An efficient synthesis of indolo[3,2-a]carbazoles via the novel acid catalyzed reaction of indoles and diaryl-1,2-diones

Article abstract of DOI:10.1039/b803009j

A mild and efficient one pot method for the synthesis of indolo[3,2-a]carbazole derivatives by the para-toluenesulfonic acid catalyzed condensation of indole with acyclic 1,2-diones is described. With cyclobutene-1,2-diones the reaction afforded indole substituted carbazole derivatives in good yield. The Royal Society of Chemistry.

Full text of DOI:10.1039/b803009j

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An efficient synthesis of indolo[3,2-a]carbazoles via the novel acid catalyzed
reaction of indoles and diaryl-1,2-diones†
Vijay Nair,*a,b Vidya Nandialath,a Keecherikunnel G. Abhilasha and Eringathodi Sureshc
Received 21st February 2008, Accepted 3rd March 2008
First published as an Advance Article on the web 8th April 2008
DOI: 10.1039/b803009j
A mild and efficient one pot method for the synthesis of indolo[3,2-a]carbazole derivatives by the
para-toluenesulfonic acid catalyzed condensation of indole with acyclic 1,2-diones is described. With
cyclobutene-1,2-diones the reaction afforded indole substituted carbazole derivatives in good yield.
Introduction
Results and discussion
The indolocarbazole framework is present in a number of alka-
loids which exhibit potent biological activities.1 Inter alia, these
include antitumor, antihistaminic and antimicrobial activities.2
It is also noteworthy that, by virtue of their thermal stability,
intense luminescence and other desirable properties, carbazole
derivatives have been widely used in molecular electronics.3 Their
potential applications as organic LEDs,4 high performance p-
channel semiconductors and thin film transistors are also worthy
of mention.5
Against this background, our studies were initiated by heating
a solution of benzil and N-methylindole in dry toluene under
reflux in the presence of para-toluenesulfonic acid (20 mol%) for
10 h. The reaction mixture, on processing, afforded a product in
74% yield which was characterized as the indolo[3,2-a]carbazole
3 (Scheme 1)
Of the five isomeric forms of indolocarbazoles,1 the [3,2-
a]carbazole framework has received the least attention, presum-
ably because of its limited access from natural sources and the
lack of efficient methods for its synthesis. In this context, the
recent one pot synthesis of the related indolo[3,2-b]carbazoles
is noteworthy.6 The first report on the natural occurrence of an
indolo[3,2-a]carbazole consists of the isolation of acorinazole by
Bergman et al., from a marine sponge Ancorina species.7 In 1951,
Tomlinson reported the first synthesis of indolo[3,2-a]carbazole
by the double Fischer indolization of bis(cyclohexanone)-m-
phenylenedihydrazone.8 Subsequently, Hall and Plant reported
another method for the synthesis of indolocarbazole by heating 5-
aminotetrahydrocarbazole with 2-chlorocyclohexanone.9 In spite
of the large amount of work on the synthesis of carbazoles,10
no direct and efficient method for the synthesis of indolo[3,2-
a]carbazole derivatives has been reported. In the context of our
interest in the chemistry of 1,2-diones11 we surmised that a con-
densation of these with indole, if successful, would provide a direct
route to the synthesis of indolo[3,2-a]carbazole. Surprisingly, such
a simple protocol has not been reported.
Scheme 1 Reaction of N-methyl indole with benzil.
The product was characterized on the basis of spectroscopic
data. In the 1H NMR the two methyl protons resonated at
d 4.51 and 3.28 ppm and, in the 13C NMR, the signals at d
35.7 and 33.2 ppm correspond to the two methyl carbons. The
high resolution mass spectrum was also in accordance with the
assigned structure. Unambiguous evidence for the structure of
the compound 3 was confirmed by single crystal X-ray analysis
(Fig. 1)‡
The reaction was investigated with various Lewis and Brønsted
acids and the results are shown in Table 1. It appears that PTSA
is the best catalyst for the condensation reaction.
