Simple and highly efficient catalytic thiocyanation of aromatic compounds in aqueous media

Article abstract of DOI:10.1002/hlca.201100244

Two simple, mild, and efficient procedures for the thiocyanation of N-heterocycles, substituted anilines (electron-rich and electron-deficient), and N-substituted aromatic amines at room temperature are reported (Table3). The first uses H₂O

Full text of DOI:10.1002/hlca.201100244

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Helvetica Chimica Acta – Vol. 95 (2012)
Simple and Highly Efficient Catalytic Thiocyanation of Aromatic Compounds
in Aqueous Media
by Ardeshir Khazaei*, Mohammad Ali Zolfigol*, Mohmmad Mokhlesi, Fateme Derakhshan Panah, and
Sami Sajjadifar
Faculty of Chemistry, Bu-Ali Sina University, P. O. Box 6517838683, Hamedan, Iran
(e-mail: khazaei_1326@yahoo.com, zolfi@basu.ac.ir)
Two simple, mild, and efficient procedures for the thiocyanation of N-heterocycles, substituted
anilines (electron-rich and electron-deficient), and N-substituted aromatic amines at room temperature
are reported (Table 3). The first uses H2O2 as pollution-free oxidant and the second H5IO6; both with the
reagent potassium thiocyanate in H2O as solvent. These procedures provided the target thiocyanates
after a short reaction time in good to excellent yields and high regioselectivity.
Introduction. – Aryl thiocyanates are useful intermediates in the synthesis of S-
containing heterocycles, in which the thiocyanate group will be readily transformed into
other S-functionalities [1]. One of the most general way for the direct introduction of
an S-atom at aromatic rings is the electrophilic thiocyanation. On the other hand, the
nucleophilic attack of thiocyanate ion at an aromatic nucleus via displacement
reactions is not an easy way to form thiocyanated compounds [2 – 5]. So, several
methods have been developed for the electrophilic thiocyanation of arenes [6][7] such
as bromine/potassium thiocyanate [8], N-thiocyanatosuccinimide [9], an thiocyanate
salt in the presence of ceric ammonium nitrate (CAN) [10], acidic montmorillonite K10
clay [11], I2/MeOH [2], Oxoneꢀ [12], pentavalent iodine, i.e., 2-iodoxybenzoic acid
(IBX) [13], or potassium peroxydisulfate/copper(II) sulfate [14]. However, these
methodologies suffer from one or more drawbacks such as the use of toxic transition
metals as oxidants [1], the toxicity of reagents in general [7][8], large excess of catalyst
[11], and performances under certain special conditions. Therefore, there is a need to
find new, fast, and useful methods with safe and clean oxidants or reagents for the
synthesis of aryl thiocyanates.
In recent years, the use of ꢁgreenꢂ oxidants such as hydrogen peroxide (H2O2) has
become highly attractive for a number of reasons: it is a cheap, mild, and an
environmentally benign oxidant with a high content of active O-atom, and H2O is
formed as only by-product. Hydrogen peroxide was used as an oxidant for a wide range
of organic functional groups such as sulfides and thiols [15], 1,4-dihydropyridines [16]
and alcohols [17], but expensive catalysts were normally required to affect O-atom
transfer from H2O2 to the substrate. Therefore, the use of cheap catalysts beside H2O2 is
a very important topic.
Also, hypervalent iodine compounds such as H5IO6 are employed to oxidize various
organic substrates with or without catalyst. Very recently, oxidation of alcohols with
ꢃ 2012 Verlag Helvetica Chimica Acta AG, Zꢄrich

