took place smoothly to deliver 3ꢀ
3
acylcoumarin in excellent yield (Scheme 1). We now present details of InCl catalyzed
3
condensation of 2ꢀhydroxybenzaldehydes 1 and βꢀketo esters 2 for synthesis of a combinatorial
library of 3ꢀacylcoumarins 3 (Scheme 1) and fluorescence emission properties of the 3ꢀ
acylcoumarins having sulfanyl, sulfone or sulfoxide functional groups. Furthermore, we
delineate an application of the 3ꢀacylcoumarin with sulfone functional group for efficiently
sensing acetate anions. In recent years, InCl has become a catalyst of choice in promoting
3
1
3
plethora of reactions where soft Lewis acid is required. Noteworthy features of InCl catalysis
3

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include high chemoꢀ and regioꢀselectivity, tolerance to sensitive functional groups, compatibility
with wide variety of solvents including water, true catalytic nature and stability of the catalyst.
Results and Discussion
We selected condensation of 2ꢀhydroxybenzaldehyde 1a and ethyl 3ꢀoxoꢀ5ꢀ
(phenylthio)pentanoate 2c to form 3ꢀacylcoumarin 3c to determine suitable solvent and
minimum quantity of InCl . When we used solvents such as toluene (nonꢀpolar), ethyl acetate
3
(moderately polar, aprotic), dimethyl formamide (polar aprotic) or acetonitrile (polar aprotic) at
their respective reflux temperatures, 2c was decomposing and the reaction mixture was turning
black. In ethanol (polar protic) reflux, however, the reaction was clean and 3c was obtained in
excellent yield (Scheme 1). Next, we varied mol% of InCl and found that with 1 mol% the
3
reaction was taking more than 24 h for completion and isolated yield of 3c was low (< 30%).
With 10 mol% of the catalyst the reaction was clean and took 6.5 h for completion to provide 3c
in 90% yield. There was no significant improvement in yield or rate of the reaction when mol%
of InCl was increased to 20%. From these experiments we concluded that 10 mol% of InCl in
3
3
EtOH reflux is optimal for preparation of 3ꢀacylcoumarins. Apart from the anticipated signals,
1
the H NMR spectrum of 3c displayed diagnostic singlet at δ 8.5 ppm for C4H of the coumarin
portion of the molecule. With the optimal conditions in hand, we examined the scope of the
condensation for synthesis of a combinatorial library of coumarins. The InCl mediated
3
condensation of five 2ꢀhydroxybenzaldehydes 1a-e and three βꢀketo esters 2a-c furnished fifteen
coumarins 3a-o without any complication (Scheme 1). The spectral and analytical data of each
one of them matched well with the parent 3a.

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Scheme 1. InCl catalyzed synthesis of 3ꢀacylcoumarins 3a-o (time taken for the reaction and the
3
yield are given in parenthesis).
Among the coumarins prepared in this study 3d-e are highly fluorescent owing to rigid
framework of the coumarin scaffold and extended conjugation emanating from electron donating
C7 diethylamino group to electron withdrawing C3 acyl group. Flourimetric detection of
fluorescent compounds is the most sensitive analytical technique and widely used in many
[
7]
scientific fields. Fluorescent emission maximum and intensity are highly susceptible to
structural changes in the molecule and environment. Indeed, such changes are the basis for
design of molecular sensors. Hitherto unknown 3f was prepared with an intention to study
influence of the sulfur atom on the fluorescence emission characteristics of the coumarin
flouropore. Similarly, coumarins 3p with sulfoxide and 3q with sulfone functional groups in the
side chain were prepared to study the influence of sulfur as well as oxygen atoms on the
fluorescence emission (Scheme 2). Furthermore, the 3ꢀacylcoumarins like 3q are suitable for ion

