Stable triazenes derived from 2-alkylaminonaphthalenes and 5-nitrobenzo[c]-1,2-thiazole-3-diazonium hydrogensulfate

Article abstract of DOI:10.1002/ejoc.200800180

A calculation using the DFT method confirmed that the extraordinary stability of the triazenes formed by an azo coupling reaction of 5-nitrobenzo[c]-1,2-thiazole-3-diazonium with primary or secondary aromatic amines is caused by the fact that these substances are protonated at the heterocyclic nitrogen atom and not at the nitrogen atom of the triazene grouping -N=N-N(R)Ar. Stable triazenes are also formed by reaction of 5-nitrobenzo[c]-1,2-thiazole-3-diazonium with 2-alkylaminonaphthalenes. In the cases of the azo coupling reaction with 2-methylamino- and 2- ethylaminonaphthalene, the content of triazenes is almost 50% in the product mixture with the isomeric azo compounds. The structures of the triazenes were confirmed by X-ray analysis. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800180

FULL PAPER
DOI: 10.1002/ejoc.200800180
Stable Triazenes Derived from 2-Alkylaminonaphthalenes and 5-Nitrobenzo[c]-
1,2-thiazole-3-diazonium Hydrogensulfate
[a][†]
[b]
[c]
[b]
ˇ
ˇ
Vladimír Machácek,* Petr Jansa, Markéta Svobodová,
Josef Prikryl,
[d]
[e]
[a]
ˇ
ˇ
˚ˇ ˇ
Ales Ruzicka, Petr Nachtigall, and Michal Cerný
Dedicated to the memory of Professor Otto Exner
Keywords: Azo compounds / Diazoniums / Triazenes / Density functional calculations / Amines
A calculation using the DFT method confirmed that the ex-
traordinary stability of the triazenes formed by an azo coup-
ling reaction of 5-nitrobenzo[c]-1,2-thiazole-3-diazonium
with primary or secondary aromatic amines is caused by the
fact that these substances are protonated at the heterocyclic
nitrogen atom and not at the nitrogen atom of the triazene
grouping –N=N–N(R)Ar. Stable triazenes are also formed by
reaction of 5-nitrobenzo[c]-1,2-thiazole-3-diazonium with 2-
alkylaminonaphthalenes. In the cases of the azo coupling re-
action with 2-methylamino- and 2-ethylaminonaphthalene,
the content of triazenes is almost 50% in the product mixture
with the isomeric azo compounds. The structures of the tri-
azenes were confirmed by X-ray analysis.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
increased up to 50% only in the reactions of the above-
mentioned diazonium salt with diphenylamine, 3-methyl-
aniline, and 3-methyl-N-alkylanilines (Scheme 1).
Introduction
In our previous papers we described the formation of
extraordinarily stable triazenes by the reaction of 5-ni-
trobenzo[c]-1,2-thiazole-3-diazonium hydrogensulfate with
primary and secondary anilines and their 3-chloro and 3-
methyl derivatives.[1–3] We found that the azo coupling reac-
tion of the said anilines with this diazonium salt gives a
mixture of triazenes 1 in addition to azo compounds 2 (i.e.
the expected isomeric azo dyestuff). Triazenes 1 are formed
as the major products in yields reaching up to 95%;
for example, see the azo coupling with N-ethylaniline
(Scheme 1). The proportion of isomeric azo compounds 2
[a] Institute of Polymeric Materials, Faculty of Chemical Technol-
ogy, University of Pardubice
[b] Department of Organic Chemistry, Faculty of Chemical Tech-
nology, University of Pardubice,
ˇ
Nám. Cs. legií 565, 53210 Pardubice, Czech Republic
Scheme 1.
Fax: +420-466-037-068
E-mail: vladimir.machacek@upce.cz
[c] Gilead Sciences & IOCB Research Centre, Centre for New
Antivirals and Antineoplastics, Institute of Organic Chemistry
and Biochemistry, Academy of Sciences of the Czech Republic,
Flemingovo nám. 2, 16610 Prague 6, Czech Republic
[d] Department of General and Inorganic Chemistry, Faculty of
Chemical Technology, University of Pardubice
[e] Center for Complex Molecular Systems and Biomolecules, In-
stitute of Organic Chemistry and Biochemistry, Academy of
Sciences of the Czech Republic,
Triazenes 1 are extraordinarily stable in acid media. Half
lives of their decomposition to the diazonium salt and
amine in 0.5 molL–1 H2SO4 in aqueous acetic acid (1:1 v/v)
are in the order of magnitude of hours.[1,3] 1,3-Diaryl-
triazenes in the benzene series decomposed at a measurable
rate in aqueous ethanol (20%v/v) in buffers at room tem-
perature already at pH Ͻ7. For example, 1,3-diphenyl-
triazene decomposes in the medium mentioned at pH 6
with a half life of 7 min.[4]
Flemingovo nám. 2, 16610 Prague 6, Czech Republic
[†] Deceased
Supporting information for this article is available on the
WWW under http://www.eurjoc.org/ or from the author.
