2,2-Azobenzothiazole as a new recyclable oxidant for heterogeneous thiocyanation of aromatic compounds with ammonium thiocyanate

Article abstract of DOI:10.1080/00397911.2010.551699

A simple synthesis of thiocyanated aromatics has been developed by using heteroaromatic azo compounds such as 2,2′-azobenzothiazole as a mild and heterogeneous oxidant. The reactions proceed at room temperature with good regioselectivity. The hydrazine by

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2,2′-Azobenzothiazole as a New
Recyclable Oxidant for Heterogeneous
Thiocyanation of Aromatic Compounds
with Ammonium Thiocyanate
Nasser Iranpoor a , Habib Firouzabadi a , Rezvan Shahin a & Dariush
Khalili a
a Department of Chemistry, College of Sciences, Shiraz University,
Shiraz, Iran
Accepted author version posted online: 27 Oct 2011.Published
online: 26 Mar 2012.
To cite this article: Nasser Iranpoor , Habib Firouzabadi , Rezvan Shahin & Dariush Khalili (2012):
2,2′-Azobenzothiazole as a New Recyclable Oxidant for Heterogeneous Thiocyanation of Aromatic
Compounds with Ammonium Thiocyanate, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 42:14, 2040-2047
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Synthetic Communications1, 42: 2040–2047, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.551699
2,2’-AZOBENZOTHIAZOLE AS A NEW RECYCLABLE
OXIDANT FOR HETEROGENEOUS THIOCYANATION
OF AROMATIC COMPOUNDS WITH AMMONIUM
THIOCYANATE
Nasser Iranpoor, Habib Firouzabadi, Rezvan Shahin, and
Dariush Khalili
Department of Chemistry, College of Sciences, Shiraz University,
Shiraz, Iran
GRAPHICAL ABSTRACT
Abstract A simple synthesis of thiocyanated aromatics has been developed by using hetero-
aromatic azo compounds such as 2,20-azobenzothiazole as a mild and heterogeneous
oxidant. The reactions proceed at room temperature with good regioselectivity. The
hydrazine by-product can be easily separated by filtration and recycled for further use.
Keywords Aromatic compounds; azobenzothiazole; heterogeneous; thiocyanation
INTRODUCTION
Thiocyanation of aromatics and heteroaromatics is an important reaction in
organic synthesis.[1] Aryl or heteroaryl thiocyanates are versatile intermediates in
the synthesis of sulfur-containing heterocycles.[2] Moreover, aryl thiocyanates can
be easily converted into various sulfur-bearing functionalities such as sulfides, aryl
thioesters,[3a,b] and cyanothiolated compounds.[3c] Several methods for the thiocya-
nation of aromatic systems using a variety of reagents such as oxone,[4] iodine,[5]
ferric chloride,[6] N-thiocyanatosuccinimide,[7] ceric ammonium nitrate,[8] acidic
K-10 clay,[9] phenyliodine(III) bis(trifluoroacetate) (PIFA),[10] PhICl2,[11] diacetoxyio-
dobenzene,[12] 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ),[13] iodic acid,[14]
Mn(OAc)3,[15] Al2O3=MeSO3H,[16] I2O5,[17] and iodoxybenzoic acid (IBX),[18] have
Received August 13, 2010.
Address correspondence to Nasser Iranpoor or Habib Firouzabadi, Department of Chemistry,
College of Sciences, Shiraz University, Shiraz 71454, Iran. E-mail: iranpoor@susc.ac.ir; firouzabadi@susc.ac.ir
2040

