N-lodosuccinimide - An effective reagent for regioselective heterocyclization of o-cyclohex-2′-enylanilines for the synthesis of hexahydrocarbazoles

Article abstract of DOI:10.1139/v04-162

A number of carbazoles are synthesized in good yields by the N-iodosuccinimide mediated heterocyclization of o-cyclohex-2′-enylanilines in acetonitrile at -10 °C for 45 min followed by heating with palladium-charcoal (10%) in benzene (80 °C) for 18-20 h.

Full text of DOI:10.1139/v04-162

63
N-Iodosuccinimide — An effective reagent for
regioselective heterocyclization of o-cyclohex-2′-
enylanilines for the synthesis of
hexahydrocarbazoles
K.C. Majumdar, U.K. Kundu, U. Das, N.K. Jana, and B. Roy
Abstract: A number of carbazoles are synthesized in good yields by the N-iodosuccinimide mediated heterocyclization
of o-cyclohex-2′-enylanilines in acetonitrile at –10 °C for 45 min followed by heating with palladium–charcoal (10%)
in benzene (80 °C) for 18–20 h.
Key words: N-iodosuccinimide, hexahydrocarbazole, heterocyclization, carbazole, Claisen rearrangement.
Résumé : On a réalisé la synthèse de plusieurs carbazoles avec de bons rendements en faisant appel à une hétérocycli-
sation, catalysée par le N-iodosuccinimide, de o-cyclohex-2′-énylanilines, à –10 °C pendant 45 min, suivie par un
chauffage avec du palladium sur charbon (10 %), dans le benzène, à 80 °C, pendant une période de 18 à 20 heures.
Mots clés : N-iodosuccinimide, hexahydrocarbazole, hétérocyclisation, carbazole, réarrangement de Claisen.
[Traduit par la Rédaction] Majumdar et al. 67
Introduction
substrates. Electron-poor nitrogen species like amides and
hydroxylic amines have so far been employed (9) for the prep-
aration of N-p-toluenesulfonyl-3-hydroxy-4-pentenamides,
N-protected 3-hydroxy-4-pentenyl-amines, 4-hydroxy-5-
hexenylamines, and dialkylsubstituted pyrrolidines and
piperidines from hydroxylic amines. Intramolecular cycli-
zation of substrates by electron-poor nitrogen nucleophiles
has been accomplished under various reaction conditions.
However, there is no report so far on the iodoamination
where arylamines have been used as electron-poor nucleo-
philes with N-iodosuccinimide. Therefore, substrate 1a was
stirred with 1 equiv. of N-iodosuccinimide in acetonitrile at
–10 °C. The reaction was completed in -45 min to give a
viscous oil in 60% yield. This was characterized as
1,2,3,4,4a,9a-hexahydro-1-iodo-9-methylcarbazole (2a) from
its elemental analysis and spectroscopic data. 13C NMR
spectral data and DEPT experiment corroborated the pro-
posed structure for product 2a. The degree of protonation of
the 10 carbons were determined by DEPT experiment,
which showed one -CH3, three -CH2-, and six >CH-. Sub-
strates 1b–1g were also similarly treated to give products
2b–2g in 55%–70% yield (Scheme 1). It was found that the
iodoamines (2a–2g) are not very stable and on standing de-
compose to intractable products. The products 2a–2e were
dehydrogenated with palladium-charcoal (10%) in refluxing
benzene for 18–20 h to give corresponding carbazoles 3a–3e
in 60%–65% yield (10–13). These were characterized as
substituted 9-methylcarbazoles (3) from their elemental
analyses and spectroscopic data. The products 2f and 2g
were similarly treated with 5% palladium–charcoal to give
products 4f and 4g in 60% and 62% yield, respectively, con-
taminated with minor amounts of fully aromatized products.
These minor products could not be isolated in pure form.
