Metal-free synthesis of 3-sulfenylindoles via an iodine-mediated electrophilic cyclisation of 2-alkynylanilines with disulfides

Article abstract of DOI:10.3184/174751912X13467835639963

An efficient and metal-free method was developed to synthesise 3-sulfenylindoles via the iodine-mediated electrophilic cyclisation of 2-alkynylanilines with disulfides. In the presence of I₂, various 3-sulfenylindoles were obtained in moderate

Full text of DOI:10.3184/174751912X13467835639963

644 RESEARCH PAPER
NOVEMBER, 644–646
JOURNAL OF CHEMICAL RESEARCH 2012
Metal-free synthesis of 3-sulfenylindoles via an iodine-mediated
electrophilic cyclisation of 2-alkynylanilines with disulfides
Li-Ming Tao*, Wen-Qi Liu, Yun Zhou and Ai-Tao Li
Hunan Provincial Key Laboratory of Xiangnan Rare-Rrecious Metals Compounds Research and Application, Department of Chemistry and
Life Science, Xiangnan University, Chenzhou 423000, P. R. China
An efficient and metal-free method was developed to synthesise 3-sulfenylindoles via the iodine-mediated electro-
philic cyclisation of 2-alkynylanilines with disulfides. In the presence of I2, various 3-sulfenylindoles were obtained in
moderate to high yields.
Keywords: metal-free, iodine, electrophilic cyclisation, 3-sulfenylindoles, 2-alkynylanilines, disulfides
The indole ring is a widespread heterocycle found in many
natural products and pharmaceutical intermediates.1–3 Among
these indole derivatives, 3-sulfenylindoles are particularly
attractive due to their therapeutic value in diseases such as
cancers, HIV, obesity, heart disease, and allergies.4–6 Although
considerable efforts have been devoted to the development of
efficient methods for 3-sulfenylindoles synthesis, the majority
are focused on direct sulfenylation at the 3-position of the
indole with thiols, disulfides, quinone mono-O,S-acetals, N-
thioalkyl(aryl)-phthalimides, and ammonium thiocyanate.7–13
Thus, the development of some novel alternative strategies
for synthesising 3-sulfenylindoles remains a challenging area.
Recently a few new efficient methods have been developed
for this purpose via the electrophilic cyclisation of 2-(1-
alkynyl)anilines. For example, Larock and co-workers have
reported a novel synthetic approach to 3-sulfenylindoles by
(n-Bu)4NI-induced electrophilic cyclisation of N,N-dialkyl-2-
(1-alkynyl)anilines with an arylsulfenyl chloride or phenylsel-
enenyl chloride [Eqn (1), Scheme 1].14 Recently, Li and
co-workers developed a simple and efficient transition metal-
promoted electrophilic annulation method for the synthesis of
3-chalcogenoindoles [Eqn (2)].15 However, transition metal-
based protocols usually have inherent limitations, including
moisture sensitivity, costly metal catalysts and environmental
toxicity. Therefore, there is still a need for developing a direct
and metal-free method to synthesise 3-sulfenylindoles. While
studying the reaction of 2-(phenylethynyl)aniline (1a) with
1,2-diphenyl disulfide (2a), we serendipitously discovered that
the annulation reaction can be accomplished in the presence of
I2 alone without the use of transition-metals. Here, we report
the metal-free synthesis of 3-sulfenylindoles via an iodine-
mediated electrophilic cyclisation of 2-alkynylanilines with
disulfides [Eqn (3), Scheme 1].15–17
We chose the reaction of 2-(phenylethynyl)aniline (1a) with
1,2-diphenyl disulfide (2a) as a model system to determine the
optimal reaction conditions, and the results are summarised in
Table 1. Initially, the amount of I2 was examined using DMSO
as solvent at 80 °C, and it turned out that 1.0 equiv. I2 was the
most suitable for the reaction (entries 1 and 2). These results
indicated that the solvent and temperature played important
roles in the formation of 3 (entries 3–10). A series of solvents,
such as MeOH, DMF, MeCN, THF, m-xylene and toluene,
were firstly investigated. Screening revealed that toluene
was the most effective solvent, increasing the yield to 81%.
(entries 3–8). The effect of the reaction temperature was also
examined. The reaction in toluene at 110 °C gave the highest
yield (entries 9 and 10): the yield of 3 was enhanced to 93% at
110 °C. Notably, the amount of 2a has no effect on the reaction
(entry 11).
