Synthesis and characterization of a tetracationic acidic organic salt and its application in the synthesis of bis(indolyl)methanes and protection of carbonyl compounds

Article abstract of DOI:10.1039/c5ra01422k

A new tetracationic acidic organic salt (TCAOS) based on DABCO was prepared, characterized and applied as an eco-friendly, powerful and reusable catalyst for the synthesis of bis(indolyl)methanes from indoles and carbonyl compounds in water with high turnover frequency (TOF). Also, this catalyst was successfully applied for acetalization of carbonyl compounds with diols under solvent-free conditions.

Full text of DOI:10.1039/c5ra01422k

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Synthesis and characterization of a tetracationic
acidic organic salt and its application in the
synthesis of bis(indolyl)methanes and protection of
carbonyl compounds†
Cite this: RSC Adv., 2015, 5, 21772
*
Azim Ziyaei Halimehjani, Seyyed Emad Hooshmand and Elham Vali Shamiri
Received 24th January 2015
Accepted 17th February 2015
A new tetracationic acidic organic salt (TCAOS) based on DABCO was prepared, characterized and applied
as an eco-friendly, powerful and reusable catalyst for the synthesis of bis(indolyl)methanes from indoles and
carbonyl compounds in water with high turnover frequency (TOF). Also, this catalyst was successfully
applied for acetalization of carbonyl compounds with diols under solvent-free conditions.
DOI: 10.1039/c5ra01422k
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the synthesis of series of tetracationic salts applicable (see
Scheme 1) as gas-phase ion-pairing agents for the detection of
Introduction
trivalent anions in positive mode electrospray ionization mass
spectrometry.37
Solid acids have found wide applications in chemistry due to
their important advantages over traditional mineral acids
including operational simplicity, environmental compatibility,
non-toxicity, reusability, low cost, and ease of isolation. Various
solid acids such as silica, montmorillonite, clay, zeolite,
Amberlyst-15, acidic polymers, polymer-supported reagents,
heteropoly acids, Naon-H, modied silica-supported acids
(silica sulfuric acid, silica phosphoric acid, NaHSO4–SiO2,
SiO2–Cl and etc.), cellulose sulfuric acid and solid organic acids
etc. have been introduced in various branches of chemistry.1–14
Indole derivatives are prevalent in numerous natural prod-
ucts and are extremely important in medicinal chemistry.38
Bis(indolyl)methanes, which contain two indole or substituted
indole units in a molecule, were found in the structure of many
biologically active compounds extracted from various plants.39
Thus, the development of facile and environmental friendly
synthetic methods for the synthesis of bis(indolyl)methanes
constitutes an active area of investigation in pharmaceutical
and organic synthesis. One of the most applied procedures for
synthesis of bis(indolyl)alkanes is based on the electrophilic
substitution reaction of indoles with various aldehydes and
ketones in the presence of a Bronsted or Lewis acid. Typical
protic acids applied for this transformation include silica
sulfuric acid (SSA), oxalic acid, zeolites HY, amberlyst, H2SO4,
HCl, CH3COOH, sulfamic acid, NaHSO4–SiO2, KHSO4, NaHSO3,
H3PW12O40, H3PW12O40–ZrO2, H3PW12O40–SiO2, –TiO2 or
¨
In recent years, Bronsted acidic organic salts have been
designed to replace solid acids and traditional mineral liquid
acids in chemical procedures.15–21 These compounds can
potentially be applied as homogenous or heterogeneous cata-
¨
lysts in organic reactions. It is well documented that Bronsted-
acidic functional groups such as SO3H and SO4H can be simply
introduced in the structure of organic compounds, especially as
anion in the structure of ionic liquids.22–24 Among the organic
salts, the mono-, di-, and tricationic ammonium salts (ionic
liquids) have been extensively applied in analytical chemistry as
ion-pairing reagents, as catalyst and solvent in organic trans-
formations and etc.25–33 Recently, Armstrong et al. reported a
facile procedure for synthesis of novel dicationic and tricationic
ionic liquids and investigated their physiochemical proper-
ties.34–36 They have shown that these compounds can be applied
in dye-sensitized solar cells, as solvent in high-temperature
reactions, and as lubricants. Also, the same group reported
Faculty of Chemistry, Kharazmi University (formerly Tarbiat Moallem University), 49
Mofateh St., Tehran, Iran. E-mail: ziyaei@khu.ac.ir; Fax: +98 21 88820992; Tel: +98
21 88848949
† Electronic supplementary information (ESI) available: Experimental procedures
and copies of 1H and 13C NMR spectra for new compounds and TGA/DTG curve for
the catalyst. See DOI: 10.1039/c5ra01422k
Scheme 1 Structures of a number of tetracationic ionic compounds
prepared by Armstrong et al.
