Ultrasound-assisted eco-friendly synthesis of triarylmethanes catalyzed by silica sulfuric acid

Article abstract of DOI:10.1007/BF03245914

An efficient and eco-friendly synthesis of triarylmethanes by the reaction of arenes with aldehydes in the presence of silica sulfuric acid as a heterogeneous and reusable catalyst under ultrasonic irradiation is reported. The advantages of this protocol are the use of green solvents, inexpensive catalyst, commercially available precursors, reusability of SSA, simple work-up, high yields and short reaction times.

Full text of DOI:10.1007/BF03245914

J. Iran. Chem. Soc., Vol. 8, No. 3, September 2011, pp. 840-850.
JOURIrNaAnLiaOnF THE
Chemical Society
Ultrasound-Assisted Eco-Friendly Synthesis of Triarylmethanes Catalyzed by Silica
Sulfuric Acid
I. Mohammadpoor-Baltorka,*, M. Moghadama,b, S. Tangestaninejada, V. Mirkhania, K. Mohammadiannejad-
Abbasabadia and M.A. Zolfigolc
aCatalysis Division, Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
bDepartment of Nanotechnology Engineering, University of Isfahan, Isfahan 81746-73441, Iran
cDepartment of Chemistry, Bu-Ali Sina University, Hamadan 65175, Iran
(Received 22 November 2010, Accepted 13 January 2011)
An efficient and eco-friendly synthesis of triarylmethanes by the reaction of arenes with aldehydes in the presence of silica
sulfuric acid as a heterogeneous and reusable catalyst under ultrasonic irradiation is reported. The advantages of this protocol are
the use of green solvents, inexpensive catalyst, commercially available precursors, reusability of SSA, simple work-up, high
yields and short reaction times.
Keywords: Triarylmethanes, Ultrasonic irradiation, Silica sulfuric acid, Reusable catalyst, Eco-friendly
INTRODUCTION
recoverable catalysts [4]. Preparation of silica sulfuric acid
(SSA) as a stable solid acid has been reported and its catalytic
activity in a wide range of organic reactions has been verified.
Easy handling, separation and work-up processes, non-
hazardous nature, recyclability and easier waste disposal are
among the most common characteristics that make it a green
catalyst [5].
Triarylmethane (TRAM) scaffold constitutes the
fundamental member of several natural products, biologically
active compounds, dyes and polymers. Typically, the
application of triarylmethanes as promising candidates in
textile industry, phenolic triarylmethanes as antioxidant and
antitumor reagents and diheteroarylmethanes as natural
components of certain foods and beverages has been reported.
Triarylmethanes are also used as protecting groups in
synthesis, bioconjugation, cross-linking, mass-spectroscopy,
fluorescence and optics [6]. Several methods such as Suzuki-
Miyaura coupling reaction of diarylmethyl carbonates with
arylbronic acid in the presence of [Pd(η3-C3H5)Cl]2-DPPPent
Many synthetic chemists have made a great deal of effort
to design sustainable and clean procedures to replace the
classical synthetic methods [1]. Application of sonochemistry
to enhance the efficiency and/or selectivity of organic
reactions is one of the most well-known challenges [2].
Ultrasound-assisted organic synthesis (UAOS) exploits a
variety of factors such as milder and more efficient conditions,
high yields and shorter reaction times, energy conservation,
formation of purer products, waste minimization and easier
manipulation. To date, many valuable organic compounds
have been synthesized under ultrasound irradiation without
need to potent conditions like the traditional methods [3].
Today, there is a great demand for solid acids instead of
conventional mineral acids such as HF, H2SO4 and BF3 in
chemical processes. Mineral acids are corrosive and non-
*Corresponding author. E-mail: imbaltork@sci.ui.ac.ir

Mohammadpoor-Baltork et al.
(1,5-bis(diphenylphosphino)pentane) [7], direct alkylation of
output power was 0-400 W through manual adjustment.
During the sonication, the reaction temperature was
maintained at 45 °C by the addition or removal of circulating
water (Fig. 1).
aryl(azaaryl)methanes with aryl halides catalyzed by
palladium [8], Friedel-Crafts reaction of arenes with aldehydes
catalyzed by InCl3/chlorodimethylsilan [9], [Ir(COD)Cl]2-
SnCl4 [10], AuCl3 or AuCl3/AgOTf [11], Yb(OTf)3 [12],
Cu(OTf)2 [13], FeCl3 [14], sulfuric acid [15], AcBr/ZnCl2/
SiO2 [16], and trifluromethansulfonic acid or trifluroacetic
acid [17] have been reported for the synthesis of TRAMs.
