Ammonium metavanadate/thiocyanate-triggered electrophilic thiocyanation of aromatic and heteroaromatic compounds in aqueous bisulfate and acetonitrile media

Article abstract of DOI:10.1080/17415993.2014.940350

The ammonium metavanadate/thiocyanate system is used as an efficient reagent for regioselective thiocyanation of aromatic and hetero aromatic compounds under conventional and nonconventional conditions such as ultrasonically assisted and microwave-assisted reactions. The reactions proceeded smoothly and afforded good yields of products with high regioselectivity. Longer reaction times (about 8 h) observed under conventional conditions were reduced to 0.5 h/30 min under sonication and to 90 s in the case of microwave-assisted reactions.

Full text of DOI:10.1080/17415993.2014.940350

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Ammonium metavanadate/thiocyanate-
triggered electrophilic thiocyanation
of aromatic and heteroaromatic
compounds in aqueous bisulfate and
acetonitrile media
N. Venkateshama, K. Rajendar Reddya, K. C. Rajannaa & P.
Veerasomaiaha
a Department of Chemistry, Osmania University (O.U), Hyderabad
500 007, Andhra Pradesh, India
Published online: 24 Jul 2014.
To cite this article: N. Venkatesham, K. Rajendar Reddy, K. C. Rajanna & P. Veerasomaiah (2014)
Ammonium metavanadate/thiocyanate-triggered electrophilic thiocyanation of aromatic and
heteroaromatic compounds in aqueous bisulfate and acetonitrile media, Journal of Sulfur Chemistry,
35:6, 606-612, DOI: 10.1080/17415993.2014.940350
To link to this article: http://dx.doi.org/10.1080/17415993.2014.940350
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Journal of Sulfur Chemistry, 2014
Vol. 35, No. 6, 606–612, http://dx.doi.org/10.1080/17415993.2014.940350
Ammonium metavanadate/thiocyanate-triggered electrophilic
thiocyanation of aromatic and heteroaromatic compounds in
aqueous bisulfate and acetonitrile media
N. Venkatesham, K. Rajendar Reddy, K. C. Rajanna∗ and P. Veerasomaiah
Department of Chemistry, Osmania University (O.U), Hyderabad 500 007, Andhra Pradesh, India
(Received 6 March 2014; accepted 26 June 2014)
The ammonium metavanadate/thiocyanate system is used as an efficient reagent for regioselective
thiocyanation of aromatic and hetero aromatic compounds under conventional and nonconventional
conditions such as ultrasonically assisted and microwave-assisted reactions. The reactions proceeded
smoothly and afforded good yields of products with high regioselectivity. Longer reaction times (about
8 h) observed under conventional conditions were reduced to 0.5 h/30 min under sonication and to 90 s in
the case of microwave-assisted reactions.
X
X
NH4VO3
SCN
NH4SCN /KHSO4/MeCN
Y
Y
i) Conventional; ii) Sonication; iii) Microwave
Where X = OH, NH2, COOH, COCH3, CHO
Y = EWG or EDG
Keywords: ammonium metavanadate; potassium thiocyanate; regioselective thiocyanation; sonication;
microwave irradiation
1. Introduction
Thiocyanation of aromatic and heteroaromatic compounds is one of the most useful electrophilic
substitution reactions in organic synthesis and provides a direct route for the formation of carbon–
sulfur bonds despite the fact that nucleophilic attack of thiocyanate ion at aromatic nuclei is a
difficult route to form thiocyanate compounds. The thiocyanate functional group is an interesting
variant of organic thiocompounds because they can be readily transformed into pharmaceutically
importantentities.[1–5]Thisparticularaspecthasbecomethestimulusforthedevelopmentofsim-
ple, safe, and fast synthetic protocols for the synthesis of thioaromatic systems.[6–17] Recently,
Sajadifar and co-workers developed simple regioselective thiocynation protocols using H2O2,
∗Corresponding author. Email: kcrajannaou@yahoo.com
© 2014 Taylor & Francis

