Water-mediated green and efficient synthesis of bis(Indolyl)methanes using ammonium iron(II) sulfate

Article abstract of DOI:10.2174/1570178616666190522121135

An efficient protocol for the synthesis of symmetrical 3,3’-bis(indolyl)arylmethanes catalyzed by green and easily available ammonium iron(II) sulfate (Mohr’s salt) has been developed in “green” water solvent afforded in excellent yield.

Full text of DOI:10.2174/1570178616666190522121135

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Letters in Organic Chemistry, 2020, 17, 61-67
RESEARCH ARTICLE
ISSN: 1570-1786
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Water-Mediated Green and Efficient Synthesis of Bis(Indolyl)methanes
Using Ammonium Iron(II) Sulfate
BENTHAM
S
CIENCE
Vikas D. Kadu1, Dinesh N. Nadimetla1, Mahesh G. Hublikar1, Dattatraya G. Raut1 and
Raghunath B. Bhosale1,*
1Organic Research Laboratory, School of Chemical Sciences, Solapur University Solapur-413255 (MH), India
A R T I C L E H I S T O R Y
Abstract: An efficient protocol for the synthesis of symmetrical 3,3’-bis(indolyl)arylmethanes
catalyzed by green and easily available ammonium iron(II) sulfate (Mohr’s salt) has been developed in
“green” water solvent afforded in excellent yield.
Received: December 29, 2018
Revised: April 15, 2019
Accepted: April 18, 2019
DOI:
10.2174/1570178616666190522121135
Keywords: Water, catalysis, Bis(indolyl)methanes, green chemistry, ammonium iron(II) sulfate, Indole.
1. INTRODUCTION
more favorable over other iron salts because of somewhat
labile to oxidation by air to iron (III) in an enormous breadth,
tuning prime attention to the field of catalysis for organic
transformations [35]. Here, we are extending our efforts to-
wards the construction of 3,3'-bis(indolyl)arylmethanes using
an environmentally benign water medium in the presence of
catalytic amount of ammonium iron(II) sulfate (Mohr’s salt).
Indoles are most feasible imperative motif among hetero-
cyclic scaffolds owing to their existence in diverse natural
commodities, agrochemicals and pharmaceuticals [1-3]. Es-
pecially, the nitrogen-containing 3,3'-bis(indolyl)methanes
(BIMs) of two indole motifs united by a single carbon at 3-
position are the well-known class of alkaloids, [4] have
been frequently found in marine [5-9] and terrestrial [10-
12] origins. For the variety of biological applications, their
structure has been proven to be constructive in search of
new therapeutic agents [13]. By virtue of their prominence
in natural capitals and the diversity of biological perfor-
mance, BIMs are essential building block having prominent
recognition and intensifying appeal in the development of
green synthetic routes. The methods accounted for the syn-
thesis of substituted BIMs by condensation of indoles with
carbonyl compounds or their synthetic equivalents in the
presence of an acid or base [14-34]. The expanding aware-
ness of greener, more sustainable technologies has directed
wakefulness on the adoption of efficient catalytic method-
ologies for the manufacture of fine chemicals and pharma-
ceuticals. Another facet of the growing recognition for al-
ternative reaction media circumvents the disputes accom-
panied by prevailing volatile organic solvents. The mani-
festo of nonconventional reaction media brings conven-
ience for promoting the improvement and recycling of the
catalyst. The ultimate in clean catalytic technologies is to pro-
duce an interesting approach in an aqueous media for green,
sustainable organic synthesis. Interestingly, Mohr's salt is
2. RESULTS AND DISCUSSION
At the outset, we started the reaction between 4-
methoxybenzaldehyde 1a (1 mmol) and indole 2 (2 mmol) in
the presence of 5 mole % of Ammonium Ferrous Sulphate
(Mohr’s salt) catalyst at room temperature using toluene
as reaction medium afforded 3,3'-((4-methoxyphenyl)-
methylene)bis(1H-indole) 3a in 38% yield after 12 h (entry
1, Table 1). In a further study, the reaction carried out using
different non-polar solvents such as chlorobenzene, DCM
and DCE also obtained 3a in yields such as 35%, 25%, and
46% (entries 2-4, Table 1). The reaction was carried out to a
greater extent in order to improve the yield of anticipated
product in divergent polar solvents like EtOH, i-PrOH, n-
BuOH, and MeOH at room temperature and product 3a fur-
nished in 45%, 40%, 30% and 60%, yields correspondingly
(entries 5-8, Table 1). Interestingly, under favorable optimi-
zation using MeOH up to 60%, the reaction attempted in
H2O medium and product 3a was afforded in 65% yield (en-
try 9, Table 1). Similarly, the reaction was executed under
reflux condition using H2O medium in which 3,3'-((4-
methoxyphenyl)methylene)bis(1H-indole) 3a procured in
excellent yield up to 88% within 3.5 h (entry 10, Table 1).