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–Al2O3, H3PO4–SiO2, PEG-supported sulfonic acid and etc. Also,
Lewis acids such as lanthanide resins, zeolite (ZnY), montmo-
rillonite clay K-10, cerium ammonium nitrate (CAN), ZrOCl2,
ZrOCl2$8H2O–SiO2, ZrCl4, InF3, InCl3, In(OTf)3, SnCl2$2H2O,
LiClO4, Bi(OTf)3 and etc. were successfully developed for this
reaction.39
The acetals have been introduced as a suitable protection
group for carbonyl compounds because of their remarkable
stability to neutral and basic conditions.40,41 Diversities of
procedures have been developed for synthesis of acetals from
carbonyl compounds in Literature.42–45
Fig. 1 TGA curve of TCAOS 5.
Results and discussion
Encouraged by the work of Armstrong et al., here we are inter-
ested to report for the rst time, the synthesis and properties of
a new tetracationic acidic organic salt 5 based on DABCO
(Scheme 2) and its application as catalyst for the synthesis of
bis(indolyl)methanes and acetals from carbonyl compounds.
We began our investigation for the synthesis of compound 5
by the reaction of an excess amount of DABCO 1 with
1,3-dibromopropane 2 in methanol at room temperature to give
the dicationic salt 3 in 97% yield. Then, the salt 3 reacted with
an excess amount of methyl iodide in neat conditions at room
temperature to give the tetracationic salt 4 in quantitative yield.
Finally, treatment of salt 4 with sulfuric acid in reuxing
dichloromethane afforded the tetracationic acidic organic salt
(TCAOS) 5 in 93% yield. This novel TCAOS contains four
hydrogen sulfate (HSO4ꢀ) groups in the structure which caused
strong acidic character. The structure of catalyst characterized
To continue, we investigate the catalytic efficiency of the
prepared TCAOS for synthesis of bis(indolyl)methanes from
indoles and carbonyl compounds (Scheme 3). We have shown
that the reactions proceed well in water at room temperature
using only 4 mol% of TCAOS. The results of these reactions
are summarized in Table 1. As shown in Table 1, indole
derivatives such as indole, 2-methyl indole, 1-methyl indole,
and 5-bromo indole were applied successfully in this reaction
and afforded the corresponding bis(indolyl)methanes in
excellent yields. Also, aromatic aldehydes containing both
electron-donating and –withdrawing groups on the phenyl
ring were found to react equally well in this protocol. Heter-
oaromatic aldehydes such as furfural and thiophene carbal-
dehyde are suitable substrates in this protocol (Table 1,
entries
6 and 7). Terephthaldehyde was also reacted
1
by IR, H and 13C NMR and CHN analysis. Also, purity of the
with indoles to give the corresponding products with four
indole groups in the structure (Table 1, entries 14 and 15).
Cyclohexanone was successfully applied in this protocol to
give the corresponding product in 80% isolated yield (Table 1,
entry 17). In addition, aliphatic aldehydes such as formal-
dehyde and acetaldehyde are suitable starting materials
for synthesis of bis(indolyl)alkanes (Table 1, entries 16
and 18). Finally, 3-formylchromone gave high yields with
indole, 1-methylindole and 2-methylindole (Table 1, entries
19 to 21).
catalyst (assay more than 98%) was examined by titration with
NaOH solution.
