Synthesis, spectral and electrochemical characterization of novel 2-(fluoroanilino)-1,4-naphthoquinones

Article abstract of DOI:10.1016/j.jfluchem.2010.12.001

The preparation of novel 2-(fluoroanilino)-1,4-naphthoquinones is presented. It takes place under mild conditions by reacting the corresponding fluoroaniline and 1,4-naphthoquinone in the presence of a Lewis acid catalyst with strong oxidation properties

Full text of DOI:10.1016/j.jfluchem.2010.12.001

Journal of Fluorine Chemistry 132 (2011) 94–101
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis, spectral and electrochemical characterization of novel
2-(fluoroanilino)-1,4-naphthoquinones
b
a
a
Elisa Leyva a, , Lluvia I. Lopez , Silvia E. Loredo-Carrillo , Margarita Rodrıguez-Kessler ,
*
´
´
Antonio Montes-Rojas a
a Facultad de Ciencias Quı´micas, Universidad Auto´noma de San Luis Potosı´, Av. Manuel Nava No. 6, Zona Universitaria, C.P. 78210, San Luis Potosı´, S.L.P., Mexico
b
´
´
´
´
´
Facultad de Ciencias Quımicas, Universidad Autonoma de Coahuila, Blvd. V, Carranza e Ing. Jose Cardenas s/n, Col. Republica, C.P. 25250, Saltillo, Coahuila, Mexico
A R T I C L E I N F O
A B S T R A C T
Article history:
The preparation of novel 2-(fluoroanilino)-1,4-naphthoquinones is presented. It takes place under mild
conditions by reacting the corresponding fluoroaniline and 1,4-naphthoquinone in the presence of a
Lewis acid catalyst with strong oxidation properties such as CeCl3ꢂ7H2O. This preparation was also
investigated under microwave irradiation. All 1,4-naphthoquinone derivatives were characterized by
UV–Vis, IR, 1H and 19F NMR, MS and cyclic voltammetry, to investigate the effect of the fluoro-
substituents on their electronic properties.
Received 26 August 2010
Received in revised form 29 November 2010
Accepted 1 December 2010
Available online 8 December 2010
Keywords:
ß 2010 Elsevier B.V. All rights reserved.
2-(Fluoroanilino)-1,4-naphthoquinone
Electrochemical oxidation
Lewis acid catalyst
Microwave
CeCl3ꢂ7H2O
1. Introduction
protic and aprotic solvents [11–13]. Other researchers have
centered their studies on the influence of structural parameters
A large number of synthetic and natural organic compounds
containing a quinone moiety in their structure have been associated
with different biological activities [1]. Many quinones have been
used as antituberculosis agents [2], antimalarial agents [3],
antimicrobials [4], antitumor agents [5], and fungicides [6,7]. In
most cases, the biological activity of quinones has been related to
their redox properties and their capacity to accept one or two
electrons to form the corresponding radical-anion (Qꢀꢁ) and
hydroquinone radical dianion (Q2ꢁ) [8]. These intermediate species
interact with crucial cellular molecules such as oxygen, DNA and
proteins modifying their biological activity [8–10]. Therefore, the
studies on mechanisms of biological action, as well as the design and
synthesis of novel compounds with more selective biological
activities, require the understanding of the factors that modify
the physicochemical properties of quinone systems.
on the redox properties of quinones [14–20].
It has been previously demonstrated that the presence of
intramolecular hydrogen bonds (O. . .H–N) is an important factor
in the acceptance of the first electron in the electrochemical
reduction of the quinone system [21]. The electrochemical studies
were performed in acetonitrile since quinones and their corre-
sponding anions are not strongly solvated and intramolecular
hydrogenbonds becomemoreimportant inthissolvent[22]. Several
investigations have suggested that aprotic solvents indeed mimic
cellular nonpolar environment where electron transfer takes place
[23].
The electron-accepting capacity and several physicochemical
properties of a given quinone can be modified by directly adding a
substituted aniline to the quinone system [18]. In this type of
compounds, the electron-attracting and donor properties of the
substituents on the aniline modify their redox properties either
facilitating or interfering with the charge transfer from the
substituent to the quinone [19]. Since fluorine atoms are known
to form hydrogen bonds and present both, electron-attracting and
donor properties, the presence of one or more fluorine atoms on
aniline, attached to the quinone structure, must have some
interesting effects on the physicochemical properties of 1,4-
naphthoquinone.
The biological importance of this kind of compounds has
motivated an extensive research to evaluate the electrochemical
behavior of a given quinone and hydroquinone system [11–20].
Many researchers studied the redox kinetics and mechanisms in
´
* Corresponding author. Present address: Facultad de Ciencias Quımicas,
Universidad Auto´ noma de San Luis Potosı´, Av. Nava No. 6, San Luis Potosı´
78290, S.L.P., Mexico. Tel.: +52 444 8262440x508; fax: +52 444 8262371.
E-mail address: elisa@uaslp.mx (E. Leyva).
There are some previous reports on the use of a Lewis acid to
catalyze the reaction of an aniline with 1,4-naphthoquinone to
0022-1139/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2010.12.001

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E. Leyva et al. / Journal of Fluorine C
]
hemistry 132 (2011) 94–101
95
Scheme 2. Reaction of 1,4-naphthoquinone with amines.
Scheme 1. Catalytic synthesis of 2-(fluoroanilino)-1,4-naphthoquinones.
chromatographic purifications since they give many side products,
especially with primary amines.
synthesize 2-(R-anilino)-1,4-naphthoquinones [19,24]. However,
even in these cases the yields varied from low to moderate and the
reactions required long times (days or weeks). In this paper, we
report the preparation of several 2-(fluoroanilino)-1,4-naphtho-
quinones, eight of which have not been previously described, by
It has been reported in the literature that when the reaction of
1,4-naphthoquinone 1 with aniline 2 is performed without a
catalyst low yields of the 2-(anilino)-1,4-naphthoquinone 3 are
obtained (0–50%) and several secondary products are formed [26].
