Determination of the promoting effect of nano SiO2 and H3PO4@nano SiO2 in the thiocyanation of N-containing aromatic compounds under solvent-free conditions

Article abstract of DOI:10.1080/17415993.2015.1004066

Silica nanoparticles/ammonium thiocyanate (nano SiO₂/NH₄SCN) and H₃PO₄ embedded on nano silica (H₃PO₄@nano SiO₂) in the presence of NH₄SCN were found to be effective systems for the thiocyanation of some arylamines and indoles to afford their corresponding thiocyanated adducts at 70C under solventfree conditions. The recovery and reusability of nano SiO₂ as a prompting system have been investigated. A simple procedure for the synthesis of H₃PO₄@nano SiO₂ has also been represented. In addition, a plausible mechanism of thiocyanation has also been suggested.

Full text of DOI:10.1080/17415993.2015.1004066

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Determination of the promoting effect
of nano SiO2 and H3PO4@nano SiO2
in the thiocyanation of N-containing
aromatic compounds under solvent-
free conditions
Kobra Nikoofara & Samareh Gorjia
a Department of Chemistry, Faculty of Physics and Chemistry,
Alzahra University, Tehran, Iran
Published online: 29 Jan 2015.
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To cite this article: Kobra Nikoofar & Samareh Gorji (2015): Determination of the promoting effect
of nano SiO2 and H3PO4@nano SiO2 in the thiocyanation of N-containing aromatic compounds under
solvent-free conditions, Journal of Sulfur Chemistry, DOI: 10.1080/17415993.2015.1004066
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Journal of Sulfur Chemistry, 2015
http://dx.doi.org/10.1080/17415993.2015.1004066
Determination of the promoting effect of nano SiO2 and
H3PO4@nano SiO2 in the thiocyanation of N-containing
aromatic compounds under solvent-free conditions
Kobra Nikoofar∗ and Samareh Gorji
Department of Chemistry, Faculty of Physics and Chemistry, Alzahra University, Tehran, Iran
(Received 10 October 2014; accepted 31 December 2014)
Silica nanoparticles/ammonium thiocyanate (nano SiO2/NH4SCN) and H3PO4 embedded on nano silica
(H3PO4@nano SiO2) in the presence of NH4SCN were found to be effective systems for the thiocyanation
of some arylamines and indoles to afford their corresponding thiocyanated adducts at 70°C under solvent-
free conditions. The recovery and reusability of nano SiO2 as a prompting system have been investigated.
A simple procedure for the synthesis of H3PO4@nano SiO2 has also been represented. In addition, a
plausible mechanism of thiocyanation has also been suggested.
SCN
Nano SiO2 / NH4SCN
Solvent-free, 70ºC
N
N
H
H
1h, 90%
Keywords: thiocyanation; indole; nano SiO2; H3PO4@nano SiO2; solvent-free conditions
1. Introduction
Thiocyanated aromatic and heteroaromatic compounds are important in both organic syn-
thesis and in pharmaceuticals.[1,2] Some organic compounds that contain thiocyanato group
have been used as precursors for agrochemicals, dyes, and drugs.[3] This moiety is a
significant functionality in several anticancer agents.[4] Most critical applications of the thio-
cyanate motif have been observed when it is used as a functional group in N-bearing (het-
ero)arene compounds. Nitrogen occurs in all living organisms, primarily in amino acids and
thus in proteins and in the nucleic acids (DNA and RNA).[5] Several methods have been
explored for the thiocyanation of (hetero)aromatic systems using various reagents such as
oxone/NH4SCN,[6] N-thiocyanosuccinimde,[7] FeCl3/NH4SCN,[8] cerric ammonium nitrate
(CAN)/NH4SCN,[9] montmorillonite K-10 clay/NH4SCN,[10] poly(4-diacetoxyiodo)styrene
(PDAIS)/NH4SCN,[11] acidic Al2O3/NH4SCN,[12] HCl/H2O2/KSCN,[13] trichloroisocyanuric
acid (TCCA)/NH4SCN/wet SiO2,[14] and citric acid/KSCN/H2O2.[15] Although the reported
*Corresponding author. Emails: knikoofar@yahoo.com, k.nikoofar@alzahra.ac.ir
c
ꢀ 2015 Taylor & Francis

