Switching azonaphthols containing a side chain with limited flexibility. Part 1. Synthesis and tautomeric properties

Article abstract of DOI:10.1016/j.dyepig.2011.07.013

A series of azo dyes, possessing amide fragments with restricted flexibility tethered to 4-(phenyldiazenyl)naphthalen-1-ol, was obtained from 1-hydroxy-2-naphthoic acid by subsequent conversion to amides and diazo coupling. It was shown that the position of the tautomeric equilibrium in solution strongly depends on the solvent in both UV and NMR concentration scale. The compounds exist as pure enol forms in chloroform and hydrocarbons, while in polar solvents (acetone, acetonitrile, alcohols) a tautomeric mixture is observed. According to the quantum-chemical calculations the aggregation of the keto tautomer is the possible reason for this shift in the position of the tautomeric equilibrium. To support the theoretical predictions, it was found that from acetone the keto form crystallizes as a dimer with hydrogen bonding between N₁-H in the one molecule and amide CO in the other forming a three-dimensional structure. The importance of the side-chain nitrogen atom on the dimer formation was confirmed by solution and solid state study of 4-(phenyldiazenyl)-2-acetylnaphthalen-1-ol. The results indicate that the new azo-dyes obtained could be suitable candidates for switching and sensing applications in non-polar solvents.

Full text of DOI:10.1016/j.dyepig.2011.07.013

Dyes and Pigments 92 (2012) 1266e1277
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Switching azonaphthols containing a side chain with limited flexibility.
Part 1. Synthesis and tautomeric properties
,
a
,
a
b
Vanya B. Kurteva a , Liudmil M. Antonov , Daniela V. Nedeltcheva , Aurélien Crochet ,
*
*
Katharina M. Fromm b, Rositsa P. Nikolova c, Boris L. Shivachev c, Maya S. Nikiforova a
a Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 9, 1113 Sofia, Bulgaria
b University of Fribourg, Department of Chemistry, Chemin du Musée 9, CH-1700 Fribourg, Switzerland
c Institute of Mineralogy and Crystallography “Acad. Ivan Kostov”, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 107, 1113 Sofia, Bulgaria
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of azo dyes, possessing amide fragments with restricted flexibility tethered to 4-(phenyl-
diazenyl)naphthalen-1-ol, was obtained from 1-hydroxy-2-naphthoic acid by subsequent conversion to
amides and diazo coupling. It was shown that the position of the tautomeric equilibrium in solution
strongly depends on the solvent in both UV and NMR concentration scale. The compounds exist as pure
enol forms in chloroform and hydrocarbons, while in polar solvents (acetone, acetonitrile, alcohols)
a tautomeric mixture is observed. According to the quantum-chemical calculations the aggregation of the
keto tautomer is the possible reason for this shift in the position of the tautomeric equilibrium. To
support the theoretical predictions, it was found that from acetone the keto form crystallizes as a dimer
with hydrogen bonding between N1eH in the one molecule and amide C]O in the other forming
a three-dimensional structure. The importance of the side-chain nitrogen atom on the dimer formation
was confirmed by solution and solid state study of 4-(phenyldiazenyl)-2-acetylnaphthalen-1-ol. The
results indicate that the new azo-dyes obtained could be suitable candidates for switching and sensing
applications in non-polar solvents.
Received 11 May 2011
Received in revised form
20 July 2011
Accepted 21 July 2011
Available online 8 August 2011
Keywords:
Azonaphthol dyes
Rigid side chain
Preferred conformations
Quantum-chemical calculations
Spectroscopy
X-ray
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
shift in the position of the tautomeric equilibrium can be achieved
through protonation and/or complexation, was proven by attaching
The design of selective and sensitive chemosensors for ions and
systems for molecular recognition has developed at the extraor-
dinary rate due to their potential application in clinical biochem-
istry, analytical chemistry and environmental science [1e9].
Organic chemosensors are molecules of abiotic nature capable
of interacting selectively with a specific substrate accompanied
with corresponding changes in one or more characteristics
of the system, i.e. to detect certain chemical stimuli in the
environment.
of a flexible nitrogen linked piperidine [18] (1, Fig. 1) or 1-aza-
15-crown-5 [19] (2) fragment to 4-(phenyldiazenyl)naphthalen-1-
ol. Both compounds were isolated as pure enol forms due to
strong hydrogen bonding between tautomeric hydroxyl group and
the side chain nitrogen atom and were fully converted to keto-
tautomers, which, to the best of our knowledge, present the first
examples of an active side-arm containing chemosensors with
significant bathochromic and hyperchromic effects of the absorp-
tion maximum upon protonation or complexation. The developed
Azo compounds are among the most widely examined coloring
and signaling systems due to their optical stability [10e17].
However, when both tautomeric forms coexist in solution at room
temperature the isolation of individual end-structures represents
a general problem, which limits their use for sensing purposes.
Recently, the concept for a tautomeric switch, where a directed
tautomeric aza-15-crown-5 system 2 has shown acceptable
complexation with small alkali and alkaline earth metal ions.
However, the selectivity toward different metals was relatively low,
most probably due to flexibility of the methylene spacer and the
azacrown moiety. In order to restrict the rotation of the side chain
the methylene group was replaced with carbonyl function, capable
of forming a strong hydrogen bond with the tautomeric hydroxyl.
Additionally this could lead to reduction of the electron density at
the heterocycle nitrogen atom as a tuning tool to model the
complexation ability of the attached heterocycle.
* Corresponding authors. Tel.: þ359 2 9606156; fax: þ359 2 8700225.
E-mail addresses: vkurteva@orgchm.bas.bg (V.B. Kurteva), lantonov@orgchm.
bas.bg (L.M. Antonov).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.07.013

V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
1267
chloride by following the protocol applied in the synthesis of
similar compounds [20] and the target amide 6a was isolated in
good yield together with slight impurity of 8a. The same result was
obtained for 6b. Finally, the excess of the reagent was reduced and
the amides 6ae6c were obtained as sole products and were directly
used in the next step. Diazo coupling was carried out via a standard
protocol (Scheme 1) and the pure products 3 were isolated by high
performance flash chromatography on silica gel in good overall
yields.
Fig. 1. Azonaphthol dyes.
The position of the tautomeric equilibrium in 3 was studied by
quantum-chemical calculations and mass spectrometry in the gas
phase and was afterward investigated by NMR and UV spectros-
copy in solution and by X-ray in the solid state.
Herein, we present the synthesis of a series of azo dyes (3)
possessing amide fragments tethered to 4-(phenyldiazenyl)naph-
thalen-1-ol and a study on their conformational preferences
and tautomeric properties in the gas phase, solutions and in the
solid state. As a comparison, the spectral data of compound 4,
where the nitrogen containing heterocycle is absent, will be
considered.
It was reported in the case of 1 [18] that the intramolecular
hydrogen bonding between the piperidine nitrogen atom and the
enol OH group leads to full shift of the equilibrium towards the enol
tautomer in chloroform, cyclohexane, acetone and acetonitrile. This
bonding is probably partially broken in the case of alcohols, where
an enol predominating tautomeric mixture is observed. The
absorption spectra of 3a, shown in Fig. 2 show a different behavior.
The tautomeric equilibrium is switched to the enol isomer
(absorption maximum at 400 nm) only in chloroform and cyclo-
hexane and increasing amount of the keto tautomer is observed in
acetone, acetonitrile and ethanol. The increase of the concentration
in acetonitrile and in acetone from 10ꢀ5 to 10ꢀ3 mol/l leads to slight
rise of the keto band at 470 nm suggesting an aggregation effect,
while in the case of the chloroform no spectral changes were
observed.
The quantum-chemical calculations performed (Fig. 3) show
that the enol form (E) can exist as two isomers depending on the
hydrogen bonding formation. Logically the side chain C]O.HO
interaction is preferred, but both isomers are substantially more
stable in comparison to the keto tautomer (K) in the gas phase. On
the other side the side chain of the keto form is out of planarity,
which gives possibility for intermolecular interaction (NeH.O]C
side chain) with another molecule and to formation of a dimer (DK),
which leads to final stabilization of appr. 2 kcal/mol in respect of
the more stable enol form (Fig. 3a).
