Oxidant-free thiocyanation of phenols and carbonyl compounds under solvent-free conditions by AlCl 3/NH 4SCN

Article abstract of DOI:10.1080/17415993.2015.1091888

A simple, efficient, solvent-free, and mild method for thiocyanation of phenols by ammonium thiocyanate (NH₄SCN) in the presence of AlCl₃ has been developed. This new methodology was used to prepare para-thiocyanated products in good to excellent yields at 70°C. In addition, the successful α-thiocyanation of various ketones has also been described. Finally, a plausible mechanism of thiocyanation has been suggested.

Full text of DOI:10.1080/17415993.2015.1091888

Journal of Sulfur Chemistry
ISSN: 1741-5993 (Print) 1741-6000 (Online) Journal homepage: http://www.tandfonline.com/loi/gsrp20
Oxidant-free thiocyanation of phenols and
carbonyl compounds under solvent-free
conditions by AlCl3/NH4SCN
Kobra Nikoofar & Samareh Gorji
To cite this article: Kobra Nikoofar & Samareh Gorji (2015): Oxidant-free thiocyanation of
phenols and carbonyl compounds under solvent-free conditions by AlCl3/NH4SCN, Journal of
Sulfur Chemistry, DOI: 10.1080/17415993.2015.1091888
To link to this article: http://dx.doi.org/10.1080/17415993.2015.1091888
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Date: 14 December 2015, At: 09:56

JOURNAL OF SULFUR CHEMISTRY, 2015
http://dx.doi.org/10.1080/17415993.2015.1091888
Oxidant-free thiocyanation of phenols and carbonyl
compounds under solvent-free conditions by AlCl3/NH4SCN
Kobra Nikoofar and Samareh Gorji
Department of Chemistry, Faculty of Physics and Chemistry, Alzahra University, Tehran, Iran
ABSTRACT
ARTICLE HISTORY
Received 12 July 2015
Accepted 4 September 2015
A simple, efficient, solvent-free, and mild method for thiocyanation
of phenols by ammonium thiocyanate (NH4SCN) in the presence of
AlCl3 has been developed. This new methodology was used to pre-
pare para-thiocyanated products in good to excellent yields at 70°C.
In addition, the successful α-thiocyanation of various ketones has
also been described. Finally, a plausible mechanism of thiocyanation
has been suggested.
KEYWORDS
Thiocyanation; phenol;
carbonyl compound; AlCl3;
solvent-free conditions
O
O
SCN
CH3
1 h, 92%
r.t.
AlCl3
NH4SCN
70°C
OH
OH
4 h, 85%
SCN
1. Introduction
Thiocyanate is a versatile synthon,[1] which can be easily transferred to other functional
groups such as sulfides,[2] thioesters,[3] mercaptoindoles,[4] and thiocarbamates.[5]
In particular, α-ketothiocyanates are useful intermediates in the synthesis of sulfur-
containing heterocycles, such as thiazoles.[6] Some of these thiazoles exhibit herbicidal and
other important biological properties.[7] Thiocyanation of aromatic and heteroaromatic
compounds is an important transformation in both organic synthesis and in the produc-
tion of pharmaceuticals.[8,9] Organic compounds that contain the thiocyano group have
been used as precursors for agrochemical, dyes, and drugs.[10] This moiety is a significant
CONTACT Kobra Nikoofar
kobranikoofar@yahoo.com
© 2015 Taylor & Francis