2
K. Nikoofar and S. Gorji
SCN
Nano SiO2/ NH4SCN
Solvent-free, 70 C
N
N
H
H
1a
2a
Scheme 1. Thiocyanation of N-containing aromatic compounds by nano SiO2/NH4SCN.
methods are efficient, some of them have disadvantages including low yields, strongly oxidiz-
ing conditions, long reaction times, high temperatures and use of expensive and toxic reagent
or solvent. So developing new protocols for the thiocyanation of organic compounds is still in
demand.
Nano catalysts offer higher surface to volume ratio and fewer coordinating sites in compar-
ison to bulk counterparts, which are responsible for their higher catalytic activity as well as
selectivity. In addition, they have distinct advantages such as high atom efficiency, easy product
purification, and reusability.[16–18] SiO2 nanoparticles (SiO2 NPs) have been a focus of exten-
sive research in the catalyst field due to their chemical stability, large surface area, non-toxicity
(LD50: 20,800 mg/kg), reasonable cost, and environmentally friendliness in the context of green
synthesis.[19–21] Furthermore, nanometer silicon dioxide (nano SiO2) has played a catalytic
role in a wide range of transformations such as 2-aryloxazines and 2-aryltetrahydropyrimidines
synthesis,[22] synthesis of 3-oxo-3-phenylpropanamid derivatives,[23] bis-Michael addition of
active methylene compounds to conjugated alkenes,[24] synthesis of 4H-pyrans and poly-
substituted aniline derivatives,[25] synthesis of thioethers, thioesters, vinyl thioethers, and
thio-Michael adducts,[26] synthesis of 2-hydroxyacetamide,[27] and synthesis of 2,3-dihydro-
1H-isoindolones.[28]
In continuation of our research interest on the thiocyanation of (hetero)aromatic organics,[29–
32] herein we report nano SiO2 as a commercially available, low-cost and non-oxidative
promoter for the thiocyanation of some N-containing heteroaromatics and N-activated arenes
in the presence of NH4SCN at 70°C under solvent-free conditions (Scheme 1 and Table 2).
2. Results and discussion
In order to determine the optimized reaction condition, initial studies were performed with indole
1a as a model substrate. The results are summarized in Table 1. Optimization of the temperature
effect confirmed that 70°C is appropriate (Entries 1–4). The effect of solvent has also been exam-
ined. The data confirmed solvent-free conditions are the most effective situation (Entries 5, 6).
By checking various amounts of nano SiO2, we determined that 0.02 g afforded the best results
(Entries 3, 7). Decreasing the NH4SCN amount to 1.5 mmol slowed the reaction progress (Entry
8). In order to be sure about the nano silica promoting role in the thiocyanation reaction, the
model reaction was examined in the presence of 2 mmol of NH4SCN without any nano SiO2
(Entry 9). Comparison of the results affirmed the unique promoting activity of nano SiO2.
Therefore, performing the reaction in the presence of substrates (1 mmol), NH4SCN (2 mmol),
and nano SiO2 (0.02 g) at 70°C under solvent-free condition has been chosen as the optimized
situation for our procedure.
Applying the optimized conditions, different aromatic and heteroaromatic have been thio-
cyanated. The results are summarized in Table 2. According to the data in Table 2, indole
1a, and its electron donating derivatives such as 2-methyl indole 1b and 1-methyl indole 1c
thiocyanated at their 3-position successfully. 5-Bromoindole 1d, as an electron-withdrawing

