BiCl3-loaded montmorillonite K10: A new solid acid catalyst for solvent-free synthesis of bis(indolyl)methanes

Article abstract of DOI:10.1007/s11164-014-1636-3

We report an efficient green process for synthesis of bis(indolyl)methane derivatives under solvent-free conditions by use of a new solid acid catalyst, BiCl₃-loaded montmorillonite K10. The catalyst was characterized by SEM, BET surface area m

Full text of DOI:10.1007/s11164-014-1636-3

Res Chem Intermed
DOI 10.1007/s11164-014-1636-3
BiCl -loaded montmorillonite K10: a new solid acid
3
catalyst for solvent-free synthesis
of bis(indolyl)methanes
K. Ravi
•
B. Krishnakumar M. Swaminathan
•
Received: 16 December 2013 / Accepted: 24 March 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract We report an efficient green process for synthesis of bis(indolyl)methane
derivatives under solvent-free conditions by use of a new solid acid catalyst, BiCl3-
loaded montmorillonite K10. The catalyst was characterized by SEM, BET surface area
measurement, and determination of its surface acidity by temperature-programmed
desorption measurements. The salient features of the procedure are its simplicity, ease of
preparation of the catalyst, cost effectiveness, excellent yield with short reaction time,
reusability of the catalyst, and enhancement of the acidity of montmorillonite K10
catalyst by BiCl loading.
3
Keywords BiCl –MK10 ꢀ Bis(indolyl)methanes ꢀ Simple grinding ꢀ
3
Solid acid catalyst ꢀ Green synthesis ꢀ Reusability
Introduction
The indole ring system is an extremely important constituent in many natural
products, pharmaceuticals, agrochemicals, and other compounds of importance [1,
2
of terrestrial and marine natural sources and they have a range of important
]. Many bis(indolyl) and tris(indolyl) methanes have been isolated from a variety
0
biological activity [3–6]. 3,3 -Bis(indolyl)methane and its derivatives are used as
dietary supplements for humans [7, 8]. The activity of bis(3-indolyl)methanes in
cancer chemotherapy [9], inhibition of bladder growth [10], and in lung cancer cells
[
11] has been studied in detail. They also have antibiotic [12], antibacterial [13],
antimetastatic [14], radical-scavenging [15] antimicrobial and antifungal [16], and
analgesic and anti-inflammatory [17] activity. Because of this vast range of
K. Ravi ꢀ B. Krishnakumar ꢀ M. Swaminathan (&)
Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar,
Chidambaram 608 002, Tamil Nadu, India
e-mail: chemres50@gmail.com
123

K. Ravi et al.
R
R
BiCl -MK10
3
+
2
N
O
grinding, rt
H
N
N
H
(a-r)
1
(a-r)
2
H
3
3
Scheme 1 BiCl –MK10 catalyzed synthesis of bis(indolyl)methanes
biological activity and associated medicinal applications, a variety of methods for
synthesis of bis(indolyl)methanes have been reported in the literature; many of these
methods use protic and Lewis acid catalysts in electrophilic substitution of indoles
with aldehydes [18–29]. However, many Lewis acids can be deactivated, and some
decompose nitrogen-containing reactants. Even when the reactions proceed as
expected, more than a stoichiometric amount of the Lewis acid is required, because
the acids are trapped by nitrogen [30].
Commercially available montmorillonite clay K10 (MK10) is an environmentally
benign, economically feasible, stable, mild, and easily reusable solid acid catalyst
which has been used for many organic transformations [31–34]. Commercially
available BiCl has been widely used as a Lewis acid catalyst in the aldol [35] and
3
hetero Diels–Alder [36] reactions, for ring opening of epoxides with aromatic
amines [37], for deoxygenative allylation of substituted benzylic alcohols with
allyltrimethylsilane [38], and for three-component synthesis of b-amino carbonyl
compounds [39]. Owing to its highly hygroscopic nature, BiCl cannot be easily
3
handled and reused; it is, therefore, loaded on suitable support materials for easy
handling and reuse. Because MK10 clay is one such material that can fulfill this
requirement, in continuation of our work on green chemical synthesis of
heterocyclic compounds [40, 41] we report herein a simple, rapid and efficient
method for synthesis of bis(indolyl)methanes by use of a newly developed catalyst,
BiCl -loaded montmorillonite K10 (BiCl –MK10), as a solid acid catalyst under
3
3
ambient and solvent free conditions (Scheme 1). To the best of our knowledge, this
is the first report of the preparation of bis(indolyl)methanes using this solid acid
catalyst.
Experimental
Materials and methods
Bismuth chloride, 2-propanol (Spectrochem, C99.50 %), montmorillonite K10
(
Aldrich, composition (wt%) SiO : 67.6; Al O : 14.6; Fe O : 2.9; MgO: 1.8) were
2 2 3 2 3
used as received. All other chemicals were of AnalaR grade and used as received. IR
spectra were recorded by use of an Avatar-330 FT-IR spectrophotometer. Product
analysis was performed by GC–MS by use of a GC–MS QP2010 plus with DB-5-
MS capillary column. Proton and carbon NMR spectra were recorded on a Bruker
AVIII FT-NMR spectrometer operating at 400 MHz.
1
23

