Heterogeneous and catalytic thiocyanation of aromatic compounds in aqueous media

Article abstract of DOI:10.1080/10426507.2011.610846

This article describes a highly efficient heterogeneous catalytic thiocyanation of indoles, substituted anilines (electron rich and electron deficient) and N-substituted aromatic amines in water under mild and green conditions. The reaction of substrates

Full text of DOI:10.1080/10426507.2011.610846

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Heterogeneous and Catalytic
Thiocyanation of Aromatic Compounds in
Aqueous Media
Mohammad Ali Zolfigol a , Ardeshir Khazaei a , Mohammad Mokhlesi a
, Hoshang Vahedi b , Sami Sajadifar a b & Mahtab Pirveysian a
a Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran
b Chemistry Department, Payame Noor University, 19395-4697,
Tehran, I. R. of Iran
Version of record first published: 31 Jan 2012.
To cite this article: Mohammad Ali Zolfigol , Ardeshir Khazaei , Mohammad Mokhlesi , Hoshang
Vahedi , Sami Sajadifar & Mahtab Pirveysian (2012): Heterogeneous and Catalytic Thiocyanation of
Aromatic Compounds in Aqueous Media, Phosphorus, Sulfur, and Silicon and the Related Elements,
187:3, 295-304
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Phosphorus, Sulfur, and Silicon, 187:295–304, 2012
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.610846
HETEROGENEOUS AND CATALYTIC THIOCYANATION
OF AROMATIC COMPOUNDS IN AQUEOUS MEDIA
Mohammad Ali Zolfigol,1 Ardeshir Khazaei,1 Mohammad
Mokhlesi,1 Hoshang Vahedi,2 Sami Sajadifar,1,2
and Mahtab Pirveysian1
1Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran
2Chemistry Department, Payame Noor University, 19395-4697 Tehran,
I. R. of Iran
GRAPHICAL ABSTRACT
SCN
Catalyst/ Oxidant
KSCN/ H2O/ rt
N
N
H
H
7-10 min
OSO3H
90-95%
SiO2
SiO2 -B(OSO3H)3
B: SBSA/H2O2
A: SSA/UHP
Abstract This article describes a highly efficient heterogeneous catalytic thiocyanation of
indoles, substituted anilines (electron rich and electron deficient) and N-substituted aromatic
amines in water under mild and green conditions. The reaction of substrates with KSCN in
the presence of silica sulfuric acid (SSA) and silica boron sulfonic acid (SBSA) as heteroge-
neous catalysts and H2O2 or UHP (urea hydrogen peroxide) as green oxidants produces aryl
thiocyanates in short reaction times and high yields. Moreover, in this article, some novel aryl
thiocyanates were synthesized.
Keywords Silica sulfuric acid; silica boron sulfonic acid; thiocyanation; water; heterogeneous
INTRODUCTION
One of the useful reactions for the introduction of sulfur into aromatic systems
is thiocyanation. Thiocyanate is a versatile synthon that can be readily transferred to
Received 6 May 2011; accepted 1 August 2011.
The authors gratefully acknowledge partial support of this work by the Research Affairs Office of Bu-
Ali Sina University, Hamedan (Grant number 32-1716 entitled development of chemical methods, reagent, and
molecules.), Iranian National Elites and Center of Excellence in Development of Chemical Method (CEDCM)
Hamedan, Iran.
Address correspondence to Mohammad Ali Zolfigol, Faculty of Chemistry, Bu-Ali Sina University, P.O.
Box 6517838683, Hamedan, Iran. E-mail: zolfi@basu.ac.ir
295

