Solvent-induced E/Z(C=N)-isomerization of imines of some hydroxy-substituted formylcoumarins

Article abstract of DOI:10.1007/s11172-008-0267-5

Aromatic imines, namely, 5-formyl-6-hydroxy-4-methyl- and 8-formyl-7-hydroxy-4-methylcoumarin derivatives, have been synthesized. A dependence of their spectral characteristics (¹H NMR spectra, electronic absorption spectra) from the solvent (DMSO, CHCl₃, DMF, acetonitrile, MeOH) has been studied. The solvatochromic effects observed for a number of imines, first of all, for 7-hydroxy-4-methyl-8-(4'-nitrophenylimino) methyl-2H-1-benzopyran-2-one, were related to their E/Z-isomerization with respect to the C=N bond based on the quantum chemical calculations by the AM1, PM3, PPP CI methods.

Full text of DOI:10.1007/s11172-008-0267-5

Russian Chemical Bulletin, International Edition, Vol. 57, No. 9, pp. 1989—1995, September, 2008
1989
Solventꢀinduced E/Z(C=N)ꢀisomerization of imines
of some hydroxyꢀsubstituted formylcoumarins
V. F. Traven', I. V. Ivanov, A. V. Panov, O. B. Safronova, and T. A. Chibisova
D. I. Mendeleev University of Chemical Technology of Russia,
9 Miusskaya pl., 125047 Moscow, Russian Federation.
Eꢀmail: traven@muctr.edu.ru
Aromatic imines, namely, 5ꢀformylꢀ6ꢀhydroxyꢀ4ꢀmethylꢀ and 8ꢀformylꢀ7ꢀhydroxyꢀ4ꢀmeꢀ
thylcoumarin derivatives, have been synthesized. A dependence of their spectral characteristics
(1H NMR spectra, electronic absorption spectra) from the solvent (DMSO, CHCl3, DMF,
acetonitrile, MeOH) has been studied. The solvatochromic effects observed for a number
of imines, first of all, for 7ꢀhydroxyꢀ4ꢀmethylꢀ8ꢀ(4´ꢀnitrophenylimino)methylꢀ2Hꢀ1ꢀbenzopyrꢀ
anꢀ2ꢀone, were related to their E/Zꢀisomerization with respect to the C=N bond based on
the quantum chemical calculations by the AМ1, РМ3, PPP CI methods.
Key words: hydroxyformylcoumarins, imines, solvatochromism, E/Zꢀisomerization.
A principal importance of studying structural changes
in organic molecules under the action of a solvent is subꢀ
stantiated by the fact that noncovalent interactions, which
are in the basis of the changes indicated, to a considerable
extent define formation and properties of supramolecular
and bioorganic structures.1,2 In this connection, examples
of isomerization with respect to the C=N bond attract
ones attention since it is an analog of the C=C bond,
however, differs by the presence of unshared pair of elecꢀ
trons on the nitrogen atom. This brings specific features to
both the tautomeric transformations and the E/Zꢀisomerꢀ
ization in the imine series, as it was mentioned, in particuꢀ
lar, during study of isomerization of azomethine dyes, benꢀ
zoylacetanilide and pyrazolone derivatives,3 as well as phoꢀ
toisomerization of imines of 3ꢀphenylnorbornadieneꢀ2ꢀ
carbaldehyde4 in nonpolar aprotic solvents.
Aromatic and heteroaromatic imines containing hyꢀ
droxy groups adjacent to the —CH=N— fragment are
characterized by increased ability to the transformations
between different isomeric forms. Some of these forms are
tautomers, other, geometrical isomers relatively to the
HC=N bond. For a number of aromatic and heteroaroꢀ
matic hydroxyaldehyde and hydroxyketone derivatives, the
corresponding isomerization transformations are studied
in detail.4—7
Results and Discussion
We synthesized a number of aromatic imines, viz.,
8ꢀformylꢀ7ꢀhydroxyꢀ4ꢀmethylꢀ (1a—g) and 5ꢀformylꢀ6ꢀ
hydroxyꢀ4ꢀmethylcoumarin (2a—j) derivatives, and studꢀ
ied their transformations induced by various solvents.
1: Ar = Ph (a), pꢀMeC6H4 (b), pꢀO2NC6H4 (c), pꢀMeOCC6H4 (d),
oꢀPhNHC6H4 (e), terphenylꢀ4ꢀyl (f), pꢀClC6H4 (g)
2: Ar = pꢀClC6H4 (a), pꢀMeC6H4 (b), oꢀMeC6H4 (c), mꢀMeC6H4 (d);
oꢀHOC6H4 (e), pꢀMeOC6H4 (f), oꢀMeOC6H4 (g), oꢀ(PhNH)C6H4 (h),
fluorenꢀ2ꢀyl (i), terphenylꢀ4ꢀyl (j)
All the hydroxyꢀsubstituted imines of the coumarin seꢀ
ries synthesized possess potential ability to exist in several
isomeric forms. For example, isomers of 8ꢀformylꢀ7ꢀhyꢀ
droxyꢀ4ꢀmethylcoumarin imines 1 are shown in Scheme 1.