Helvetica Chimica Acta – Vol. 95 (2012)
107
H5IO6/2,2,6,6-tetramethylpiperidin-1-yl-oxy (TEMPO) [18], H5IO6/FeIII/2-picolinic
acid [19], or H5IO6/CrO3 [20] and oxidation of sulfides to sulfoxides with H5IO6/FeCl3
[21], H5IO6/manganese [22] were also reported. The reagent H5IO6 has several
advantages, such as cost effectiveness, nontoxicity, and an exceedingly simple and clean
workup of products.
We now report the thiocyanation of aromatic and heteroaromatic compounds with
potassium thiocyanate as a thiocyanating agent. For this purpose, we designed and run
the reaction on two general routes: on the one hand with H2O2 as oxidant in
conjunction with HCl as inexpensive catalysts in H2O (Method A), and on the other
hand with H5IO6 as oxidant [23 – 30] in H2O (Method B; Scheme 1).
Scheme 1. Conversion of 1H-Indole to 1H-Indol-3-yl Thiocyanate
Results and Discussion. – The conversion of 1H-indole into 1H-indol-3-yl
thiocyanate at room temperature was considered as a model reaction and used to
study the solvent influence on the rate and yield of the reaction. Thus the conversions
according to Methods A and B with the same concentration of the reactants but in other
solvents then H2O, i.e., in MeOH, EtOH, MeCN, CH2Cl2, CHCl3, and AcOEt
suggested that H2O was the most favorable solvent for the thiocyanation (Table 1).
Table 1. Thiocyanation of 1H-Indole in Various Solvents
Solvent
Time [min]
Yield [%]a)
A
B
A
B
MeOH
EtOH
MeCN
CHCl3
CH2Cl2
H2O
< 5
< 5
< 5
30
45
8
5
91
94
89
83
75
91
82
93
91
90
85
80
90
90
7
10
25
35
10
20
AcOEt
20
a) Yield of isolated 1H-indol-3-yl thiocyanate.
In further experiments, different amounts of catalyst and oxidant were tested in the
model conversion. Thus, in the case of Method A, only 1 – 2 drops of conc. aqueous HCl
solution was needed to complete the reaction. Changing the molar ratio of the oxidant
(Table 2) revealed that generally increasing the amount of H2O2 or H5IO6 shortened
the reaction time and increased the yield, the optimum being reached with 3 mmol-

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Table 2. Optimization of Catalyst and Oxidant for the Thiocyanation of 1H-Indole
Method A
Method B
a
a
H2O2
)
Time [min]
Yield [%]b)
H5IO6
)
Time [min]
Yield [%]b)
1
1.5
2
2.5
3
240
240
90
30
10
25
45
55
80
91
0.25
0.5
0.6
0.75
1
240
90
50
15
15
30
55
79
90
89
a) Amount in mmol per mmol of 1H-indole. b) Yield of isolated 1H-indol-3-yl thiocyanate.
equiv. of H2O2 after 10 min Method A, and with 0.75 mmol-equiv. of H5IO6 after 15 min
(Method B).
Having established the optimal reaction conditions, we treated various N-hetero-
cycles and aromatic amines, including such compounds with electron-releasing and
electron-withdrawing substituents as well as halogen atoms at their aromatic rings, with
potassium thiocyanate in the presence of H2O2/HCl or H5IO6 to afford the
corresponding aryl thiocyanates in high to excellent yields (75 – 95%) within relatively
short reaction times (8 – 120 min; Table 3). The products from 1H-indoles (Entries 1 –
4) showed that the monothiocyanation uniquely occurred at the 3-position of 1H-
indole. However, the thiocyanation of ethyl 1H-indole-2-carboxylate in the presence of
H2O2/HCl gave only 35% yield, this lower yield being probably due to the steric
hindrance of 2-substituted 1H-indoles (Scheme 2). The 1H-pyrrole (Table 3, Entry 5)
was also easily transformed into 1H-pyrrol-2-yl thiocyanate within 10 – 16 min.
Scheme 2. Thiocyanation of Ethyl 1H-Indole-2-carboxylate
Aromatic amines were readily converted to the monothiocyanated products with
high para-selectivity (Table 3, Entries 6 – 20). The influence of electron-releasing
substituents, electron-withdrawing substituents, and halogen atoms at the aromatic ring
of amines was also studied. Thus, electron-donating substituents increased the reaction
rate and gave the products in high yields (Table 3, Entries 9 – 11). When amines with
electron-withdrawing substituents were thiocyanated, the reaction times were longer
and the yields slightly decreased (Table 3, Entries 16 – 20). N-Substituted amines such
as N-methylaniline, N,N-dimethylaniline, N,N-diethylaniline, N-phenylmorpholine, 1-
phenyl-1-aza[15]crown-5, and 2,3-dihydro-1H-indole furnished the corresponding aryl
thiocyanates in short reaction times and good yields (Table 3, Entries 6 – 8 and 12 – 15).