296
M. A. ZOLFIGOL ET AL.
other functional groups such as sulfide,1 aryl nitrile,2 thiocarbamate,3 and thionitrile.4
Therefore, it is important to find new, safe, fast, and useful methods for the synthesis of aryl
thiocyanates.5–11 Various methods have been reported for the thiocyanation of aromatic and
heteroaromatic systems such as, N-thiocyanatosuccinimide,12 iodine/methanol,13 diethyl
azodicarboxylate,14 IL-OPPh2,15 I2O5,16 pentavalent iodine,17 IBX,18 and SelectfluorTM 19
.
However, most of the reported methods for the synthesis of aryl thiocyanates are associated
with one or more of the following drawbacks: (1) low yields, (2) long reaction times,
(3) the use of a large amount of catalyst, (4) the use of toxic or expensive catalysts, (5) a
tedious work-up procedure, and (6) performances under certain special conditions. Thus,
finding an efficient, inexpensive, and nonpolluting method for the synthesis of this class of
compounds is still of practical importance.
In recent years, the application of eco-friendly applicable industrial and green cata-
lysts has received considerable interest.20 Thus, green chemistry has been defined as a set
of principles that reduces or eliminates the use or generation of hazardous substances and
catalysts throughout the entire life of chemical materials. Along this line, using heteroge-
neous catalysts as green, inexpensive and nontoxic catalysts, have found more attention.
They have been used as an efficient acidic catalyst in organic synthesis. For example,
they have efficiently catalyzed oxidation of sulfides,21 thioacetalization of carbonyl com-
pounds,22 von Pechmann Reaction,23 thioacetalization of carbonyl compounds,24 synthesis
of benzimidazoles,25 synthesis of 3,4-dihydropyryimidinones,26 α-bromination of carbonyl
compouns,27 and acetalization of carbonyl compounds.28
According to our previously reported results on the applications of heterogeneous
catalysts such as silica sulfuric acid (SSA) in organic reactions,29,30 now, we wish to report
an efficient and convenient procedure for the thiocyanation of aromatic compounds using
silica supported catalysts in conjunction with UHP (urea hydrogen peroxide) or hydrogen
peroxide under green conditions (Scheme 1).
Scheme 1
RESULT AND DISCUSSION
For the synthesis of aryl thiocyanate, conversion of indole into its corresponding
indole thiocyanate has been considered as a model reaction. At first, the reaction was exam-
ined in the presence of some different catalysts such as SiO2, p-Tol-SO3H, SSA, and silica
boron sulfonic acid (SBSA) at room temperature. The results are summarized in Table 1.
As Table 1 indicates, in the absence of a catalyst, when indole was treated with hydrogen
peroxide and KSCN, no product was obtained. If SiO2 was directly used as catalyst, the

HETEROGENEOUS AND CATALYTIC THIOCYANATION OF AROMATIC COMPOUNDS 297
Table 1 The thiocyanation of indole using different catalysts
Entry
Catalyst
Time (min)
Yield (%)a
>1
2
3
4
5
6
7
8
9
—
SiO2
180
180
180
180
180
100
180
30
No reaction
b
15
30
50
60
75
40
92
95
89
90
4-Me-Ph-SO3H (5%)
4-Me-Ph-SO3H (10%)
4-Me-Ph-SO3H (15%)
4-Me-Ph-SO3H (20)
SSA (0.05 g)
SSA (0.1 g)
SSA (0.2g)
SBSA (0.01 g)
SBSA (0.05 g)
7
90
10
10
11
aIsolated yields. bSiO2 (50–60 mesh).
yield of product was 15% (entry 2). The use of p-tol-SO3H was also examined whereby
a moderate yield of product was obtained but it needs a long reaction time (entries 3–6).
Then, we examined heterogeneous catalysts, and as shown in Table 1, higher yields and
shorter reaction times were obtained when SSA and SBSA were utilized as catalysts (Table
1, entries 7–11). The optimal amounts of the catalysts were 10 mol% (0.2 g) and 5 mol%
(0.05 g) respectively.
In the next step, we examined the effect of solvents on the reaction. Water, methanol,
ethanol, and acetonitrile were used as solvents and surprisingly the same results were
obtained (Table 2, entries 1–4). In other organic solvents reaction were proceeded in longer
reaction time (Table 2, entries 5–7). Therefore, water was chosen as the preferred solvent.
In continuation, a wide range of aromatic and hetero-aromatic compounds were used
for the thiocyanation. The reaction was mild and clean for hetero aromatics, electron rich,
and deficient anilines and N-substituted amines. High regioselectivity, excellent yields
and short reaction times were obtained in all cases (Table 3). As shown in Table 3, in two
methods, indole and its derivatives as electron rich heterocycles, are produced as the product
with good to excellent yields (Table 3, 1a–1d). Also, 5-bromoindole reacted with KSCN to
afford the corresponding 5-bromo-3- thiocyanato indole in good yields, but required longer
Table 2 Optimization of solvent for thiocyanation reaction
Time (min)
Yield (%)a
Entry
Solvent
A
B
A
B
1
2
3
4
5
6
7
H2O
MeOH
EtOH
CH3CN
CH2Cl2
CHCl3
EtOAc
7
10
<5
<5
< 5
90
95
91
91
95
55
45
75
90
94
89
93
45
30
50
<5
<5
< 5
90
90
90
90
90
aIsolated yields.