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sensing properties, and such property can be followed through changes in fluorescence emission.
Oxidation of the coumarin 3f with 30% aqueous H O provided sulfoxide 3p. On the other hand,
2
2
oxidation of 3f with potassium monoperoxosulfate (Oxone) provided sulfone 3q (Scheme 2).
Scheme 2. Oxidation of sulphide 3f to sulfone 3q and to sulfoxide 3p.
Absorption and emission characteristics of 3-acylcoumarins 3d, 3f, 3p and 3q
The UV absorption spectra of the parent 3d, sulfide 3f, sulfoxide 3p and sulfone 3q were
recorded in five different types of solvents, namely, hexane (nonꢀpolar), ethyl acetate (EtOAc,
moderately polar aprotic), methanol (MeOH, polar protic), acetonitrile (MeCN, polar aprotic)
and dimethylformamide (DMF, highly polar and aprotic; Figure 1, Table 1 and supplementary
information). The solvatochromic studies showed that absorption behavior of the chromophore in
the parent 3d, sulfide 3f, sulfoxide 3p and sulfone 3q are similar in each one the solvents. The
maximum bathochromic shift from nonꢀnonpolar solvent hexane to polar solvent acetonitrile is
about 16 nm. Similarity of absorption spectra of sulfone 3q and its parent 3d in each solvent
indicates that sulfur present in the side chain of 3q has little influence on the absorption
characteristics of the chromophore.

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1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1.2
1
0
0
0
0
.0
.8
.6
.4
.2
Hexane 417 nm
Ethy acetate 424 nm
Acetonitrile 428 nm
Methanol 432 nm
DMF 432 nm
Hexane 421 nm
Ethy acetate 428 nm
Acetonitrile 438 nm
Methanol 432 nm
DMF 435 nm
0.0
3
60
450
540
630
300
400
500
600
700
800
Wavelength (nm)
Wavelength nm
Figure 1. Absorption spectra of sulfone 3q (9.8 µM) and parent 3d (10.8 µM) in hexane (black),
EtOAc (red), MeCN (green), MeOH (blue), DMF (cyan)
Table 1. Absorption characteristics of parent 3d, sulfide 3f, sulfoxide 3p and sulfone 3q in
hexane, EtOAc, acetonitrile, methanol and DMF
ꢀ
1
ꢀ1
Compound
λmax (ε in cm mol )
∆λmax
17
Hexane
421
EtOAc
DMF
MeOH
432
MeCN
438
74352
434
74678
433
76734
428
3
q
428
79346
424
79545
425
435
6
6
6
7
9424
418
8924
417
9345
417
76871 767202
3
f
432
77347
433
430
76802
432
78402
432
16
3
3
p
d
16
81246
424
75487
432
15
0126
88426
79234
89240
77456
Fluorescence spectra of parent 3d, sulfide 3f, sulfoxide 3p and sulfone 3q are gathered in
Figure 2 and the data in Table 2. Solvatochromic effects on fluorescence emission of the parent
3d, sulfide 3f, sulfoxide 3p and sulfone 3q was evaluated in five solvents namely hexane,
EtOAc, MeOH, DMF and MeCN. Similar to that of the parent 3d, solutions of sulfide 3f,
sulfoxide 3p and sulfone 3q were strongly fluorescent in EtOAc and noticeable to naked eye. We
1
4
calculated quantum yield with reference to parent 3d. Compared to the parent 3d (Φ = 66%)

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the sulfide 3f exhibited higher quantum yield (Φ = 80%) in EtOAc, which indicated that the
sulfur in the side chain had positive influence on emission, possibly by stabilizing S excited
0
state. Solvent polarity profoundly influenced emission intensities of sulfide 3f, sulfoxide 3p and
sulfone 3q. In MeOH (polar protic) and DMF (polar aprotic), the emission was much lower than
EtOAc (moderately polar aprotic). Similar to the parent 3d, the sulfide 3f, sulfoxide 3p and
sulfone 3q exhibited maximum Stokes shift of about 50 nm in DMF which indicates that in all of
them the dipolar excited state is stabilized in polar solvents. Interestingly, the sulfone 3q
displayed hundredꢀfold increase in emission intensity in hexane compared to others. Its quantum
yield (Φ = 30%) was nearly five times higher than that of the sulfoxide 3p (Φ = 6%).
6
8
7
6
5
4
3
2
1
x10
x10
x10
x10
x10
x10
x10
x10
6
6
6
6
6
6
6
6
8
x10
6
6
6
6
6
6
6
7x10
6x10
5x10
4x10
3x10
2x10
Hexane
Ethy acetate
Acetonitrile
Methanol
DMF
Hexane
Ethy acetate
Acetonitrile
Methanol
DMF
O
O
S
Et N
O
2
1
x10
3f
0
0
5
00
600
700
800
5
00
600
700
800
Wavelength (nm)
Wavelength (nm)
6
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
5
.0x10
.5x10
.0x10
.5x10
.0x10
.5x10
.0x10
.5x10
.0x10
.5x10
.0x10
.5x10
.0x10
.5x10
.0x10
.0x10
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
5
9x10
8x10
7x10
6x10
5x10
4x10
Hexane
Ethyl acetate
Acetonitrile
methanol
DMF
Hexane
Ethy acetate
Acetonitrile
Methanol
DMF
3
x10
2x10
1x10
0
.0
0
5
00
600
700
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)