The products (4f and 4g) were characterized as 1,2,3,4-tetra-
There are several reports for the synthesis of carbazoles
(1). Carbazole has been synthesized from diphenylamine ei-
ther by photochemical cyclization (2) or by thermal
cyclization (3), and also in the presence of iodine (4) at
350 °C. We have reported the synthesis of 1-alkoxy tetra-
hydrocarbazoles by the Hg(II)-mediated heterocyclization
(5) of o-cyclohex-2′-enylanilines. This prompted the evalua-
tion of the efficacy of N-iodosuccinimide for the hetero-
cyclization of o-cyclohex-2′-enylanilines.
o-Cyclohex-2′-enylanilines 1a–1g were prepared in 50%–
90% yields by the acid-catalyzed aza-Claisen rearrangement
of 3-N-alkyl, N-arylamino cyclohexene in refluxing ethano-
lic hydrochloric acid following the literature procedure (5).
Results and discussion
It was envisaged that heterocyclization of o-cyclohex-2′-
enylanilines (1a–1g) would afford hexahydrocarbazoles.
Previously, we have successfully utilized pyridinehydrotri-
bromide (6) and hexaminehydrotribromide (7) for the
heterocyclization of o-cyclohex-2′-enylphenols. However,
our attempts at cyclization of o-cyclohex-2′-enylanilines
with these reagents were of no avail (8). At this point our at-
tention was drawn to N-iodosuccinimide, which is reported
to effect the cyclization of unsaturated nitrogen-containing
Received 24 June 2004. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 5 February 2005.
K.C. Majumdar,1 U.K. Kundu, U. Das, N.K. Jana, and
B. Roy. Department of Chemistry, University of Kalyani,
Kalyani-741 235, W.B, India.
1Corresponding author (e-mail: kcm@klyuniv.ernet.in).
Can. J. Chem. 83: 63–67 (2005)
doi: 10.1139/V04-162
© 2005 NRC Canada

64
Can. J. Chem. Vol. 83, 2005
Scheme 1. Reagents and conditions: (i) NIS, CH3CN, –10 °C, 45 min.
Scheme 2. Reagents and conditions: (i) Pd-C (10%), C6H6, 98, 18–20 h; (ii) Pd-C (5%), 98, 18–20 h.
Scheme 3.
hydrocarbazoles from their elemental analyses and spectro-
scopic data (Scheme 2).
The regioselective formation of hexahydrocarbazoles 2a–
2g from substrates 1a–1g may be easily explained by the ini-
tial formation of an intermediate iodonium ion (5a–5g),
which may undergo a “5-exo” cyclization to give products
2a–2g. An alternative “6-endo” cyclization to give bicyclic
products is ruled out as no such product could be isolated
from the reaction mixture (Scheme 3).
The relative stereochemistry of the product could not be
determined from the observed coupling constants of protons
(C4a-H, C9a-H, and C1-H) alone. However, we could assign
© 2005 NRC Canada

Majumdar et al.
65
reported in cm–1. 1H NMR (300 MHz) and 13C NMR
(75.5 MHz) spectra were recorded in CDCl3 (TMS was used
as an internal standard) on a Bruker DPX-300 instrument at
the Indian Institute of Chemical Biology, Kolkata, India
(chemical shifts in δ ppm). Elemental analyses and recording
of mass spectra were carried out by RSIC (CDRI) Lucknow
on a JEOL D-300 (El) instrument. Silica gel (60–120 mesh,
Spectrochem, India) was used for chromatographic separa-
tion. Petroleum ether refers to the fraction boiling between
40 and 60 °C. Palladium on activated charcoal (5% and
10%) was procured from S.D. Fine Chemicals Ltd. (India)
and Loba Chemie, respectively.
Fig. 1. Stereostructure of compound 2.
the stereochemistry of the product by considering the mech-
anism of the reaction. As the product 2a is formed via an
iodonium intermediate, the N9—C9a and C1—I bonds in 2a
are antiperiplanar. Again, Pd-C-catalyzed dehydrogenation
occurs via cis-elimination. Therefore, the C9a—H and C1—
I bonds are expected to be cis. C4a-H is suggested to be ax-
ial because shifting of the double bond is facilitated by the
axial hydrogen. Hence, C9a-H and C4a-H should be trans
(J = 6.5 Hz because of the presence of nitrogen heteroatom).
