Rapid synthesis of a 13C6-benzothiazolium salt from 13C6-aniline

Article abstract of DOI:10.1002/jlcr.1854

¹³C₆-labeled aniline was used as a starting material for the facile synthesis of ¹³C₆-benzothiazolium salt (1). Copyright

Full text of DOI:10.1002/jlcr.1854

Research Article
Received 29 June 2010,
Revised 14 September 2010,
Accepted 19 October 2010 Published online 25 March 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.1854
Rapid synthesis of a 13C6-benzothiazolium salt
from C6-aniline
13
Ã
Chris V. Galliford, Kimberly Voronin, David Hesk, David Koharski, and
Paul McNamara
13C6-labeled aniline was used as a starting material for the facile synthesis of 13C6-benzothiazolium salt (1).
Keywords: benzothiazolium; carbon-13; thione
carried out under an atmosphere of argon or nitrogen in flame-
dried glassware with magnetic stirring. Reagents were purified
Introduction
The chemistry of benzothiazolium salts generated by alkylation
of benzothiazole thiones has been studied significantly.1 These
salts have been reported to inhibit nitric oxide production and
in the treatment of coronaviril infection,2 and are also useful
intermediates in the synthesis of bioactive 2-imino-benzothia-
zolines.3 The hydrolysis of these salts can be effected under a
variety of conditions leading to exclusively N-alkyl substituted
benzothiazoline structures which are useful building blocks in
the synthesis of compounds containing this heterocyclic motif.4
The synthesis of the benzothiazolium salts is readily achieved
by double alkylation of an appropriate ambidentate electrophile
by benzothiazole thione (2).5 2 is itself an important accelerator
in the vulcanization of rubber among other applications.6
Additionally, 2 has been utilized as a convenient leaving group
for the introduction of the Fmoc-protecting group (Figure 1)7.
13C6-labeled aniline was selected as a convenient precursor
(Scheme 1), as the direct conversion of aniline to benzothiazole
thione (2) has been well studied. First reported in 1923 by
Sebrell and Boord,8 (Scheme 2) the laboratory synthesis of this
compound by various strategies has been the subject of
extensive study in the literature.1,9
On an industrial scale, the most efficient methods to prepare
2 involve the direct reaction of near stoichiometric quantities of
aniline (3), elemental sulfur and carbon disulfide at high
temperature and pressure.10 Improved yields have been
reported using Brønsted acid catalysis.11 Nitrobenzene has also
been used as a precursor, undergoing in situ reduction to
generate aniline under the reaction conditions.12 Numerous
procedures for continuous recycling of unused starting materials
and reprocessing of crude material have also been reported.13
prior to use unless otherwise stated. Purification of reaction
products was carried out by flash chromatography using EM
Reagent silica gel 60 (230–400 mesh), or using an automated
ISCO CombiFlash Rf machine. Analytical thin layer chromato-
graphy was performed on EM Reagent 0.25 mm silica gel 60-F
plates. Visualization was accomplished with UV light and
anisaldehyde, ceric ammonium nitrate stain, or phospho-
molybdic acid followed by heating. 1H NMR spectra were
recorded on a Varian Mercury (400 MHz) spectrometer and are
reported in ppm using solvent as an internal standard (DMSO-
d6). Data are reported as (ap = apparent, s = singlet, d = doublet,
t = triplet, q = quartet, m = multiplet, b = broad; coupling con-
stant(s) in Hz; integration). Proton-decoupled 13C-NMR spectra
were recorded on a Mercury 400 (100 MHz) spectrometer and
are reported in ppm using solvent as an internal standard
(DMSO-d6). Mass spectrometry data was acquired on a JEOL
double focusing sector mass spectrometer operating in the FAB
ionization mode. PEG-600 was used as the reference for the
high-resolution measurements. Samples were dissolved in
DMSO and 3-NBA was used as the FAB matrix. The chemical
purities of 1 and 2 were determined by high-performance liquid
chromatography using a Waters 2487 Dual programmable
wavelength detector equipped with a Gemini NX 3 m C18
column, 4.6 mm ꢀ 50 mm, 254 nm and 323 nm, (50:50:0.1)
methanol:water:trifluoroacetic acid at 1 mL/min for 6 min
followed by a gradient to 100% methanol elution.
Benzothiazole thione (2)
Aniline, sulfur and carbon disulfide were transferred and
combined inside a stainless steel bomb reactor with an internal
volume of 18 mL. The reaction apparatus was heated to an oil
Experimental
General methods
Merck Research Laboratories, 2015 Galloping Hill Road, K-15-A4545, Kenilworth,
NJ 07033, USA
Pressure reactions were carried out in stainless steel screw-
capped bomb reactor obtained from Parr Instruments Co.
Caution: heating these reagents at high pressure requires a
closed fume hood and blast shield. All other reactions were
*Correspondence to: Chris V. Galliford, Merck Research Laboratories, 2015
Galloping Hill Road, K-15-A4545, Kenilworth, NJ 07033, USA.
E-mail: chris.galliford@spcorp.com
J. Label Compd. Radiopharm 2011, 54 229–232
Copyright r 2011 John Wiley & Sons, Ltd.