Journal of Sulfur Chemistry
Table 1. Optimization of the reaction conditions for the synthesis of 3-thiocyanatoindole 2a.
3
Entry
Conditiona
Temperature (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8d
9
Nano SiO2 (0.02 g)/solvent-free
Nano SiO2 (0.02 g)/solvent-free
Nano SiO2 (0.02 g)/solvent-free
Nano SiO2 (0.02 g)/solvent-free
Nano SiO2 (0.02 g)/MeOHc
Nano SiO2 (0.02 g)/H2Oc
Nano SiO2 (0.0015 g)/solvent-free
Nano SiO2 (0.02 g)/solvent-free
Solvent-free
r.t.
80
70
1
1
0.5
1
1
2
0.45
1
57
90
90
64
51
–
90
82
< 20
60
Reflux
Reflux
70
70
70
5
aIndole/NH4SCN molar ratio of 1/2 has been used.
bIsolated yields.
c5 ml of solvent was used.
dIndole/NH4SCN molar ratio of 1/1.5 has been used.
candidate of indole derivatives, yielded its corresponding 3-thioyanato-5-bromoindole 2d. Isatin
1e was another N-bearing heteroaromaitc which thiocyanated at 5-position successfully. It must
be mentioned that the literature reports for isatin thiocyanation are rare. Carbazole 1f pro-
duced its correspondence 4-thiocyanato derivative in good yield. This result also demonstrates
the regioselectivity of the method since 4,4ꢁ-dithiocyanatocarbazole was not formed. To illus-
trate the efficacy of nano SiO2 for the thiocyanation of a wide range of N-containing organic
compounds, we conducted the thiocyanation reaction of aniline 1 g as an example of a N-
activated aromatic substrate. 4-Thiocyantoaniline 2 g was the only product. N-methylaniline
and N,N-dimethylaniline also thiocyanated exclusively at their para position (Table 2, 2 h–2i).
Diphenylamine 1j also monothiocyanted at its para position to afford 2j. This case is another
demonstration of the regioselectivity of the protocol, since 4,4ꢁ-dithiocyanatodiphenylamine
was not obtained. Utilizing 3-methylindole 1 k as another substrate, which contains a blocked
3-position, produced 3-methyl-2-thiocyantoindole 2 k.
The reusability of nano SiO2 in the thiocyanation of indole (as an example) has been examined.
For this reason, after completion of the reaction, the nano silica was filtered from the reaction
mixture by adding MeOH (15 ml) and washed with further methanol (15 ml). The solid was
heated in oven at 100°C for 30 min. This recovered catalyst was used for three runs without any
loss of activity (Table 2, Entry 1).
In the next step, in order to increase the promoting role of nano SiO2, concentrated H3PO4
was loaded on nano SiO2 via a simple procedure. The acidic capacity of the synthesized
H3PO4@nano SiO2 was determined to be 15.38 meq g−1 via titration of 0.1 g of the catalyst with
a 0.1 N standard solution of NaOH. The nano catalyst was then characterized by FT-IR spectra.
As shown in Figure 1, the Si–O–H and Si–O–Si stretching bands appear at 933 and 1093 cm−1
.
Stretching bands of P–O–H, P–O, and P–O are also found at 814, 1033, and 1639 cm−1, respec-
tively. The bands in the range of 2827–3307 cm−1 are related to the free silanol group of nano
SiO2 and also hydroxyl moieties of phosphoric acid. Based on the obtained results, a structure
for H3PO4@nano SiO2 has been offered. According to Scheme 2, the synthesized reagent has
both Lewis and Bronsted acid moieties.
The size of H3PO4@nano SiO2 NPs has been determined from the scanning electron
microscopy (SEM) image. As can be seen from Figure 2, the average diameter of nano particles
is in the range of 30–50 nm.
In order to investigate the efficacy of H3PO4@nano SiO2, the thiocyanation of some N-
bearing aromatic and heteroaromatic compounds have been performed in the presence of this
nanopromoter with NH4SCN at 70°C under solvent-free conditions. The results are summarized
in Table 3.

