Synthesis and structure of N-(4-dialkylaminophenyl)hexafluoro-1,4- naphthoquinone 4-imines

Article abstract of DOI:10.1134/S1070428013070105

The reaction of heptafluoro-1-naphthol with N,N-dialkyl-p-nitrosoanilines or N,N-dialkyl-p-phenylenediamines in the presence of HIO₃ gave the corresponding polyfluorinated N-aryl-1,4-naphthoquinone 4-imine derivatives which exist in solution as

Full text of DOI:10.1134/S1070428013070105

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2013, Vol. 49, No. 7, pp. 1019–1024. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © L.V. Ektova, A.D. Bukhtoyarova, I.Yu. Bagryanskaya, I.V. Beregovaya, I.P. Chuikov, 2013, published in Zhurnal Organicheskoi
Khimii, 2013, Vol. 49, No. 7, pp. 1035–1040.
Synthesis and Structure of N-(4-Dialkylaminophenyl)hexafluoro-
1,4-naphthoquinone 4-Imines
L. V. Ektova, A. D. Bukhtoyarova, I. Yu. Bagryanskaya, I. V. Beregovaya, and I. P. Chuikov
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia
e-mail: bad@nioch.nsc.ru
Received December 5, 2012
Abstract—The reaction of heptafluoro-1-naphthol with N,N-dialkyl-p-nitrosoanilines or N,N-dialkyl-p-phe-
nylenediamines in the presence of HIO3 gave the corresponding polyfluorinated N-aryl-1,4-naphthoquinone
4-imine derivatives which exist in solution as equilibrium mixtures of Z and E isomers. 2,3,5,6,7,8-Hexafluoro-
N-(4-dimethylaminophenyl)-1,4-naphthoquinone 4-imine in crystal has exclusively the Z-isomer structure.
DOI: 10.1134/S1070428013070105
naphthoquinone 4-imines are generally obtained by
reaction of α-naphthols with nitrosoarenes and by
oxidative amination of α-naphthols [8]. Heptafluoro-1-
naphthol (I) reacted with N,N-dialkyl-p-nitrosoanilines
IIa and IIb in ethanol at room temperature to give in
2 h 2,3,5,6,7,8-hexafluoro-N-(4-dialkylaminophenyl)-
1,4-naphthoquinone 4-imines IIIa and IIIb in ~30%
yield (Scheme 1). We failed to improve the yield of
IIIa and IIIb by raising the reaction temperature or
increasing the reaction time. We succeeded in obtain-
ing compounds IIIa and IIIb in 73 and 61% yield,
respectively, by oxidative arylamination of fluorinated
naphthol I with N,N-dialkyl-p-phenylenediamines in
the presence of HIO3 according to the procedure
proposed by us previously for the synthesis of N-aryl-
5-hydroxy-1,4-naphthoquinone imines [9]. This reac-
tion is likely to follow ionic mechanism involving
electrophilic attack by HIO3 on the hydroxy group of
heptafluoro-1-naphthol to give complex A [10] which
reacts with aromatic amine to produce compound IIIa
or IIIb (Scheme 2).
N-(4-Dialkylaminophenyl)-1,4-naphthoquinone
4-imines are used in color photography [1]. Introduc-
tion of electron-withdrawing substituents into the
quinoid fragment gives rise to a strong charge-transfer
band in the near-IR region of the electronic spectrum,
which makes these compounds sensitive to semicon-
ductor laser irradiation [2]. The resulting IR dyes are
used in laser filters and printers [3] and were also
proposed as materials for optical data recording [4, 5].
Among halogen-containing derivatives of N-(4-di-
alkylaminophenyl)-1,4-naphthoquinone 4-imines,
2-chloro- and 2-bromo-substituted analogs of Naphthol
Blue [6] and 2,6-dibromo-4-(dialkylaminophenyl)-5-
hydroxy-1,4-naphthoquinone imine [7] have been
reported. Naphthoquinone imines having fluorine
atoms in the naphthoquinone fragment were not
described previously.
The goal of the present work was to synthesize
polyfluorinated N-aryl-1,4-naphthoquinone 4-imines
starting from heptafluoro-1-naphthol. N-Aryl-1,4-
Scheme 1.
F
OH
F
O
F
F
F
F
NO
F
F
F
+
Alk2N
F
F
F
F
N
C6H4NAlk2-4
I
IIa, IIb
IIIa, IIIb
II, III, Alk = Me (a), Et (b).
1019

EKTOVA et al.
1020
Scheme 2.