3272
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 3272–3278

Stable Triazenes Derived from 2-Alkylaminonaphthalenes and Diazonium Salts
The decomposition of triazenes 1 obtained by azo coup- ylamine does not possess any C–H group to allow the azo
ling with N-alkylanilines gives the original 5-nitrobenzo[c]- coupling reaction in the aromatic ring. The high inclination
1,2-thiazole-3-diazonium, whereas the decomposition of tri- of 5-nitrobenzo[c]-1,2-thiazole-3-diazonium to undergo the
azenes derived from primary anilines produces substituted azo coupling reaction to give stable triazenes allows the
benzenediazonium salts and 3-amino-5-nitrobenzo[c]-1,2- possible formation of triazenes in the case of the azo coup-
thiazole.[1,3] The reason for the extraordinary stability of ling reaction with naphthylamines to also be considered. In
triazenes 1 probably lies in the fact that they are protonated order to avoid complications with potential reaction at the
at a different site in comparison to the protonation of com- 2- and 4-positions in the azo coupling with 1-naphth-
mon 1,3-diaryltriazenes: whereas 1,3-diaryltriazenes are ylamine, we chose 2-(N-alkyl)naphthylamines as the sub-
protonated at the nitrogen atom adjacent to the azo strates. The azo coupling reaction of 5-nitrobenzo[c]-1,2-
group,[4–6] triazenes 1 are presumably protonated mostly at thiazole-3-diazonium hydrogensulfate with 1-(2-hydroxy-
the heterocyclic nitrogen atom,[1–3] which leads to the for- ethylamino)naphthalene was described by Georgiadou and
mation of a structure stabilized by a classic benzene ring Tsatsaroni,[10] who did not mention any formation of poten-
(Scheme 2). However, this presumption cannot be con- tial triazene besides the expected azo compound, viz.
firmed experimentally. Therefore, the aim of this work is to 3-[4-N-(2-hydroxyethylamino)naphthalen-1-yldiazenyl]-5-ni-
study the structure and stability of triazenes 1 by means of trobenzo[c]-1,2-thiazole.
quantum-chemical methods.
Results and Discussion
Calculations
The stability of triazenes 1 in acid media has been as-
cribed to a different site of protonation of these sub-
stances[1–3] in comparison to common 1,3-diphenyl-
triazenes. Protonation at the heterocyclic nitrogen atom
(Scheme 2) has been proposed. In order to verify this hy-
pothesis, DFT calculations were performed for the model
compound 3-methyl-3-phenyl-1-(5-nitrobenzo[c]-1,2-thiaz-
ol-3-yl)triazene. The calculations were performed by using
the hybrid exchange-correlation functional B3LYP[11,12] and
valence-triple-ζ basis set with polarization functions
(TZVP).[13] The effect of water as the solvent was investi-
gated by using the polarizable continuum model (PCM).[14]
Natural bond orbital analysis (NBO, Version 3.1)[15] was
Scheme 2.
used for discussion of atomic charges. Calculations were
performed with the Gaussian 03 program suite.[16]
In the benzene series, triazenes are formed by azo coup-
The relative stability of various products of protonation
ling with anilines in neutral to slightly acidic media. Ac-
were calculated for 3-methyl-3-phenyl-1-(5-nitrobenzo[c]-
1,2-thiazol-3-yl)triazene in the gas phase and in aqueous
cording to Zollinger,[6] N-alkylnaphthylamines do not give
[7]
ˇ
triazenes under such conditions. Sterba also states that
ˇ
solution (Table 1). The solvent effect does not change the
relative stabilities of the individual protonated forms
(Table 1). The results of these calculations unambiguously
indicate that the protonation preferably takes place at the
heterocyclic nitrogen atom (N6, atom-numbering scheme is
defined in Scheme 2). Triazene protonated at this site is
more stable by 103 kJmol–1 than the same compound pro-
the main reaction of naphthylamines with diazonium salts
is C coupling. Bagal[8] presumes that the primary attack of
the nitrogen atom of the diazonium group takes place at
the nitrogen atom of the naphthylamines, but the triazenes
in the naphthalene series are unstable and have been neither
isolated nor identified so far. The reluctance of naphth-
ylamines to produce triazenes by the azo coupling reaction
is obviously due to the lower aromaticity of naphthalene
relative to that of benzene and, hence, also the lower acti-
vation energy of the azo coupling reaction in the aromatic
ring. Therefore, the reversible azo coupling reaction at the
nitrogen atom does not make itself felt. Only Abezgauz et
al.[9] reported a diazotization reaction of heptafluoro-2-
naphthylamine giving the product of elemental composition
C20HN3F14 corresponding to 1,3-bis(heptafluoronaphth-
alen-2-yl)triazene. In the presence of sodium 2-naphtholate,
this compound gives 1-(heptafluoronaphthalen-2-yl)di-
azenyl-naphthalen-2-ol. Of course, heptafluoro-2-naphth-
Table 1. Relative energies [kJmol–1] of 3-methyl-3-phenyl-1-(5-ni-
trobenzo[c]-1,2-thiazol-3-yl)triazene molecule protonated at vari-
ous positions with respect to the energy of most stable species (pro-
tonated at N6); calculation at the B3LYP/TZVP level (for notation
see Scheme 2).
Site of protonation
Gas phase
Solution
N6
O
N1
S5
0.0
62.0
112.6
207.7
0.0
83.7
103.4
190.9
Eur. J. Org. Chem. 2008, 3272–3278
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3273

V. Machácˇek et al.
FULL PAPER
tonated at the nitrogen atom of the triazene grouping (N1). Table 3. Electron spectra in chloroform of triazenes 1a–d and azo
compounds 2a–d.