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Figure 2. Fluorescence emission of parent 3d (2.3 µM), sulfide 3f (2.2 µM) , sulfoxide 3p (2.2
µM) and sulfone 3q (2.3µM) in hexane (black), EtOAc (red), MeCN (green), MeOH (blue) and
DMF (cyan)
Table 2. Emission characteristics of of parent 3d, sulfide 3f, sulfoxide 3p and sulfone 3q in
hexane, EtOAc, MeOH, DMF and MeCN.
Coumarin
λmax (emission)
∆λmax
Φ
Hexane EtOAc MeCN MeOH DMF
Hexane EtOAc MeCN MeOH DMF
Parent
428
461
467
466
468
475
481
480
478
480
483
482
484
480
483
480
483
52
50
51
51
0.05
(5%)
0.03
(3%)
0.06
6%
0.66
(66%) (1%)
0.80 0.018 0.016 0.024
(80%) (1.8%) (1.6%) (2.4%)
0.025 0.021 0.031
14
3
d
(2.1%) (3.1%)
Sulfide 433
3
f
Sulfoxide 432
0.68
68%
0.51
0.028 0.013 0.023
2.8% 1.3% 2.5%
0.030 0.023 0.035
3% 2.3% 3.5%
3
p
Sulfone 432
0.298
29.8% 51%
3
q
Acetate anion sensing properties of sulfone 3q
Coumarin based fluorescent chemosensors are highly valuable tools for detection of
1
5
various cations, anions and biologically relevant molecules. Functional groups present on 3ꢀ
acyl coumarin scaffold namely lactone conjugated to cinnamoyl moiety and the sideꢀchain on the
acyl groups can be fineꢀtuned to recognize target ions. For example, recently Lin and coworkers
have demonstrated that coumarin 4 selectively recognizes acetate anion present in water or in
1
6
DMSO through binding mode (4A) as shown in Figure 3. They used colorimetric titration
method for this purpose. As the coumarins made in this study namely, sulfide 3f, sulfoxide 3p
and sulfone 3q possess structural similarities to 4 we considered to study their acetate anion
sensing properties by following fluorescence emission.

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Figure 3. Structure of fluorescent coumarin 4, its acetate complex 4A and sulfone acetate
complex 5.
Initially, absorption and emission spectra of sulfide 3f, sulfoxide 3p and sulfone 3q were
recorded in MeOH in the absence (blank) and in presence of excess (4 equivalents) of potassium
acetate (KOAc) and the results are shown in Figure 4. It is evident from the spectra that only the
emission of sulfone 3q responds to potassium acetate, while absorption and emission maxima
(λmax) and the respective intensities of other coumarins 3f and 3p are more or less remained as
such. Even in the case of sulfone 3q the maximum for both absorption and emission remained
unchanged, although emission intensity doubled by a factor of 2 indicating that the fluorescence
emission is sensitive to the presence of potassium acetate.

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6
1.5x10
6
1.5x10
Sulfoxide Blanc
KOAc (4 eqiv)
Sulfoxide Blanc
KOAc (4 eqiv)
6
1.0x10
6
1.0x10
5
5.0x10
5
5.0x10
0.0
0.0
500
600
700
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
B
B
6
6
2.0x10
1.5x10
6
1.5x10
parent coumarin Blank
KOAc (4 eqiv)
sulfone Blank
KOAc (4 eqiv)
6
1.0x10
6
1.0x10
5
5.0x10
5
5.0x10
0.0
0.0
500
600
700
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
C
D
ꢀ7
Figure 4. A: Emission spectra of the sulfide 3f (blank) (1.42 × 10 molar) and with excess (4
equiv) potassium acetate (KOAc) in MeOH. B: Emission spectra of the sulfoxide 3p (blank)
ꢀ7
(
1.52 × 10 molar) and with excess KOAc in MeOH. C: Emission spectra of the sulfone 3q
ꢀ7
(
blank) (1.32 × 10 molar) and with excess KOAc in MeOH. D. Emission spectra of the 3ꢀacyl
ꢀ7
coumarin 3d (blank) (1.47 × 10 molar) and with excess KOAc in MeOH
To ascertain the enhancement of emission intensity is due to the presence of acetate anion
rather than potassium cation, we changed counter cations. We recorded emission spectra of 3q in