The iodine atom and N-R groups are bulky and likely to
occupy equatorial positions (Fig. 1). The same hetero-
cyclization was attempted with N-bromosuccinimide without
any success, while pyridine hydrotribromide and hexamine
hydrotribromide afforded the addition products along with
ring-brominated products, and there also, no cyclization was
observed (8). N-Iodosuccinimide is a mild reagent and reacts
at low temperature bringing about regioselective heterocycli-
zation of o-cyclohex-2′-enylanilines to give hexahydrocar-
bazoles. For the cases where pyridine hydrotribromide and
hexamine hydrotribromide afforded bicyclic products,
cyclizations occurred by a 6-endo mode. In contrast, N-
iodosuccinimide gave linear hexahydrocarbazoles by a 5-exo
cyclization. Recently, we reported the formation of bicylic
heterocycles by N-iodosuccinimide-mediated reactions in
cases of 2-cyclohex-2′-enyl phenols (14) and 3-cyclohex-2′-
enyl-4-hydroxy coumarins (15). 2-(Cyclopent-2′-enyl)ani-
lines have recently been cyclized using iodine (16) in the
presence of NaHCO3. Synthesis of tetrahydrocarbazoles by
Ullmann cross coupling of o-halonitrobenzene and various
related nitrobenzenes with a range of α-halo cyclohexenones
followed by reaction with hydrogen in the presence of Pd on
C has also been recently reported (17). The present work
clearly demonstrates the viability of N-iodosuccinimide as
an effective reagent for regioselective heterocyclization of o-
allylanilines. In the cases of cyclization of electron-poor ni-
trogen species like amides and hydroxylic amines, different
types of stereoisomers were obtained (9). However, in the
present study only one type of regioselective product was
obtained. Therefore, this is an effective reagent for the syn-
thesis of hexahydrocarbazoles. We have, therefore, success-
fully demonstrated the use of aryl amines as electron-poor
nucleophiles in the N-iodosuccinimide-mediated iodoamina-
tions for the first time.
General procedure for the synthesis of compounds 2a–
2g
A solution of compound 1 (1a–1g) (1 mmol) in dry
CH3CN (50 mL) was stirred at –10 °C with N-
iodosuccinimide (0.23 g, 1 mmol) for 45 min. The solvent
was removed under reduced pressure at room temperature.
The residual mass was dissolved in CHCl3 (50 mL). The
CHCl3 was washed with a saturated solution of NaHSO3
(2 × 25mL), water (2 × 25 mL), and dried (Na2SO4). A
gummy residue was obtained after evaporation of the
solvent. This was purified by column chromatography over
silica gel. Elution of the column with petroleum ether (40–
60 °C) gave compounds 2a–2g as viscous oils.
Compound 2a
Viscous oil; yield 60%. UV (EtOH, nm) λmax: 217, 265,
303. IR (neat, cm–1) νmax: 530, 1260, 1500, 1590, 2940,
1
3120. H NMR (CDCl3, 300 MHz) δ: 1.73–1.89 (m, 4H),
1.97–2.01 (m, 2H), 2.75 (s, 3H, N-Me), 3.29 (dt, J = 6.5,
11.4 Hz, 1H, ArCH), 3.57 (dd, J = 6.5, 2.5 Hz, 1H, -NCH),
4.86 (d, J = 2.5 Hz, 1H, -CHI), 6.40–6.43 (m, 1H, ArH),
7.14–7.21 (m, 2H, ArH). 13C NMR (CDCl3, 75.5 MHz) δ: at
21.24 (C3), 29.89 (C4), 30.83 (C1), 31.28 (C2), 35.19
(>NCH3), 38.57 (C4a), 74.30 (C9a), 110.82 (C8), 126.37
(C5), 130.49 (C7), 137.03 (C6), 138.12 (C4b), and 148.03
(C8a). MS m/z: 391, 393 (M+, M+ + 2). Anal. calcd. for
C13H15BrIN (%): C 39.79, H 3.82, N 3.57; found: C 39.95,
H 3.72, N 3.43.