In fact, the protonation on N1 is even less likely than pro-
tonation at the nitro group oxygen atom (about 20 kJmol–1
difference in stability of triazene protonated at the N1 and
O atoms).
Compound
λmax [nm]
ε [Lg–1 cm–1]
ε [Lmol–1 cm–1]
1a
1b
478
481
483
480
628
629
632
631
67.67
75.60
73.05
67.33
80.08
82.50
76.20
78.33
24590
28533
29620
26490
29102
31173
30897
30818
Atomic charges at selected atoms calculated for gas- 1c
1d
phase and solvated triazene (by using Mulliken, NBO, and
ESP analysis) are summarized in Table 2. The electron den-
sity on the N1 triazene nitrogen atom is significantly lower
than that on the N6 heterocyclic nitrogen atom and that on
the O atom of the nitro group (NBO analysis for the sol-
vated triazene). Thus, protonation on the N6 heterocyclic
nitrogen atom appears to be the most likely on the basis of
orbital population analysis; this is in very good agreement
with predictions based on the calculated stabilities of indi-
vidual protonated forms.
2a
2b
2c
2d
The compounds were identified by means of their 1H
NMR spectra. On the basis of the presence of an AMX
system of the protons of the naphthalene residue {δ = 7.75
3
(d, 1Ј-H), 7.78 (dd, 3Ј-H), 7.91 (d, 4Ј-H) ppm, J = 8.9 Hz,
4J = 2.3 Hz, Table S2}, the orange substance was assigned
the structure of triazene 1c (Figure S1). The 3Ј-H and 4Ј-H
protons of the naphthalene residue of the blue substance
form two doublets with δ = 7.05 (d, 3Ј-H), 7.78 (d 4Ј-H)
Table 2. Atomic charges at selected atoms of 3-methyl-3-phenyl-1-
(5-nitrobenzo[c]-1,2-thiazol-3-yl)triazene molecule calculated at the
B3LYP/TZVP level (for notation see Scheme 2).
3
ppm, J = 9.4 Hz. This compound was identified as azo
Atom
Mulliken
(gas phase)
Mulliken
(solution)
NBO
(solution)
ESP
(solution)
compound 2c (Figure S2). Also, 2-methylaminonaphtha-
lene, 2-ethylaminonaphthalene, and 2-(2-hydroxyethyl-
amino)naphthalene behaved similarly as 2-(but-1-yl-
amino)naphthalene in their azo coupling reactions with
5-nitrobenzo[c]-1,2-thiazole-3-diazonium hydrogensulfate
(Scheme 3). In all the cases, the reactions produced mix-
tures of triazenes 1a,b,d and azo compounds 2a,b,d. The
ratios of triazenes to azo compounds in the azo coupling
products are presented in Table S1. Because of the very low
solubility of both triazenes 1a–d and azo compounds 2a–d
in CDCl3 and their near insolubility in [D6]DMSO, high-
N6
O
N1
S5
–0.16
–0.18
0.02
–0.16
–0.27
–0.03
0.24
–0.61
–0.41
–0.11
0.62
–0.55
–0.49
0.47
0.37
0.14
On the basis of relative energies calculated at the DFT
level, triazenes of the 3-methyl-3-phenyl-1-(5-nitrobenzo[c]-
1,2-thiazol-3-yl)triazene type are protonated in acid media
predominantly at the heterocyclic nitrogen atom. The re-
sults of NBO analysis of Lewis structures of this protonated
form show a double bond between N1–N2 and between
N3–C4. The Wiberg bond indices calculated for the N1–
N2, N2–N3, and N3–C4 bonds are 1.436, 1.421, and 1.296,
respectively.
1
quality H NMR spectra could only be measured for tri-
azenes 1a,b,c and azo compounds 2b,c. The 13C NMR spec-
tra were only measured with the above-mentioned triazenes
and azo compound 2c. The values of the δ(1H) and δ(13C)
chemical shifts are presented in Table S2.
As a result of its high energy, the form that can decom-
pose to give the diazonium ion is present at only a negligible
concentration, which explains the stability of the triazene
in acid medium. The results of DFT calculations confirm
the previously formulated hypothesis[1–3] about the proton-
ation site and explain the unexpected stability of these triaz-
enes in acid media.
Scheme 3.
The chemical shifts of the protons of the heterocyclic res-
idue in azo compounds 2b,c differ from those of the same
protons in the azo compounds prepared earlier by azo
Reaction of 5-Nitrobenzo[c]-1,2-thiazole-3-diazonium
Hydrogensulfate with 2-Alkylaminonaphthalenes
The reaction of 5-nitrobenzo[c]-1,2-thiazole-3-diazonium coupling reaction of 5-nitrobenzo[c]-1,2-thiazole-3-diazo-
hydrogensulfate with 2-(but-1-ylamino)naphthalene gives nium hydrogensulfate with secondary amines of the ben-
two products differing on TLC (Silufol; toluene/ethyl ace- zene series.[3] These differences may be due to the existence
tate, 5:1): an orange substance (RF ≈ 0.7) and a blue sub- of intramolecular hydrogen bonds in naphthalene deriva-
stance (RF ≈ 0.6) (Table 3). The products were separated on tives 2b,c [δ(NH) ≈ 12 ppm] and its absence in the benzene
preparative scale on the basis of solubility differences in derivatives, or to a different position of the equilibrium for
acetone (see Experimental Section). Elemental analysis in- the azo–hydrazone tautomer in the two series of com-
dicates that they are isomers (Table S1).
pounds (in connection with different solvent used; CDCl3
3274
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 3272–3278

Stable Triazenes Derived from 2-Alkylaminonaphthalenes and Diazonium Salts
vs. [D6]DMSO). The calculation shows that azo compound of values of parameters for triazene 1c belong to two inde-
2b is only present in the form containing an intramolecular pendent molecules in the crystal unit. For numbering of
hydrogen bond and a trans configuration at the azo group atoms see Figures 2 and 3.
