Simple and efficient method for synthesis of bis(indolyl) methanes with Cu(BF4)2sio2 under mild conditions

Article abstract of DOI:10.1080/00397910902916064

A simple and efficient synthesis of bis(indolyl) methanes by the reaction of indoles with various carbonyl compounds in the presence of a catalytic amount of cupric fluoroborate [Cu(BF4)2SiO2] afforded the corresponding bis(indolyl) methanes in excellent yields. The remarkable selectivity under mild and neutral conditions and commercially available, inexpensive catalyst are attractive features of this method.

Full text of DOI:10.1080/00397910902916064

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Simple and Efficient Method for
Synthesis of Bis(indolyl) Methanes with
Cu(BF4)2·SiO2 Under Mild Conditions
G. A. Meshram a & Vishvanath D. Patil a
a Organic Chemistry Research Laboratory, Department of Chemistry,
Vidyanagari, Santacruz (E), Mumbai, Maharashtra, India
Version of record first published: 09 Dec 2009.
To cite this article: G. A. Meshram & Vishvanath D. Patil (2009): Simple and Efficient Method
for Synthesis of Bis(indolyl) Methanes with Cu(BF4)2·SiO2 Under Mild Conditions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
40:1, 29-38
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Synthetic Communications1, 40: 29–38, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902916064
SIMPLE AND EFFICIENT METHOD FOR SYNTHESIS OF
BIS(INDOLYL) METHANES WITH Cu(BF4)2 ꢀ SiO2 UNDER
MILD CONDITIONS
G. A. Meshram and Vishvanath D. Patil
Organic Chemistry Research Laboratory, Department of Chemistry,
Vidyanagari, Santacruz (E), Mumbai, Maharashtra, India
A simple and efficient synthesis of bis(indolyl) methanes by the reaction of indoles with
various carbonyl compounds in the presence of a catalytic amount of cupric fluoroborate
[Cu(BF4)2 ꢀ SiO2] afforded the corresponding bis(indolyl) methanes in excellent yields.
The remarkable selectivity under mild and neutral conditions and commercially available,
inexpensive catalyst are attractive features of this method.
Keywords: Bis(indolyl) methanes; cupric fluoroborate; dichloromethane; indole
INTRODUCTION
The development of simple, efficient, environmentally benign, and
economically viable chemical process or methodologies for widely used organic com-
pounds are in great demand. Bis(indolyl) alkanes are a biologically useful class of
organic compounds, and a large number of these indolyl methanes have been iso-
lated from terrestrial and marine natural sources such as tunicates and sponges.[1]
For example, vibrindole A (3,30-diindolyl ethane) exhibits antibacterial activity,
and 3,30-diindolyl methane has potent carcinogenic properties.[2] Because of their
wide occurrence as natural products and various biological activities, synthesis of
these bis(indolyl) methanes have attracted attention. Recently, oxidized bis(indolyl)
methanes containing a conjugated bis(indolyl) skeleton have acted as colorimetric
sensors and chromogenic sensors.[3] In addition,1,1-bis(30-indolyl)-1-(p-substituted
phenyl) methanes containing p-trifluoromethyl (DIM-C-p-CF3Ph), p-t-butyl
(DIM-C-p-tBuPh), and p-phenyl (DIMC-p-C6H5Ph) substituents have been ident-
ified as a new class of peroxisome proliferator-activated receptors (PPAR) agonists
that exhibit antitumorigenic activity.[4] Various methods have been developed for
their synthesis using Lewis acid catalysts such as LiClO4,[5] In(OTf)3,[6] Dy(OTf)3,[7]
Sc(OTf)3,[8,9] CAN,[10] ZrOCl2,[11] InCl3,[12] and AlPW12O40,[13] ionic liquids,[14]
trichloro-1,3,5-triazine,[15] potassium hydrogen sulphate,[16] La(NO3)3 ꢀ 6H2O,[17]
Received January 4, 2009.
Address correspondence to G. A. Meshram, Organic Chemistry Research Laboratory, Department
of Chemistry, Vidyanagari, Santacruz (E), Mumbai 400 098, Maharashtra, India. E-mail: vdp148@
yahoo.com
29

