2-Nitrosobenzothiazoles: useful synthons for new azobenzothiazole dyes

Article abstract of DOI:10.1016/j.tetlet.2008.09.096

Novel azobenzothiazole dyes have been synthesized by condensation of 2-nitrosobenzothiazole and 6-nitro-2-nitrosobenzothiazole with aniline, anthranilic acid, 3-hydroxymethylaniline, 2-, 3- and 4-chloroaniline, 4-fluoroaniline, 4-iodoaniline or 4-nitroaniline. The new synthetic approach described is advantageous over the classic diazotization process commonly used for the preparation of related disperse dyes, since the presence of an electron-donating group at the para-position, or equivalent, of the coupling component is no more a pre-requisite for the success of the condensation reaction.

Full text of DOI:10.1016/j.tetlet.2008.09.096

Tetrahedron Letters 49 (2008) 6907–6909
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Tetrahedron Letters
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2-Nitrosobenzothiazoles: useful synthons for new azobenzothiazole dyes
Hélio Faustino a, Reda M. El-Shishtawy a, Lucinda V. Reis b, Paulo F. Santos b, Paulo Almeida a,
*
a Departamento de Química and Unidade de I&D de Materiais Têxteis e Papeleiros, Universidade da Beira Interior, 6201-001 Covilhá, Portugal
b Departamento de Qu´ımica and Centro de Qu´ımica, Vila Real, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel azobenzothiazole dyes have been synthesized by condensation of 2-nitrosobenzothiazole and 6-
nitro-2-nitrosobenzothiazole with aniline, anthranilic acid, 3-hydroxymethylaniline, 2-, 3- and 4-chloro-
aniline, 4-fluoroaniline, 4-iodoaniline or 4-nitroaniline. The new synthetic approach described is advan-
tageous over the classic diazotization process commonly used for the preparation of related disperse
dyes, since the presence of an electron-donating group at the para-position, or equivalent, of the coupling
component is no more a pre-requisite for the success of the condensation reaction.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 5 August 2008
Accepted 17 September 2008
Available online 20 September 2008
Keywords:
Disperse dyes
Benzothiazole
Azo
Nitroso condensation
Azo compounds constitute the largest class of compounds
amongst all known families of dyes. The structural versatility
inherent to the methodology of preparation and the application
in a variety of fields, being the use as textile fiber dyestuff the most
prominent one, are undoubtedly on the basis of their popularity.
Despite the endless number of patents and papers describing the
synthesis and dyeing properties of this group of dyes that can be
found in the literature, the diazotization of an aromatic or hetero-
aromatic primary amine, followed by coupling with an electron-
donating aromatic compound are the two steps by which
practically almost all azo dyes are prepared.1
Azo dyes based on heterocyclic amines, in particular, have
found great success due to their higher tinctorial strength and
brighter dyeing in relation to diazo dyes based uniquely on ani-
line.2 Most disperse heteroarylazo and disazo dyes of technical
interest for application to textiles are constituted by five-mem-
bered rings containing one sulfur atom and a diazotizable amino
group. Additionally, this heterocyclic ring may possess one or
two nitrogen atoms and be fused to an aromatic ring. Systems of
this type, with emphasis on thiazoles, benzothiazoles, isothiazoles,
benzoisothiazoles, thiadiazoles, and thiophenes, have been well re-
viewed by Towns.3
both in industry and in academia.5 Aside from the continuous
interest on benzothiazole disperse dyes for textiles dying, a diverse
range of non-textile applications have also emerged. As an exam-
ple, thiazole and azobenzothiazole dyes have found application
in liquid crystal technology,6 reprography,7 and non-linear optics
(NLO),8 and more recently have been investigated as potential sen-
sitizers for photodynamic therapy (PDT).9 Nevertheless, the major-
ity of the developments concerning azobenzothiazole dyes have
tended to focus on series of compounds where the substitution
pattern was varied and the color–structure relationship, as well
as the effect of the solvent, temperature, and acidity in the visible
spectrum, was studied.