2. Results and discussion
The target azo dyes 3 were obtained from 1-hydroxy-2-
naphthoic acid 5 by subsequent conversion to amides 6 and diazo
coupling as a one-pot two step protocol without intermediate
purification, as shown on Scheme 1. Piperidine (a), morpholine (b),
and N-phenylpiperazine (c) were chosen as N-heterocyclic units in
order to examine the influence of the presence and the type of
the additional heteroatoms on the position of the tautomeric
equilibrium.
As a first step, the conditions for the preparation of the amides 6,
which was expected to be attended with synthetic problems due
to the presence of the reactive phenolic hydroxyl group in the
acid molecule, were optimized using the piperidino derivative
by checking different approaches. The direct amide coupling in
the presence of N-(3-dimethylaminopropyl)-N0-ethylcarbodiimide
hydrochloride (EDCI) and 1-hydroxybenzotriazole (HOBT) led to
a side product 8a formation with the participation of the hydroxyl
group (Scheme 2). Its structure was assigned by 1D and 2D NMR
spectroscopy, where characteristic ester and amide resonances
were observed.
The overall process can be summarized as follows:
E %K%DK
The classical protocol for the preparation of the amides via acyl
chlorides, heating of the acid in thionyl chloride followed by reac-
tion with piperidine, resulted in a very complex mixture. The amide
6a was isolated in low yield after several subsequent chromatog-
raphy purifications, along with some side-products, the chlorinated
compound 7a and the already obtained ester-amide 8a. The
formation of these side products, which could be achieved only
during the preparation of the acid chloride, showed that the harsh
classical conditions are not efficient in this case. The chlorination
was further performed in solution with 4-fold excess of thionyl
In gas phase the tautomeric equilibrium is shifted to the enol
isomer, but in solution the situation could be determined by the
polarity and/or proton-acceptor/donor ability of the solvent. At
the same time, the dimer formation requires availability of the
keto tautomer. According to the quantum-chemical calculations the
keto tautomer is slightly more polar (3.5D against 3.1D for the enol
form) and it is the form that gives better opportunities for inter-
action with proton-acceptor solvents (free NeH). In chloroform,
a bulky, strong proton-donor and low polar solvent, there are no
Scheme 1. 1) see Scheme 2; ii) PhNH2, conc. HCl, NaNO2, acetone-H2O, NaOH.

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V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
Scheme 2. i) piperidine, EDCI, HOBT, CH2Cl2, rt; ii) 1. SOCl2, reflux; 2. piperidine, CH2Cl2, rt; iii) SOCl2 (4 equiv), DMF, heptane, reflux, 4 h; 2. piperidine or morpholine, i-Pr2NEt,
CH2Cl2, rt; iv): 1. SOCl2 (2.5 equiv), DMF, heptane, reflux, 4 h; 2. piperidine or morpholine, i-Pr2NEt, CH2Cl2, rt.
possibilities to shift the equilibrium to the more polar keto form, in
which C]O group is sterically hindered for hydrogen bonding with
the solvent. However, in acetonitrile and in acetone (due their
higher polarity and substantial proton-acceptor abilities [21])
a proportion of the keto tautomers are available giving the back-
ground for dimer formation.
As seen from Fig. 3a, the energies of the structures in acetonitrile
field (PCM calculations) suggest substantial stabilization of the keto
tautomer in respect of the enol forms (relative energy lows from 4.1
to 0.8 kcal/mol and from 8.1 to 3.4 kcal/mol) and the dimeric
structure remains most stable at the same time. The interactions of
the tautomers with acetone (Fig. 3b) dramatically change the
situation leading to comparably stable keto monomer, keto dimer
and C]O.HO enol form. The leading force is the intermolecular
hydrogen bonding between the tautomeric proton and the acetone
C]O group.
which results from the C]O.HO interaction hindering migration
of the tautomeric proton towards the piperidine unit. The mass
spectrum from methanol-d4 solution additionally shows that both
isomers of the enol tautomer coexist in gas phase, because the
molecular ion radical (m/z 360) again gives fragment m/z 274 (loss
of deuterated piperidine, which requires intramolecular hydrogen
bonding involving the piperidine nitrogen atom [21]) along with
fragment with m/z 255.
The mass spectra of 3 not only confirm the existence of the enol
tautomer in gas phase [21], but give indications for co-existence of
the C]O.HO and N.HO isomers. While in the case of 1 the
fragmentation begins with loss of piperidine [22], in 3a (Scheme 3
as a representative example for 3) this fragmentation pathway
(appearance of m/z 274 and subsequent fragmentation) is accom-
panied by cleavage of the ]NeCnaph bond (m/z 254 and m/z 105),
Fig. 3. The values of the relative energy (HF/6e31G**), relative energy
correction and DDG (kcal/mol) of the tautomers of 3a: a) in gas phase; the relative
þ
ZPE
energies in acetonitrile environment are given in brackets; b) with addition of
a
molecule acetone; the relative energies in acetone environment are given in
Fig. 2. Absorption spectra of 3a in various solvents.
brackets.

V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
1269
Scheme 3. Fragmentation of 3a.
The NMR spectra of the products in 10ꢀ1 M concentration in
deuterochloroform showed sharp and well defined signals consis-
tent with enol forms. It can be suggested that azo dyes 3 exist in
chloroform as single enol form, as was observed for analogues with
flexible side-chain 1 [18] and 2 [19], and contrary to unsubstituted
4-phenylazo-1-napthol and 2-phenylazo-1-napthol, where both
tautomers coexist and the populations of the keto forms are
calculated from 13C and 15N spectra [23,24]. The preferred confor-
mations of amide side chains (Fig. 4) were studied by 1He1H NOESY
experiments. Strong interactions of the singlet for CH-3 with the
ring methylene groups were observed in all spectra, while no
CH2.OH interactions were detected, which are compatible with
conformers 3B.
of the products 3ae3c were recorded in 10ꢀ1 M solutions in
deuteroacetone. The proton spectra showed broad signals and the
hydroxyl group resonances were not detected, which is consistent
with slow exchange between two sites. The latter was supported
by the fact that the complete carbon spectra were not detected
within a reasonable time-scale. The spectra of a selected sample
(3b) in deuteroacetone were recorded at low temperature (213 K)
at 5 ꢁ 10ꢀ2 M concentration in order to avoid the product crys-
tallization (crystal formation was observed at 278 K in 10ꢀ1
M
solution) and clear signals for both exchangeable forms were
detected. The assignment of the signals was achieved by 2D
techniques. The HMBC cross peaks explicitly show that the
carbonyl resonances at 180.6, 169.4, 165.9, and 158.0 belong to C]
O,K, NC]O,E, NC]O,K, and CqeOH, respectively. The double
signals observed for a part of the E carbons at 213 K is most
probably due to 3A-3B (Figure 4) slow exchange, as both forms
differ significantly in relative energy due to strong H-bond in 3B.
The results in chloroform were in full agreement with the
UVeVis data in respect of the enol stabilization. In order to study if
the influence of the solvent on the position of the tautomeric
equilibrium is dependent on the concentration, the NMR spectra

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V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
Fig. 4. Conformations of the amide side chain in 3 and important 1He1H NOESY interactions in: a) 3a; b) 3b; c) 3c; d) 4.
The spectra of 3b were additionally compared with the spectra in
deuteroacetone of already published [25] O-methyl and N-methyl
derivatives of 4-phenylazo-1-naphthol, model enol and keto
analogues, respectively. The carbonyl resonances of 3b are in full
accordance with those of the model compounds, as illustrated on
Fig. 5. So, it can be suggested that the exchange observed in 3 is
due to keto-enol equilibrium. These results show that the tauto-
meric preferences are strongly dependent on the solvent both in
UV and in NMR concentration scales.
The crystals of 3b were obtained by slow diffusion of hexane into
acetone solution of the product. Unfortunately, all other solvent
systems used led to small crystal formation and X-ray analyses were
not successful. It was found that the product crystallize in keto form
as a dimer with tautomeric proton “delocalized” between N1eH in
the one molecule and amide C]O in the other (Fig. 6) thus forming
an interesting three-dimensional structure (Fig. 7), where a mole-
cule of acetone is captured [26]. This result strongly supports the
suggestions made on the base of the quantum-chemical calculations
for 3a and the observed spectral properties in acetone.
analogue of 3 possessing methyl group instead of N-heterocycle
was chosen as useful candidate. Thereby, 4-phenylazo-2-acetyl-1-
naphthol 4 was obtained from commercially available 2-acetyl-1-
naphthol 9 by diazo coupling (Scheme 4).