IR spectra (KBr), 1H NMR spectra (DMSOꢀd6,
CD3CN, CDCl3), and electronic absorption spectra (DMF,
DMSO, MeCN, and CHCl3) of new hydroxyꢀsubstitutꢀ
ed imines were recorded and analyzed, whereas, their
solutions in acetonitrile were subjected to irradiation
(λ = 360 nm). Photochromic properties of the imines were
not found. A part of them possess fluorescence, whereas
some of them undergo solvatochromic transformations.
In the literature, the data on the isomerization transꢀ
formations of imines, derivatives of hydroxyꢀsubstituted
acylcoumarins, are absent. At the same time, the study of
such transformations is of interest since many coumarin
hydroxy and amino derivatives possess pronounced fluꢀ
orescence, which is subjected to modulation due to the
structural changes of the fluorophore molecules.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1954—1960, September, 2008.
1066ꢀ5285/08/5709ꢀ1989 © 2008 Springer Science+Business Media, Inc.

1990
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
Traven' et al.
Scheme 1
firmed by the signal for the proton of the hydroxy group at
δ 14.82, by the absence of the signal for the proton of the
NH group in the spectrum of this imine, which should be
in the region δ 9.5—12.0 characteristic of aminovinyl keꢀ
tones, and by the signal at δ 9.19, which suggests the presꢀ
ence of the methine proton in compound 1a.
The structure of compound 1a in the Eꢀ(hydroxy)imine
form suggested from the results of analysis of its IR and
1H NMR spectra agrees with the analysis data of its elecꢀ
tronic absorption spectrum. The electronic absorption
spectrum of compound 1a in solvents varying in polarity is
given in Figure 1. It is characterized by an intensive band
at 321 nm and lowꢀintensive band at 455 nm. Imine 1a, as
most of other imines of the series 1 and 2, turned out to be
insensitive to the changes of solvents. However, imines 1f,
2h, 2i and, especially, imine 1c, displayed a pronounced
dependence of their spectral characteristics from the solꢀ
vent composition (Table 1).
In the IR spectrum of compound 1c (in KBr),
the absorption bands with frequencies 1610, 1720, and
3420 cm–1 are observed, they are responsible for the
stretching vibrations of the —CH=N— fragment, the pyꢀ
rone ring, and the hydroxy group participating in the forꢀ
mation of intramolecular hydrogen bond, that corresponds
to the structure of Eꢀ(hydroxy)imine I (Scheme 2).
Quantum chemical calculations of formation enthalpies for
different tautomeric forms of the imines and transition enerꢀ
gies in their electronic absorption spectra were performed.
In the IR spectra of all the imines 1 and 2, the bands
with frequencies of 1690—1750 and 1585—1630 cm–1 reꢀ
sponsible for the stretching vibrations of the αꢀpyrone ring
and the imine group, as well as intensive bands in the region
2700—3400 cm–1, which can be related to the stretching
vibrations of both the OH and the NH groups, are present.
However, the vibrations indicated differ in the form of the
band. For example, in the IR spectrum of compound 1a,
1
According to the H NMR spectroscopy, imine 1c in
chloroform also exists in the same form I. The signal for
the hydroxy group, bonded by intramolecular hydrogen
bond with the lone electron pair of the nitrogen atom of
the imine group, is recorded in the upfield region at δ 14.13.
A singlet signal for the CH proton of the imine group is
observed at δ 9.30. In the region of aromatic protons, four
doublet signals are present, the two of which at δ 6.95 and
7.92 are related to the coumarin fragment (the integral
intensity of each signal corresponds to one proton), whereꢀ
absorption bands with frequencies of 1600 and 1710 cm–1
,
responsible for the stretching vibrations of the imine group
and the pyrone ring, are present, whereas, a narrow intenꢀ
sive band, which, most likely, is related to the stretching
vibrations of the hydroxy group participating in the formaꢀ
tion of intramolecular hydrogen bond, is observed at
3400 cm–1 (form I in Scheme 1). As it is known, more
broad bands correspond to the stretching vibrations of NH
groups in IR spectra.
Signals for the protons of OH and NH groups can be
more reliably distinguished in 1H NMR spectra. In the 1H
NMR spectra of all the imines synthesized, recorded in
CDCl3, a broadened signal in the region δ 14.38—14.95
characteristic of the OH group was observed, whereas the
signal for the methine proton of the CH=N group is obꢀ
served in the region δ 9.20—9.41. The broadening the sigꢀ
nals for the OH group observed suggests that this group
participates in the intramolecular hydrogen bond.
A
3
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
2
1
300
350
400
450
500 λ/nm
For imine 1a, the preference of the Eꢀ(hydroxy)imine
form I with the intramolecular hydrogen bond was conꢀ
Fig. 1. Electronic absorption spectra of compound 1a in the solꢀ
vents of various polarity: DMF (1), DMSO (2), MeCN (3).