Furthermore, performing this reaction under refluxing conditions
leads to even lower yields (0–30%). In fact, the variety of
compounds obtained in this reaction are due to two major effects:
(a) the presence of several electrophilic centers of comparable
reactivity, on the structure of 1,4-naphthoquinone 1, leading to the
formation of 1,4-addition adduct 3 and 1,2-addition adduct 4 as
shown in Scheme 2, (b) the redox properties of 1,4-naphthoqui-
none 1 favoring the formation of an inert 1,4-naphthodiol 5.
There are few reports on the use of Lewis acids to catalyze the
reaction of anilines with 1,4-naphthoquinone [19,24]. We recently
reported a bentonitic clay assisted preparation of 2-(chloroani-
lino)-1,4-naphthoquinones [28]. However, to the best of our
knowledge there are no reports on the optimal conditions to
prepare 2-(fluoroanilino)-1,4-naphthoquinones. The reaction of
1,4-naphthoquinone 1 with several fluoroanilines 2b–2j gave the
corresponding adducts 3b–3j in low to moderate yields (30–60%).
Only when the reaction (Table 2) was carried out in the presence of
CeCl3ꢂ7H2O, there was a high conversion of reactant species and
the naphthoquinone derivatives 3b–3j were obtained in very good
yields (70–91%). This latter reaction was performed under
microwave irradiation and moderate to good yields (55–90%) of
adducts 3b–3j were obtained in few minutes (1–3).
In the case of cerium, due to the low redox potential between
the two ions (Ce3+/Ce4+, 1.7 V) in solution, the strong oxidant Ce4+
predominates in an oxidative atmosphere [29]. Therefore, in this
particular reaction the high yield observed is most likely due to the
presence of a strong Lewis acid and a strong oxidizing agent. It is
envisioned that the Ce4+ assisted synthesis proceeds via a four
steps mechanism (Scheme 3). First, the quinone 1 is activated by
reaction with the Lewis acid catalyst to form an organic complex
6a. Second, and due to resonance electronic interactions, there is a
charge transfer in the activated complex with the 2-position of the
quinone ring becoming positively charged 6b. Third, the activated
reaction of
a fluoroaniline with 1,4-naphthoquinone in the
presence of a strong Lewis acid and a good oxidizing agent such
as CeCl3ꢂ7H2O. We also investigated this reaction under microwave
irradiation (Scheme 1).
All the compounds prepared (Table 1) were characterized by IR,
UV–Vis, 1H and 19F NMR, mass spectrometry and cyclic voltam-
metry. By means of these studies, the structure and effect of the
substituents on the electronic properties of 1,4-naphthoquinone
were investigated. Since the biological activity of quinone
compounds is related to the redox potentials of the quinone
moiety, a systematic investigation of substituent effects on the
redox properties of 2-(R-anilino)-1,4-naphthoquinone would be
useful for designing new molecules with greater and more specific
biological activity.
2. Results and discussion
2.1. Synthesis and reaction pathway
The synthesis of simple alkyl or arylamino derivatives of
naphthoquinone and related compounds is of interest due to their
broad biological activity [1]. Furthermore, the aminonaphthoqui-
none moiety is a component of the molecular framework of many
natural compounds and has been used as a key intermediate for the
synthesis of several compounds with important biological applica-
tions [25].
There are two general methods for their preparation. In the first
one, an already halogenated naphthoquinone molecule is reacted
with an amine to produce the corresponding derivative by
nucleophilic displacement [26]. In the second one, an amine is
added, in a 1,4-type manner, to the naphthoquinone structure [27].
However, both methods are unpractical and require tedious
Table 1
Table 2
Structure of 2-(fluoroanilino)-1,4-naphthoquinones synthesized.
Effect of reaction conditions in the yield of 2-(fluoroanilino)-1,4-naphthoquinones.
Compound
R1
R2
R3
R4
Compound
Yield (%)a
Yield (%)b
Yield (%)c
PAN 3aa,c
H
F
H
H
F
H
H
H
H
H
H
H
F
PAN 3a
60
30
50
55
60
40
35
30
40
45
85
91
72
85
85
70
70
74
70
74
57
55
60
83
56d
55
73
90
78
55
2-FPAN 3bb
3-FPAN 3c
H
2-FPAN 3b
3-FPAN 3c
4-FPAN 3d
2,3-FPAN 3e
2,4-FPAN 3f
2,5-FPAN 3g
3,4-FPAN 3h
3,5-FPAN 3i
2,4,5-FPAN 3j
H
H
F
H
4-FPAN 3db
2,3-FPAN 3eb
2,4-FPAN 3fb
2,5-FPAN 3gb
3,4-FPAN 3hb
3,5-FPAN 3ib
2,4,5-FPAN 3jb
4-MeOPAN 3kc
H
F
F
H
F
H
H
F
F
F
H
H
H
F
F
H
F
F
H
H
H
F
F
H
CH3O
H
a
Without catalyst and 7 days.
a
b
c
It is also known as 2-[(phenyl) amine]-1,4-naphthalenediona or PAN.
These compounds have not been previously described.
These compounds were prepared for comparison.
With catalyst CeCl3ꢂ7H2O and 4 h.
b
c
With catalyst CeCl3ꢂ7H2O under microwave irradiation and 1–3min.
d
This reaction required microwave irradiation for 10 min.