Furthermore, catalyst loading was accomplished to evaluate
the well-optimized condition for 7 mole % where 3a pro-
duced 92% yield in 3.5 h (entry 11, Table 1) and the same
*Address correspondence to this author at the School of Chemical Sciences,
Solapur University Solapur, Solapur-Pune National Highway, Kegoan,
Solapur-413255, Maharashtra, Pin Code: 413255, India;
E-mail: bhosale62@yahoo.com.
1875-6255/20 $65.00+.00
© 2020 Bentham Science Publishers

62 Letters in Organic Chemistry, 2020, Vol. 17, No. 1
Kadu et al.
Table 1.
Optimization of reaction conditionsa.
OMe
O
(NH4)2Fe(SO4)2.6H2O
solvent
H
+
N
H
MeO
N
H
N
H
2
3a
1a
Entry
Solvent
Catalyst (mol %)
Temp (°C)
Time (h)
Yieldb (%)
1
2
Toluene
Chlorobenzene
DCM
5
5
r.t.
r.t.
24
24
24
24
48
48
48
24
24
3.5
3.5
3.5
6.0
30
34
25
38
45
40
30
60
65
88
92
92
80
3
5
r.t.
4
DCE
5
r.t.
5
EtOH
5
r.t.
6
i-PrOH
n-BuOH
MeOH
H2O
5
r.t.
7
5
r.t.
8
5
r.t.
9
5
r.t.
10
11
12
13
H2O
5
reflux
reflux
reflux
reflux
H2O
7
H2O
10
2
H2O
aReaction conditions: 1a (1.2 mmol), 2a (2.0 mmol), solvent (5 mL). bIsolated yield.
result was obtained by further increasing catalyst loading up
to 10 mole % (entry 12, Table 1). On the other hand, the de-
crease in catalyst loading from 5 mole % to 2 mole % of-
fered anticipated product 3a in lower yield of up to 80% in
6 h (entry 13, Table 1).
Mohr’s salt at carbonyl oxygen of aldehydes generates a
driving force for electrophilic substitution reaction with in-
dole molecule 2a towards construction of C-3 alkylated in-
termediate 4. In a further step, intermediate 4 undergoes de-
hydration, resulted in to the formation of azafulvene inter-
mediate 5, which reacts with another molecule of indole 2a
leading to formation of 3,3’-bis(indolyl)methanes 3.
With the optimized condition, the substrate scope of sub-
stituted aromatic aldehydes and indoles illustrated is shown
in Table 2. The substrates containing electron-donating i.e.
_
_Me, _OH and OMe groups were well-tolerated and pro-
3. EXPERIMENTAL SECTION
3.1. General Information
duced the desired products in moderate to excellent yields.
_
_
Along with the electron-withdrawing i.e. CN, NO2 groups
were found competent offering the corresponding products in
excellent yields. Noticeably, the halogen substituents were
unharmed after the reaction, leading to construction of more
halogenated derivatives of BIMs.