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presence of excess (4 equiv) metal acetate salts such as sodium acetate, copper(II) acetate,
magnesium acetate, manganese(II) acetate, cobalt(II) acetate, zinc acetate and lead(II) acetate
(Figure 5A). The spectra indicate that cations do not have much effect on the acetate sensing
characteristics of the sulfone 3q except possibly Zn(II) and Pb(II) where the intensity
enhancement is not much. After these set of experiments, we recorded emission spectra of 3q in
presence of different potassium salts to establish acetate anion’s role in emission intensity
enhancement. The spectra of sulfone 3q were recorded with different monodentate salts like
potassium bisulfate, potassium chloride, potassium azide, potassium nitrate, and bidentate salt
like potassium tartarate (Figure 5B). The spectra clearly show that only acetate anion has
influence in fluorescence emission enhancement.
6
6
6
5
6
6
6
5
2.0x10
1.5x10
1.0x10
5.0x10
NaOAc
KOAc
CuOAc2
MgOAc2
MnOAc2
2.0x10
1.5x10
1.0x10
5.0x10
sodium pottasium tartarate
NaHSO3
KCl
NaN3
CoOAc2
KOAc
NaNO3
ZnOAc2
PbOAc2
0.0
0.0
500
600
700
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
B
A
ꢀ7
Figure 5. A) Emission of sulfone 3q (1.32 × 10 molar) in MeOH in presence of different
ꢀ7
cations associated with cetate. B) Emission of sulfone 3q (1.32 × 10 molar) in MeOH in
presence of different anions associated with potassium.
After establishing fluorescence emission enhancement by acetate anion we set out
determine stoichiometric ratio of sulfone 3q to that of acetate anion. We titrated sulfone 3q with
increasing amount of potassium acetate and recorded fluorescence spectra after each titration

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(Figure 6A). The emission enhancement showed Sigmoidal relationship (Figure 6B) until
potassium acetate concentration reached two molar equivalents. Further addition of KOAc did
not change emission intensity indicating that two units of acetate anion complexes with the
coumarin 3q.
Blank
-
7
-9
6
6
6
6
6
6
5
5
5
5
1.32 X 10 M Sulfone : 1.00 X 10 M KOAc
2
1
1
1
1
1
8
6
4
2
.0x10
.8x10
.6x10
.4x10
.2x10
.0x10
.0x10
.0x10
.0x10
.0x10
-7
-9
1
1
1
1
.32 X 10 M Sulfone : 1.25 X 10 M KOAc
6
6
6
6
6
5
-7
-9
1.8x10
1.6x10
1.4x10
1.2x10
.32 X 10 M Sulfone : 2.5 X 10 M KOAc
-7 -9
.32 X 10 M Sulfone : 5.0 X 10 M KOAc
-7 -9
.32 X 10 M Sulfone : 7.5 X 10 M KOAc
-7
-8
1.32 X 10 M Sulfone : 1.0 X 10 M KOAc
-8
-7
1.32 X 10 M Sulfone : 2.5 X 10 M KOAc
-7
-8
1
1
1
.32 X 10 M Sulfone : 5.0 X 10 M KOAc
-7
-8
.32 X 10 M Sulfone : 7.5 X 10 M KOAc
-7 -7
.32 X 10 M Sulfone : 1.0 X 10 MKOAc
-7 -7
1
.0x10
.0x10
1.32 X 10 M Sulfone : 2.5 X 10 M KOAc
-7
-7
1.32 X 10 M Sulfone : 5.0 X 10 M KOAc
8
0.0
-
8
-8
-8
-7
-7
-7
-7
-7
450
500
550
600
650
700
750
800
0.0
3.0x10 6.0x10 9.0x10 1.2x10 1.5x10 1.8x10 2.1x10 2.4x10
Wavelength (nm)
Concentration of KOAc M
A
B
ꢀ7
Figure 6. A) Emission of sulfone 3q (1.32 × 10 ) in MeOH in presence of different
ꢀ9
ꢀ7
concentrations of KOAc (1 × 10 to 5.0 × 10 molar). B) Sigmoidal plot of fluorescence
ꢀ9
ꢀ7
intensity and concentration of KOAc (1 × 10 to 5.0 × 10 molar).
We determined the detection limit of acetate ions by the sensor coumarin 3q by keeping
concentration of sensor constant and varying acetate ion concentration gradually (Figure 6A).
ꢀ9
Minimum concentration of KOAc required for its detection was 1.25 x 10 mol per the sensor
ꢀ7
3
q concentration of 1.32 × 10 mol. This study indicates that 3q can detect up to about one
hundredth concentration of acetate ions compared to its own concentration.
Generally, the mechanism of recognition of target ions by coumarin based chemosensors
could be through (i) intramolecular charge transfer (ICT), (ii) photoinduced electron transfer
1
7
(
PET) and (iii) fluorescence resonance energy transfer (FRET). Among the three, ICT