3
BiCl loaded montmorillonite K10
Preparation of BiCl –MK10
3
BiCl –MK10 catalyst was prepared by a simple solid dispersion method. MK10
3
-
4
(
2.7 g ) was dispersed in 50 mL 2-propanol and 2.5 9 10 mol BiCl (0.081 g) in
3
10 mL 2-propanol was added to the MK10–2-propanol mixture. The mixed
suspension was stirred for 4 h at room temperature, then the solvent was evaporated
to furnish the product. The solid obtained was dried at 110 °C for 3 h. The BiCl3–
MK10 catalyst was obtained as a fine powder. This catalyst contained 3 wt% BiCl .
3
Characterization of BiCl –MK10
3
BiCl -loaded montmorillonite K10 catalyst was characterized by SEM–EDS, BET
3
surface area measurement, and determination of surface acidity by ammonia TPD
measurements. SEM images of bare MK10 and BiCl –MK10, and the SEM–EDS
3
spectrum of BiCl –MK10 are shown in Fig. 1. Agglomeration of BiCl on the
3
3
MK10 catalyst is apparent from SEM images recorded at three different
magnifications (Fig. 1b–d), and the EDS spectrum confirms the presence of Bi in
the BiCl –MK10 catalyst (Fig. 1e). In BET surface area measurements, the BiCl –
3
3
MK10 catalyst had a lower surface area than MK10, because of impregnation of the
MK10 surface with BiCl ; BiCl loading leads to reduced pore volume and an
increase in median pore width (Table 1).The efficiency of solid acid catalysts
3
3
depends on their surface acidity. The surface acidity of BiCl –MK10 (Table 2) was
3
determined by temperature-programmed desorption (TPD), using ammonia as probe
molecule, in the temperature range 120–600 °C. The results indicated that the
acidity of BiCl –MK10 was approximately 18 times larger than that of MK10 [42];
3
this large increase is because of loading of the MK10 with BiCl3.
0
General procedure for synthesis of 3,3 -bis(indolyl)methane
BiCl –MK10 catalyst (0.1 g) was added to a mixture of indole (2 mmol) and
3
aldehyde (1 mmol), and the mixture was ground in a mortar with a pestle at room
temperature. After completion of the reaction (tested by thin-layer chromatography,
TLC), ethyl acetate was added and the insoluble catalyst was removed by filtration.
The filtrate was dried over anhydrous Na SO . The solvent was evaporated under
2
4
vacuum to furnish the product. All the products were characterized by GC–MS, and
1
13
selected compounds were characterized by H and C NMR. NMR data for three
representative compounds are given below.
0
3
,3 -(Phenylmethylene)bis(1H-indole) (3a)
1
Pink solid; H NMR (400 MHz, CDCl ) d 5.87 (1H, s, Ar–CH), 6.60 (2H, s),
6
3
1
3
.98–7.01 (2H, t), 7.24–7.26 (3H, m), 7.28–7.39 (8H, m), 7.85 (2H, bs, NH);
C
NMR (100 MHz, CDCl ) 40.20, 111.08, 119.25, 119.69, 120.76, 121.95, 123.68,
3
?
24.20, 126.18, 127.07, 128.27, 136.68, 144.02; GC–MS (m/z) = 322(M );
1
123