(Figure 1). The form without a hydrogen bond and the
forms with cis arrangement at the azo group were not iden-
tified, not even as local minima at the potential energy sur-
face. The carbon atoms of the naphthalene residue in com-
pound 2c exhibit very similar chemical shifts to those in 2-
amino-1-phenyldiazenylnaphthalene[17] and 2-ethylamino-
1-(4-nitrophenyldiazenyl)naphthalene (3) (Table S2). The
1H NMR spectra of the aromatic protons of triazene 1c
and azo compound 2c are given in Figures S1 and S2.
Figure 2. ORTEP view of triazene 1b showing the thermal ellip-
soids at 50% probability; selected bond lengths [Å] and angles [°]:
S1–N4 1.646(3), S1–C1 1.707(3), N1–1.279(4), N1–C1 1.379(4),
N4–C3 1.338(4), N3–N2 1.323(4), N3–C8 1.412(4), N3–C18
1.483(4), O1–N5 1.214(4), N5–O2 1.224(4), N5–1.457(4); N4–S1–
C1 96.50(16), N2–N1–C1 111.0(3), C3–N4–S1 108.9(2), N2–N3–
C8 116.0(3), N2–N3–C18 120.0(3), C8–N3–C18 123.8(3), N1–N2–
N3 114.4(3), N1–C1–C2 124.1(3), N1–S1–127.8(2), C2–C1–S1
108.1(3), O1–N5–O2 122.8(3), O1–N5–C5 119.0(3), O2–N5–C5
Figure 1. Structure of azo compound 2b calculated by using the
118.2(3), C4–C5–N5 118.6(3), C6–C5–N5 118.3(3), C13–C8–N3
B3LYP/TZVP method.
120.2(3), N3–C8–C9 120.1(3), N4–C3–C7 124.5(3), N4–C3–C2
115.9(3), N3–C18–C19 111.9(3).
The structures of triazenes 1b and 1c were confirmed by
X-ray analysis. Azo compounds 2a–d could not be prepared
in the crystalline form that is needed for X-ray analysis.
Triazenes 1a–d are brilliant orange-red substances with λmax
≈ 480 nm and high absorptivity, εmax ≈ 2.8ϫ104 Lmol–1cm–1.
Absorption spectra (Table 3, Figure S3) were measured in
chloroform. The substances are not very soluble in other
solvents (methanol, ethanol, acetone, ethyl acetate). Iso-
meric azo dyestuffs 2a–d exhibit λmax values around
630 nm: they are blue pigments with a strong green hue.
The absorptivity εmax ≈ 3.1ϫ104 Lmol–1 cm–1 is ca. 10%
higher than that of the isomeric triazenes.
Solid-State Study
Selected bond lengths and angles are given in Figures 2
Figure 3. ORTEP view of one of two independent molecules of
and 3. Compound 1b crystallizes in the monoclinic space triazene 1c showing the thermal ellipsoids at 50% probability; se-
lected bond lengths [Å] and angles [°]: S1–N4 1.651(3), S1–C1
group, whereas 1c crystallizes in an orthorhombic space
1.706(3), S1Ј–N4Ј 1.644(3), S1Ј–C1Ј 1.708(4), N3–N2 1.314(4), N3–
group with two geometrically independent molecules in the
C8 1.419(4), N3–C18 1.480(4), N3Ј–N2Ј 1.320(4), N3Ј–C8Ј
crystal unit. Views of 1b and 1c (Figures 2 and 3) show
1.429(4), N3Ј–C18Ј 1.473(4), N1–N2 1.295(4), N1–C1 1.372(4),
an almost planar arrangement of both molecules where the
triazene group adopts the C–N=N–N trans configuration.