E/ZꢀIsomerization of imines
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
1991
Table 1. Electronic absorption and fluorescence spectra of 8ꢀformylꢀ7ꢀhydroxyꢀ4ꢀmethylcoumarin 1a—g
and 5ꢀformylꢀ6ꢀhydroxyꢀ4ꢀmethylcoumarin 2a,b,e,f,h—j derivatives
Comꢀ
pound
Electronic absorption spectrum, λmax/nm (logε)
Fluorescence
spectrum, λ /nm*
fl
DMSO
DMF
EtOH
1a
1b
1c
1d
1e
1f
1g
2a
2b
2e
2f
322 (3.88), 445 (3.55)
348 (4.09), 450 (3.22)
386 (4.19), 495 (3.15)
320 (4.14), 435 (3.20)
310 (4.11), 410 (3.56)
340 (4.06), 455 (3.61)
334 (4.27), 450 (3.57)
332 (3.92), 395 (3.63)
392 (4.18), 423 (3.30)
370 (4.12), 410 (3201)
344 (3.91), 410 (2.82)
395 (4.39), 430 (3.48)
380 (4.46), 412 (3.45)
303 (4.06), 442 (3.27)
324, 443
340, 450
352, 494
321, 435
316, 408
347, 455
328, 448
331, 395
390, 422
368, 410
342, 408
368, 430
375, 411
306, 440
331 (3.75), 460 (3.54)
356 (3.89), 458 (3.27)
330 (4.02), 505 (3.15)
329 (4.10), 470 (3.23)
345 (4.11), 458 (3.62)
330 (4.01), 470 (3.68)
355 (4.25), 464 (3.58)
325 (3.90), 460 (3.65)
350 (4.12), 465 (3.34)
356 (4.09), 440 (3.52)
335 (3.90), 480 (2.85)
375 (4.36), 460 (3.48)
365 (4.32), 440 (3.45)
315 (4.02), 450 (3.29)
—
538
462
—
476
—
529
—
536
—
—
2h
2i
2j
600
608
419
* A hyphen means the absence of fluorescence.
Scheme 2
ferent solvents. It turned out that on going from acetoniꢀ
trile to DMF and further to DMSO, the longꢀwave abꢀ
sorption band of imine 1c is bathochromically shifted. The
spectral curves obtained in solvents of various composition
(from 100% DMF to 100% DMSO) intersect in the isosꢀ
bestic point (Fig. 3).
To clarify the character of isomeric transformations of
imine 1c during the change of solvent composition, we
compared the data obtained with the results of quantum
chemical calculations. The electronic absorption spectra
of this compound tautomers were calculated by the PPP
EꢀH(9)
H(2´)
δ
as the signals at δ 7.73 and 8.33, each being proportional to
2 H, are related to the aniline fragment. On suppression of
the signal for the methyl group, the nuclear Overhauser
effect (NOE) is observed for both the signal of the proton
H(3) and the signal at δ 7.92. Based on this fact, the latter
was assigned to H(5) of the coumarin fragment, whereas
the signal at δ 6.95, interacting with it (J = 10.0 Hz), was
assigned to H(6) of the coumarin fragment. On suppresꢀ
sion of the signal at δ 9.30, the NOE is observed for the
doublet signal at δ 7.73, that allows one to assign this
signal to the protons H(2´) and H(6´) of the aniline fragꢀ
ment. A doublet signal at δ 8.33, interacting with the preꢀ
ceding signal by the spinꢀspin coupling constant of 10 Hz,
is related to the protons H(3´) and H(5´) of the aniline
fragment. The diagram of mutually interacting signals by
NOE is shown in Figure 2.
6.5
7.0
7.5
8.0
H(2´)
8.5
9.0
EꢀH(9)
9.5
10.0
10.5
10
9
8
7
δ
Isomeric transformations of imine 1c reveal themselves
on comparison of its electronic absorption spectra in difꢀ
Fig. 2. Diagram of mutually interacting signals for the protons in
the structure of Eꢀ(hydroxy)imine 1cꢀI according to the NOE.

1992
1.6
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
Traven' et al.
A
wave absorption band is 60 nm on going from the form I to
the form IV (λmax = 386 and 327 nm, respectively), whereꢀ
as the observed value is equal to 64 nm on going from
DMSO to DMF (λmax = 385 and 321 nm, respectively).
The longꢀwave absorption maximum values of the experiꢀ
mental curves of EAS and positions of the longꢀwave abꢀ
sorption maxima of tautomers 1cꢀI and 1cꢀIV resulted
from the PPP CI calculations are compared in Table 3.
Analysis of the eigenvalue coefficients of the frontier
orbitals shows that the most longꢀwave transition in the
EAS is accompanied by the transfer of electron density
from the coumarin fragment of the molecule to its 4ꢀniꢀ
troaniline fragment.
1
2
1.2
0.8
3
5
4
5
0.4
320
370
420
λ/nm
Fig. 3. Electronic absorption spectra of compound 1c in DMSO
(1), in the mixtures DMF (30%) + DMSO (70%) (2),
DMF (50%) + DMSO (50%) (3), DMF (70%) + DMSO (30%)
(4), and in DMF (5).
The conclusion on the mutual transformation of isoꢀ
mers 1cꢀI and 1cꢀIV does not contradict to the results of
their formation energy calculations (the AМ1 method)
since the difference in the values of ΔHf for all the four
forms does not exceed 4 kcal mol–1
.
CI method using the standard parameters accounting the
intramolecular hydrogen bonding, respectively, in the fragꢀ
ments —O—H...N(Ar)=CH— and =O...H—N(Ar)—CH=
(the parameter values used are given in Table 2).8
An alternative explanation for the spectral properties of
imine 1c observed would have served a gradual (as the
solvent composition changes) turning of the imine fragꢀ
ment around the CAr—N bond. The weakening this bond
could have been related to the effect of the pꢀNO2 group.