O
I
OH
O–
F
F
OH
F
F
O
F
F
F
O
F
F
F
F
F
F
F
F
F
F
O–
I
[O]
+
–IO2, –H2O
O
O
F
F
F
NHAr
F
H2NAr
I
A
F
F
O
F
F
F
F
–HF
NAr
IIIa, IIIb
The structure of IIIa and IIIb was determined by
spectral methods. Their high-resolution mass spectra
contained strong peaks from the molecular ions with
m/z values corresponding to the calculated ones. In the
IR spectra of IIIa and IIIb we observed absorption
bands in the region 1550–1700 cm–1 due to stretching
vibrations of the C=O and C=N groups. Compounds
IIIa and IIIb in chloroform solution displayed in the
electronic absorption spectra strong absorption bands
at λmax 662 (IIIa) and 680 nm (IIIb), which were dis-
placed toward long-wave region by 67 and 63 nm,
respectively, relative to those of their fluorine-free
analogs [11]; on the other hand, the electronic spectra
of IIIa and IIIb resemble those of 2,3-dicyano-N-(4-
methoxyphenyl)- and 2,3-dicyano-N-(2,4-dimethoxy-
phenyl)-1,4-naphthoquinone 4-imines [2]. Obviously,
the observed red shift is related to the electron-
acceptor character of the fluorinated fragment.
1
The 19F and H chemical shifts in the NMR spectra
of quinone imines IIIa and IIIb are generally con-
sistent with the proposed structures. Nevertheless, the
spectra recorded at room temperature contained two
sets of signals with different intensities. In particular,
in the 19F NMR spectrum of IIIa (Fig. 1, Table 1) at
room temperature we observed six diffuse signals
together with less intense broadened signals at δF ~12–
13, ~23–25, and ~42 ppm. These findings suggest the
existence of two structures occurring in dynamic equi-
librium with each other. By analogy with published
data on the isomerization of N-aryl-1,4-benzoquinone
imines [8], it was reasonable to presume that these
structures are Z and E isomers with respect to the
exocyclic C=N bond (Scheme 3). With a view to verify
this assumption, we examined the temperature depend-
ence of the 19F NMR spectra of IIIa.
(a)
(b)
(c)
2-F
3-F
5-F 8-F
6-F
7-F
2-F
3-F
*
7-F
*
5-F
*
8-F
*
6-F
*
*
The spectrum recorded at –50°C contained two sets
of signals with an intensity ratio of ~73:27. Each set
included 6 signals whose resolution was sufficient to
analyze their fine structure. Raising the temperature to
55°C lead to disappearance of the minor signals, the
signal at δF 25.9 ppm belonging to the major isomer
becomes strongly broadened (up to complete disap-
pearance, while the other signals remain almost un-
40
30
20
δF, ppm
Fig. 1. 19F NMR spectra of 2,3,5,6,7,8-hexafluoro-N-(4-di-
methylaminophenyl)-1,4-naphthoquinone 4-imine (IIIa) in
CDCl3 recorded at (a) 55, (b) 25, and (c) –50°C. Signals
from the minor E isomer in the bottom spectrum are marked
with an asterisk.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 7 2013

SYNTHESIS AND STRUCTURE OF N-(4-DIALKYLAMINOPHENYL)HEXAFLUORO-...
1021
Scheme 3.
variation of chemical shifts. The largest difference in
the fluorine chemical shifts between the Z and E
isomers in the spectrum recorded at –50°C is observed
for 5-F (ΔδF ~17 ppm), while the chemical shifts of
2-F, 7-F, and 8-F in the two isomers differ by no more
than 2 ppm, and the ΔδF value for 3-F and 6-F amounts
to a few tenth ppm. Compound IIIa displayed no
F
F
O
N
F
O
F
F
F
F
F
F
F
F
F
N
C6H4NAlk2-4
C6H4NAlk2-4
E Isomer
1
signals from the E isomer in the H NMR spectrum
Z Isomer
recorded at room or higher temperature, and the corre-
sponding signals became visible only at –50°C at the
same intensity ratio as in the 19F NMR spectrum.
changed. The subsequent cooling to room temperature
restores the initial spectrum.
The 19F and 1H NMR spectra of IIIb showed analo-
gous patterns (Table 1). The isomer ratio for IIIb was
77:23 at –50°C.