The interplanar dihedral angles in triazene and benzoisothi-
azole moieties are 4.9(1) (for 1b) and 7.0(1), 11.8(1)° (for
1c), whereas those between the triazene and the naphthyl
N2Ј–N1Ј 1.284(4), N1Ј–C1Ј 1.363(4), C3–N4 1.325(5), N5–O2
1.200(5), N5–O1 1.213(4), N5–C5 1.473(5), N4Ј–C3Ј 1.336(5), N5Ј–
O2Ј 1.212(6), N5Ј–O1Ј 1.230(5), N5Ј–1.477(6); N4–S1–C1
97.12(16), N4Ј–S1Ј–C1Ј 96.95(17), N2–N3–C8 116.1(3), N2–N3–
C18 121.2(2), C8–N3–C18 122.7(3), N2Ј–N3Ј–C8Ј 115.7(3), N2Ј–
N3Ј–C18Ј 121.1(3), C8Ј–N3Ј–C18Ј 123.1(3), N2–N1–C1 109.7(3),
ring are 2.9(1)° and 2.5(1), 2.9(1)° in 1b and 1c, respectively. N1Ј–N2Ј–N3Ј 114.3(3), N2Ј–N1Ј–C1Ј 110.7(3), N1–N2–N3
114.8(3), C13–C8–N3 120.5(3), C9–C8–N3 119.3(3), N4–C3–C7
123.9(3), N4–C3–C2 117.5(3), C3–N4–S1 108.1(2), C13Ј–C8Ј–N3Ј
120.5(3), C9Ј–C8Ј–N3Ј 119.7(3), N1Ј–C2Ј–124.5(3), N1Ј–C1Ј–S1Ј
128.3(3), C2Ј–C1Ј–S1Ј 107.2(3), C20–C19–C18–113.6(3), N3–C18–
The interplanar angles between both rings are 2.7(1) and
9.4(1), 13.6(1)°. These values are significantly smaller than
those found for analogous phenyl derivatives.[1] All these
values support the concept of highly extended conjugation
between the heterocyclic and triazene moieties. Small values
of the dihedral angles between the benzoisothiazole ring
and the nitro group [5.7(1) and 4.5(1), 4.7(1)° for 1b and
1c, respectively] are additional proof for high resonance
C19 111.9(3), N3Ј–C18Ј–C19Ј 112.0(3), O2–N5–O1 124.5(4), O2–
N5–C5 118.7(3), O1–N5–C5 116.8(4), N1–C1–C2 124.4(3), N1–
C1–S1 127.7(3), C2–C1–S1 107.9(2), C4–C5–N5 118.3(3), C6–C5–
N5 118.6(3), C3Ј–N4Ј–S1Ј 108.8(3), C10Ј–C9Ј–C8Ј 120.3(3), N4Ј–
C3Ј–C2Ј 116.1(3), N4Ј–C3Ј–C7Ј 124.6(4), C2Ј–C7Ј–119.2(4), O2Ј–
N5Ј–O1Ј 123.7(5), O2Ј–N5Ј–C5Ј 118.8(4), O1Ј–N5Ј–C5Ј 117.5(5),
contribution to a structure of resonance hybrid. A couple C4Ј–C5Ј–N5Ј 117.6(4), C6Ј–C5Ј–N5Ј 118.9(4).
Eur. J. Org. Chem. 2008, 3272–3278
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3275

V. Machácˇek et al.
FULL PAPER
The π conjugation through both aromatic rings (benzo- of different structures in solution: both the derivatives have
isothiazole and naphthyl ones) and the triazene moiety is almost identical spectra in the aromatic region. We continue
further supported by delocalization of electron density to deal with this problem.
within the triazene moiety. The lengthening of the N1=N2
Table 4. Kinetic parameters for the decomposition of triazenes 1 in
bond lengths, at 1.279(4) for 1b and 1.295(4), 1.284(4) Å for
1c, relative to the standard N=N distance of 1.24 Å[18] is
similar to that found for phenyl derivatives studied pre-
viously.[1] Further similarities with structure of previously
published compounds were found in the shortening of the
N2–N3 distances, at 1.323(4) for 1b and 1.314(4),
1.320(4) Å for 1c, relative to the N(sp2)–N(sp3) single bond
of 1.40 Å,[18] and they are attributable to the resonance in-
herent in the triazene moiety. All studied and comparative
compounds[19–25] display close delocalization within the tri-
azene moieties, and structures 1b and 1c display C1–N1
bond lengths of 1.379(4) and 1.372(4), 1.363(5) Å, which
are much shorter than the standard C(aryl)–N single bond
(1.44 Å[18]).
These data, together with the resonance within the
C1=C2–C3=N4 atomic chain, provide evidence that the
benzoisothiazole ring takes part significantly in the triazene
π conjugation. Furthermore, compounds 1b and 1c each
display a shortening of the N3–C8 bond [1.412(4) for 1b
and 1.419(4), 1.429(4) Å for 1c] with respect to the standard
N(sp3)–C(sp2) distance of 1.44 Å,[18] which is attributable to
a small extension of triazene delocalization to the N3–C8
bond, where C8 belonging to the naphthyl group is almost
coplanar with the triazene moiety.
H2SO4 (1 molL–1) in aqueous CH3COOH (1:1, v/v) at 25 °C.
Triazene
Rate constant k [h–1]
Half life t1/2 [h]
1a
1b
1c
1d
2.005ϫ10–2
28.190ϫ10–2
4.937ϫ10–2
4.889ϫ10–2
34.58
2.46
14.04
14.18
The decomposition of triazenes derived from secondary
amines in acid medium produces the original 5-nitro-
benzo[c]-1,2-thiazole-3-diazonium, which can (to a small
extent) undergo the azo coupling reaction in the aromatic
ring of the amine formed by the decomposition of triazene
(Scheme 4). However, this reaction is of little probability, as
the concentration of the amine formed in the solution dur-
ing kinetic measurements is very low and the amine is in
effect present only in its unreactive protonated form.
Both molecules of 1b and 1c display a layered structure
without classical H-bonding connections but by π–π stack-
ing or short contacts between the nitro group of one mole-
cule and the methyl fragment of the alkyl group of the other
one with interplanar distances of 3.390 and 3.470 Å for 1b
and 3.467 Å for 1c; supramolecular architectures are shown
in Figures S4 and S5. Compound 1c shows a cavity in area
between the alkyl chain and the benzoisothiazole moiety
(Figure S6).