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Table 3
Spectroscopic data for 2-(R-anilino)-1,4-naphthoquinones synthesized.
a
a
a
Compound
(
l1max
)
(l2max
)
(l4max)
p
–p*
p–
p*
CT + h–p*
PAN (3a)
207
208
208
207
209
208
208
208
209
211
203
200
271
268
272
270
268
266
268
269
272
265
273
245
466
447
458
462
448
443
441
456
451
438
480
329
2-FPAN (3b)
3-FPAN (3c)
4-FPAN (3d)
2,3-FPAN (3e)
2,4-FPAN (3f)
2,5-FPAN (3g)
3,4-FPAN (3h)
3,5-FPAN (3i)
2,4,5-FPAN (3j)
4-MeOPAN (3k)
NQ
a
Scheme 3. Reaction mechanism for the synthesis of 2-(fluoroanilino)-1,4-
The UV–Vis spectra were determined using CH3OH as solvent and
are in nanometers.
lmax values
naphthoquinones.
complex undergoes a selective nucleophilic attack, by the aniline 2
to give intermediate 7. Fourth, under ethanolic conditions, it easily
loses a proton and decomposes to produce a reduced ion Ce3+ and
an oxidized compound or 2-(R-anilino)-1,4-naphthoquinone 3. As
stated above (Table 2), this adduct is obtained in lower yields (30–
60%) when the reaction is performed without a catalyst due to the
competitive formation of several secondary products. Indeed, Ce4+
provides a unique catalytic effect, promoted by Lewis acid and
redox properties, with formation of the desired products in very
good yields (70–91%). In all the reactions with CeCl3ꢂ7H2O, no
secondary products were observed and some amounts of the
starting materials were recovered.
330 nm (
l
3). We also observed a broad and low energy band in the
4). This latter
visible region centered between 438 and 480 nm (
absorption is typical of amino substituted quinones and it has been
previously assigned to charge-transfer (CT) transitions and weak
l
h–p* electron transitions of the carbonyl group in the quinone
[31]. This absorption is strongly shifted to the visible region in PAN
and FPAN derivatives relative to 1,4-naphthoquinone (NQ). Indeed,
when a substituted aniline is added to the C2 position of 1,4-
naphthoquinone, bathochromic shifts of the absorption bands are
observed with changes in the intensity and width [19]. In this case,
the substitution of a fluoroaniline on NQ to give an FPAN
compound, produces bathochromic shifts (relative to NQ) due to
an increase in both, the
interactions between the
p
–
h
p
* electron transition probability and
electrons of the amine with the
2.2. Mass spectrometry characterization
p
system. This behavior has been previously discussed indicating
that substituents on the aniline have an important role modulating
the electronic properties of PAN compounds.
All the quinones obtained were characterized by mass spectrom-
etry and the molecular radical ion for each quinone was determined.
In addition, all the compounds showed analogous fragments
(Scheme 4) suggesting a common fragmentation pattern [30].
Indeed, electron-accepting and electron-donating substituents
move the
configuration, to longer wavelengths [32]. However we observed
(Table 3) that the two bands ( 1 and 2) frequently associated with
these absorptions are not strongly shifted by modification of the
substituent. In contrast, the low energy band ( 4) observed in the
p–p* absorptions, contributed from the HOMO-LUMO
2.3. UV–Vis spectroscopy studies
l
l
In general, PAN derivatives containing fluoro-substituents on
their structure were found to be less soluble than PAN in
acetonitrile and ethanol. PAN and FPAN derivatives gave orange
to dark red solutions in ethanol (Table 3). UV–Vis electronic
l
visible region centered between 438 and 480 nm shifted when the
substituent ismodified. Aswementioned earlier, thislatterbandhas
been assigned to CT transitions and weak
h–p* transitions [31]. Both
absorption spectra of 3a–3k (Fig. 1) showed the expected
electron transitions bands associated with benzene and naphtho-
quinone in the region 203–211 nm ( 1) and around 265–273 nm
2). Furthermore, a weak * transition band is observed at 310–
p–p*
types of transitions are expected to be modified by nonbonding
[)(TD$FIG]
l
(
[()TD$FIG]
l
h–p
Fig. 1. UV–Vis electronic absorption spectra of 2-anilino-1,4-naphthoquinone and
1,4-naphthoquinone in methanol.
Scheme 4. MS fragmentation pattern for 2-(2-fluoroanilino)-1,4-naphthoquinone.

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E. Leyva et al. / Journal of Fluorine Chemistry 132 (2011) 94–101
97
Several parameters were evaluated from the voltammetric
curves obtained over a potential range between ꢁ600 and
ꢁ2020 mV with a sweep rate of 100 mV/s. The two half-wave
potentials are associated with waves I and II, E1/2 = (Epa + Epc)/2,
where Epa corresponds to the anodic peak potential and Epc
corresponds to the cathodic peak potential. With the exception of
2,5-FPAN that presented a more complex cyclic voltammogram,
the electrochemical behavior of all FPAN derivatives studied was
quite similar to PAN indicating that all quinones oxidize by means
of the same mechanism [14]. The cathodic peak current (ipc) versus
the square root of the sweep rate (v
1=2), produced a linear
relationship with zero intercept for both peaks indicating no
complications [34]. In addition to this, there was consistency of the
voltammetric function with the sweep rate. In the voltammetric
studies, the concentration of a given FPAN compound varied due to
differences in solubility. Voltammetric function values were
Fig. 2. Voltammetric response of PAN obtained at 100 mV/s in Et4NBF4 0.1 M/AcN.
standardized for concentration and reported as ipcvꢁ1=2
function, ipc represents the cathodic peak current at the first or
second reduction peak, is the scan rate and c is the concentration
of compound in mol/cm3.
c
ꢁ1. In this
electrons (
h
electrons) present in the nitrogen atom of aniline. When
v
these electrons are more available for interactions with the
h
p
system of the quinone, larger bathochromic shifts are observed.
Some substituent effect on the position of this band is observed in
Table 3. A strong bathochromic shift (relative to PAN) was observed
for p-methoxyaniline due to an electron-donating effect of the
In most of the cases, the values of the current function, ipc, for
both waves are approximately the same. This is generally
associated with a one electron transfer for each wave and quite
similar diffusion coefficients [35]. Indeed this result is expected,
since most of the FPAN studied present a quite similar structure. In
addition to this, the reversibility of the process is indicated by the
relationship ipa/ipc and it has a value close to one for most cases
substituentthatmakesthehelectronsmoreavailable. Incontrast,all
FPAN derivatives exhibited hypsochromic shifts (relative to PAN) to
shorter wavelenghts due to an electron-accepting effect of the
substituents. This shift increased with the number of fluorine atoms
in the aniline and the largest shift was observed for 2,4,5-FPAN.