However, the observation of clear isosbestic point for the
spectral curves recorded for various ratios of DMF and
DMSO contradicts to such an explanation. The isosbestic
point suggests the equilibrium of two forms, each of which
is characterized by its own extinction coefficient in the
isosbestic point. The gradual change of geometry suggestꢀ
ed as the alternative should have been accompanied by
a monotonous change of the extinction coefficient and, as
a consequence, by the absence of the isosbestic point on
the curves. At the same time, taking into account obvious
steric hindrance arising in the plane Zꢀform, one can sugꢀ
gest that isomeriic transformation of Eꢀform under the
action of the solvent is accompanied by the formation of
spatially distorted Zꢀconfiguration.
A comparison of calculated and experimental values of
positions of the longꢀwave absorption band maxima exꢀ
cludes the possibility to explain the spectral properties by
transformation of (hydroxy)imine I to ketoenamine II:
according to the calculations, such a transformation would
have been accompanied by a considerable bathochromic
shift of the longꢀwave absorption band. The most likely
reason for the hypsochromic shift of the longꢀwave abꢀ
sorption band observed is a E/Zꢀisomerization of imine 1c.
The calculated value of the hypsochromic shift of the longꢀ
Table 2. The atomic and bond parameters* used in the quanꢀ
tum chemical calculations of the enol forms by the PPP CI
method for 1c
Bond
IP
A
–β
r•10–1
/nm
Examples of isomerization of imines and their analogs
with respect to the C=N double bond induced by the acꢀ
tion of solvation effects are few. Recently,9 an equilibrium
of four isomers of an open form of 5´ꢀhydroxyꢀ1,3,3ꢀ
eV
C=C (arom.)
С=С (aliph.)
С—С= (aliph.)
С=О (free)
С=О (H bonded)
С—ОH (free)
С—ОH (H bonded)
С—О (heterocycl.)
С—ОМе
С=N
С—NH
C—NH (H bonded)
N—O (nitro)
С—N (nitro)
11.16
11.16
11.16
15.00
17.70
32.90
28.00
40.00
32.90
15.00
21.90
15.00
16.30
24.80
0.03
0.03
0.03
0.71
2.47
11.40
10.00
14.50
11.40
0.97
10.15
8.60
1.80
2.40
2.60
2.30
2.46
2.46
2.60
2.60
2.40
2.60
2.80
2.75
2.75
3.05
2.00
1.40
1.35
1.45
1.22
1.22
1.36
1.36
1.35
1.36
1.26
1.38
1.38
1.21
1.49
Table 3. Heat of formation values (the AМ1 method) and the
values of the longꢀwave absorption maxima in the EAS (the PPP
CI method) of various tautomers of 1c
Tautomer
–ΔHf
λmax/nm
/kcal mol–1
Calculated
Experimental
DMSO DMF
1cꢀI
27.34
26.76
25.55
24.73
386.23
461.83
509.76
327.43
—
—
—
385
12.50
1cꢀII
1cꢀIII
1cꢀIV
—
—
—
* Ionization potential (IP), affinity to electron (A), resonance
integral (β), bond length (r).
321

E/ZꢀIsomerization of imines
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
1993
(conc. HCl—H2O, 84 : 100, 75 mL). The solution obtained was
refluxed for 30 min and cooled to 25 °C followed by addition of
water (375 mL). The reaction mixture was extracted with chloroꢀ
form (1×500 mL, 1×250 mL), the extract was washed with brine
(125 mL), dried with Na2SO4, then chloroform was evaporated
to obtain a substance (1.1 g) with m.p. 174—175 °C. Pure
8ꢀformylꢀ7ꢀhydroxyꢀ4ꢀmethylcoumarin was obtained after reꢀ
crystallization from ethanol, m.p. 189—190 °C,13 Rf 0.61 (a Siluꢀ
fol UVꢀ254 plate, eluent, chloroform—acetone, 5 : 1). The yield
was 20%.
5ꢀFormylꢀ6ꢀhydroxyꢀ4ꢀmethylcoumarin. Urotropine (10 g,
0.07 mol) was added to a hot solution of 6ꢀhydroxyꢀ4ꢀmethylꢀ
coumarin (5 g, 0.03 mol) in glacial AcOH (100 mL). The reacꢀ
tion mixture was refluxed for 10 h followed by addition of aq. HCl
(conc. HCl—H2O, 1 : 1, 75 mL). The solution obtained was
heated for 2 h at 95 °C, cooled to 25 °C, and treated as described
in the preceding experiment to obtain a substance (1.1 g) with
m.p. 186—187 °C. After recrystallization from ethanol, the prodꢀ
uct with m.p. 190—191 °C13 was obtained, Rf 0.61 (a Silufol
UVꢀ254 plate, eluent, chloroform—acetone, 5 : 1). The yield
was 22%.
trimethylspiro[indolylꢀ2,3´ꢀ[3H]naphtho[1,2ꢀb][1,4]ꢀ
oxazine] at –50 °C in CDCl3 has been reported, two of
which are the Eꢀ and Zꢀisomers with respect to the C=N
bond, whereas two other, the Eꢀ and Zꢀisomers with reꢀ
spect to the C=C bond.