298
M. A. ZOLFIGOL ET AL.
Table 3 Thiocyanation of heterocycle compounds in aqueous media
SSA/UHp
SBSA/H2O2
Melting point
Found (lit)
Product
Time (min)
Yield (%)a
Time (min)
Yield (%)a
7
95
10
20
10
90
(70–72)
(71–73)8
50
15
84
85
80
87
(97–99)
(99–101)6
(79–81)
(83–84)6
45
78
25
87
113–115)
(127–129)6
40
15
88
90
10
45
91
92
liquid
(liquid)6
(62–64)
(−)b
10
45
94
89
40
45
89
83
(70–71)
(−)b
(191–194)
(−)b
40
86
50
90
(69–71)
(71–73)8

HETEROGENEOUS AND CATALYTIC THIOCYANATION OF AROMATIC COMPOUNDS 299
Table 3 Thiocyanation of heterocycle compounds in aqueous media (Continued)
SSA/UHp
SBSA/H2O2
Melting point
Found (lit)
Product
Time (min)
Yield (%)a
Time (min)
Yield (%)a
35
55
90
55
60
85
(58–60)
(60–61)6
88
82
(58–60)
(−)b
75
70
40
79
71
91
3.5 (h)
4.5 (h)
60
71
76
89
(97–99)
(−)b
(148–150)
(−)b
(74–76)
(−)b
10
95
60
85
(77–80)
(−)b
45
83
4 (h)
79
(66–69)
(−)b
aIsolated yields. bThis compound is new.

300
M. A. ZOLFIGOL ET AL.
reaction times (Table 3, 1d). This observation can be attributed to the lower electron density
of such substrates.
Thiocyanation of pyrrole was also investigated. In two methods, pyrrole was produced
the 2-thiocyanato pyrrole in short reaction time and high efficiency (Table 3, 1e).
After our investigation of the thiocyanation reaction of heterocycles, we examined
the ability of aromatic amines in the thiocyanation reaction (Scheme 2). In all cases, the
HO
OH
O
O
HO
OH
N
N
SSA/UHP/KSCN
H2O/r.t
N
+
SCN
100%
0%
Scheme 2
substitution was highly regioselective at the para-position of the aromatic ring. The reaction
of potassium thiocyanate with electron rich amines such as o-toluidine, m-toluidine, and
1-amino naphthalene, occurred under the same conditions (Table 3, entries 1f–1h). Our
studies showed that electron deficient amines are also converted in this reaction, but they
need more time (Table 3, 1i–1m). In accord, on reaction times, we investigated the effect
of halogens and inductive and resonance withdrawing groups. As shown in Table 3, in the
presence of halogens groups such as Cl and Br, the reactions took place at longer time but
in good yields (Table 3, 1i, 1j). Similarly, the effect of inductive and resonance withdrawing
groups such as CF3 and CN was studied. In both, the reaction times were longer but the
yields are as acceptable as other substrates (Table 3, 1k–1m). In the case of N-substituted
amines, when N-phenyl morpholine and N-phenyl-15-crown-5 were used as the substrate,
the products were produced in a short reaction time and excellent yields.
Oxidizable functional groups such as alcoholic hydroxyl group remain intact under the
reaction conditions (Scheme 2). For example, N-phenyl-2, 2-iminodiethanol, in the presence
of KSCN, UHP and SSA, was converted to 4-thiocyanato N-phenyl-2, 2-iminodiethanol
and oxidation of the hydroxyl to aldehyde did not occur. The above mentioned fact was
proven by spectroscopy techniques. In its infrared spectrum, the presence of a sharp and
single peak at 2153 cm−1 confirmed the presence of the SCN group.
Furthermore, a broad peak related to the OH group exists in 3300 cm−1. Around
1650–1800 cm−1, the C O group peak was not observed.
A proposed reaction mechanism is shown in Scheme 3. In the first step, H2O2 reacts
with H+ and generates the hydrogen peroxonium ion (H3O2+) in situ.31 Subsequently,
reaction of this cation with SCN− generates HOSCN,32 which in the presence of H+ is
able to produce thiocyanium ion (S-CN)+ and H2O. In the last step, the thiocyanium ion
(S-CN)+ attacks indole and produces the corresponding indole thiocyanate.
Finally, we compared our results with other reported methods using other reagents
that require refluxing conditions, ultrasonic irradiation, toxic solvent, or oxidant. As shown
in Table 4, the described methods in this article occurred under mild and green reaction
conditions.