Particularly larger hypsochromic shifts were observed in FPAN
compounds with an ortho-fluoro-substituted aniline.
studied [19]. The value
D
Ep = (Epa ꢁ Epc) approaches the theoretical
value reported for one-electron reversible system. In terms of the
similarities in the voltammetric measurements (Table 4), we can
presume that all the quinones studied are reduced by the same
mechanism [14]. Therefore, one would expect to find consistency
in several physicochemical properties such as substituent elec-
tronic effects and spectroscopic parameters.
2.4. Electrochemical studies
Redox potentials of all FPAN evaluated in this study were
measured by cyclic voltammetry at 25 8C using a Pt disk electrode
and 0.1 M tetraethylammonium tetrafluoroborate in acetonitrile
as the electrolytic medium [33]. The typical electrochemical
behavior of PAN and FPANs was investigated and presented two
reversible waves (Fig. 2). In the first wave, a semiquinone radical
anion (Qꢀꢁ) is generated by the addition of an electron to the
quinone structure (Q). In the second wave, a dianion hydroquinone
Q2ꢁ is produced by the subsequent addition of an electron to the
semiquinone radical anion [11–13]. These oxidation–reduction
reactions could be represented by the following equations:
A particularly different voltammetric behavior was observed in
the case of 2,5-FPAN (Fig. 3). In this case, it was not possible to
determine the Epa and Epc values since it was observed an
intercrossing of current in the cathodic side of the voltammogram.
This behavior could be due to an autocatalytic reaction [36]. In fact,
there are several examples in the literature in which this type of
chemistry has been reported.
More negative E1/2 potentials were obtained for PAN and FPANs
than the one reported for naphthoquinone. Therefore, the
substitution of aniline and fluoroaniline increases electron density
on the quinone structure [21]. In the FPAN compounds the lone
pair on nitrogen (
a to the C1 = O1 carbonyl) is displaced towards
Q þ eꢁ ¼ Qꢂꢁ EI1=2
(1)
(2)
the enone structure. For a single fluoro-substituent, the E1/2
potentials, corresponding to the first wave and second wave,
presented the same trend. A moderate effect was observed to less
negative values, with the following order 2F > 3F > 4F. An increase
Qꢂꢁ þ eꢁ ¼ Q2ꢁ
E
II
1=2
Table 4
Electrochemical parametersa of 2-(fluoroanilino)-1,4-naphthoquinones.
Compound
ipavꢁ1=2cꢁ1
ipavꢁ1=2cꢁ1
Epa
Epa
ipavꢁ1=2cꢁ1
ipavꢁ1=2cꢁ1
Epc
Epc
ipa/ipc
ipa/ipc
D
Ep
DEp
Wave I
Wave II
Wave I
Wave II
Wave I
Wave II
Wave I
Wave II
Wave I
Wave II
Wave I
Wave II
PAN
369
335
823
254
187
471
359
545
553
841
329
491
ꢁ1133
ꢁ1094
ꢁ1102
ꢁ1091
ꢁ1124
ꢁ1102
ꢁ1051
ꢁ1587
ꢁ1555
ꢁ1551
ꢁ1642
ꢁ1604
ꢁ478
ꢁ503
ꢁ893
ꢁ368
ꢁ471
ꢁ520
ꢁ378
ꢁ324
ꢁ443
ꢁ556
ꢁ419
ꢁ453
ꢁ645
ꢁ368
ꢁ461
ꢁ1204
ꢁ1178
ꢁ1198
ꢁ1215
ꢁ1169
ꢁ1197
ꢁ1169
ꢁ1203
ꢁ1110
ꢁ1172
ꢁ1693
ꢁ1717
ꢁ1727
ꢁ1755
ꢁ1774
0.77
0.66
0.92
0.69
0.4
1.3
71
84
106
162
176
113
170
2-FPAN
1.2
3-FPAN
1.3
96
4-FPAN
0.89
1.06
124
45
2,3-FPAN
2,4-FPAN
2,5-FPAN
3,4-FPAN
3,5-FPAN
2,4,5-FPAN
500
b
ꢁ1568
ꢁ393
ꢁ1704
0.9
1.27
b
95
136
b
b
b
b
0.95
118
236
b
470
648
546
ꢁ1130
ꢁ1593
ꢁ1597
ꢁ1556
ꢁ429
ꢁ738
ꢁ431
ꢁ1723
ꢁ1691
ꢁ1667
0.73
b
1.09
0.88
1.27
73
b
130
94
b
422
ꢁ1072
0.76
100
111
a
Determined by cyclic voltammetry at 100 mV/s. The potentials are given with respect to the Fc/Fc+ redox couple.
b
Since the waves were not well defined, these values could not be accurately determined.

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I
Scheme 6. Resonance hybrids showing stabilization of the radical anion (Qꢂꢁ)
where the substituent F acts as an electron-acceptor and a hydrogen bond forming
atom.
Fig. 3. Voltammetric response of 2,5-FPAN obtained at 100 mV/s in Et4NBF4 0.1 M/
AcN.
than in para-position. This confirmed that the inductive effect of
the halogen in this meta-position is more pronounced than the
resonance effect. In previous studies on anilino-1,4-naphthoqui-
nones [19] it was found that the first reduction corresponds to the
in number of the fluoro-substituents also shows a quite moderate
effect to less negative values. These results are in agreement with
previous studies on PAN and its derivatives [19–21]. In fact, the
introduction of electron-donor groups displaced E1/2 potentials for
the first wave to more negative values than the one obtained for
PAN. In contrast, introduction of electron-accepting substituents
changed the E1/2 potentials to less negative region. Electron-
accepting substituents strongly unsaturated like cyano, acetyl and
nitro were more powerful electron-acceptors than halogens. In
general, electron-acceptor groups reduced the electron density of
the electroactive group and facilitated the reduction process.
