Catalytic Thiourea Promoted Electrophilic Thiocyanation of Indoles and Aromatic Amines with NCS/NH4SCN

Article abstract of DOI:10.1002/cjoc.201600344

A simple and efficient protocol for the electrophilic thiocyanation of indoles and aromatic amines with thiourea/NCS/NH₄SCN system has been developed. The major features of the present procedure are the mild conditions, good yields, short reaction times, and the use of inexpensive and readily available organocatalyst. Moreover, N-chlorosuccinimide (NCS) was found to be indispensable, and thiourea could greatly promote the reaction.

Full text of DOI:10.1002/cjoc.201600344

COMMUNICATION
DOI: 10.1002/cjoc.201600344
Catalytic Thiourea Promoted Electrophilic Thiocyanation of
Indoles and Aromatic Amines with NCS/NH4SCN
Cancan Wang, Zhonghao Wang, Liang Wang,* Qun Chen, and Mingyang He*
School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China
A simple and efficient protocol for the electrophilic thiocyanation of indoles and aromatic amines with thiou-
rea/NCS/NH4SCN system has been developed. The major features of the present procedure are the mild conditions,
good yields, short reaction times, and the use of inexpensive and readily available organocatalyst. Moreover,
N-chlorosuccinimide (NCS) was found to be indispensable, and thiourea could greatly promote the reaction.
Keywords thiocyanation, indoles, aromatic amines, catalytic thiourea, N-chlorosuccinimide
Introduction
strong oxidizing agents, and the use of metal catalysts.
Recently, we are devoted to developing milder and
more convenient procedures for the electrophilic thio-
cyanation reactions. Conventionally, these procedures
involved the use of rather unstable (SCN)2 or XSCN (X
＝halogen) which required the use of the molecular
halogen as coreagent. N-Thiocyanatosuccinimide, gen-
erated in situ from N-halosuccinimide and an inorganic
thiocyanate, was also employed as an electrophilic rea-
gent in the aromatic thiocyanation[17,18] and ring-expan-
sion substitution reactions of 1,3-dithiolanes and 1,3-di-
thianes.[19] To the best of our knowledge, only limited
substrates were investigated for the aromatic thiocya-
nation using N-thiocyanatosuccinimide. Meanwhile,
N-chlorosuccinimide (NCS) was found to show much
lower reactivity towards the aromatic thiocyanation
which was due to its strong N—Cl bond. Herein, we
wish to report an effective and versatile electrophilic
thiocyanation of indoles and aromatic amines with
cheap and readily available NCS/NH4SCN combination
using catalytic thiourea as activator (Scheme 1).
Organic thiocyanates are important compounds as
thiocyano groups widely existing in natural products
and pharmaceutical ingredients such as 9-thiocyanato
pupukeanane, Psammaplin B and Cefamandol.[1,2] They
also serve as synthetic intermediates or pseudohalides to
access sulfur-containing compounds.[3-14] Thus, the
introduction of a thiocyano group into a molecule is of
significant importance. Over the past decades, tremen-
dous efforts have been made in direct oxidative thiocy-
anation of (hetero)aromatic compounds including
indoles, aromatic amines as well as other hetero-
cycles.[15] Thiocyanation and thiocyanooxygenation of
functionalized alkenes were also realized.[16] Notably,
these procedures utilized cheap and broadly available
thiocyanate salts as the thiocyanation reagent. While
acknowledging the pioneering work in this field, the
aforementioned methods still suffered some disad-
vantages including the long reaction times, unsatisfac-
tory yields, the use of a large excess of expensive or
Scheme 1 Thiourea-promoted thiocyanation of indoles and aromatic amines
*
E-mail: lwcczu@126.com; Tel.: 0086-0519-86330263; Fax: 0086-0519-86330251
Received June 3, 2016; accepted August 1, 2016; published online XXXX, 2016.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600344 or from the author.
Chin. J. Chem. 2016, XX, 1—5
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Wang et al.
COMMUNICATION
Experimental
indoles using 1a as the model substrate (Table 1). As
shown from Table 1, the choice of solvents is vital for
this transformation. Reaction in acetonitrile, tetrahy-
drofuran (THF, Entry 2), dimethyl sulfoxide (DMSO,
Entry 5) and dioxane (Entry 6) gave the desired product
2a in low to moderate yields. Only trace amount of 2a
was obtained in dichloromethane (DCM, Entry 3) and
N,N-dimethylformamide (DMF, Entry 4). Switching the
solvent to methanol afforded the desired product in 71%
yield after 12 h (Entry 8). Notably, only trace amount of
chlorinated indole was observed as the by-product,
which might be attributed to the fact that the reaction
between NCS and NH4SCN to form N-thiocyanato-
succinimide is much faster than the chlorination of in-
dole.[6,8] Pleasingly, addition of catalytic amount of thio-
urea significantly accelerated the reaction, and almost
quantitative yield was obtained within 30 min (Entry 9).