Compound 2b
Viscous oil; yield 67%. UV (EtOH, nm) λmax: 217, 264,
303. IR (neat, cm–1) νmax: 555, 1266, 1506, 1615, 2929,
1
3132. H NMR (CDCl3, 300 MHz) δ: 1.15 (t, J = 7.5 Hz,
3H), 1.28–1.89 (m, 4H), 2.28–2.33 (m, 2H), 2.46 (q, J =
7.5 Hz, 2H), 2.88 (s, 3H, N-Me), 3.29 (brs, 1H, -NCH), 3.83
(brs, 1H, ArCH), 4.84 (brs, 1H, -CHI), 6.43 (d, J = 8.7 Hz,
1H, ArH), 6.67–7.22 (m, 2H, ArH). MS m/z: 341 (M+).
Anal. calcd. for C15H20IN (%): C 52.78, H 5.86, N 4.10;
found: C 52.68, H 5.79, N 4.15.
Compound 2c
Experimental
Viscous oil; yield 65%. UV (EtOH, nm) λmax: 217, 264,
Melting points were determined in an open capillary and
are uncorrected. UV absorption spectra were recorded in
EtOH on a Shimadzu UV-2401 PC spectrophotometer (λmax
in nm), and IR spectra were obtained on a PerkinElmer L
120-000A grating IR spectrophotometer and the frequency
303. IR (neat, cm–1) νmax: 550, 1260, 1500, 1610, 2921,
1
3130. H NMR (CDCl3, 300 MHz) δ: 1.22–1.91 (m, 4H),
2.20 (s, 3H), 2.24–2.38 (m, 2H), 2.88 (s, 3H, N-Me), 3.05
(brs, 1H, -NCH), 3.48 (brs, 1H, ArCH), 4.83 (brs, 1H,
-CHI), 6.42 (d, J = 8.7 Hz, 1H, ArH) 6.65–6.93 (m, 2H,
© 2005 NRC Canada

66
Can. J. Chem. Vol. 83, 2005
ArH). MS m/z: 327 (M+). Anal. calcd. for C14H18IN (%): C
51.37, H 5.50, N 4.28; found: C 51.29, H 5.48, N 4.32.
(M+, M+ + 2). Anal. calcd. for C13H10BrN (%): C 60.00, H
3.84, N 5.38; found: C 60.11, H 3.76, N 5.32.
Compound 2d
Compound 3b
Viscous oil; yield 58%. UV (EtOH, nm) λmax: 218, 265,
Viscous oil; yield 60%. UV (EtOH, nm) λ max: 222, 265,
304. IR (neat, cm–1) νmax: 590, 1261, 1470, 1600, 2924,
297. IR (neat, cm–1) νmax: 1247, 1488, 1605, 2926, 3133. H
1
1
3132. H NMR (CDCl3, 300 MHz) δ: 1.28–2.00 (m, 4H),
NMR (CDCl3, 300 MHz) δ: 1.31 (t, J = 7.5 Hz, 3H), 2.79 (q,
J = 7.5 Hz, 2H), 3.88 (s, 3H, N-Me), 7.17–7.47 (m, 5H,
ArH), 7.91–8.08 (m, 2H, ArH). 13C NMR (75.5 MHz,
CDCl3) δ: at 17.03 (C12), 29.37 (C11), 29.50 (>NCH3),
108.60 (C1), 108.75 (C8), 118.95 (C6), 119.47 (C5), 120.61
(C2), 123.10 (C4a), 123.25 (C4b), 125.86 (C7), 126.37 (C4),
135.29 (C3), 139.91 (C9a), and 141.66 (C8a). MS m/z: 209
(M+). Anal. calcd. for C15H15N (%): C 86.12, H 7.17, N
6.69; found: C 86.22, H 7.11, N 6.60.