THIOCYANATION OF AROMATIC AMINES
2041
been explored. Recently, we have reported on the use of diethylazodicarboxylate
(DEAD) as an oxidant for this reaction.[19a] However, DEAD is explosive, photosensi-
tive, toxic, shock sensitive, and thermally unstable.[19b] Because of these practical issues,
there is a requirement to develop new efficient and simple routes for thiocyanation of
aromatic and heteroaromatic compounds. In this context, we report a novel, simple,
and efficient protocol for the thiocyanation of electron-rich arenes such as indoles,
pyrrole, and aryl amines using NH4SCN in conjunction with 2,20-azobenzothiazole in
CH3CN at room temperature. This azo reagent has several advantages over the previous
reagents and also DEAD including easy handling, simple synthesis, and heterogeneity.
To have easily prepared oxidants, we first synthesized some heteroaromatic azo com-
pounds A1–A6[20] and studied their applicability for thiocyanation of aromatic com-
pounds. The reaction of N,N-dimethylaniline (1 eq) with ammonium thiocyanate (3
eq) in the presence of these azo compounds (A1–A6: 1.9 eq) at room temperature was
chosen as a model reaction. Among these azo compounds, 2,20-azobenzothiazole A6
was found to be the most suitable oxidant for this reaction in acetonitrile (Table 1).
A major advantage of using 2,20-azobenzothiazole A6 is the heterogeneous
nature of this azo and also its hydrazine, which facilitates the purification of the pro-
duct. In addition, A6 is readily prepared in one step by the treatment of 2-aminoben-
zothiazole with commercial sodium hypochlorite (aqueous 6–14%).[20a] The produced
hydrazine (Fig. 1, II) can be easily removed by filtration and reoxidized to the original
azo A6 by oxidation with PhI(OAc)2 in dimethylsufoxide (DMSO) at 60 ꢀC. With the
optimized reaction conditions in hand, we probed the scope of our reagent system for
thiocyanation of various activated aromatic rings (Table 2).
Aniline (Table 2, entry 2) and some of its N-substituted derivatives (Table 2,
entries 3–5) reacted efficiently with ammonium thiocyanate in the presence of
Table 1. Thiocyanation of N,N-dimethylaniline utilizing different heterocyclic azo compounds A1–A6
Entry
Azo
Yield of 2a (%)a
Time (h)
1
2
3
4
5
6
A1
A2
A3
A4
A5
A6
59
45
77
60
12
87
12
12
8
5
24
2
aIsolated yields.

2042
N. IRANPOOR ET AL.
Figure 1. Proposed mechanism for the thiocyanation of aromatic amines with 2,20-azobenzothiazole.
2,20-azobenzothiazole A6 and resulted in the formation of arene thiocyanates 2b–e in
good yields. Generation of ammonia in these reactions has no effect on the produced
product. In the case of aryl amines 1a–e, the products were obtained with excellent
para-selectivity. Even under optimized condition, when 3-methoxyaniline 1f was
used, no reaction occurred and all the starting material was recovered after 24 h
(Table 2, entry 6). Like N,N-dimethylaniline, pyrrole also afforded the correspond-
ing 2-thiocyanatopyrroles 2g in good yield (Table 2, entry 7). In the case of pyrrole, a
minor amount of 2,4-dithiocyanatopyrrole 2g0 was formed, which increased with a
longer reaction time. We also studied the conversion of indole (Table 2, entry 8)
and its derivatives (Table 2, entries 8–13) to their corresponding thiocyanato indoles
under similar reaction conditions. Substituted indoles such as N-methylindole
(Table 2, entry 9), 2-methylindoles (Table 2, entry 10), 5-methoxyindole (Table 2,
entry 11), 5-bromoindole (Table 2, entry 12), and 5-nitroindole predominantly gave
the corresponding thiocyanato products. Indole with an electron-withdrawing group
on the aryl ring affords a lesser product yield. Consistent with this observation, the
product yield of 5-nitroindole is generally less than for 5-methoxyindole. No reaction
took place when we used phenol and anisol as substrate (Table 2, entries 14 and 15).
As in the present work, thiocyanation of aromatic amines with 2,20-azobenzothiazole
can be carried out under heterogeneous conditions, which provides easier workup,
although its reactivity is comparable with those reagents that have been used for this
reaction under homogeneous condition (Table 3).
The proposed reaction mechanism is shown in Fig. 1. Addition of ammonium
thiocyanate to azo A6 produces the corresponding thiocyanato hydrazine I, and the
red color of the reaction changes to dark green. In a subsequent reaction, aromatic
amine reacts with I to furnish the end product together with the formation of hydra-
zine II. In addition to the formation of ammonia and isolation of the corresponding
hydrazine II, to have more evidence in support of the proposed mechanism, we
1
performed a H and 13C NMR study (in DMSO-d6) of the reaction mixture of
azo A6 and ammonium thiocyanate prior to the addition of arene. The presence
of the NH proton and N-SCN group at 8.46 ppm and 126.32 ppm supports the
intermediacy of I in the proposed mechanism.