Although, the available methods are put to good use in
certain synthetic conditions, many of them have drawbacks
such as multistep processes [13], formation of by-products
[17], low yields and selectivity, and tedious work-up process
[10,12,14-17]. On the other hands, some of these methods
require inert atmosphere [11], long reaction times
[7,8,11,12,14] and high temperatures [7,8,11,13,14]. The use
of corrosive reagents [14-16], toxic solvents [7,8,11,13-17],
large amounts of catalyst [15,17] and non-reusable catalysts
[7-17] are other problems with some of the reported methods.
Therefore, the development of an efficient and eco-friendly
method for the synthesis of TRAMs is clearly justified.
General Procedure for Ultrasound-Promoted
Synthesis of TRAMs
To a mixture of veratrole 1 (828 mg, 6 mmol) and
aldehyde (2 mmol) in cyclohexene/ethyl acetate (6 ml/2 ml)
was added SSA (600 mg) and exposed to US irradiation at 45
°C for the appropriate time according to Tables 2 and 3. The
progress of the reaction was monitored by TLC (eluent:n-
hexane/ethyl acetate, 10:4). After completion of the reaction,
the solvent was evaporated. Then, absolute ethanol (2 × 10 ml)
Scheme 1
In continuation of our previous works on developing new
and environmentally benign methods for the preparation of
fine chemicals [18], here, we report a novel, efficient and eco-
friendly method for the synthesis of triarylmethanes by the
reaction of arenes with aldehydes catalyzed by silica sulfuric
acid (SSA) under ultrasound irradiation (Scheme 1).
EXPERIMENTAL
Commercial solvents were used in the reactions after
drying and distillation. SSA was prepared according to the
literature [5a]. All other chemicals were obtained from Merck
chemical company and used without further purification.
Melting point was determined using Stuart Scientific SMP2
apparatus. 1H NMR and 13C NMR were recorded on a Bruker-
AC 500 MHz spectrometer in CDCl3. FT-IR spectra were
obtained as KBr pellets using a Nicolet-Impact 400D
instrument in the range of 400-4000 cm-1. Mass spectra were
recorded on a Micromass Platform II spectrometer. A UP
400S ultrasonic processor equipped with a 3 mm wide and 100
mm long probe (made of titanium, Sonotrode H3), which was
immersed directly into the reaction mixture, was used for
sonication. The operating frequency was 24 KHz and the
Fig. 1. Ultrasonic experimental set-up.
841

Ultrasound-Assisted Eco-Friendly Synthesis of Triarylmethanes
was added and the catalyst was filtered. The product was
obtained by recrystallization from EtOH or EtOH/H2O (10:2).
287.12 (9.77), 139.05 (30.80), 94.00 (37.72), 78.99 (100),
77.00 (50.89). Anal. Calcd. for C23H22Cl2O4: C, 63.75; H,
5.12. Found: C, 63.49; H, 5.09.
Spectral Data
1-(Bis(3,4-dimethoxyphenyl)methyl)-3-nitrobenzene
(Table 2, entry 2). M.p.: 154-155.5 ºC. FT-IR (KBr)ν
1,2-Dimethoxy-4-((3,4-dimethoxyphenyl)(3-methoxy-
phenyl)methyl)benzene (Table 2, entry 8). M.p.: 120-122
°C. IR (KBr)ν 3010, 2935, 1602, 1581, 1463, 1317, 1246,
max
max
1
3084, 3003, 2947, 1589, 1516, 1463, 1342, 1028, 916, 869,
1139, 1091, 1026, 958, 860, 767 cm-1. H NMR (500 MHz,
1
736 cm-1. H NMR (500 MHz, CDCl3) δ 8.05-8.08 (m, 1H),
CDCl3) δ 7.21 (t, J = 7.95 Hz, 1H), 6.78 (d, 8.25 Hz, 2H), 6.76
(dd, J = 8.2 Hz, J = 2.2 Hz, 1H), 6.72 (d, J = 7.65 Hz, 1H),
6.68-6.69 (m, 3H), 6.62 (dd, J = 8.25 Hz, J = 1.9 Hz, 2H), 5.41
(s, 1H), 3.86 (s, 6H), 3.77 (s, 6H), 3.74 (s, 3H). 13C NMR (125
MHz, CDCl3) δ 159.63, 148.83, 147.57, 146.01, 136.62,
129.16, 121.85, 121.44, 115.46, 112.89, 111.32, 111.00,
55.96, 55.88, 55.86, 55.13. MS (EI) m/z: 395.10 ([M+1]+,
73.79), 394.10 ([M]+, 100), 364.07 (71.56), 363.07 (98.93),
287.05 (93.51), 257.06 (81.08), 241.05 (57.30), 225.05
(57.30), 181.07 (68.11), 152.01 (69.73), 138.97 (64.86),
114.99 (51.35), 94.87 (39.46), 76.98 (55.68). Anal. Calcd. for
C24H26O5: C, 73.08; H, 6.64. Found: C, 72.89; H, 6.65.