Screening the solubility of the TCAOS in polar and nonpolar
solvents revealed that it is only miscible with water, MeOH and
DMSO, but immiscible in PhCH3, EtOAc, THF, Et2O, CH2Cl2,
CCl4, CHCl3, isopropanol (IPA) and hexane.
The thermal stability of the TCAOS was studied using ther-
mogravimetry analysis (TGA). The TGA curves revealed that the
ꢁ
TCAOS is stable until 220 C and then started to lose only its
10% of weight until 300 ꢁC. At 305–315 ꢁC, it lost more than 60%
of its weight. Decomposition nally completed at temperature
The reusability of the catalyst in the synthesis of bis(indolyl)
methane from benzaldehyde and indole was investigated
through separating the product from the reaction mixture by
ꢁ
higher than 315 C (Fig. 1).
Scheme 3 Synthesis of bis(indolyl)methanes catalyzed by TCAOS in
Scheme 2 Synthesis of TCAOS 5.
water.
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Table 1 Diversity in the synthesis of bis(indolyl)methanes in the presence of TCAOS
Entry
Indole
Carbonyl compound
Yielda (%)
M.p. (ꢁC) (L)
1
2
3
4
5
6
7
8
A
A
A
A
A
A
A
B
B
B
B
C
D
A
B
A
A
A
A
B
C
C6H5CHO
93
148–149 (151–152)46
75–76 (72–74)47
2-ClC6H4CHO
4-ClC6H4CHO
4-NO2C6H4CHO
4-CH3C6H4CHO
Thiophene carbaldehyde
Furfural
Quant.
Quant.
92
92
Quant.
95
75–78 (74–77)47
216–217 (217–219)48
94–95 (95–97)49
183–186 (186–188)50
114–115 (113–115)51
185–188 (182–185)52
220–221
C6H5CHO
96
9
2-ClC6H4CHO
4-ClC6H4CHO
4-CH3C6H4CHO
C6H5CHO
4-NO2C6H4CHO
Terephthaldehyde
Terephthaldehyde
Formaldehyde
Cyclohexanone
Acetaldehyde
Quant.
Quant.
95
90
85
80
75
98
80
10
11
12
13
14
15
16
17
18
19
20
21
187–189 (186–188)53
175–176 (174–175)54
187–189 (184–187)54
294–297 (290–294)54
137–140 (138–140)55
189–191 (188–190)56
168–170 (150)57
116–118 (118–120)49
150–153 (148–150)58
235–236 (233–234)59
311–312 (310–312)59
266–267 (265–266)59
92
3-Formylchromone
3-Formylchromone
3-Formylchromone
78b
70b
75b
a
Isolated yield. b The reaction was performed in the presence of 5 mol% of the catalyst at 70 ꢁC.
CH2Cl2, then reusing the aqueous phase in subsequent reac- 23.75) is among the best catalysts reported to date for this
tions. We conrmed that the catalyst can be reused at least ve transformation.
times without signicant drop in reaction yield.
In addition, we have shown that the TCAOS can be applied as
The catalytic efficiency of TCAOS in the synthesis of bis(in- an efficient catalyst for preparation of acetals from aldehydes
dolyl)methanes from indole and benzaldehyde is compared and diols. The best conditions for this reaction include heating
with those reported in the literatures and summarized in the starting materials at 80 ꢁC in the presence of 4 mol% of the
Table 2. As revealed from Table 2, the TOF of the TCAOS (TOF ¼ catalyst for 3 h under solvent-free conditions. The results are
summarized in Scheme 4.
Table 2 Comparison of the catalytic efficiency of various catalysts reported in the literature for the reaction of indole with benzaldehyde
Catalyst
Solvent
Catalyst amount (mol%)
Time (h)
Yield (%)
TOF (hꢀ1
)
Ref.