THIOCYANATION OF AROMATIC AMINES
2043
Table 2. Thiocyanation of activated arenes promoted by 2,20-azobenzothiazole A6 in the presence of
NH4SCNa
Entry
Arene
Productb
Time (h)
Yield (%)c
1
2
3
4
5
2[13b]
5[13b]
4[13b]
4[5]
87
80
81
85
86
5[5]
6
—
—
—
80
5
7d
5.5[13b]
8
9
2[8]
83
90
3[13b]
10
11
4.5[13b]
85
83
80
3[5]
12
13
5[13b]
6[9]
75
(Continued )

2044
N. IRANPOOR ET AL.
Table 2. Continued
Entry
Arene
Productb
Time (h)
Yield (%)c
14
15
Unreacted starting material
Unreacted starting material
—
—
—
—
aMolar ratio of arene: A6:NH4SCN was 1:1.9:3.
bAll the products are known compounds and were identified from their spectral data and comparison
with known samples.
cIsolated yields.
dAfter 18 h, the bis-adduct was obtained in 12% yield.
In summary, we have developed a simple, convenient, and efficient protocol for
the preparation of aryl and hetero aryl thiocyanates. The use of 2,20-
azobenzothiazole as a new, stable, and heterogeneous oxidant provides mild
conditions for the regioselective electrophilic thiocyanation of aromatic and hetero-
aromatic amines at room temperature. The simple preparation of this oxidant and
ease of removal with the possibility of reoxidation of the produced hydrazine can
be considered as other advantages of this system.
EXPERIMENTAL
All the solvents and reagents were purchased from Fluka or Merck chemical
companies. The products were purified by column or prepartory thin-layer chroma-
tographic (TLC) techniques and identified by comparison of their spectral data with
those of known compounds. Fourier transform infrared (FTIR) spectra were
recorded on a Shimadzu DR-8001 spectrometer. NMR spectra were recorded on a
Brucker Avance DPX 250-MHz instrument. Elemental analyses were determined
in our department using a ThermoFinnigan Flash EA 1112 series instrument.
Typical Procedure
The arene (1.0 mmol, 0.12) was added slowly in a dropwise manner to a stirred
solution of ammonium thiocyanate (3 mmol, 0.23 g), and 2,20-azobenzothiazole
Table 3. Comparison of the reactivity of 2,20-azobenzothiazole for heterogeneous thiocynation of N,N-
dimethylaniline and homogeneous thiocyanation using some other reagents
Entry
Oxidant
Condition
Time
Yield (%)
Reference
1
2
3
4
5
6
2,20-Azobenzothiazole
Rt=CH3CN
2 h
1.5 h
87
81
78
87
75
95
This work
DEAD
PhI(OAc)2
I2
Rt=CH3CN
Rt=CH3CN
Rt=MeOH
19a
12
5
2 h
20 min.
15 min.
15 min.
CAN
DDQ
Rt=MeOH
Sonication=MeOH
8
13b

