ZnO nanorods as an efficient catalyst for the green synthesis of indole derivatives using isatoic anhydride

Article abstract of DOI:10.1007/s10593-015-1734-1

[Figure not available: see fulltext.] Indole derivatives have been synthesized in high yields by the reaction of isatoic anhydride and alkyl bromides in the presence of ZnO nanorods and an equimolar amount of an alcohol as a catalyst under solvent-free co

Full text of DOI:10.1007/s10593-015-1734-1

DOI 10.1007/s10593-015-1734-1
Chemistry of Heterocyclic Compounds 2015, 51(6), 541–544
ZnO nanorods as an efficient catalyst for the green synthesis
of indole derivatives using isatoic anhydride
Sayyed Zahra Sayyed-Alangi1*, Zinatossadat Hossaini2
1 Department of Chemistry, Azadshahr Branch, Islamic Azad University,
Azadshahr, Iran; e-mail: zalangi@gmail.com
2 Department of Chemistry, Qaemshahr Branch, Islamic Azad University,
P.O. Box: 163, Qaemshahr, Iran; e-mail: zshossaini@yahoo.com
Published in Khimiya Geterotsiklicheskikh Soedinenii,
2015, 51(6), 541–544
Submitted May 31, 2015
Accepted June 19, 2015
O
OH
ZnO nanorods (20 mol %)
ROH (1 equiv)
O
O
O
+
Br
R1
R1
Solvent-free
80°C, 12 h
N
N
H
O
H
R = Me, Et; R1 = Ar, CO2Et
Indole derivatives have been synthesized in high yields by the reaction of isatoic anhydride and alkyl bromides in the presence of ZnO
nanorods and an equimolar amount of an alcohol as a catalyst under solvent-free conditions at 80°C. The reaction work-up is simple, and
the products can be easily separated from the reaction mixture. The ZnO nanocatalyst proved to be reusable and exhibited a significant
yield enhancement in comparison with bulk ZnO and some other nanocatalysts.
Keywords: alkylbromide, indole, ZnO nanorods, green synthesis, solvent-free conditions.
Indole nucleus is important in medicinal chemistry and
is considered as one of the so-called advantageous scaf-
folds.1 Consequently, the synthesis of indoles derivatives
with high selectivity have been the focus of active research
in recent years.2–8 Indole derivatives comprise an important
class of compounds of interest in medicinal chemistry
involving anticancer,9 antioxidant,9,10 antirheumatic, and
antiHIV activities11,12 and influence on the immune
system.13,14 Many indole derivatives are considered as
effective scavengers of free radicals.15 The avoidance of
using hazardous organic solvents in organic syntheses is
the most important goal in green chemistry.16–18 Solvent-
free organic reactions make syntheses simpler, save energy,
and avoid environmental hazards due to toxic solvent
waste. Also, in recent years, there has been an increased
interest in catalysis by nanomaterials. These materials
display better catalytic activity compared to the same
substances consisting of bulk-size particles.19,20 Nano-
crystalline zinc oxide is a non-hygroscopic, cheap, and
non-toxic material which has been used as a catalyst of
various organic reactions.21–26 Continuing our efforts
directed towards the simple preparation of biologically
active target molecules through one-pot reactions,27–29 we
present here the synthesis of some novel indoles via a one-
pot reaction of isatoic anhydride, alcohols, and alkyl
bromide in the presence of zinc oxide nanorods (NR-ZnO)
at 80°C in a solvent-free reaction medium (Scheme 1).