8.00 (s, 1H), 7.44-7.46 (m, 2H), 6.81 (d, J = 8.25 Hz, 2H),
6.65 (d, J = 1.95 Hz, 2H), 6.57 (dd, J = 8.25 Hz, J = 1.95 Hz,
2H), 5.53 (s, 1H), 3.86 (s, 6H), 3.77 (s, 6H). 13C NMR (125
MHz, CDCl3) δ 149.16, 148.44, 148.04, 146.68, 135.35,
135.17, 129.12, 124.05, 121.51, 121.39, 112.72, 111.26,
55.91, 55.59. MS (EI) m/z: 410.12 ([M+1]+, 25.86), 409.11
(M+, 100), 378.06 (46.26), 287.18 (40.23), 226.13 (9.99),
152.10 (7.33), 139.08 (6.90), 76.93 (21.83). Anal. Calcd. for
C23H23NO6: C, 67.47; H, 5.66; N, 3.42. Found: C, 67.34; H,
5.64; N, 3.42.
1-(Bis(3,4-dimethoxyphenyl)methyl)-4-nitrobenzene
(Table 2, entry 3). M.p.: 116-118 °C. FT-IR (KBr)ν 3074,
2-(Bis(3,4-dimeyhoxyphenyl)methyl)naphthalene
max
2956, 2835, 1587, 1514, 1463, 1346, 1265, 1138, 1024, 806,
742, 700 cm-1. 1H NMR (500 MHz, CDCl3) δ 8.10 (d, J = 8.75
Hz, 2H), 7.27 (d, J = 8.7 Hz, 2H), 6.79 (d, J = 8.3 Hz, 2H),
6.63 (d, J = 2 Hz, 2H), 6.56 (dd, J = 8.25 Hz, J = 2 Hz, 2H),
5.51 (s, 1H), 3.84 (s, 6H), 3.75 (s, 6H). 13C NMR (125 MHz,
CDCl3) δ 152.12, 149.14, 148.04, 146.51, 135.09, 130.09,
123.47, 121.41, 112.73, 111.25, 55.89, 55.76. MS (EI) m/z:
410.07 ([M+1]+, 46.48), 409.07 ([M]+, 100), 378.05 (89.44),
287.05 (78.87), 151.98 (35.39), 138.94 (36.97), 78.90 (29.05).
Anal. Calcd. for C23H23O6N: C, 67.47; H, 5.66; N, 3.42.
Found: C, 67.39; H, 5.66; N, 3.40.
4-((2,4-Dichlorophenyl)(3,4-dimethoxyphenyl)methyl)-
1,2-dimethoxybenzene (Table 2, entry 5). Mp 151-152 °C.
IR (KBr)νmax 3072, 2999, 2933, 1587, 1516, 1463, 1328, 1250,
1138, 1028, 960, 866, 815, 754, 638 cm-1. 1H NMR (500 MHz,
CDCl3) δ 7.39 (d, J = 2.15 Hz, 1H), 7.15 (dd, J = 8.5 Hz, J =
2.15 Hz, 1H), 6.88 (d, J = 8.4 Hz, 1H), 6.78 (d, J = 8.3 Hz,
2H), 6.61 (d, J = 2 Hz, 2H), 6.51 (dd, J = 8.25 Hz, J = 1.95
Hz, 2H), 5.76 (s, 1H), 3.86 (s, 6H), 3.77 (s, 6H). 13C NMR
(125 MHz, CDCl3) δ 149.03, 147.85, 140.76, 135.15, 134.82,
132.77, 131.74, 129.41, 126.83, 121.41, 112.89, 111.03,
55.89, 55.86, 52.16. MS (EI) m/z: 436.09 ([M+4]+, 2.40),
434.06 ([M+2]+, 10.16), 432.08 ([M]+, 18.97), 365.07 (7.59),
(Table 2, entry 9). Mp 133-135 °C. FT-IR (KBr)ν
3005,
max
2954, 1629, 1587, 1462, 1365, 1261, 1184, 1028, 964, 862,
1
761 cm-1. H NMR (500 MHz, CDCl3) δ 7.81-7.83 (m, 1H),
7.79 (d, J = 8.5 Hz, 1H), 7.73-7.75 (m, 1H), 7.52 (s, 1H), 7.46
(d, J = 3.5 Hz, 1H), 7.44 (d, J = 3.35 Hz, 1H), 7.36 (dd, J =
8.52 Hz, J = 1.6 Hz, 1H), 6.81 (d, J = 8.3 Hz, 2H), 6.79 (d, J =
1.85 Hz, 2H), 6.69 (dd, J = 8.25, 1.85, 2H), 5.65 (s, 1H), 3.88
(s, 6H), 3.79 (s, 6H). 13C NMR (125 MHz, CDCl3) δ 148.97,
147.70, 142.07, 136.60, 133.49, 132.25, 128.09, 127.93,
127.84, 127.63, 127.60, 126.04, 125.66, 121.68, 113.05,
111.12, 56.13, 55.91, 55.90. MS (EI) m/z: 415.11 ([M+1]+,
69.51), 414.09 ([M]+, 100), 383.07 (91.97), 287.05 (80.00),
277.07 (65.88), 245.07 (57.06), 215.06 (52.94), 127.96
(61.18), 119.46 (68.24), 77.00 (25.29). Anal. Calcd. for
C27H26O4: C, 78.24; H, 6.32. Found: C, 77.95; H, 6.34. 52.94),
127.96 (61.18), 119.46 (68.24), 77.00 (25.29).