The NMR spectra of 4 showed sharp signals compatible with
individual form, i.e. with absence of exchange. From the other side,
the NOESY interaction between CH-3 and CH3-group (Fig. 4d)
shows that in chloroform solution at room temperature the product
exists as 4B form. The spectra in deuteroacetone with the same
concentration (10ꢀ1 M) showed again sharp signals compatible
with enol tautomer, which indicates that the side-chain nitrogen in
3 has significant influence on the conformational preferences of the
azo dyes.
The UV spectral data in various solvents show that in all cases
the equilibrium is shifted to the enol tautomer. These results are
supported by the quantum-chemical calculations shown in Fig. 8.
The enol form is strongly stabilized through intramolecular
hydrogen bonding, where the tautomeric proton is “delocalized”
between the carbonyl groups. The crystal structure of 4 [26] (Fig. 9)
supports the calculations and spectral results, showing the exis-
tence of the enol tautomer, stabilized through strong C]O.HeO
To ascertain the influence of the side chain heteroatoms on
the conformational preferences of the products, the simplified

V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
1271
C=O
185
180
175
70
65
60
155
150
pm
Cq-OMe
185
180
175
70
65
60
155
150
pm
C=O
Cq-OH
NC=O, E NC=O, K
185
180
175
170
165
160
155
150
ppm
Fig. 5. Carbonyl area of 13C NMR spectra in deuteroacetone of 3b at 213 K (down) and O-methyl (middle) and N-methyl (up) derivatives of 4-phenylazo-1-naphthol at 298 K.
bonding (according to Gilli et al. [27] in the light of the bond
distances of 0.820 Å for the OeH bond and 1.816 Å for H.O]C
bond). This hydrogen bonding is much stronger that the N.HeO
interaction in 1 [18], which leads the better stabilization of the enol
tautomer (9.8 vs 4.2 kcal/mol).
According to the theoretical calculations, the keto tautomer is
essentially planar and the formation of dimers, although stabilize
the keto form by itself, do not lead to stabilization in respect of the
enol tautomers.
3. Conclusions
We report herein on the synthesis, tautomeric properties and
conformational preferences of switching azonaphthols containing
a side chain with limited flexibility. The target azo dyes possessing
amide fragments tethered to 4-(phenyldiazenyl)naphthalen-1-ol 3
were obtained in good overall yields from 1-hydroxy-2-naphthoic
acid by consecutive conversion to amides 6 and diazo coupling as
a one-pot two step protocol without intermediate purification. It
was shown by quantum-chemical calculation and mass spectra that
the products exist in enol form in gas phase, while the position of
the equilibrium in solution is strongly dependent on the solvent
both in UV and NMR concentration scale. It was found that the keto
form of 3b crystallizes from acetone as dimer with hydrogen
Fig. 6. View of molecular structure of 3b, 50% of probability, H-bond are drawn in
dashed blue, solvent molecules and some hydrogen atoms are omitted for clarity, #1:
1ꢀx, y, 0.5ꢀz. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Fig. 7. 3D structure of 3b; ellipsoids are in 30% probability.

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V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
Scheme 4. i) PhNH2, conc. HCl, NaNO2, acetoneeH2O, NaOH.
bonding between N1eH in the one molecule and amide C]O in the
other. A study on the switching and sensing properties of the
products is in progress and will be reported in due course.
Resolution Magnetic Sector MS Thermo Fisher Scientific GmbH
(Bremen, Germany) instrument operating in EI mode at 70 eV with
an ion source temperature 220 ꢂC. The accurate mass measure-
ments were performed using perfluorokerosene as a reference.
4. Experimental section
4.1.2. Amide coupling. Synthesis of 2-(piperidine-1-carbonyl)
naphthalen-1-yl-1-hydroxy-2-naphthoate 8a
4.1. Synthesis
A mixture of 1-hydroxy-2-naphthoic acid 5 (5 mmol), piperidine
(6 mmol), EDCI (6 mmol), and HOBT (6 mmol) in dichloromethane
(25 ml) was stirred at room temperature for 18 h. The reaction
mixture was poured into ice-water and the solid phase was filtered
off and washed with CH2Cl2. The products were partitioned
between CH2Cl2 and water. The organic layer was washed with
brine, dried over Na2SO4, and purified by flash chromatography on
silica gel. Mobile phase with a gradient of polarity from CH2Cl2 to
ether:CH2Cl2 ¼ 1:9: 54% yield of 8a; Rf 0.54 (CH2Cl2:acetone 95:5);
red crystals, m. p. 165e166 ꢂC; 1H NMR 1.439 (bm, 6H, CH2-3, CH2-4
and CH2-5 piperidine), 3.272 (bm, 2H, ½ CH2-2 and CH2-6 piperi-
dine), 3.503 (bs, 2H, ¼ CH2-2 and CH2-6 piperidine), 3.579 (bs,
2H, ¼ CH2-2 and CH2-6 piperidine), 7.347 (d, 1H, J 8.7, CH-4), 7.380
(d, 1H, J 8.7, CH-30), 7.480 (m, 3H, CH-8, CH-70 and CH-80), 7.591
(ddd, 1H, J 1.2, 7.0, 8.2, CH-7), 7.759 (d,1H, J 8.2, CH-6), 7.788 (d, 1H, J
8.5, CH-40), 7.831 (dd, 1H, J 1.2, 8.5, CH-90), 7.847 (dd, 1H, J 1.2, 8.4,
CH-60), 8.062 (d, 1H, J 8.7, CH-3), 8.364 (d, 1H, J 8.3, CH-9), 11.492
(s, 1H, OH);13C NMR 23.37 (CH2-3, CH2-4 or CH2-5 piperidine),
24.54 (CH2-3, CH2-4 or CH2-5 piperidine), 25.42 (CH2-3, CH2-4 or
CH2-5 piperidine), 41.68 (CH2-2 or CH2-6 piperidine), 47.31 (CH2-2
or CH2-6 piperidine), 103.45 (Cquat-2), 118.35 (CH-4), 120.75 (CH-90),
122.87 (CH-30), 123.01 (CH-9), 123.32 (CH-3), 123.69 (Cquat-10),
125.08 (CH-8), 125.69 (Cquat-100), 125.94 (CH-40), 126.05 (Cquat-20),
126.34 (CH-70), 126.47 (CH-80), 126.62 (CH-6), 127.11 (CH-60), 129.04
(CH-7), 133.52 (Cquat-50), 136.70 (Cquat-5), 141.50 (Cquat-10), 161.09
4.1.1. General
All reagents were purchased from Aldrich, Merck and Fluka and
were used without any further purification. Fluka silica gel/TLC-
cards 60778 with fluorescent indicator 254 nm were used for TLC
chromatography and Rf-values determination. The high perfor-
mance flash chromatography (HPFC) purifications were carried out
on a Biotage HorizonÔ system (Charlottesville, Virginia, USA) on
silica gel. The melting points were determined in capillary tubes on
SRS MPA100 OptiMelt (Sunnyvale, CA, USA) automated melting
point system. The NMR spectra were recorded on a Bruker Avance
IIþ 600 spectrometer (Rheinstetten, Germany) with fid resolution
0.3 Hz, i.e. 5.10ꢀ4 ppm; the chemical shifts were quoted in ppm in
d-values against tetramethylsilane (TMS) as an internal standard
and the coupling constants were calculated in Hz. The assignment
of the signals was confirmed by applying 2D techniques. The
spectra were recorded as 10ꢀ1 M solutions in deuterochloroform or
deuteroacetone (where indicated) except NOESY experiments
where 3 ꢁ 10ꢀ2 M solutions were used in order to avoid trans-
molecular interactions. For simplicity, the ester naphthol unit in 8 is
described as basic, while the amide substituted moiety is desig-
nated as (C,H)’. The MS-spectra were recorded on a DFS High
Fig. 9. View of the molecular structure of 4, 30% of probability. H atoms are presented
with spheres of arbitrary radii. H-bond is shown in dashed blue. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version
of this article.)
Fig. 8. Relative stability (HF/6e31G**) of the tautomers of 4. The values of the relative
energy, relative energy þ ZPE correction and DDG are given in kcal/mol units.