K. Ravi et al.
3 3
Fig. 1 SEM images of a MK10 (950,000), b MK10 (9100,000), c BiCl –MK10 (950,000), d BiCl –
3
MK10 (9100,000), and e EDS spectrum of BiCl –MK10
3
Table1 Surface area and pore size of MK10 and BiCl –MK10 catalysts
Measurement
MK10 catalyst
3
BiCl –MK10 catalyst
2
BET surface area (m g
3
Maximum pore volume (cm g
-1
)
241
170.27
0.286
77.15
-1
)
0.36
57.52
˚
Median pore width (A)
1
23

3
BiCl loaded montmorillonite K10
3
Table2 Surface acidity of MK10 and BiCl –MK10 catalysts
-1
-1
3
BiCl –MK10 catalyst (mmol g )
Temperature (°C)
MK10 catalyst (mmol g
)
1
2
20–200
00–600
0.55
0.37
0.92
9.48
6.90
Total
16.38
0
3
,3 -(3-Bromophenylmethylene)bis(1H-indole)(3e)
1
Pink solid; H NMR (400 MHz, CDCl ) d 5.838 (1H, s, Ar–CH), 6.611 (2H, s),
3
6
.99–7.03 (2H, t), 7.10–7.19 (4H, m), 7.24–7.27 (2H, m), 7.33–7.38 (3H, m), 7.49
1
3
(
1H,s), 7.89 (2H, bs, NH) C NMR (100 MHz, CDCl ) 39.96, 111.16, 118.95,
3
1
1
19.42, 119.79, 122.13, 122.44, 123.71, 126.88, 127.42, 129.41, 129.89, 131.73,
?
36.69, 146.52; GC–MS (m/z) = 402(M );
0
3
,3 -(3-Methoxyphenylmethylene)bis(1H-indole) (3l)
1
Pink solid; H NMR (400 MHz, CDCl ) d 3.717 (3H, s, OCH ), 5.83 (1H, s),
3
3
6
7
1
1
.70–6.74 (3H, m), 6.91–6.96 (4H, m), 7.08–7.12 (2H,t), 7.15–7.19 (2H, t),
1
.35–7.37 (3H, m), 7.39 (2H, s, NH) C NMR (100 MHz, CDCl ) 40.64, 54.99,
3
3
11.21, 114.64, 118.77, 119.55, 121.27, 123.84, 126.96, 128.92, 136.78, 146.19,
?
59.37; GC–MS (m/z) = 352(M ).
Results and discussion
0
To determine the optimum conditions and amount of catalyst, 3,3 -(phenylmeth-
ylene)bis(1H-indole) was synthesized by reaction of indole and benzaldehyde under
different conditions. No reaction occurred when 2 mmol indole and 1 mmol
benzaldehyde were ground without catalyst. The yield of product with 0.1 g bare
MK10 catalyst after simple grinding for 10 min was only 55 %. When the reactants
were ground with 0.1 g 3 wt% BiCl –MK10 catalyst for 3 min, the product yield
3
was 99 %. Reaction of 2 mmol indole and 1 mmol benzaldehyde with 0.05 g of
BiCl –MK10 catalyst under simple grinding resulted in 60 % product yield in
3
3 min. Increasing the amount of catalyst beyond 0.1 g/mmol did not result in any
significant improvement in reaction yield and time. These results clearly indicate
that simple grinding is efficient and 0.1 g BiCl –MK10 is the optimum amount of
3
catalyst for the reaction to proceed with high yield (Fig. 2).
To assess the scope of the method, this reaction was performed with a variety of
substituted aromatic aldehydes 1a–1r, indole 2, and BiCl –MK10 catalyst under the
3
optimized conditions by simple grinding, and the corresponding bis(indolyl)meth-
anes 3a–3r were obtained (Table 3). There was no significant change in yield with
the electronic nature of the substituents (yields were more than 90 % with p-NO2
123