Some significant azobenzothiazole derivatives hitherto
described are alkoxy,6,8a,10 alkyl,8a,11 (di and tri)chloro,8a,11 dialkyl-
amino,12 (di)nitro,2b,8a,c,10c,d,f,11a,c,13 methylsulfonyl,11c thio-
cyanates,12 and compounds bearing multiple combinations of
different substituents,8a,11a,d in addition to the unsubstituted
one.2b,8a,10a,c,f,12,13b Of this group of dyes, molecules produced by the
coupling of diazotized 2-aminobenzothiazole to aniline derivatives
are by far the most explored case. Other suitably substituted couplers
lessfrequentlyusedarealkoxybenzenes,6 azulenes,8a benzopyranon-
es,2b imidazoles,11c naphthalenes,10d,13b naphthalimides,14 pyra-
zoles,10a pyrazolones,10b pyridones,10f and thiophenes.13a
Benzothiazole-based disperse dyes are considered to be the first
example of the successful commercial exploitation of heterocyclic
amines, by using the 2-aminobenzothiazole nucleus as diazonium
component in the production of red dyes.4 Due to their cheapness,
brightness, and dyeing performance, this type of dyes has become
economically important, motivating a substantial research effort,
Invariably, azo- and disazobenzothiazole disperse dyes have
been prepared by diazotization of an aminobenzothiazole, fol-
lowed by coupling with an electron-donating aromatic molecule.
Consequently, the presence of an electron-donating group or
equivalent in the aromatic coupler is always a precondition for
the success of the coupling reaction, being the location of the
new bond limited to the para-position in relation to the donating
group. To extend the access to new azobenzothioazole dyes,
* Corresponding author. Tel.: +351 275 319 761; fax: +351 275 319 730.
E-mail address: paulo.almeida@ubi.pt (P. Almeida).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.096

6908
H. Faustino et al. / Tetrahedron Letters 49 (2008) 6907–6909
Table 1
especially to those bearing electron-withdrawing groups at the
coupling component, we have envisioned an alternative synthetic
route to this family of dyes based on the condensation of a reactive
2-nitrosobenzothiazole with an aromatic amine, for which a strong
electron-donating capability is dispensable.
Yields, melting points, and kmax of azobenzothiazole dyes 4a–l
Dye
Yield (%)
Mp (°C)
kmax (nm) (MeOH)
R1
R2
4a
4b
4c
4d
4e
4f
4g
4h
4i
H
H
H
H
2-Cl
3-CH2OH
H
2-CO2H
2-Cl
3-Cl
3-CH2OH
4-F
4-Cl
4-I
30
10
16
53
63
58
63
43
72
82
77
33
99–100
105–107
87–89
328
332
328
340
340
340
336
340
344
346
370
332
Amongst the wide variety of available synthetic procedures for
the oxidation of an aromatic primary amine to the nitroso group,15
the use of Caro’s acid related oxidant, commercially available as
OxoneÒ,16 has revealed to be the most effective one. Other com-
mon oxidation reagents, such as peracetic acid17 or meta-chloro-
perbenzoic acid,18 have failed due to a lack of efficiency or to the
production of very complex reaction mixtures. Therefore, 2-nitro-
sobenzothiazoles 2a,b were prepared by oxidation of the corre-
sponding 2-aminobenzothiazoles 1a,b with OxoneÒ, in an
aqueous–organic system (H2O/MeOH/CHCl3—1:1:5).19 In these
conditions, the success of the reaction was found to critically de-
pend on the solubility of the aniline in water, since the reaction oc-
curs in the organic phase. In fact, 6-nitro-2-nitrosobenzothiazole
(2b) was prepared in a much higher yield than the parent unsub-
stituted 2a, due to the less solubility of 2-amino-6-nitrobenzo-
thiazole (1b) in water when compared to that of 1a. A large
volume of organic solvents was also used to avoid undesirable con-
densation reactions (Scheme 1).
To assess the generality of our methodology both 2-nitro-
sobenzothiazoles 2a,b were condensed with aniline (3a), anthra-
nilic acid (3b), 2-chloroaniline (3c), 3-chloroaniline (3d),
3-hydroxymethylaniline (3e), 4-chloroaniline (3f), 4-fluoroaniline
(3g), 4-iodoaniline (3h), or 4-nitroaniline (3i) (Scheme 2).20 The
condensation reaction was carried out at room temperature in gla-
cial acetic acid for 0.5 h to 3 days. The azobenzothiazole dyes 4a–l
are precipitated from the reaction mixture, and could be readily
isolated by simple filtration in 10–82% non-optimized yield (Table
1).