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Finally, we compared our results with reported methods which involve other
reagents, and toxic solvents or oxidants, some under refluxing conditions or ultrasonic
irradiation. As shown in Table 4, the methods described in this work occur under very
mild and eco-friendly reaction conditions.
Table 4. Conversion of 1H-Indole to 1H-Indol-3-yl Thiocyanate with a Thiocyanate Salt by Various
Methods
System
Time [min]
Yield [%]
H5IO6/H2O
HCl/H2O2/H2O
I2/MeOH
10
8
50
90
91
85 [2]
DDQ/MeOHa)
DEAD/MeCNb)
Mn(OAc)3/AcOH
Oxoneꢀ/MeOH
K10 clay/MeOHc), 08
50
45
120
43
120
99 [31a][31b]
85 [6]
83 [31c]
98 [12]
85 [11]
a) DDQ ¼ 4,5-Dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile. b) DEAD ¼ Diethyl azodicar-
boxylate ¼ diethyl diazenedicarboxylate. c) K10 clay ¼ montmorillonite K10 clay.
Conclusions. – We developed a mild and straightforward procedure for the
synthesis of organic thiocyanate derivatives by a reaction with KSCN in the presence of
HCl/H2O2 (Method A) or H5IO6 (Method B). The thiocyanates were obtained at room
temperature after short reaction times in good to excellent yields and high
regioselectivity.
The authors gratefully acknowledge partial support of this work by the Research Affairs Office of
the Bu-Ali Sina University, Hamedan (Grant number 32-1716 entitled development of chemical
methods, reagent and molecules), Iranians National Elites Foundation and Center of Excellence in
Development of Chemical Method (CEDCM), Hamedan, I. R. Iran.
Experimental Part
General. All chemicals were purchased from Merck or Fluka. All known compounds were identified
by comparison of their melting points and spectral data with those reported in the literature. TLC: silica
gel SIL G/UV 254 plates. Column chromatography (CC): silica gel 60 (0.063 – 0.200 mm) No. 1.07734,
Merck KGaA. M.p.: Bꢀchi-B-545 apparatus; open capillary tubes. IR Spectra: PerkinꢀElmer-17259 FT-
1
IR spectrometer, KBr disks; ˜n in cmꢀ1. H- and 13C-NMR Spectra: Bruker-Avance-DPX-250 and FT-
NMR spectrometer; at 300 (1H) and 75 MHz (13C); d in ppm rel. to Me4Si as internal standard, J in Hz.
Microanalysis: Perkin-Elmer-240-B microanalyzer.
Thiocyanation of 1H-Indole to 1H-Indol-3-yl Thiocyanate. Method A (Typical Procedure). A
suspension of 1H-indole (0.117 g, 1 mmol), potassium thiocyanate (0.294 g, 3 mmol), 36% HCl soln.
(0.04 – 0.07 g, 1 – 2 drops) in H2O (5 – 7 ml) was stirred at r.t. for 5 min. Then, 30% H2O2 soln. (3 mmol)
was added dropwise (2 – 5 min). After completion of the reaction (TLC monitoring), the mixture was
extracted with CHCl3 (2 ꢁ 10 ml), the extract dried (Na2SO4 (5 g)) for 20 min and concentrated, and the
residue purified by CC (hexane/AcOEt 5 :1): 0.159 g (91%) of 1H-indol-3-yl thiocyanate.
Method B (Typical Procedure). A suspension of 1H-indole (0.117 g, 1 mmol), potassium thiocya-
nate (0.228 g, 3 mmol), and HIO4 · 2 H2O (0.143g, 0.75 mmol) in H2O (5 – 7 ml) was stirred at r.t. for
10 min. After completion of the reaction (TLC monitoring), the mixture was worked up as described for
Method A: 0.158 g (90%) of 1H-indol-3-yl thiocyanate. (Table 3, Entry 1). IR: 2159 (SCN), 3289 (NH).