THIOCYANATION OF AROMATIC AMINES
2045
(1.9 mmol, 0.56 g) in dry CH3CN (5 mL), and the mixture was stirred at room tem-
perature for the appropriate time (Table 2). After completion of the reaction as indi-
cated by TLC (2 h), the reaction mixture was filtered to remove the hydrazine
by-product. Evaporation of the filtrate and flash chromatography with EtOAc=
n-hexane (1:5) gave N,N-dimethyl-4-thiocyanatoaniline 2a in 87% yield (0.15 g).
Compound 2a
Mp 73–74 ꢀC lit.[12b] 71–72 ꢀC IR (KBr, cmꢁ1): 2158 (SCN). 1H NMR
(250 MHz, CDCl3) d: 7.33 (d, 2H, J ¼ 5.4 Hz), 6.60 (d, 2H, J ¼ 5.5 Hz), 2.91 (s,
6H). 13C NMR (62.9 MHz, CDCl3) d: 151.6, 134.5, 113.1, 112.8, 106.7, 40.1. Anal.
calcd. for C9H10N2S: C, 60.64; H, 5.65; N, 15.72%. Found: C, 61.02; H, 5.11; N,
16.12%.
4-Thiocyanatoaniline (2b)
IR (KBr, cmꢁ1): 3366 (NH2), 2145 (SCN). 1H NMR (250 MHz, CDCl3) d: 7.43
(d, 2H, J ¼ 8.5 Hz), 6.71 (d, 2H, J ¼ 8.5 Hz), 3.94 (br, s, 2H, NH2). 13C NMR
(62.9 MHz, CDCl3) d: 148.9, 133.5, 116.8, 112.4, 109.4. Anal. calcd. for C7H6N2S:
C, 55.97; H, 4.03; N, 18.65%. Found: C, 56.20; H, 4.19; N, 18.47%.
4-Thiocyanato-N-methylaniline (2c)
IR (KBr, cmꢁ1): 3380 (NH), 2140 (SCN). 1H NMR (250 MHz, CDCl3) d: 7.35
(d, 2H, J ¼ 8.6 Hz), 6.63 (d, 2H, J ¼ 8.6 Hz), 4.09 (br, s, 1H, NH), 2.80 (s, 3H). 13C
NMR (62.9 MHz, CDCl3) d: 150.4, 131.7, 117.2, 115.0, 112.6, 29.8. Anal. calcd. for
C8H8N2S: C, 58.51; H, 4.91; N, 17.06%. Found: C, 58.75; H, 4.63; N, 17.42%.
3-Thiocyanato-1H-indole (2h)
IR (KBr, cmꢁ1): 3315 (NH), 2159 (SCN). 1H NMR (250 MHz, CDCl3) d: 8.96
(br, s, 1H, NH), 7.81 (1H, d, J ¼ 9.1 Hz), 7.43–7.28 (4H, m). 13C NMR (62.9 MHz,
CDCl3) d: 136.1, 131.3, 127.7, 123.8, 121.8, 118.6, 112.3, 112.1, 91.6. Anal. calcd. for
C9H6N2S: C, 62.05; H, 3.47; N, 16.08%. Found: C, 62.41; H, 3.17; N, 15.78%.
1-Methyl-3-thiocyanato-1H-indole (2i)
1
IR (KBr, cmꢁ1): 2148 (SCN). H NMR (250 MHz, CDCl3) d: 7.81 (d, 1H,
J ¼ 8.5 Hz), 7.45–7.38 (m, 4H), 3.80 (s, 3H). 13C NMR (62.9 MHz, CDCl3) d:
136.9, 133.0, 128.6, 122.3, 121.9, 120.8, 113.1, 112.3, 88.4, 37.1. Anal. calcd. for
C10H8N2S: C, 63.80; H, 4.28; N, 14.88%. Found: C, 63.56; H, 4.48; N, 14.79%.
2-Methyl-3-thiocyanato-1H-indole (2j)
IR (KBr, cmꢁ1): 3326 (NH), 2139 (SCN). 1H NMR (250 MHz, CDCl3) d: 8.48
(br, s, 1H, NH), 7.82 (d, 1H, J ¼ 7.5 Hz), 7.35 (d, 1H, J ¼ 7.63 Hz), 7.29–7.20 (m,
2H), 2.62 (s, 3H). 13C NMR (62.9 MHz, CDCl3) d: 138.4, 135.0, 128.6, 123.1,

2046
N. IRANPOOR ET AL.
120.8, 119.5, 112.2, 111.4, 86.6, 12.5. Anal. calcd. for C10H8N2S: C, 63.80; H, 4.28;
N, 14.88%. Found: C, 64.00; H, 4.07; N, 14.66%.
5-Methoxy-3-thiocyanato-1H-indole (2k)
IR (KBr, cmꢁ1): 3304 (NH), 3138, 2150 (SCN). 1H NMR (250 MHz, CDCl3) d:
8.77 (br, s, 1H, NH), 7.42 (d, 1H, J ¼ 2.4 Hz), 7.29 (d, 1H, J ¼ 8.5 Hz), 7.14 (d, 1H,
J ¼ 2.1 Hz), 6.97 (dd, 1H, J ¼ 8.5 Hz, J ¼ 2.8 Hz), 3.93 (s, 3H); 13C NMR (62.9 MHz,
CDCl3) d: 157.1, 131.9, 130.5 127.9, 114.0, 113.0, 112.1, 98.9, 93.5, 55.5. Anal. calcd.
for C10H8N2OS: C, 58.80; H, 3.95; N, 13.72%. Found: C, 58.49; H, 3.76; N, 14.03%.
5-Bromo-3-thiocyanato-1H-indole (2l)
IR (KBr, cmꢁ1): 3318 (NH), 2135 (SCN). 1H NMR (250 MHz, CDCl3) d: 8.69
(br, s, 1H, NH), 7.90 (s, 1H), 7.53 (d, 1H, J ¼ 2.8 Hz), 7.37 (dd, 1H, J ¼ 8.5, 1.6 Hz),
7.26 (d, 1H, J ¼ 8.5 Hz). 13C NMR (62.9 MHz, CDCl3) d: 137.7, 132.8, 129.5, 127.6,
123.1, 117.1, 113.8, 112.5, 93.4. Anal. calcd. for C9H5BrN2S: C, 42.71; H, 1.99; N,
11.07%. Found: C, 42.55; H, 2.12; N, 11.30%.
ACKNOWLEDGMENTS
We are thankful to the Organization of Management and Planning of Iran and
Shiraz University Research Council for the partial financial support of this work.
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