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
137–139
188–190
194–196
138–140
127–130
155–156
166–167
190–192
173–175
4j
4k
4l
4-NO2
coupling with the weakest electron-withdrawing aniline, the one
isolated with the higher yield.
The influence of the substitution pattern of azobenzothiazole
disperse dyes possessing an aromatic amine coupler with typical
push-groups on their absorption spectrum, based on conventional
donor–acceptor interactions, is well documented.3 However, the
effect of substitution in the benzene ring acting as coupling com-
ponent, as in dyes 4a–l, to the best of our knowledge has never
been studied.
All the synthesized azobenzothiazole dyes displayed large
absorption bands, with kmax within the range 328–370 nm (Table
1). As expected, the presence of a nitro group in the benzothiazole
moiety leads to a bathochromic shift due to the extension of con-
jugation. Accordingly, nitrobenzothiazoles 4d,f,h have shown
ranging from 8 to 12 nm with respect to their parent non-substi-
tuted analogues 4a–c.
Dk
As expected, the presence of the powerful electron-withdraw-
ing nitro substituent in the heteroaryl coupler has shown to in-
crease the reactivity of the nitroso component toward anilines. In
fact, whereas 2-nitroso-6-nitrobenzothiazole (2b) afforded the cor-
responding azobenzothiazole dyes 4d–l in 33–82% yield, being the
lower yield obtained for the compound bearing the two pull nitro
groups in both aromatic moieties, its unsubstituted analogue 2a
was only able to condense with the less electron-withdrawing ani-
lines 3a,c,e in 10–30% yield. The corresponding azo dyes 4a–c were
obtained in poor yields, being compound 4a, resulting from the
The influence of the substitution pattern of the aniline coupling
moiety of the dyes in their visible absorption spectra seems to de-
pend on a balance between the bathochromic electron donating
and hypsochromic electron-withdrawing effects. Hence, while
dyes 4c,h, bearing the benzene ring substituted with alkyl-type
groups, have kmax identical to those of the correspondent non-
substituted counter-parts 4a,d. The presence of a second strong
electron-withdrawing nitro group in 4l induces an hypsochromic
shift of 8 nm. Halogen substitution revealed to have a net batho-
chromic effect, resulting from the electron-donating outcome by
resonance, attenuated by the opposite inductive electron with-
drawal. Thus, the halogen para-substituted dyes 4i–k revealed
R1
R1
S
S
D
k ranging from 4 to 30 nm, the larger shift being displayed by
Oxone
NH2
N
O
4k, which holds the less electronegative halogen atom, with the
higher electronic density. Consistently, the absorption of 2- and
4-chloro dyes 4f,j, which hold the halogen atom in a position di-
rectly conjugated with the pulling azobenzothiazole moiety, is
more red-shifted than that of the 3-substituted parent 4g. The
crowding effect over the chlorine atom of the 2-substituted dye
4f, being superior to that in the 4-substituted compound 4j, prob-
ably explains the larger red shift of the latter.
H2O:MeOH: CHCl3
(1:1:5), 65 ºC
N
N
2a R1 = H (16%)
b R1 = NO2 (16%)
1a R1 = H
b R1 = NO2
Scheme 1. Synthesis of 2-nitrosobenzothiazoles 2a,b.
In conclusion, it can be claimed that 2-nitrosobenzothiazole
(2a) and 6-nitro-2-nitrosobenzothiazole (2b), whose synthesis is
disclosed herein, are useful synthons for the preparation of new
azobenzothiazole dyes, extending the access to molecules possess-
ing electron-withdrawing groups in different positions of the aro-
matic coupling component. This synthetic methodology presents
the advantage of being a more general approach to the preparation
of this class of azoheterocyclic dyes than the classic diazotization
process used for the preparation of azo disperse dyes, since the
presence of an activating electron-donating group suitably located
in the coupling component is no longer crucial.
HOAc
r.t.
2a-b
+
H2N
R2
3a-i
R1
S
N
N
R2
N
4a-l
Scheme 2. Synthesis of azobenzothiazoles 4a–l.

H. Faustino et al. / Tetrahedron Letters 49 (2008) 6907–6909
6909
Tsatsaroni, E.; Xisai, M. Dyes Pigments 1992, 20, 41–51; (f) Peters, A. T.; Taebi,
A.; Yang, S. S. J. Soc. Dyers Colour. 1993, 109, 397–401.