1H and 13C spectra were recorded at 400 and 100 MHz
respectively. Chemical shifts were reported in parts per mil-
lion (ppm) downfield from an internal TMS (tetramethylsilane)
reference. Coupling constants (J) were reported in hertz (Hz),
and spin multiplicities represented by the symbol s (singlet), br s
(broad singlet), d (doublet), t (triplet), q (quartet), p (pentet) and
m (multiplet). Aromatic ring C-s of different aldehydes in
the 4-substituted cases numbered as C11, C12, C13 and C14,
and in the 2-substituted or other cases as C11, C12,
We proposed cautious reaction mechanism for the syn-
thesis of bis(indolyl)methanes from benzaldehyde 1 and in-
dole 2a using environmentally benign water medium as rep-
resented in Scheme (1). At first, the protonation using

Water-Mediated Green and Efficient Synthesis of Bis(Indolyl)methanes
Letters in Organic Chemistry, 2020, Vol. 17, No. 1 63
Table 2. Substrate scopea.
14
13
12
3
15
R1
16
3
O
H
5
5
11
10
4
4
9
9
(NH4)2Fe(SO4)2.6H2O
H2O, reflux
6
6
R1
+
R2
7
N
H
7
8
8
N
N
R2
R2
2
2
H
H
1
1
3
1
2
M. P. (°C)
Reported
R1
R2
Time (h)
Yieldb (%)
Product
Entry
Observed
1
2
H
H
H
3.5
3.5
3.5
3.5
3.0
3.5
3.5
4.0
4.0
3.5
3.5
3.5
3.5
93
90
92
94
95
91
85
90
93
91
92
93
92
139-141
210-212
77-79
140-142 [25]
210-211 [26]
78-80 [27]
3b
3c
3d
3e
3f
4_OH
4_Cl
3
H
4
4_F
H
71-73
73-75 [28]
5
4_CN
2_Cl
H
209-210
72-73
208-209 [29]
74-76 [25]
6
H
3g
3h
3i
7
2_OMe
2_OH
2_OH, 5_Br
H
H
136-138
340-342
137-139
246-248
236-238
146-148
240-242
134-136 [30]
341-343 [31]
136-138 [32]
247-248 [25]
238-240 [26]
147-150 [33]
241-243 [34]
8
H
9
H
3j
10
11
12
13
Me
Me
Me
Me
3k
3l
4_OH
4_OMe
4_Cl
3m
3n
aReaction conditions: 1 (1.0 mmol), 2 (2.0 mmol), (NH4)2Fe(SO4)2.6H2O (0.027 g, 7 mole %), H2O
(5 mL), at reflux condition. bIsolated yield.
3.3. Spectral Data
C13, C14, C15 and C16. Bond positions were reported in
reciprocal centimeters (cm-1). Thin layer chromatography
(TLC) was performed by using commercially prepared 0.25
mm silica plates, and visualization was observed with a short
wavelength UV lamp (254 nm). All commercially available
reagents were used directly without purification unless oth-
erwise stated. All the solvents used in the reactions were
distilled for purity.
3.3.1. Synthesis of 3,3'-(4-methoxyphenyl)methylene)
bis(1H-indole) (3a)
This compound was synthesized following the above
mentioned general procedure using 4-methoxybenzaldehyde
(0.136 g, 1.0 mmol), indole (0.234 g, 2.0 mmol),
(NH4)2Fe(SO4)2.6H2O (0.027 g, 7.0 mole %), and H2O (5
mL). Yield: 323 mg (92%) as brown colored solid com-
pound. Chemical Formula: C24H20N2O; Molecular Weight:
352.4284; M.P.: 183-185oC (Lit. [25] 185-187oC); IR (cm-1):
3408, 1609, 1507, 1455, 1416, 1337, 1300, 1242, 1173,
1124, 1092, 1027, 813, 793, 740; 1H NMR (400 MHz,
CDCl3): δ 7.94 (br s, 2H, NH), 7.41 (d, J = 7.6 Hz, 2H,
Ar-H), 7.38 (d, J = 8.4 Hz, 2H, Ar-H), 7.28 (d, J = 6.6 Hz,
2H, Ar-H), 7.19 (t, J = 7.8 Hz, 2H, Ar-H), 7.03 (t, J = 7.5
Hz, 2H, Ar-H), 6.84 (d, J = 6.8 Hz, 2H, Ar-H), 6.68 (s, 2H,
C2H), 5.87 (s, 1H, C10H), 3.81 (s, 3H, OCH3) ppm; 13C
NMR (100 MHz, CDCl3): δ 157.8 (C14), 136.6 (C8), 136.2
3.2. General Procedure for the Synthesis of BIMs (3a-n)
To a round bottom flask, a mixture of aldehyde 1 (1.0
mmol), indole 2 (2.0 mmol), (NH4)2FeSO4 (0.027 g, 7 mole
%), and H2O (5 mL) was stirred at reflux condition for ap-
propriate time-period. The reaction progress was monitored
by thin layer chromatography and the reaction mixture was
cooled to room temperature after completion of the reaction.
