First general solvent-free synthesis of symmetrical triindolylmethanes using acid-washed montmorillonite clay

Article abstract of DOI:10.1081/SCC-120034161

Dry reaction of indoles and alkylindoles (1a-h) with triethyl orthoformate (2.5 equiv.) on acid-washed montmorillonite K10 clay at room temperature afforded in < 5 min-4 hr the respective symmetrical TIMs (2a-h) in good to high yields along with the formylindole 3 and the formyl-TIM 4 from 1g and 1h, respectively.

Full text of DOI:10.1081/SCC-120034161

SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 10, pp. 1801–1810, 2004
First General Solvent-Free Synthesis of
Symmetrical Triindolylmethanes Using
Acid-Washed Montmorillonite Clay
1
2
Manas Chakrabarty,1, Sandipan Sarkar, Anthony Linden,
*
and Bridget K. Stein3
1Department of Chemistry, Bose Institute, Kolkata, India
2Institute of Organic Chemistry, University of Zu¨rich, Zu¨rich, Switzerland
3Department of Chemistry, EPSRC National Mass Spectrometry Service
Centre, University of Wales Swansea, Swansea, UK
ABSTRACT
Dry reaction of indoles and alkylindoles (1a–h) with triethyl orthoformate
(2.5equiv.) on acid-washed montmorillonite K10 clay at room temperature
afforded in ,5 min–4 hr the respective symmetrical TIMs (2a–h) in good
to high yields along with the formylindole 3 and the formyl-TIM 4 from 1g
and 1h, respectively.
Key Words: Triindolylmethanes; Solvent-free synthesis; Acid-washed
montmorillonite clay; Orthoester.
*
Correspondence: Manas Chakrabarty, Department of Chemistry, Bose Institute, 93/1,
A.P.C. Road, Kolkata 700009, India; Fax: þ91-33-2350-6790; E-mail: chakmanas@
yahoo.co.in or manas@bosemain.boseinst.ac.in.
1801
DOI: 10.1081/SCC-120034161
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Chakrabarty et al.
INTRODUCTION
Montmorillonite clay with its large specific surface area and high Bronsted
¨
and Lewis acidity has emerged as an environmentally benign solid acid catalyst,
replacing many hazardous acidic catalysts in organic syntheses and reactions,
both in laboratory[1,2] and in industry.[3] We have utilised earlier the commer-
cially available montmorillonite K10 clay in the solvent-free synthesis of diin-
dolylalkanes[4] and in the study of the reaction of 3-formylindole with indoles,
which led to the formation of a mixture of both symmetrical and unsymmetrical
triindolylmethanes.[5] In recent years, the triarylalkanes, specially the symmetri-
cal ones, have become increasingly important as dyes, for physicochemical
studies, for the preparation of cage compounds, and for photochemical trans-
formations[6–11] and also because of the recent identification of a bacterial
metabolite as a symmetrical triindolylalkane.[12] Of the various symmetrical
triarylalkanes synthesised, the symmetrical triindolylmethanes (sym. TIMs)
constitute a major class. Although, there are isolated reports of the synthesis
of individual sym. TIMs by the acid-catalysed condensation of indoles with
various acylium ion equivalents in solution,[13–16] there exist only three general
syntheses, which involve the acid-catalysed reaction of indoles with either
triethyl orthoformate or 2-alkoxydioxolanes in solution in presence of protic
acids.[17–19] Since all the aforesaid syntheses were carried out in solution and
in view of the growing need for green chemistry for a cleaner environment
and the ever increasing contribution of acidic solid surfaces, specially montmor-
illonite clay, towards this end, we have now developed a general, solvent-free
synthesis of sym. TIMs using modified montmorillonite clay. Our findings
have been briefly presented below.
RESULTS AND DISCUSSION
Initially, we attempted the dry reaction of indole with triethyl orthofor-
¨
mate (1–3 equiv.) on montmorillonite K10 clay (Bronsted acidity: Hammet
acidity function, H0 ¼ 25.5 to 25.9) at room temperature and also under
microwave irradiation (2450 MHz using domestic oven), but the reaction
was extremely sluggish. In order to overcome this difficulty, we utilised the
fact that washing of this clay with mineral acids increases its acidity sharply
(H0 ¼ 26 to 28).[1] Indeed, acid-treated clay has been previously used for
a few organic reactions.[20] Therefore, we washed montmorillonite K10 clay
with concentrated hydrochloric acid, which, after usual work-up, furnished
what we designate “acid-clay”. It was found to be devoid of any free acid
and free chloride ions. We next used this acid-clay in the targeted reactions
and achieved success, as described below.