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1
8
mechanism is predominant for anion sensing. The coumarin prepared by Lin and coworkers
appear to sense acetate anions through ICT mechanism. Hydrogen bonding interactions of the
acetate with hydrogen atoms present on two adjacent nitrogen atoms stabilizes coumarin scaffold
[
16]
in a particular conformation (Structure 4A, Figure 3). In our case the acetate anion sensing
could be through hydrogen bonding and electrostatic interactions of the enolate tautomer of the
acetate anion as shown in structure 5 (Figure 3). Such an intermediate could restrict
conformational mobility of 3q leading to enhancement of fluorescence emission (Figure 5).
Conclusion
In summary, we have developed an efficient InCl catalyzed condensation reaction of 2ꢀ
3
hydroxybenzaldehydes and βꢀketo esters to furnish coumarins 3a-o. It is noteworthy that the
condensation worked under InCl catalysis in cases where other catalysts failed. The coumarins
3
with C3ꢀacyl group and C7ꢀdiethylamino group were highly fluorescent in ethyl acetate and
hexane. One of the coumarins with sulfone group in the C3ꢀacyl sideꢀchain, 3q detects acetate
anion efficiently.
Experimental procedure:
General: Analytical thinꢀlayer chromatography (TLC) was performed on inꢀhouse coated silica
gel glass plates (0.25 mm, silica gel G, LOBA Chemicals, UV silica gel GF 254). TLC spots
were visualized under UV light and iodine. Column chromatography was carried out using silica
gel 100ꢀ200 mesh (LOBA Chemicals) and eluent mixture of increasing amounts of ethyl acetate
1
13
in hexanes. H and C NMR spectra were recorded on a Bruker Avance 400 spectrometer as
dilute solutions in CDCl + CCl (1:1) mixture. The chemical shifts values (δ) are expressed in
3
4
1
13
parts per million relative to the residual solvent peak (δ = 7.26 ppm for H, δ = 77.0 ppm for C)
where possible or alternatively to SiMe (δ = 0.00 ppm) as internal standard. Coupling constants
4

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(J) are given in Hz and multiplicities are designated as s (singlet), d (doublet), t (triplet), q
(quartet), m (multiplet) or broad (br). DEPTꢀ135 NMR spectra were recorded for all the samples
to determine number of hydrogen atoms present on each carbon. IR spectra were recorded as
KBr pellets on a Thermo Nicoletꢀ6700 spectrometer. Samples for melting point determination
were recrystallized from hexane and ethyl acetate (9:1). Melting points were determined by using
openꢀended capillary tubes on VEEGO VMPꢀDS instrument and are uncorrected. The 2ꢀ
hydroxybenzaldehdes 1a-e and βꢀketo esters 2a-b were procured from commercial sources and
were used as obtained. The βꢀketo ester 2c was prepared via Blaise reaction according our
1
9
reported procedure. Coumarins 3c, 3f, 3i, 3l and 3o are unknown compounds whose
preparation and spectral data are given in the following section. Rest of the coumarins prepared
2
0
in this study are known compounds.
Representative procedure for synthesis of coumarins: 3-(3-(Phenylthio)propanoyl)-2H-
chromen-2-one 3c:
To 2ꢀhydyroxy benzaldehyde 1a (58.2 mg, 0.47 mmol) and ethyl 3ꢀ(phenylthio)propanoate 2c
(100.6 mg, 0.39 mmol) taken in a 10 mL test tube indium(III) chloride (8.9 mg, 0.03 mmol) in
ethanol (2 mL) was added at rt. The reaction vessel was then placed in a preꢀheated oil bath (80
o
C) for 6.5 h by which time TLC indicated completion of the reaction. Contents of the testꢀtube
were transferred into a roundꢀbottomed flask and ethanol was removed under reduced pressure.
Remaining reaction mixture was diluted with 20 mL dichloromethane (DCM). The DCM
solution was washed with water and brine solution followed by removal of DCM under reduced
pressure. Trituration of the remaining viscous liquid with hexane lead to formation of 3ꢀ(3ꢀ