Scheme 4.
Therefore, the predominant part of the diazonium salt
undergoes decomposition. If 4,5-dihydroxynaphthalene-2,7-
disulfonate (which is able to react in the sense of azo coup-
ling even in acidic medium) is added to the reaction mix-
ture, it acts as a scavenger for the diazonium salt and pro-
duces a new azo compound (λmax ≈ 570 nm). Spectral re-
cords of decomposition of triazene 1c in the presence of
4,5-dihydroxynaphthalene-2,7-disulfonate, in its absence,
and the kinetic curve of decomposition of this triazene are
presented in Figures S7–S9. The identity of azo compound
2c formed during decomposition of triazene 1c in the ab-
sence of 4,5-dihydroxynaphthalene-2,7-disulfonate was
proved chromatographically.
Decomposition of Triazenes 1a–d in Acid Media
Like in the case of the earlier described triazenes ob-
tained by the azo coupling reaction of 5-nitrobenzo[c]-1,2-
thiazole-3-diazonium with N-alkylanilines,[1,3] triazenes 1a–
d formed by azo coupling with 2-alkylaminonaphthalenes
also exhibit unexpected stability in acid medium. In the
solution of 1 molL–1 H2SO4 in aqueous acetic acid (1:1
v/v), these compounds decompose with half lives of up to
several tens of hours at 25 °C (Table 4). The decomposition
half lives of individual triazenes 1a–d considerably differ,
and these differences cannot be explained by polar or steric
effects of the N-alkyl groups. The differences may be due
to the preservation of certain supramolecular structures of
compounds containing hydrophobic naphthalene rings in
strongly polar media (H2O, H2SO4, CH3COOH). A certain
difference between molecular arrangements of ethyl and bu-
Conclusions
DFT calculations confirmed the hypothesis that 1-(5-ni-
tyl derivatives 1b and 1c in crystals was confirmed by X- trobenzo[c]-1,2-thiazol-3-yl)-3-alkyl-3-phenyltriazenes are
1
ray analysis, but the H NMR spectra of these compounds protonated at the nitrogen atom of the isothiazole ring. The
measured in less polar CDCl3 did not prove the existence nitrogen atom adjacent to the azo group [–N=N–N(R)Ar]
3276
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 3272–3278

Stable Triazenes Derived from 2-Alkylaminonaphthalenes and Diazonium Salts
is protonated very little, which explains the high stability of Table 5. Crystallographic data for triazenes 1c and 1d.
these triazenes in acidic media. Strong inclination of the
Compound
1c
1d
5-nitrobenzo[c]-1,2-thiazol-3-diazonium cation to undergo
azo coupling reactions with aromatic primary and second-
ary aromatic amines at the nitrogen atom was also mani-
Empirical formula
Crystal system
Space group
C19H15N5O2S
triclinic
¯
P1
C21H19N5O2S
orthorhombic
Pca21
fested by the fact that triazenes are formed by the reaction a [Å]
7.4000(14)
8.3910(12)
14.081(3)
94.208(12)
98.864(15)
96.809(12)
2
854.1(3)
1.468
0.4ϫ0.3ϫ0.2
red block
0.216
45.5310(5)
4.8850(14)
17.519(3)
90
90
90
b [Å]
of this diazonium cation with 2-alkylaminonaphthalenes.
c [Å]
α [°]
β [°]
γ [°]
The structure of triazenes of this so far unknown type was
1
confirmed by H and 13C NMR spectroscopy and X-ray
structure analysis.
Z
8
V [Å3]
3896.6(13)
1.382
0.57ϫ0.128ϫ0.067
red needle
0.195
Dcalcd. [gcm–3]
Crystal size [mm]
Crystal form
µ [mm–1]
F (000)
h; k; l range
θ range [°]
Reflns measured
Experimental Section
General: 1H and 13C NMR spectra were recorded with a Bruker
Avance spectrometer at 500.13 (1H) and 125.77 MHz (13C) in
CDCl3. 1H and 13C chemical shifts (Table S2) were referenced to
TMS. All 2D experiments (gradient-selected gs-COSY, gs-HSQC,
gs-HMBC) were performed by using the software of the manufac-
turer (XWINNMR 3.1). Proton–proton connectivities were found
by using gs-COSY. Protonated carbon atoms were assigned by gs-
HSQC and quaternary carbon atoms by gs-HMBC spectra.