Journal of Sulfur Chemistry 607
H5IO6, or silica boron sulfonic acid/hydrogen peroxide in the presence of Potassium thiocyanide
(KSCN).[16,17]These procedures provided the target thiocyanate derivatives in short duration fol-
lowed by good to excellent yields and high regioselectivity. However, some of these methods still
suffer from the disadvantages such as the use of strongly acidic or oxidizing conditions, low yields,
long reaction time and/or high temperature, and also the use of expensive reagents. These reasons
together with the versatility of the thiocyanide (SCN) functionality stimulated us to take up the
present study in which laboratory desktop chemicals such as ammonium metavanadate (NH4VO3)
and ammonium thiocyanate (NH4SCN) are used to achieve thiocyanation of aromatic and hetero
aromatic compounds. In order to attain greenery of the study, we have used potassium hydrogen
sulfate (KHSO4) as a catalyst, because it has received considerable attention in several organic
protocols as an inexpensive, eco friendly, highly reactive, easy to handle and non-toxic substance.
Baghernejad’s recent review[18] on this aspect provides an excellent literature survey on the use
of KHSO4 as a solid catalyst. Since preliminary results indicated longer reaction times, efforts
were also made to increase the rate of the reactions. The success and advantages of ultrasound-
assisted,[19–24] and microwave-assisted chemical reactions,[25–28] reported in earlier investi-
gations prompted us to explore their utility in the present study to achieve effective thiocyanation.
2. Results and discussion
The thiocyanation of aromatic compounds with ammonium metavanadate (NH4VO3) as a catalyst
in the presence of (0.10 mol) ammonium thiocyanate and KHSO4 afforded thiocyanate derivatives
in very good yield (Scheme 1).
X
X
NH4VO3
SCN
NH4SCN /KHSO4/MeCN
Y
Y
i) Conventional; ii) Sonication; iii) Microwave
Where X = OH, NH2, COOH, COCH3, CHO
Y = EWG or EDG
Scheme 1. Thiocyanation of aromatic compounds in the presence of NH4VO3.
The reaction has been conducted using different concentrations of NH4VO3 (0.001–0.01 mol)
in different solvents such as dichloromethane (DCM), dichloroethane (DCE), and acetonitrile
(MeCN), in order to optimize the conditions. Table 1 shows reaction times and percentage (%)
yields of thiocyanation of phenol under different reaction conditions. The reaction went smoothly
inDCE when 0.005 mmol of NH4VO3 wasusedasa catalyst. Consequently, 0.001 mol of NH4VO3
is used throughout the study.
We have also performed control experiments under different conditions: (i) using KHSO4 but
excluding NH4VO3 and (ii) using NH4VO3 but excluding KHSO4. In both cases the reactions
were too sluggish even under drastic conditions. Practically no trace of the product was observed
in these two cases.
A wide range of active, moderately active, and non-active aromatic compounds and heterocyclic
compounds were chosen for thiocyanation under conventional and non-conventional conditions
and the corresponding results are presented in Table 2. The data revealed that thiocyanation of
aromatic compounds occurred smoothly with good regioselectivity and yields in the presence

608 N. Venkatesham et al.
Table 1. Effect of solvent and NH4VO3 on thiocyanation of phenol under conventional conditions.
NH4VO3
(mol)
Time
(h)
Yield
(%)
S. No.
1
Solvent
ACN
0.001
0.002
0.005
0.01
0.001
0.002
0.005
0.01
0.001
0.002
0.005
0.01
6–8
6–8
6–8
6–8
12–14
12–14
12–14
12–14
12–14
12–14
12–14
12–14
75
70
75
75
50
55
60
60
50
55
55
55
2
3
DCE
DCM
Table 2. Ultrasonic and microwave-assisted thiocyanation of certain aromatic and heteroaromatic compounds using
(KHSO4/NH4VO3/NH4SCN) in MeCN.
Conventional
(6–8 h)
Sonication
(30–45 min) (5–7 min)
MWAR
S. No.
Substrate
Product
Yield (%)
Yield (%)
Yield (%)
1
2
3
4
5
6
7
8
Phenol
o-Cresol
m-Eresol
4-Bromophenol
Aniline
o-Chlorophenol
m-Methoxy-aniline
N-Methylaniline
N,N-dimethylaniline
N,N-diethylaniline
Diphenylamine
Diphenylamine
Indole
2-Thiocyanatophenol
75
79
79
66
78
80
79
79
79
75
69
80
75
72
85
77
17
78
82
80
77
82
80
70
79
83
80
79
82
78
73
83
78
74
86
79
15
81
79
78
80
83
80
74
80
85
80
80
83
82
76
87
86
77
86
81
10
83
80
79
2-Methyl-4-thiocyanatophenol
3-Methyl-4-thiocyanatophenol
4-Bromo-2-thiocyanato-phenol
4-Thiocyanatoaniline
2-Chloro- 4-thiocyanatoaniline
3-Methoxy-4-thiocyanato-aniline
4-Thiocyanato-N-methylaniline
4-Thiocyanato-N,N-dimethylaniline
4-Thiocyanato-N,N-diethylaniline
4-Thiocyanatodiphenylamine
Dithiocyanatodiphenylamine
3-Thiocyanato-1H-indole
1-Methyl-3-thiocyanatoindole
5-Bromo-3-thiocyanatoindole
2-Thiocyanato-1H-pyrrole
2,5-Dithiocyanato-1H-pyrrole
2-Thiocyanatothiophene
9
10
11
12
13
14
15
16
17
18
19
20
1-Methyl-indole
5-Bromo-indole
Pyrrole
Pyrrole
Thiophene
1-Methoxy naphthalene 1-Methoxy-4-thiocyanatonaphthalene
N-Methyl-indole 3-Thiocyanato-N-methyl-indole
of NH4VO3, but are associated with longer reaction times (8–12 h). It is of interest to note that
para-substituted compounds underwent thiocyanation only at the ortho position, while ortho-
substituted compounds afforded the para derivative in a very good yield. Even though, benzene
failed to undergo thiocyanation; it is also of interest to note that aromatic hydrocarbons, such
as α-methoxynaphthalene, underwent smooth thiocyanation with fairly good yields. When this
protocol is extended to carbonyl compounds, thiocyanation went smoothly without oxidation or
any other side reaction. The reaction was clean with no attack observed on the alkyl portion of
the ketones. To further check the regioselectivity of the reaction, the reaction was carried out with
different para-substituted aromatic carbonyl and related compounds, which afforded only meta
thiocyanate products in good yields.
In an attempt to enhance the rates of the reactions they were also conducted with nonconven-
tional activation techniques, such as ultrasonically assisted and microwave-assisted methods.