C. V. Galliford et al.
13C6-Benzothiazole thione (13C6-2)
bath temperature of 2701C (using fresh 710 CS heat-transfer
fluid). The mixture was then heated for a further 12–48 h as per
the entries in Table 1. The reaction vessel was removed from the
heat source and allowed to cool over a 1 h period, before being
brought to ambient temperature using an ice bath. The bomb
reactor was carefully opened and the crude mixture purified
directly by flash column chromatography using gradient elution
0–30% MTBE/hexanes (product Rf = 0.29, 20% MTBE/hexanes,
visualized by ceric ammonium nitrate). Samples of the
intermediates phenyl isothiocyanate (4), N,N-diphenyl thiourea
(5) (Rf = 0.09 and Rf = 0.29 in 20% MTBE/hexanes respectively)
were also isolated from the reaction mixture. 1H NMR
(tautomeric mixture, DMSO-d6) d, ppm: 8.15 s, 1H 7.6–7.48 d,
2H; 7.41–7.18 m, 1H. 13C NMR (DMSO-d6) d, ppm: 190.0, 141.3,
128.2, 26.9, 124.0, 121.4, 112.1. MS: M1 173.99 (100%) found
174.15.
13C6-Aniline, sulfur and carbon disulfide were transferred and
combined inside a stainless steel bomb reactor with an internal
volume of 18 mL. The reaction apparatus was heated to an oil
bath temperature of 2701C (using fresh 710 CS heat-transfer
fluid). The mixture was then heated for a further 12 h. The
reaction vessel was removed from the heat source and allowed
to cool over a 1 h period, before being brought to ambient
temperature using an ice bath. The bomb reactor was carefully
opened and the crude mixture purified directly by flash column
chromatography using gradient elution 0–30% MTBE/hexanes
(product Rf = 0.29, 20% MTBE/hexanes, visualized by ceric
ammonium nitrate). After SiO2 chromatography, 5.89 g of 13C6-
2 (86% yield) was isolated with spectral and chromatographic
analyses consistent with the data above.
2,3-dihydrothiazolo[2,3-b] benzothiazolium bromide (1)
To a mixture of 2-benzothiazoline thione (250 mg, 1.44 mmol) in
DMF (1.5 mL), was added NaOH pellets (60 mg, 1.51 mmol). The
resulting solution was stirred at room temperature until a
homogenous solution was observed. This solution was then
added dropwise into a solution of ethylene dibromide (1.24 ml,
14.43 mmol) in DMF (1 mL) at 751C over 10 min via syringe
pump. A white solid precipitated at the end of the addition and
the reaction mixture was stirred at 751C for 30 min, and then at
room temperature overnight. The white solid was then filtered
and washed with CH2Cl2 (2 ꢀ 10 mL), followed by drying in a
vacuum overnight to give the product as a white crystalline
solid (185 mg, 46% yield), and an oily residue containing the
remaining product that was not further manipulated. 1: 1H NMR
(DMSO-d6) d, ppm: 8.31–8.29 (d; J = 7.5 Hz; 1H), 7.99–7.96 d;
J = 7.5 Hz; 1H), 7.80–7.74 t; J = 7.5 Hz; 1H). 7.61–7.66 t; J = 7.5 Hz;
1H), 5.00–4.95 t; J = 7.9 Hz; 2H), 4.27–4.22 t; J = 7.9 Hz; 2H). 13C
NMR (DMSO-d6) d, ppm: 182.0, 137.4, 133.9, 128.6, 126.6, 124.7,
115.6, 51.3, 37.1. HR MS: M1 194.0098 (100%) found 194.0097.
13C6-1: 1H NMR (DMSO-d6) d, ppm: 8.31–8.29 (m; 1H),
7.99–7.96 m; 1H), 7.80–7.74 m; 1H). 7.61–7.66 m; 1H), 5.00–4.95
Figure 1. Benzothiazolium salt.
Scheme 1.
Scheme 2.
Table 1. Table of optimization
Entry
PhNH2:S:CS2
Reaction conditions (h)a
Scaleb
Yield %c
1
2
3
4
5
6
7
8
1:1.05:1.1
1:1.05:1.1
1:1.05:1.1
1:1.05:1.1
1:1.05:1.1
1:1.5:1.5
0.8:1.05:1.5
1.2:1.05:1.1
1:1.05:1.1
1:1.05:1.5
11:1.05:1.1
12
12
16
48
12
20
20
20
20
20
20
32
16
8
8
39.6
8
8
8
4
4
79
70
44
48
86d
39
27
22e
25
6
9
10
11
4
0f
aAll reactions were performed in a sealed Parr stainless steel bomb reactor heated by an oil bath containing 710 CS heat transfer
fluid at an oil bath temperature of 260–2701C for the indicated duration.
bBased on mmol PhNH2.
cIsolated yields after SiO2 chromatography.
dScale used for production of 13C6-2.
eIn addition to 5, an unidentified impurity, close in Rf to the product was isolated from this crude reaction mixture.
fAniline hydrochloride used in place of aniline.
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J. Label Compd. Radiopharm 2011, 54 229–232