The solid crude product 3 was afforded sufficiently pure for
further step by simple filtration of the reaction mixture.

64 Letters in Organic Chemistry, 2020, Vol. 17, No. 1
Kadu et al.
Scheme (1). Plausible reaction mechanism.
(C11), 129.6 (C12), 127.1 (C9), 123.5 (C2), 121.9 (C6),
120.0 (C5), 119.9 (C4), 119.2 (C13), 113.5 (C3), 111.0 (C7),
55.2 (OCH3), 39.3 (C10) ppm.
indole 2a (0.234 g, 2.0 mmol), (NH4)2Fe(SO4)2.6H2O (0.027
g, 7.0 mole %), and H2O (5 mL). Yield: 289 mg (90%) as
pink colored solid compound. Chemical Formula:
C23H18N2O; Molecular Weight: 338.4018; M. P.: 210-212oC
(Lit. [26] 210-211oC); IR (cm-1): 3412, 2924, 2853, 1611,
1510, 1456, 1338, 1217, 1093, 786, 744, 598; 1H NMR (400
MHz, CDCl3): δ 7.90 (br s, 2H, NH), 7.37-7.32 (m, 4H),
7.19-7.12 (m, 4H), 6.98 (t, J = 6.8 Hz, 2H), 6.72 (s, 2H,
C2H), 6.63 (t, J = 6.8 Hz, 2H), 5.81 (s, 1H, C10H), 4.80 (br
s, 1H, OH) ppm; 13C NMR (100 MHz, CDCl3): δ 154.5
(C14), 136.6 (C8), 135.5 (C11), 129.4 (C12), 126.9 (C9),
123.5 (C2), 121.3 (C6), 119.7 (C5), 118.6 (C4), 119.2 (C13),
113.5 (C3), 111.0 (C7), 39.3 (C10) ppm.
3.3.2. Synthesis of 3,3'-(phenylmethylene)bis(1H-indole)
(3b)
This compound was synthesized following the above men-
tioned general procedure using benzaldehyde (0.106 g, 1.0
mmol), indole (0.234 g, 2.0 mmol), (NH4)2Fe(SO4)2.6H2O
(0.027 g, 7.0 mole %), and H2O (5 mL). Yield: 299 mg (93%)
as pink colored solid compound; Chemical Formula:
C23H18N2; Molecular Weight: 322.4024; M. P.: 139-141oC
(Lit. [25] 140-142oC); IR (cm-1): 3309, 1508, 1456, 1417,
1
1302, 1244, 1174, 1028, 814, 792; H NMR (400 MHz,
CDCl3): δ 7.85 (br s, 2H, NH), 7.45-7.19 (m, 11H, Ar-H),
7.05 (t, J = 8.0 Hz, 2H), 6.64 (s, 2H, C2H), 5.93 (s, 1H,
C10H) ppm; 13C NMR (100 MHz, CDCl3): δ 144.0 (C11),
137.7 (C8), 128.8 (C12), 128.3 (C13), 127.1 (C14), 126.2
(C9), 123.7 (C2), 121.9 (C6), 120.0 (C5), 119.2 (C4), 112.1
(C3), 111.1(C7), 40.2 (C10) ppm.