1,1-Bis(3,4-dimethoxyphenyl)butane (Table 2, entry
12). Oil. IR (neat)ν
2995, 2931, 1589, 1463, 1425, 1259,
max
1141, 1028, 952, 856, 808, 754 cm-1. H NMR (500 MHz,
CDCl3) δ 6.79 (s, 4H), 6.74 (s, 2H), 3.88 (s, 6H), 3.87 (s, 6H),
3.81 (t, J = 6.83 Hz, 1H), 1.97 (q, J = 7.65 Hz, 2H), 1.29 (sext,
J = 7.55 Hz, 2H), 0.93 (t, J = 7.32 Hz, 3H). 13C NMR (125
MHz, CDCl3) δ 148.87, 147.36, 138.28, 119.58, 111.50,
1
842

Mohammadpoor-Baltork et al.
111.24, 55.89, 50.17, 38.34, 21.19, 14.07. MS (EI) m/z:
125.94, 125.51, 111.39, 56.44, 43.61, 21.24. MS (EI) m/z:
384.15 ([M+2]+, 9.55), 383.14 ([M+1]+, 56.50), 382.04 ([M]+,
97.56), 367.07 (47.97), 351.02 (81.30), 260.14 (33.33), 140.96
(100), 135.08 (60.98), 104.96 (73.98), 77.05 (17.99). Anal.
Calcd. for C27H26O2: C, 84.78; H, 6.85. Found: C, 84.31; H,
6.86.
331.15 ([M+1]+, 29.55), 330.14 (M+, 64.09), 288.13 (62.27),
287.13 (84.55), 166.94 (90.91), 148.90 (100), 127.95 (61.82),
114.81 (66.36), 112.98 (85.91), 103.89 (65.91), 82.92 (80.45),
76.89 (49.09).
1,2-Dimethoxy-4-(1-(3,4-dimethoxyphenyl)-2-phenyl-
propyl)benzene (Table 2, entry 13). Oil. IR (neat)ν 3057,
4-Chloro-2-((5-chloro-2-methoxyphenyl)(phenyl)
methyl)-1-methoxybenzene (Table 3, entry 4). M.p.: 138-
max
2931, 1589, 1463, 1334, 1263, 1184, 1028, 856, 810, 761, 700
cm-1. 1H NMR (500 MHz, CDCl3) δ 7.15-7.2 (m, 4H), 7.08 (t,
J = 6.9 Hz, 1H), 6.96 (dd, J = 8.22 Hz, J = 1.8 Hz, 1H), 6.88
(d, J = 1.7 Hz, 1H), 6.85 (d, J = 8.2 Hz, 1H), 6.68 (dd, J = 8.22
Hz, J = 1.85 Hz, 1H), 6.61 (d, J = 8.3 Hz, 1H), 6.55 (d, J =
1.75 Hz, 1H), 3.97 (d, J = 11.1 Hz, 1H), 3.89 (s, 3H), 3.86 (s,
3H), 3.67 (s, 3H), 3.66 (s, 3H), 3.45-3.49 (m, 1H), 1.24 (d, J =
6.8 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 148.99, 148.26,
147.58, 146.94, 146.06, 136.97, 136.78, 128.12, 127.69,
125.82, 120.25, 120.16, 111.95, 111.45, 110.92, 58.39, 55.97,
55.91, 55.74, 55.69, 44.89, 21.93. MS (EI) m/z: 393.06
([M+1]+, 2.04), 392.05 ([M]+, 7.72), 287.09 (100), 257.03
(75.72), 197.04 (62.17), 167.01 (64.78), 152.03 (71.30),
128.04 (72.17), 115.00 (80.43), 105.02 (90.87), 77.00 (85.65).