The reaction appears to be general with the analogs of benzil
and the results are presented in Table 2. Similar reactivity was also
observed in the reaction of indole with 2,3-butanedione 21, but
the product 22 was obtained in low yield.
aOrganic Chemistry Section, National Institute for Interdisciplinary Science
and Technology (formerly known as RRL) (CSIR), Trivandrum, 695019,
Kerala, India
‡ Crystal data for compound 3: C32H24N2, Mr = 436.53, triclinic, space
bJawaharlal Nehru Centre for Advanced Scientific Research, Bangalore,
560064, India. E-mail: vijaynair_2001@yahoo.com; Fax: +91) 471-
2491712
˚
group P–1, a = 9.819(3), b = 11.498(3), c = 11.946(3) A, a = 68.553(4),
◦
3
˚
b = 69.635(4), c = 69.955(4) , V = 1140.4(5) A , Z = 2, qcalcd
=
1.271 g cm−3, l = 0.074 mm−1, T = 293(2) K, reflections collected =
7966, independent reflections = 3975, Rint = 0.0295, R1 = 0.0591, wR2 =
0.1389 [I > 2r(I)]. CCDC-658189.† These data can also be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
cAnalytical Sciences Discipline, Central Salt and Marine Chemicals Research
Institute, Bhavnagar, 364 002, Gujarat, India
† CCDC reference numbers 658189 and 659431. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b803009j
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Table 1 Condensation reaction of indole and benzil with different
catalysts
Entry
Acids
Mol %
Product %
1
2
3
4
5
6
7
PTSA
AuCl3
Cu(OTf)3
Yb(OTf)3
SnCl4
Sc(OTf)3
BF3OEt
20 mol %
1 mol %
20 mol %
20 mol %
20 mol %
20 mol %
20 mol %
62%
58%
57%
28%
29%
52%
No reaction
i = catalyst, toluene, reflux, argon, 10 h.
Scheme 2 Proposed mechanism for the formation of indolocarbazole.
Table 2 Condensation reaction of indole and N-substituted indole with
various benzils
and subsequent cyclization and aromatization affords the product
3. The possibility of the diarylation followed by pinacol-type
rearrangement is excluded since it will lead to the formation of
only indolo[2,3-a]carbazole.
The success of the above reaction prompted us to explore the
indole condensation reaction with cyclobutene-1,2-diones. In the
event of 3,4-dimesitylcyclobutene-1,2-dione 29 reacting with N-
methylindole 1 the carbazole derivative 30 was obtained in 64%
yield (Scheme 3).
Entry R1, R2
R3
Product %
1
2
3
4
5
6
7
8
9
R1 = R2 = H (4)
Phenyl (2)
3,4-Dichlorophenyl (6)
4-Fluorophenyl (8)
5 (62)
7 (76)
9 (57)
R1 = R2 = H (4)
R1 = R2 = H (4)
R1 = H, R2 = CH3 (1) 4-Trifluoromethylphenyl (10) 11 (79)
R1 = H, R2 = CH3 (1) 3,4-Dichlorophenyl (6)
R1 = H, R2 = CH3 (1) 4-Bromophenyl (13)
R1 = H, R2 = allyl (15) Phenyl (2)
12 (67)
14 (67)
16 (60)
18 (64)
20 (74)
22 (12)
R1 = H, R2 = ethyl (17) Phenyl (2)
R1 = Br, R2 = H (19)
R1 = R2 = H (4)
Phenyl (2)
Methyl (21)
10
Scheme 3 Reaction of N-methyl indole with cyclobutene-1,2-dione.