V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
1273
(Cquat-1), 165.23 (Cquat-CO’), 168.41 (Cquat-CO); COSY cross peaks
1.439/3.272, 1.439/3.503, 1.439/3.579, 7.347/8.062, 7.380/7.788,
7.480/7.591, 7.480/7.831, 7.480/7.847, 7.480/8.364, 7.591/7.759;
NOESY cross peaks 1.439/3.272, 1.439/3.503 (weak), 1.439/3.579
(weak), 3.272/3.503, 3.272/3.579, 3.503/7.380, 7.347/7.759, 7.347/
8.062, 7.380/7.788, 7.480/7.591, 7.480/7.831, 7.480/7.847, 7.480/
8.364, 7.591/7.759, 7.788/7.847, 7.831/11.492 (weak), 8.364/11.492;
HSQC cross peaks 1.439/24.54, 1.439/25.42, 1.439/23.37, 3.272/
47.31, 3.503/41.68, 3.579/41.68, 7.347/118.35, 7.380/122.87, 7.480/
125.08, 7.480/126.34, 7.480/126.47, 7.591/129.04, 7.759/126.62,
7.788/125.94, 7.831/120.75, 7.847/127.11, 8.062/123.32, 8.364/
123.01; HMBC cross peaks 7.347/103.45, 7.347/123.69, 7.347/
126.62, 7.347/136.70, 7.347/161.09 (weak), 7.380/133.52, 7.380/
141.50, 7.380/165.23, 7.480/120.75, 7.480/123.69, 7.480/125.69,
7.480/126.62, 7.480/127.11, 7.480/133.52, 7.591/123.01, 7.591/
136.70, 7.759/118.35, 7.759/123.69, 7.759/125.08, 7.759/136.70
(weak), 7.759/161.09 (weak), 7.788/125.69, 7.788/126.05, 7.788/
127.11, 7.788/133.52 (weak), 7.788/141.50 (weak), 7.831/126.34,
7.831/133.52, 7.831/141.50, 7.847/125.69, 7.847/125.94, 7.847/
126.47, 8.062/136.70, 8.062/161.09, 8.062/168.41, 8.364/129.04,
8.364/136.70, 8.364/161.09, 11.492/103.45, 11.492/123.69, 11.492/
136.70 (weak), 11.492/161.09; m/z (EI) 425 (4, Mþ), 255 (100), 238
(7), 170 (88), 143 (13), 114 (48%); HRMS (EI): Mþ, found 425.1626.
C27H23NO4 requires 425.1627.
yellow crystals, m. p. 118e119 ꢂC; 1H NMR 1.622 (m, 4H, CH2-3, and
CH2-5 piperidine), 1.667 (m, 2H, CH2-4 piperidine), 3.623 (dd, 4H,
J 5.2, 5.7, CH2-2 and CH2-6 piperidine), 7.316 (s, 1H, CH-3), 7.508
(ddd, 1H, J 1.2, 6.9, 8.2, CH-8), 7.615 (ddd, 1H, J 1.3, 6.9, 8.3, CH-7),
8.084 (dt, 1H, J 0.7, 8.4, CH-6), 8.343 (ddd, 1H, J 0.6, 1.2, 8.3, CH-9),
11.240 (s, 1H, OH); 13C NMR 24.51 (CH2-4 piperidine), 26.12 (CH2-3
and CH2-5 piperidine), 47.19 (CH2-2 and CH2-6 piperidine), 109.54
(Cquat-2), 120.64 (Cquat-4), 124.00 (CH-9), 124.05 (CH-3), 124.22
(CH-6), 126.42 (CH-8), 126.62 (Cquat-10), 129.43 (CH-7), 132.47
(Cquat-5), 157.33 (Cquat-1), 170.95 (C]O); COSY cross peaks 1.622/
1.667, 1.622/3.623, 1.667/3.623, 7.508/7.615, 7.508/8.343, 7.615/
8.084; NOESY cross peaks 1.622/1.667, 1.622/3.623, 3.623/7.316,
7.508/7.615, 7.508/8.343, 7.615/8.084, 8.343/11.240; HSQC cross
peaks 1.622/26.12, 1.667/24.51, 3.623/47.19, 7.316/124.05, 7.508/
126.42, 7.615/129.43, 8.084/124.22, 8.343/124.00; HMBC cross
peaks 1.622/24.51, 1.622/26.12, 1.622/47.19 (weak), 1.667/26.12,
3.623/24.51, 3.623/26.12, 3.623/47.19, 3.623/170.95, 7.316/120.64,
7.316/132.47, 7.316/157.33, 7.316/170.95, 7.508/124.22, 7.508/
126.62, 7.615/124.00, 7.615/132.47, 8.084/120.64, 8.084/126.42,
8.343/120.64 (weak), 8.343/129.43, 8.343/132.47, 8.343/157.33,
11.240/109.54 (weak), 11.240/126.62, 11.240/157.33 (weak); m/z (EI)
289 (100, Mþ), 254 (16), 203 (72), 170 (7), 149 (44), 113 (21%); HRMS
(EI): Mþ, found 289.0864. C16H16ClNO2 requires 289.0870.
4.1.3.2. Method 2. To a solution of 1-hydroxy-2-naphthoic acid 5
(5 mmol) in DMF (0.1 ml) and heptane (20 ml) SOCl2 (20 mmol) was
added and the solution was refluxed for 4 h. The excess of SOCl2 and
solvents were evaporated to dryness. To the crude acyl chloride in
CH2Cl2 (20 ml) EtN(i-Pr)2 (15 mmol) and piperidine or morpholine
(6 mmol) was added at 0 ꢂC and the mixture was stirred at room
temperature for 4e5 h. The products were partitioned between
CH2Cl2 and aq. Na2CO3. The organic phase was washed with brine,
2N HCl, and again with brine, was dried over Na2SO4, and evapo-
rated to dryness. The product was purified by HPFC on silica gel:
44% 6a and 13% 8a; 42% 6b and 16% 8b.
4.1.3. Synthesis of amides 6
4.1.3.1. Method 1. A solution of 1-hydroxy-2-naphthoic acid 5
(5 mmol, 911 mg) in SOCl2 (10 ml) was refluxed for 4 h. The excess
of SOCl2 was evaporated to dryness. To a solution of piperidine
(20 mmol, 2 ml) in dry CH2Cl2 (40 ml) a solution of the crude acyl
chloride in CH2Cl2 (10 ml) was added at 0 ꢂC and the mixture was
stirred at room temperature overnight. The products were parti-
tioned between CH2Cl2 and aq. Na2CO3. The organic phase was
washed with brine, 2N HCl, and again with brine, was dried over
Na2SO4, and evaporated to dryness. The product was purified by
flash chromatography on silica gel. Pure products were isolated
after the third purification: 25% 6a, 13% 7a, and <1% 8a.
4.1.3.1.1. (1-Hydroxynaphthalen-2-yl)(piperidin-1-yl)methanone
6a. Rf 0.71 (CH2Cl2:acetone 95:5), Rf 0.30 (CH2Cl2); light yellow
crystals, m. p. 108e109 ꢂC; 1H NMR 1.700 (m, 6H, CH2-3, CH2-4 and
CH2-5 piperidine), 3.695 (dd, 4H, J 5.2, 5.7, CH2-2 and CH2-6
piperidine), 7.266 (d, 1H, J 8.5, CH-4), 7.290 (d, 1H, J 8.6, CH-3), 7.505
(ddd, 1H, J 1.3, 6.9, 8.3, CH-8), 7.559 (ddd, 1H, J 1.3, 6.8, 8.1, CH-7),
7.758 (d, 1H, J 8.0, CH-6), 8.377 (dt, 1H, J 0.6, 8.3, CH-9), 11.273 (s, 1H,
OH); 13C NMR 24.59 (CH2-4 piperidine), 26.17 (CH2-3 and CH2-5
piperidine), 47.14 (CH2-2 and CH2-6 piperidine), 109.52 (Cquat-2),
117.50 (CH-4), 123.56 (CH-9), 124.44 (CH-3), 125.53 (Cquat-10),
125.63 (CH-8), 127.28 (CH-6), 128.37 (CH-7), 135.48 (Cquat-5), 158.14
(Cquat-1), 172.11 (C]O); COSY cross peaks 1.700/3.695, 3.659/7.290,
7.266/7.290, 7.505/7.559, 7.505/8.377, 7.559/7.758; NOESY cross
peaks 1.700/3.695, 7.266/7.290, 7.266/7.758, 7.266/7.758, 7.505/
7.559, 7.505/8.377, 7.559/7.758, 8.377/11.273; HSQC cross peaks
1.700/24.59, 1.700/26.17, 3.695/47.14, 7.266/117.50, 7.290/124.44,
7.505/125.63, 7.559/128.37, 7.758/127.28, 8.377/123.56; HMBC
cross peaks 1.700/24.59, 1.700/26.17, 1.700/47.14, 3.695/24.59,
3.695/26.17, 3.695/47.14, 3.695/172.11, 7.266/109.52, 7.266/125.53,
7.266/127.28, 7.290/135.48, 7.290/158.14, 7.290/172.11, 7.505/
125.53, 7.505/127.28, 7.559/123.56, 7.559/135.48, 7.758/117.50,
7.758/125.53, 7.758/125.63, 7.758/135.48 (weak), 8.377/128.37,
8.377/135.48, 8.377/158.14, 11.273/109.52 (weak), 11.273/125.53,
11.273/158.14 (weak); m/z (EI) 255 (88, Mþ), 238 (11), 170 (100), 143
(22), 115 (81%); HRMS (EI): Mþ, found 255.1257. C16H17NO2 requires
255.1259.