30
G. A. MESHRAM AND V. D. PATIL
Fe(DS)3.[18] However, many of these reported methods suffer from one or more
disadvantages such as harsh experimental procedure and reagents that are expensive,
moisture sensitive, or highly toxic in nature. A mild and efficient catalyst for the syn-
thesis of bis(indolyl) methanes is highly desirable. Consequently, there is an opport-
unity for further development toward mild conditions increased variation of the
components, and better yields. In this communication, we report a simple and
efficient method for the synthesis of bis(indolyl) methanes using cupric fluoroborate
[Cu(BF4)2 ꢀ SiO2] as a solid-supported catalyst.
RESULTS AND DISCUSSION
The catalytic activity of Cu(BF4)2 ꢀ SiO2 for the synthesis of bis(indolyl)
methanes using benzaldehyde (2 m mol) with indole (4 mmol) at 80ꢁC was studied,
and it was found that application of less than 0.1 mmol (30 mg) of Cu(BF4)2 ꢀ SiO2
in dichloromethane (DCM) (5 ml) gave a moderate yield of the corresponding bis
(indolyl) methanes (Table 1, entries 8 and 9), whereas use of more than 0.1 mmol
(30 mg) gave excellent yields (Table 1, entries 10–13). The reactions in tetrahydro-
turan (THF), Ch3CN, CHCI3, Et2O, EtOAc, and dimrthyformanide (DMF)
(Table 2, entries 1–7) were less effective. Thus, we carried out the reaction in the
presence of CH2Cl2 as solvent to get the excellent yield (96%, entry 10).
To find the most effective catalyst for the synthesis of bis(indolyl) methanes, we
employed different catalysts with benzaldehyde and indole (1:2 equimolar) at 80ꢁC
(Table 2). According to the results obtained, Cu(BF4)2 ꢀ SiO2 was the most efficient
catalyst (Table 2, entries 7 and 8). However, other catalysts such as FeCl3 ꢀ 6H2O,
CuCl2, CuO, Cu2O3, SiO2, and CaO exhibit and less significant catalytic property
in the synthesis of bis(indolyl) methanes.
A wide variety of compounds were applied to the optimal reaction conditions
to prepare a wide range of bis(indolyl) methanes (Scheme 1). The results are sum-
marized in Table 3. The reaction proceeded efficiently and smoothly at 80ꢁC in
Table 1. Reactions of benzaldehyde with indole under various conditions
Cu(BF4)2 ꢀ SiO2
Time
(h)
Temp.
(ꢁC)
Yielda
(%)
Entry
Solvent
(mmol)
1
2
Neat
THF
—
0.1
6
80
80
10
50
75
65
70
85
80
75
85
96
96
96
96
2
3
CH3CN
CHCl3
Et2O
0.1
0.1
2
80
80
4
2
5
0.1
0.1
2.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
80
80
6
EtOAc
DMF
7
0.1
80
80
8
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
0.01
0.05
0.10
0.10
0.15
0.20
9
80
80
10
11
12
13
Reflux
80
80
aIsolated yield of the corresponding bis(indolyl) methanes.

