Synthesis of aryl thiocyanates using Al2O3/MeSO 3H (AMA) as a novel heterogeneous system

Article abstract of DOI:10.3184/030823408X324706

Indoles and various aromatic and heteroaromatic compounds undergo smooth thiocyanation with ammonium thiocyanate in the presence of a mixture of Al ₂O₃, MeSO₃H (AMA) under mild conditions without use of any organic solvent

Full text of DOI:10.3184/030823408X324706

318 RESEARCH PAPER
JUNE, 318–321
JOURNAL OF CHEMICAL RESEARCH 2008
Synthesis of aryl thiocyanates using Al2O3/MeSO3H (AMA) as a novel
heterogeneous system
Mona Hosseini-Sarvari* and Mina Tavakolian
Department of Chemistry, College of Science, Shiraz University, Shiraz 71454, I.R. Iran
Indoles and various aromatic and heteroaromatic compounds undergo smooth thiocyanation with ammonium
thiocyanate in the presence of a mixture of Al2O3/MeSO3H (AMA) under mild conditions without use of any organic
solvents to afford the corresponding aryl thiocyanates in excellent yields and with high selectivity. The reactions
proceed rapidly at room temperature.
Keywords: Al2O3, MeSO3H, aromatic compounds, thiocyanation, indoles, solvent-free conditions
Aromatic or heteroaromatic thiocyanates are useful
intermediates in the synthesis of sulfur-containing
heterocycles.1,2 Furthermore, aryl thiocyanates can be
easily transformed into various sulfur functional groups3,4
such as thiophenols by reduction with lithium aluminium
hydride and aryl nitriles/disulfides by reaction with aromatic
Grignard reagents. Thus, the direct thiocyanation of aromatic
systems is of importance. In view of the versatility of the
thiocyanate group in heterocycle construction,2,5 it is of
significance to probe the direct thiocyanation of aromatic
and heteroaromatic compounds. Several methods have been
developed for the thiocyanation of arenes and indoles,6-15
including bromine/potassium thiocyanate6 and N-
thiocyanatosuccinimide (only for 5-methoxy-2-methylindole
and accompanied by two bis-thiocyanates)7 and (each with
ammonium thiocyanate) ceric ammonium nitrate (CAN),8
acidic K-10 clay,9 iodine/MeOH (only for indoles and aryl
amines),10 oxone®11 and iron(III)chloride (only for indoles
and aryl amines).12 However, these methodologies suffer
from one or more drawbacks such as the lack of availability
or difficult preparation of starting materials,6,7 the requirement
for a large excess of strong oxidising reagents, low yields
for some compounds,8 and reactions under certain special
conditions.9 Furthermore, some require high temperatures
and the use of a large amount of organic solvents to obtain
satisfactory results. Since organosulfur compounds have
become increasingly useful and important in the field of drugs
and pharmaceuticals, the development of simple, convenient
and efficient approaches are desirable. In the present work,
we report a novel, efficient, and regioselective thiocyanation
of aromatic and heteroaromatic compounds using ammonium
thiocyanate in the presence of Al2O3/MeSO3H (AMA).
R
R
Al2O3/ MeSO3H
r.t., solvent free
+ NH4SCN
SCN
2
1
Scheme 1
synthesis of macrocyclic polyether-diesters21 and the synthesis
of coumarins.22 This paper reports its application in the
thiocyanation of various aromatic compounds (Scheme 1).
Initially, the reaction of anisole (1a) with ammonium
thiocyanate was chosen as a model reaction and its behaviour
was studied under a variety of conditions by TLC and
1H NMR spectroscopy (Table 1).
As shown in Table 1, the best results were obtained using
a mixture of Al2O3 and MeSO3H in 1:5 molar ratio, when
carried out at room temperature for 10 min (entry 7).
A typical experimental procedure is as follows: to a mixture
of MeSO3H (1 ml, 15 mmol) and Al2O3 (acidic type 540 C,
0.27 g, 3 mmol) was added anisole (1a) (1 mmol) and
ammonium thiocyanate (1 mmol). The mixture was stirred
for 10 min. Then the reaction mixture was extracted with
ethyl acetate and after evaporation of the organic layer
4-thiocyanatoanisole (2a) was obtained in an excellent yield.