4.1.3.2.1. (1-Hydroxynaphthalen-2-yl)(morpholine-1-yl)methanone
6b. Rf 0.61 (CH2Cl2:acetone 95:5), Rf 0.12 (CH2Cl2); light yellow
crystals, m. p. 131e132 ꢂC; 1H NMR 3.759 (m, 4H, CH2-O morpho-
line), 3.801 (m, 4H, CH2-N morpholine), 7.257 (d, 1H, J 8.7, CH-3),
7.281 (d, 1H, J 8.7, CH-4), 7.520 (ddd, 1H, J 1.2, 6.8, 8.3, CH-8), 7.576
(ddd, 1H, J 1.3, 6.9, 8.2, CH-7), 7.761 (dt, 1H, J 0.8, 8.1, CH-6), 8.385
(ddd, 1H, J 0.6, 1.3, 8.3, CH-9), 11.177 (s, 1H, OH); 13C NMR 46.48
(CH2eN morpholine), 66.93 (CH2eO morpholine), 108.56 (Cquat-2),
117.77 (CH-4), 123.60 (CH-9), 124.01 (CH-3), 125.53 (Cquat-10),
125.80 (CH-8), 127.30 (CH-6), 128.64 (CH-7), 135.63 (Cquat-5), 158.52
(Cquat-1), 172.30 (C]O); COSY cross peaks 3.759/3.801, 7.257/7.281,
7.520/7.576, 7.520/8.385, 7.576//7.761; NOESY cross peaks 3.759/
3.801, 3.759/7.257, 3.759/11.177 (weak), 3.801/7.257, 3.801/11.177
(weak), 7.257/7.281, 7.281/7.761, 7.520/7.576, 7.520/8.385, 7.576/
7.761, 8.385/11.177; HSQC cross peaks 3.759/66.93, 3.801/46.48,
7.257/124.01, 7.281/117.77, 7.520/125.80, 7.576/128.64, 7.761/
127.30, 8.385/123.60; HMBC cross peaks 3.759/46.48, 3.759/66.93,
3.801/46.48, 3.801/66.93, 3.801/172.30, 7.257/135.63, 7.257/158.52,
7.257/172.30, 7.281/108.56, 7.281/125.53, 7.281/127.30, 7.520/
125.53, 7.520/127.30, 7.576/123.60, 7.576/135.63, 7.761/117.77,
7.761/125.53, 7.761/125.80, 7.761/135.63 (weak), 8.385/128.64,
8.385/135.63, 8.385/158.52, 11.177/108.56, 11.177/125.53, 11.177/
158.52; m/z (EI) 257 (68, Mþ), 213 (3), 170 (100), 143 (8), 115 (16%);
HRMS (EI): Mþ, found 257.1048. C15H15NO3 requires 257.1052.
4.1.3.2.2. 2-(Morpholine-1-carbonyl)naphthalen-1-yl-1-hydroxy-
2-naphthoate 8b. Rf 0.42 (CH2Cl2:acetone 95:5); light yellow crys-
tals, m. p.142.5e143 ꢂC; 1H NMR 3.448 (m, 4H, CH2eN morpholine),
3.620 (m, 4H, CH2eO morpholine), 7.450 (d, 1H, J 8.9, CH-4), 7.458
(d, 1H, J 8.5, CH-30), 7.585 (m, 3H, CH-8, CH-70 and CH-80), 7.688
(ddd, 1H, J 1.3, 7.0, 8.2, CH-7), 7.849 (d, 1H, J 8.2, CH-6), 7.883 (d, 1H,
4.1.3.1.2. (4-Chloro-1-hydroxynaphthalen-2-yl)(piperidin-1-yl)
methanone 7a. Rf 0.68 (CH2Cl2:acetone 95:5), Rf 0.43 (CH2Cl2); light

1274
V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
J 8.4, CH-40), 7.938 (dd, 1H, J 1.2, 8.4, CH-60), 7.943 (dd, 1H, J 0.9, 8.3,
CH-90), 8.146 (d, 1H, J 8.8, CH-3), 8.454 (ddd, 1H, J 0.5, 1.0, 8.3, CH-9),
11.511 (s, 1H, OH); 13C NMR 47.69 (CH2eN morpholine), 66.92
(CH2eO morpholine), 104.33 (Cquat-2), 119.59 (CH-4), 121.83 (CH-
90), 123.77 (CH-30), 124.13 (CH-9), 124.19 (CH-3), 124.75 (Cquat-10),
125.62 (Cquat-100), 126.25 (CH-8), 127.07 (Cquat-20), 127.10 (CH-40),
127.69 (CH-6), 127.72 (CH-70 and CH-80), 128.18 (CH-60), 130.24 (CH-
120.71, 6.865/129.33 (weak), 6.865/150.83, 7.223/108.81, 7.223/
116.70, 7.223/120.71 (weak), 7.223/125.55, 7.223/129.33, 7.223/
135.66, 7.223/150.83, 7.223/158.55, 7.223/172.38, 7.441/125.55,
7.441/127.37, 7.499/123.66, 7.499/135.66, 7.693/117.80, 7.693/
125.55, 7.693/125.85, 7.693/135.66 (weak), 8.316/128.69, 8.316/
135.66, 8.316/158.55, 11.170/108.81, 11.170/125.55, 11.170/158.55;
m/z (EI) 332 (75, Mþ), 213 (97), 145 (31), 132 (100%); HRMS (EI): Mþ,
found 332.1522. C21H20N2O2 requires 332.1525.
7), 134.77 (Cquat-50), 137.81 (Cquat-5), 142.87 (Cquat-10), 162.28 (Cquat
-
1), 166.66 (Cquat-CO0), 169.48 (CquateCO); COSY cross peaks 3.448/
3.620, 7.450/8.146, 7.458/7.883, 7.585/7.688, 7.585/7.938, 7.585/
7.943, 7.585/8.454, 7.688/7.849; NOESY cross peaks 3.448/3.620,
3.448/7.458, 3.620/7.458 (weak), 3.620/11.511 (weak), 7.450/7.849,
7.450/8.146, 7.458/7.883, 7.585/7.688, 7.585/7.938, 7.585/7.943,
7.585/8.454, 7.688/7.849, 7.883/7.938, 8.454/11.511; HSQC cross
peaks 3.448/47.69, 3.620/66.92, 7.450/119.59, 7.458/123.77, 7.585/
126.25, 7.585/127.72, 7.688/130.24, 7.849/127.69, 7.883/127.10,
7.938/128.18, 7.943/121.83, 8.146/124.19, 8.454/124.13; HMBC cross
peaks 3.448/66.92, 3.448/166.66, 3.620/47.69, 7.450/104.33, 7.450/
124.75, 7.450/127.69, 7.458/134.77, 7.458/142.87, 7.458/166.66,
7.585/121.83, 7.585/124.75, 7.585/125.62, 7.585/127.69, 7.585/
128.18, 7.585/134.77, 7.688/124.13, 7.688/137.81, 7.849/119.59,
7.849/124.75, 7.849/126.25, 7.883/125.62, 7.883/127.07, 7.883/
128.18, 7.883/134.77 (weak), 7.883/142.87 (weak), 7.938/125.62,
7.938/127.10, 7.938/127.72, 7.943/127.72, 7.943/134.77, 7.943/
142.87, 8.146/137.81, 8.146/162.28, 8.146/169.48, 8.454/130.24,
8.454/137.81, 8.454/162.28, 11.511/104.33, 11.511/124.75, 11.511/
162.28; m/z (EI) 427 (3, Mþ), 257 (51), 213 (4), 170 (100), 143 (12),
114 (37%); HRMS (EI): Mþ, found 427.1414. C26H21NO5 requires
427.1420.