2.20–2.50 (m, 2H), 2.88 (s, 3H, N-Me), 3.04 (brs, 1H,
-NCH), 3.49 (brs, 1H, ArCH), 4.75 (brs, 1H, -CHI), 6.39 (d,
J = 8.7 Hz, 1H, ArH), 6.80–7.03 (m, 2H, ArH). MS m/z:
347, 349 (M+, M+ + 2). Anal. calcd. for C13H15ClIN (%): C
44.89, H 4.31, N 4.02; found: C 44.78, H 4.29, N 4.12.
Compound 2e
Viscous oil; yield 55%. UV (EtOH, nm) λ max: 218, 264,
303. IR (neat, cm–1) νmax: 560, 1260, 1470, 1630, 2920,
1
3130. H NMR (CDCl3, 300 MHz) δ: 1.26–2.01 (m, 4H),
Compound 3c
2.23–2.49 (m, 2H), 2.87 (s, 3H, N-Me), 3.08 (brs, 1H,
-NCH), 3.42 (brs, 1H, ArCH), 4.76 (brs, 1H, -CHI), 6.53–
7.49 (m, 4H, ArH). MS m/z: 313 (M+). Anal. calcd. for
C13H16IN (%): C 49.84, H 5.11, N 4.47; found: C 49.83, H
5.08, N 4.52.
Pale yellow crystalline solid, mp 81 to 82 °C (lit. value
(11) mp 84 °C); yield 61%. UV (EtOH, nm) λ max: 222, 265,
297. IR (KBr, cm–1) νmax: 1264, 1494, 1590, 2931, 3134. H
1
NMR (CDCl3, 300 MHz) δ: 2.53 (s, 3H, ArCH3), 3.83 (s,
3H, N-Me), 7.17–7.47 (m, 5H, ArH), 7.89–8.07 (m, 2H,
ArH). MS m/z: 195 (M+). Anal. calcd. for C14H13N (%): C
86.15, H, 6.66, N 7.17; found: C 86.31, H 6.59, N 7.23.
Compound 2f
Viscous oil; yield 70%. UV (EtOH, nm) λ max: 217, 264,
304. IR (neat, cm–1) νmax: 550, 1250, 1500, 1610, 2920,
1
3110. H NMR (CDCl3, 300 MHz) δ: 1.14–1.19 (m, 3H),
Compound 3d
1.30–1.89 (m, 4H), 2.30–2.34 (m, 2H), 2.98 (brs, 1H, -NCH),
3.05–3.42 (m, 2H), 3.52 (brs, 1H, ArCH), 4.75 (brs, 1H,
-CHI), 6.50–7.09 (m, 4H, ArH). MS m/z: 327 (M+). Anal.
calcd. for C14H18IN (%): C 51.37, H 5.50, N 4.28; found: C
51.28, H 5.43, N 4.32.
Pale yellow crystalline solid, mp 38–40 °C (lit. value (12)
mp 38–40 °C); yield 55%. UV (EtOH, nm) λ max: 221, 266,
1
297. IR (KBr, cm–1) νmax: 1246, 1474, 1596, 2939, 3130. H
NMR (CDCl3, 300 MHz) δ: 3.84 (s, 3H, N-Me), 7.22–7.44
(m, 4H, ArH), 7.47–8.05 (m, 3H, ArH). MS m/z: 215, 217
(M+, M+ + 2). Anal. calcd. for C13H10ClN (%): C 72.38, H
4.64, N 6.49; found: C 72.29, H 4.61, N 6.42.
Compound 2g
Viscous oil; yield 63%. UV (EtOH, nm) λ max: 217, 265,
303. IR (neat, cm–1) νmax: 560, 1260, 1490, 1590, 2930,
Compound 3e
1
3100. H NMR (CDCl3, 300 MHz) δ: 1.13–1.17 (m, 3H),
Pale yellow crystalline solid, mp 83 to 84 °C (lit. value
(13) mp 85 °C); yield 65%. UV (EtOH, nm) λ max: 222, 265,
1.28–1.80 (m, 4H), 2.26–2.40 (m, 2H), 2.99 (brs, 1H, -NCH),
3.14–3.36 (m, 2H), 3.51 (brs, 1H, ArCH), 4.65 (brs, 1H,
-CHI), 6.30 (d, J = 8.7 Hz, 1H, ArH), 7.09–7.31 (m, 2H,
ArH). MS m/z: 361, 363 (M+, M+ + 2). Anal. calcd. for
C14H17ClIN (%): C 46.47, H 4.70, N 3.87; found: C 46.52,
H 4.68, N 3.79.