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(phenylthio)propanoyl)ꢀ2Hꢀchromenꢀ2ꢀone 3c (99.6 mg) as a colorless solid. Analytical sample
was obtained by recrystallization with ethyl acetate and hexane (95:5). Yield 90%; mp 134 °C;
ꢀ1 1
IR Data (ν) 3055, 2930, 1726, 1675, 1573,1171, 752 cm ; H NMR (400 MHz, CDCl + CCl ) δ
3
4
8.48 (s, 1H), 7.66ꢀ7.63 (m, 2H), 7.37ꢀ7.31 (m, 4H), 7.28ꢀ7.26 (m, 2H),7.24ꢀ7.16 (m, 1H), 3.49ꢀ
1
3
3
.46 (t, J = 6.8 Hz, 2H), 3.29ꢀ3.25 (t, J = 7.2 Hz, 2H) ppm; C NMR (100 MHz, CDCl + CCl )
3
4
δ 195.9, 155.4, 147.8, 136.1, 134.6, 130.4, 129.8, 129.1, 126.4, 125.1, 124.3, 118.4, 116.9, 96.3,
+
4
2.6, 28.2 ppm; HRMS (ESI ) calcd for C H O SNa (M + Na) 333.0561; found 333.0558.
1
8
14
3
7-(Diethylamino)-3-(3-(phenylthio)propanoyl)-2H-chromen-2-one 3f:
ꢀ1 1
Yield 95%; Mp 148 °C; IR Data (ν) 3045, 2965, 1715, 1665, 1572, 1171, 815 cm ; H NMR
(
400 MHz, CDCl + CCl ) δ 8.42 (s, 1H), 7.39ꢀ7.35 (m, 3H), 7.28ꢀ7.24 (m, 2H), 7.16ꢀ7.14 (m,
3 4
1
1
1
H), 6.66ꢀ6.58 (m, 1H), 6.45ꢀ6.44 (m, 1H), 3.48ꢀ3.43 (m, 6H), 3.29ꢀ3.25 (t, J = 7.2 Hz, 2H),
1
3
.26ꢀ1.22 (t, J = 8.0 Hz, 6H) ppm; C NMR (100 MHz, CDCl + CCl ) δ 195.9, 160.6, 158.9,
3
4
53.2, 148.3, 136.8, 132.1, 129.5, 129.1, 126.1, 115.9, 109.9, 108.5, 96.8, 45.99, 42.4, 28.4, 12.7
+
ppm; HRMS (ESI ) calcd for C H NO SNa (M + Na) 404.1296; found 404.1295.
2
2
23
3
7-Methoxy-3-(3-(phenylthio)propanoyl)-2H-chromen-2-one 3h:
ꢀ1 1
Yield 75%; Mp 142 °C; IR Data (ν) 3064, 2985, 1713, 1656, 1566, 1171, 792 cm ; H NMR
(
400 MHz, CDCl + CCl ) δ 8.47 (s, 1H), 7.54ꢀ7.52 (d, J = 8.8 Hz, 1H), 7.37ꢀ7.34 (m, 2H), 7.28ꢀ
3 4
7.24 (m, 2H),7.17ꢀ7.15 (m, 1H), 6.90ꢀ6.80 (m, 2H), 3.91 (s, 3H), 3.48ꢀ3.44 (t, J = 6.8 Hz, 2H),