4
K. Nikoofar and S. Gorji
Table 2. Thiocyanation of aromatic and heteroaromatic compounds with NH4SCN in the presence of nano SiO2 under
solvent-free conditions at 70°C.
Substrate
Product
Time (h) Yield (%)
m.p. (°C)
SCN
1a
1b
2a
1
90
70
101–102, Lit. [12]: 103–105
N
H
N
H
SCN
CH3
2b 0.25
99–101, Lit. [8]: 102–103
N
H
CH3
N
H
SCN
N
1c
2c 0.5
67
70
83–84, Lit. [11]: 80–81
N
CH3
CH3
CH3
Br
Br
1d
2d
2e
2f
2
8
8
127–129, Lit. [11]: 125–126
N
N
H
H
O
O
NCS
1e
1f
32
62
201, Lit. [8]: 203–204
90–92, Lit. [10]: 89–90
O
O
N
N
H
H
SCN
N
N
H
H
1 g
1 h
2 g
2 h
2
4
80
83
51–52, Lit. [14]: 50–51
Liquid, Lit. [14]: liquid
NH2
NCS
NCS
NCS
NH2
CH3
H
N
N
CH3
CH3
H
CH3
CH3
N
1i
1j
2i
1
78
50
71–72, Lit. [14]: 72–74
62–64, Lit. [29]: 62–64
N
CH3
H
H
N
2j 0.75
N
SCN
CH3
CH3
1 k
2 k
5
64
86, –
SCN
N
N
H
H
aAll products were characterized by comparison of their spectroscopic data (IR, 1H-NMR) with those reported in the literature.
bYield of isolated product.
cReference of known compounds.

Journal of Sulfur Chemistry
5
Figure 1. FT-IR spectra of synthesized H3PO4@nano SiO2.
OH
HO
Bronsted acid site
Si
P
O
O
O
HO
O
Si
Si
Si
O
O
O
Lewis acid site
Scheme 2. The structure of H3PO4@nano SiO2.
Figure 2. The SEM image of H3PO4@nano SiO2.
Although the actual mechanism of thiocyanation is not yet clear, we have suggested a plausi-
ble mechanism for nano SiO2-promoted thiocyanation (Scheme 3). It seems that nano silica as
Lewis acid can interact with thiocyanate anion to form I. Interaction of I with NH4SCN lead to
thiocyanogen II. An examination of the mechanisms, that were previously suggested, shows that
thiocyanogen is an important dimer in thiocyanation procedures.[33,34] As shown in Scheme

6
K. Nikoofar and S. Gorji
Table 3. Thiocyanation of aromatic and heteroaromatic compounds with NH4SCN in the presence of H3PO4@nano
SiO2 under solvent-free conditions at 70°C.
Substrate
Product
SCN
Time (h) Yield (%)
m.p. (°C)
1a
1b
2a
0.5
90
68
101–102, Lit. [12]: 103–105
N
H
N
H
SCN
2b 0.25
99–101, Lit. [8]: 102–103
N
H
CH3
N
H
SCN
1c
2c
0.5
0.5
63
75
83–84, Lit. [11]: 80–81
N
N
CH3
CH3
CH3
Br
Br
1d
2d
127–129, Lit. [11]: 125–126
N
N
H
H
O
O
NCS
1e
2e
8
38
201, Lit. [8]: 203–204
O
O
N
N
H
H
1 g
1 h
2 g
2 h
5
7
70
87
51–52, Lit. [14]: 50–51
Liquid, Lit. [14]: liquid
NCS
NH2
NH2
H
CH3
N
NCS
N
CH3
H
H
H
N
N
1j
2j 0.75
80
80
62–64, Lit. [29]: 62–64
SCN
CH3
CH3
SCN
1 k
2 k
5
5
86, –
N
N
H
H
NH2
NH2
1l
2l
78
60
190–191, Lit. [13]: 191–194
Oil, Lit. [14]: oil
SCN
1 m
2 m 6.5
SCN
N
H
N
H
aAll products were characterized by comparison of their spectroscopic data (IR, 1H-NMR) with those reported in the literature.
bYield of isolated product.
cReference of known compounds.