1,4-Bis(bis(4-methoxyphenyl)methyl)benzene (Table 3,
140 °C. FT-IR (KBr) ν 3007, 2935, 1593, 1485, 1240, 1176,

max
1026, 887, 702, 644 cm-1. 1H NMR (500 MHz, CDCl3) δ 7.29-
7.31 (m, 2H), 7.23-7.27 (m, 1H), 7.21 (dd, J = 8.7 Hz, J = 2.61
Hz, 2H), 7.07 (d, J = 7.21 Hz, 2H), 6.82 (d, J = 8.7 Hz, 2H),
6.77 (d, J = 2.56 Hz, 2H), 6.10 (s, 1H), 3.71 (s, 6H). 13C NMR
(125 MHz, CDCl3) δ 156.21, 142.43, 134.29, 130.10, 129.62,
128.70, 127.77, 126.79, 125.73, 112.43, 56.39, 43.83. MS (EI)
m/z: 375.94 ([M+4]+, 4.76), 373.95 ([M+2]+, 26.15), 371.95
([M]+, 38.30), 337.00 (33.03), 195.06 (15.83), 165.05 (16.51),
121.12 (23.85), 91.02 (100), 77.03 (12.44). Anal. Calcd. for
C21H18O2Cl2: C, 67.57; H, 4.86. Found: C, 67.48; H, 4.85.
4-Chloro-2-((5-chloro-2-methoxyphenyl)(4-nitrophenyl)
methyl)-1-methoxybenzene (Table 3, entry 5). M.p.: 168-
170 °C. FT-IR (KBr)νmax 3007, 2935, 1593, 1485, 1435, 1402,
1290, 1240, 1120, 1068, 1026, 887, 835, 794, 702, 644 cm-1.
1H NMR (500 MHz, CDCl3) δ 8.16 (d, J = 8.56 Hz, 2H), 7.26
(dd, J = 8.68 Hz, J = 2.34 Hz, 2H), 7.23 (d, J = 8.57 Hz, 2H),
6.86 (d, J = 8.71 Hz, 2H), 6.72 (d, J = 2.28 Hz, 2H), 6.15 (s,
1H), 3.72 (s, 6H). 13C NMR (125 MHz, CDCl3) δ 156.06,
150.83, 147.04, 132.34, 130.25, 129.99, 128.53, 125.96,
123.96, 112.51, 56.29, 44.03. MS (EI) m/z: 420.95 ([M+4]+,
12.25), 418.96 ([M+2]+, 68.42), 417.98 ([M+1]+, 23.48),
416.95 ([M]+, 100), 381.98 (90.28), 155.10 (93.93), 121.14
(78.95). Anal. Calcd. for C21H17Cl2NO4: C, 60.30; H, 4.10; N,
3.35. Found: C, 60.17; H, 4.12; N, 3.34.
entry 1). M.p.: 114-116 °C. FT-IR (KBr)ν
3001, 2951,
max
1608, 1581, 1462, 1301, 1246, 1111, 1033, 813, 759 cm-1. 1H
NMR (500 MHz, CDCl3) δ 7.04-7.08 (m, 12H), 6.85 (d, J =
16.7 Hz, 8H), 5.46 (s, 2H), 3.81 (s, 12H). 13C NMR (125
MHz, CDCl3) δ 158.39, 142.76, 137.02, 130.70, 129.56,
114.06, 113.94, 55.67, 55.30. MS (EI) m/z: 532.29 ([M+2]+,
7.58), 531.26 ([M+1]+, 28.79), 530.26 ([M]+, 100), 423.20
(16.48), 303.14 (66.67), 227.19 (60.98), 152.10 (37.12),
121.18 (51.14), 77.17 (19.03). Anal. Calcd. for C36H34O4: C,
81.48; H, 6.46. Found: C, 81.36; H, 6.45.