Acknowledgments
12. Rangnekar, D. W.; Chaudhari, M. B. Dyes Pigments 1989, 10, 173–181.
13. (a) Ming, S. Y.; Waqng, I. J. Dyes Pigments 2004, 61, 243–250; (b) Tsatsaroni, E.
G. Dyes Pigments 1996, 31, 301–307; (c) Peters, A. T.; Yang, S. S. Dyes Pigments
1995, 28, 151–164; (d) Sokolowska-Gadja, J.; Freeman, H. S. Dyes Pigments
1992, 20, 137–145.
The authors thank Fundação para a Ciência e a Tecnologia (Por-
tugal) POCI 2010 and FEDER for the funding of the Project POCI/
QUI/57913/2004.
14. Peters, A. T.; Mehdi, S. D. M. J. Soc. Dyers Colour. 1990, 106, 275–280.
15. Gowenlock, B. G.; Richter-Addo, G. B. Chem. Rev. 2004, 104, 3315–3340, and
references cited therein.
References and notes
16. (a) Yu, B.-C.; Shirai, Y.; Tour, J. M. Tetrahedron 2006, 62, 10303–10310; (b)
Priewisch, B.; Rück-Braun, K. J. Org. Chem. 2005, 70, 2350–2352.
1. Zollinger, H. Color Chemistry. Synthesis, Properties and Application of Organic
Dyes; Verlag Chemie: Weincheim, 1987.
2. (a) Sokolowska, J.; Podsiadly, R.; Sochocka, E. Dyes Pigments 2007, 72, 223–227;
(b) Karci, F.; Ertan, N. Dyes Pigments 2005, 64, 243–249; (c) Yen, M. S.; Wang, I. J.
Dyes Pigments 2004, 61, 243–250.
17. Holmes, R. R.; Bayer, R. P. J. Am. Chem. Soc. 1959, 82, 3454–3456.
18. Taylor, E. C.; Tseng, C.-P.; Rampal, J. B. J. Org. Chem. 1982, 47, 552–555.
19. Synthesis of 6-nitro-2-nitrosobenzothiazole (2b). Typical procedure. OxoneÒ
(23.16 g, 37.67 mmol) in water (130 mL) was added to a solution of 1b (2.39 g,
11.88 mmol) in MeOH/CHCl3 (1:5) (670 mL) and the resulting mixture was
heated under reflux for 24 h. After cooling, the reaction mixture was filtrated
under reduced pressure to remove the insoluble material. The organic layer
was separated by decantation, washed with brine, dried over anhydrous
Na2SO4, and evaporated to dryness. The residue was subjected to c.c. (silica gel,
CH2Cl2) to afford 2b as green needles. Yield: 68%. Mp 97–99 °C. Vis (MeOH)
kmax (nm): 392. 1H NMR (250.13 MHz, CDCl3): d 8.61 (1H, dd, J = 8.5, 2.0 Hz,
CH), 8.76 (1H, d, J = 2.0 Hz, CH), 9.14 (1H, d, J = 8.5 Hz, CH). 13C NMR
(62.90 MHz, CDCl3): d 108.3 (C), 120.2 (C), 123.9 (CH), 124.0 (CH), 124.9 (C),
3. Towns, A. D. Dyes Pigments 1999, 42, 3–28.
4. Shuttleworth, L.; Weaver, M. A. In The Chemistry and Application of Dyes;
Waring, D. R., Hallas, G., Eds.; Plenum Press: New York, 1990; Chapter 4. p 137.
5. Hallas, G.; Towns, A. D. Dyes Pigments 1997, 35, 219–277.
6. Belmar, J.; Parra, M.; Zúñiga, C.; Pérez, C.; Muñoz, C. Liquid Crystals 1999, 26,
389–396.
7. (a) BASF. European patent 442 360, 1991.; (b) Nippon Kayaku. Japanese Patent
08 207 456, 1996; Chem. Abstr. 125, 288864e.; (c) Nippon Kayaku. Japanese
patent 09 255 884-5, 1997; Chem. Abstr. 127, 332786y-7z.; (d) Mistsubishi.
Japanese Patent 08 166 689, 1996; Chem. Abstr. 125, 208403r.; (e) Sharp.