Under the best reaction conditions described above, various
aromatic and heteroaromatic compounds (Table 2, 1–15)
were converted into the corresponding aryl thiocyanates.
The products were identified by H and 13C NMR, MS and
IR analysis. The IR spectra showed the characteristic peak of
–SCN near 2080 cm-1 and the –C–S stretching near 730 cm-1.
The results are listed in Table 2.
The results summarised in Table 2 reveal that excellent
yields were obtained with aromatic and heteroaromatic
compounds. All of the aromatic compounds reacted very
rapidly within 5–73 min. In all cases, the reactions proceeded
smoothly at room temperature with high regioselectivity.
This method is very clean and free from side products.
1
We have recently reported that (AMA) was an effective
reagent for many useful organic reactions such as the
Fries-rearrangement,16 the Beckmann rearrangement,17 the
preparation of amides from nitriles,18 the mono-esterification
of diols,19 the conversion of aldehydes into monoesters,20 the
Table 1 Thiocyanation of anisole (1 mmol) with ammonium thiocyanate (1 mmol) under various reaction conditions
Entry
Catalyst/reagent
Solvent
Time
Yield/%a
1
2
I2, r.t.
MeOH
CH2Cl2
None
None
None
None
None
None
None
None
24 h
24 h
24 h
24 h
24 h
24 h
10 min
1 h
0
5
FeCl3, r.t.
3
Sulfamic acid
0
4
Alumina sulfonic acid (ASA)23,24
Al2O3 (3 mmol), r.t.
0
0
65
98
80
80
0
5
6
MeSO3H, (15 mmol), r.t.
Al2O3 (3 mmol)/MeSO3H (15 mmol), r.t.
Al2O3 (2 mmol)/MeSO3H (15 mmol), r.t.
Al2O3 (1 mmol)/MeSO3H (15 mmol), r.t.
No catalyst/reagent
7
8
9
2 h
10
24 h
aIsolated yields.
* Correspondent. E-mail: hossaini@susc.ac.ir

JOURNAL OF CHEMICAL RESEARCH 2008 319
Table 2 Thiocyanation of aromatic compounds using AMA
Entry
Substrates
Products
Time/min
10
Yield/%a
98
OMe
Me
OMe
1
1a
1b
2a
2b
NCS
NCS
Me
2
65
80
NH2
Cl
NH2
Cl
3
4
5
6
1c
1d
1e
1f
2c
2d
2e
2f
4
35
5
98
95
97
85
NCS
NCS
NCS
NCS
OH
OH
NMe2
NO2
NMe2
10
7
8
1g
1h
No reaction
OMe
OMe
2h
2i
25
45
98
98
MeO
MeO
HO
SCN
OH
OH
9
1i
1j
HO
SCN
NH2
NH2
10
2j
5
94
Me
Me
SCN
OMe
OMe
11
12
13
1k
1l
2k
2l
20
15
10
94
94
98
SCN
SCN
O
O
1m
2m
SCN
SCN
N
H
N
H
14
15
1n
1o
2n
2o
20
73
90
81
N
H
N
H
SCN
N
H
N
H
16
17
18
1p
1q
1r
No reaction
No reaction
SCN
2r
30
84
aYields are of the isolated compounds.

320 JOURNAL OF CHEMICAL RESEARCH 2008
4-Thiocyanatotoluene (2b): Yellow crystalline solid; m.p. 46–55°C
(lit.14 54 °C); IR(KBr) (νmax/cm-1): 3168, 2077, 1613; 790; 1H NMR
(250 MHz, CDCl3): d 7.88 (m, 2H, J* = 8.8 Hz, ArH), 7.45 (m, 2H,
J* = 8.8 Hz, ArH), 3.38 (s, 3H, –CH3); 13C NMR (62.9 MHz, CDCl3):
d 61.7(–CH3), 119.3 (–SCN), 128.8, 131.8, 132.8, 155.4; GC-Ms/EI:
m/z (%) = 151 (M+ + 2, 96.0), 150 (M+ + 1, 23.7), 149 (M+, 11.7), 135
(43.2), 134 (100), 133 (21.9), 83 (41.0), 57 (51.0); Anal. Calcd for
C8H7NS: C, 64.4; H, 4.7. Found: C, 64.2; H, 4.5%.