392
1696
–9, 9; –10, 10; –18, 18 –58, 56, –5, 6, –22, 21
2.95; 27.5
17076
3910 (0.1601)
1808
3.55; 27.5
34882
8610 (0.0657)
7020
523
0.271/–0.337
1.173
[a]
– indep. (Rint
– obs. [IϾ2σ(I)]
)
Parameters refined 244
Max/min ∆ρ [eÅ–3] 0.353/–0.327
X-ray data for crystals of compounds 1b and 1c (Table 5) were
obtained at 150 K by using an Oxford Cryostream low-temperature
device with a Nonius Kappa CCD diffractometer and Mo-Kα radi-
ation (λ = 0.71073 Å), a graphite monochromator, and φ and the
χ scan mode. Date reduction were performed with the Denzo-SMN
package.[26] The absorption was corrected by integration meth-
ods.[27] Structures were solved by direct methods (SIR92)[28] and
refined by full-matrix least-squares based on F2 (SHELXL-97).[29]
Hydrogen atoms were mostly localized on a difference Fourier map;
however, to ensure uniformity of treatment of the crystal, all hydro-
gen atoms were recalculated into idealized positions (riding model)
and assigned temperature factors Hiso(H) = 1.2 Ueq (pivot atom)
or of 1.5 Ueq for the methyl moiety, with C–H = 0.96, 0.97, and
0.93 for the methyl, methylene, and hydrogen atoms in the aromatic
ring, respectively. CCDC-632632 for (1b) and -610595 (for 1c) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
GOF[b]
1.025
0.0693/0.1167
R[c]/wR
0.0584/0.1305
[c]
[a] Rint = Σ|Fo – Fo,mean2|/ΣFo2. [b] GOF = [Σ(w(Fo – Fc ) )/
2
2
2 2
(Ndiffrs – Nparams)]1/2 for all data. [c] R(F) = Σ||Fo| – |Fc||/Σ|Fo| for
observed data, wR(F2) = [Σ(w(Fo2 – Fc ) )/(Σw(Fo2)2)]1/2 for all data.
2 2
washed with water. After dilution with dichloromethane, drying,
distilling off of volatile components, and vacuum distillation, the
product (16.2 g, 95%) was sufficiently pure for subsequent applica-
tion.
For the preparation of 2-methylaminonaphthalene, a 40% aqueous
solution of methylamine was used. n-Butylamine and ethanolamine
were used as neat liquids. The preparation and diazotization of 3-
amino-5-nitrobenzo[c]-1,2-thiazole was previously described.[1]
1-(5-Nitrobenzo[c]-1,2-thiazol-3-yl)-3-ethyl-3-(naphthalen-2-yl)triaz-
ene (1b) and 3-(2-N-Ethylaminonaphthalen-1-yldiazenyl)-5-nitro-
benzo[c]-1,2-thiazole (2b): A solution of 2-ethylaminonaphthalene
(9.67 g, 0.0525 mol) and emulsifier (sodium C12–C13 alkyltriethoxy-
sulfate, 2 g) in 99% acetic acid (10 mL) was added with stirring to
a mixture of CH3COONa·3H2O (68 g, 0.5 mol) in water (50 mL)
and finely crushed ice (450 g). The emulsion obtained was stirred
and treated with a solution of 5-nitrobenzo[c]-1,2-thiazole-3-diazo-
nium hydrogensulfate in 96% sulfuric acid (0.05 mol) added drop
by drop. The reaction mixture was stirred for another 3 h, where-
upon the precipitated brown solid was collected by suction. The
The electron spectra were measured with a diode array spectropho-
tometer Hewlett Packard 8453. The molar absorption coefficients
were determined by using ca. 5ϫ10–5 molL–1 solutions of the sub-
stances in chloroform (Table 3). The stability of the triazenes in
acidic media was measured with the use of ca. 3ϫ10–5 molL–1
solution of the substance in 1.0 molL–1 H2SO4 in aqueous acetic
acid (1:1, v/v) at 25 °C. A similar experiment was carried out in the
presence of 4,5-dihydroxynaphthalene-2,7-disulfonic acid of
2.5ϫ10–3 molL–1 concentration.
The starting 2-alkylaminonaphthalenes were prepared by the Buch- filter cake was washed with distilled water (500 mL) and dried at
erer reaction of 2-naphthol with primary alkylamines in aqueous
solution of NaHSO3 under pressure in an autoclave in pressure
vessels with Teflon lining of laboratory dyeing apparatus Linitest
(Original Hanau Comp.) as in the case of 2-ethylaminonaphtha-
lene. A 250-mL cooled pressure vessel was charged with 2-naphthol
(14.4 g, 0.1 mol), Na2S2O5 (47.75 g, 0.25 mol), ethylamine (22.5 g,
0.5 mol) and ice (70 g). The closed pressure vessel was agitated in
the apparatus at 150 °C for 10 h. The course of conversion was
monitored by TLC (Silufol; n-heptane/acetone, 5:3, v/v). After 10 h,
2-naphthol completely disappeared from the reaction mixture. The
reaction mixture was diluted with water and heated to boiling. The
aqueous layer was removed, and the organic phase was repeatedly
the temperature of 60 °C. Yield of crude product 16.2 g (86%).
TLC (Silufol; toluene/ethyl acetate, 5:1, v/v) showed that the azo
coupling reaction product is a mixture of compounds 1b and 2b in
a ca. 1:1 ratio. The molar ratio of product 1b/2b 46:54 was then
estimated by HPLC. The more soluble triazene 1b was separated
from azo dyestuff 2b by repeated extraction of the mixture with
boiling acetone and subsequent dilution of the extract with aque-
ous (ca. 5%) ammonia. The separated precipitate of triazene was
then purified by repeated recrystallization from chloroform (m.p.
241–242 °C). Azo dyestuff 2b was obtained from the residue after
extraction of triazene by repeated recrystallization from chloroform
(m.p. 262–263 °C).
Eur. J. Org. Chem. 2008, 3272–3278
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3277

V. Machácˇek et al.
FULL PAPER
[15] A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88,
899–926.