The structure of the product 30 was established on the basis of
spectroscopic data. The peak at 3523 cm−1 in the IR is diagnostic
of the OH group. In the 1H NMR the OH proton resonated at d
5.29 ppm (exchangeable with D2O) and the two N-methyl protons
resonated at d 4.27 and 3.64 ppm. In the 13C NMR the signals at d
29.8 and 29.4 ppm correspond to the two methyl carbons attached
to nitrogen. The structure of the compound was finally confirmed
by single crystal X-ray analysis (Fig. 2)§
The reaction appears to be general with analogs of cyclobutene-
1,2-diones and the results are presented in Table 3.
Fig. 1 ORTEP digram for the compound 3 (40% probability factor for
the thermal ellipsoids).
§ Crystal data for compound 30 : C41H40Cl2N2O, Mr = 647.65, triclinic,
˚
space group P–1, a = 8.2446(10), b = 14.36◦09(17), c = 15.8071(19) A,
Mechanism
3
˚
a = 111.220(2), b = 95.001(2), c = 105.513(2) , V = 1645.5(3) A , Z = 2,
qcalcd = 1.307 g cm−3, l = 0.234 mm−1, T = 100(2) K, reflections collected =
10889, independent reflections = 5147, Rint = 0.0431, R1 = 0.1089, wR2 =
0.2814 [I > 2 r(I)], GOF = 1.091. CCDC-659431.† These data can also
be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
A mechanistic rationalization for the reaction is given in Scheme 2.
Indole 1 first reacts with benzil to form the intermediate 23
which loses one molecule of water to form the highly electrophilic
species 24. The latter reacts with another molecule of indole
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Table 3 Condensation reaction of indole and N-substituted indoles with
various cyclobutene-1,2-diones
Conclusions
In conclusion, we have devised a direct and efficient acid catalyzed
protocol for the annulation of indole with acyclic 1,2-diones and
cyclobutene-1,2-diones, affording indolo[3,2-a]carbazoles and in-
dole substituted carbazole derivatives. The experimental simplicity
and the mild reaction conditions are noteworthy.
Experimental section
Entry
R1
Ar
Product (%)
General methods
1
2
3
4
5
6
H (4)
Phenyl (31)
32 (50)
33 (72)
34 (75)
35 (54)
37 (52)
39 (62)
All reactions were carried out in oven-dried glassware under argon
atmosphere. Progress of reaction was monitored by thin layer
chromatography (visualization was effected by exposure to UV
light or iodine), while purification was effected by silica gel column
chromatography. NMR data were collected at room temperature
in CDCl3 or (CD3)2SO at an operating frequency of 300 MHz
(75 MHz for 13C NMR). IR spectra were recorded on Bomem MB
series FT-IR spectrophotometer. Melting points were recorded
on a Buchi melting point apparatus and are uncorrected. High
resolution mass spectra were recorded under EI/HRMS (at 5000
resolution) using JOEL JMS 600H mass spectrometer.
Allyl (15)
Methyl (1)
Allyl (15)
Methyl (1)
Ethyl (38)
Mesityl (29)
Phenyl (31)
Phenyl (31)
4-Methylphenyl (36)
4-Methylphenyl (36)
General experimental procedure for the synthesis of carbazole
derivatives
To a solution of indole derivatives (2.5 equiv.) and 1,2-diones
(1 equiv.) in dry toluene was added 20 mol% of para-
toluenesulfonic acid under an atmosphere of argon. After refluxing
the reaction mixture for 10 h, the solvent was removed in vacuo
and the residue was purified by silica gel column chromatography
(60–120 mesh, hexane–ethyl acetate solvent mixtures) to afford the
product as colorless crystalline solid.
Fig. 2 ORTEP digram for the compound 30 (30% probability factor for
the thermal ellipsoids, lattice dichloromethane molecule is omitted for
clarity).