2-(Bis(2-methoxy-5-methylphenyl)methyl)naphthalene
(Table 3, entry 3). M.p.: 137.5-139 °C. FT-IR (KBr)
 ν
max
3049, 2935, 1608, 1492, 1436, 1363, 1234, 1105, 1035, 804,
742 cm-1. 1H NMR (500 MHz, CDCl3) δ 7.84-7.86 (m, 1H),
7.78 (d, J = 8.78 Hz, 1H), 7.74-7.77 ( m, 1H), 7.47 (d, J = 3.24
Hz, 1H), 7.45 (d, J = 3.26 Hz, 1H), 7.44 (s, 1H), 7.34 (dd, J =
7.4 Hz, J = 1.71 Hz, 1H), 7.07 (dd, J = 8.22 Hz, J = 1.96 Hz,
2H), 6.85 (d, J = 8.24 Hz, 2H), 6.71 (d, J = 2.08 Hz, 2H), 6.38
(s, 1H), 3.71 (s, 6H), 2.24 (s, 6H). 13C NMR (125 MHz,
CDCl3) δ 155.79, 142.41, 133.90, 132.66, 132.53, 131.40,
129.68, 129.14, 128.33, 128.08, 127.94, 127.79, 127.64,
RESULTS AND DISCUSSION
We initially optimized the reaction conditions in the
reaction of veratrole 1 with 3-nitrobenzaldehyde 2b as a
model. As shown in Table 1, the reaction parameters such as
the kind of solvent, reaction temperature, amount of catalyst,
molar ratio of reagents and the power of ultrasonic irradiation
were optimized. Previously, toxic solvents such as chloroform,
dichloromethane and benzene have been used in the synthesis
843

Ultrasound-Assisted Eco-Friendly Synthesis of Triarylmethanes
of TRAMs. To find a green and suitable solvent, the model
reaction time (Table 1, entry 9).
reaction was triggered in several solvents and the best results
were obtained in 3:1 mixture of n-hexane/ethyl acetate or
cyclohexane/ethyl acetate (Table 1, entries 1-8).
The model reaction was explored with different amounts of
SSA and the best results were obtained with 300 mg of SSA
(Table 1, entries 8, 11 and 12). To confirm the catalytic role of
SSA, the model reaction was repeated in the absence of the
catalyst and no considerable improvement was observed
(Table 1, entries 17). Further, the model reaction was studied
in varied intensities of the ultrasound processor. The results
showed that the reaction was completed at 80% of full power
of US processor (Table 1, entries 8, 15 and 16).
Since, cyclohexane is greener than n-hexane [19], all
reactions were allowed to take place in cyclohexane/ethyl
acetate (3:1) mixture. To find the molar ratio of reactants, the
model reaction was investigated with different molar ratios of
veratrole and aldehyde (Table 1, entries 8-10). The reaction
was completed with 3:1 molar ratio of veratrole to aldehyde.
As illustrated in Table 1, the use of a lower amount of
veratrole led to lower yields (Table 1, entry 10), while higher
amount did not have considerable effect on the yield and
The effect of the reaction temperature was also studied. As
can be seen (Table 1, entries 8, 13 and 14), the 45 °C was
chosen as the optimal reaction temperature. In order to show
Table 1. Optimization Conditions in the Synthesis of TRAMs under Ultrasound Irradiation
NO2
CHO
OCH3
OCH3
H3CO
H3CO
OCH3
OCH3
SSA
+
.
)
)
)
)
NO2
3b
1
2b
Entry
SSA
(mg)
300
300
300
300
300
300
300
300
300
300
350
250
300
300
300
300
-
Veratrole
(mmol)
Aldehyde
(mmol)
Solvent
Intensity
T
(°C)
45
45
45
45
45
45
45
45
45
45
45
45
60
RT
45
45
45
Yield
(%)a
93
20
20
40
88
88
95
95
95
74
95
88
95
66
95
Time
(min)
20
30
30
30
25
25
20
20
20
25
20
30
20
30
20
(%)
80
80
80
80
80
80
80
80
80
80
80
80
80
80
100
60
80
1
2
3
4
5
6
7
8
3
3
3
3
3
3
3
3
3.5
2.5
3
3
3
3
3
3
3
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
CHCl3
EtOH
EtOAc (EA)
CH3CN
n-Hexane
Cyclohexane
n-Hexane/EA
Cyclohexane/EA
Cyclohexane/EA
Cyclohexane/EA
Cyclohexane/EA
Cyclohexane/EA
Cyclohexane/EA
Cyclohexane/EA
Cyclohexane/EA
Cyclohexane/EA
Cyclohexane/EA
9
10
11
12
13
14
15
16
17
83
trace
30
60
aIsolated yield.
844

Mohammadpoor-Baltork et al.
the efficiency of SSA, the model reaction was allowed to
reaction of 2-naphthaldehyde with veratrole, the
corresponding TRAM was obtained in high yield (Table 2,
entry 9). Aliphatic aldehydes such as butanal and 2-phenyl
propionaldehyde were also converted to their corresponding
diveratylmethanes in high yields under the same reaction
conditions (Table 2, entries 12 and 13). However, when
terephthalaldehyde 2g was used under these conditions, only
one aldehyde moiety reacted with veratrole. Therefore, this
method can be used for the selective synthesis of mono-
TRAM 3g without the formation of tetrakis(veratryl) adduct 4
(Scheme 2).
occur in the presence of sulfuric acid under similar
experimental conditions (molar ratio and temperature); the
corresponding triarylmethane 3b was obtained in only 27%
yield. In addition, SSA as a solid acid, in comparison with
sulfuric acid, affords the advantages of easy separation, non-
corrosiveness and low pollution effects.