Having established the optimal reaction conditions, we
investigated the scope of both 2-alkynylanilines 1 and disul-
fides 2 for the annulation reaction (Table 2).18 Initially, a
number of 2-alkynylanilines 1a–1i was tested in the reaction
with 1,2-diphenyl disulfide (2a). We found that a series of
functional groups at the terminal position of the alkyne moiety
of 2-alkynylaniline were well-tolerated under the optimal
conditions but steric hindrance disfavoured the reaction in
terms of yields. For example, 2-bromophenyl and alkyl groups
favoured the reactions providing moderate to good yields
(entries 2–4), but 2-methoxyphenyl and 4- acetylphenyl groups
gave low yields (entries 1 and 5). Substrates 1g, having a
methyl group on the corresponding aromatic ring of the anil-
ine, also underwent the annulation reaction with disulfide 2a
smoothly in 72% yield (entry 6). Notably, N–Me substituted
Table 1 Screening optimal conditionsa
Entry
I2 (equiv.)
Solvent
T/ °C
Isolated yields/%
1
2
0.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
DMSO
DMSO
MeOH
80
80
49
68
37
26
72
25
61
81
88
93
94
3
80
4
DMF
80
5
MeCN
THF
m-Xylene
Toluene
Toluene
Toluene
Toluene
80
6
80
7
80
8
80
9
100
110
110
10
11b
Scheme 1 Synthesis of 3-sulfenylindoles.
a Under otherwise indicated, the reaction conditions were as
follows: 1a (0.2 mmol), 2a (0.1 mmol), I2, solvent (2 mL) for 8 h
under air atmosphere. b 2a (0.2 mmol).
* Correspondent. E-mail: taoliming2005@yahoo.com.cn

JOURNAL OF CHEMICAL RESEARCH 2012 645
Table 2 I2-Mediated annulation reactions of 2-alkynylanilines (1) with disulfides (2)a
Entry
2-Alkynylaniline 1
Disulfide 2
Time/h
Yield/%b
1
2
R1 = R3 = R4 = H, R2 =2-MeOC6H4 (1b)
R1 = R3 = R4 = H, R2 = 2-BrC6H4 (1c)
R1 = R3 = R4 = H, R2= n-C6H13 (1d)
R1 = R3 = R4 = H, R2= t-C4H9 (1e)
R1 = R3 = R4 = H, R2= 4-CH3COC6H4 (1f)
R1 = 4-Me, R2 = Ph, R3 = R4 = H (1g)
R1 = R4 = H, R2 = Ph, R3 = Me (1h)
R1 = H, R2 = Ph, R3 = R4 = Me (1i)
R1 = R3 = R4 = H, R2 = Ph (1a)
R5= Ph (2a)
8
15
8
44 (4)
R5= Ph (2a)
77 (5)
3
R5= Ph (2a)
73 (6)
4
R5= Ph (2a)
15
8
87 (7)
5
R5= Ph (2a)
24 (8)
6
R5= Ph (2a)
8
72 (9)
7
R5= Ph (2a)
8
71 (10)
81 (10)
59 (11)
75 (12)
95 (13)
93 (14)
77 (15)
82 (16)
8
R5= Ph (2a)
8
9
R5= 4-MeC6H4 (2b)
R5= 4-MeOC6H4 (2c)
R5= 4-ClC6H4 (2d)
R5= 4-NO2C6H4 (2e)
R5= pyridin-2-yl (2f)
R5 = Me (2g)
10
8
10
11
12
13
14
R1 = R3 = R4 = H, R2 = Ph (1a)
R1 = R3 = R4 = H, R2 = Ph (1a)
8
R1 = R3 = R4 = H, R2 = Ph (1a)
8
R1 = R3 = R4 = H, R2 = Ph (1a)
8
R1 = R3 = R4 = H, R2 = Ph (1a)
8
a Reaction conditions: 1 (0.2 mmol), 2 (0.1 mmol), I2 (1 equiv.), toluene (2 mL) at 110 °C under air atmosphere.
b Isolated yield.
substrates 1h and 1i, which had previously been reported as
efficient substrates,15,16 also displayed high reactivity under the
metal-free conditions (entries 7 and 8).
Experimental
NMR spectroscopy was performed on a Bruker Avance spectrometer
operating at 300 MHz (1H NMR) and 75 MHz (13C NMR). TMS
(tetramethylsilane) was used an internal standard and CDCl3 was used
as the solvent. Mass spectrometric analysis was performed on GC-MS
analysis (Shimadzu GCMS-QP2010).