Japanese Patent 10 36 692, 1998; Chem. Abstr. 128, 193735m.
8. (a) Razus, A. C.; Birzan, L.; Surugiu, N. M.; Corbu, A. C.; Chiraleu, F. Dyes
Pigments 2007, 74, 26–33; (b) Matsui, M.; Kushida, M.; Funabiki, K.; Shibata, K.;
Muramatsu, H.; Hirota, K.; Hosoda, M.; Tai, K. Dyes Pigments 1998, 38, 283–289;
(c) Moylan, C. R.; Twieg, R. J.; Lee, V. Y.; Swanson, S. A.; Betterton, K. M.; Miller,
R. D. J. Am. Chem. Soc. 1993, 115, 12599–12600.
133.5 (CH), 159.3 (C). IR (KBr) m
max (cmÀ1): 3095 (w), 3070 (w), 1532 (s), 1483
(m), 1390 (m), 1353 (s), 1248 (m), 1165 (s), 1151 (m), 1107 (s), 897 (m), 846
(m), 829 (s), 743 (m). TOFHRMS: calcd for C7H3N3O3S [M]+: 208.9895; found
208.9894.
20. Synthesis of (6-nitrobenzothiazol-2-yl)phenyldiazene (4d). Typical procedure. A
solution of 2b (0.21 g, 1.00 mmol) in glacial AcOH (1.5 mL), prepared in an
ultrasound bath at 40 °C, was added dropwise (3 min) to a solution of aniline
(3a) (0.09 mL, 1.0 mmol) in the same solvent (0.5 mL) and the mixture was
stirred at rt for 4 h. The resulting yellow-orange precipitate was collected by
filtration under reduce pressure, washed with petroleum or diethyl ether, and
dried. Recrystallization from MeOH/CH2Cl2 afforded 4d as orange needles.
Yield: 53%. Mp 137–139 °C. Vis (MeOH) kmax (nm): 340. 1H NMR (250.13 MHz,
CDCl3): d 7.56–7.65 (3H, m), 7.98 (2H, d, J = 6.50 Hz), 8.13 (1H, d, J = 8.50 Hz),
8.33 (1H, dd, J = 8.75 Hz, J = 1.75 Hz), 8.73 (1H, d, J = 1.50 Hz) 13C NMR
(62.90 MHz, CDCl3): d 109.8 (C), 123.3 (CH), 123.6 (CH), 123.7 (CH), 124.2
9. Salvador, M. A.; Reis, L. V.; Almeida, P.; Santos, P. F. Tetrahedron 2008, 64, 209–
303.
10. (a) Karci, F.; Karci, F. Dyes Pigments 2006, 76, 147–157; (b) Karci, F.; Ertan, N.
Dyes Pigments 2002, 55, 99–108; (c) Georgiadou, K. L.; Tsatsaroni, E. G. Dyes
Pigments 2002, 53, 73–78; (d) Georgiadou, K. L.; Tsatsaroni, E. G. Dyes Pigments
2001, 50, 93–97; (e) Deligeorgiev, T. G.; Simov, D. Dyes Pigments 1998, 38, 115–
125; (f) Ertan, N.; Eyduran, F. Dyes Pigments 1995, 27, 313–320; (g) Penchev, A.;
Simon, D.; Gadjev, N. Dyes Pigments 1991, 16, 77–81.
11. (a) Peters, A. T.; Yang, S. S. Dyes Pigments 1996, 30, 291–299; (b) Peters, A. T.;
Chisowa, E. Dyes Pigments 1996, 31, 131–139; (c) Peters, A. T.; Wu, C. T.;
Viscardi, G.; Barni, E. Dyes Pigments 1995, 29, 103–115; (d) Peters, A. T.;
Gbadamosi, N. M. A. Dyes Pigments 1992, 18, 115–123; (e) Peters, A. T.;
(CH), 127.4 (C), 129.6 (CH), 133.8 (CH), 148.9 (C), 150.5 (C), 151.2 (C). IR (KBr)
m
(cmÀ1): 3094 (w), 1525 (s), 1485 (m), 1469 (m), 1442 (m), 1345 (s), 1319 (m),
1310 (m), 882 (m), 774 (m), 708 (m), 683 (m). FABHRMS (3-NBA) calcd for
C13H9N4O2S+ [M+H]+: 285.0446; found 285.0435.