Characterization data of 3
Mp: 240–242 ◦C. IR (KBr) mmax: 3049, 1579, 1500, 1479, 1444,
1417, 1386, 1323, 1255 cm−1. H NMR (CDCl3 + DMSO-d6): d
1
8.59 (d, 1H, J = 8.0 Hz), 7.46–7.17 (m, 16H), 6.87 (uneven triplet,
1H, J1 = 7.9 Hz, J2 = 7.1 Hz), 6.48 (d, 1H, J = 7.9 Hz), 4.51 (s,
3H), 3.28 (s, 3H). 13C NMR (CDCl3 + DMSO-d6): d 142.7, 142.1,
140.4, 139.2, 132.6, 130.6, 128.2, 127.5, 126.9, 126.8, 124.9, 124.6,
124.1, 122.9, 121.4, 121.2, 119.6, 119.4, 115.4, 109.4, 109.1, 107.5,
35.7, 33.2. HRMS (EI): calculated: 436.1939; found: 436.1909.
Mechanism
A mechanistic rationalization for the reaction is given in Scheme 4.
Presumably, the diarylation of cyclobutene-1,2-dione takes place
to form an intermediate which then undergoes ring opening to
form the ketene, which conceivably undergoes ring closure and
subsequent aromatization to afford the product 30.
Characterization data of 5
Mp: 320–322 ◦C. IR (KBr) mmax: 3452, 3402, 1635, 1610, 1462, 1442,
1
1371, 1327, 1255, 1153, 1022 cm−1. H NMR (CDCl3 + DMSO-
d6): d 10.46 (bs, 1H), 8.65 (bs, 1H), 8.52 (d, 1H, J = 7.5 Hz),
7.60 (d, 1H, J = 8.0 Hz), 7.48–7.23 (m, 14H), 6.88 (m, 1H, J =
7.1 Hz), 6.80 (d, 1H, J = 7.8 Hz). 13C NMR ((CD3)2CO): d 141.4,
141.0, 140.5, 139.3, 138.8, 132.3, 131.6, 129.0, 128.8, 127.7, 127.4,
125.2, 124.2, 123.1, 121.7, 120.1, 119.6, 118.7, 115.3. HRMS (EI):
calculated: 408.1625; found: 408.1665.
Characterization data of 12
Mp: 220–221 ◦C. IR (KBr) mmax: 3053, 2431, 1612, 1548, 1583,
1490, 1469, 1444 cm−1. 1H NMR (CDCl3 + DMSO-d6): d 8.56 (d,
Scheme 4 Proposed mechanism for the formation of indolylcarbazole.
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1H, J = 8.1 Hz), 7.51–7.23 (m, 9H), 7.08–6.96 (m, 3H), 6.58 (d,
1H, J = 7.9 Hz), 4.46 (s, 3H), 3.31 (s, 3H). 13C NMR (CDCl3 +
DMSO-d6): d 142.4, 139.7, 133.8, 133.7, 132.3, 131.5, 131.4, 131.3,
130.4, 129.7, 129.6, 129.4, 124.9, 124.4, 123.8, 122.8, 120.8, 120.6,
119.9, 119.7, 109.3, 109.2, 35.5, 33.4. LRMS (FAB): calculated:
572.0831; found: 572.2324.
5.13–5.05 (m, 2H), 4.90–4.87 (m, 2H), 4.63–4.51 (m, 2H), 4.34
(d, 1H, J = 17.1 Hz), 2.22 (s, 3H), 2.06 (s, 3H), 2.04 (s, 3H), 1.98
(s, 3H), 1.93 (s, 3H), 1.57 (s, 3H). 13C NMR (CDCl3): d 139.6,
138.4, 137.6, 137.5, 137.3, 134.4, 133.1, 130.3, 129.0, 128.8, 127.8,
127.6, 125.1, 123.6, 121.1, 119.0, 118.5, 115.8, 109.3, 47.9, 47.5.
LRMS (FAB): calculated: 614.82; found: 614.94.