Journal of Sulfur Chemistry 609
Ultrasonically assisted reactions (USAR) and microwave-assisted reactions (MWAR) have
become popular with physical and organic chemists because of their immense importance to
promote and enhance a broad spectrum of synthetic organic reactions.[25–28] The use of these
non-traditional tools has been found to be of great help in overcoming most of the difficulties
associated with conventional reactions related to their environmental shortcomings. We were
extremely pleased to discover that all of the reactions underwent dramatic rate enhancements.
In addition, the kinetically enhanced reactions maintained their desirable selectivity and ease of
experimental manipulation. The reaction times were reduced from 8 to 12 h (under conventional
conditions) to about 30–45 min in USA reactions and about 5–7 min in MWA reactions.
Rate accelerations of the ultrasonically assisted thiocyanation reactions (USNR) in the present
study are due to cavitation, a physical process that creates, enlarges, and implodes gaseous and
vaporous cavities in an irradiated liquid.[19–24] Cavitation is a process in which mechanical
activation destroys the attractive forces of molecules in the liquid phase. Applying ultrasound,
compression of the liquid is followed by rarefaction (expansion), in which a sudden pressure drop
forms small, oscillating bubbles of gaseous substances. These bubbles expand with each cycle
of the applied ultrasonic energy until they reach an unstable size; they can then collide and/or
violently collapse. Cavitation induces very high local temperatures in the liquid and enhances
mass transfer.
On the other hand, the observed rate and yield enhancements in MWA reactions [28] are
probably due to a combination of thermal effects, arising from the heating rate, superheating
or ‘hot spots’, and the selective absorption of radiation by polar substances, which ultimately
causes bulk activation of molecules. It is important to note that the rate of the reaction has a direct
dependence on the fraction of activated/energized species.
3. Conclusion
In summary, the authors have developed ammonium metavanadate/thiocyanate as an efficient
reagent to trigger electrophilic thiocyanation of aromatic and heteroaromatic compounds in aque-
ous bisulfate medium under conventional and nonconventional, such as ultrasonically assisted
(USA) and micro wave-assisted (MWA), conditions. These newly developed methods have sev-
eral advantages such as the use of water as solvent, simple workup procedure, good selectivity,
short reaction times, and high product yields. The longer reaction times of conventional methods
(6–8 h) were remarkably reduced to about 30–45 min in sonicated and (5–7 min) in MW-assisted
reactions.
4. Experimental
4.1. General
Chemicals were obtained from S.D fine Chemicals (India), Avra fine Chemicals (India), and
Aldrich Chemical Company and used without further purification.
4.2. A typical procedure
In a typical experiment, a suspension of phenol (0.1 mol), ammonium metavanadate (0.001 mol),
ammonium thiocyanate (0.1 mol), and aqueous KHSO4 (0.1 mol), in about 20 mL solvent (MeCN)
was stirred at room temperature. The progress of the reaction was monitored by thin-layer chro-
matography (TLC) (ethyl acetate:n-hexane 1:9). After completion of the reaction, the reaction