3.3.4. Synthesis of 3,3'-((4-chlorophenyl)methylene)bis(1H-
indole) (3d)
The compound was synthesized following the representa-
tive procedure using 4-chlorobenzaldehyde 1d (0.140 g, 1.0
mmol), indole 2a (0.234 g, 2.0 mmol), (NH4)2Fe(SO4)2.6H2O
(0.027 g, 7.0 mole %), and H2O (5 mL). Yield: 327 mg
(92%) as pink colored solid compound. Chemical Formula:
C23H17ClN2; Molecular Weight: 356.8475; M. P.: 77-79oC
(Lit. [27] 76-77oC); IR (cm-1): 3411, 3055, 2923, 2848, 1617,
3.3.3. Synthesis of 3,3'-(4-Hydroxybenzylidene)bis(1H-
indole) (3c)
The compound was synthesized following the representa-
tive procedure using 4-hydroxybenzaldehyde 1c (1.0 mmol),
1
1417, 1337, 1089, 1013, 743; H NMR (400 MHz, CDCl3):

Water-Mediated Green and Efficient Synthesis of Bis(Indolyl)methanes
Letters in Organic Chemistry, 2020, Vol. 17, No. 1 65
3.3.8. Synthesis of 3,3'-((2-methoxyphenyl)methylene)
bis(1H-indole) (3h)
7.91 (br, s, 2H, NH), 7.14-7.36 (m, 8H, Ar-H), 7.02 (t, 2H,
Ar-H), 6.97 (t, 2H, Ar-H), 6.63 (s, 2H, C2H), 5.86 (s, 1H,
C10H); 13C NMR (100 MHz, CDCl3,): δ 142.4 (C8), 136.7
(C11), 131.8 (C14), 130.0 (C12), 128.3 (C13), 126.9 (C9),
123.4 (C2), 122.1 (C6), 119.8 (C5), 119.4 (C4), 119.2 (C3),
111.0 (C7), 39.6 (C10) ppm.
The compound was synthesized following the representa-
tive procedure using 2-methoxybenzaldehyde 1h (0.136 g, 1.0
mmol), indole 2a (0.234 g, 2.0 mmol), (NH4)2Fe(SO4)2.6H2O
(0.027 g, 7.0 mole %), and H2O (5 mL). Yield: 299 mg
(85%) as white solid compound. Chemical Formula:
C24H20N2O; Molecular Weight: 352.4284; M. P.: 136-138oC
(Lit. [30] 134-136oC); IR (cm-1): 3414, 3057, 2933, 1597,
3.3.5. Synthesis of 3,3'-((4-fluorophenyl)methylene)bis(1H-
indole) (3e)
1
1489, 1456, 1338, 1243, 1092, 1028, 743, 600; H NMR
The compound was synthesized following the representa-
tive procedure using 4-fluorobenzaldehyde 1e (0.124 g, 1.0
mmol), indole 2a (0.234 g, 2.0 mmol), (NH4)2Fe(SO4)2.6H2O
(0.027 g, 7.0 mole %), and H2O (5 mL). Yield: 319 mg
(94%) as pink colored solid compound. Chemical Formula:
C23H17FN2; Molecular Weight: 340.3929; M. P.: 71-73oC
(Lit. [28] 73-75oC); IR (cm-1): 3413, 3056, 2926, 1603, 1506,
1456, 1217, 1094, 863, 744, 582; 1H NMR (400 MHz,
CDCl3): δ 7.88 (br s, 2H, NH), 7.30 (d, J = 8.4 Hz, 4H),
7.23-7.19 (m, 2H), 7.11 (t, J = 6.8 Hz, 2H), 6.96-6.87 (m,
4H), 6.58 (s, 2H, C2H), 5.80 (s, 1H, C10H) ppm; 13C NMR
(100 MHz, CDCl3): δ 139.7 (C14), 136.7 (C8), 130.1 (C11),
130.0 (C12), 126.9 (C9), 123.5 (C2), 122.0 (C6), 119.8 (C5),
119.6 (C4), 119.3 (C13), 115.0 (C3), 111.0 (C7), 39.4 (C10)
ppm.