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Synthesis of Triindolylmethanes
1803
Thus, when a solution of indole (1a) in 2.5 equiv. (since the reaction was
very slow up to 2 equiv.) of triethyl orthoformate was adsorbed on acid-clay at
room temperature, there was an almost instantaneous sharp change of colour
of the mixture, and indole was fully consumed (TLC). Leaching of the clay
with dichloromethane furnished tris(indol-3-yl)methane (2a) in high yield.
A similar, separate treatment of each of N-methylindole (1b), N-ethylindole
(1c), skatole (1d), 2-methylindole (1e), and 1,2-dimethylindole (1f) produced
the respective TIMs (2b–f) in comparable yields (Sch. 1, Table 1). As
expected, only skatole furnished the corresponding tris(indol-2-yl)methane
(2d), and the rest (2b, c, e, f) were tris(indol-3-yl)methanes. Understandably,
the reaction with 1d–f required more time for completion, of which the reac-
tion with skatole (1d) was most sluggish.
From 1,3-dimethylindole (1g), however, the expected sym. TIM (2g) was
obtained only as a minor product with the corresponding 2-formylindole (3) as
the major one. Conceivably, the steroelectronic factors arising out of the two
methyl groups are responsible for both the observed low yield of 2g and the
simultaneous occurrence of an alternate reaction course, viz., the substitution
of one ethoxy group of the orthoester, leading to the unisolated acetal 30, and
subsequent in situ hydrolysis to form 3 (Sch. 2).
With 5-bromoindole (1h), two products were formed expeditiously—the
sym. TIM (2h; 60%) and a minor product (4; 10%) (Sch. 3).
1
The latter, C26H16N3OBr3 (HR MS), displayed characteristic H NMR
signals for three 5-substituted indol-3-yl moieties, but with only two indolic
NHs (d 8.066, s) and a N-formyl group (d 8.909, s) in addition to a triaryl-
methine proton (d 5.904, s), which suggested it to be N-formyl-tris(5-bromoin-
dol-3-yl)methane (4). This structure was fully supported by the two sets of
peaks at m/z 401, 403, 405 and m/z 400, 402, 404, observed in its mass spec-
trum and resulting from the loss of 222 m.u. (5-bromo-N-formylindol-3-yl
moiety) and 223 m.u., respectively, from the molecular ion peaks. The poor
solubility of 4 in chloroform and its instability in dimethyl sulphoxide solution
precluded its correct 13C NMR analysis. Finally, a single crystal x-ray diffrac-
tion analysis revealed that the correct structure of the crystalline material is
Scheme 1.

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Chakrabarty et al.
Table 1. Sym. TIMs (2a–f) from indoles (1a–f) using acid-clay.
Indoles
R0
Sym. TIMs
R0
Yield
(%)
R
R00
R
Time
1a
1b
1c
1d
1e
1f
H
H
H
2a
2b
2c
2d
2e
2f
H
H
,5 min
,5 min
,5 min
4 hr
85
87
92
63
74
82
Me
Et
H
H
Me
Et
H
H
H
H
H
H
Me
H
—
H
—
Me
Me
H
Me
Me
Me
2 hr
2 hr
H
Me
N-formyl-tris(5-bromoindol-3-yl)methane-dichloromethane (1/1) (ORTEP
diagram: Fig. 1).
In order to test the reusability of the acid-clay, the clay, recovered from
the reaction of indole (1a) with triethyl orthoformate, was washed thoroughly,
dried at 110–1208C for 12 hr and reused in the same reaction on the same
scale. The product 2a was obtained in a somewhat lower yield (82%) in the
same time span (,5 min) as in the experiment using fresh clay. Each of 1b
and 1c reacted similarly with triethyl orthoformate using the regenerated
clay to furnish 2b and 2c, respectively, in comparable yields (85% and
90%) both within 5 min. The reusability of acid-clay in these reactions was
thus demonstrated at least for the second cycle.
Compared to the three previous general syntheses in solution phase,[17–19]
the present solvent-free synthesis of sym. TIMs encompasses a larger number
of substrates and requires appreciably lesser time to furnish the TIMs in
Scheme 2.

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Synthesis of Triindolylmethanes
1805
Scheme 3.
comparable yields. Pertinently, two points are worth noting. Firstly, formylation
took place in only two cases, one involving C-formylation and the other
N-formylation. Secondly, in contrast to our earlier method involving the dry
reaction of 3-formylindole with indoles on montmorillonite K10 clay furnishing
a mixture of both sym. and unsym. TIMs,[5] the present method is the first,
general solvent-free synthesis of only sym. TIMs starting from single
indolic components, that too using acid-washed clay. It has the added advantage
that it employs an environment-friendly solid acidic catalyst, involves a
simple work-up and the products are obtained in good to high yields. The
fate of a similar treatment of indoles with other orthoesters remains to be seen.
EXPERIMENTAL
Melting points (8C) were recorded on a Toshniwal apparatus and are uncor-
rected. The IR spectra were recorded on a Nicolet FT-IR I-410 spectrophotometer,
the 1H (500 MHz) and 13C (125 MHz) NMR spectra on a Bruker DRX 500 NMR
spectrometer, and the mass spectra on JEOL JMS-700T MStation (LR EI, HR
FAB) or AEI MS 30 (LR EI) mass spectrometers. Microanalyses were carried
¨
out on a Dr. Hans Hosli Analyser (Type A1; No. 1058). Analytical and prepara-
tive TLCs were carried out on silica gel G plates (Merck, India).
Preparation of Acid-Clay
Montmorillonite K10 clay (25.0 g) was dipped in conc. HCl (20.0 mL) and
kept for 4 days. The supernatant acid was then decanted off, excess water added