4-Thiocyanatoaniline (2c): Yellow crystalline solid; m.p. 52–55°C
(lit.11 49–51°C); IR(KBr) (νmax/cm-1): 3623, 3412, 2071, 782; 1H NMR
(250 MHz, d6-DMSO): d 6.94 (m, 2H, J* = 8.5 Hz, ArH), 7.46 (m,
2H, J* = 8.5 Hz, ArH), 4.56 (s, brs, 2H, –NH2); 13C NMR (62.9 MHz,
d6-DMSO): d 115.9 (–SCN), 122.7, 128.3, 128.5, 156.6; GC-Ms/
EI: m/z (%) = 151 (M+ + 1, 2.4), 150 (M+, 36.2), 149 (M+, 38.6),
108 (31.5), 71 (64.6), 84 (44.1), 57 (100), 56 (50.4); Anal. Calcd for
C7H6N2S: C, 56.0; H, 4.0. Found: C, 55.9; H, 4.0%.
It was very exciting to find that unactivated benzenes such as
chlorobenzene reacted smoothly in the presence of AMA to
afford the corresponding thiocyanato aromatics in high yields
(Table 2, entry 4). Thicyanation occurs exclusively at the
position para to –OMe, –Me, –OH, –NH2, –NMe2 and –Cl for
all of the compounds studied in excellent yields. However, in
cases where the para positions are blocked (Table 2, entries
8–11), the thiocyano group is introduced in the ortho position.
This procedure can also be used for the thiocyanation of
heterocyclic compounds such as furan, pyrrole, and indoles
(entries 12–15). In the case of indoles, the products were
obtained with mono-thiocyanation at the 3-position of the
indole ring. The thiocyanation of nitrobenzene, anthracene,
and naphthalene with ammonium thiocyanate seemed more
difficult to perform perhaps because of inherent reactivity or
mixing problems (entries 7, 16 and 17). The present method
provides a useful means for the regioselective thiocyanation
of aromatic compounds. We believe that this constitutes the
first satisfactory method for the direct conversion of arenes
to the corresponding thiocyanoarenes. Exhaustive search of
Chemical Abstracts has led us to this conclusion.
Another interesting behaviour of alumina lies in the fact
that it can be reused after simple washing with AcOEt and
H2O, thus rendering the process more economic. The yields
of 4-thiocyanatoanisole (1a) in the 2nd, 3rd, 4th, and 5th use
of the alumina were almost the same as that in the 1st use.
It should be noted that MeSO3H was not adsorbed on the
alumina during the reaction and, after extraction with AcOEt,
MeSO3H was not found on the alumina; thus, it is necessary
to add MeSO3H again in the use of recovered alumina.
1-Chloro-4-thiocyanatobenzene (2d): Yellow crystalline solid;
m.p. 73–78°C; IR(KBr) (νmax/cm-1): 2080, 7879; 1H NMR (250 MHz,
CDCl3): d 7.36 (m, 2H, J* = 8.5 Hz, ArH), 7.08 (m, 2H, J* = 8.5 Hz,
ArH); 13C NMR (62.9 MHz, CDCl3): d 113.1(–SCN), 121.8, 129.3,
132.3, 134.6; GC-Ms/EI: m/z (%) = 169 (M+, 19.6), 167 (18.6), 149
(52.0), 129 (20.6), 97 (30.4),81 (44.1), 57 (100), 55 (81.4); Anal.
Calcd for C7H4ClNS: C, 49.6; H, 2.4. Found: C, 49.2; H, 2.1%.
4-Thiocyanatophenol (2e): Yellow crystalline solid; m.p. 148–
1
152°C; IR(KBr) (νmax/cm-1): 3341, 2077, 782; H NMR (250 MHz,
CDCl3): d 7.46 (m, 2H, J* = 7.6 Hz, ArH), 6.81 (m, 2H, J* = 7.6 Hz,
ArH), 4.14 (brs, 1H, –OH). 13C NMR (62.9 MHz, CDCl3): d 114.3
(–SCN), 117.3, 122.7, 129.6, 160.5; GC-Ms/EI: m/z (%) = 153 (M+ +
2, 4.2), 152 (M+ + 1, 14.1), 151 (M+, 13.2), 150 (60.4), 137 (43.60),
134 (100), 123 (6.7), 91 (4.7), 57 (56.1); Anal. Calcd for C7H5NOS:
C, 55.6; H, 3.3. Found: C, 55.4; H, 3.1%.