C. V. Galliford et al.
t; J = 7.9 Hz; 2H), 4.27–4.22 t; J = 7.9 Hz; 2H). 13C NMR (DMSO-d6)
d, ppm: 182.0, 137.4, 133.9, 128.6, 126.6, 124.7, 115.6, 51.3, 37.1.
MS: M1 200.0229 (100%) found 200.0303.
13C6-2,3-dihydrothiazolo[2,3-b] benzothiazolium bromide
(13C6-1)
Figure 2. Isolated side products.
To a mixture of 2-benzothiazoline thione (250 mg, 1.44 mmol) in
DMF (1.5 mL), was added NaOH pellets (60 mg, 1.51 mmol). The
resulting solution was stirred at room temperature until a
homogenous solution was observed. This solution was then
added dropwise into a solution of ethylene dibromide (1.24 ml,
14.43 mmol) in DMF (1 mL) at 75 1C over 10 min via syringe
pump. A white solid precipitated at the end of the addition and
the reaction mixture was stirred at 75 1C for 30 min, and then at
room temperature overnight. The white solid was then carefully
filtered and washed with CH2Cl2 (2 ꢀ 10 mL), followed by drying
in a vacuum overnight to give the product as a white crystalline
solid (300 mg, 74% yield). The isolated product provided spectral
and chromatographic analyses consistent with the data above.
Scheme 3.
of each reactant (Entries 6–8 and 10). This deleterious effect
upon lowering the reaction scale is most likely due to decreased
pressure in the reaction chamber.
With optimized conditions for the synthesis of 2 in hand, we
were able to prepare 5.89 g (86% isolated yield) of 13C6-2
(Table 1, entry 5). Completion of the synthesis was achieved by
treatment of 13C6-2 with an excess dibromoethane in dimethyl-
formamide to yield in 13C6-1 74% yield (Scheme 3).
Results and discussion
Unfortunately, our initial attempts to replicate this process using
either screw-capped pressure reaction vials or microwave vials
either failed completely or gave only trace quantities of product.
Either insufficient heat was provided to start the reaction, or in
the case where 41701C was used as the reaction temperature,
the resultant high pressures generated as the reaction
proceeded caused the seal of the vial to fail.
Conclusion
A convenient and rapid synthesis of the 13C6-2,3-dihydrothiazo-
lo[2,3-b] benzothiazolium salt (1) was accomplished. A high-
yielding preparation of benzothiazole thione (2) in 13C6-labeled
form was also developed.
Successfully down-sizing this industrial process to a laboratory
scale was eventually achieved by utilizing a stainless steel bomb
reactor with an 18 mL internal capacity reaction chamber. This References
removed the hazards associated with sealed tube reactions in
glass vessels and also allowed for sufficiently high pressures of
hydrogen disulfide to be generated to allow the reaction to
proceed to completion.10 Our initial run under these conditions
furnished an isolated yield of 2 in 44% yield after column
chromatography (Table 1, entry 3). Gratifyingly, by using the
same apparatus and conditions, we were able to investigate the
effect of internal pressure by simply increasing the scale of
reaction within the same bomb reactor. In this way the optimal
scale on which to perform our reaction was identified (Table 1).
We were able to observe conversion of aniline to the
intermediates phenyl isothiocyanate (4) and N,N0-diphenyl
thiourea (5) by LC-MS of the reaction mixtures, and by isolation
of these compounds as side products (Figure 2). In most cases
the mass balance was completely accounted for by the
formation of these side products.
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J. Label Compd. Radiopharm 2011, 54 229–232