The major set of signals was assigned to the Z iso-
mer taking into account the observed spin–spin cou-
pling of 3-F with the ortho-protons (2′-H, 6′-H) in the
6
6
The difference in the Gibbs energies of the Z and E
isomers of IIIa and IIIb was estimated by the equation
ΔG0 = –RTlnKeq at 0.44 and 0.53 kcal/mol, respec-
tively, from the equilibrium isomer ratio Keq at –50°C,
which was determined from the signal intensities in the
19F NMR spectrum. Processing of the 19F NMR spec-
trum recorded at 25°C with the use of TOPSPIN 2.1
allowed us to estimate the Z→E isomerization rate
constants: 240 and 290 s–1 for IIIa and IIIb, respec-
tively. On the basis of these values we calculated by
the Eyring equation the Gibbs energies of activation
for the transformation of the Z isomer into E isomer:
ΔG = 14.2 (IIIa), 14.1 kcal/mol (IIIb).
4-dimethylaminophenyl group: J3-F,2′-H = J3-F,6′-H
=
5.3 Hz. This coupling was identified by the 19F–{1H}
and H–{19F} double-resonance techniques. We previ-
1
6
ously [9] revealed the coupling constant J3-H 2′-F
,
=
6J3-H 6′-F
= 2.5 Hz for 5-hydroxy-N-pentafluorophenyl-
,
1,4-naphthoquinone 4-imine, where the N-aryl frag-
ment is oriented syn with respect to the quinoid
protons owing to strong intramolecular hydrogen bond
between the peri-hydroxy group and nitrogen atom of
the C=N bond. It is also known that the long-range
6
coupling constant JFH may reach 11.9 Hz if the inter-
acting nuclei appear spatially close to each other [12].
The minor set of signals in the spectrum of IIIa does
not contradict the E-isomer structure. We observed no
long-range coupling between 5-F and 2′(6′)-H in the
4-dimethylaminophenyl group (7J5-F,2′(6′)-H), which
could unambiguously indicate the isomer structure. On
the other hand, change of the magnetic environment of
5-F in going from the Z to E isomer follows from the
1
The temperature dependence of the H NMR spec-
tra was reported previously for N-aryl-2,3-bis(me-
thoxycarbonyl)-1,4-naphthoquinone 4-imines [2].
However, no signal broadening was observed at room
1
temperature in the H NMR spectra of N-aryl-1,4-
naphthoquinone 4-imines containing no fluorine atoms
1
Table 1. Chemical shifts in the 19F and H NMR spectra of 2,3,5,6,7,8-hexafluoro-N-(4-dialkylaminophenyl)-1,4-naphtho-
quinone 4-imines IIIa and IIIb
Chemical shifts δF, δ, ppm
Tempera- Comp.
ture, °C
no.
2-F
3-F
5-F
6-F
7-F
8-F
2′-H, 6′-H 3′-H, 5′-H
CH3, CH2
3.08 (3H)
–25
Z-IIIa
13.5
32.6
25.9
16.3
10.0
22.1
7.03
6.68
E-IIIa 10.7–12.7 32.6 41.1–42.6 16.3 10.7–12.7 23.0–24.4
Z-IIIb 12.5 32.1 25.4 15.9 9.3 21.6
E-IIIb 09.9–11.9 32.1 40.9–42.9 15.9 09.9–11.9 22.1–24.1
7.03
6.66
1.22 (3H), 3.44 (2H)
–50
–55
Z-IIIa
E-IIIa
Z-IIIb
E-IIIb
E,Z-IIIa
12.7
10.7
11.8
10.0
13.5
31.2
31.3
31.0
31.3
24.6
41.5
24.4
41.9
16.6
17.0
16.1
16.4
9.9
11.9
9.1
21.1
22.7
20.8
22.5
22.7
22.3
7.07
6.93
6.69
6.61
6.67
6.59
6.71
6.68
3.12 (3H)
3.10 (3H)
7.07
1.22 (3H), 3.45 (2H)
1.20 (3H), 3.43 (2H)
3.07 (3H)
11.1
10.3
9.6
6.80–7.05
7.02
33.0 25.0–29.5 16.2
32.5 24.0–28.5 15.8
E,Z-IIIb 12.6
7.03
1.23 (3H), 3.44 (2H)
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 7 2013

EKTOVA et al.
1022
Table 2. Coupling constants in the 19F and 1H NMR spectra of 2,3,5,6,7,8-hexafluoro-N-(4-dialkylaminophenyl)-1,4-naphtho-
quinone 4-imines IIIa and IIIb, recorded at –50°C
Coupling constant J, Hz
Compound
no.