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DOI: 10.1039/C4RA09228G
1
3
3
.28ꢀ3.25 (t, J = 6.8 Hz, 2H) ppm; C NMR (100 MHz, CDCl + CCl ) δ 195.8, 165.5, 159.5,
3 4
157.9, 148.2, 136.3, 131.7, 129.7, 129.1, 126.3, 120.4, 114.1, 112.2, 100.4, 56.1, 42.5, 28.2 ppm;
+
HRMS (ESI ) calcd for C H O SNa (M + Na) 363.0667; found 363.0663.
19
16
4
6-Chloro-3-(3-(phenylthio)propanoyl)-2H-chromen-2-one 3l:
ꢀ1 1
Yield 88%; Mp 139 °C; IR Data (ν) 3064, 2985, 1713, 1656, 1566, 1171, 792 cm ; H NMR
(
400 MHz, CDCl + CCl ) δ 8.31 (s, 1H), 7.55ꢀ7.50 (m, 2H), 7.29ꢀ7.18 (m, 5H), 7.11ꢀ7.10 (m,
3 4
1
3
1
H), 3.41ꢀ3.38 (t, J = 6.8 Hz, 2H), 3.28ꢀ3.25 (t, J = 6.8 Hz, 2H) ppm; C NMR (100 MHz,
CDCl + CCl ) δ 195.6, 158.4, 153.7, 146.4, 136.1, 134.4, 130.5, 129.9, 129.2, 129.1, 126.5,
3
4
+
1
3
6
25.2, 119.3, 118.3, 42.6, 28.1 ppm; HRMS (ESI ) calcd for C H ClO SNa (M + Na)
18 13 3
67.0172; found 367.0175.
-Bromo-3-(3-(phenylthio)propanoyl)-2H-chromen-2-one 3o:
ꢀ1 1
Yield 95%; Mp 135 °C; IR Data (ν) 3030, 2930, 1740, 1678, 1558, 1295, 758 cm ; H NMR
(
400 MHz, CDCl + CCl ) δ 8.37 (s, 1H), 7.77ꢀ7.70 (m, 2H), 7.36ꢀ7.34 (m, 2H), 7.26ꢀ7.16 (m,
3 4
1
3
4
H), 3.48ꢀ3.44 (t, J = 6.8 Hz, 2H), 3.28ꢀ3.24 (t, J = 6.8 Hz, 2H) ppm; C NMR (100 MHz,
CDCl + CCl ) δ 195.5, 158.3, 154.2, 146.4, 137.2, 136.0, 132.4, 129.8, 129.1, 126.4, 125.1,
3
4
+
1
19.8, 118.5, 117.7, 42.6, 28.1 ppm; HRMS (ESI ) calcd for C H BrO SNa (M + Na)
18 13 3
410.9666; found 410.9669.

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7-(Diethylamino)-3-(3-(phenylsulfinyl)propanoyl)-2H-chromen-2-one 3p:
O
S
O
Et N
O
O
2
To the coumarin 3f (1.15 g, 3.01 mol) solution in acetonitrile (20 mL) aqueous H O solution
2
2
(30%, 0.5 mL) was added and the reaction mixture was stirred at rt for 22 h. TLC showed the
absence of 3f. Acetonitrile was evaporated and the resulting solid was washed with water and
dried under vacuum to get 3p as a faint yellow solid. Yield = 1.10 g (90%); Mp 165 °C; IR Data
ꢀ1 1
(
ν) 3045, 2964, 1713, 1665, 1572, 1171, 815 cm ; H NMR (400 MHz, CDCl + CCl ) δ 8.36 (s,
3 4
1H), 7.33ꢀ7.29 (m, 3H), 7.21ꢀ7.17 (m, 2H), 7.10ꢀ7.08 (m, 1H), 6.55ꢀ6.52 (m, 1H), 6.39ꢀ6.38 (m,
13
1
H), 3.41ꢀ3.35 (m, 6H), 3.23ꢀ3.19 (t, J = 7.2 Hz, 2H), 1.18ꢀ1.15 (t, J = 8.0 Hz, 6H) ppm; C
NMR (100 MHz, CDCl + CCl ) δ 195.9, 160.7, 158.9, 153.2, 148.3, 136.6, 132.1, 129.4, 129.1,
3
4
+
1
26.1, 115.7, 109.9, 108.4, 96.8, 45.3, 42.3, 28.3, 12.5 ppm; HRMS (ESI ) calcd for
C H NO SNa (M + Na) 420.4788; found 436.4785
22
23
4
7-(Diethylamino)-3-(3-(phenylsulfonyl)propanoyl)-2H-chromen-2-one 3q:
To the coumarin 3f (1.20 g, 3.21 mol) taken in MeOH (10 mL) oxone (5.52 g, 9.0 mmol)
dissolved in 10 mL of water was added slowly during 10 min. Resulting reaction mixture was
stirred for 8 h at rt by which time 3f was absent in TLC. Methanol and water were removed in
reduced pressure and the resulting solid was washed with water and dried under vacuum to get
yellow solid 3q. Yield = 1.27 g (97%); Mp 175 °C; IR Data (ν) 3045, 2965, 1712, 1618, 1572,
ꢀ1 1
1
171, 815 cm ; H NMR (400 MHz, CDCl ) δ 8.37 (s, 1H), 7.97ꢀ7.95 (m, 2H), 7.64ꢀ7.62 (m,
3