To check the generality of this method, a wide range of
aldehydes were treated with veratrole 1 and the corresponding
TRAMs were synthesized in good to excellent yields in short
reaction times.
As shown in Table 2, the nature of the substitutes on the
aromatic aldehydes has a significant effect on the yields.
Electron-withdrawing groups such as nitro, chloro, cyano and
formyl increased the yields and diminished the reaction times
(Table 2, entries 2-8), while electron-donating substituents
such as hydroxy and methoxy groups behaved differently in
the synthesis of TRAMs (Table 2, entries 10 and 11). In the
In general, US has chemical and mechanical effects. The
chemical effects of ultrasound do not derive from a direct
coupling of the acoustic field with the chemical species on a
molecular level. Rather, sonochemistry and sonoluminescence
derive principally from acoustic cavitation: the formation,
growth and implosive collapse of bubbles in liquids irradiated
with high-intensity ultrasound. Bubble collapse during
Table 2. Ultrasound-Assisted Synthesis of Diveratrylmethanes in the Presence of SSAa
Entry
RCHO
Time
(min)
30
20
20
25
20
30
9
30
30
60
60
20
20
Yield
(%)b
75 (23)c
95 (48)c
93
product
M.p. (°C) [Ref.]
1
2
3
4
5
6
7
8
PhCHO (2a)
3a
3b
3c
3d
3e
3f
3g
3h
3i
122-124 [16]
154-155.5
116-118
155-157 [16]
151-152
115-116 [15]
128-129 [15]
120-122
133-135
-
3-NO2C6H4CHO (2b)
4-NO2C6H4CHO (2c)
4-ClC6H4CHO (2d)
2,4-Cl2C6H3CHO (2e)
4-NCC6H4CHO (2f)
4-OHCC6H4CHO (2g)
3-CH3OC6H4 CHO (2h)
2-Naphthaldehyde (2i)
4-HOC6H4CHO (2j)
4-CH3OC6H4CHO (2k)
n-PrCHO (2l)
90
89
88 (36)c
87
78
9
93 (29)c
Trace
Trace
89
10
11
12
13
3j
3k
3l
-
Oil
Oil
2-Phenylpropionaldehyde (2m)
90
3m
aReaction conditions: veratrole (6 mmol), aldehyde (2 mmol), SSA (600 mg), cyclohexane/ethylacetate
(3:1), T = 45 °C, intensity (80%). bIsolated yield. cReaction was performed under thermal conditions and
vigorous stirring.
845

Ultrasound-Assisted Eco-Friendly Synthesis of Triarylmethanes
The creation of the so-called hot spots in the reaction
mixture produces intense local temperatures and high
pressures generated inside the cavitation bubble and its
interfaces when it collapses (the rise in the reaction
temperature is an evidence of this). Since this system is a
heterogeneous catalytic one, it seems that another part of US is
due to its mechanical effect. On the other hand, US waves can
break up the surface structure and facilitate the penetration of
reactants and/or release of materials from the surface. The
accelerated motion of the suspended particles enhances the
mass transfer [21]. Another effect of ultrasonic irradiation on
the catalytic activity enhancement may be due to the break-up
of the agglomerates during the sonication process. To explore
this point, the catalytic activity of a sonicated sample of SSA
Scheme 2
was studied in the reaction of veratrole
1 with 3-
cavitation serves as an effective means of concentrating the
diffuse energy of sound: compression of a gas generates heat.
When the compression of bubbles occurs during cavitation,
heating is more rapid than thermal transport, creating a short-
lived localized hot spot. There is a nearly universal consensus
that this hot spot is the source of homogeneous sonochemistry
[20].
The mechanical effects of ultrasound offer an opportunity
to overcome the following types of problems associated with
the conventional solid/metal reactions: break-up of the surface
structure allows penetration of reactants and/or release of
materials from the surface, degradation of large solid particles
due to shear forces induced by shock waves and
microstreaming leads to the reduction of particle size and
increase of surface area and accelerated motion of suspended
particles leads to better mass transfer [21].
nitrobenzaldehyde 2b under thermal conditions. It was found
that the reaction yield increased from 48% to 67%. These
results show that the break-up of the agglomerates and
thorough mixing of the reactants are important factors in the
acceleration of the reactions.