Solvent-free stirred mixture of isatoic anhydride (1) and
alkyl bromides 2a–f in the presence of equimolar amount
of alcohol and ZnO nanorods led to indole derivative 3a–f
Scheme 1
O
OH
NR-ZnO (20 mol %)
ROH (1 equiv)
O
O
O
+
Br
R1
R1
Solvent-free
80°C, 12 h
N
N
H
O
H
3a–f
2a–f
1
R = Me, Et;
2, 3 a R1 = 4-MeOC6H4, b R1 = 4-BrC6H4, c R1 = 4-MeC6H4,
d R1 = 4-O2NOC6H4, e R1 = 3-MeOC6H4, f R1 = CO2Et
0009-3122/15/5106-0541©2015 Springer Science+Business Media New York
541

Chemistry of Heterocyclic Compounds 2015, 51(6), 541–544
Table 1. Optimization of reaction conditions
for the synthesis of compound 3a
in 85–90% yields (Scheme 1). The alcohol used was
methanol (for the synthesis of compounds 3a,c,e,f) or
ethanol (for the synthesis of compounds 3b,d). The
products were obtained as solids, and no impurities were
observed by TLC. Structures of products were confirmed
Entry
Catalyst
Catalyst load, mol % Yield, %
1
2
3
4
5
6
7
8
9
None
None
10
0
1
by IR, H and 13C NMR spectroscopy. For example, the
CM-ZnO
CM-ZnO
NR-ZnO
NR-ZnO
NR-ZnO
NR-ZnO
65
70
75
78
85
80
55
67
55
55
1H NMR spectrum of compound 3a exhibited singlets at
3.85, 11.23, and 11.74 ppm for OMe, NH, and OH protons,
respectively. The 13C NMR spectrum of compound 3a
featured the carbonyl group signal at 189.3 ppm and the
signal of the methoxy group carbon at 55.6 ppm in
agreement with the proposed structure.
15
10
15
20
25
The method that was used for the synthesis of NR-ZnO
upon the interaction of NaOH and Zn(OAc)2 in water has
been already reported in literature.29 The average crystal
size for NR-ZnO is about 30 nm.30,31 The nanoparticle
morphology can be controlled using sodium dodecylsulfate
(SDS). The morphology of the products was confirmed by
SEM (Fig. 1). Formation of ZnO with different morpho-
logies, like nanosheets and nanorods, involve the inter-
action between crystallization nucleus and SDS bilayers or
micelles to control the growth rate in a certain direction,
i.e., at various faces of the preformed nucleus.32 It seems
that the NR-ZnO morphology results under the influence of
cylindrical inverse micelles in aqueous solutions of SDS.33
The X-ray diffraction (XRD) pattern of NR-ZnO is
shown in Figure 2. All the characteristic peaks in the
pattern correspond to the wurtzite structure of ZnO, which
can be indexed on the basis of JCPDS file No. 36-1451. No
other characteristic peaks of the impurities are observed,
indicating the high purity of the NR-ZnO catalyst.
NP–KF/clinoptilolite
NP–KF/clinoptilolite
10
15
10 Nanozeolite clinoptilolite
11 Nanozeolite clinoptilolite
15
20
In order to find the best reaction conditions, the reaction
of isatoic anhydride (1), methanol, and 4-methoxyphenacyl
bromide (2a) was selected as a model reaction (Scheme 1)
and commercial zinc oxide (CM-ZnO) (CAS Number:
1314-13-2) was used as a catalyst. It was found to give
70% yield of product at 80°C under solvent-free condition
(Table 1, entry 2). Confident by this outcome, further
optimization was performed by NR-ZnO as catalyst.
The result of our optimization studies in catalyst
charging are showed in Table 1. The reaction did not take
place without any catalyst (entry 1), and the yield of the
product was increased when NR-ZnO was used (entries 4–7)
in comparison to the use of bulk ZnO or some two other
nanocatalysts. The yield increased along with the catalyst
load up to 20%, but further increase in the amount of
catalyst led to the decrease of product yield. The best
reaction temperature when using NR-ZnO was 80°C. The
catalyst can be separated by filtration and reused at least
six times without significant loss of activity. The
reusability of the catalyst was checked for the synthesis of
compound 3a. The catalyst was separated after each run
and washed thoroughly with ethyl acetate. Then it was
dried at room temperature for 24 h and used for the next
catalytic cycle.
Figure 1. SEM image of NR-ZnO particles.