2
K. NIKOOFAR AND S. GORJI
OH
OH
AlCl3
70 0C, Solvent-free
NH4SCN
+
SCN
1a
2a
O
O
SCN
AlCl3
CH3
NH4SCN
+
Solvent-free, r.t.
1f
2f
Scheme 1. Thiocyanation of phenols and carbonyl compounds.
functionality in several anticancer agents.[11] Several methods for the α-thiocyanation
of carbonyl compounds and phenols using various reagents such bromo dimethyl-
sulfonium bromide/ammonium thiocyanate,[12] (dichloroiodo)benzene and potassium
thiocyanate,[13] heteopolyacid/ammonium thiocyanate,[14] (dichloroiodo)benzene/lead
(II) thiocyanate,[15] potassium peroxydisulfate/copper (II) complex,[16] I2 /ammonium
thiocyanate,[17] NH4VO3/KHSO4/ammonium thiocyanate,[18] and bromodimethylsol-
fonium bromide/ammonium thiocyanate[19] have been reported. However, most of these
reported methods involve the use of a large excess of strong oxidizing agents and toxic
metal thiocyanates. Since organosulfur compounds have become increasingly useful and
important in the field of drugs and pharmaceuticals,[20] the development of simple,
convenient, and efficient approaches for their synthesis is desirable.
Anhydrous AlCl3 is probably the most commonly used Lewis acid. This white rela-
tively nontoxic powder with LD50 = 3730 mg kg−1 is a potent moisture-adsorbent. Water
removal can be achieved by sublimation to produce anhydrous AlCl3. It has been exten-
sively used as a strong Lewis acid in many organic reactions such as Friedel-Crafts
reactions and Fries rearrangement.[21,22] Recently, AlCl3 has been employed for some
other transformations such as generation of highly strained cycloheptadienones,[23] selec-
tive synthesis of 2-aryl-2H- and 4-aryl-4H-3,5-diformylpyrans,[24] asymmetric aldehydes
arylation,[25] 1,4-dihydropyridines synthesis,[26] aldol cyclocondensation,[27] 1,2,3,4-
tetrahydroquinolines synthesis,[28] and substituted guanidines synthesis.[29]
In a continuation of our research interest on thiocyanation of organic compounds,
[30–34] and following our successful results in thiocyanation of N-containing compounds,
[35] herein we report AlCl3 as a commercially available and low-cost promoter for the thio-
cyanation of some phenols and ketones in the presence of NH4SCN, as thiocyanate ion
source, under solvent-free conditions (Scheme 1).
2. Results and discussion
Thiocyanation of phenols were performed in the presence of AlCl3, at 70°C under solvent-
free conditions. Initial studies were done using phenol as a substrate model. The results are
summarized in Table 1. By checking various amounts of NH4SCN and AlCl3, we deter-
mined that a NH4SCN/AlCl3 molar ratio (2/1) afforded the best results (Entries 1–4).
We also optimized the temperature and confirmed that 70°C is the most effective (Entries

JOURNAL OF SULFUR CHEMISTRY
3
Table 1. Optimization of the reaction conditions for the synthesis of 4-thiocyanatophenol 1a.
Entry
Conditiona
Temp. (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
AlCl3/NH4SCN molar ratio (0.5/2)/solvent-free
AlCl3/NH4SCN molar ratio (1/2)/solvent-free
AlCl3/NH4SCN molar ratio (1.5/2)/solvent-free
AlCl3/NH4SCN molar ratio (2/2)/solvent-free
AlCl3/NH4SCN molar ratio (1/2)/solvent-free
AlCl3/NH4SCN molar ratio (1/2)/solvent-free
AlCl3/NH4SCN molar ratio (1/2)/solvent-free
AlCl3/NH4SCN molar ratio (1/2)/MeOHc
AlCl3/NH4SCN molar ratio (1/2)/EtOHc
r.t.
r.t.
r.t.
r.t.
40
60
70
r.t.
r.t.
r.t.
1.5
0.5
0.75
1
2
2
1
2.5
4
80
85
80
75
65
70
92
45
70
50
AlCl3/NH4SCN molar ratio (1/2)/CH3CNc
2
a1 mmol of phenol was used.
bIsolated yields.
c5 mL of solvent was used.
5–7). The effect of solvent has also been examined (Entries 8–10). The data confirmed that
solvent-free conditions gave the best results.
Therefore, the reactions were performed with these experimentally determined opti-
mized conditions using 1 mmol of substrate, 1 mmol of AlCl3, and 2 mmol of NH4SCN
at 70°C under solvent-free conditions. According to the data in Table 2, phenol 1a and
its electron-donating derivative 3-methyl phenol 1b successfully thiocyanated at their 4-
positions. 4-Bromo phenol 1c with an electron-withdrawing substituent yielded its corre-
sponding 4-bromo-2-thiocyanatophenol 2c. On the other hand, 4-bromophenol is blocked
at the para-position, so the thiocyanation occurred at C2 of the phenol ring. 1-Naphthol
1d and 2-naphthol 1e, as other samples of phenolic group, produced 4-thiocyanato-1-
naphthol 2d and 1-thiocyanato-2-naphthol 2e, respectively. For the case of 2-naphthol 1e,
oxathiole has previously been reported as the main product instead of 2e.[13]
We have also examined and successfully achieved α-thiocyanation of some carbonyl
compounds in the presence of AlCl3, at room temperature under solvent-free conditions.
Initially, acetophenone 1f has been chosen as the model substrate for this study. α-
Thiocyanation has been performed at various molar ratios of AlCl3/NH4SCN at room tem-
perature under solvent-free conditions (Table 3, Entries 1–4). The optimal AlCl3/NH4SCN
ratio was determined to be 2/1 (Entry 2). Increasing the temperature did not enhance the
reaction progress (Entries 5,6). In addition, performing the reaction in the presence of dif-
ferent solvents did not improve the thiocyanation (Entries 7–9). The result presented in
Table 3 confirms that doing the thiocyanaion reactions under these optimized conditions
using a 1/2 molar ratio of AlCl3/NH4SCN at room temperature without a solvent is the
most appropriate reaction protocol.
Thiocyanation of 1f occurred at the α-position of the carbonyl moiety using these opti-
mized conditions to afford the 1-phenyl-2-thiocyanatoethanone 2f within 4 h in 85% yield
(Scheme 1). Similarly, other substituted acetophenones, such as 2-hydroxy-, 4-nitro-, 4-
methoxy-, and 4-bromo derivatives 1g–1j underwent smooth thiocyanation to furnish
their corresponding α-thiocyanoketones 2g–2j in good yields (Table 4). The excellent
reactivity of AlCl3 in the thiocyanation of different aryl methyl ketones prompted us to
extend this method to 4-methyl cyclohexanone 1k as a representative cyclic ketone. This
compound gave the corresponding 2-thiocyano-4-methylcyclohexanone 2k in excellent
yield.

