Sulfated Zirconia: An Efficient and Reusable Heterogeneous Catalyst in the Friedel–Crafts Acylation Reaction of 3-Methylindole

Article abstract of DOI:10.1007/s10562-017-2057-x

Abstract: An efficient process for the Friedel–Crafts acylation of 3-methylindole with acid anhydrides in the presence of sulfated zirconia was developed. This catalyst shows excellent catalytic activity with good conversion and selectivity towards formation of the products of 2-acylation (3-methyl-1H-indol-2-yl)ketones. Other advantages related to this process are the simple work-up procedure and smaller production of chemical waste. The catalyst was easily recycled and reused with a minimal loss in activity through three reaction cycles. The catalyst was characterized by FT-IR spectroscopy, X-Ray diffraction and superficial acidity. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2057-x

Catal Lett
DOI 10.1007/s10562-017-2057-x
Sulfated Zirconia: An Efficient and Reusable Heterogeneous
Catalyst in the Friedel–Crafts Acylation Reaction
of 3-Methylindole
Darío A. Vargas1 · Leticia J. Méndez1,2 · Alicia S. Cánepa1 · Rodolfo D. Bravo1
Received: 20 January 2017 / Accepted: 17 April 2017
© Springer Science+Business Media New York 2017
Abstract An efficient process for the Friedel–Crafts
acylation of 3-methylindole with acid anhydrides in the
presence of sulfated zirconia was developed. This catalyst
shows excellent catalytic activity with good conversion and
selectivity towards formation of the products of 2-acyla-
tion (3-methyl-1H-indol-2-yl)ketones. Other advantages
related to this process are the simple work-up procedure
and smaller production of chemical waste. The catalyst was
easily recycled and reused with a minimal loss in activity
through three reaction cycles. The catalyst was character-
ized by FT-IR spectroscopy, X-Ray diffraction and superfi-
cial acidity.
1 Introduction
Indole derivatives present wide biological importance in
a variety of therapeutic areas such as anti-inflammatory,
anticonvulsant, cardiovascular, antitumor, antibacterial,
antiparkinsonian, antiviral and antidiabetics agents, among
others [1].
In particular, acylindoles present appreciable interest,
not only for their biological properties but they are also
widely used as intermediates in the synthesis of alkaloid
and different families of heterocyclic compounds [2, 3].
There are different methods in the synthesis of acylin-
doles, among them the Friedel–Crafts reactions [4–7], Vils-
meier–Haack reaction [8] and the use of Grignard reactions
[9].
Graphical Abstract
O
(RCO)2O/ SO4-2/ZrO2
Some of these methods present advantages and short-
comings that limit their scope and performance. The Vils-
meier–Haack acylations also give good yields, but the
amides used are limited (e.g. formamide, alkylcarboxam-
ide) and a large amount of POCl3 is used, causing compli-
cations in the environment.
+
R
N
N
H
N
H
COR
3
1
2
Keywords Sulfated zirconia · 3-Methylindole · Ketones ·
Recyclability · Acylation reaction
Regarding the Friedel–Crafts reaction, the electro-
philic substitution preferably occurs at the C-3 instead of
the C-2 position. Acylation at the C-2 position can occur
for indoles 1,3-disubstituted or 3-substituted. However,
in 3-substituted indole compounds, N-acylation may also
occur [10]. The use of N-protecting groups is generally the
chosen strategy to avoid the formation of 3-acyl derivatives
[11]. The disadvantage of this process is the largest number
of synthetic steps, with the initial preparation of the N-pro-
tected derivative, subsequent acylation and removal of the
protecting group to give the final product.
* Alicia S. Cánepa
ascanepa@quimica.unlp.edu.ar
1
Centro de Estudio de Compuestos Orgánicos, Departamento
de Química, Facultad de Ciencias Exactas, Universidad
Nacional de La Plata, Calle 47 y 115, 1900 La Plata,
Argentina
Our research group is interested in obtaining 2-acyl-
3-methylindoles, which are used as substrates in the
synthesis of complex heterocyclic compounds. Thus,
2
CINDECA (CONICET-UNLP), Facultad de Ciencias
Exactas, 47 No. 257, 1900 La Plata, Argentina
Vol.:(0123456789)
1 3

D. A. Vargas et al.
Friedel–Crafts acylation arises as a good synthetic way
of mentioned compounds from 3-methylindole, using
Lewis or Brønsted acid catalysts. Homogeneous Lewis
acids used for these reactions include AlCl3, BF3, TiCl4,
ZnCl2, SnCl2, SnCl4 and ZrCl4 among others [7, 12–14].