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Chakrabarty et al.
Figure 1. ORTEP diagram of compound 4.
to the clay and, after a brief stirring, the clay was filtered under suction, washed
successively with water till free from acid (pH) and free chloride ions (silver
nitrate test) and subsequently washed with 10% aq. EtOH, dried under water-
suction and then in a vacuum dessicator overnight and finally activated at
1108C for 8 hr to furnish what we describe as “acid-clay.”
General Procedure for the Reaction with Orthoester
A solution of the indole (1 mM) in triethyl orthoformate (2.5 mM; 0.4 mL)
was adsorbed on acid-clay (2.0 g) and kept at room temperature, upon which
there was almost immediate generation of a red, orange, pink or violet colour.
On completion of the reaction (TLC), the clay was leached with CH2Cl2
(3 ꢀ 10 mL) and the products were purified by either crystallisation (2a–f)

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Synthesis of Triindolylmethanes
1807
or prep. TLC on silica gel (2g, 2h, 3, 4). All the products were identified by IR,
1
LR MS, H, 13C, and DEPT 135 NMR spectra and, for new compounds,
additionally by elemental analyses/HR FAB-MS data. The spectral and analy-
tical data of only those triindolylmethanes, whose spectral data have not been
reported earlier, in addition to those of the formylindole (3) and the N-forrnyl-
trisindole (4) have been presented below.
2a. Colourless crystals, m.p. 238–2408C (CH2Cl2–pet. ether) (Ref.[17]
m.p. 242–2448C).
2b. Orange needles, m.p. 256–2588C (dec.)(CH2Cl2–pet. ether)
1
([Ref.[17] 1508C (dec.)]; H NMR (CDCl3): d 7.493 (3H, d, J 8 Hz), 7.287
(3H, d, J 8 Hz), 7.192 (3H, t, J 7.5 Hz), 6.990 (3H, t, J 7.5 Hz), 6.635 (2H,
s), 6.156 (1H, s), 5.298 (lH, s), 3.663 (9H, s).
2c. Orange needles, m.p. 216–2188C (CH2Cl2–pet. ether) (Ref.[5] m.p.
216–2188).
2d. Green prisms, m.p. 319–3208C (CH2Cl2–pet. ether) (Ref.[21] m.p.
1
320–3218C); IR(nujol): 3403, 1460, 744 cm21; H NMR (CDCl3): d 7.704
(3H, br s, NH), 7.577 (3H, d, J 7 Hz), 7.251 (3H, dd, J 7.5, 2 Hz), 7.184 and
7.154 (3H each, dt, J 7.5, 1 Hz,), 6.214 (1H, s), 2.17 (9H, s); 13C NMR: d
135.7, 131.9, 129.9, 109.3 (all Ar-C), 122.4, 120.1, 119.0, 111.5 (all Ar-CH),
34.0 (Ar3CH), 8.7 (Ar-CH3); EI-MS: m/z 403 (Mþ, 34%), 272 (17), 271
(18), 257 (100), 130 (15).
2e. Pink needles, m.p. 3368C (dec.) (CH2Cl2–pet. ether) [Ref.[19] m.p.
.3308C (dec.)].
2f. Pink needles, m.p. 250–2528C (CH2Cl2–pet. ether) (Ref.[19] m.p.
255.28C).
3. Yellow liquid; IR (neat): 1659, 1613, 1572, 1474, 1381, 1208, 883,
744 cm21; 1H NMR (CDCl3): d 10.153 (1H, s, CHO), 7.683 (1H, d, J 8 Hz),
7.414 (1H, dt, J 7.5, 1 Hz), 7.32 (1H, d, J 8.5 Hz), 7.146 (1H, dt, J 7.5,
0.5 Hz), 4.035 and 2.628 (3H each, s); 13C NMR: d 181.9 (CHO), 140.1,
131.6, 127.2, 126.8 (all Ar-C), 127.7, 121.6, 120.4, 110.5 (all Ar-CH), 31.9
(N-CH3), 8.9 (Ar-CH3); EI-MS: m/z 173 (Mþ, 100%), 172 (48), 158 (28),
144 (70), 130 (18), 128 (15), 77 (30).
2g. Colourless needles, m.p. 230–2328C (CH2Cl2–pet. ether); IR
1
(nujol): 3403, 1460, 744 cm21; H NMR (CDCl3): d 7.525 (3H, d, J 8 Hz),
7.296 (3H, d, J 8 Hz), 7.238 (3H, t, J 7.5 Hz), 7.125 (3H, t, J 7.5 Hz), 6.10
(1H, s), 3.503 (9H, s), 2.175 (9H, s); EI-MS: m/z 445 (Mþ, 100%), 430
(30), 300 (33), 285 (76), 270 (72), 144 (19); HR FAB-MS: obs.: m/z
445.2515; calc. for C31H31N3: 445.2515.
2h. Orange prisms, m.p. 260–2648C (CH2Cl2–pet. ether); IR (nujol):
1
3443, 1096, 777 cm21; H NMR (d6-DMSO): d 10.986 (1H, s, NH), 7.501
(3H, d, J 1.5 Hz), 7.318 (3H, d, J 8.5 Hz), 7.124 (3H, dd, J 8.5, 1.5 Hz),
6.980 (3H, d, J 2 Hz), 6.051 (1H, s); 13C NMR: d 136.1, 129.2, 118.3, 111.6