However, in the case of fluoro-substituents this effect is rather
weak due to simultaneous electron-attracting and donor proper-
ties of the substituents (Scheme 5).
In the case of PAN, electron-donor groups favor displacement of
the free pair of electrons on nitrogen towards the quinone system,
while electron-accepting groups facilitate displacement of electron
density of nitrogen toward the aniline ring. A slightly larger
electron-attracting effect is observed when an ortho-fluoro-
substituent is present in the aniline. In this case, the intramolecular
hydrogen bond C55O. . .H. . .F (Scheme 6) favors the reduction by
enhancing the inductive effect of the substituent. Indeed, there are
previous reports in the literature that evidence the thermodynam-
ic stability in a three centered hydrogen bond present in similar
aromatic ortho-substituted amides [37].
reduction of the carbonyl (C155O)
a to the amino group and the
second reduction is associated with the other carbonyl (C455O).
Displacement of the electron pair in amine nitrogen mainly affects
the electron density on (C455O) carbonyl and a greater susceptibil-
ity of the second reduction to the electronic effect of the
substituents is observed (Table 4). In general, it has been observed
that increasing the amount of fluorine atoms in the structure favors
both reduction waves but a small effect is still observed.
3. Conclusions
We synthesized nine FPANs, eight of which have not been
previously reported. These compounds were prepared by reaction
of the corresponding fluoroaniline with 1,4-naphthoquinone in the
presence of a Lewis acid catalyst with strong oxidation properties
such as CeCl3ꢂ7H2O. This preparation was also achieved under
microwave irradiation in several minutes. UV–Vis spectra of FPANs
show the existence of an intramolecular electronic transfer from
the fluoro-substituent to the 1,4-naphthoquinone moiety. In
agreement with this donor-acceptor character, the cyclic voltam-
mograms of FPANs exhibit two one-electron reduction waves to
the corresponding radical-anion and dianion. In general, fluoro-
substituents reduced the electron density of the electroactive
group and facilitated the reduction process. However, this effect is
rather weak due to simultaneous electron-attracting and donor
properties of the substituents. Furthermore, a slightly larger effect
is observed when an ortho-fluoro-substituent is present in the
aniline due to an intramolecular three centered hydrogen bond. A
different voltammetric behavior was observed for 2,5-FPAN that
presented an intercrossing of current in the cathodic side of the
voltammogram indicating an autocatalytic reaction.
Analyzing substituent effects, we observe that the anodic shift
(Table 4) was greater with a fluoro-substituent in meta-position
[()TD$FIG]
4. Experimental
4.1. General methods
Melting points were measured with a Fisher Johns apparatus.
UV–Vis spectra were obtained on a Shimadzu UV-2401 PC
spectrophotometer. IR spectra were recorded on a Nicolet Nexus
470 FTIR spectrophotometer. NMR spectra were obtained on a
Varian, Mercury 400 MHz nuclear magnetic resonance spectrom-
eter. 1H NMR spectra were recorded in ppm from tetramethylsi-
lane and 19F NMR spectra were recorded in ppm from
trifluoroacetic acid. Mass spectra were recorded on an Finnigan
MAT 8200 spectrometer at 70 eV. All the starting compounds were
Scheme 5. Resonance hybrids showing stabilization of the radical anion (Qꢂꢁ) when
R is an electron-donor (ED) or an electron-acceptor (EA).

E. Leyva et al. / Journal of Fluorine Chemistry 132 (2011) 94–101
99
purchased from Aldrich. Reactions under microwave irradiation
were performed in a CEM microwave reactor, Discover System No.
DU8756.
naphthoquinone was formed. The resulting solid was filtered and
washed with cold ethanol. After recrystallization from ethanol, the
desired product was obtained as a pure and crystalline colored solid.
Comparing with previous reports, it was found that the yields of this
reaction were notoriously improved. In terms of the proposed
mechanism, the order in which the reagents are added is quite
important.
4.2. Electrochemical procedure. Solvent and supporting electrolyte
Acetonitrile was dried overnight with CaCl2 and purified by
distillation on P2O5 under vacuum [38]. Traces of water in the
˚
solvent were eliminated by contact with a molecular sieve 3 A in
4.4.3. Preparation of 2-(fluoroanilino)-1,4-naphthoquinones by
method C
the absence of light. Tetraethylammonium tetrafluoroborate
(Et4NBF4) was dried under vacuum at 60 8C.
In a flask were placed 1,4-naphthoquinone (1 mmol), the
corresponding aniline (1 mmol), CeCl3ꢂ7H2O (0.1 mmol) and 5 mL
of ethanol. The reaction mixture was placed in a microwave reactor
and it was irradiated (20–50 W) in an open atmosphere system.
The temperature was set at 65 8C for 1–3 min. After this time, the
reaction mixture flask was placed in an ice-water bath for 1 h to
induce precipitation. A crystalline product was separated by
filtration and washed with cold ethanol.
4.3. Electrodes, apparatus and instrumentation
Cyclic voltammetry measurements were carried out in a
conventional BASS model C3 three-electrode cell. A polished Pt-
disk BASS electrode with a diameter of 0.2 cm was used as a working
electrode. Prior to measurements, this electrode was polished and
cleaned with 15
mm and 3 mm alumina, washed with distilled water
and sonicated in ethanol for 3 min. The auxiliary electrode consisted
of a piece of platinum wire with an area of 0.475 cm2. The reference
electrode was a silver wire in a solution of AgNO3 3 M (Ag/Ag+) with
acetonitrile. This electrode was isolated from the main cell body by a
Luggin tube filled with 0.1 M Et4NBF4/acetonitrile.