N,N-Dimethyl-4-thiocyanatonaniline (2f): Yellow crystalline solid;
m.p. 73–75°C (lit.12 72–74°C); IR(KBr) (νmax/cm-1): 3234, 3060,
2092, 765; 1H NMR (CDCl3): d 6.88 (m, 2H, J* = 8.4 Hz, ArH), 6.97
(m, 2H, J* = 8.4 Hz, ArH) 3.7 (s, 6H, –CH3); 13C NMR (62.9 MHz,
CDCl3): d 55.2 (–CH3), 113.0 (–SCN), 114.5, 116.2, 130.4, 153.2;
GC-Ms/EI: m/z (%) = 179 (M+ + 1, 1.1), 178 (M+, 0.5), 165 (8.7),
129 (12.9), 111 (17.0), 83 (36.3), 57 (100), 55 (90.1); Anal. Calcd for
C9H10N2S: C, 60.6; H, 5.7. Found: C, 60.5; H, 5.4%.
In conclusion, we have demonstrated that a readily available
and inexpensive reagent AMA is very effective and highly
selective for the thiocyanation of aromatic compounds. The
extremely high regioselectivity of this reaction may be very
useful in organic synthesis. The low cost and availability
of the reagent, the easy procedure and work-up, the lack of
solvent in the reaction step, and the high yields and short
reaction times make this method a useful addition to the
present methodologies. Hence, we believe that it will find
wide application in organic synthesis as well as in industry.
1,4-Dimethoxy-2-thiocyanatobenzene (2h): Orange crystalline
solid; m.p. 72–75°C (lit.15 68–69°C); IR(KBr) (νmax/cm-1): 3404,
1
3184, 2073, 1631, 711; H NMR (250 MHz, CDCl3): d 6.76–6.77
(m, 3H, ArH), 3.70 (s, 6H, –OMe); 13C NMR (62.9 MHz, CDCl3):
d 55.8 (–OCH3), 56.6(–OCH3), 113.1 (–SCN), 114.6, 116.9, 119.6,
120.4, 151.1, 153.7; GC-MS/EI: m/z (%) = 196 (M+ + 1, 5.9), 195
(M+, 8.3), 166 (24.6), 165 (7.3), 150 (27.7), 123 (16.6), 97 (37.2), 71
(44.8), 55 (100); Anal. Calcd for C9H9NO2S: C, 55.4; H, 4.7. Found:
C, 55.1; H, 4.3%.
Experimental
General
1,4-Dihydroxy 2-thiocyanatobenzene-1,4-diol (2i): Yellow
crystalline solid; m.p. 126–128°C; IR(KBr) (νmax/cm-1): 3348, 3168,
2092, 1618, 759; 1H NMR (250 MHz, d6-DMSO): d 8.82 (s, 1H,
–OH), 8.61 (s, 1H, –OH), 6.13–6.37 (m, 3H, ArH); 13C NMR (62.9
MHz, d6-DMSO): d 115.1 (-SCN), 115.6, 116.1, 120.6, 123.3, 149.1,
154.7; GC–MS/EI: m/z (%) = 168 (M+ + 1, 1.2), 167 (M+, 1.2), 150
(21.9), 135 (21.4), 107 (14.7), 85 (43.9), 57 (100); Anal. Calcd for
C7H5NO2S: C, 50.3; H, 3.0. Found: C, 50.0; H, 3.0%.
4-Methyl-2-thiocyanatoaniline (2j): Yellow crystalline solid; m.p.
146–150°C; IR(KBr) (νmax/cm-1): 3224, 3130, 2077, 1631, 844;
1H NMR (250 MHz, d6-DMSO): d 7.29 (d, 1H, J = 8.3 Hz, ArH),
7.20 (s, 1H, ArH), 6.88 (d, 1H, J = 8.3 Hz, ArH), 4.18 (brs, 2H, –
NH2), 3.13 (s, 3H, –CH3); 13C NMR (62.9 MHz, d6-DMSO): d 25.7
(–CH3), 111.4, 114.6(-SCN), 121.7, 125.8, 130.2, 135.9, 150.2; GC–
MS/EI: m/z (%) = 164 (M+, 20.8), 132 (21.9), 107 (27.9), 97 (31.7),
71 (48.6), 59 (89.1), 57 (69.4), 55 (100); Anal. Calcd for C8H8N2S:
C, 58.5; H, 4.9. Found: C, 58.1; H, 4.65%.