2-F/3-F 3-F/2′(6′)-H 5-F/6-F
5-F/7-F
7.5
5-F/8-F
12.8
6-F/7-F
21.0
6-F/8-F
10.4
7-F/8-F
21.0
2-H/3-H
Z-IIIa
E-IIIa
Z-IIIb
E-IIIb
13.4
15.0
14.2
15.5
5.3
–
20.5
24.0
20.6
24.0
–
–
9.0
11.0
21.0
09.5
20.8
5.7
–
7.0
12.7
21.5
10.0
21.0
7.0
7.0
8.6
12.0
21.0
09.2
21.0
or other strong electron-withdrawing substituents,
which may be interpreted as the absence of Z/E iso-
merization under the given conditions [7, 13]. Accord-
ing to the X-ray diffraction data, the substituent on the
imino nitrogen atom in N-(4-methoxyphenyl)-2-(4-
methoxyphenylamino)-1,4-naphthoquinone 4-imine in
crystal is oriented opposite to the aromatic ring [14];
analogous configuration was observed for some other
N-substituted 1,4-naphthoquinone 4-imines [15]. In
order to rationalize the observed differences, we per-
formed DFT calculations of the relative stabilities of
the Z and E isomers of IIIa and its fluorine-free
analog, as well as of the energy barriers to mutual
isomer transformations. The calculations were carried
out with the PBE/3z functional using Priroda software
[16]. The Z isomer of IIIa turned out to be more stable
than the E isomer by 1.3 kcal/mol, and the energy
barrier to the transformation of Z-IIIa into E-IIIa was
estimated at 12.6 kcal/mol. The corresponding isomer
of the fluorine-free analog, in which the substituent on
the imino nitrogen atom is oriented opposite to the
aromatic ring, is more stable than the other isomer by
4.7 kcal/mol, and the energy of activation for the iso-
merization is 14.4 kcal/mol. Thus, in going from the
fluorine-free to fluorinated compound the energies of
formation of the isomers become closer, and the barrier
to their interconversion decreases. This favors in-
creased fraction of the E isomer and makes it possible
to observe that isomer experimentally by NMR spec-
troscopy at reduced temperature.
The crystalline structure of compound IIIa was
determined by X-ray analysis; suitable single crystals
were obtained by crystallization from benzene–hexane
(Fig. 2). According to the X-ray diffraction data, mole-
cule IIIa in crystal has Z configuration, the naphtho-
quinone fragment is planar, and the deviations of
atoms from the mean-square plane including all atoms
of that fragment do not exceed 0.030 Å. The phenyl
ring is turned through a dihedral angle of 55.90° with
respect to the naphthoquinone plane. The dimethyl-
amino group is also planar; the mean-square deviation
from the N2C4′C11C12 plane is 0.024 Å. No shortened
non-valence contacts (as compared to the sum of the
corresponding van der Waals radii) were observed in
the crystal packing of IIIa. Molecules IIIa are linked
to dimers via π-stacking interaction between the per-
fluorinated naphthoquinoid fragment (C1–C10) and
nonfluorinated C1′–C6′ ring [intercentroid distance
3.546(1) Å, interplane angle 3.54(8)°]. The experimen-
tal X-ray powder pattern of IIIa confirmed the pres-
ence of only one isomer in the crystalline state.
F6
O1
F5
C8
C1
F1
C7
C6
C9
C2
C3
C10
F2
C5
F3
C4
F4
N1
C1′
C2′
C3′
C4′
C6′
C5′
N2
C12
C11
Thus N-(4-dialkylaminophenyl)hexafluoro-1,4-
naphthoquinone 4-imines in chloroform solution exist
as a dynamic equilibrium mixture of the Z and E iso-
Fig. 2. Structure of the molecule of 2,3,5,6,7,8-hexafluoro-
N-(4-dimethylaminophenyl)-1,4-naphthoquinone 4-imine
(IIIa) according to the X-ray diffraction data.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 7 2013

SYNTHESIS AND STRUCTURE OF N-(4-DIALKYLAMINOPHENYL)HEXAFLUORO-...
1023
mers, the former prevailing. Dimethylamino derivative
IIIa in the crystalline state exists exclusively as the Z
isomer.
compound IIIa was deposited to the Cambridge Crys-
tallographic Data Centre (entry no. CCDC 888443)
and is available at www.ccdc.cam.ac.uk/data_request/
cif. The bond lengths and bond angles in molecule IIIa
are consistent with the corresponding standard values
[19] within 3σ.