In the literature, several pathways for the mutual transꢀ
formations of imine Eꢀ and Zꢀisomers are under discusꢀ
sion. An inversion mechanism is considered the most prefꢀ
erable.10,11 This is the mechanism which provides a conꢀ
siderable decrease (to 10—20 kcal mol–1) of the isomerꢀ
ization barrier with respect to the C=N bond in compariꢀ
son with a significantly higher (40—60 kcal mol–1) isomerꢀ
ization barrier with respect to the C=C bond in substituted
ethylenes. It is supposed that in the course of isomerizaꢀ
tion accompanied by a transfer of substituent R, the unꢀ
shared pair of electrons of the nitrogen atom is transferred
from sp2ꢀ to рꢀorbitals, owing to which substituent R gets
a possibility to move in the plane of the molecule with
respect to the double bond (Scheme 3). A stretched conꢀ
figuration along the —C=N—R bond corresponds to the
intermediate state of isomerization.
Condensation of 8ꢀformylꢀ7ꢀhydroxyꢀ4ꢀmethylcoumarin with
aromatic amines (general procedure). A mixture of 8ꢀformylꢀ7ꢀ
hydroxyꢀ4ꢀmethylcoumarin (2 g, 0.01 mol) and aromatic amine
(0.01 mol) in ethanol (6—15 mL) was heated for 1—3 h, cooled
to 25 °C, a precipitate formed was filtered off, washed with ethꢀ
anol, and dried.
Scheme 3
7ꢀHydroxyꢀ4ꢀmethylꢀ8ꢀphenyliminomethylꢀ2Hꢀ1ꢀbenzopyrꢀ
anꢀ2ꢀone (1a). The yield was 80%, m.p. 165—166 °C, Rf 0.61.
Found (%): C, 73.85; H, 5.04; N, 4.72. C17H13NO3. Calculated
(%): C, 73.12; H, 4.66; N, 5.02. 1H NMR, δ: 6.16 (s, 1 H, H(3));
2.37 (s, 3 H, 4ꢀMe); 7.76 (d, 1 H, H(5), J5,6 = 9.0 Hz); 6.89 (d, 1 H,
H(6), J6,5 = 9.0 Hz); 9.19 (s, 1 H, —CH=N); 14.82 (s, 1 H,
7ꢀOH); 7.26—7.45 (m, 5 H, H(2´), H(3´), H(4´), H(5´), H(6´)).
IR, ν/cm–1: 1720 (αꢀpyrone), 1620 (—CH=N). EAS, λmax/nm
(logε): 460 (3.54). MS, m/z (Irel (%)): 279 (65).
7ꢀHydroxyꢀ4ꢀmethylꢀ8ꢀ(4´ꢀmethylphenylimino)methylꢀ2Hꢀ
1ꢀbenzopyranꢀ2ꢀone (1b). The yield was 75%, m.p. 188—189 °C,
Rf 0.73. Found (%): C, 73.99; H, 5.01; N, 4.62. C18H15NO3.
Calculated (%): C, 73.72; H, 5.12; N, 4.78. 1H NMR, δ: 6.16 (s, 1 H,
H(3)); 2.46 (s, 3 H, 4ꢀMe); 7.78 (d, 1 H, H(5), J5,6 = 9.0 Hz);
6.90 (d, 1 H, H(6), J6,5 = 9.0 Hz); 9.35 (s, 1 H, —CH=N); 14.89
(s, 1 H, 7ꢀOH); 2.39 (s, 3 H, 4´ꢀMe); 7.30—7.44 (m, 4 H, H(2´),
H(3´), H(5´), H(6´)). IR, ν/cm–1: 1730 (αꢀpyrone), 1610
(—CH=N). EAS, λmax/nm (logε): 458 (3.27). MS, m/z (Irel (%)):
293 (65).
7ꢀHydroxyꢀ4ꢀmethylꢀ8ꢀ(4´ꢀnitrophenylimino)methylꢀ2Hꢀ1ꢀ
benzopyranꢀ2ꢀone (1c). The yield was 63%, m.p. 267—268 °C,
Rf 0.81. Found (%): C, 62.79; H, 3.90; N, 8.62. C17H12N2O5.
Calculated (%): C, 62.96; H, 3.70; N, 8.64. 1H NMR, δ: 6.27 (s,
1 H, H(3)); 2.43 (s, 3 H, 4ꢀMe); 7.92 (d, 1 H, H(5), J5,6 = 8.8
Hz); 6.95 (d, 1 H, H(6), J6,5 = 8.8 Hz); 9.30 (s, 1 H, —CH=N);
14.13 (s, 1 H, 7ꢀOH); 7.73 (d, 2 H, H(2´), H(6´), J2´,3´;6´,5´ = 8.0 Hz);
8.33 (d, 2 H, H(3´), H(5´), J3´,2´;5´,6´ = 8.0 Hz). IR, ν/cm–1: 1720
(αꢀpyrone), 1610 (—CH=N), 3420 (OH). EAS, λmax/nm (logε):
505 (3.15). MS, m/z (Irel (%)): 324 (65).