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1808
Chakrabarty et al.
(all Ar-C), 125.8, 124.1 122.2, 114.4 (all Ar-CH), 31.0 (Ar3CH); EI-MS: m/z
601 (36%), 599 (98), 597 (100), 595 (Mþ, 36), 520 (20), 518 (16), 516 (6), 404
(40), 402 (55), 400 (23), 323 (28), 321 (25); HR FAB-MS: obs.: m/z 594.8865;
calc. for C25H16N739Br3: 594.8895.
4. Pale brown prisms, m.p. 186–1888C (CH2Cl2–pet. ether); IR (KBr):
1
3431, 3332, 1693, 1600, 1456, 802, 734 cm21; H NMR (CDCl3): d 8.909
(1H, s, N-CHO), 8.305 (1H, d, J 8 Hz), 8.066 (2H, s, D2O-exchangeable,
2 ꢀ NH), 7.553 (3H, s), 7.466 (2H, d, J 8 Hz), 7.316–7.2 (3H, m), 6.865
(1H, s), 6.787 (2H, s), 5.904 (1H, s, Ar3CH); EI-MS: m/z 629 (42%), 627
(100), 625 (100), 623 (Mþ, 41), 548 (6), 546 (11), 544 (6), 405 (7), 404
(11), 403 (16), 402 (19), 401 (20), 400 (9), 323 (19), 321 (19), 242 (23),
179 (33); HR FAB-MS: obs.: m/z 622.8817; calc. for C26H16N3O79Br3:
622.8844.
Crystallographic Data for 4 CH2Cl2
.
Crystals from CH2Cl2, C26H16N3OBr3 CH2Cl2, Mr ¼ 711.08, mono-
˚
clinic, space group P21/n, a ¼ 13.4698(2), b ¼ 11.1017(2), c ¼ 17.7796(3)A,
3
b ¼ 99.6172(7)8, V ¼ 2621.35(8) A , Z ¼ 4, Dc ¼ 1.802 Mg m23, m(Mo
˚
Ka) ¼ 4.861 mm21, F(0 0 0) ¼ 1392, T ¼ 160(1) K, crystal dimensions:
0.12 ꢀ 0.15 ꢀ 0.25 mm3, Nonius KappaCCD diffractometer, Mo Ka radi-
˚
ation, l ¼ 0.71073 A, umax ¼ 268, 43,452 measured reflections 5164 unique,
4003 with I . 2s(I). Numerical absorption correction, structure solution by
using SIR92, structure refined on F2 using SHELXL97, non-H atoms refined
anisotropically, amine H-atoms refined isotropically, all other H-atoms riding
in calculated positions. Refinement of 333 parameters using 5164 reflections
gave R(F) [I . 2s(I) data] ¼ 0.0513, Rw(F2) [all data] ¼ 0.1418, goodness
of fit on F2 ¼ 1.041, Drmax ¼ 1.90 e A23. CCDC-196483 contains the sup-
˚
plementary crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; Fax: þ44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk).
ACKNOWLEDGMENTS
The authors sincerely thank the Director, Bose Institute for laboratory
facilities, the C.S.I.R., Govt. of India for providing fellowship (S.S.), Prof.
Y. Harigaya, School of Pharmaceutical Sciences, Kitasato University,
Tokyo, Japan, Mr. B. Majumder, NMR Lab., Mr. J. Chatterjee, R.S.I.C. and
Mr. P. Dey, Microanalytical Lab., all of B.I., for recording the spectra.

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Synthesis of Triindolylmethanes
1809
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Received in India December 4, 2003

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