The effect of US intensity was also investigated in the
reaction of veratrole with 3-nitrobenzaldehyde. The results
showed that the highest yield was obtained at 80% intensity
(Fig. 2).
In order to show the generality of the presented method,
To verify the effect of ultrasound, some of these reactions
were allowed to occur under thermal conditions and vigorous
stirring. As can be seen in Table 2 (entries 1, 2, 6 and 9), the
lower yields were obtained under the same reaction
conditions. These results clearly indicate that the energy
provided by ultrasound accelerates the reactions. Increasing
the yields and reducing the reaction times may be a
consequence of the specific effects of ultrasound. The
observed effect on the reaction is due to cavitation, a physical
process that creates, enlarges, and implodes gaseous and
vaporous cavities in an irradiated liquid, thus enhancing the
mass transfer [21,22] and allowing chemical reactions to
occur.
Fig. 2. The effect of US intensity on the synthesis of TRAMs.
846

Mohammadpoor-Baltork et al.
different arenes such as anisole and its derivatives were
CATALYST RECOVERY AND REUSE
allowed to react with different aldehydes under US irradiation.
As shown in Table 3, the corresponding TRAMs were
obtained in high yields and short reaction times.
In recent years, employing environmentally-friendly
processes has attracted considerable attention in laboratories
Table 3. Ultrasound-Assisted Synthesis of TRAMs in Presence of SSAa
Entry
1
Ar-H
Ar'CHO
Product
Time
(min)
Yield
(%)b
MeO
MeO
OMe
OMe
120c
94
CHO
OHC
5a
2g
6a
Cl
OMe
CHO
2
3
35
40
94
92
OMe
Cl
5b
5b
2d
2i
MeO
6b
OMe
OMe
OMe
CHO
OMe
MeO
6c
OMe
CHO
4
5
Cl
40
35
71
90
Cl
Cl
5c
5c
MeO
2a
Cl
6d
NO2
CHO
OMe
O2N
Cl
2c
MeO
Cl
6e
847

Ultrasound-Assisted Eco-Friendly Synthesis of Triarylmethanes
Table 3. Continued
CHO
Cl
OMe
OMe
Cl
6
40
15
88
94
Cl
Cl
2n
5c
MeO
Cl
6f
MeO
OMe
CHO
7
8
Br
2o
OMe
5d
6g
Cl
CHO
MeO
OMe
15
93
Cl
OMe
OMe
2d
OMe
5d
OMe
MeO
OMe MeO
6h
aRection conditions: arene (6 mmol), aldehyde (2 mmol), SSA (600 mg), cyclohexane/ethyl acetate (3:1),
T = 45 °C, intensity (80%). bIsolated yield. cThe reaction was carried out by 8 mmol anisol and 1 mmol
terephthalaldehyde in the presence of 600 mg of SSA.
Fig. 3. Investigation of catalyst reusability in the synthesis of TRAMs.
848

Mohammadpoor-Baltork et al.
Kumar, A. Sharma, M. Sharma, U.K. Sharma, A.K.
and industrial chemistry. In fact, the reusability of a catalyst is
one of the most important factors which determine its potential
in commercial applications. So, we examined the reusability of
SSA by the sequential reaction of veratrole 1 with 3-
nitrobenzaldehyde 2b (Table 2, entry 2). After completion of
the reaction, the solvent was evaporated and absolute EtOH
was added. The SSA catalyst was easily separated by simple
filtration, washed with absolute EOH and dried at 70 °C and
reused at least five successive times successfully (Fig. 3).
Shina, Tetrahedron 63 (2007) 9718; c) A.K. Shina, B.P.
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Joshi, Tetrahedron 63 (2007) 960; e) F. Brümmer, J.
Brenner, T. Bräumer, D.F. Hülser, Ultrasound Med.
Biol. 15 (1989) 229.
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[4]
[5]
[6]
CONCLUSIONS
In conclusion, a novel, efficient and convenient method for
the synthesis of triarylmethanes catalyzed by SSA under US
irradiation is reported. It is noteworthy that the toxic solvents
which were used in the previous methods have been
successfully replaced with cyclohexane/ethyl acetate as green
solvents. In addition, the use of a cheap and reusable catalyst,
commercially available precursors, easy work-up, high yields
and short reaction times are other advantages of this procedure
which make it readily applicable for the preparation of
triarylmethanes.
ACKNOWLEDGEMENTS
[7]
[8]
We are grateful to the Research Council and Center of
Excellence of Chemistry of the University of Isfahan for the
financial support of this work.
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T. Miyai, Y. Onishi, A. Baba, Tetrahedron 55 (1999)
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[10] S. Podder, J. Choudhury, U.K. Roy, S. Roy, J. Org.
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