EXPERIMENTAL
The spectral studies were carried out at the Shared
Chemical Service Center, Siberian Branch, Russian
Academy of Sciences. The IR spectra were recorded in
KBr on a Bruker Vector 22 spectrometer. The elec-
tronic absorption spectra were measured from solu-
tions in chloroform on a Hewlett Packard 4853 spec-
trophotometer. The NMR spectra were obtained on
Compounds IIIa and IIIb (general procedure).
a. A mixture of 0.27 g (1 mmol) of heptafluoro-1-
naphthol (I) and 2 mmol of N,N-dimethyl- or N,N-di-
ethyl-p-nitrosoaniline hydrochloride in 20 ml of
ethanol was stirred for 2 h at 20°C. The mixture was
poured into water, the precipitate was filtered off,
washed with water, dried in air, and dissolved in
chloroform, and the solution was subjected to column
chromatography on silica gel (gradient elution with
carbon tetrachloride–chloroform, 0 to 100 vol % of the
latter). Compounds IIIa and IIIb were isolated from
the main blue zone.
1
Bruker Avance 300 (300.13 MHz for H and
282.4 MHz for 19F) and Avance 400 spectrometers
1
(400.13 MHz for H and 376.5 MHz for 19F) from
solutions in CDCl3 using the residual proton signal of
1
the solvent (CHCl3, δ 7.24 ppm) as reference (for H).
The 19F chemical shifts were determined relative to
C6F6 as external reference. The mass spectra were
recorded on DFS and Finnigan MAT-8200 instruments;
the molecular weights and elemental compositions
were determined from the precise m/z values of the
molecular ions. The progress of reactions and the
purity of products were monitored by TLC on Silufol
UV-254 plates using chloroform as eluent.
4-(4-Dimethylaminophenylimino)-2,3,5,6,7,8-
hexafluoro-1,4-dihydronaphthalen-1-one (IIIa).
Yield 32%, mp 190–192°C. IR spectrum, ν, cm–1: 1650
1
(C=O), 1625 (C=N), 1606 (C=C). The 19F and H
NMR spectra are given in Tables 1 and 2. Electronic
absorption spectrum (CHCl3): λmax 662 nm (logε 4.34).
Found: m/z 384.07044 [M]+. C18H10F6N2O. Calculated:
M 384.06972.
The X-ray diffraction data and X-ray powder pat-
tern for compound IIIa were obtained at room tem-
perature on a Bruker Kappa Apex II diffractometer
equipped with a two-coordinate CCD detector (MoKα
irradiation, graphite monochromator). A 0.06×0.35×
0.90-mm single crystal of IIIa was analyzed by ω–φ
scanning in the range 2θ < 50°. Monoclinic crystal
system: a = 12.9314(5), b = 8.0283(3), c =
15.7845(6) Å; β = 106.190(2)°; V = 1573.7(1) Å3;
4-(4-Diethylaminophenylimino)-2,3,5,6,7,8-hexa-
fluoro-1,4-dihydronaphthalen-1-one (IIIb). Yield
30%, mp 166–167°C. IR spectrum, ν, cm–1: 1665
1
(C=O), 1629 (C=N), 1606 (C=C). The 19F and H
NMR spectra are given in Tables 1 and 2. Electronic
absorption spectrum (CHCl3): λmax 680 nm (logε 4.57).
Found: m/z 412.10086 [M]+. C20H14F6N2O. Calculated:
M 412.10102.
b. A solution of 2 mmol of HIO3 in 15 ml of water
was added dropwise under stirring to a suspension of
0.27 g (1 mmol) of heptafluoro-1-naphthol (I) and
1.1 mmol of N,N-dimethyl-p-phenylenediamine hydro-
chloride or N,N-diethyl-p-phenylenediamine sulfate in
50 ml of ethanol, and the mixture was stirred for 2 h at
20°C and treated as described above in a. Yield 73%
(IIIa), 61% (IIIb).
space group P21/n; Z = 4; C18H10F6N2O; dcalc
=
1.622 g/cm3; μ = 0.151 mm–1. Intensities of 2783 in-
dependent reflections were measured. A correction for
absorption was applied using SADABS program [17]
(transmission 0.81–0.86). The structure was solved by
the direct method using SHELXS-97 and was refined
by the least-squares procedure in full-matrix aniso-
tropic approximation for all non-hydrogen atoms using
SHELXL-97 [18]. The positions of hydrogen atoms
were calculated in each iteration cycle from the coor-
dinates of the corresponding carbon atoms. The final
refinement against F2 was carried out up to wR2 =
0.1278; goodness of fit S = 1.07; 246 variables (R =
0.0361 for 2124 reflections with F > 4σ). The CIF file
containing the complete set of crystallographic data for
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