4.1.4. Diazo coupling
To aniline (4 mmol) ice, conc. HCl (1.2 ml), and a solution of
NaNO2 (4.4 mmol) in H2O (10 ml) were subsequently added and the
mixture was stirred at 0 ꢂC for 5 min to form a diazonium salt. To
a solution of crude amide 6 or ketone 9 (4 mmol) in acetone (25 ml)
10% aq. NaOH (4 ml) and then the solution of the diazonium salt
were subsequently added at 0 ꢂC. The products were partitioned
between water and CH2Cl2. The organic phase was washed with
water, dried over Na2SO4, and purified by HPFC on silica gel.
4.1.4.1. (1-Hydroxy-4-(phenyldiazenyl)naphthalen-2-yl)(piperidin-1-
yl)methanone 3a. Starting from crude 6a; 52% overall yield; Rf
0.53 (CH2Cl2:acetone 95:5); red crystals, m. p. 180.5e181 ꢂC; 1H
NMR 1.726 (bs, 6H, CH2-3, CH2-4 and CH2-5 piperidine), 3.731
(bs, 4H, CH2-2 and CH2-6 piperidine), 7.455 (t, 1H, J 7.3, CH-4 Ph),
7.537 (t, 2H, J 7.4, CH-3 and CH-5 Ph), 7.600 (ddd, 1H, J 1.1, 6.9, 8.2,
CH-8 Ar), 7.729 (ddd, 1H, J 1.2, 6.8, 8.2, CH-7 Ar), 7.937 (s, 1H, CH-3
Ar), 7.974 (d, 2H, J 7.5, CH-2 and CH-6 Ph), 8.432 (d, 1H, J 8.3, CH-9
Ar), 8.889 (d, 1H, J 8.4, CH-6 Ar), 11.789 (bs, 1H, OH); 13C NMR
24.55 (CH2-4 piperidine), 26.08 (CH2-3 and CH2-5 piperidine),
47.23 (CH2-2 and CH2-6 piperidine), 112.14 (CH-3 Ar), 116.15
(Cquat-2 Ar), 122.59 (CH-2 and CH-6 Ph), 123.01 (CH-6 Ar), 123.78
(CH-9 Ar), 125.76 (Cquat-10 Ar), 126.22 (CH-8 Ar), 129.11 (CH-3 and
CH-5 Ph), 129.51 (CH-7 Ar), 130.28 (CH-4 Ph), 133.99 (Cquat-5 Ar),
138.84 (Cquat-4 Ar), 152.78 (Cquat-1 Ph), 162.10 (Cquat-1 Ar), 171.58
(C]O); COSY cross peaks 1.726/3.731, 7.455/7.537, 7.537/7.974,
7.600/7.729, 7.600/8.432, 7.729/8.889; NOESY cross peaks 1.726/
3.731, 1.726/7.937, 3.731/7.937, 7.455/7.537, 7.537/7.974, 7.600/
7.729, 7.600/8.432, 7.729/8.889; HSQC cross peaks 1.726/24.55,
1.726/26.08, 3.731/47.23, 7.455/130.28, 7.537/129.11, 7.600/126.22,
7.729/129.51, 7.937/112.14, 7.974/122.59, 8.432/123.78, 8.889/
123.01; HMBC cross peaks 1.726/24.55, 1.726/47.23 (weak), 3.731/
24.55, 3.731/26.08, 3.731/47.23 (weak), 3.731/171.58, 7.455/
122.59, 7.537/122.59 (weak), 7.537/129.11, 7.537/152.78, 7.600/
123.01, 7.600/125.76, 7.729/123.78, 7.729/133.99, 7.937/133.99,
7.937/138.84 (weak), 7.937/162.10 (weak), 7.937/171.58, 7.974/
122.59, 7.974/130.28, 7.974/152.78 (weak), 8.432/129.51, 8.432/
133.99, 8.889/125.76, 8.889/138.84 (weak); 1H NMR (CD3COCD3)
1.705 (bm, 4H, CH2-3 and CH2-5 piperidine), 1.752 (bm, 2H, CH2-4
piperidine), 3.719 (bs, 4H, CH2-2 and CH2-6 piperidine), 7.477 (bs,
1H, CH-4 Ph), 7.588 (t, 2H, J 7.6, CH-3 and CH-5 Ph), 7.669 (t, 1H,
J 7.6, CH-8 Ar), 7.811 (ddd, 1H, J 1.2, 6.9, 8.2, CH-7 Ar), 7.967
(bs, 2H, CH-2 and CH-6 Ph), 8.028 (s, 1H, CH-3 Ar), 8.374 (d, 1H,
J 7.9, CH-9 Ar), 8.912 (bs, 1H, CH-6 Ar); m/z (EI) 359 (100, Mþ), 289
(5), 274 (77), 254 (36), 246 (28), 218 (8), 197 (26), 169 (82), 141
(29), 113 (27), 105 (12%); HRMS (EI): Mþ, found 359.1628.
C22H21N3O2 requires 359.1634.
4.1.3.3. Method 3. To a solution of 1-hydroxy-2-naphthoic acid 5
(5 mmol) in DMF (0.1 ml) and heptane (20 ml) SOCl2 (12.5 mmol)
was added and the solution was refluxed for 4 h. The excess of SOCl2
and solvents were evaporated to dryness. To the crude acyl chloride
in CH2Cl2 (20 ml) EtN(i-Pr)2 (15 mmol) and amine (6 mmol) was
added at 0 ꢂC and the mixture was stirred at room temperature for
4e5 h. The products were partitioned between CH2Cl2 and aq.
Na2CO3. The organic phase was washed with brine, 2N HCl, and
again with brine, was dried over Na2SO4, and evaporated to dryness
to give the crude products which were directly used in the next
step. Small portion of 6c was purified by HPFC on silica gel for
characterization.
4.1.3.3.1. (1-Hydroxynaphthalen-2-yl)(4-phenylpiperazine-1-yl)
methanone 6c. Rf 0.69 (CH2Cl2:acetone 95:5), Rf 0.22 (CH2Cl2); m. p.