TCAOS
Water
Water
Solvent-free
CH3OH
CH3CN
Solvent-free
Water
Water
CH3CN
CH2Cl2
4
18
13
100
10
20
14
100
10
20
5
1
3
95
90
92
90
90
72
84
96
96
89
96
71
93
92
86
23.75
3.75
9.43
0.36
1.8
–
Squaric acid
SiO2–H2SO4
KHSO4
LiClO4
I2
Heteropoly acid
Oxalic acid
ZrCl4
NaHSO4–SiO2
In(OTf)3
InCl3
54
60
61
49
47
62
63
55
50
51
51
51
64
65
0.75
2.5
5
0.16
2
0.75
0.5
2.5
0.25
1
1.5
0.25
2.5
22.5
2
1.28
19.2
1.78
76.8
7.1
6.2
36.8
1.72
[Omim][PF6]
[Omim][PF6]
[Omim][PF6]
CH3CN
10
10
10
20
BiCl3
TCT
HMTAB
Water
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Synthesis of 4,40-(propane-1,3-diyl)bis(1-methyl-1,4-diaza bicyclo-
[2.2.2]octane-1,4-diium) dibromide diiodide (compound 4)
Compound a (20 mmol) and CH3I (60 mmol) were mixed and
stirred at room temperature for 6 h. Then the reaction mixture
was ltered off and washed several times with ethyl acetate to
remove any remained CH3I. The products were collected and
dried under vacuum. Colorless solid; yield: (99%); 1H NMR
(300 MHz, D2O): d (ppm) 2.48 (2H, m), 3.32 (6H, s), 3.68 (4H, t, J:
8.4 Hz), 4.03 (24H, s); 13C NMR (75 MHz, D2O): 16.5, 51.7, 52.1,
53.1, 60.5; Anal. Calcd. (%) for C17H36Br2I2N4: C, 28.75; H, 5.11;
N, 7.89. Found: C, 29.18; H, 5.29; N, 7.89.
Scheme
solvent-free conditions.
4 TCAOS-catalyzed acetalization of aldehydes under
Synthesis of 4,40-(propane-1,3-diyl)bis(1-methyl-1,4-diaza bicyclo-
[2.2.2]octane-1,4-diium) tetra hydrogen sulfate (compound 5)
Conclusions
In conclusion, a simple and efficient procedure for synthesis of
a new tetracationic acidic organic salt (TCAOS) based on DABCO
and its characterization is described in this paper. This
A solution of H2SO4 (20 mmol) in CH2Cl2 (5 mL) was added
dropwise to a suspension of compound 4 (5 mmol) in 5 mL
CH2Cl2, and the reaction mixture was stirred at 0 ꢁC for 10 min.
Then, the resulting reaction mixture was reuxed for 20 h. Aer
completion of the reaction, solvent was removed by evaporation
under reduced pressure and the residue was washed several
times with ethyl acetate to give the pure tetracationic acidic
organic salt 5. Colorless solid; yield: (93%); 1H NMR (300 MHz,
D2O): d (ppm) 2.51 (2H, m), 3.37 (6H, s), 3.71 (4H, m), 4.08 (24H,
s); 13C NMR (75 MHz, D2O) d (ppm) 15.9, 51.4, 52.4, 53.1, 60.5;
Anal. Calcd. (%) for C17H40N4O16S4: C, 29.82; H, 5.89; N, 8.18;
found: C, 29.88; H, 5.89; N, 8.12.
ꢀ
compound contains four HSO4 groups with strong acidic
character. Due to its acidic character, we have shown that it can
be applied as an eco-friendly, powerful and reusable catalyst for
the synthesis of bis(indolyl)methanes in water with high turn-
over frequency (TOF) and for acetalization of aldehydes with
diols. Also, this compounds may be applicable as electrolyte in
metal–acid batteries.