Subsequently, we studied the reactions of disulfide 2a–g
with 2-(phenylethynyl)aniline (1a) under the optimal condi-
tions. We were pleased to observe that several functional
groups, such as methyl, methoxy, chloro, nitro and pyridine
groups on the aromatic moiety of disulfides were perfectly
tolerated, and disulfides having electron-deficient aryl groups
gave the better results. Treatment of substrate 1a with disulfide
2b–f, afforded the corresponding products 11–15 in 59%,
75%, 95%, 93% and 77% yields, respectively (entries 9–13).
Gratifyingly, an excellent yield was still achieved from the
reaction of substrate 1a with aliphatic disulfide 2g in the
presence of I2 and toluene (entry 14).
A possible mechanism for the present transformation is out-
lined in Scheme 2.14–17 Initially, the reaction of disulfide with I2
would yield R2SI A in situ.15,16 Then, the electrophilic addition
of R2SI A with the 2-alkynylaniline would afford intermediate
B. Annulation of intermediate B gives intermediate C, and
then hydrogen can be removed from intermediate C with the
aid of I− to afford the target product.
In summary, we have developed a simple and metal-free
protocol for the construction of 3-sulfenylindoles via the
iodine-mediated electrophilic cyclisation of 2-alkynylanilines
and disulfides. In the presence of I2, a variety of 3-sulfenylin-
doles were synthesised from the reaction of 2-alkynylanilines
derivatives with different disulfides in moderate to good yields.
Importantly, these reactions were conducted under metal-free
conditions.
Iodine-mediated annulation of 2-alkynylanilines with disulfides;
general procedure
2-Alkynylaniline derivative 1 (0.2 mmol), disulfide 2 (0.5 equiv), I2
(1 equiv.) and toluene (2 mL) were added to a Schlenk tube. Then the
tube was stirred at 80 °C in air for the indicated time until complete
consumption of starting material was revealed as monitored by TLC
or GC-MS analysis. After the reaction finished, the reaction mixture
was filtered through a glass filter and washed with ethyl acetate. The
solution was washed with saturated aqueous Na2S2O3 (5×2 mL) and
extracted with diethyl ether (3×5 mL). The combined organic layers
were dried with anhydrous Na2SO4 and concentrated under vacuum.
The resulting residue was purified by column chromatography on
silica gel (hexane/ethyl acetate) to afford the desired product.
2-Phenyl-3-(phenylthio)-1H-indole (3):18 Pale yellow oil. 1H NMR
(300MHz, CDCl3) δ: 8.57 (brs, 1H), 7.78 (d, J = 8.2 Hz, 2H), 7.67 (d,
J = 7.8 Hz, 1H), 7.49–7.42 (m, 4H), 7.30–7.27 (m, 1H), 7.22–7.12 (m,
6H); 13C NMR (75 MHz, CDCl3) δ 142.1, 139.3, 135.9, 131.4, 131.2,
128.9, 128.8, 128.7, 128.1, 125.6, 124.6, 123.4, 121.2, 120.0, 111.2,
99.4; LRMS (EI, 70 eV) m/z (%): 301 (M+, 100); HRMS m/z (ESI)
Calcd for C20H16NS ([M+H]+) 302.0998; found 302.0993.
2-(2-Methoxyphenyl)-3-(phenylthio)-1H-indole (4):16 Yellow oil.
1H NMR (300 MHz, CDCl3) δ: 9.44 (brs, 1H), 7.97 (d, J = 7.9 Hz,
1H), 7.66 (d, J = 7.9 Hz, 1H), 7.48 (d, J = 7.9 Hz, 1H), 7.38–7.27 (m,
2H), 7.20–7.12 (m, 5H), 7.06–7.04 (m, 3H), 3.91 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ 156.7, 139.5, 139.4, 135.4, 131.8, 130.4, 130.0,
128.7, 125.5, 124.4, 123.0, 121.1, 120.7, 119.7, 111.5, 111.1, 99.3,
Scheme 2 Possible mechanism.

646 JOURNAL OF CHEMICAL RESEARCH 2012
55.8; LRMS (EI, 70 eV) m/z (%): 331 (M+, 100); HRMS m/z (ESI)
Calcd for C21H18ONS ([M+H]+) 332.1104; found 332.1109.
2-(2-Bromophenyl)-3-(phenylthio)-1H-indole (5):19 Pale yellow oil.