HETEROGENEOUS AND CATALYTIC THIOCYANATION OF AROMATIC COMPOUNDS 301
H2O
SCN
H
HO SCN
O
H
O
H
H
H2O
H
S-CN
H2O2
SCN
N
H
N
H
Scheme 3
In conclusion, we have developed an efficient, simple, and green mediated thiocya-
nation of aromatic compounds with high regioselectivity. The described methods have
advantages such as simple work-up, short reaction time, metal free, mild reaction condi-
tions, and clean production of the desired products in high yields.
EXPERIMENTAL
Preparation of Silica Boron Sulfonic Acid
A 50 mL suction flask was equipped with a constant pressure dropping funnel. The
gas outlet was connected to a vacuum system through an adsorbing solution (water) and
an alkali trap. Boric acid (1.55 g, 25 mmol) was charged in the flask and chlorosulfonic
Table 4 Thiocyanation of indole in various systems
Entry
System
SSA/UHP
SBSA/H2O2
I2/MeOH
DDQ
Time (min)
Yield% [Ref.]
1
2
3
4
5
6
7
8
7
10
50
50
45
2 (h)
43
2 (h)
95 (This work)
90 (This work)
85 [13]
94 [5]
85 [14]
83 [14]
98 [8]
85 [6]
DEAD
Mn(OAc)3
Oxone
m.K10clay, 8 0 ◦C

302
M. A. ZOLFIGOL ET AL.
acid (8.74 g, ca. 5 mL, 75 mmol) in CH2Cl2 (5 mL) was added dropwise over a period
of 1 h at room temperature under N2 gas. HCl evolved immediately. After completion of
the addition, the mixture was shaken for 85 min, while the residual HCl was eliminated
by suction.33 Then, the mixture was washed with diethyl ether to remove the unreacted
chlorosulfonic acid and then SiO2 was added (14.4 g) and mixed. Finally, dried and a
grayish solid material was obtained in 95.6% yield (21.6 g).
General Procedure for the Preparation of Aryl Thiocyanate Using SBSA
and H2O2
A suspension of substrate (1 mmol), potassium thiocyanate (0.228 g, 3 mmol) and
SBSA (0.05g) in H2O (5–7 mL) was stirred at room temperature for 10 min. Then, a
solution of H2O2 (30%, 0.282 g, 3 mmol) was added dropwise (2–5 min). The progress of
the reaction was monitored by TLC. After completion of the reaction, the reaction mixture
was extracted with CHCl3 (2 × 10 mL). Anhydrous Na2SO4 (2 g) was added to the organic
layer and filtered off after 20 min. The chloroform was removed under vacuum. The crude
product was adsorbed onto silica and purified by short column chromatography on silica
gel using n-hexane/EtOAc to afford pure product.
General Procedure for the Preparation of Aryl Thiocyanate Using SSA
and UHP
A suspension of substrate (1 mmol), potassium thiocyanate (0.291 g, 3 mmol) and
SSA (0.2g) in H2O (5–7 mL) was stirred at room temperature for 10 min. Then, the solution
of UHP (0.282 g, 3 mmol in 3–5 mL H2O) was added dropwise (2–5 min). The progress
of the reaction was monitored by TLC. After completion of the reaction, the reaction
mixture was extracted with CHCl3 (2 × 10 mL). Anhydrous Na2SO4 (2 g) was added to
the organic layer and filtered off after 20 min. CHCl3 was removed and further purification
was achieved by silica gel short column chromatography hexane/EtOAc.
Note: compounds 1f, 1h, 1k, 1l, 1m, 1n, 1o, and 1p are previously unreported.
SELECTED SPECTRAL DATA
2-Methyl-4-thiocyanato Aniline (1f)
IR (KBr): 2153, 3369, 3456 cm−1, 1H NMR (CDCl3): δ 6.64–7.37 (m, 3H), 3.99 (s,
br, 2H, NH2), 2.12 (s, 3H). 13C NMR (CDCl3): δ 147.3, 135.1, 132.1, 123.9, 115.8, 112.8,
108.8, 17.2. MS: m/z = 164 (M+).
3-Methyl-4-thiocyanato Aniline (1g)
IR (KBr): 2146, 3219, 3337 cm−1, 1H NMR (CDCl3): δ 6.64–7.37 (m, 3H), 3.98 (s,
br, 2H, NH2), 2.34 (s, 3H). 13C NMR (CDCl3): δ 149.7,143, 136.3, 117.3, 112.4, 108.4,
21.2. MS: m/z = 164 (M+).