Although we have not established the mechanism of the
reaction between the isatoic anhydride (1) and alkyl
bromides 2 in the presence of alcohol and NR-ZnO, a
possible explanation is suggested in Scheme 2. NR-ZnO
has Lewis-acidic sites (Zn2+) and Lewis-basic sites (O2–).17,34 It
is possible that the reaction includes the initial formation of
anthranilic ester 4 from the alcoholysis of isatoic anhydride
(2) on the surface of NR-ZnO and elimination of carbon
dioxide. Then alkyl bromide 2 reacts with the amine group
of intermediate 4 producing adduct 5. Possibly, the genera-
tion of enol form of the intermediate 5 and the subsequent
intramolecular attack on the ester carbonyl group in the
presence of NR-ZnO eliminates alcohol and generates
diketone 6 which finally produces 3-hydroxyindole 3 by
enolization.23
Figure 2. XRD pattern of NR-ZnO.
542

Chemistry of Heterocyclic Compounds 2015, 51(6), 541–544
Scheme 2
O
O
Br
O
R1
O
2
OR
O
OR
+
ROH
R1
– HBr
– CO2
– ROH
N
H
N
H
O
NH2
O
4
5
1
O
ZnO nanorod
Lewis-acidic site
Lewis-basic site
OH
O
O
H R1
N
R1
H
N
H
6
3
stirred for about 12 h. After completion of reaction (TLC
monitoring), the viscous residue was purified by column
chromatography on silica gel (Merck 230–400 mesh) using
n-hexane–EtOAc, 7:1, as eluent.
In summary, the procedure described here provides an
acceptable one-pot method for the preparation of indole
derivatives from the reaction of isatoic anhydride and alkyl
bromides in the presence of alcohol and NR-ZnO.
Moreover, easy work-up, solvent-free conditions, and
reusability of catalyst make this method as an interesting
alternative to other approaches.
(3-Hydroxy-1H-indol-2-yl)(4-methoxyphenyl)methanone
(4a). Yield 0.46 g (87%), yellow powder, mp 152–154°C
(mp 157–158°C35). IR spectrum, ν, cm–1: 3345, 1728, 1725,
1696, 1478, 1225. 1H NMR spectrum, , ppm (J, Hz): 3.85
3
Experimental
(3H, s, CH3O); 6.94 (2H, d, J = 7.4, H Ar); 7.18 (1H, t,
3
3J = 7.5, H Ar); 7.48 (1H, t, J = 7.5, H Ar); 7.58 (1H, d,
IR spectra were recorded on a Nicollet Magna 550-FT
spectrometer in KBr. 1H and 13C NMR spectra were
recorded on a Bruker FT-500 spectrometer (500 and
100 MHz, respectively) in CDCl3, and TMS was used as
internal standard. Electron ionization mass spectra were
recorded on a Finnigan Mat TSQ-70 spectrometer ope-
rating at an ionization potential 70 eV. Elemental analyses
were carried out with a Perkin-Elmer model 240-C appa-
ratus. Melting points were measured on an Electrothermal
9100 apparatus. The morphology of nanorods of ZnO was
established by utilizing a Holland Philips XL30 scanning
electron microscope. X-ray diffraction analysis was per-
formed at room temperature on a Holland Philips Xpert
X-ray powder diffractometer with CuKα radiation
(λ 1.5406 Å), over the 2θ collection range of 20–80°.23
Average crystal size of the nanoparticles was calculated
using the Scherrer formula: D = 0.9λ/(βcosθ),23 where D is
the diameter of the nanoparticles and β is the full-width at
half-maximum of the diffraction lines.17 All chemicals were
purchased from Fluka or Merck and were used without
further purification.
Preparation of ZnO nanorods. Sodium hydroxide (0.44 g,
1.1 mmol) was dissolved in distilled water (75 ml) under
vigorous stirring at room temperature. Afterwards, sodium
dodecylsulfate (1.57 g, 5 mmol) and Zn(OAc)2·2H2O (0.6 g,
3 mmol) were added, and the solution (pH 14) was refluxed
for 1.5 h at 80°C. The product was filtered off and washed
with distilled water and ethanol (96%) several times.30
Yield 89%.
Preparation of indoles 3a–f (General method).