The half-wave potentials were measured at room temperature
in acetonitrile solutions using 0.1 M Et4NBF4 as the supporting
electrolyte. The concentration for the PAN solutions were
5 ꢃ 10ꢁ3 M. Voltametric curves were recorded using a Tacussel
1MT 101 Electrochemical Analyzer with a signals generator DEA
332 interfaced with a personal computer with a Voltamaster 2-
X9725-2.0 program. Measurements were made over a potencial
range between ꢁ600 and ꢁ2020 mV with a sweep rate of 100 mV/
s. Prior to the experiments, solutions were purged with nitrogen.
All potentials were determined under the same conditions in order
to obtain a consistent data set [39].
4.5. Characterization of 2-(2-fluoroanilino)-1,4-naphthoquinone (3b)
Itwasobtainedasanorangesolid with m.p. 155 8C; IR (KBr, cmꢁ1):
3314 (–NH–), 1667.6, 1644.5 (C55O), 1607, 1594 (C55Caromatic),1502
(–NH–), 1353 (C–N), 1300 (C–CO–C), 1104 (C–F); UV–Vis (CH3OH, nm,
log
d6)
e
d
): 447 (3.73), 268 (4.55), 208 (4.29); 1H NMR (400 MHz, DMSO-
(ppm): 6.19 (1H, s, vinyl H), 7.28 (1H, m, aromatic H), 7.37 (2H, m,
aromatic H), 7.44 (1H, t, JHF = 8.5, JHH = 7.42 Hz, aromatic H), 7.79 (1H,
td, J = 7.42, 1.37 Hz, aromatic H), 7.85 (1H, td, J = 7.42, 1.37 Hz,
aromatic H), 7.93 (1H, dd, J = 7.42, 1.37 Hz, aromatic H), 8.06 (1H, dd,
J = 7.42, 1.37 Hz, aromatic H), 9.15 (1H, s, N–H); 19F NMR (DMSO-d6)
d
(ppm): ꢁ120.07 (m, J = 8.3, 5.95 Hz aromatic F); EIMS (probe) 70 eV,
m/z (rel. int.): 267 [M+] (100), 250 [M+–OH] (4.5), 238 [M+–COH]
(30.4), 222 [M+–OH–CO] (12.5), 162 [M+–COH–C6H4] (20.2), 105[M+–
COH–C8H4FN] (17.6), 76 [M+–COH–C8H4FN–COH] (13.2). The exact
mass was 267.0696 amu, the mass observed was 267.0691 amu for
C
16H10FNO2.
4.6. Characterization of 2-(3-fluoroanilino)-1,4-naphthoquinone (3c)
It was obtained as a red solid with m.p. 200 8C; IR (KBr, cmꢁ1
4.4. Synthetic procedures
4.4.1. Preparation of 2-(fluoroanilino)-1,4-naphthoquinones by
method A
)
The substituted naphthoquinones were prepared by the method
reported in the literature [24] with several modifications. 1,4-
Naphthoquinone (1 mmol) was dissolved in ethanol (30 mL). A
solution of fluoroaniline (1 mmol) in ethanol (30 mL) was slowly
added and the reaction mixture was allowed to react for seven days.
The reaction vessel was equipped with a reflux condenser to minimize
ethanol losses during the experiment. The solution turned deep red or
orange-yellow when the corresponding 2-(fluoroanilino)-1,4-
naphthoquinone was formed. The resulting solid was filtered and
washed with cold ethanol. After recrystallization from ethanol, the
desired product was obtained as a pure and crystalline colored solid.
3192 (–NH–), 1681.5, 1624.9 (C55O), 1599.5 (C55C aromatic), 1529 (–
NH–), 1358 (C–N), 1300 (C–CO–C), 1221 (C–F); UV–Vis (CH3OH, nm,
log
(ppm): 6.46 (1H, s, vinyl H), 6.9 (1H, td, JHF = 8.2, JHH = 2.34 Hz,
e
): 458 (3.61), 272 (4.38), 208 (4.119); 1H NMR (400 MHz, CDCl3)
d
aromatic H), 7.01 (1H, dt, JHF = 9.77, JHH = 2.01 Hz, aromatic H), 7.05
(1H, dd, J = 8.01, 1.1 Hz, aromatic H), 7.37 (1H, m, JHH = 8.2,
JHF = 6.5 Hz, aromatic H), 7.56 (1H, s, N–H), 7.68 (1H, td, J = 7.62,
1.17 Hz, aromatic H), 7.77 (1H, td, J = 7.62, 1.37 Hz, aromatic H), 8.12
(2H, td, J = 7.42, 0.8 Hz, aromatic H); 19F NMR (CDCl3)
d (ppm):
ꢁ110.77 (sextet, J = 9.3, 6.7 Hz, aromatic F); EIMS (probe) 70 eV, m/z
(rel. int.): 267 [M+] (100), 250 [M+–OH] (7), 238 [M+–COH] (30), 222
[M+–OH–CO] (18), 162 [M+–COH–C6H4] (15), 105 [M+–COH–
C8H4FN] (16). The exact mass was 267.0696 amu, the mass observed
was 267.0690 amu for C16H10FNO2.