146e147 ꢂC; 1H NMR 3.179 (t, 4H, J 5.1, CH2-3 and CH2-5 pipera-
zine), 3.853 (t, 4H, J 5.1, CH2-2 and CH2-6 piperazine), 6.844 (tt, 1H,
J 0.9, 7.3, CH-4 NePh), 6.865 (dd, 2H, J 0.9, 8.9, CH-2 and CH-6
NePh), 7.223 (m, 4H, CH-3 Ar, CH-4 Ar, and CH-3 and CH-5 NePh),
7.441 (ddd, 1H, J 1.2, 6.8, 8.2, CH-8 Ar), 7.499 (ddd, 1H, J 1.3, 6.8, 8.2,
CH-7 Ar), 7.693 (d, 1H, J 8.2, CH-6 Ar), 8.316 (dt, 1H, J 0.6, 8.3, CH-9
Ar),11.170 (s,1H, OH); 13C NMR 45.99 (CH2-2 and CH2-6 piperazine),
49.74 (CH2-3 and CH2-5 piperazine), 108.81 (Cquat-2 Ar), 116.70
(CH-2 and CH-6 NePh), 117.80 (CH-4 Ar), 120.71 (CH-4 NePh),
123.66 (CH-9 Ar),124.26 (CH-3 Ar),125.55 (Cquat-10 Ar),125.85 (CH-
8 Ar), 127.37 (CH-6 Ar), 128.69 (CH-7 Ar), 129.33 (CH-3 and CH-5
NePh), 135.66 (Cquat-5 Ar), 150.83 (Cquat-1 NePh), 158.55 (Cquat-1
Ar), 172.38 (C]O); COSY cross peaks 3.179/3.853, 6.844/7.223,
6.865/7.223, 7.441/7.499, 7.441/8.316, 7.499/7.693; NOESY cross
peaks 3.179/3.853, 3.179/7.223 (weak), 3.853/7.223, 6.844/7.223,
6.865/7.223, 7.223/7.693, 7.441/7.499, 7.441/8.316, 7.499/7.693,
8.316/11.170 (weak); HSQC cross peaks 3.179/49.74, 3.853/45.99,
6.844/120.71, 6.865/116.70, 7.223/117.80, 7.223/124.26, 7.223/
129.33, 7.441/125.85, 7.499/128.69, 7.693/127.37, 8.316/123.66;
HMBC cross peaks 3.179/45.99, 3.179/49.74, 3.179/150.83, 3.853/
45.99, 3.853/49.74, 3.853/172.38, 6.844/116.70, 6.865/116.70, 6.865/
4.1.4.2. (1-Hydroxy-4-(phenyldiazenyl)naphthalen-2-yl)(morpholin-
1-yl)methanone 3b. Starting from crude 6b; 44% overall yield; Rf
0.28 (CH2Cl2:acetone 95:5); red crystals, m. p. 179e180 ꢂC; 1H NMR
3.712 (m, 4H, CH2-2 and CH2-6 morpholine), 3.769 (m, 4H, CH2-3
and CH2-5 morpholine), 7.395 (t, 1H, J 7.3, CH-4 Ph), 7.469 (dd, 2H,
J 7.3, 7.8, CH-3 and CH-5 Ph), 7.540 (ddd, 1H, J 1.1, 6.8, 8.1, CH-8 Ar),
7.668 (ddd, 1H, J 1.2, 6.9, 8.2, CH-7 Ar), 7.784 (s, 1H, CH-3 Ar), 7.897
(d, 2H, J 7.6, CH-2 and CH-6 Ph), 8.363 (d, 1H, J 8.3, CH-9 Ar), 8.812

V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
1275
(d, 1H, J 8.2, CH-6 Ar), 11.648 (bs, 1H, OH); 13C NMR 45.52 (CH2-3
and CH2-5 morpholine), 65.87 (CH2-2 and CH2-6 morpholine),
110.23 (CH-3 Ar), 113.01 (Cquat-2 Ar), 121.68 (CH-2 and CH-6 Ph),
122.08 (CH-6 Ar), 122.80 (CH-9 Ar), 124.83 (Cquat-10 Ar), 125.44
(CH-8 Ar), 128.15 (CH-3 and CH-5 Ph), 128.76 (CH-7 Ar), 129.49
(CH-4 Ph), 132.86 (Cquat-5 Ar), 138.28 (Cquat-4 Ar), 151.69 (Cquat-1
Ph), 160.89 (Cquat-1 Ar), 170.73 (C]O); COSY cross peaks 3.712/
3.769, 7.395/7.469, 7.469/7.897, 7.540/7.668, 7.540/8.363, 7.668/
8.812; NOESY cross peaks 3.712/3.769, 3.769/7.784, 7.395/7.469,
7.469/7.897, 7.540/7.668, 7.540/8.363, 7.668/8.812; HSQC cross
peaks 3.712/65.87, 3.769/45.52, 7.395/129.49, 7.469/128.15, 7.540/
125.44, 7.668/128.76, 7.784/110.23, 7.897/121.68, 8.363/122.80,
8.812/122.08; HMBC cross peaks 3.712/65.87, 3.769/45.52 (weak),
3.769/170.73, 7.395/121.68, 7.469/121.68 (weak), 7.469/128.15,
7.469/151.69, 7.540/122.08, 7.540/124.83, 7.668/122.80, 7.668/
132.86, 7.784/122.80 (weak), 7.784/132.86, 7.784/138.28, 7.784/
160.89 (weak),7.784/170.73, 7.897/121.68, 7.897/129.49, 8.363/
128.76, 8.363/132.86, 8.812/125.44; 1H NMR (CD3COCD3) 3.749
(bm, 4H, CH2-2 and CH2-6 morpholine), 3.773 (bm, 4H, CH2-3 and
CH2-5 morpholine), 7.437 (bs, 1H, CH-4 Ph), 7.583 (t, 2H, J 7.6, CH-3
and CH-5 Ph), 7.672 (t, 1H, J 7.6, CH-8 Ar), 7.815 (ddd, 1H, J 1.2, 6.9,
8.2, CH-7 Ar), 7.959 (bs, 2H, CH-2 and CH-6 Ph), 8.027 (s, 1H, CH-3
Ar), 8.370 (d, 1H, J 7.9, CH-9 Ar), 8.899 (bs, 1H, CH-6 Ar); m/z (EI) 361
(92, Mþ), 274 (100), 256 (15), 246 (30), 218 (10), 197 (32), 169 (92),
141 (42), 113 (45), 105 (24%); HRMS (EI): Mþ, found 361.1435.
C21H19N3O3 requires 361.1426.
CH-4 Ph), 7.598 (t, 2H, J 7.4, CH-3 and CH-5 Ph), 7.696 (t,1H, J 7.4, CH-8
Ar), 7.841 (ddd,1H, J 1.0, 7.3, 8.3, CH-7 Ar), 7.984 (bs, 2H, CH-2 and CH-
6 Ph), 8.103 (s,1H, CH-3 Ar), 8.401 (d,1H, J 7.7, CH-9 Ar), 8.932 (bs,1H,
CH-6 Ar); m/z (EI) 436 (100, Mþ), 331 (10), 274 (39), 246 (15), 218 (9),
197 (14),169 (44),141 (28),113 (31),105 (29%); HRMS (EI): Mþ, found
436.1902. C27H24N4O2 requires 436.1899.
4.1.4.4. 4-Phenylazo-2-acetyl-1-naphthol 4. Starting from 9; 82%
yield; Rf 0.36 (hexane:CH2Cl2 1:1); orangeered crystals; m. p.
145.5e146 ꢂC (lit. 136 ꢂC [28], 143 ꢂC [29]); 1H NMR 2.812 (s, 3H,
CH3), 7.535 (tt, 1H, J 1.9, 7.3, CH-40 Ph), 7.598 (ddt, 2H, J 1.3, 1.7, 7.3,
7.8, CH-30 and CH-50 Ph), 7.666 (ddd, 1H, J 1.2, 6.9, 8.1, CH-8 Ar),
7.825 (ddd, 1H, J 1.4, 6.9, 8.2, CH-7 Ar), 8.050 (ddd, 2H, J 1.3, 1.7,
7.8, CH-20 and CH-60 Ph), 8.153 (s, 1H, CH-3 Ar), 8.548 (d, 1H, J 8.2,
CH-9 Ar), 8.922 (d, 1H, J 8.4, CH-6 Ar), 14.287 (s, 1H, OH);13C NMR
27.11 (CH3), 111.37 (CH-3 Ar), 112.59 (Cquat-2), 123.01 (CH-20 and
CH-60 Ph), 123.32 (CH-6 Ar), 124.54 (CH-9 Ar), 125.46 (Cquat-10),
126.61 (CH-8 Ar), 129.20 (CH-30 and CH-50 Ph), 130.86 (CH-40 Ph),
131.00 (CH-7 Ar), 135.46 (Cquat-5), 139.78 (Cquat-4), 153.04 (Cquat-10
Ph), 164.72 (Cquat-1), 205.49 (C]O); COSY cross peaks 7. 535/
7.598, 7.598/8.050, 7.666/7.825, 7.666/8.548; NOESY cross peaks
2.812/8.153, 2.812/14.287 (weak), 7.535/7.598, 7.598/8.050, 7.666/
7.825, 7.666/8.548, 7.825/8.922, 8.548/14.287; HSQC cross peaks
2.812/27.11, 7. 535/130.86, 7.598/129.20, 7.666/126.61, 7.825/
131.00, 8.050/123.01, 8.153/111.37, 8.548/124.54, 8.922/123.32;
HMBC cross peaks 2.812/115.59, 2.812/205.49, 7.535/123.01,
7.598/129.20, 7.598/153.04, 7.666/123.32, 7.666/125.46, 7.825/
124.54, 7.825/135.46, 8.050/123.01, 8.050/130.86, 8.050/153.04
(weak), 8.153/135.46, 8.153/139.78, 8.153/164.72, 8.153/205.49,
8.548/131.00, 8.548/135.46, 8.548/164.72, 8.922/125.46, 8.922/
139.78, 14.287/112.59, 14.287/125.46, 14.287/164.72; 1H NMR
(CD3COCD3) 2.829 (s, 3H, CH3), 7.607 (tt, 1H, J 1.9, 7.2, CH-40 Ph),
7.665 (ddt, 2H, J 1.2, 1.7, 7.3, 7.9, CH-30 and CH-50 Ph), 7.743 (ddd,
1H, J 0.9, 6.9, 8.0, CH-8 Ar), 7.906 (ddd, 1H, J 1.2, 6.9, 8.2, CH-7 Ar),
8.072 (ddd, 2H, J 1.0, 1.9, 7.4, CH-20 and CH-60 Ph), 8.253 (s, 1H,
CH-3 Ar), 8.496 (d, 1H, J 8.4, CH-9 Ar), 8.954 (d, 1H, J 8.4, CH-6 Ar),
14.423 (s, 1H, OH).