Nevertheless, replacing thiourea with urea delivered the
corresponding product in 73% yield after 12 h. Different
amounts of NCS and thiourea were also investigated. It
was worth noting that increasing the amount of NCS led
to the formation of chlorinated indole by-product, and
meanwhile, 5 mol% of thiourea is enough to promote
this reaction (Entries 10－14). Finally, a comparable
Chemicals were used as received without special pu-
rification unless stated otherwise. H and 13C NMR
1
spectra were recorded on a Bruker Advance 400 or 500
analyzer. Melting points (m.p.) are determined with an
Optimelt MPA 100 apparatus and are not corrected.
Mass spectrometry was performed on an LCMS-2010
EV (Shimadzu) instrument with an ESI source. All the
products are known compounds and were identified by
comparing of their physical and spectra data with those
reported in the literature.
General procedure for the thiocyanation of indoles
and aromatic amines
Thiourea (2 mg, 5 mol%) was added to a solution of
NCS (0.6 mmol) and ammonium thiocyanate (1 mmol)
in a solvent (2 mL) at room temperature. The mixture
was stirred for 5 min. Then indole or aromatic amine
(0.5 mmol) was added immediately, and the reaction
mixture was stirred for a certain time. After reaction
(monitored by TLC), the reaction mixture was quenched
with water (10 mL) and extracted with dichloromethane
(10 mL×3). The combined extracts were dried over
anhydrous Na2SO4 and concentrated in vacuo. The
crude product was purified by column chromatography
on silica gel with petroleum ether/ethyl acetate to afford
the pure desired product.
Table 1 Optimization of reaction conditions for thiocyanation
of indolea
Selected characterization data
3-Thiocyanato-1H-indole (2a)[15f] Light yellow
solid, m.p. 71－73 ℃; 1H NMR (400 MHz, DMSO-d6)
δ: 12.03 (s, 1H), 7.99 (d, J＝2.8 Hz, 1H), 7.73－7.65 (m,
1H), 7.56 (dd, J＝6.8, 1.3 Hz, 1H), 7.33－7.20 (m, 2H);
13C NMR (100 MHz, DMSO-d6) δ: 136.4, 133.2, 127.5,
122.9, 121.1, 117.8, 112.9, 112.3, 89.4; MS (ESI) m/z:
175.0 [M＋H]＋.
Entry Solvent NCS/equiv. Thiourea/equiv. Time/h Yieldb/%
1
CH3CN 1.2
THF 1.2
—
12
12
12
12
12
12
12
12
24
2
—
30
3-Methylindoline-2-thione (2k)[20] Light yellow
solid, m.p. 116－118 ℃; 1H NMR (500 MHz, CDCl3) δ:
8.95 (s, 1H), 7.21 (dd, J＝11.7, 4.3 Hz, 2H), 7.07－7.00
(m, 1H), 6.92 (d, J＝7.6 Hz, 1H), 3.47 (q, J＝7.6 Hz,
1H), 1.51 (d, J＝7.7 Hz, 3H); 13C NMR (125 MHz,
CDCl3) δ: 181.5, 141.2, 131.2, 127.9, 123.8, 122.3,
109.7, 41.1, 15.2. MS (ESI) m/z 164.1 [M＋H]＋.
4-Thiocyanatoaniline (4a)[15f] Light yellow solid,
m.p. 54－55 ℃; 1H NMR (400 MHz, CDCl3) δ: 7.34 (d,
J＝8.6 Hz, 2H), 6.65 (d, J＝8.6 Hz, 2H), 3.99 (s, 2H);
13C NMR (100 MHz, CDCl3) δ: 148.8, 134.4, 116.0,
112.4, 109.3. MS (ESI) m/z 151.0 [M＋H]＋.