NR-ZnO (20 mol %) was added to a stirred mixture of
isatoic anhydride (1) (0.32 g, 2 mmol) and methanol or
ethanol (2 mmol). After 40 min, alkyl bromide 2a–f
(2 mmol) was added at 80°C. The reaction mixture was
3
3J = 7.6, H Ar); 8.12 (2H, d, J = 7.4, H Ar); 8.32 (1H, d,
3J = 7.6, H Ar); 11.23 (1H, s, NH); 11.74 (1H, s, OH).
13C NMR spectrum, δ, ppm: 55.6 (CH3O); 97.3 (C); 113.2
(CH); 114.5 (2CH); 116.4 (CH); 117.6 (CH); 120.4 (C);
123.2 (CH); 129.7 (C); 132.4 (2CH); 135.4 (C); 159.5
(C–OH); 161.7 (C); 189.3 (C=O). Mass spectrum, m/z
(Irel, %): 267 [M]+ (15), 135 (100), 132 (45), 31 (100).
Found, %: C 71.83; H 4.82; N 5.14. C16H13NO3.
Calculated, %: C 71.90; H 4.90; N 5.24.
(4-Bromophenyl)(3-hydroxy-1H-indol-2-yl)methanone
(4b). Yield 0.54 g (85%), yellow powder, mp 178–180°C.
IR spectrum, ν, cm–1: 3365, 1732, 1727, 1695, 1462, 1275.
3
1H NMR spectrum, , ppm (J, Hz): 7.18 (1H, t, J = 7.4,
H Ar); 7.62 (1H, t,3J = 7.4, H Ar); 7.75 (1H, d, 3J = 7.5, H Ar);
7.87 (2H, d, 3J = 7.8, H Ar); 7.92 (2H, d, 3J = 7.8, H Ar); 8.42
(1H, d, 3J = 7.4, H Ar); 11.27 (1H, s, NH); 11.85 (1H, s, OH).
13C NMR spectrum, δ, ppm: 97.5 (C); 113.5 (CH); 118.7 (CH);
119.5 (CH); 120.4 (C); 123.6 (CH); 129.4 (C); 130.4 (2 CH);
131.6 (2CH); 134.2 (C); 135.6 (C); 160.3 (C–OH); 188.6
(C=O). Mass spectrum, m/z (Irel, %): 315 [M]+ (6), 184 (100),
132 (65). Found, %: C 56.87; H 3.12; N 4.35. C15H10BrNO2.
Calculated, %: C 56.99; H 3.19; N 4.43.
(3-Hydroxy-1H-indol-2-yl)(4-methylphenyl)methanone
(4c). Yield 0.43 g (85%), pale-yellow powder, mp 148–
150°C (mp 154°C35). IR spectrum, ν, cm–1: 3342, 1725,
1720, 1687, 1464, 1247. 1H NMR spectrum, , ppm
(J, Hz): 2.37 (3H, s, CH3); 7.16 (1H, t, 3J = 7.5, H Ar); 7.38
3
(2H, d, 3J = 7.6, H Ar); 7.56 (1H, t, J = 7.5, H Ar); 7.72
3
3
(1H, d, J = 7.5, H Ar); 8.14 (2H, d, J = 7.6, H Ar); 8.52
3
(1H, d, J = 7.5, H Ar); 11.15 (1H, s, NH); 11.62 (1H, s,
OH). 13C NMR spectrum, δ, ppm: 21.3 (CH3); 96.4 (C);
112.8 (CH); 118.2 (CH); 119.4 (CH); 120.5 (C); 123.4
(CH); 129.4 (2CH); 130.4 (2CH); 133.7 (C); 135.2 (C);
543

Chemistry of Heterocyclic Compounds 2015, 51(6), 541–544
4. (a) Yoshiaki, N.; Masato, Y.; Youichi, I.; Masnobu, H.;
142.4 (C); 158.9 (C–OH); 188.6 (C=O). Mass spectrum, m/z
(Irel, %): 251 [M]+ (10), 132 (85), 119 (100). Found, %:
C 76.34; H 5.12; N 5.48. C16H13NO3. Calculated, %:
C 76.48; H 5.21; N 5.57.
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(3-Hydroxy-1H-indol-2-yl)(4-nitrophenyl)methanone
(4d). Yield 0.48 g (85%), yellow powder, mp 185–187°C.