HETEROGENEOUS AND CATALYTIC THIOCYANATION OF AROMATIC COMPOUNDS 303
4-Thiocyanato-1-naphthyl Amine (1h)
IR (KBr): 2146, 3358, 3442 cm−1, 1H NMR (CDCl3): δ 6.71–7.85 (m, 6H), 4.58 (s,
br, 2H). 13C NMR (CDCl3): δ 136.2, 128.2, 125.8, 125.6, 121.6, 108.9. MS: m/z = 200
(M+).
2-Cyano-4-thiocyanato aniline (1k)
1
IR (KBr): 2156, 2222, 3360 cm−1, 3444, H NMR (CDCl3): δ 7.28–7.58 (m, 2H),
6.83 (s, 1H), 4.96 (s, br, 2H, NH2). 13C NMR (CDCl3): δ 151.7, 138.4, 137.2, 117.8, 116,
111.3, 109.3, 96.7. MS: m/z = 175 (M+).
3-Triflouro methyl-4-thiocyanato Aniline (1l)
IR (KBr): 2160, 3275, 3369 cm−1, 1H NMR (CDCl3): δ 7.33–7.82 (m, 3H), 4.01 (s,
br, 2H, NH2). 13C NMR (CDCl3): δ 144.6, 135.3, 125.5, 118.8, 117.4, 115.1, 111.7. MS:
m/z = 218 (M+).
2-Triflouro methyl-4-thiocyanato Aniline (1m)
IR (KBr): 2158, 3395, 3435 cm−1, 1H NMR (CDCl3): δ 7.28–7.68 (m, 2H), 6.809 (s,
1H), 4.574 (s, br, 2H, NH2). 13C NMR (CDCl3): δ 146.7, 137.5, 131.9, 125.6, 114.3, 111.6,
109.2. MS: m/z = 218 (M+).
4-Thiocyanato N-phenyl Morpholine (1n)
IR (KBr): 2156 cm−1, 1H NMR (CDCl3): δ 7.48 (d, 2H, J = 8.29 Hz), 6.92 (d, 2H,
J = 8.42 Hz), 3.88 (t, 4H, J = 4.69 Hz), 3.24–3.21 (t, 4H, J = 4.83 Hz). 13C NMR (CDCl3):
δ 152.5, 133.7, 116.1, 111.9, 111.1, 67.1, 48.1. MS: m/z = 200 (M+).
4-Thiocyanato N-phenyl 15-crown-5 (1o)
1
IR (KBr): 2147, 1094 cm−1, H NMR (CDCl3): δ 7.35 (d, 2H, J = 5.5 Hz), 6.64
(d, 2H, J = 6 Hz), 3.57–3.72 (m, 20H). 13C NMR (CDCl3): δ 149.3, 134.7, 112.8, 105.9,
112.6, 71.2, 70.2, 69.9, 68.0, 52.6. MS: m/z = 352 (M+).
4-Thiocyanato N-phenyl-2,2-iminodiethanol (1p)
IR (KBr): 2153, 3300 cm−1, 1H NMR (CDCl3): δ 7.43 (d, 2H, J = 6.5 Hz), 6.70 (d,
2H, J = 7.1 Hz), 3.75–3.60 (m, 4H), 3.55–3.35 (m 4H), 2.15 (s, 2H). 13C NMR (CDCl3):
δ 151.8, 132.1, 116.5, 114.3, 111.7, 58.7, 55.9. MS: m/z = 238 (M+).
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