8ꢀ(4´ꢀAcetylphenylimino)methylꢀ7ꢀhydroxyꢀ4ꢀmethylꢀ2Hꢀ1ꢀ
benzopyranꢀ2ꢀone (1d). The yield was 68%, m.p. 232—233 °C,
Rf 0.80. Found (%): C, 71.02; H, 4.68; N, 4.32. C19H15NO4.
Calculated (%): C, 71.03; H, 4.67; N, 4.36. 1H NMR, δ: 6.24
(s, 1 H, H(3)); 2.48 (s, 3 H, 4ꢀMe); 2.63 (s, 3 H, 4´ꢀCOMe); 7.85
It cannot be excluded that especially pronounced abiliꢀ
ty of imine 1c to the isomerization found is explained by
a strong electronꢀwithdrawing effect of the pꢀnitro group,
that leads to a decrease in electron density in the region of
the double C=N bond and can facilitate isomerization by
both the torsion and the inversion mechanisms.
Experimental
1
H NMR spectra were recorded on a Bruker WPꢀ200ꢀSY
spectrometer (200 МHz) in CDCl3. IR spectra were recorded on
a Specord Mꢀ80 spectrophotometer in KBr pellets. Electronic
absorption spectra were recorded on a Specord Mꢀ400 spectroꢀ
photometer in ethanol in 1ꢀcm pathlength cuvettes. Mass spectra
were recorded on a GCQ mass spectrometer with the energy of
electron ionization of 70 eV and the source temperature of 200 °C.
The ab initio quantum chemical calculations (AM1,
ZINDO/S) were performed using the MOPAC program packꢀ
age.12 Initial optimization of geometry was made using the method
of molecular mechanics in the ММ+ version. Quantum chemiꢀ
cal calculations of electronic absorption spectra for the imine
series were made also by the PPP CI method using standard
values of the atom and bond parameters (see Table 3).8
8ꢀFormylꢀ7ꢀhydroxyꢀ4ꢀmethylcoumarin. A solution of 7ꢀhyꢀ
droxyꢀ4ꢀmethylcoumarin (5 g, 0.03 mol) and urotropine (10 g,
0.07 mol) in glacial AcOH (37.5 mL, 0.65 mol) was heatꢀ
ed for 5.5 h at 95 °C followed by addition of aqueous HCl

1994
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
Traven' et al.
(d, 1 H, H(5), J5,6 = 9.0 Hz); 6.95 (d, 1 H, H(6), J6,5 = 9.0 Hz);
9.41 (s, 1 H, —CH=N); 14.41 (s, 1 H, 7ꢀOH); 7.62 (d, 2 H,
H(2´), H(6´), J2´,3';6´,5´ = 9.0 Hz); 8.12 (d, 2 H, H(3´), H(5´),
1590 (—CH=N). EAS, λmax/nm (logε): 460 (3.48). MS, m/z
(Irel (%)): 293 (100).
6ꢀHydroxyꢀ4ꢀmethylꢀ5ꢀ(3´ꢀmethylphenylimino)methylꢀ2Hꢀ
1ꢀbenzopyranꢀ2ꢀone (2d). The yield was 61%, m.p. 178—179 °C,
Rf 0.72. Found (%): C, 72.98; H, 5.16; N, 4.85. C18H15NO3.
Calculated (%): C, 73.72; H, 5.12; N, 4.78. 1H NMR, δ: 6.37 (s, 1 H,
H(3)); 2.42 (s, 3 H, 4ꢀMe); 14.90 (s, 1 H, 6ꢀOH); 7.07—7.41
(m, 6 H, H(7), H(8), H(2´), H(4´), H(5´), H(6´)); 9.34 (s, 1 H,
—CH=N); 2.68 (s, 3 H, 3´ꢀMe). IR, ν/cm–1: 1750 (αꢀpyrone),
1560 (—CH=N). EAS, λmax/nm (logε): 460 (3.37). MS, m/z
(Irel (%)): 293 (56).
6ꢀHydroxyꢀ(2´ꢀhydroxyphenylimino)methylꢀ4ꢀmethylꢀ5ꢀ2Hꢀ
1ꢀbenzopyranꢀ2ꢀone (2e). The yield was 75%, m.p. 263—264 °C,
Rf 0.72. Found (%): C, 69.20; H, 4.40; N, 4.79. C17H13NO4.
Calculated (%): C, 69.15; H, 4.41; N, 4.75. 1H NMR, δ: 6.28 (s, 1 H,
H(3)); 2.64 (s, 3 H, 4ꢀMe); 15.55 (s, 1 H, 6ꢀOH); 7.81 (m, 6 H,
H(7), H(8), H(3´), H(4´), H(5´), H(6´)); 9.58 (s, 1 H, —CH=N);
9.25 (s, 1 H, 2´ꢀOH). IR, ν/cm–1: 1690 (αꢀpyrone), 1610
(—CH=N). EAS, λmax/nm (logε): 440 (3.52). MS, m/z (Irel (%)):
295 (56).
6ꢀHydroxyꢀ4ꢀmethylꢀ5ꢀ(4´ꢀmethoxyphenylimino)methylꢀ2Hꢀ
1ꢀbenzopyranꢀ2ꢀone (2f). The yield was 71%, m.p. 193—194 °C,
Rf 0.72. Found (%): C, 69.92; H, 5.02; N, 4.47. C18H15NO4.