(400 MHz, CDCl3): δ 7.83 (br s, 2H, NH), 7.39 (d, J = 8.4
Hz, 2H), 7.32 (d, J = 7.6 Hz, 2H), 7.15-7.11 (m, 4H), 6.97 (t,
J = 6.8 Hz, 2H), 6.92 (d, J = 8.4 Hz, 1H), 6.79 (t, J = 8.0 Hz,
1H), 6.63 (s, 2H, C2H), 6.34 (s, 1H, C10H), 3.80 (s, 3H,
OCH3) ppm; 13C NMR (100 MHz, CDCl3): δ 156.9 (C12),
136.7 (C8), 132.3 (C16), 129.7 (C9), 127.2 (C14), 127.1
(C11), 123.5 (C2), 121.7 (C6), 120.4 (C15), 120.0 (C5),
119.6 (C4), 119.0 (C13), 110.9 (C3), 110.6 (C7), 55.7
(OCH3), 32.0 (C10) ppm.
3.3.9. Synthesis of 2-(di(1H-indol-3-yl)methyl)phenol (3i)
The compound was synthesized following the representa-
tive procedure using 2-hydroxybenzaldehyde 1i (0.122 g, 1.0
mmol), indole 2a (0.234 g, 2.0 mmol), (NH4)2Fe(SO4)2.6H2O
(0.027 g, 7.0 mole %), and H2O (5 mL). Yield: 304 mg
(90%) as pink colored solid compound. Chemical Formula:
C23H18N2O; Molecular Weight: 338.4018; M. P.: 340-342oC
(Lit. [31] 341-343oC); IR (cm-1): 3818, 3749, 3494, 2931,
3.3.6. Synthesis of 4-(di(1H-indol-3-yl)methyl)benzonitrile
(3f)
1
2360, 1743, 1365, 1326, 748; H NMR (250 MHz, DMSO-
The compound was synthesized following the representa-
tive procedure using 4-cyanobenzaldehyde 1f (0.131 g, 1.0
mmol), indole 2a (0.234 g, 2.0 mmol), (NH4)2Fe(SO4)2.6H2O
(0.027 g, 7.0 mole %), and H2O (5 mL). Yield: 329 mg
(95%) as pink colored solid compound. Chemical Formula:
C24H17N3; Molecular Weight: 347.4119; M. P.: 209-210oC
(Lit. [29] 211oC); IR (cm-1): 3396, 3165, 2940, 2253, 1602,
1407, 757; 1H NMR (400 MHz, CDCl3): 7.96 (br s, 2H, NH),
7.56 (d, J = 8.2 Hz, 2H, Ar–H), 7.44 (d, J = 8.2 Hz, 2H, Ar–
H), 7.1–7.5 (m, 8H, Ar–H), 6.63 (s, 2H, C2H), 5.92 (s, 1H,
C10H) ppm; 13C NMR (100 MHz, CDCl3): δ 149.7 (C11),
136.7 (C8), 132.2 (C13), 129.5 (C12), 126.7 (C9), 123.7
(C2), 122.3 (C6), 119.6 (C5), 119.5 (C4), 119.2 (C3), 118.2
(CN), 111.2 (C7), 110.0 (C14), 40.4 (C10) ppm.
d6): δ 10.75 (s, 2H, NH), 9.42 (s, 1H, OH), 7.27-7.36 (m, 4H,
Ar-H), 6.96-7.09 (m, 4H, Ar-H), 6.85 (t, 3H, J = 7.2 Hz, Ar-H),
6.75 (s, 2H, C2H) 6.65 (t, 1H, J = 7.0 Hz, Ar-H), 6.21 (s, 1H,
C10H) ppm; 13CNMR (62.5 MHz, DMSO-d6): δ 154.2
(C12), 136.5 (C8), 130.8 (C16), 129.3 (C9), 126.8 (C14),
126.5 (C11), 123.4 (C2), 120.7 (C6), 118.9 (C15), 118.5
(C5), 118.0 (C4), 117.9 (C13), 114.9 (C3), 111.3 (C7), 31.4
(C10) ppm.