Characterization data of 34
Characterization data of 16
Mp: 135–137 ◦C. IR (KBr) mmax: 3523, 3051, 2929, 1598, 1541,
1481, 1454, 1436, 1386, 1354 cm−1. 1H NMR (CDCl3): d 7.38–7.24
(m, 11H), 7.23–6.73 (m, 8H), 6.55 (s, 1H), 5.20 (s, 1H), 4.26 (s,
3H), 3.68 (s, 3H). 13 C NMR (CDCl3): d 142.3, 140.9, 132.1, 131.5,
130.8, 129.0, 128.3, 127.4, 126.7, 126.2, 125.3, 125.0, 123.3, 122.9,
121.3, 120.7, 119.1, 118.5, 108.9, 108.1, 32.7, 31.9. HRMS (EI):
calculated: 478.1888; found: 478.1835.
Mp: 224–226 ◦C. IR (KBr) mmax: 3080, 2951, 1610, 1589, 1538,
1
1440, 1400, 1350, 1250 cm−1. H NMR (CDCl3 + DMSO-d6): d
8.44 (d, 1H, J = 7.7 Hz), 7.43–7.16 (m, 15H), 6.86 (uneven triplet,
1H, J1 = 7.8 Hz, J2 = 7.2 Hz), 6.59–6.51 (m, 1H), 6.44 (d, 1H, J =
7.8 Hz), 5.62–5.43 (m, 5H), 4.93 (d, 1H), 4.58 (d, 1H), 4.37–4.36
(m, 2H). 13C NMR (CDCl3 + DMSO-d6): d 142.4, 141.2, 140.1,
138.4, 137.3, 135.8, 134.5, 133.2, 131.9, 130.4, 127.9, 127.3, 126.9,
124.9, 123.9, 121.2, 119.8, 119.6, 118.5, 115.7, 109.9, 107.5, 49.5,
46.7. HRMS (EI): calculated: 488.2252; found: 488.2252.
Characterization data of 35
IR (thin film) mmax: 3515, 3048, 2935, 1620, 1555, 1490, 1389, 1240,
1110, 1019 cm−1. 1H NMR (CDCl3): d 7.53–7.08 (m, 11H), 6.84–
6.72 (m, 8H), multiplet centered around 6.13 (s, 1H), 5.74–5.72 (m,
1H), 5.56 (s, 1H), 5.46–5.29 (m, 2H), 5.17–5.12 (m, 2H), 5.00–4.98
(m, 1H), 4.60–4.55 (m, 3H). 13C NMR (CDCl3): d 141.5, 139.6,
136.6, 131.1, 130.8, 130.7, 129.8, 128.1, 127.7, 126.5, 126.3, 125.3,
125.1, 123.5, 122.5, 120.1, 120.0, 118.7, 109.8, 108.9, 108.5, 48.4,
47.6. LRMS (FAB): calculated: 530.66; found: 530.89.
Characterization data of 22
Mp: 261–263 ◦C. IR (KBr) mmax: 3437, 3412, 3049, 1639, 1498,
1458, 1373, 1250, 1100 cm−1. H NMR (CDCl3 + DMSO-d6): d
1
9.95 (bs, 1H), 9.17 (bs, 1H), 8.36 (d, 1H, J = 7.6 Hz), 8.23 (d, 1H,
J = 7.8 Hz), 7.61 (d, 1H, J = 7.9 Hz), 7.54 (d, 1H, J = 7.9 Hz),
7.39–7.20 (m, 4H), 2.96 (s, 3H), 2.62 (s, 3H). 13C NMR (DMSO-
d6): d 139.5, 139.3, 138.9, 132.1, 128.5, 124.2, 122.1, 121.7, 121.1,
120.7, 119.7, 118.9, 113.9, 111.5, 110.3, 110.1, 104.2, 13.4, 13.2.
HRMS (EI): calculated: 284.1313; found: 284.1367.