Calculated (%): C, 69.90; H, 4.85; N, 4.53. 1H NMR, δ: 6.37 (s, 1 H,
H(3)): 2.69 (s, 3 H, 4ꢀMe); 14.93 (s, 1 H, 6ꢀOH); 6.98 (d, 1 H,
H(7), J7,8 = 8.7 Hz); 7.37 (d, 1 H, H(8), J8,7 = 8.7 Hz); 9.34
(s, 1 H, —CH=N); 7.20—7.35 (m, 4 H, H(2´), H(3´), H(5´),
H(6´)); 3.85 (s, 3 H, 4´ꢀOMe). IR, ν/cm–1: 1720 (αꢀpyrone),
1590 (—CH=N). EAS, λmax/nm (logε): 480 (2.85). MS, m/z
(Irel (%)): 309 (65).
6ꢀHydroxyꢀ4ꢀmethylꢀ5ꢀ(2´ꢀmethoxyphenylimino)methylꢀ2Hꢀ
1ꢀbenzopyranꢀ2ꢀone (2g). The yield was 39%, m.p. 228—229 °C,
Rf 0.72. Found (%): C, 69.89; H, 5.05; N, 4.54. C18H15NO4.
Calculated (%): C, 69.90; H, 4.85; N, 4.53. 1H NMR, δ: 6.36 (s, 1 H,
H(3)); 2.69 (s, 3 H, 4ꢀMe); 15.51 (s, 1 H, 6ꢀOH); 6.98 (d, 1 H,
H(7), J7,8 = 8.7 Hz); 7.37 (d, 1 H, H(8), J8,7 = 8.7 Hz); 9.48
(s, 1 H, —CH=N); 6.36—7.39 (m, 4 H, H(3´), H(4´), H(5´),
H(6´)); 3.94 (s, 3 H, 4´ꢀOMe). IR, ν/cm–1: 1710 (αꢀpyrone),
1585 (—CH=N). EAS, λmax/nm (logε): 490 (3.53). MS, m/z
(Irel (%)): 309 (56).
J
3´,2';5´,6´ = 9.0 Hz). IR, ν/cm–1: 1740 (αꢀpyrone), 1620 (—CH=N).
EAS, λmax/nm (logε): 470 (3.23). MS, m/z (Irel (%)): 321 (60).
7ꢀHydroxyꢀ4ꢀmethylꢀ8ꢀ(2´ꢀphenylaminophenylimino)methylꢀ
2Hꢀ1ꢀbenzopyranꢀ2ꢀone (1e). The yield was 61%, m.p. 169—170 °C,
Rf 0.91. Found (%): C, 74.79; H, 5.02; N, 7.27. C23H18N2O3.
Calculated (%): C, 74.59; H, 4.86; N, 7.57. 1H NMR, δ: 6.12 (s, 1 H,
H(3)); 2.40 (s, 3 H, 4ꢀMe); 7.55 (d, 1 H, H(5), J5,6 = 8.9 Hz);
6.85 (d, 1 H, H(6), J6,5 = 8.9 Hz); 9.31 (s, 1 H, —CH=N); 14.85
(s, 1 H, 7ꢀOH); 6.21 (s, 1 H, NH); 6.92—7.42 (m, 9 H, H(3´),
H(4´), H(5´), H(6´), H(2″), H(3″), H(4″), H(5″), H(6″)). IR,
ν/cm–1: 1700 (αꢀpyrone), 1600 (—CH=N). EAS, λmax/nm (logε):
458 (3.62). MS, m/z (Irel (%)): 370 (65).
7ꢀHydroxyꢀ4ꢀmethylꢀ8ꢀ([1,1´;4´,1″]terphenylꢀ4ꢀylimiꢀ
no)methylꢀ2Hꢀ1ꢀbenzopyranꢀ2ꢀone (1f). The yield was 60%, m.p.
283—284 °C, Rf 0.61. Found (%): C, 80.68; H, 4.90; N, 3.62.
C29H21NO3. Calculated (%): C, 80.74; H, 4.87; N, 3.25.
1H NMR, δ: 6.16 (s, 1 H, H(3)); 2.43 (s, 3 H, 4ꢀMe); 6.95 (d, 1 H,
H(6), J6,5 = 9.2 Hz); 9.44 (s, 1 H, —CH=N); 14.65 (s, 1 H,
7ꢀOH); 7.35—7.77 (m, 14 H, H(5), H(2´), H(3´), H(5´), H(6´),
H(2″), H(3″), H(5″), H(6″), H(1´´´), H(2´´´), H(3´´´), H(5´´´),
H(6´´´)). IR, ν/cm–1: 1720 (αꢀpyrone), 1630 (—CH=N). EAS,
λ
max/nm (logε): 470 (3.68). MS, m/z (Irel (%)): 431 (100).
(4´ꢀChlorophenylimino)methylꢀ7ꢀhydroxyꢀ4ꢀmethylꢀ8ꢀ2Hꢀ1ꢀ
benzopyranꢀ2ꢀone (1g). The yield was 62%, m.p. 193—194 °C,
Rf 0.61. Found (%): C, 65.06; H, 3.85; N, 4.48. C17H12ClNO3.