2-Methoxy-3-thiocyanatonaphthalene (2k): Yellow crystalline
solid; m.p. 150–152°C; IR(KBr) (νmax/cm -1): 3390, 3257, 2077,
1622, 7598; 1H NMR (250 MHz, CDCl3): d 8.04 (d, 1H, J = 7.8 Hz,
ArH), 7.83 (s, 1H, ArH), 7.75 (s, 1H, ArH), 7.25–7.40 (m, 3H, ArH),
3.99 (s, 3H, –OCH3); 13C NMR (62.9 MHz, CDCl3): d 56.8 (–OCH3),
108.2, 111.4, 113.1 (–SCN), 118.1, 124.0, 124.2, 127.5, 127.8, 130.7,
139.6, 150.2; GC–MS/EI: m/z (%) = 217 (M+ + 2, 52.5), 216 (M+ + 1,
61.7), 215 (M+, 5.0), 183 (39.2), 140 (50.8), 113 (25.8), 83 (37.5), 60
(43.3), 57 (100), 55 (80.0); Anal. Calcd for C12H9NOS: C, 66.95; H,
4.2. Found: C, 66.7; H, 4.0%.
2-Thiocyanatofuran (2l): Yellow crystalline solid; m.p. 65–67°C;
IR(KBr) (νmax/cm-1): 3379, 3276, 2100, 1626, 767. 1H NMR (250
MHz, d6-DMSO): d 7.57 (s, 1H, ArH), 6.98 (s, 1H, ArH), 6.35 (s, 1H,
ArH); 13C NMR (62.9 MHz, DMSO): d 112.6 (–SCN), 112.9, 116.8,
1H NMR and 13C NMR spectra were measured on a Bruker Advance
DPX FT 250 and 62.9 MHz spectrometer with TMS as an internal
standard. IR spectra were obtained on Perkin–Elmer or FTIR-800
instruments. Mass spectra were obtained on a Shimadzu GCMS0QP
1000EX at 20 and/or 70 eV. Elemental analyses were performed on
PerkinElmer 240-B microanalyser. Al2O3 (acidic type 540 C) was
purchased from Merck company.
General procedure for thiocyanation of aromatic and heteroaromatic
compounds
To a mixture of MeSO3H (1.0 ml) and Al2O3 (0.27 g, 3.0 mmol) were
added the aromatic compound (1 mmol) and NH4SCN (1 mmol).
The mixture was stirred at room temperature for the appropriate time
(Table 2). Then the mixture was poured into water and extracted
twice with ethyl acetate or chloroform (20 ml). The organic layer
was washed with a saturated solution of sodium bicarbonate (20 ml).
Then the organic layer dried over CaCl2 and evaporated under
reduced pressure. The resulting product was purified by column
chromatography on silica gel (Merck, 100-2—mesh, EtOAc–hexane,
1
1:9) to afford the pure thiocyanato derivative. For H NMR spectra
of AA'XX' systems J* = J23 + J25.
4-Thiocyanatoanisole (2a): Yellow crystalline solid; m.p. 131–
133°C;IR(KBr)(νmax/cm-1):3168, 2076, 1619, 790;1HNMR(250MHz,
d6-DMSO): d 7.98 (m, 2H, J* = 8.7 Hz, ArH), 6.99 (m, 2H,
J* = 8.7 Hz, ArH), 3.84 (s, 3H, –OCH3); 13C NMR (62.9 MHz, d6-
DMSO): d 55.3 (–OCH3), 112.9 (–SCN), 113.4, 129.3, 131.2, 161.8;
GC-Ms/EI: m/z (%) = 167 (M+ + 2, 97.8), 166 (M+ + 1, 24.9), 165 (M+,
12.0), 151 (43.2), 150 (13.7), 134 (100), 83 (20.1), 76 (11.3), 57 (48.4);
Anal. Calcd for C8H7NOS: C, 58.2; H, 4.3. Found: C, 58.0; H, 4.0%.