SYNTHESIS OF BIS(INDOLYL) METHANES
31
Table 2. Synthesis of bis(indolyl) methanes by reaction of benzaldehyde and indole in the presence of
different metal salt catalyst with CH2Cl2 solvent at 80ꢁC
Amount
[mmol (mg)]
Time
(h)
Yielda
(%)
Entry
Catalyst
1
2
3
4
5
6
7
8
FeCl3 ꢀ 6H2O
CuCl2
CuO
0.2 (27)
0.2 (13)
0.2 (80)
0.2 (11)
0.2 (6)
3
68
78
20
15
10
10
96
96
3
3
Cu2O3
SiO2
3
3
CaO
Cu(BF4) ꢀ SiO2
Cu(BF4) ꢀ SiO2
0.2 (6)
0.2 (60)
0.1 (30)
3
0.5
0.5
aIsolated yield.
the presence of DCM, and the products were obtained in excellent yields. Further-
more, the reaction conditions we are very mild, and no by-products were observed.
Cu(BF4)2 ꢀ SiO2 is highly oxophilic, forms a labile bond with carbonyl oxygen, and
initiates the formation of a CꢂC bond with carbonyl compounds. Aromatic
aldehydes reacted more rapidly than aliphatic aldehyde. The effect of electron
deficiency and nature of the substituents on the aromatic ring showed some effect
on this conversion. The nitro-substituted aryl aldehydes required slightly longer
reaction times to produce comparable yields than those of their simple and
electron-rich counterparts (Table 3, entries 1–14). Electron-rich aldehydes such as
anisaldehyde, piperonal, and veratraldehyde reacted rapidly with indole to give cor-
responding products in excellent yields within 40 min (Table 3, entries 3, 4, and 15).
Aliphatic aldehydes such as n-hexanal, n-heptanal and cyclohexane carboxaldehyde
showed somewhat less reactivity (Table 3, entries 16–18). Substituted indoles reacted
rapidly with aryl aldehydes with considerable yields (Table 3, entries 19, 20, and 21).
Further, the reaction of ketones proceeded with more reaction time under the same
reaction conditions, and the corresponding products were obtained in moderate
yields (Table 3, entries 22–25).
In conclusion, this article describes a method in which silica-supported cupric
fluoroborate [Cu(BF4)2 ꢀ SiO2] is highly efficient catalyst for the synthesis of bis
(indolyl) methanes using various substrates of aldehydes, ketones, and indoles: The
Scheme 1.

32
G. A. MESHRAM AND V. D. PATIL
Table 3. Synthesis of bis(indolyl) methanes using Cu(BF4)2 ꢀ SiO2
Time Yieldb
Entry
Aldehyde
Indoles
Product (a)
(min)
(%)
1
30
96
2
30
96
3
30
95
4
5
6
40
40
40
93
90
92
(Continued )

SYNTHESIS OF BIS(INDOLYL) METHANES
33
Table 3. Continued
Time Yieldb
Entry
Aldehyde
Indoles
Product (a)
(min)
(%)
7
40
93
8
30
95
9
40
94
10
60
87
11
40
94
12
60
85
(Continued )

34
G. A. MESHRAM AND V. D. PATIL
Table 3. Continued
Time Yieldb
(min) (%)
Entry
Aldehyde
Indoles
Product (a)
13
14
15
16
75
92
75
40
90
90
94
90
17
18
90
90
92
90
(Continued )

SYNTHESIS OF BIS(INDOLYL) METHANES
35
Table 3. Continued
Time Yieldb
Entry
Aldehyde
Indoles
Product (a)
(min)
(%)
19
50
94
20
50
93
21
40
95
22
23
120
120
87
89
24
25
120
120
85
86
aAll products were identified by their IR and 1H NMR spectra.
bIsolated yields after column chromatography.