4.4.2. Preparation of 2-(fluoroanilino)-1,4-naphthoquinones by
method B
The substituted naphthoquinones were prepared by the method
reported in the literature [24] with several modifications. 1,4-
Naphthoquinone (1 mmol) was dissolved in ethanol (10 mL) and
CeCl3.7H2O (0.1 mmol) was added. The reaction mixture was stirred
at least 60 min to allow the reaction between the Lewis base (1,4-
naphthoquinone) and the Lewis acid (CeCl3ꢂ7H2O). A solution of
fluoroaniline (1 mmol) in ethanol (10 mL) was slowly added and the
mixture was allowed to react for 4 h. The reaction vessel was
equipped with a reflux condenser to minimize ethanol losses during
the experiment. The progress of the reaction was monitored by TLC
and only one product was formed. The solution turned deep red or
orange-yellow when the corresponding 2-(fluoroanilino)-1,4-
4.7. Characterization of 2-(4-fluoroanilino)-1,4-naphthoquinone (3d)
It was obtained as a violet solid with m.p. 244 8C; IR (KBr, cmꢁ1
)
3317.1 (–NH–), 1676.8, 1645 (C55O), 1609.7 (C55C aromatic), 1509
(–NH–), 1354 (C–N), 1300 (C–CO–C), 1226 (C–F); UV–Vis (CH3OH,
nm, log
(ppm): 5.98 (s, 1H, vinyl H), 7.27 (2H, t, JHF = 8.79, JHH = 6.75,
e
): 462 (3.678), 270 (4.45), 207 (4.19); 1H NMR (DMSO-d6)
d
2.34 Hz, aromatic H), 7.4 (2H, dd, JHH = 7.23, 2.15 JHF = 5.27 Hz,
aromatic H), 7.77 (1H, td, J = 7.62, 1.37 Hz, aromatic H), 7.84 (1H,
td, J = 7.62, 1.37 Hz, aromatic H), 7.93 (1H, dd, J = 7.62, 1.37 Hz,
aromatic H), 8.04 (1H, dd, J = 7.62, 1.37 Hz, aromatic H), 9.26 (1H, s,

100
E. Leyva et al. / Journal of Fluorine Chemistry 132 (2011) 94–101
N–H); 19F NMR (DMSO-d6)
d
(ppm): ꢁ117.04 (s, aromatic F); EIMS
(3.54), 269 (4.35), 208 (4.06); 1H NMR (CDCl3)
d (ppm): 6.31 (1H, s,
(probe) 70 eV, m/z (rel. int.): 267 [M+] (100), 250 [M+–OH] (6), 238
[M+–COH] (29), 222 [M+–OH–CO] (14), 162 [M+–COH–C6H4] (21),
105 [M+–COH–C8H4FN] (22). The exact mass was 267.0696 amu,
the mass observed was 267.0691 amu for C16H10FNO2.
vinyl H), 7.01 (1H, m, J = 8.6 Hz, aromatic H), 7.13 (1H, ddd,
JHF = 10.98, 6.74, JHH = 2.64 Hz, aromatic H), 7.22 (1H, q,
JHF = 9.57, JHH = 8.79 Hz, aromatic H), 7.44 (1H, s, N–H), 7.69 (1H,
td, J = 7.62, 1.37 Hz, aromatic H); 7.78 (1H, td, J = 7.42, 1.17 Hz,
aromatic H), 8.11 (2H, t, J = 7.62, 0.78 Hz, aromatic H); 19F NMR
4.8. Characterization of 2-(2,3-difluoroanilino)-1,4-naphthoquinone
(CDCl3)
d
(ppm): ꢁ134.46 and ꢁ140.24 (s, J = 9.3, 6.7 Hz, aromatic
(3e)
F); EIMS (probe) 70 eV, m/z (rel. int.): 285 [M+] (100), 268 [M+–OH]
(19.5), 256 [M+–COH] (63), 240 [M+–CO–OH] (44.5), 180 [M+–
COH–C6H4] (45.5), 105 [M+–COH–C8H3F2N] (49.5), 76 [M+–COH–
C8H3F2N–COH] (13.2). The exact mass was 285.0601 amu, the
mass observed was 285.06 amu for C16H9F2NO2.
It was obtained as an orange solid with m.p. 154 8C; IR (KBr,
cmꢁ1) 3316 (–NH–), 1666 (C55O), 1608 (C55C aromatic), 1525 (–
NH–), 1348 (C–N), 1301 (C–CO–C), 1248 (C–F); UV–Vis (CH3OH,
): 448 (3.65), 268 (4.47), 209 (4.23); 1H NMR (CDCl3)
nm, log e d
(ppm): 6.32 (1H, s, vinyl H), 7.21 (2H, m, aromatic H), 7.45 (1H, m,
aromatic H), 7.55 (1H, s, N–H), 7.7 (1H, td, J = 7.44, 1.5 Hz, aromatic
H), 7.77 (1H, td, J = 7.47, 1.48 Hz, aromatic H), 8.13 (2H, m,
4.12. Characterization of 2-(3,5-difluoroanilino-1,4-naphthoquinone
(3i)
J = 7.25 Hz, aromatic H); 19F NMR (CDCl3)
d
(ppm): ꢁ124.98 (s,
It was obtained as an orange solid with m.p. 261 8C; IR (KBr, cmꢁ1
)
3322 (–NH–), 1666.3, 1646.3 (C55O), 1614 (C55C aromatic), 1536 (–
NH–), 1304 (C–N), 1248 (C–CO–C), 1116 (C–F); UV–Vis (CH3OH, nm,
log e d(ppm):
): 451 (3.42), 272 (4.21), 209 (3.95); 1H NMR (DMSO-d6)
aromatic F), ꢁ125.01 (s, aromatic F); EIMS (probe) 70 eV, m/z (rel.
int.): 285 [M+] (100), 266 [M+–F] (8.3), 256 [M+–COH] (23.7), 180
[M+–COH–C6H4] (22%), 105 [M+–COH–C8H3F2N] (20.6), 76 [M+–
COH–C8H3F2N–COH] (14). The exact mass was 285.0601 amu, the
mass observed was 285.06 amu for C16H9F2NO2.