K. Ravi et al.
1
00
8
6
4
2
0
0
0
0
0
0.05
0.10
0.15
0.20
0.25
Catalyst amount (g)
3
Fig. 2 Effect of catalyst loading. Catalyst = BiCl –MK10, benzaldehyde = 1 mmol, indole = 2 mmol,
time = 3 min (grinding under solvent-free conditions at room temperature)
and p-OCH benzaldehyde; Table 3, entries i and k) but the size of the substituent
3
had some effect on reaction time and yield. Thus, chloro benzaldehydes gave higher
yields than bromo and iodo benzaldehydes (Table 3, entries b–h). For 4-fluoro-3-
phenoxybenzaldehyde (Table 3, entry m) the yield was 82 % after grinding for
4
0 min. The longer reaction time is due to the steric effect of the phenoxy group.
Naphthaldehydes react more slowly than benzaldehydes; this could be because of a
steric effect. Furan-2-aldehydes and thiophene-2-aldehyde gave good yields.
The efficiency of BiCl –MK10 was compared with that of other catalysts
3
0
reported [43–47] for synthesis of 3,3 -(phenylmethylene)bis(1H-indole) (Table 4).
BiCl –MK10 was found to be more efficient than other catalysts. The greater
3
activity can be attributed to the high surface acidity of the catalyst. As is apparent
from Table 2, the surface acidity of BiCl –MK10 is approximately 18 times that of
3
bare MK10 and it catalyzes the reaction more efficiently. Based on the Lewis acidity
of the catalyst, an acid catalyzed mechanism was proposed (Scheme 2). This
mechanism involves:
1
attack at the carbonyl group of the aldehyde by BiCl –MK10 producing
3
intermediate a;
nucleophilic addition of one molecule of indole to the intermediate a, giving
2
intermediate b which, after aromatization, forms the azafulvene intermediate c;
and
1
23

3
BiCl loaded montmorillonite K10
Table 3 Percentage yields of bis(indolyl)methanes from different aldehydes
a
Yield % Observed mass M
?
Entry Aldehyde (1)
Product (3)
Grinding
time in min
M ? 1/M - 1
a
O
3
99
92
322
N
H
N
H
b
H
O
Cl
4
356
Cl
N
N
H
H
c
O
4
4
92
84
356
400
Cl
Cl
N
N
H
H
d
H
O
Br
N
N
H
H
Br
e
f
Br
4
4
80
80
402
400
O
N
H
N
H
Br
H
O
Br
Br
N
N
H
H
g
O
I
4
60
448
N
H
N
H
I
h
I
4
5
60
90
448
367
O
I
N
H
N
H
i
H
O
NO
2
N
H
N
NO
2
H
123

K. Ravi et al.
Table 3 continued
a
Yield % Observed mass M
?
Entry Aldehyde (1)
Product (3)
Grinding
time in min
M ? 1/M - 1
j
4
99
336
H
O
Me
Me
N
H
N
H
k
H
O
MeO
4
99
352
N
H
N
H
OMe
l
OMe
4
99
82
352
432
H
O
N
H
N
H
OMe
m
H
O
F
40
O
O
F
N
H
N
H
n
o
O
OH
5
8
90
83
338
372
N
H
N
H
OH
O
N
H
N
H
p
q
8
5
60
99
372
312
O
N
H
N
H
O
O
O
N
H
N
H
1
23