1H NMR (300 MHz, CDCl3) δ 8.68 (brs, 1H), 7.72–7.66 (m, 2H),
7.49–7.47 (m, 2H), 7.36–7.31 (m, 3H), 7.22–7.14 (m, 6H); 13C NMR
(75 MHz, CDCl3) δ 141.3, 138.8, 135.7, 133.22, 133.16, 132.4, 130.5,
129.6, 128.7, 127.2, 125.8, 124.6, 123.5, 123.4, 121.1, 120.0, 111.4,
101.6; LRMS (EI, 70 eV) m/z (%): 381 (M++2, 30), 379 (M+, 31), 300
(100); HRMS m/z (ESI) Calcd for C20H15BrNS ([M+H]+) 382.0083;
found 382.0085.
128.8, 128.1, 126.8, 123.5, 121.3, 119.8, 111.3, 98.9; LRMS (EI,
70 eV) m/z (%): 337 (M++2, 43), 335 (M+, 100); HRMS m/z (ESI)
Calcd for C20H15ClNS ([M+H]+) 336.0608; found 336.0602.
3-(4-Nitrophenylthio)-2-phenyl-1H-indole (14):19 Yellow solid,
1
m.p. 155.9–157.5 °C (uncorrected) (lit.19 155.0–157.1 °C). H NMR
(300 MHz, CDCl3) δ: 8.81 (brs, 1H), 8.01 (d, J = 8.9 Hz, 2H), 7.71
(d, J = 8.2 Hz, 2H), 7.56–7.43 (m, 5H), 7.34–7.22 (m, 2H), 7.16 (d,
J = 8.9 Hz, 2H); 13C NMR (75 MHz, CDCl3) δ 149.8, 144.9, 142.7,
135.9, 130.9, 130.4, 129.1, 128.9, 128.1, 125.1, 124.0, 123.8, 121.6,
119.4, 111.6; LRMS (EI, 70 eV) m/z (%): 346 (M+, 100); HRMS m/z
(ESI) Calcd for C20H15N2O2S ([M+H]+) 347.0849; found 347.0847.
2-Phenyl-3-(pyridin-2-ylthio)-1H-indole (15): White solid, m.p.
184.5–186.0 °C (lit.19 185.4–185.7 °C) (uncorrected). 1H NMR
(300 MHz, CDCl3) δ 9.32 (s, 1H), 8.40–8.38 (m, 1H), 7.79 (d, J =
8.1 Hz, 2H), 7.65–7.63 (m, 1H), 7.47–7.18 (m, 7H), 6.95–6.91 (m,
1H), 6.97 (d, J = 7.9 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 162.7,
149.3, 142.3, 136.8, 136.1, 131.3, 130.8, 128.7, 128.7, 128.2, 123.4,
121.2, 119.71, 119.7, 119.3, 111.5, 97.8; LRMS (EI, 70 eV) m/z (%):
302 (M+, 100); HRMS m/z (ESI) Calcd for C19H15N2S ([M+H]+)
303.0950; found 303.0959.
1
2-Hexyl-3-(phenylthio)-1H-indole (6):19 Pale yellow oil. H NMR
(300 MHz, CDCl3) δ 8.26 (brs, 1H), 7.58 (d, J = 7.8 Hz, 1H), 7.39
(d, J = 7.8 Hz, 1H), 7.28–7.14 (m, 4H), 7.09–7.04 (m, 3H), 2.94 (t,
J = 7.6 Hz, 2H), 1.71–1.61 (m, 2H), 1.34–1.29 (m, 6H), 0.89 (q, J =
6.8 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 145.5, 139.5, 135.4, 130.3,
128.6, 125.4, 124.4, 122.1, 120.6, 119.0, 110.7, 98.9, 31.5, 29.7, 29.5,
28.9, 26.4, 22.5; LRMS (EI, 70 eV) m/z (%): 309 (M+, 100); HRMS
m/z (ESI) Calcd for C20H24NS ([M+H]+) 310.1624; found 310.1627.
2-tert-Butyl-3-(phenylthio)-1H-indole (7): Pale yellow solid,
1
m.p. 114.5–115.5 °C (uncorrected) (lit.19 115.0–115.6 °C). H NMR
3-(Methylthio)-2-phenyl-1H-indole (16):21 Pale yellow oil. 1H NMR
(300 MHz, CDCl3) δ 8.34 (brs, 1H), 7.86 (d, J = 8.5 Hz, 2H), 7.69–
7.67 (m, 1H), 7.47–7.40 (m, 2H), 7.53–7.50 (m, 2H), 7.30–7.27 (m,
2H), 2.34 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 139.7, 135.6, 131.9,
128.6, 128.3, 127.7, 125.1, 123.0, 120.6, 120.2, 110.9, 99.9, 19.7;
LRMS (EI, 70 eV) m/z (%): 239 (M+, 100); HRMS m/z (ESI) Calcd
for C15H14NS ([M+H]+) 240.0841; found 240.0847.