4.1.4.3. (1-Hydroxy-4-(phenyldiazenyl)naphthalen-2-yl)(4-phenylpi
perazine-1-yl)methanone 3c. Starting from crude 6c; 47% overall
yield; Rf 0.48 (CH2Cl2:acetone 95:5); red crystals, m. p.183e183.5 ꢂC;
1H NMR 3.308 (t, 4H, J 5.1, CH2-3 and CH2-5 piperazine), 3.993 (t, 4H,
J 5.1, CH2-2 and CH2-6 piperazine), 6.928 (tt, 1H, J 0.9, 7.3, CH-4
NePh), 6.955 (dd, 2H, J 0.9, 8.7, CH-2 and CH-6 NePh), 7.304 (dd, 2H,
J 7.3, 8.7, CH-3 and CH-5 NePh), 7.492 (tt,1H, J 1.3, 7.3, CH-4 Ph), 7.556
(dd, 2H, J 7.3, 8.5, CH-3 and CH-5 Ph), 7.636 (ddd, 1H, J 1.0, 6.8, 8.1,
CH-8 Ar), 7.765 (ddd,1H, J 1.2, 6.8, 8.2, CH-7 Ar), 7.919 (s,1H, CH-3 Ar),
8.005 (dd, 2H, J 1.3, 8.6, CH-2 and CH-6 Ph), 8.471 (d, 1H, J 8.2, CH-9
Ar), 8.941 (d, 1H, J 8.4, CH-6 Ar), 11.825 (s, 1H, OH); 13C NMR 46.01
(CH2-2 and CH2-6 piperazine), 49.61 (CH2-3 and CH2-5 piperazine),
108.41 (Cquat-2 Ar), 110.93 (CH-3 Ar), 116.66 (CH-2 and CH-6 NePh),
120.70 (CH-4 NePh), 123.01 (CH-2 and CH-6 Ph), 123.15 (CH-6 Ar),
123.77 (CH-9 Ar),125.70 (Cquat-10 Ar),126.44 (CH-8 Ar),129.19 (CH-3
and CH-5 Ph), 129.35 (CH-3 and CH-5 NePh), 129.72 (CH-7 Ar),
130.78 (CH-4 Ph), 133.86 (Cquat-5 Ar), 139.30 (Cquat-4 Ar), 150.75
(Cquat-1 NePh),153.04 (Cquat-1 Ph),161.55 (Cquat-1 Ar),172.03 (C]O);
COSY cross peaks 3.308/3.993, 6.928/7.304, 6.955/7.304, 7.492/7.556,
7.556/8.005, 7.636/7.765, 7.636/8.471, 7.765/8.941; NOESY cross
peaks 3.308/3.993, 3.308/6.955, 3.308/7.919 (weak), 3.993/7.919,
6.928/7.304, 6.955/7.304, 7.492/7.556, 7.556/8.005, 7.636/7.765,
7.636/8.471, 7.765/8.941, 8.471/11.825 (weak); HSQC cross peaks
3.308/49.61, 3.993/46.01, 6.928/120.70, 6.955/116.66, 7.304/129.35,
7.492/130.78, 7.556/129.19, 7.636/126.44, 7.765/129.72, 7.919/110.93,
8.005/123.01, 8.471/123.77, 8.941/123.15; HMBC cross peaks 3.308/
49.61, 3.308/172.03 (weak), 3.993/49.61, 3.993/172.03, 6.928/116.66,
6.928/129.35 (weak), 6.955/116.66, 6.955/120.70, 6.955/150.75
(weak), 7.304/116.66 (weak), 7.304/129.35, 7.304/150.75, 7.492/
123.01, 7.556/123.01 (weak), 7.556/129.19, 7.556/153.04, 7.636/
123.15, 7.636/125.70, 7.765/123.77, 7.765/133.86, 7.919/133.86, 7.919/
139.30, 7.919/161.55, 7.919/172.03, 8.005/123.01, 8.005/129.19,
8.005/153.04 (weak), 8.471/129.72, 8.471/133.86, 8.471/161.55,
8.941/125.70, 8.941/126.44, 8.941/139.30, 11.825/108.41, 11.825/
125.70, 11.825/161.55; 1H NMR (CD3COCD3) 3.351 (bt, 4H, CH2-3 and
CH2-5 piperazine), 3.953 (bs, 4H, CH2-2 and CH2-6 piperazine), 6.862
(t, 1H, J 7.3, CH-4 NePh), 7.037 (dd, 2H, J 0.9, 8.4, CH-2 and CH-6
NePh), 7.273 (dd, 2H, J 7.4, 8.4, CH-3 and CH-5 NePh), 7.485 (bs, 1H,
4.2. Quantum chemical calculations
Ab initio HF/6e31G** calculations were performed by using the
Gaussian 09 program suite [30]. The effect of the solvent was
described using the Polarizable Continuum Model (PCM) model
[31]. The individual tautomeric forms and the keto dimers in gas
phase were optimized without restrictions and then were charac-
terized as true minima by vibrational frequency calculations. In
acetone environment the dimereacetone complex was optimized
taking the X-ray structure as initial approximation.
The use of the HF method has been justified by a comparative
study of various levels of theory in the description of the tautomeric
equilibrium in azonaphthols [32,33].
4.3. UVeVis spectroscopy
The absorption spectra were measured on Jasco V-570 UVeVise
NIR spectrophotometer equipped with Julabo ED thermostat at
constant temperature of 20 ꢂC. Spectral grade solvents were used
(Scharlau Multisolvent). Typical concentration range solutions of
10ꢀ5 mol/l were measured.
4.4. Crystallography
4.4.1. (1-Hydroxy-4-(phenyldiazenyl)naphthalen-2-yl)(morpholin-
1-yl)methanone 3b
The crystal of 3b was mounted of on a loop and all geometric
and intensity data were taken from this crystal. Data collection

1276
V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
using Mo-Ka radiation (
l
¼ 0.71073 Å) was performed at 150 K on
Acknowledgments
a STOE IPDS-II diffractometer equipped with an Oxford Cryosystem
open flow cryostat [34]. Absorption correction was partially inte-
grated in the data reduction procedure [35]. The structure was
solved by SIR 2004 and refined using full-matrix least-squares on F2
with the SHELX-97 package [36,37].
The generous financial support by The Bulgarian Science Fund,
projects TK-X-1716 (synthesis and spectral studies), UNA-17/2005
(NMR spectrometer), RNF01/0110 (quantum-chemical calcula-
tions) and DRNF-02/1 (crystallography), is gratefully acknowl-
edged. We also thank Dr. Nikolay Vassilev from Institute of Organic
Chemistry of Bulgarian Academy of Sciences for carrying out the
low temperature NMR experiments.
Crystal data of: monoclinic, C2/c (No. 15), a ¼ 26.521(3) Å,
b ¼ 8.1089(6) Å, c ¼ 19.766(2) Å,
b
¼ 111.006(8)ꢂ. V ¼ 3968.3(7) Å3,
Z ¼ 4, d ¼ 1.307 Mg mꢀ3, 5519 reflections collected, 2981 unique,
R1 ¼ 0.0857 (I > 2 s (I)), wR2 ¼ 0.2056 (all data).
Compound 3b crystallizes in the monoclinic space group C2/c
(Nꢂ 15) with two molecules of 3b and one acetone molecule per
asymmetric unit (Fig. 5). The backbone of the compound consists in
a phenyl ring attached to a functionalized naphthalene by a diazenyl
function with C1eN1, C7eN2, N1eN2 bond lengths of respectively
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V.B. Kurteva et al. / Dyes and Pigments 92 (2012) 1266e1277
1277
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