Helvetica Chimica Acta – Vol. 95 (2012)
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1H-NMR: 8.87 (s, 1 H); 7.83 (t, J ¼ 3.3, 1 H); 7.46 – 7.23 (m, 4 H). 13C-NMR: 136.0; 131.2; 127.6; 123.8;
121.8; 118.6; 112.2; 91.7. MS: 174 (Mþ).
1-Methyl-1H-indol-3-yl Thiocyanate (Entry 2): IR: 2146 (SCN). 1H-NMR: 7.84 – 7.36 (m, 5 H); 3.74
(s, 3 H). 13C-NMR: 137.1; 135.2; 128.4; 123.4; 121.6; 118.8; 112.1; 110.3; 33.4. MS: 188 (Mþ).
2-Methyl-1H-indol-3-yl Thiocyanate (Entry 3): IR: 2151 (SCN), 3395 (NH), 3395. 1H-NMR: 8.55 (s,
1 H); 7.71 (d, J ¼ 6.9, 1 H); 7.33 – 7.23 (m, 3 H); 2.49 (s, 3 H). 13C-NMR: 142.1; 135.1; 128.6; 122.9; 121.5;
118.0; 111.3; 88.7; 12.0. MS: 188 (Mþ).
5-Bromo-1H-indol-3-yl Thiocyanate (Entry 4): IR: 2146 (SCN), 3351 (NH). 1H-NMR: 8.87 (s, 1 H);
7.90 – 7.15 (m, 4 H). 13C-NMR: 134.6; 132.2; 129.3; 123.1; 121.2; 115.3; 113.7; 111.9; 102.2. MS: 251 (Mþ),
253 (M þ 2).
1H-Pyrrole-2-yl Thiocyanate (Entry 5): IR: 2160 (SCN), 3247 (NH). 1H-NMR: 9.04 (s, 1 H), 7.04 –
7.00 (m, 1 H); 6.68 – 6.59 (m, 1 H); 6.30 – 6.29 (m, 1 H). 13C-NMR: 121.2; 120.2; 112.2; 110.9; 109.6. MS:
124 (Mþ).
4-(Diethylamino)phenyl Thiocyanate (Entry 6): IR: 2151 (SCN). 1H-NMR: 7.50 – 7.34 (m, 2 H);
7.20 – 7.13 (m, 1 H), 6.63 – 6.61 (m, 1 H); 3.50 (q, J ¼ 7.2, 2 H); 3.37 (q, J ¼ 6.9, 2 H); 1.16 (t, J ¼ 5.4,
6 H). 13C-NMR: 149.1, 134.9, 125.2, 121.7, 112.6, 44.5, 12.3. MS: 206 (Mþ).
1
4-(Diethylamino)phenyl Thiocyanate (Entry 7): IR: 2146 (SCN). H-NMR: 7.44 (d, J ¼ 8.7, 2 H);
6.69 (d, J ¼ 8.7, 2 H); 3.00 (s, 6 H). 13C-NMR: 151.6; 134.5; 113.1; 112.7; 106.3; 40.1. MS: 178 (Mþ).
1
4-(Methylamino)phenyl Thiocyanate (Entry 8): IR: 2153 (SCN), 3413 (NH). H-NMR: 7.15 – 7.12
(m, 2 H); 7.06 – 6.82 (m, 2 H); 4.70 (s, 1 H); 3.32 (s, 3 H). 13C-NMR: 150.9; 134.6; 132.8; 117.6; 116.4;
114.8; 113.6; 30.3. MS: 164 (Mþ).
4-Amino-3-methylphenyl Thiocyanate (Entry 9): IR: 2153 (SCN), 3369, 3456 (NH2). 1H-NMR:
6.64 – 7.37 (m, 3 H); 3.98 (s, 2 H); 2.12 (s, 3 H). 13C-NMR: 147.3; 135.1; 132.1; 123.9; 115.8; 112.8; 108.81;
17.2. MS: 164 (Mþ).
4-Amino-2-methylphenyl Thiocyanate (Entry 10): IR: 2146 (SCN), 3219, 3337 (NH2). 1H-NMR:
7.37 – 6.64 (m, 3 H); 3.98 (s, 2 H); 2.34 (s, 3 H). 13C-NMR: 149.7; 143.0; 136.3; 117.3; 112.4; 108.4; 21.2.
MS: 164 (Mþ).
4-Aminonaphthalen-1-yl Thiocyanato (Entry 11): IR: 2146 (SCN), 3358, 3442 (NH2). 1H-NMR:
7.85 – 6.70 (m, 6 H); 4.58 (s, 2 H). 13C-NMR: 136.2; 128.2; 125.8; 125.6; 121.6; 108.9. MS: 200 (Mþ).
4-[Bis(2-hydroxyethyl)amino]phenyl Thiocyanate (Entry 12): IR: 2153 (SCN), 3300 (OH).
1H-NMR: 7.42 (d, J ¼ 8.7, 2 H); 6.70 (d, J ¼ 8.4, 2 H); 4.43 (s, 2 H); 3.75 – 3.60 (m, 4 H); 3.55 – 3.35
(m, 4 H). 13C-NMR: 151.8; 132.1; 116.5; 114.3; 111.7; 58.7; 55.9. MS: 238 (Mþ).
4-(Morpholin-4-yl)phenyl Thiocyanate (Entry 13): IR: 2156 (SCN). 1H-NMR: 7.48 (d, J ¼ 8.1, 2 H);
6.92 (d, J ¼ 8.4, 2 H); 3.88 (t, J ¼ 4.5, 4 H); 3.24 (t, J ¼ 5.1, 4 H). 13C-NMR: 152.5; 133.7; 116.1; 111.9;
111.1; 67.1; 48.0. MS: 220 (Mþ).
4-(1-Aza[15]-crown-5)1-yl Thiocyanate (¼1,4,7,10-Tetraoxa-13-azacyclopentadec-13-yl Thiocya-
1
nate; Entry 14): IR: 2147 (SCN), 1094 (C – O). H-NMR: 7.37 (d, J ¼ 8.4, 2 H); 6.64 (d, J ¼ 8.1, 2 H);
3.72 – 3.57 (m, 20 H). 13C-NMR: 149.3; 134.7; 112.8; 105.9; 112.6; 71.2; 70.2; 69.9; 68.0; 52.6. MS: 352
(Mþ).
2,3-Dihydro-1H-indol-5-yl Thioanate (Entry 15): IR: 2216 (SCN), 3420 (NH). 1H-NMR: 7.19 – 6.92
(m, 3 H); 5.2 (s, 1 H); 3.23 (t, J ¼ 8.4, 2 H); 2.18 (t, J ¼ 10.5, 2 H). 13C-NMR: 142.1; 128.1; 127.8; 127.4;
122.3; 114.8; 110.3; 50.7; 28.5. MS: 176 (Mþ).
4-Amino-3-chlorophenyl Thiocyanate (Entry 16): IR: 2159 (SCN), 3379, 3435 (NH2). 1H-NMR:
7.37 – 7.19 (m, 3 H); 6.80 (s, 1 H); 4.47 (s, 2 H). 13C-NMR: 145.3; 132.7; 133.8; 119.6; 116.5; 111.8; 109.7.
MS: 184 (Mþ), 186 (M þ 2).
4-Amino-2-bromophenyl Thioanate (Entry 17): IR: 2156 (SCN), 3233, 3376 (NH2). 1H-NMR: 7.23 –
7.50 (m, 3 H); 6.65 (s, 1 H); 4.12 (s, 2 H). 13C-NMR: 144.8; 134.5; 132.6; 117.5; 116.1; 112.7; 109.2. MS: 228
(Mþ), 230 (M þ 2).
4-Amino-3-cyanophenyl Thiocyanate (Entry 18): IR: 2156 (SCN), 2222 (CN), 3360, 3444 (NH2).
1H-NMR: 7.58 – 7.28 (m, 2 H); 6.83 (s, 1 H); 4.96 (s, 2 H). 13C-NMR: 151.7; 138.4; 137.2; 117.8; 116.3;
111.3; 109.3; 96.7. MS: 175 (Mþ).

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4-Amino-2-(trifluoromethyl)phenyl Thiocyanate (Entry 19): IR: 2160 (SCN), 3275, 3369 (NH2).
1H-NMR: 7.82 – 7.33 (m, 3 H); 4.01 (s, 2 H). 13C-NMR: 144.6; 135.3; 125.5; 118.8; 117.4; 115.1; 111.7. MS:
218 (Mþ).
4-Amino-3-(trifluoromethyl)phenyl Thiocyanate (Entry 20): IR: 2158 (SCN) 3395, 3435 (NH2).
1H-NMR: 7.68 – 7.28 (m, 2 H); 6.80 (s, 1 H); 4.57 (s, 2 H). 13C-NMR: 146.7; 137.5; 131.9; 125.6; 114.3;
111.6; 109.2. MS: 218 (Mþ).
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