Characterization data of 37
Mp: 218–220 ◦C. IR (KBr) mmax: 3520, 3053, 2928, 1588, 1532,
Characterization data of 30
1
1486, 1360, 1210, 1098, 1010 cm−1. H NMR (CDCl3): d 7.27–
Mp: 255–257 ◦C. IR (KBr) mmax: 3520, 3053, 2928, 1588, 1532,
1486, 1360, 1210, 1098, 1010 cm−1. 1H NMR (CDCl3): d 7.36–7.25
(m, 5H), 7.13 (uneven triplet, 2H, J1 = 8.7 Hz, J2 = 8.4 Hz), 6.86
(uneven triplet, 2H, J1 = 7.2 Hz, J2 = 7.8 Hz), 6.72 (s, 1H), 6.67
(s, 1H), 6.49 (s, 2H), 6.31 (s, 1H), 5.29 (s, 1H), 4.27 (s, 3H), 3.64
(s, 3H), 2.24 (s, 3H), 2.16 (s, 3H), 2.09 (s, 3H), 2.05 (s, 3H), 2.00
(s, 3H), 1.94 (s, 3H). 13C NMR (CDCl3): d 139.6, 137.5, 137.4,
137.3, 136.8, 136.3, 134.4, 130.3, 128.9, 128.7, 127.6, 126.7, 125.1,
123.5, 123.2, 121.6, 120.9, 118.8, 118.3, 113.8, 109.0, 107.9, 29.8,
29.4, 26.9, 21.8, 21.5, 21.1, 20.9, 20.8. LRMS (FAB): calculated:
562.2984; found: 562.78.
7.15 (m, 7H), 6.97–6.92 (m, 3H), 6.66–6.51 (m, 7H), 5.21 (s, 1H),
4.21 (s, 3H), 3.64 (s, 3H), 2.27 (s, 3H), 2.06 (s, 3H). 13C NMR
(CDCl3): d 142.3, 138.4, 137.9, 136.8, 136.4, 133.9, 132.9, 131.9,
130.8, 129.9, 129.3, 129.1, 128.1, 124.7, 123.3, 122.9, 121.2, 120.7,
119.1, 118.4, 114.1, 108.9, 32.6, 31.9, 21.2, 21.1. LRMS (FAB):
calculated: 506.64; found: 506.90.
Acknowledgements
The authors thank the Council of Scientific and Industrial
Research (CSIR) and Department of Science and Technology
(DST), New Delhi for financial assistance.
Characterization data of 32
Notes and references
IR (thin film) mmax: 3510, 3419, 3063, 2923, 1590, 1540, 1454, 1316,
1215, 1094, 1012 cm−1 1H NMR (CDCl3): d 8.39 (bs, 1H), 7.93
1 For a comprehensive review on carbazoles see: H.-J. Kno¨lker and K. R.
Reddy, Chem. Rev., 2002, 102, 4303.
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(bs, 1H), 7.41–7.14 (m, 11H), 6.86–6.70 (m, 9H), 5.29 (bs, 1H). 13
C
NMR (CDCl3): d 140.8, 135.4, 131.6, 130.9, 128.9, 128.5, 128.3,
127.8, 126.7, 126.3, 125.4, 125.2, 123.9, 122.9, 121.8, 120.5, 119.7,
119.1, 110.9, 110.5. HRMS (EI): calculated: 450.1732; found:
450.1733.
Characterization data of 33
IR (thin film) mmax: 3545, 3075, 2925, 1628, 1568, 1550, 1440, 1420,
1
1390, 1018 cm−1. H NMR (CDCl3): d 7.33–7.11 (m, 5H), 6.89–
6.69 (m, 5H), 6.53 (s, 1H), 6.47 (s, 1H), 6.32 (m, 1H), 6.14–6.08
(m, 1H), 5.69–5.64 (m, 1H), 5.49–5.42 (m, 1H), 5.29–5.23 (m, 1H),
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1742 | Org. Biomol. Chem., 2008, 6, 1738–1742
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