4
K. NIKOOFAR AND S. GORJI
Table 2. Thiocyanation of phenols 1a–1e with NH4SCN (2 mmol) in the presence of AlCl3 (1 mmol)
under solvent-free conditions and at 70°C.
Substrate
Producta
Time (h) Yieldb (%)
m.p.c (°C)
1a
2a
1
92
52–53, Lit. [19]: 51–52
1b
2b
0.5
1.5
1
80
83
68
122–125
133–135
1c
2c
1d
2d
146–147, Lit. [36]: 147–148
1e
2e
1
45
104, Lit. [37]: 107
aAll products were characterized by comparison of their spectroscopic data (IR, 1H-NMR, and CHN analysis) with those
reported in literature.
bYield of isolated product.
cReference of known compounds.
Although the actual mechanism of the thiocyanation is not obvious, we suggested a
plausible route in Scheme 2. Anhydrous AlCl3, as Lewis acid, could form a complex A
with NH4SCN. Nucleophilic attack of phenol 1a on A produces B, which is followed by a
proton-release leading to the corresponding thiocyanated product 2a.
3. Conclusion
In conclusion, we report AlCl3 as a commercially available, inexpensive, and non-
volatile promoter for the thiocyanation of various phenols and carbonyl compounds. This

JOURNAL OF SULFUR CHEMISTRY
5
Table 3. Screening the reaction conditions for the synthesis of α-thiocyanatoacetophenone 1f.
Entry
Conditiona
Temp. (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
AlCl3/NH4SCN molar ratio (0.5/2)/solvent-free
AlCl3/NH4SCN molar ratio (1/2)/solvent-free
AlCl3/NH4SCN molar ratio (1.5/2)/solvent-free
AlCl3/NH4SCN molar ratio (2/2)/solvent-free
AlCl3/NH4SCN molar ratio (1/2)/solvent-free
AlCl3/NH4SCN molar ratio (1/2)/solvent-free
AlCl3/NH4SCN molar ratio (1/2)/MeOHc
AlCl3/NH4SCN molar ratio (1/2)/EtOHc
r.t.
r.t.
r.t.
r.t.
50
70
r.t.
r.t.
r.t.
1.25
4
5.5
4.5
2.5
3
3.25
5
7
75
85
45
60
50
65
70
54
-
AlCl3/NH4SCN molar ratio (1/2)/CH3CNc
a1 mmol of acetophenone was used.
bIsolated yields.
c5 mL of solvent was used.
non-corrosive and commonly found Lewis acid promotes the activation of NH4SCN for
the nucleophilic attack of the substrates. The regioselectivity of the procedure was spec-
ified in para-thiocyanation of phenols and α-thiocyanation of carbonyl compounds. In
addition, the absence of hazardous oxidants and non-green solvents, easy work-up, and its
wide-range thiocyanating power for different substrates are some noteworthy advantages
of the reported procedure.
4. Experimental section
4.1. General
Chemicals and solvents were purchased from Merck, Aldrich, and Alfa Aesar and used
without further purifications. Commercial AlCl3 was heated and its sublimated anhy-
drous form was used. Melting points were determined using a Stuart Scientific apparatus
and are uncorrected. All products were identified by their spectral data. IR spectra (KBr
discs, 500–4000 cm−1) were recorded using a Bruker FT-IR model Tensor 27 spectrometer.
1H-NMR spectra were recorded in CDCl3 solvent on a Brucker 400 MHz spectrome-
ter. Elemental analyses were determined using a Thermo-Finnigan Flash EA 1112 Series.
Preparative layer chromatography (PLC) was carried out on 20 × 20 cm2 plates, coated
with a 1 mm layer of Merck silica gel PF254, prepared by applying the silica as slurry and
drying in air. Yields refer to isolated products.
4.2. General procedure for thiocyanation
A mixture of substrates 1a–1e (1 mmol), anhydrous AlCl3 (1 mmol), and NH4SCN
(2 mmol) was stirred at 70°C. The progress of the reaction was monitored by TLC (eluent:
n-hexane:EtOAc, 3:1). After completion (Table 2, 0.5–1.5 h), the residue was diluted
with distillated water (20 mL) and extracted with chloroform (2 × 20 mL). The combined
organic layer was dried over MgSO4 and evaporated. The resulting crude product was
purified by plate chromatography on silica gel (PLC) to afford the pure products 2a–2e
(45–92%). In the case of carbonyl compounds, a mixture of substrates 1f–k (1 mmol),
anhydrous AlCl3 (1 mmol), and NH4SCN (2 mmol) was stirred at room temperature for
the appropriate time (Table 4, 1–5 h). The work-up procedure is the same as that of phe-
nols. The pure products were obtained in 68–85%. All the products were characterized by