JOURNAL OF CHEMICAL RESEARCH 2008 321
145.5, 151.7; GC–MS/EI: m/z (%) = 127 (M+ + 2, 8.0), 126 (M+ + 1,
3.7) 125 (M+, 5.7), 97 (25.0), 96 (10.9), 95 (18.0), 73 (30.7), 71
(52.2), 69 (65.9), 57 (100), 55 (70.2); Anal. Calcd for C5H3NOS: C,
48.0; H, 2.4. Found: C, 47.7; H, 2.2%.
Received 12 February 2008; accepted 20 May 2008
Paper 08/5104 doi: 10.3184/030823408X324706
2-Thiocyanato-pyrrole (2m): Colourless liquid11 IR(KBr)
References
1
(νmax/cm-1): 3379, 3276, 2100, 1626, 767; H NMR (250 MHz, d6-
1
J.L. Wood, Organic reactions, Wiley: New York, 1967. Vol. III, pp. 240-
266.
DMSO): d 7.05 (s, 1H, ArH), 6.87 (s, 1H, ArH), 6.03 (s, 1H, ArH);
13C NMR (62.9 MHz, d6-DMSO): d 112.5 (–SCN), 113.1, 117.2,
145.4, 151.6; GC–MS/EI: m/z (%) = 126 (M+ + 2, 17.6), 125 (M+ +
1, 3.5) 124 (M+, 12.6), 107 (27.6), 97 (31.2), 82 (50.8), 57 (100), 55
(87.9); Anal. Calcd for C5H4N2S: C, 48.4; H, 3.25. Found: C, 48.2;
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4
5
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6
7
1
758; H NMR (250 MHz, CDCl3): d 8.9 (s, brs, 1H, –NH), 7.88 (d,
1H, J = 8.0 Hz, ArH), 7.32–7.70 (m, 4H, ArH); 13C NMR (62.9 MHz,
CDCl3): d 83.0, 113.2, 113.5 (–SCN), 118.9, 122.4, 122.8, 127.3,
127.6, 145.4; GC–MS/EI: m/z (%) = 175 (M+ + 1, 41.2), 174 (M + ,
30), 155 (21), 141 (30), 97 (20), 85 (22), 71 (35), 57 (98), 43 (100);
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1
1620, 750; H NMR (250 MHz, CDCl3): d 8.50 (s, brs, 1H, –NH),
7.73 (d, 1H, J = 8.0 Hz, ArH), 7.20–7.54 (m, 3H, ArH), 2.52 (s, 3H,
–CH3); 13C NMR (62.9 MHz, CDCl3): d 16.3 (–CH3), 78.0, 113.6
(–SCN), 119.5, 122.2, 122.8, 127.5, 129.1, 135.4, 139.1; GC–MS/
EI: m/z (%) = 189 (M+ + 1, 32.2), 188 (M + , 24.3), 156 (15.0), 57
(98), 55 (100); Anal. Calcd for C10H8N2S: C, 63.80; H, 4.3. Found:
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18 H. Sharghi and M. Hosseini Sarvari Synth. Commun., 2003, 33, 207.
19 H. Sharghi and M. Hosseini Sarvari Tetrahedron, 2003, 59, 3627.
20 H. Sharghi and M. Hosseini Sarvari J. Org. Chem., 2003, 68, 4096.
21 H. Sharghi and M. Hosseini Sarvari Synthesis, 2003, 6, 879.
22 H. Sharghi and M. Jokar Heterocycles, 2007, 71, 2721.
23 H. Sharghi, M. Hosseini-Sarvari and E. Eskandari J. Chem. Res., 2005,
482.
1
212°C; IR(KBr) (νmax/cm-1): 3162, 2074, 1615, 792; H NMR (250
MHz, d6-DMSO): d 7.53 (d, 2H, J = 8.0 Hz, ArH), 7.32 (m, 3H,
ArH); 13C NMR (62.9 MHz, d6-DMSO): d 113.2 (–SCN), 125.1,
130.4, 131.5,131.8; GC–MS/EI: m/z (%) = 137 (12.0), 136 (14.9),
135 (M + ,53.2), 121 (33.2), 83 (100); Anal. Calcd for C8H7NOS: C,
62.2; H, 3.7. Found: C, 62.0; H, 3.5%.
We gratefully acknowledge the support of this work by the
Shiraz University Research Council.
24 M. Hosseini-Sarvari and H. Sharghi J. Chem. Res., 2006, 205.