3.3.10. Synthesis of 4-bromo-2-(di(1H-indol-3-yl)methyl)
phenol (3j)
The compound was synthesized following the representa-
tive procedure using 2-hydroxy-5-bromobenzaldehyde 1j
(199.9 g, 1.0 mmol), indole 2a (0.234 g, 2.0 mmol),
(NH4)2Fe(SO4)2.6H2O (0.027 g, 7.0 mole %), and H2O (5
mL). Yield: 386 mg (93%) as pink colored solid compound.
Chemical Formula: C23H17BrN2O; Molecular Weight:
3.3.7. Synthesis of 3,3'-((2-chlorophenyl)methylene)bis(1H-
indole) (3g)
The compound was synthesized following the representa-
tive procedure using 2-chlorobenzaldehyde 1g (0.140 g, 1.0
mmol), indole 2a (0.234 g, 2.0 mmol), (NH4)2Fe(SO4)2.6H2O
(0.027 g, 7.0 mole %), and H2O (5 mL). Yield: 323.9 mg
(91%) as pink colored solid compound. Chemical Formula:
C23H17ClN2; Molecular Weight: 356.8475; M. P.: 72-73oC
(Lit. [25] 74-76oC); IR (cm-1): 3413, 3057, 2924, 1618, 1456,
1
417.2979; M.P.: 137-139oC (Lit. [32] 136-138oC); H NMR
(400 MHz, CDCl3): 10.81 (s, 1H, OH), 9.80 (brs, 2H, NH),
7.35-6.76 (m, 11H, Ar-H), 6.13 (s, 2H, C2H), 5.72 (s, 1H,
C10H) ppm; 13C NMR (100 MHz, CDCl3): δ 153.7 (C12),
136.8 (C8), 132.4 (C16), 131.6 (C9), 130.8 (C11), 126.6
(C13), 123.67 (C2), 122.4 (C6), 119.6 (C5), 118.4 (C4),
116.5 (C14), 115.6 (C15), 112.9 (C3), 111.3 (C7), 60.5
(C10) ppm.
1
1337, 1216, 1093, 1038, 743, 599; H NMR (400 MHz,
CDCl3): δ 7.84 (br s, 2H, NH), 7.36-7.28 (m, 5H), 7.16-7.01
(m, 5H), 6.94 (t, J = 6.8 Hz, 2H), 6.57 (s, 2H, C2H), 6.27 (s,
1H, C10H) ppm; 13C NMR (100 MHz, CDCl3): δ 141.3
(C11), 136.7 (C8), 133.9 (C12), 130.3 (C16), 129.4 (C13),
127.5 (C9), 127.0 (C14), 126.6 (C15), 123.7 (C2), 122.0
(C6), 119.8 (C5), 119.3 (C4), 118.3 (C3), 111.0 (C7), 36.6
(C10) ppm.
3.3.11. Synthesis of 3,3'-(phenylmethylene)bis(2-methyl-
1H-indole) (3k)
The compound was synthesized following the representa-
tive procedure using benzaldehyde 1k (0.106 g, 1.0 mmol),

66 Letters in Organic Chemistry, 2020, Vol. 17, No. 1
Kadu et al.
2-methylindole 2b (0.262 g, 2.0 mmol), (NH4)2Fe(SO4)2.6H2O
(0.027 g, 7.0 mole %), and H2O (5 mL). Yield: 318 mg
(91%) as pink colored solid compound. Chemical Formula:
C25H22N2; Molecular Weight: 350.4556; M.P.: 246-248oC
(Lit. [25] 247-248oC); IR (cm-1): 3393, 3050, 1458, 1298,
384.9006; M. P.: 240-242oC (Lit. [34] 241-243oC); IR (cm-1):
1
3384, 3050, 1486, 1457, 1429, 1015, 747; H NMR (400
MHz, CDCl3): δ 7.75 (s, 2H, NH), 7.24-7.21 (m, 6H, Ar-H),
7.08-7.04 (m, 2H, Ar-H), 6.98 (d, J = 8.0 Hz, 2H, Ar-H),
6.90-6.86 (m, 2H, Ar-H), 5.96 (s, 1H, C10H), 2.07 (s, 6H,
2xCH3) ppm; 13C NMR (100 MHz, CDCl3): δ 142.8 (C11),
135.0 (C8), 131.8 (C14), 131.6 (C2), 130.4 (C12), 128.7
(C13), 128.2 (C9), 120.7 (C6), 119.2 (C5), 118.7 (C4), 112.8
(C3), 110.0 (C7), 38.6 (C10), 12.4 (CH3) ppm.