6
K. NIKOOFAR AND S. GORJI
Table 4. α-Thiocyanation of carbonyl compounds1f–1k withNH4SCN(2 mmol) inthe presence ofAlCl3
(1 mmol) under solvent-free conditions at room temperature.
Substrate
Producta
Time (h) Yieldb (%)
m.p.c (°C)
1f
2f
4
85
69–70, Lit. [38]:
68–70
1g
1h
2g 3.5
70
70
112–114
2h
5
118–120,
Lit. [12]:
119–120
1i
2i
4
4.5
1
68
73
82
120–122,
Lit. [12]:
121–125
1j
2j
123–125,
Lit. [39]:
122–125
1k
2k
Semi solid
aAll products were characterized by comparison of their spectroscopic data (IR, 1H-NMR) with those reported in literature.
bYield of isolated product.
cReference of known compounds.

JOURNAL OF SULFUR CHEMISTRY
7
+
OH
NH3
OH
_
+
NH4SCN
HCl Al
NH3SCN
AlCl3
+
+
3
_
HCl3Al
H
SCN
1a
A
B
OH
H2
AlCl3
+
+
SCN
2a
Scheme 2. The proposed mechanism of thiocyanation by AlCl3/NH4SCN.
comparison of their spectroscopic data (FT-IR, 1HNMR, and CHN analysis) with those of
the authentic samples in literature. The spectral data of new compounds are given below.
4-Thiocyanato-3-methylphenol (2b): FT-IR (KBr): ν = 3423 (OH), 2220 (SCN), 1547,
1420, 1098, 656 (C-S) cm−1. 1H NMR (CDCl3): δ = 3.72 (s, 3H, CH3), 5.22 (s, 1H), 6.43-
(s, 1H, OH), 7.23–6.91 (m, 2H) ppm. Anal. Calcd for C8H7SNO: C 59.37%, H 7.27%, N
9.68%. Found: C 59.25%, H 7.95%, N 9.75%.
4-Bromo-2-thiocyanatophenol (2c): FT-IR (KBr): ν = 3421 (OH), 2256 (SCN), 1461,
1
1241, 825 (C–S) cm−1. H NMR (CDCl3): δ = 6.43 (s, 1H, OH), 6.75–6.84 (m, 2H),
6.89–6.92 (m, 1H) ppm. Anal Calcd. for C7H4SNOBr: C 36.54%, H 1.75%, N 6.95%. Found:
C 37.01%, H 1.83%, N 7.21%.
α-Thiocyanato-2-hydroxyacetephenone (2g): FT-IR (KBr): ν = 3428 (OH), 2177
1
(SCN), 1728(C=O), 1644, 1458, 1108, 790 (C–S) cm−1. H NMR (CDCl3): δ = 3.57
(s, 2H), 6.91–7.12 (m, 2H), 7.13–7.20 (m, 2H), 7.61 (s, 1H, OH) ppm. Anal. Calcd for
C9H7SNO2: C 55.95%, H 3.65%, N 7.25%. Found: C 55.01%, H 3.78%, N 7.78%.
α-Thiocyanato-4-methylcyclohexanone (2k): FT-IR (KBr): ν = 2068 (SCN), 1634
1
(C=O), 1416, 1104, 794, 671 (C–S) cm−1. H NMR (CDCl3): δ = 2.49 (s, 3H, CH3),
1.49–1.99 (m, 2H), 1.33–1.45 (m, 1H), 1.14–1.30 (m, 5H) ppm. Anal. Calcd for C8H11SNO:
C 56.77%, H 6.55%, N 9.45%. Found: C 56.93%, H 6.41%, N 9.24%.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The authors acknowledge the financial support from the Research Council of Alzahra University.
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