36
G. A. MESHRAM AND V. D. PATIL
advantages include the low cost, ease of catalyst handling, requirement of very small
amount of catalyst such as 0.1 mmol (30 mg), mild reaction conditions and excellent
yields. The remarkable selectivity under mild and neutral conditions and the commer-
cially available, inexpensive catalyst are attractive features of this method.
General Experimental Procedure for Bis(Indolyl) Methanes
A mixture of benzaldehyde (2 mmol), indole (4 mmol), and [Cu(BF4)2 ꢀSiO2]
(0.1 mmol, 30 mg) was stirred magnetically at 80ꢁC with DCM, and the progress
of the reaction was monitored by thin-layer chromatography (TLC). After com-
pletion of the reaction (TLC), the catalyst was separated by filtration. The filtrate
was dried over anhydrous Na2SO4 and then evaporated under a vacuum to afford
the crude product, which was further purified by column chromatography. In all
the cases, the product obtained after the usual work up gave satisfactory spectral
data.
Spectral Data for Selected Compounds
3,30-Bis(indolyl) phenyl methane (1b). Palered solid, yield 96%, mp
1
123–125.; IR (KBr): 735, 1094, 1342, 1458, 1483, 2927, 2956, 3045, 3387 cmꢂ1; H
NMR (300 MHz, CDCl3): d ¼ 7.78 (br s, 2H), 7.36 (d, J ¼ 8.1 Hz, 2H), 7.10–7.36
(m, 9H), 6.95 (t, J ¼ 7.5 Hz, 2H), 6.46 (d, J ¼ 1.8 Hz, 2H), 5.82 (s, 1H): 13C NMR
(CDCl3): 143.0, 136.2, 128.5, 128.6, 127.3, 126.5, 123.4, 121.3, 119.4, 119.0, 118.9,
111.3, 40.6.
3,30-Bis(indolyl)-4-methoxyl phenyl methane (3b). Brown needles, yield
95%, mp 193–195, IR (KBr): 740, 780, 1015, 1090, 1132,1341, 1450, 1607, 2922,
1
3052, 3386, 3422 cmꢂ1: H NMR (300 MHz, CDCl3): d ¼ 7.90 (br s, 2H), 6.9–7.9
(m, 12H), 6.4 (s, 2H), 6.1 (s, 1H), 3.95(s, 3H): 13C NMR (CDCl3): 39.4, 55.6,
102.3, 111.3, 119.7, 122, 123.8, 128.2, 129.9, 135.8,136.8, 157.8: EIMS m=z 352.
3,30- Bis(indolyl)-4-dimethylamino phenyl methane (7b). Pink solid,
yield 93%, mp 168–170; IR (KBr): 740, 760, 1015, 1095, 1222, 1350, 1456, 1512,
1
2920, 3062, 3342, 3418, 3520 cmꢂ1; H NMR (300 MHz, DMSO) d: 11.30 (br s,
2H), 8.30 (d, J ¼ 2.0 Hz, 2H), 8.02 (dd, J ¼ 8.8, 2.0 Hz, 2H), 7.50 (dd, J ¼ 9.2,
1.6 Hz, 2H), 7.43 (d, J ¼ 7.6 Hz, 2H), 7.36 (t, J ¼ 7.6 Hz, 2H), 7.25 (d, J ¼ 7.2 Hz,
1H), 7.20 (s, 2H), 6.23(s, 1H); 13C NMR (DMSO): 148.6, 145.2, 144.2, 133.1,
133.5, 132.6, 131.7, 130.2, 125.8, 121.4, 121.5, 117.3, 43.7, MS (ESI) m=z: 411.34.
3,30-Bis(indolyl)-4-methyl phenyl methane (8b). Pale red solid, yield 95%,
mp 95–98; IR (KBr): 742, 788, 999, 1090, 1225, 1340, 1457, 1508, 2925, 3056, 3386,
3420 cmꢂ1; 1H NMR (300 MHz, CDCl3) d: 7.84 (br s, 2H), 7.42 (d, J ¼ 8.0 Hz, 2H),
7.30 (d, J ¼ 8.3 Hz, 2H), 7.26 (d, J ¼ 8.0 Hz, 2H), 7.15 (dd, J ¼ 7.6 Hz, 2H), 7.03 (d,
J ¼ 8.0 Hz, 2H), 7.00 (t, J ¼ 7.2 Hz, 2H), 6.64 (s, 2H), 5.83 (s, 1H), 2.33 (s, 3H); 13C
NMR (CDCl3) d: 141.2, 136.5, 136.5, 129.2, 128.5, 127.3, 123.4, 122.1, 120.2, 119.5,
119.2, 111.2, 40.2, 21.3.
3,30-Bis(indolyl)-4-chloro phenyl methane (11b). Pale solid, yield 94%, mp
74–76: IR (KBr): 743, 787, 1010, 1100, 1130, 1325, 1456, 1603, 2920, 3050, 3382,