610 N. Venkatesham et al.
mixture was treated with DCE and dried over anhydrous Na2SO4 and concentrated to obtain the
end product. The products were identified by IR, 1H NMR, and MS spectra. IR spectra showed
the characteristic peaks of the –SCN group between 2120 and 2150 cm−1 and the C–S stretching
between 650 and 750 cm−1. The regio-chemistry of substitution was deduced by interpretation of
1H NMR spectra and by their comparison with the spectral and physical data of those of authentic
samples.[4–18,29–34] Mass and 1H NMR data for typical examples are given below:
Spectroscopic data of representative compounds
(1) 2-Thiocyanato phenol: 1H NMR (CDCl3): δ 7.14 (d, J = 8.1 Hz, 1H), 6.73 (m, J = 7.9 Hz,
2H), 6.54 (d, J = 8.1 Hz, 1H), 4.83 (s, 1H), m/z = 151.
(2) 2-methyl-4-thiocyanato-phenol: 1H NMR (CDCl3): δ 6.63–7.24 (m, 3H), 6.13 (brs, 1H), 3.26
(s, 3H), m/z = 165 (mp 70–74◦C).
(3) 3-methyl-4-thiocyanato-phenol: 1H NMR (CDCl3): δ 7.13 (d, J = 7.9 Hz, 1H), 6.93 (m, 2H),
4.84 (s, 1H), 2.32 (s, 1H), m/z = 165.
(4) 4-bromo-2-thiocyanato-phenol: 1H NMR (CDCl3): δ 7.22 (s, 1H), 7.03 (d, J = 8.1 Hz, 1H),
6.53 (d, J = 8.1 Hz, 1H), 4.84 (s, 1H) m/z = 230.
1
(5) 4-Thiocyanatoaniline: H NMR (CDCl3): δ 7.38 (d, J = 8.7 Hz, 2H), 6.66 (d, J = 8.7 Hz,
2H), 3.94 (brd s, 2H), 13C NMR (75 Hz, CDCl3): δ 148.7, 134.3, 116.1, 112.3, 109.5, m/z = 150
(m.p.51–54◦C.)
1
(6) 2-Choloro-4-thiocyanatoaniline: H NMR (CDCl3): δ 7.52 (d, J = 8.2 Hz, 2H), 7.26 (dd,
J = 8.1 Hz, 1H), 6.76 (d, J = 8.2 Hz, 1H), 4.37 (brd s, 2H). m/z = 184 (m.p. 59–61◦C).
1
(7) 3-Methoxy-4-thiocyanatoaniline: H NMR (CDCl3): δ 7.29 (d, J = 8.3 Hz, 1H), 6.28 (dd,
J = 8.3 Hz, 1H), 6.24 (d, J = 8.2 Hz, 1H), 3.98 (s, 2H), 3.87 (s, 3H). m/z = 180 (m.p. 99–101◦C)
1
(8) 4-Thiocyanato-N-methylaniline: H NMR (CDCl3): δ 7.37 (d, J = 8.64 Hz, 2H), 6.59 (d,
J = 8.68 Hz, 2H), 4.11 (brd s, 1H), 2.85 (s, 3H). m/z = 164.
1
(9) 4-Thiocyanato-N, N-dimethylaniline: H NMR (CDCl3): δ 7.44 (d, J = 8.8 Hz, 2H), 6.66
(d, J = 8.8 Hz, 2H), 3.02 (s, 6H), 13C NMR (75 MHz, CDCl3): 151.5, 134.3, 113.2, 112.9, 106.4,
40.2. m/z = 178 (m.p. 72–74◦C).
(10) 4-Thiocyanato-N, N-diethylaniline: 1H NMR (CDCl3): δ 7.5–7.35 (m, 2H), 7.2–7.11 (m, 1H),
6.62–6.60 (m, 1H), 3.38 (q, J = 7.12 Hz, 4H), 1.16 (t, J = 5.4 Hz, 6H). 13C NMR(CDCl3): δ
149.2, 134.8, 125.3, 121.6, 112.8, 44.7, 12.3 (m/z) = 206 (M+) (mp 82–84◦C).
(11) 4-Thiocyanatodiphenylamine: 1H NMR (CDCl3): δ 7.43 (d, J = 8.1 Hz, 2H), 7.34 (t, J =
8.2 Hz, 2H), 7.14 (d, J = 8.1 Hz, 2H), 7.08 (t, J = 8.2 Hz, 1H), 7.03 (d, J = 8.2 Hz, 2H), 5.94
(brd s, 1H). m/z = 226 (m.p. 62–63◦C).
(12) Dithiocyanatodiphenylamine: 1H NMR (CDCl3): δ 7.49 (d, J = 8.4 Hz, 4H), 7.13 (d, J =
8.4 Hz, 4H), 6.09 (brd s, 1H). m/z = 283 (m.p. 110–111◦C).
(13) 3-thiocyanato-1H-indole: 1H NMR (CDCl3): δ 7.30 (m, 2H, J = 9.9 Hz, J = 6.9 Hz), 7.42
(m, 1H, J = 9.9 Hz, J = 7.2 Hz), 7.48 (d, 1H, J = 3 Hz), 7.09 (dd, 1H, J = 5.7 Hz, J = 3 Hz),
8.72 (br s, 1H), 13 C NMR (75 MHz, CDCl3): δ 135.9, 131.1, 127.4, 123.7, 121.8, 118.5, 112.3,
112.1, 91.53. m/z = 174 (mp 72–73◦C).
1
(14) 1-Methyl-3-thiocyanatoindole: H NMR (CDCl3): δ 7.83–7.36 (m, 5H), 3.74 (s, 3H).
13C NMR (CDCl3): δ 137.2, 135.2, 128.4, 123.4, 121.6, 118.8, 112.1, 110.3, 33.4 (m/z) = 188.
mp 79–81◦C.
1
(15) 5-Bromo-3-thiocyanatoindole: H NMR (CDCl3): δ 8.87 (br s, 1H), 7.92–7.15 (m, 5H).
13C NMR (CDCl3): δ 134.6, 132.2, 129.3, 123.1, 121.2, 115.4, 113.7, 111.9, 102.2 (m/z) =
251(M = 1), 253 (M + 2) (mp 126–127◦C).