3
BiCl loaded montmorillonite K10
Table 3 continued
a
Yield % Observed mass M
?
Entry Aldehyde (1)
Product (3)
Grinding
time in min
M ? 1/M - 1
r
5
99
328
S
S
O
N
H
N
H
3
Reaction conditions: benzaldehyde 1 mmol, indole 2 mmol, catalyst BiCl –MK10, 0.10 g simple phys-
ical grinding
a
Yields based on aldehyde
Table 4 Comparison of the efficiency of BiCl
bis(indolyl)methanes from benzaldehyde and indole
3
–MK10 and other catalysts used for synthesis of
a
Yield (%)
Entry
Reaction conditions
Reaction time (min)
Catalyst qty
b
1
2
3
4
5
6
7
8
a
Bare MK10, RT physical grinding
10
3
0.1 g
55
b
99
BiCl
AlPW12
La(PFO)
LiClO /CH
3
-MK10, RT, physical grinding
40/CH CN
/EtOH
CN
0.1 g
O
3
15
30
300
60
35
60
2–7 mol %
5 mol %
10 mol %
–
92 [43]
90 [44]
90 [20]
90 [45]
89 [46]
85 [47]
3
4
3
c
HFIP , RT
ZrOCl /CH
HY-Zeolite/CH
2
3
CN
10 mol %
0.20 g
2
Cl
2
Yield based on benzaldehyde
This work
b
c
HFIP: 1,1,1,3,3,3-Hexafluoro-2-propanol
3
addition of another molecule of indole to intermediate c then aromatization to
give the corresponding condensed product d.
The solid acid catalyst BiCl –MK10 promotes deprotonation and dehydration.
3
The reusability of the catalyst was tested for five runs. The catalyst was separated
from the reaction mixture by filtration and dried at 100 °C for 1 h. This recovered
catalyst was used again. The results revealed that the catalyst can be used
continuously for the reaction. The yield of the products is [95 % even in the 5th
run. Percentage yields of the product for five runs are given in Table 5.
We tested the stability of the catalyst by checking for desorption of BiCl by use
3
of atomic absorption spectroscopy (AAS). The concentration of bismuth in the fresh
catalyst was 1.95 % (equivalent to 2.95 % as BiCl ); that in the catalyst recovered
3
after the 5th run was 1.82 % (equivalent to 2.75 % as BiCl ). This shows that no
3
significant desorption of BiCl occurred.
3
123

K. Ravi et al.
Scheme 2 Plausible reaction mechanism for synthesis of bis(indolyl)methane catalyzed by BiCl
3
–MK10
3
Table 5 BiCl –MK10 Catalyst Reusability
Run
1
2
3
4
5
Yield (%)
99.0
98.0
96.0
94.0
94.0
3
Reaction conditions: indole = 2 mmol, benzaldehyde = 1 mmol, BiCl –MK10 = 0.1 g, room temper-
ature, grinding. Yield based on benzaldehyde
Conclusion
A new solid acid catalyst, BiCl -loaded MK10, was prepared by simple solid
3
dispersion method and characterized by SEM, EDS, BET surface area, and surface
3
acidity measurements. SEM EDS confirmed the presence of Bi in the catalyst. The
?
surface acidity of the BiCl –MK10 catalyst was approximately 18 times more than
3
that of bare MK10, and BiCl –MK10 catalyzes the reaction more efficiently. BiCl –
3
3
MK10 is introduced as an excellent catalyst for synthesis of bis(indolyl)methane
derivatives from a variety of substituted aldehydes by simple grinding at room
temperature. BiCl –MK10 promotes both dehydration and deprotonation in the
3
reaction. This novel and practical method has the advantages of mild conditions,
excellent yield of products, and short reaction time. Recovery and reusability of the
catalyst make this a green process. This method can easily scaled up for large-scale
1
23

3
BiCl loaded montmorillonite K10
production of bis(indolyl)methanes using commercially available grinding equip-
ment. We believe this novel catalyst will have wide applications in organic synthesis.
Acknowledgments One of the authors, M.S., is grateful to CSIR, New Delhi, India, for financial
support through research Grant number 21(0799)/10/EMR-II.
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