6.31 (1H, s, vinyl H), 7.03 (1H, tt, JHF = 9.38, JHH = 2.34, aromatic H),
7.15 (2H, dd, JHF = 9.18, JHH = 2.34 Hz, aromatic H), 7.79 (1H, td,
J = 7.42, 1.37 Hz, aromatic H), 7.86 (1H, td, J = 7.42, 1.37 Hz, aromatic
H), 7.95 (1H, dd, J = 7.62, 1.17 Hz, aromatic H), 8.05 (1H, dd, J = 7.62,
4.9. Characterization of 2-(2,4-difluoroanilino)-1,4-naphthoquinone
(3f)
1.17 Hz, aromatic H), 9.38 (1H, s, N–H); 19F NMR (DMSO-d6)
d (ppm):
ꢁ109.19 (t, J = 9.29 Hz, aromatic F); EIMS (probe) 70 eV, m/z (rel. int.):
285 [M+] (100), 268 [M+–OH] (9), 256 [M+–COH] (41), 240 [M+–CO–
OH] (26.6), 180 [M+–COH–C6H4] (16.4), 105 [M+–COH–C8H3F2N]
(17.4), 76 [M+–COH–C8H3F2N–COH] (13.6). The exact mass was
285.0601 amu, the mass observed was 285.06 amu for C16H9F2NO2.
It was obtained as an orange solid with m.p. 205 8C; IR (KBr,
cmꢁ1) 3313 (–NH–), 1665, 1643 (C55O), 1612 (C55C aromatic), 1530
(–NH–), 1355 (C–N), 1306 (C–CO–C), 1148 (C–F); UV–Vis (CH3OH,
): 443 (3.63), 266 (4.55), 208 (4.21); 1H NMR (CDCl3)
nm, log e d
(ppm): 6.13 (1H, s, vinyl H), 6.96 (2H, m, aromatic H), 7.33 (1H, s,
N–H), 7.38 (1H, m, J = 7.23, JHF = 5.67 Hz, aromatic H), 7.68 (1H, td,
J = 7.62, 1.37 Hz, aromatic H), 7.76 (1H, td, J = 7.62, 1.37 Hz,
aromatic H), 8.10 (1H, dd, J = 7.62, 0.78 Hz, aromatic H), 8.13 (1H,
4.13. Characterization of 2-(2,4,5-trifluoroanilino)-1,4-
naphthoquinone (3j)
dd, J = 7.62, 0.78 Hz, aromatic H); 19F NMR (CDCl3)
d
(ppm): ꢁ111.6
It was obtained as an orange solid with m.p. 202 8C; IR (KBr,
cmꢁ1) 3186 (–NH–), 1686 (C55O), 1607, 1619 (C55C aromatic), 1502
(s, aromatic F), ꢁ119.02 (s, aromatic F); EIMS (probe) 70 eV, m/z
(rel. int.): 285 [M+] (100), 266 [M+–F] (6.6), 256 [M+–COH] (22.7),
180 [M+–COH–C6H4] (19.8), 105 [M+–COH–C8H3F2N] (25), 76 [M+–
COH–C8H3F2N–COH] (12). The exact mass was 285.0601 amu, the
mass observed was 285.06 amu for C16H9F2NO2.
(–NH–), 1347 (C–N), 1277 (C–F); UV–Vis (CH3OH, nm, log
e): 438
(3.49), 265 (4.34), 211 (4.12); 1H NMR (CDCl3)
d
(ppm): 6.21 (1H, s,
vinyl H), 7.10 (1H, sextet, JHF = 9.8, 7.3 Hz, aromatic H), 7.28 (1H, m,
JHF = 10.36, 7.6 Hz, aromatic H), 7.39 (1H, s, N–H), 7.70 (1H, td,
J = 7.62, 1.17 Hz, aromatic H), 7.78 (1H, td, J = 7.62, 1.17 Hz, aromatic
H), 8.12 (2H, t, J = 7.82 Hz, aromatic H); 19F NMR (CDCl3)
d (ppm):
4.10. Characterization of 2-(2,5-difluoroanilino)-1,4-
naphthoquinone (3g)
ꢁ125.58 (s, aromatic F), ꢁ135.95 (s, aromatic F), ꢁ139.63 (s,
aromatic F); EIMS (probe) 70 eV, m/z (rel. int.): 303 [M+], (100), 275
[M+–CO] (15), 274 [M+–COH] (19), 258 [M+–CO–OH] (9.5), 198 [M+–
COH–C6H4] (30), 127 [M+–CO–OH–C6H2F3] (26). The exact mass was
303.0507 amu, the mass observed was 303.050 amu for
It was obtained as an orange solid with m.p. 190 8C; IR (KBr,
cmꢁ1) 3318 (–NH–), 1666.8, 1642.9 (C55O), 1600 (C55C aromatic),
1505 (–NH–), 1348 (C–N), 1301 (C–CO–C), 1251 (C–F); UV–Vis
(CH3OH, nm, log
e
): 441 (3.65), 268 (4.42), 208 (4.24); 1H NMR
C16H8F3NO2.
(DMSO-d6) (ppm): 5.63 (1H, s, vinyl H), 7.21 (1H, m, JHF = 9.6, 5.34,
d
JHH = 9, 3.4 Hz, aromatic H), 7.34 (1H, m, JHH = 8.8, 3.1, JHF = 6.2 Hz,
aromatic H), 7.43 (1H, m, JHF = 9.65, 5, JHH = 9.45 Hz, aromatic H),
7.79 (1H, td, J = 7.4, 1.3 Hz, aromatic H), 7.86 (1H, td, J = 7.4, 1.25 Hz,
aromatic H), 7.94 (1H, d, J = 7.4 Hz, aromatic H), 8.055 (1H, d, J = 7.4,
Acknowledgements
Financial support by CONACyT (Grant 45936-Q) is gratefully
acknowledged. We thank Professor Alan Weedon from the
University of Western Ontario for help with MS and NMR
measurements.
aromatic H), 9.17 (1H, s, N–H); 19F NMR (DMSO-d6)
d (ppm):
ꢁ117.24 (s, aromaticF), ꢁ124.59 (s, aromaticF);EIMS (probe) 70 eV,
m/z (rel. int.):285 [M+](100), 266[M+–F](7.5), 256[M+–COH](22.6),
180[M+–COH–C6H4](18.5), 105[M+–COH–C8H3F2N](20.4),76[M+–
COH–C8H3F2N–COH] (13.2). The exact mass was 285.0601 amu, the
mass observed was 285.06 amu for C16H9F2NO2.
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