Journal of Sulfur Chemistry 611
1
(16) 2-thiocyanato-1H-pyrrole: H NMR (CDCl3): δ 6.27 (dd, H, J = 3 Hz, J = 6.3 Hz), 6.64
(m, 1H, J = 1.5 Hz, J = 3.6 Hz, J = 3.9 Hz), 6.96 (m, 1H, J = 1.5 Hz, J = 3 Hz, J = 4.5 Hz),
8.9 (s, 1H), 13CNMR(75 MHz, CDCl3): δ 124.3, 120.1, 111.1, 110.9, 102.8. m/z = 124.
(17) 2, 5-dithiocyanato-1H-pyrrole: 1H NMR (CDCl3) δ 1.49 (d, 2H, J = 2.4 Hz), 4.76 (s, 1H),
13C NMR (75 Hz, CDCl3): δ 121.2, 110.9, 109.4, m/z = 181 (mp 99–103◦C).
(18) 2-Thiocyanatothiophene: (Oil). 1H NMR (CDCl3): δ 7.45–8.10 (m, 3H).
1
(19) 1-Methoxy-4-thiocyanatonaphthalene: H NMR (300 MHz, CDCl3): δ 4.07 (s, 3H), 6.72–
6.74 (m, 1H), 7.60–8.31 (m, 5H) (mp 105–107◦C).
4.3. Procedure for USAR thiocyanation of organic compounds
Methodology for USNR of organic compounds is similar to the one described above. A reaction
flask containing a suspension of phenol (0.1 mol), ammonium metavanadate (0.001 mol), ammo-
nium thiocyanate (0.1 mol), and aqueous KHSO4 (0.1 mol), in about 20 mL solvent (MeCN) was
placed in a sonicator and the progress of the reaction was monitored by TLC. After completion
of the reaction, the reaction mixture was filtered and washed with water. The remaining work up
was similar to the one adopted for the conventional approach. All products are characterized by
comparison of their IR, 1H-NMR, and mass spectra and also with their physical data.
4.4. Procedure for microwave-assisted (MWAR) thiocyanation of organic compounds
A CEM microwave reactor was used, which was equipped with temperature, pressure, and
microwave power control units. An oven-dried microwave vial was charged with a mixture
containing a suspension of phenol (0.1 mol), ammonium metavanadate (0.001 mol), ammonium
thiocyanate (0.1 mol), aqueous KHSO4 (0.1 mol), and silica gel slurry, and was irradiated in a
microwave (power input 140W) at 150◦C for few minutes. After completion of the reaction, as
ascertained by TLC, the reaction mixture was treated with sodium bicarbonate; the organic layer
was diluted with DCM, and separated from the aqueous layer. The crude product mixture was
purified by recrystallization with an ethyl acetate DCM mixture. The purity was checked with
TLC. The products were identified by characteristic spectroscopic data.
Acknowledgements
The authors are indebted to Professor T. Navaneeth Rao (Former Vice-Chancellor, O.U.), Professor P.K. Saiprakash
(Former Science Faculty Dean, O.U.), Head, Department of Chemistry, O.U. for constant encouragement and facilities.
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