Analogous procedures were used for the preparation of 1a, 1c, 1d,
2a, 2c, and 2d. The electron spectra of compounds 1a–d and 2a–d
are described in Table 3; the elemental analyses, melting points, and
ratios of components in crude reaction mixture after the azo coup-
ling reaction are presented in Table S1.
[16] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr, T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tom-
asi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratch-
ian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gom-
perts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.
Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A.
Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dap-
prich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q.
Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayak-
kara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, GAUSSIAN 03, re-
vision C.02, Gaussian, Inc., Wallingford, CT, 2004..
[17] A. Lycˇka, Collect. Czech. Chem. Commun. 1983, 48, 3104–
3111.
[18] F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G.
Orpen, R. Taylor, J. Chem. Soc. Perkin Trans. 2 1987, S1–S19.
[19] A. R. Walton, T. C. Jenkins, S. Neidle, Acta Crystallogr., Sect.
B 1991, 47, 771–775.
[20] D. C. Zhang, Z.-H. Fei, T.-H. Zhang, Y.-Q. Zhang, K.-B. Yu,
Acta Crystallogr., Sect. C 1999, 55, 102–104.
[21] M. Hörner, I. C. Casagrande, J. Bordinhao, C. M. Mössmer,
Acta Crystallogr., Sect. C 2002, 58, 193–194.
[22] I. R. Pottie, K. Vaughan, M. J. Zaworotko, J. Chem. Crys-
tallogr. 2002, 31, 143–148.
Supporting Information (see footnote on the first page of this arti-
cle): Analytical data, 1H and 13C NMR spectroscopic data, 1H
NMR and UV VIS spectra of the compounds 1c, 2c; supramolec-
ular architecture of compounds 1b, 1c in crystals; spectral records
of decomposition of triazene 1c in acidic media.
Acknowledgments
The authors are indebted to the Ministry of Education, Youth and
Sports of the Czech Republic for financial support (Research pro-
ject CI MSM 002 162 7501) and to the project MSMT LC 512
(P. N.).
[1] J. Prˇikryl, A. Lycˇka, V. Bertolasi, M. Holcˇapek, V. Machácˇek,
Eur. J. Org. Chem. 2003, 4413–4421.
[2] V. Kettmann, J. Lokaj, J. Kozísˇek, J. Prˇikryl, V. Machácˇek,
ˇ
Acta Crystallogr., Sect. C 2004, 60, 143–145.
ˇ
[3] J. Prˇikryl, M. Cerný, H. Beˇlohlavová, V. Machácˇek, A. Lycˇka,
Dyes Pigm. 2007, 72, 392–402.
[4] J. Benesˇ, V. Beránek, J. Zimprich, P. Vetesˇník, Collect. Czech.
Chem. Commun. 1977, 42, 702–710.
[5] K. H. Saunders, R. M. L. Allen, Aromatic Diazo Compounds,
3rd ed., London, Arnold, 1985.
[6] H. Zollinger, Diazo Chemistry I, Aromatic and Heteroaromatic
Compounds, Weinheim, Wiley-VCH, 1994, ch. 13.2, 13.3, pp. [23] F. S. Neidle, D. E. V. Wilman, Acta Crystallogr., Sect. B 1992,
391–444.
48, 213–217.
ˇ
[7] V. Steˇrba in The Chemistry of Functional Groups: The Chemis-
[24] W. L. Bullerwell, L. R. MacGillivray, M. J. Zaworotko, K.
Vaughan, D. E. V. Wilman, Acta Crystallogr., Sect. C 1995, 51,
2624–2627.
try of Diazonium and Diazo Groups (Ed.: S. Patai), Wiley,
Chichester 1978, ch. 2, p. 87.
[8] I. L. Bagal, P. A. Stovpovoy, A. B. Eltsov, Zhur. Obsch. Khim.
1986, 56, 489–513.
[9] F. I. Abezgauz, S. V. Sokolov, S. N. Ezerskij, Zhur. Vsesoyuz.
Chim. Obszcz. Im. Mendeleeva 1965, 10, 113–114.
[25] F. R. Fronczek, C. Hansch, S. F. Watkins, Acta Crystallogr.,
Sect. C 1988, 44, 1651–1653.
[26] Z. Otwinowski, W. Minor, Methods Enzymol. 1997, 276, 307–
326.
[10] K. L. Georgiadou, E. G. Tsatsaroni, Dyes Pigm. 2001, 50, 93– [27] P. Coppens in Crystallographic Computing (Eds.: F. R. Ahmed,
97.
S. R. Hall, C. P. Huber), Copenhagen, Munksgaard, 1970, pp.
255–270.
[28] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J.
Appl. Crystallogr. 1993, 26, 343–350.
[29] G. M. Sheldrick, SHELXL-97, University of Göttingen,
Göttingen, 1997.
[11] C. Lee, W. Yang, R. Parr, Phys. Rev. B: Condens. Matter. 1988,
37, 785–789.
[12] A. D. Becke, J. Chem. Phys. 1993; 98, 5648–5652.
[13] A. Schaefer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100,
5829–5835.
[14] M. Cossi, V. Barone, B. Mennucci, J. Tomasi, Chem. Phys. Lett.
1998, 286, 253–260.
Received: February 15, 2008
Published Online: May 21, 2008
3278
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 3272–3278