1
297. IR (KBr, cm–1) νmax: 1245, 1468, 1600, 2926, 3133. H
NMR (CDCl3, 300 MHz) δ: 3.86 (s, 3H, N-Me), 7.20–7.46
(m, 4H, ArH), 7.48–8.11 (m, 4H, ArH). MS m/z: 181 (M+);
Anal. calcd. for C13H11N (%): C 86.18, H 6.07, N 7.73;
found: C 86.12, H 6.11, N 7.68.
General procedure for the synthesis of compounds 3a–
3e and 4f and 4g
Compound 4f
Viscous oil; yield 60%. UV (EtOH, nm) λ max: 229, 263,
Compound 2 (2a–2e, 0.2 g) was refluxed with 10% palla-
dium on charcoal (0.02 g), and in the case of compounds 2f
and 2g, 5% palladium on charcoal was used in benzene
(20 mL) for 18–20 h. The mixture was filtered and benzene
was distilled off. The residual mass was directly chromato-
graphed over silica gel. Elution of the column with petro-
leum ether (40–60 °C) gave the compounds 3a–3e and 4f
and 4g. The solid compounds were recrystallized from etha-
nol.
293. IR (neat, cm–1) νmax: 1231, 1484, 1598, 2928, 3053. H
1
NMR (CDCl3, 300 MHz) δ: 1.40–1.45 (m, 3H), 1.54–1.93
(m, 4H), 2.70–2.72 (m, 2H), 4.02–4.09 (m, 2H), 4.33–4.40
(m, 2H), 7.02–8.11 (m, 4H, ArH). MS m/z: 199 (M+). Anal.
calcd. for C14H17N (%): C 84.42, H 8.54, N 7.03; found: C
84.48, H 8.48, N 7.12.
Compound 4g
Viscous oil; yield 62%. UV (EtOH, nm) λ max: 221, 265,
293. IR (neat, cm–1) νmax: 1231, 1485, 1599, 2928, 3052. H
1
Compound 3a
White crystalline solid, mp 76 to 77 °C (lit. value (10) mp
78 to 79 °C); yield 63%. UV (EtOH, nm) λ max: 222, 265,
NMR (CDCl3, 300 MHz) δ: 1.41–1.45 (m, 3H), 1.60–2.02
(m, 4H), 2.71–2.74 (m, 2H), 4.03–4.13 (m, 2H, =CCH2),
4.23–4.41 (m, 2H), 7.05–8.11 (m, 3H, ArH). MS m/z: 233,
235 (M+, M+ + 2). Anal. calcd. for C14H16ClN (%): C 71.94,
H 6.85, N 5.99; found: C 71.83, H 6.87, N 5.23.
1
297. IR (KBr, cm–1) νmax: 1246, 1493, 1605, 2925, 3134. H
NMR (CDCl3, 300 MHz) δ: 3.86 (s, 3H, N-Me), 7.71–7.48
(m, 4H, ArH), 7.50–8.11 (m, 3H, ArH). MS m/z: 259, 261
© 2005 NRC Canada

Majumdar et al.
67
6. (a) K.C. Majumdar and A.K. Kundu. Indian J. Chem. 32B,
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Acknowledgements
We thank the Council of Scientific and Industrial Re-
search (CSIR, New Delhi) for financial assistance. Two of us
(U.D. and N.K.J) are grateful to CSIR (New Delhi) for fel-
lowships. We also thank Dr. S.K. Samanta for his assistance.
7. K.C. Majumdar, A.K. Kundu, and P. Chatterjee. Synth.
Commun. 26, 893 (1996).
8. K.C. Majumdar, U. Das, and A.K. Kundu. Synth. Commun.
28, 1593 (1998).
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