3
DCM 1.2
DMF 1.2
DMSO 1.2
dioxane 1.2
EtOAc 1.2
MeOH 1.2
MeOH 1.2
MeOH 1.1
MeOH 1.5
MeOH 1.2
MeOH 1.2
MeOH 1.2
MeOH 1.2
MeOH 1.2
—
trace
trace
35
4
—
5
—
6
—
49
7
—
54
8
—
71
9
0.05
0.05
0.05
0.02
0.1
0.2
0.05
0.05
30 min 99 (73)c
10
11
12
13
14
15
16
30 min 91
30 min 84 (10)d
30 min 87
6-Methylbenzo[d]thiazol-2-amine (5a)[21] Light
30 min 99
1
yellow solid, m.p. 134－135 ℃; H NMR (400 MHz,
30 min 99
30 min 97e
30 min 88f
DMSO) δ: 7.42 (s, 1H), 7.35 (s, 2H), 7.21 (d, J＝8.1 Hz,
1H), 7.04－6.97 (m, 1H), 2.30 (s, 3H); 13C NMR (100
MHz, DMSO) δ: 165.7, 150.7, 131.0, 129.9, 126.5,
120.8, 117.4, 20.8. MS (ESI) m/z: 165.0 [M＋H]＋.
a Unless otherwise specified, all reactions were carried out using
1a (0.5 mmol), NH4SCN (1 mmol) and NCS in a solvent (2 mL)
with an additive at room temperature. b Isolated yield. c Urea as
additive for 12 h. d Chlorinated indole was observed as by-product.
e NaSCN (1 mmol) was used. f KSCN (1 mmol) was used.
Results and Discussion
Our initial studies focused on the thiocyanation of
2
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Chin. J. Chem. 2016, XX, 1—8

Electrophilic Thiocyanation of Indoles and Aromatic Amines with NCS/NH4SCN
yield was obtained when NH4SCN was replaced by
NaSCN; however, a slight decrease in the product yield
was observed for KSCN (Entries 15 and 16). Thus, we
chose the less toxic NH4SCN as the thiocyanation rea-
gent.
anion or Cl－ to give compound B, followed by rear-
rangement to provide the compound 2k (Schemes 2 and
4). Moreover, only trace amount of the desired product
was observed for the N-Boc indole.
With the optimized reaction conditions in hand, we
began to examine the scope and generality of the pre-
sent method. As illustrated in Table 2, indole substrates
bearing different functional groups such as chloro, bro-
mo, methoxyl, as well as methyl were well tolerated
under the standard conditions, providing the corre-
sponding products in good to excellent yields with short
reaction times at room temperature. However, substrates
with electron-withdrawing groups showed less activity,
and a prolonged reaction time (ca. 12 h) was thereby
required to obtain a higher yield (2f, 69%; 2g, 45%, re-
spectively). While N-methyl indole and 2-methyl indole
gave good yields of the desired products, an unexpected
product 2k was obtained in 77% yield when using
3-methyl indole as the substrate. This may be because
the thiocyanation reaction takes place at the 2-position
of 3-methyl indole due to the steric hindrance. The
compound A may be not as stable as 3-thiocyanated
indoles, thereby it may be attacked by succinimide
Scheme 2 Plausible pathway for the synthesis of compound 2k
After successfully developed the thiocyanation reac-
tions of indoles, we then explored the thiocyanation re-
actions of aromatic amines. The reaction of aniline 3a
with ammonium thiocyanate under the same conditions
described above afforded the product 4a in 71% yield
after 2 h. After screening the conditions, an 89% yield
of 4a was obtained when using acetonitrile as the sol-
vent. The results for other substituted anilines are sum-
marized in Table 3. As shown from Table 3, anilines
3b－3e reacted smoothly to provide the corresponding
products 4b－4e in good yields. N-Methylaniline 3f also
afforded product 4f in 81% yield. It should be pointed
out that the thiocyanato group is selectively added to the
para position of the amino group. Interestingly, ben-
zo[d]thiazol-2-amines (5a－5c) were obtained as the
major products in moderate to good yields when using
para-substituted anilines as the substrates. It is assumed
that ortho-thiocyanation reaction occurs at the first step,
followed by the addition of amino group to the thiocya-
nato group.[22,23]
Table 2 Reaction scope for indolesa
SCN
SCN
SCN
MeO
Cl
N
N
N
H
H
H
2a, 99%
2b, 95%
SCN
2c, 85%
SCN
SCN
MeOOC
Br
Several control experiments using 1a as an example
were performed to gain insight into the mechanism
(Scheme 3). No reaction occurred in the absence of
NCS, indicating NCS is indispensable for this reaction
(Scheme 3, a). In order to eliminate the possibility that
the products are formed through substitution reactions
of chlorinated indole with SCN－, we prepared the chlo-
rinated indole 6 in the thiourea/NCS reaction system. It
was found that the chlorination process was very quick
(10 min, 99%), while no reaction took place between
substrate 6 and ammonium thiocyanate (Scheme 3, b).