These days, a number of factors demand the use of
heterogeneous catalysts because of problems associated
with the handling, disposal of homogeneous acids and
environmental contamination.
2 Experimental
2.1 Materials
Melting points were determined with a Buchi apparatus.
1HNMR and 13CNMR spectra were recorded on a Bruker
AVIII 600 Biospin in CDCl3 or DMSO-d6. Thin layer chro-
matography was performed on silica gel sheets 60 F254
(Merck A.G.). Silica gel 60 (70–230 mesh) (Fluka) was
used for column chromatography. Commercial 3-meth-
ylindole (98%) (Aldrich) was recrystallized from hexane
before use. All reagents were purchased from Merck A.G.
and Aldrich; 1,2-dichloroethane and 1,1,2-trichloroethane
were distilled over phosphorous pentoxide and stored over
4 Å molecular sieves. Commercial SO4 −2/ZrO2 from Mel
Chemical Co was dried at 110°C for 2 h in an air and was
subsequently calcined for 5 h in air atmosphere at 550°C,
based on previous experiences [21]. All yields refer to iso-
lated products.
We have studied the synthesis of different compounds
through the use of homogeneous or heterogeneous acid
catalysts. So, sulfuric acid, methanesulfonic acid (MSA)
and trifluoroacetic acid (TFA) were used as homogene-
ous catalysts [15, 16]. Moreover, many of these pro-
cesses were performed using heterogeneous catalysts to
compare their viability. The catalysts used were tung-
sten and molybdenum heteropolyacids (H3PWO40 and
H3PMoO40) supported on silica [17], Amberlyst 15 and
Amberlyst XN-1010 resins [15, 18–21], sulfated zirco-
nia [22, 23], P2O5 and HClO4 supported on silica [24].
In almost all cases, yields increased when heterogeneous
catalysts were used, with simplicity in the purification
process and a smaller amount of polluting waste.
Among those mentioned sulfated zirconia has
attracted much attention in recent years because of
its good catalytic activity, super-acidity, non-toxicity
and several advantages such as short reaction times,
high selectivity and the easiness of work-up procedure.
Some interesting methods using this catalyst include
Friedel–Crafts acylation [25], esterification reactions
[26–28], synthesis of flavones [29], synthesis of cou-
marins by the Pechmann reaction [30] and 1,5-benzodi-
azepines [31].
2.2 Catalyst Characterization
The solids were characterized by FT-IR spectroscopy,
X-Ray diffraction (XRD) and superficial acidity.
X-ray diffractograms were obtained in a Philips
PW-1732 equipment, using Cu Kα radiation Ni filter;
20 mA and 40 kV as the high voltage source, scanning
angle between 5° and 50° 2θ, and scanning rate 1°/min.
FT-IR spectra were obtained using a Bruker IFS 66 FTIR
spectrometer.
The determination of total acid sites of the catalysts was
measured by potentiometric titration with n-butylamine in
acetonitrile as a non-aqueous medium. The solid (0.05 g)
was suspended in acetonitrile, and stirred for 3 h. The sus-
pension was titrated with a 0.05 N solution of n-butylamine
in acetonitrile using a Metrohm 794 Titrino apparatus with
a double junction electrode.
Continuing with our work on the application of het-
erogeneous catalysts for development of useful synthetic
methods, we decided to study the acylation of 3-methyl-
indole using sulfated zirconia as a catalyst.
Referring to the Friedel–Crafts acylation of indole
derivatives, mono- and bi-substituted products can be
obtained. In relation to obtaining mono acylated prod-
ucts, these may be substituted at the C-2 position (2), at
the N-position (3) and other carbon atoms in the benzene
ring (4) (Scheme 1).