Experimental
General information
General procedure for preparation of bis(indolyl)methanes in
water catalyzed by tetra cationic acidic organic salt (TCAOS)
All materials were purchased from Merck and Fluka. All NMR
spectra's were recorded on a Bruker AMX 300 MHz spectrometer
and are referenced to the deuterated solvent used. Elemental
analysis performed by a Perkin-Elmer 2004 (II) CHN analyzer.
Infrared spectra were recorded in potassium bromide pellets on
a Perkin Elmer FT spectrum RX1 over the range 400–4000 cmꢀ1
and melting points were determined with a Branstead Electro-
thermal 9200 apparatus. TGA measurement has been per-
formed by a NETZSCH TG 209 F1 Iris instrument. The
measurement has been performed under a ow of nitrogen and
the sample (cꢁa. 5.90 mg) was scanned between room tempera-
ture and 600 C.
In a 25 mL round bottom ask equipped with magnetic stir bar
were added an indole (2.0 mmol), an aldehyde and water (5 mL).
To this mixture the catalyst [TCAOS (0.04 mmol, 0.03 g)] was
added and the mixture was stirred at room temperature for 2 h.
Progress of reaction was monitored by TLC (3 : 7, ethyl acetate–
petroleum ether). Aer completion of the reaction, the product
was extracted with diethyl ether (3 ꢂ 15) and the combined
organic phases were dried with anhydrous Na2SO4 and evapo-
rated under reduced pressure to give the crude product which
was puried via recrystallization in hexane, or ethanol–water
mixture. 3-((2-chlorophenyl)(2-methyl-1H-indol-3-yl)methyl)-2-
methyl-1H-indole: m.p. 220–221 ꢁC; 1H NMR (300 MHz, CDCl3)
d (ppm) 2.04 (6H, s), 6.21 (1H, s), 6.85–7.37 (12H, m), 7.73 (2H,
brs); 13C NMR (300 MHz, CDCl3) d (ppm) 11.8, 36.9, 109.5, 111.5,
118.5, 118.7, 120.2, 126.0, 127.1, 128.5, 129.0, 130.5, 131.5,
134.1, 134.5, 140.9; Anal. Calcd. (%) for C25H21ClN2: C, 78.01; H,
5.50; N, 7.28. Found: C, 78.21; H, 5.30; N, 7.32.
Synthesis of 1,100-(propane-1,3-diyl)bis(1,4-diazabicyclo[2.2.2]-
octan-1-ium) dibromide (compound 3)
A solution of DABCO (4.62 g, 40 mmol) and 1,3-dibromopro-
pane (3.45 g, 15 mmol) in methanol (50 mL) was le stirring for
20 h at rt. The reaction mixture was then evaporated in vacuo to
give a yellowish residue which was triturated with warm acetone
(30 mL, two times) to provide a white precipitate. Finally, the
precipitate was collected by ltration and dried under vacuum.
General procedure for acetalization of aldehydes in the
presence of TCAOS
White solid; yield: (97%); 1H NMR (300 MHz, D2O): d (ppm) 2.56 In a test tube equipped with a magnetic stir bar, aldehyde
(2H, m), 3.13 (12H, t, J ¼ 7.3 Hz), 3.27 (4H, t, J ¼ 8.7 Hz), 3.37 (1 mmol), a diol (1.1 mmol) and TCAOS (4 mol%) was added and
(12H, t, J ¼ 7.9 Hz); 13C NMR (75 MHz, D2O): 15.2, 44.0, 52.3, the mixture was heated at 80 ꢁC for 3 h. progress of reaction was
60.3; Anal. Calcd. (%) for C15H30Br2N4: C, 42.27; H, 7.09; N, monitored by TLC. Aer completion, ethyl acetate (10 mL) and
13.14 found: C, 42.07; H, 6.73; N, 12.89.
water (10 mL) were added to the reaction mixture. The organic
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Acknowledgements
We are grateful to the Iran National Science Foundation: INSF,
Grant number 92023919, for supporting this work. We also
thank the research council of Kharazmi University (formerly
Tarbiat Moallem University) for nancial support.
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