IR spectrum, ν, cm–1: 3352, 1734, 1730, 1685, 1482, 1263.
6. Bao, B.; Sun, Q.; Yao, X.; Hong, J.; Lee, C. O.; Sim, C. J.;
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3
1H NMR spectrum, , ppm (J, Hz):7.32 (1H, t, J = 7.6,
3
H Ar); 7.42 (2H, d, 3J = 7.8, H Ar); 7.58 (1H, t, J = 7.5,
3
3
H Ar); 7.72 (1H, d, J = 7.5, H Ar); 8.38 (2H, d, J = 7.8,
8. Kaniwa, K.; Arai, M. A.; Li, X.; Ishibashi, M. Bioorg. Med.
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3
H Ar); 8.53 (1H, d, J = 7.6, H Ar); 11.16 (1H, s, NH);
11.68 (1H, s, OH). 13C NMR spectrum, δ, ppm: 89.6 (C);
112.8 (CH); 118.4 (CH); 119.2 (CH); 120.7 (C); 123.4
(CH); 124.7 (2CH); 131.6 (2CH); 135.6 (C); 138.9 (C);
149.3 (C); 158.7 (C–OH); 189.5 (C=O). Mass spectrum, m/z
(Irel, %): 282 [M]+ (15), 150 (100), 132 (58). Found, %:
C 63.92; H 3.63; N 9.98. C15H10N2O4. Calculated, %:
C 63.83; H 3.57; N 9.93.
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(3-Hydroxy-1H-indol-2-yl)(3-methoxyphenyl)methanone
(4e). Yield 0.46 g (87%), yellow powder, mp 156–158°C.
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1H NMR spectrum, , ppm (J, Hz): 3.87 (3H, s, CH3O),
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3
6.97 (1H, d, J = 7.3, H Ar); 7.22 (1H, t, 3J = 7.5, H Ar);
7.38 (1H, t, 3J = 7.4, H Ar); 7.52 (1H, s, H Ar); 7.62 (2H, t,
3
3J = 7.4, H Ar); 7.75 (1H, d, J = 7.5, H Ar); 8.52 (1H, d,
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3J = 7.4, H Ar); 11.25 (1H, s, NH); 11.63 (1H, s, OH).
13C NMR spectrum, δ, ppm:54.2 (CH3O); 96.5 (C); 113.5 (CH);
118.2 (CH); 119.4 (CH); 119.7 (CH); 120.8 (C); 122.4 (CH);
123.7 (CH); 125.3 (CH); 130.2 (CH); 135.2 (C); 136.3 (C);
158.7 (C–OH); 159.6 (C); 188.7 (C=O). Mass spectrum, m/z
(Irel, %): 267 [M]+ (15), 135 (100), 132 (45), 108 (65), 31 (100).
Ethyl 2-(3-hydroxy-1H-indol-2-yl)-2-oxoacetate (4f).
Yield 0.42 g (90%), white powder, mp 123–125°C.
IR spectrum, ν, cm–1: 3352, 1748, 1725, 1587, 1432, 1259.
1H NMR spectrum, , ppm (J, Hz):1.32 (3H, t, 3J = 7.4, CH3);
4.25 (2H, q, 3J = 7.4, CH2O); 7.32 (1H, d, 3J = 7.6, H Ar); 7.65
(1H, t, 3J = 7.5, H Ar); 7.78 (1H, t, 3J = 7.5, H Ar); 8.63 (1H, t,
3J = 7.5, H Ar); 10.34 (1H, s, NH), 10.67 (1H, s, OH).
13C NMR spectrum, δ, ppm: 13.4 (CH3); 62.2 (CH2O); 102.7
(C); 112.4 (CH); 116.8 (CH); 119.2 (CH); 122.3 (C); 123.7
(CH); 137.4 (C); 159.6 (C–OH); 163.6 (C=O); 178.6 (C=O).
Mass spectrum, m/z (Irel, %): 233 [M]+ (20), 188 (68), 45 (100).
Found, %: C 61.92; H 4.83; N 6.15. C12H11NO4. Calculated, %:
C 61.80; H 4.75; N 6.01
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