2.3 Reaction Study
Initially, the reaction between 3-methylindole (1)
(1.0 mmol) and acetic anhydride as the acylating reagent
(variable amounts) was investigated as a model to optimize
the experimental conditions. Sulfated zirconia was used as
a heterogeneous catalyst. Parallel reactions are performed
Scheme 1 Possible products
O
in mono acylation of 3-meth-
R
+
acylating reagent /
O
ylindole
catalyst
+
R
N
H
N
H
N
N
H
COR
3
4
1
2
1 3

Sulfated Zirconia: An Efficient and Reusable Heterogeneous Catalyst in the Friedel–Crafts…
3000
using acetic acid as an example of a homogeneous catalyst
in order to compare their behavior (Scheme 1). These were
2500
carried out in a batch reactor with magnetic stirring under
Ar atmosphere. The progress of the reaction was monitored
reused catalyst
fresh catalyst
2000
by TLC (9:1 hexane/ethyl acetate). When the reaction was
complete, the catalyst was separated by filtration, and the
1500
solvent was removed in vacuum. The crude products were
purified by column chromatography on silica gel (9:1 hex-
1000
ane/ethyl acetate). The known reactions products were
characterized by comparison of their physical and 1HNMR
and 13CNMR spectra and data with those of reported ones.
500
The identification of new compound was made through
1HNMR and 13CNMR spectra and elemental analysis.
0
0
10
20
30
40
50
60
70
80
Different reaction conditions were checked, catalyst
2θ(2 theta )
amount, substrate/acylating reagent ratio and reaction time,
for both type of catalysts.
Fig. 1 XRD patterns of sulfated zirconia for fresh sample calcined at
The reactions were carried out either at 82°C using
1,2-dichloroethane as a solvent or at 140°C using
1,1,2,2-tetrachloroethane as a solvent.
550°C and a reused sample
600
catalyst fresh
2.4 Acylating Reagent
500
1 cycle
2 cycle
3 cycle
4 cycle
400
300
200
100
0
With the optimized conditions in hand, we probed the
scope of acylation of 3-methylindole using other examples
of acylating agents such as propionic anhydride, n-butyric
anhydride and benzoic anhydride.
2.5 Reusability of Catalyst
The used catalysts were washed, according to the case, with
1,2-dichloroethane or 1,1,2,2-tetrachloroethane, then meth-
anol and drying in air at 120°C for 2 h and reused in subse-
quent following experiments.
-100
-200
0,0
0,2
0,4
0,6
0,8
meq/g n-butylamine/ g catalyst
Fig. 2 Potentiometric titration of n-butylamine in acetonitrile for
3 Results and Discussion
3.1 Catalyst Characterization
sample calcined at 550°C and after being used in different cycles
the initial electrode potential (Ei) indicates the maximum
acid strength of the surface site, and the materials with
Ei values higher than 100 mV are defined as very strong
solid acids. Additionally, the range where the plateau was
reached [meq(n-butylamine) g−1] shows the [32, 33].
The measurements were performed on an original
sample and after being used in different cycles for the
acylation reaction using acetic anhydride as acylating
reagent. The potentiometric titration curves are presented
in Fig. 2.
The properties of sulfated zirconia were examined by pow-
der XRD, FT-IR spectroscopy and superficial acidity in
catalysts calcined at 550°C, without use and after being
used.
The XRD patterns of samples calcined at 550°C with-
out use (a) and after being used (b) are shown in Fig. 1. As
can be seen from Fig. 1 sulfated zirconia calcined at 550°C
contained only tetragonal phases (2θ=30.16°, 34.96°,
50.22° and 59.98°). The catalyst recovered and reused
shown the same pattern of bands, without changes after
three catalytic cycles.
It can be observed in the Fig. 2 that the initial electrode
potential Ei is 516.2, 303.3, 290.6, 257 and 244.2 mV for
the original sample of SO4/ZrO2 and samples used in four
catalytic cycles respectively. These results indicate the
presence of very strong acid sites.
The total number and strength of Brønsted and Lewis
acids sites were measured by nonaqueous potentiometric
titration with n-butylamine in acetonitrile. In this method,
1 3

D. A. Vargas et al.
The amount of n-butylamine consumed was 0.76 meq/g
for the original sample of SO4/ZrO2 to 0.63, 0.80, 0.74 and
0.75 meq/g for SO4/ZrO2 after four cycles respectively.
The FT-IR spectra of catalysts calcined at 550°C, with-
out use and after being used are shown in Fig. 3. The FT-IR
homogeneous and heterogeneous catalysts. As shown in
Table 1 the best results were obtained by the use of sulfated
zirconia. Two products were formed, generated mainly by
the substitution at C-2 position (2a) and a lower yield of
N-acylation product (3a) (Scheme 1).