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1H), 7.58ꢀ7.54 (m, 2H), 7.38ꢀ7.36 (d, J = 9 Hz, 1H) 6.62ꢀ6.60 (m, 1H), 6.45ꢀ6.44 (m, 1H), 3.55ꢀ
13
3
.53 (m, 4H), 3.46ꢀ3.43 (m, 4H), 1.25ꢀ1.24 (t, J = 8.0 Hz, 6H) ppm; C NMR (100 MHz,
CDCl ) δ 193.3, 160.6, 159.1, 153.5, 148.5, 139.1, 133.8, 132.2, 129.4, 128.4, 114.7, 110.2,
3
+
1
08.3, 96.7, 51.3, 45.3, 35.9, 12.5 ppm; HRMS (ESI ) calcd for C H NO SNa (M + Na)
2
2
23
5
436.1195; found 436.1192.
Quantum yield calculation:
ꢀAcetyl coumarin 3d was taken as reference to calculate quantum yields of the coumarins
3
3
f, 3p, 3q, since all of them share the chromophore. Indeed, the absorption characteristics of
f3f, 3p, 3q overlapped with that of the parent 3d. Quantum yields were calculated using the
3
formula given below by keeping the absorbance maximum between 0.02 – 0.08 to avoid inner
filter effects and to ensure linear response on the intensity.
ꢂ ꢄꢃ ꢅꢆ
ꢀ
= ꢀꢁ ꢂꢃ ꢄ ꢅꢆ
ꢃ
I = Integrated fluorescence intensity
A = Absorbance at the excitation wavelength
η = refractive index of the solvent
Q = quantum yield of the reference compound
R
It is to be noted that refractive index does not have a role in the above formula as
absorption and emission spectra of 3ꢀacetylꢀ7ꢀdiethylaminocourmain 3d (standard) and the
coumarins 3f, 3p, 3q were freshly recorded in respective solvents. Therefore, the refractive index
values for each solvent nullify as they appear both in the numerator and the denominator.
Excitation wavelengths and calculated quantum yields of reference 3d and the courmains 3f, 3p,
3q are given in Table 1 and Table 2 respectively.
Acknowledgement

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H. S. P. R thanks UGC, UGCꢀSAP, DSTꢀFIST for financial assistance. A. D. thanks CSIR for
fellowship. We thank CIF, Pondicherry University, and IISc, Bangalore for spectra.
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0
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The indium(III) chloride catalyzed synthesis of sulfur incorporated 3-acylcoumarins; their
photochromic and acetate sensing properties.
H. Surya Prakash Rao*, Avinash Desai
Department of Chemistry, Pondicherry University, Pondicherry – 605 014 India.
E.mail: hspr.che@pondiuni.edu.in; Telephone: +91413 2654411; Fax: +91413 2656230
6
2
1
1
5
.0x10
.5x10
.0x10
.0x10
sodium pottasium tartarate
NaHSO3
6
6
5
KCl
NaN3
KOAc
NaNO
O
O
O
O
R1
R2
3
CHO
OH
InCl , 10 mol%
R1
R2
3
+
SPh
SPh
EtOH, 80 o
.5-8 h, 74-95%
C
EtO
O
6
0.0
500
600
700
800
Wavelength (nm)