SYNTHESIS OF BIS(INDOLYL) METHANES
37
1
3420 cmꢂ1; H NMR (300 MHz, CDCl3): d ¼ 7.7 (br s, 2H), 7.3 (d, 2H, J ¼ 7.8),
7.01–7.15 (m, 8H), 6.86–6.97 (m, 2H), 6.35 (s, 2H), 5.73 (s, 1H): 13C NMR (CDCl3);
44.4, 111.1, 112.2, 119.1, 120.4, 121.6, 122.8, 128.8, 130.7, 131.5, 135.9, 136.6.
3,30-Bis(indolyl)-4-nitro phenyl methane (13b). Yellow solid, yield 92%,
mp 219–222; IR (KBr): 745, 789, 1010, 1093, 1221, 1339, 1455, 1507, 2922, 3052,
1
3386, 3422 cmꢂ1; H NMR (300 MHz, CDCl3): d ¼ 8.1 (bs, 2H), 7.3–7.5 (m, 8H),
7.2 (t, 2H, J ¼ 7.0 Hz), 7.1 (t, 2H, J ¼ 7.0 Hz), 6.7 (s, 2H), 6.0 (s, 1H), 13C NMR
(CDCl3): 153.6, 147.5, 134.9, 131.2, 127.6, 125.1, 124..5, 122.4, 120.2, 119.8, 118.5,
112.4, 41.6.
3,30-Bis(indolyl)-cyclohexyl methane (16b). Pale solid, yield 90%, mp
219–221.; IR (KBr): 746, 789, 1011, 1093, 1227, 1342, 1459, 1510, 2927, 3058,
1
3385, 3422 cmꢂ1; H NMR (300 MHz, CDCl3) d: 7.82 (br s, 2H), 7.4 (d, J ¼ 8.0 Hz,
Hz, 2H), 7.32 (d, J ¼ 8.3 Hz, 2H), 7.26 (d, J ¼ 8.0 Hz, 2H), 7.18 (dd, J ¼ 7.6 Hz, 2H),
7.10 (d, J ¼ 8.0 Hz, 2H), 7.0 (t, J ¼ 7.2 Hz, 2H), 6.65 (s, 2H), 5.85 (s, 1H), 2.30 (s, 3H);
13C NMR (CDCl3) d: 141.5, 136.6, 136.8, 129.5, 128.4,127.1, 123.5, 122.2, 120.3,
119.5, 119.4, 112.8, 40.3, 21.2.
3,30-[5-Nitro-bis(indolyl)] phenyl methane (21b). Red solid, yield 95%, mp
294–298; IR (KBr): 741, 782, 1010, 1090, 1220, 1458, 1512, 2921, 3061, 3418 cmꢂ1
;
1H NMR (300 MHz, DMSO) d: 11.70 (br s, 2H), 8.35 (d, J ¼ 2.0 Hz, 2H), 8.05
(dd, J ¼ 8.8, 2.0 Hz, 2H), 7.58 (dd, J ¼ 9.2, 1.6 Hz, 2H), 7.45 (d, J ¼ 7.6 Hz, 2H),
7.37 (t, J ¼ 7.6 Hz, 2H), 7.26 (d, J ¼ 7.2 Hz, 1H), 7.18 (s, 2H), 6.23 (s, 1H); 13C
NMR (DMSO) d: 148.7, 145.0, 144.5, 133.5, 133.1, 132.2, 131.5, 130.7, 125.4,
121.3, 121.0, 117.1, 43.5. MS (ESI) m=z: 411.35.
ACKNOWLEDGMENT
We thank the faculty of chemistry, University of Mumbai, for supporting
this work.
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