−2
spectra shown bands of the SO4 group in the region of
1200–900 cm−1, with peaks at 999, 1041, 1072, 1142 and
1235 cm−1, which are in agreement with those reported
in the literature for sulfated zirconia assigned to asym-
metric and symmetric stretching frequencies of ionized
S=O double bonds and S–O bonds. An additional broad
peak at 3408 cm−1 corresponds to the stretching vibrations
hydroxo- and aquo-OH of hydroxyl groups and adsorbed
water accompanied by the band at 1628 cm−1. In addition,
the weak unresolved band between 800 and 520 cm−1 is
attributed to Zr–O stretching modes.
3.2.2 Effect of Different Amounts of Sulfated Zirconia
in the Acetylation of 3-Methylindole
The effect of different amounts of sulfated zirconia was
studied in the acetylation of 3-methylindole in the range
25–100% (w/w). An increase in the yield of the reaction
could be observed with the increase of the proportion of
catalyst employed, obtaining the best results with 100%
(w/w) of sulfated zirconia (Table 1).
FT-IR spectra of reused zirconia sample washed and
dried at 120°C showed features similar to those observed
for fresh sulfated zirconia. In addition, very low intensity
bands at 1456, 2850 and 2927 cm−1 can be assigned to the
bending/scissoring and stretching vibration of C–H and
CH3 groups. The latter were generated, due to traces of
adsorbed reagent.
3.2.3 Influence of the Temperature
To study the effect of temperature, the reaction was carried
out at 82 and 140°C. The results indicate better results at
140°C using 1,1,2,2-tetrachloroethane as a solvent. At this
temperature the products are formed in a short reaction
time (0.25 h), greater yields and regioselectivity towards
formation of product 2a, substituted at the C-2 position
(Table 1).
3.2 Reaction Studies
3.2.1 Comparison of Homogeneous and Heterogeneous
Acids as Catalyst in the Acetylation
of 3-Methylindole
3.2.4 Influence of Molar Ratio of 3-Methylindole
and Acylating reagent
The key stage of this reaction mechanism involves the
generation of acylium ion intermediate, obtained from the
acylating reagent in the presence of a catalyst. As can be
found described in literature, acylation reactions can be
catalyzed by Brønsted or Lewis acid. On this occasion,
acetic acid and sulfated zirconia were used as examples of
The effect of the molar ratio between 3-methylindole and
acylating reagent was studied in the range 1:2–1:4. Experi-
ments showed that a ratio of 1:4 can be considered optimal
to perform this process. As could be observed in Table 1,
by the use a lower ratio of acylating reagent (1:2) the yields
are lower with recovery of reagent in some cases.
ν= stretching
δ = flexion
3.2.5 Acylating Reagent
0,72
0,48
0,24
0,00
reused catalyst
νO-H
The reaction of 3-methylindole (1) using other anhydrides
as acylating reagents were performed in order to study
the selectivity in relation to the substituent. In this regard,
reactions using anhydrides with alkyl substituent (R=Me,
Et and n-propyl) showed limited selectivity with higher
yields of C-2 acylated products, relative to the N-acylated
products as shown in Table 2. In contrast, aromatic anhy-
dride (R=Ph) provide a higher selectivity, with very good
yield of C-2 acylated product (80%), without formation of
another isomer. The stabilizing effect of the phenyl group
in the acylium ion generated, combined with a lower steric
bulk of this group could lead to the better performance.