Journal of Sulfur Chemistry
7
-
NCS
H+
d
..
N
H
SiO2
NH4SCN
NCS
SCN
NH
SCN
+
+
II
d
SCN
III
I
- H+
N
H
Scheme 3. The proposed mechanism of thiocyanation.
3, electrophilic attack of indole on (SCN)2 leads to III which is followed by a loss of a proton
to obtain the corresponding thiocyanated product. In the case of H3PO4@nano SiO2, the Bron-
sted site, in conjunction with the Lewis acid moiety, can accelerate thiocyanogen formation.
The shorter reaction time in the presence of H3PO4@nano SiO2, relative to sole nano silica, is
evidence for this explanation.
3. Conclusion
In summary, nano SiO2/NH4SCN has been employed as a novel and highly efficient system
for the thiocyanation reaction of N-bearing (hetero)aromatic compounds at 70°C under solvent-
free conditions. Nano silica is a commercially available, inexpensive, and non-corrosive Lewis
acid that can promote the thiocyanation of various N-containing substrates. Regioselectivity has
been demonstrated in the synthesis of mono-thiocyanated carbazole and p-thiocyanated anilines.
H3PO4@nano SiO2, as another promoting reagent for thiocyanation reactions, has been syn-
thesized via a simple procedure. Examining the thiocyanation of various indoles, anilines, and
pyrrole in the presence of H3PO4@nano SiO2/NH4SCN, confirms it increased efficacy in com-
parison to nano silica under the same conditions. The reusability of nano SiO2 has also been
demonstrated in several runs without any activity loss. We suggest that it is the acidic nature of
the reagents that are the driving force of the reactions. In addition, the absence of hazardous and
non-green solvents and oxidants, easy work-up procedure, and wide-range thiocyanating power
for different substrates are some other noteworthy advantages of the reported procedure.
4. Experimental section
4.1. General
Chemicals and solvents were purchased from Merck and Aldrich and used without further
purifications except some liquid anilines which have been distilled before usage. Commercial
amorphous nano SiO2 with average particle diameter of 20–30 nm and specific surface area of
180–270 m2/g was purchased from Tecnan Company. Melting points were determined using a
Stuart Scientific apparatus and are uncorrected. FT-IR spectra (KBr discs, 500–4000 cm−1) were
1
recorded using a Bruker FT-IR model Tensor 27 spectrometer. H NMR spectra were recorded
in CDCl3 solvent on a Bruker 400 MHz spectrometer. Mass spectra were obtained on Platform II
spectrometer from Micromass; EI mode at 70 eV. Preparative layer chromatography (PLC) car-
ried out on 20 × 20 cm2 plates, coated with a 1 mm layer of Merck silica gel PF254, prepared by

8
K. Nikoofar and S. Gorji
applying the silica as slurry and drying in air. The SEM recorded by EM 3200, kyky apparatus.
Yields refer to isolated products.
4.2. Preparation of H3PO4@nano SiO2
To a solution of nano SiO2 (2.5 g) in dried diethyl ether (15 ml), concentrated H3PO4 (1 ml)
has been added. The mixture was stirred at room temperature for 100 min. After vaporization of
Et2O, the solid residue heated in an oven at 110°C for 3 h. The pale gray powder is H3PO4@nano
SiO2, which has been characterized by titration, FT-IR spectra, and also SEM image.
4.3. General procedure for the thiocyanation of N-containing hetero(aromatic) compounds
by nano SiO2 or H3PO4@nano SiO2 in the presence of NH4SCN
A mixture of substrates 1a–1 m (1 mmol), NH4SCN (2 mmol), and nano SiO2 or H3PO4@nano
SiO2 (0.02 g) was stirred at 70°C in an oil bath. The progress of the reaction was monitored by
TLC (eluent: n-hexane:EtOAc, 9:2). After completion, CH3OH (20 ml) was added and filtered.
The solid residue was washed with further CH3OH (10 ml). The resulting solution was purified
by plate chromatography on silica gel (PLC) to afford the pure products 2a–2 m. All the products
were characterized by comparison of their spectroscopic data (FT-IR, 1H-NMR, and Mass spec-
tra) with those of the authentic samples in the literature. The spectral data of a selected compound
are given below.
2-Thiocyanato-3-methylindole (2k): FT-IR (KBr): v = 3418 (NH), 2218 (SCN), 1457, 1098,
1
797 (C-S) cm−1. H NMR (400 MHz, DMSO): δ = 2.48 (s, 3H, CH3), 7.01 (t, J = 9.46 Hz,
1H), 7.18 (t, J = 9.4 Hz, 1H), 7.40 (d, J = 10.24 Hz, 1H), 7.58 (d, J = 10.6 Hz, 1H), 8.81 (br
s, 1H, NH) ppm. EI-MS m/e (%):188 [M+] (100), 173 [M+-CH3] (37), 162 [M+-CN] (27), 146
[M+-HCN, -CH3] (40), 130 [M+-SCN] (12).
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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