We also prepared N-thiocyanatosuccinimide 7,[24] which
was believed as the key intermediate in the reported
electrophilic thiocyanation processes.[17,19] As expected,
1a went through the thiocyanation with compound 7
smoothly to afford the desired product 2a in 86% yield
within 10 min (Scheme 3, c and d). Finally, reactions of
1a in the presence of radical inhibitor and radical
quencher such as 2,6-di-tert-butyl-4-methylphenol
N
N
N
H
Cl
H
H
2e, 86%
2f, 69%b
2d, 83%
SCN
SCN
SCN
O2N
N
H
N
H
N
H
2g, 45%b
2i, 87%
2h, 91%
SCN
SCN
N
S
N
H
N
Boc
2j, 93%
2k, 77%
2l, trace
a Reaction conditions: 1 (0.5 mmol), thiourea (2 mg, 5 mol%),
NCS (0.6 mmol, 1.2 equiv.), NH4SCN (1 mmol, 2 equiv.), MeOH
(2 mL), room temperature, 30 min, isolated yields. b Reaction for
12 h.
Chin. J. Chem. 2016, XX, 1—8
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3

Wang et al.
COMMUNICATION
Table 3 Reaction scope for aromatic aminesa
On the basis of above experimental results and liter-
ature reports, a plausible mechanism for this thiocya-
nation in the presence of NCS/NH4SCN is proposed in
Scheme 4. Initially, N-thiocyanatosuccinimide 7 is
formed by the reaction of NCS with NH4SCN. Then
regioselective electrophilic attack on the C-3 position of
1a generates intermediate C, which provides the final
product 2a via deprotonation. In these processes, thiou-
rea may activate NCS via either hydrogen bonding
mode (I)[25] or a halogen bond interaction mode (II)[26]
to accelerate the formation of compound 7. The thiourea
may also accelerate the electrophilic thiocyanation pro-
cess via activation of compound 7 to release the SCN
cation. However, there is no direct evidence to confirm
this conclusion currently, and further studies are still
necessary to examine the nature of the thiourea-NCS
complex.
NH2
NH2
NH2
NH2
NH2
NCS
NCS
NCS
OMe
NCS
NCS
4b, 88%
4e, 83%
4a, 89%
4c, 91%
4f, 81%
H
N
NCS
Cl
4d, 82%
Scheme 4 Plausible mechanism
N
N
S
N
S
NH2
NH2
NH2
NCS + NH4SCN
S
MeO
SCN
SCN
5b, 61%
5a, 78%
5c, 70%
-H+
+SCN
a Reaction conditions: 3 (0.5 mmol), thiourea (2 mg, 5 mol%),
NCS (0.6 mmol, 1.2 equiv.), NH4SCN (1 mmol, 2 equiv.),
CH3CN (2 mL), room temperature, 2 h, isolated yields.
N
N
N
H
H
H
1a
2a
C
S
H
H
N
N
Scheme 3 Control experiments
O
δ
Cl
H
H
H
SCN
N
O
S
N
without NCS
H
(a)
O
N
NH4SCN
N
Cl
N
H
N
H
H
H
1a
O
2a, NR
I
II
Cl
(b)
NH4SCN
Conclusions
thiourea
NCS
2a, NR
N
H
N
In conclusion, we have developed a simple and effi-
cient protocol for the electrophilic thiocyanation of in-
doles and aromatic amines with thiourea/NCS/NH4SCN
system. Most of the reactions proceeded smoothly to
afford the corresponding products in good to excellent
yields except those with strong electron-withdrawing
groups. Catalytic amount of thiourea, which may acti-
vate the NCS via hydrogen bonding or a halogen bond
interaction, was found to significantly accelerate the
reaction.
H
1a
6, 99%
O
O
15 min
CH3CN, r.t.
(c)
(d)
N
SCN
+
N
Cl + NH4SCN
NH4Cl
SCN
O
7, 95%
O
O
thiourea
(5 mol%)
N
SCN
+
MeOH, r.t.
N
H
N
H
O
2a
1a
1a
7,1.2 equiv.
10 min, 86%
Acknowledgement
SCN
This project was financially supported by the Na-
tional Natural Science Foundation of China (No.
21302014).
Standard conditions
(e)
with 1 equiv. of BHT: 83%
N
H
N
H
with 1 equiv. of TEMPO: 90%
2a
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(BHT) and 2,2,6,6-tetramethylpiperidine N-oxide
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sulted in a slight decrease in yield of 2a (Scheme 3, e).
Thus, a radical pathway of the reaction may be ruled
out.
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