δ Ο−Η
fresh catalyst
νO-H
δ Ο−Η
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber (cm-1)
Fig. 3 FT-IR of fresh sulfated zirconia calcined at 550°C and reused
sample
1 3

Sulfated Zirconia: An Efficient and Reusable Heterogeneous Catalyst in the Friedel–Crafts…
Table 1 Optimization
of synthesis of acetyl-3-
methylindole 1 using acetic
O
(CH CO) O / catalyst
3
2
anhydride as acylating reagent
+
N
H
N
N
H
COCH
3
3a
1
2a
Catalyst w/w (%) Ratio
Tem-
Time (h) Yield (%) (2a) Yield (%) (3a) Unreacted
substrate/
perature
reagent (%)
(CH3CO)2O
(0C)
(1)
AcOH
100
1:4
82
1
3
6
1
3
6
24
1
1
0.25
1
1
0.25
1
0.25
1
–
–
–
–
–
–
5
40
49
51
46
53
51
49
60
61
–
–
–
–
95
93
91
91
89
28
26
27
10
–
–
–
10
9
–
140
–
32
43
28
25
46
35
26
25
27
30
30
ZS
25
50
1:4
1:4
82
82
140
82
100
1:2
1:4
1:2
140
140
1:4
–
Table 2 Acylation of
3-methylindole 1 and acylating
reagents at 140°C using sulfated
zirconia (100% w/w)
O
(RCO) O / catalyst
2
+
R
N
H
N
H
N
COR
3
1
2
Entry (RCO)2O
Yield (%) (2) Mp (oC) (2) Found/Lit.* Yield (%) (3) Mp (°C) (3) Found/Lit.*
1
2
3
4
(MeCO)2O
(EtCO)2O
(n-PrCO)2O 60 (2c)
(PhCO)2O 80 (2d)
60 (2a)
40 (2b)
147–148/147–149 [34]
166–167/161 [35]
144–146/150–152 [36]
140–140.5/138–139 [37]
30 (3a)
39 (3b)
38 (3c)
–
70–72/66-67 [38]
53–54/45 [39]
56–58
–
3.3 Reusability of Catalyst
Table 3 Catalyst reusability studied
Reusability of sulfated zirconia was also studied in the
reaction to obtain 3a. The catalyst recovered after the
reaction was employed in new processes. Experiments
show that the sulfated zirconia can be reused without
significant loss of activity until three reaction cycles
(Table 3; Fig. 4).
Reaction cycle
Yield (%) (2a)
Yield (%) (3a)
1
2
3
4
60
58
57
40
30
28
26
18
1 3

D. A. Vargas et al.
Acknowledgements The authors are grateful to UNLP and CICBA
for financial support and Lic. Omar Guaymas for the NMR measure-
ments. D A V and L J M are a holder of CONICET fellowship.
%2a
%3a
60
50
40
30
20
10
0
References
1. Sravanthi TV, Manju SL (2016) Eu J Pharm Sci 91:1–10
2. Knölker H-J (ed) (2016) The alkaloids: chemistry and biology,
vol 76. Academic Press, New York 1–340
3. Sundberg RJ (ed) (1996) Indoles. Academic, London
4. Stalick WM, Wynne JH, Lloyd CT, Jensen SD, Boson S (2004)
Synthesis 14:2277–2282
5. Metwally MA, Shaaban S, Abdel-Wahab BF, El-Hiti GA (2009)
Curr Org Chem 13 (14): 1475–1496
1
2
3
4
6. Bai G, Li T, Yang Y, Zhang H, Lan X, Li F, Han J, Ma Z, Chen
Q, Chen G (2012) Catal Commun 29:114–117
Cycles
7. Guchhait SK, Kashyap M, Kamble H (2011) J Org Chem
76:4753–4758
Fig. 4 Catalyst reusability
8. Chatterjee A, Biswas KM (1973) J Org Chem 38:4002–4004
9. Bergman J, Venemalm L (1987) Tetrahedron Lett 28:3741–3744
10. Saxton JE (1952) J Chem Soc 3592–3598
11. Ketcha DM, Gribble GW (1985) J Org Chem 50:5451–5457
12. Ottoni O, de V F Neder A, Dias AKB, Cruz RPA, Aquino LB
(2001). Org Lett 3(7):1005–1007
3.3.1 Physical and Spectral Data
for N-Butyryl-3-methylindole (3c)
13. Pal M, Batchu VR, Parasuraman K, Yeleswarapu KR (2003) J
Org Chem 68:6806–6809
Mp: 56-58 °C; 1H NMR (CDCl3, 500 MHz) δ (ppm): 8.48
(d, J = 8.2 Hz, 1 H, H-7), 7.53 (ddd, J = 7.7, 1.4, 0.7 Hz,
1 H, H-4), 7.38 (ddd, J = 8.4, 7.2, 1.4 Hz, 1 H, H-6), 7.32
(td, J = 7.5, 1.1 Hz, 1 H, H-5), 7.25 (s, 1 H, H-2), 2.88 (t,
J = 7.4 Hz, 2 H, CH2CO), 2.32 (d, J = 1.4 Hz, 3 H, CH3),
1.89 (h, J = 7.4 Hz, 2 H, CH3CH2CH2), 1.10 (t, J = 7.4 Hz,
3 H, CH3CH2). 13C NMR (CDCl3, 126 MHz) δ (ppm):
171.11(C = O), 135.92 (C-7a), 131.32(C-3a), 125.10
(C-6), 123.26(C-5), 121.65 (C-2), 118.74(C-4), 118.15
(C-3), 116.65 (C-7), 37.77(CH2CO), 18.19(CH2CH2),
13.83(CH3CH2), 9.72(CH3). Anal. Calcd for C13H15NO:
C 77.58, H 7.51, N 6.96. Found: C 77.50, H 7.55, N 6.93.