1
1220, 747; H NMR (400 MHz, DMSO-d6): δ 10.74 (s, 2H,
NH), 7.24-7.18 (m, 7H, Ar-H), 6.88 (t, J = 7.5 Hz, 2H, Ar-
H), 6.80 (d, J = 8.0 Hz, 2H, Ar-H), 6.67 (t, J = 7.5 Hz, 2H,
Ar-H), 5.92 (s, 1H, C10H), 2.06 (s, 6H, 2xCH3) ppm; 13C
NMR (100 MHz, DMSO-d6): δ 144.7 (C11), 135.5 (C8),
132.5 (C2), 129.2 (C12), 128.7 (C13), 128.4 (C9), 126.2
(C14), 120.0 (C6), 118.9 (C5), 118.4 (C4), 117.9 112.6 (C3),
110.8 (C7), 39.1 (C10), 12.4 (CH3) ppm.
CONCLUSION
In this study, we have developed a straightforward and
facile approach for the synthesis of discrete symmetrical
BIMs in a water medium. In view of this, the catalytic per-
formance of more accessible Mohr’s salt highlighting its
potential for the synthesis of imperative biologically perti-
nent BIMs scaffolds in medicinal chemistry.
3.3.12. Synthesis of 4-(bis(2-methyl-1H-indol-3-yl)methyl)
phenol (3l)
The compound was synthesized following the repre-
sentative procedure using 4-hydroxybenzaldehyde 1l (0.122
g, 1.0 mmol), 2-methylindole 2b (0.262 g, 2.0 mmol),
(NH4)2Fe(SO4)2.6H2O (0.027 g, 7.0 mole %), and H2O (5
mL). Yield: 336 mg (92%) as pink colored solid compound.
Chemical Formula: C25H22N2O; Molecular Weight:
366.4550; M. P.: 236-238oC (Lit. [26] 238-240oC); IR (cm-1):
CONSENT FOR PUBLICATION
Not applicable.
FUNDING
1
3425, 3058, 2971, 1456, 1335, 1225, 1097, 745; H NMR
None.
(400 MHz, CDCl3): δ 9.63 (s, 2H, NH), 8.61 (s, 1H, OH),
7.15 (d, 2H, J = 7.7 Hz), 7.01 (d, 2H, J = 7.6 Hz), 6.73 (m,
4H), 6.51 (m, 4H), 5.64 (s, 1H, C10H), 1.83 (s, 6H, 2xCH3)
ppm; 13C NMR (100 MHz, CDCl3): δ 155.2 (C14), 135.1
(C8), 134.5 (C2), 131.8 (C11), 129.7 (C12), 128.5 (C9),
119.7 (C6), 118.8 (C5), 118.1 (C4), 114.8 (C13), 113.1 (C3),
110.2 (C7), 38.1 (C10), 12.3 (CH3) ppm.
CONFLICT OF INTEREST
The authors declare no conflict of interest, financial or
otherwise.
ACKNOWLEDGEMENTS
Declared none.
3.3.13. Synthesis of 3,3'-((4-methoxyphenyl)methylene)
bis(2-methyl-1H-indole) (3m)
SUPPLEMENTARY MATERIAL
The compound was synthesized following the representa-
tive procedure using 4-methoxybenzaldehyde 1m (0.136 g,
1.0 mmol), 2-methylindole 2b (0.262 g, 2.0 mmol),
(NH4)2Fe(SO4)2.6H2O (0.027 g, 7.0 mole %), and H2O (5
mL). Yield: 353 mg (93%) as pink colored solid compound.
Chemical Formula: C26H24N2O; Molecular Weight:
380.4816; M. P.: 146-148oC (Lit. [33] 147-150oC); IR (cm-1):
Supplementary material is available on the publisher’s
web site along with the published article.
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