(300 MHz, CDCl3) δ 8.39 (brs, 1H), 7.57 (d, J = 7.9 Hz, 1H), 7.43 (d,
J = 7.9 Hz, 1H), 7.27–7.14 (m, 4H), 7.09–7.06 (m, 3H), 1.60 (s, 9H);
13C NMR (75 MHz, CDCl3) δ 151.4, 139.8, 133.9, 131.5, 128.5, 125.1,
124.1, 122.1, 120.7, 118.8, 96.7, 33.4, 29.8; LRMS (EI, 70 eV) m/z
(%): 281 (M+, 100); HRMS m/z (ESI) Calcd for C18H20NS ([M+H]+)
282.1311; found 282.1317.
1-(4-(3-(Phenylthio)-1H-indol-2-yl) phenyl)ethanone (8): Yellow
1
solid, m.p. 158.0–159.6 °C (uncorrected) (lit.19 158.2–160.3 °C). H
NMR (300 MHz, DMSO-D6) δ 12.25 (brs, 1H), 8.06 (d, J = 9.4 Hz,
2H), 8.02 (d, J = 9.4 Hz, 2H), 7.54 (d, J = 8.0 Hz, 1H), 7.46 (d, J =
8.0 Hz, 1H), 7.13–7.04 (m, 5H), 7.01–6.98 (m, 2H), 2.59 (s, 3H);
13C NMR (75 MHz, DMSO-D6) δ: 197.9, 141.0, 138.9, 136.9, 136.6,
135.8, 131.0, 129.6, 128.9, 128.5, 125.5, 125.4, 123.8, 121.2, 119.3,
112.7, 98.6, 27.2; LRMS (EI, 70 eV) m/z (%): 343 (M+, 100); HRMS
m/z (ESI) Calcd for C22H18NOS ([M+H]+) 344.1104; found 344.1108.
5-Methyl-2-phenyl-3-(phenylthio)-1H-indole (9): Pale yellow solid,
We thank the Hunan Provincial Natural Science Foundation of
China (No. 11JJ3019) for financial support.
Received 22 July 2012; accepted 22 August 2012
Paper 1201423 doi: 10.3184/174751912X13467835639963
Published Online: 12 November 2012
1
m.p. 117.0–118.6 °C (uncorrected) (lit.19 117.5–118.2 °C). H NMR
References
(300 MHz, CDCl3) δ 8.50 (brs, 1H), 7.76 (d, J = 8.3 Hz, 2H), 7.47–
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(300 MHz, CDCl3) δ 8.54 (brs, 1H), 7.79 (d, J = 8.2 Hz, 2H), 7.67
(d, J = 7.9 Hz, 1H), 7.47–7.42 (m, 4H), 7.30–7.20 (m, 2H), 7.05 (d,
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128.7, 128.6, 128.1, 125.7, 123.3, 121.1, 120.0, 111.1, 99.9, 20.9;
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3-(4-Methoxyphenylthio)-2-phenyl-1H-indole (12):20 Pale yellow
oil. 1H NMR (300 MHz, CDCl3) δ 8.52 (brs, 1H), 7.81–7.78 (m, 2H),
7.69 (d, J = 7.8 Hz, 1H), 7.49–7.40 (m, 4H), 7.31–7.20 (m, 2H), 7.10
(d, J = 8.5 Hz, 2H), 6.75 (d, J = 8.5 Hz, 2H), 3.74 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ 157.5, 141.5, 135.8, 131.5, 131.1, 129.8, 128.7,
128.6, 128.1, 127.7, 123.2, 121.0, 119.9, 114.6, 111.1, 100.8, 55.3;
LRMS (EI, 70 eV) m/z (%): 331 (M+, 100); HRMS m/z (ESI) Calcd
for C21H18NOS ([M+H]+) 332.1104; found 332.1106.
3-(4-Chlorophenylthio)-2-phenyl-1H-indole (13):18–20 Yellow oil.
1H NMR (300 MHz, CDCl3) δ 8.60 (brs, 1H), 7.71 (d, J = 8.2 Hz, 2H),
7.61 (d, J = 7.8 Hz, 1H), 7.45–7.40 (m, 4H), 7.28–7.18 (m, 2H), 7.11
(d, J = 8.5 Hz, 2H), 7.00 (d, J = 8.5 Hz, 2H); 13C NMR (75 MHz,
CDCl3) δ: 142.2, 137.9, 135.9, 131.2, 130.9, 130.4, 129.3, 128.9,

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