14. Pal M, Dakarapu R, Padakanti
69:2913–2916
S (2004) J Org Chem
15. Cánepa AS, Bravo RD (2004) J Heterocycl Chem 41:979–982
16. Cánepa AS, Bravo RD (2006) J Heterocycl Chem 43:235–238.
17. Cánepa AS, Sasiambarrena LD, Bravo RD (2008) Synth Com-
mun 38:2655–2661
18. Cánepa AS, Bravo RD (2002) Synth Commun 32:3675–3680
19. Sasiambarrena LD, Cánepa AS, Bravo RD (2011) Synth Com-
mun 41:200–205
20. Sasiambarrena LD, Cánepa AS, Luna CN, Bravo RD (2015) Tet-
rahedron Lett 56(16):2054–2058
21. Sasiambarrena LD, Cánepa AS, Bravo RD (2015) Tetrahedron
Lett 56(52):7184–7189
22. Sasiambarrena LD, Méndez LJ, Ocsachoque MA, Cánepa AS,
Bravo RD, González MG (2010) Catal Lett 188:180–186
23. Méndez LJ, Cánepa AS, Gónzalez MG, Bravo RD (2012) Tetra-
hedron Lett 53(8):688–690
24. Méndez LJ, Cánepa AS, Rimada R, Bravo RD (2013) Lett Org
Chem 10 (4):240–244
4 Conclusion
25. Yadav GD, Pujari AA (1999) Green Chem 1:69–74
26. Saravanan K, Tyagi B, Shukla RS, Bajaj HC (2016) Fuel 165
298–305
In conclusion, an efficient method for the Friedel–Crafts
acylation of 3-methylindole with acid anhydrides employ-
ing sulfated zirconia as catalyst has been developed. This
catalyst shows an excellent catalytic activity, with good
conversion and selectivity towards formation of the
2-acylation products, (3-methyl-1H-indol-2-yl) ketones.
Moreover, the catalyst is air stable and easy to handle
with easy work up. In addition, sulfated zirconia can be
reused up to three cycles. The characterization of the cat-
alyst performed before and after its use reveals that it did
not undergo structural changes. Compared with the use
of acetic acid, a better behavior and a smaller production
of chemical waste were observed when sulfated zirconia
was used, which gives it an additional advantage.
27. Patel A, Brahmkhatri V, Singh N (2013) Renew Energy
51:227–233
28. Osatiashtiani A, Durndell LJ, Manayil JC, Lee AF, Wilson K
(2016) Green Chem 18:5529–5535
29. Dar BD, Ahmad N, Patial J, Sharma P, Bindu K, Maity S, Singh
B (2014) J Saudi Chem Soc 18:464–468
30. Yadav GD, Ajgaonkar NP, Varma A (2012) J Catal 292:99–110
31. Reddy BM, Sreekanth PM, Lakshmanan P (2005) J Mol Catal A
Chem 237:93–100
32. Kuzminska M, Kovalchuk TV, Backov R, Gaigneaux EM (2014)
J Catal 320:1–8
33. Altass HM, Khder AERS (2016) J Mol Catal A Chem
411:138–145.
34. Fuerstner A, Jumbam DN (1993) J Chem Soc Chem Commun
2:211–212
35. Jones CD, Suárez T (1972) J Org Chem 37(23):3622–3623
1 3

Sulfated Zirconia: An Efficient and Reusable Heterogeneous Catalyst in the Friedel–Crafts…
36. Grinev AN, Shvedov VI, Krichevskii ES, Romanova OB, Altuk-
hova LB, Kurilo GN, Andreeva NI, Golovina SM, Mashkovskii
MD (1984) Pharm Chem J 18 (2):94–98
37. Jones CD, Suarez T (1972) J Org Chem 37:3622–3623
38. Geissman A (1952) J Am Chem Soc 74:3916–3918
39. Oddo G (1913) Gazzetta Chimica Italiana 43 (II): 198–213
1 3