Calculated (%): C, 65.07; H, 3.83; N, 4.47. 1H NMR, δ: 6.15 (s, 1 H,
H(3)); 2.41 (s, 3 H, 4ꢀMe); 7.60 (d, 1 H, H(5), J5,6 = 8.9 Hz);
6.93 (d, 1 H, H(6), J6,5 = 8.9 Hz); 9.34 (s, 1 H, —CH=N); 14.85
(s, 1 H, 7ꢀOH); 7.30—7.44 (m, 4 H, H(2´), H(3´), H(5´), H(6´)).
IR, ν/cm–1: 1710 (αꢀpyrone), 1605 (—CH=N). EAS, λmax/nm
(logε): 464 (3.58). MS, m/z (Irel (%)): 313 (55).
(4´ꢀChlorophenylimino)methylꢀ6ꢀhydroxyꢀ4ꢀmethylꢀ5ꢀ2Hꢀ1ꢀ
benzopyranꢀ2ꢀone (2a). The yield was 61%, m.p. 197—198 °C,
Rf 0.61. Found (%): C, 64.97; H, 3.89; N, 4.52. C17H12ClNO3.
Calculated (%): C, 65.07; H, 3.83; N, 4.47. 1H NMR, δ: 6.34 (s, 1 H,
H(3)); 2.61 (s, 3 H, 4ꢀMe); 14.45 (s, 1 H, 7ꢀOH); 6.94 (d, 1 H,
H(7), J7,8 = 9.2 Hz); 7.62 (d, 1 H, H(8), J8,7 = 9.2 Hz); 7.26 (d, 2 H,
H(2´), H(6´), J2´,3´;6´,5´ = 8.9 Hz); 7.58 (d, 2 H, H(3´), H(5´),
J3´,2´;5´,6´ = 8.9 Hz); 9.36 (s, 1 H, —CH=N). IR, ν/cm–1: 1730
(αꢀpyrone), 1550 (—CH=N). EAS, λmax/nm (logε): 460 (3.65).
MS, m/z (Irel (%)): 313 (56).
6ꢀHydroxyꢀ4ꢀmethylꢀ5ꢀ(4´ꢀmethylphenylimino)methylꢀ2Hꢀ
1ꢀbenzopyranꢀ2ꢀone (2b). The yield was 38%, m.p. 191—192 °C,
Rf 0.74. Found (%): C, 73.72; H, 5.10; N, 4.79. C18H15NO3.
Calculated (%): C, 73.72; H, 5.12; N, 4.78. 1H NMR, δ: 6.05 (s, 1 H,
H(3)); 2.43 (s, 3 H, 4ꢀMe); 14.65 (s, 1 H, 6ꢀOH); 7.09 (d, 1 H,
H(7), J7,8 = 8.9 Hz); 7.19 (d, 1 H, H(8), J8,7 = 8.9 Hz); 9.37 (s, 1 H,
—CH=N); 6.63 (d, 2 H, H(2´), H(6´), J2´,3´;6´,5´ = 9.2 Hz); 8.04
(d, 2 H, H(3´), H(5´), J3´,2´;5´,6´ = 9.2 Hz); 2.38 (s, 3 H, 4´ꢀMe).
IR, ν/cm–1: 1700 (αꢀpyrone), 1550 (—CH=N). EAS, λmax/nm
(logε): 465 (3.34). MS, m/z (Irel (%)): 293 (65).
6ꢀHydroxyꢀ4ꢀmethylꢀ5ꢀ(2´ꢀmethylphenylimino)methylꢀ2Hꢀ
1ꢀbenzopyranꢀ2ꢀone (2c). The yield was 68%, m.p. 189—190 °C,
Rf 0.71. Found (%): C, 73.70; H, 5.10; N, 4.82. C18H15NO3.
Calculated (%): C, 73.72; H, 5.12; N, 4.78. 1H NMR, δ: 6.37 (s, 1 H,
H(3)); 2.44 (s, 3 H, 4ꢀMe); 15.11 (s, 1 H, 6ꢀOH); 2.70 (s, 3 H,
4ꢀMe); 6.94—7.41 (m, 6 H, H(7), H(8), H(3´), H(4´), H(5´),
H(6´)); 9.33 (s, 1 H, —CH=N). IR, ν/cm–1: 1710 (αꢀpyrone),
6ꢀHydroxyꢀ4ꢀmethylꢀ5ꢀ(2´ꢀphenylaminophenylimino)methylꢀ
2Hꢀ1ꢀbenzopyranꢀ2ꢀone (2h). The yield was 62%, m.p. 246—247 °C.
Rf 0.72. Found (%): C, 74.62; H, 5.02; N, 7.44. C23H18N2O3.
Calculated (%): C, 74.59; H, 4.86; N, 7.57. 1H NMR, δ: 6.38 (s, 1 H,
H(3)); 2.69 (s, 3 H, 4ꢀMe); 14.38 (s, 1 H, 6ꢀOH); 6.94—7.41
(m, 12 H, H(7), H(8), H(3´), H(4´), H(5´), H(6´), NH, H(2″),
H(3″), H(4″), H(5″), H(6″)); 9.41 (s, 1 H, —CH=N). IR, ν/cm–1
:
1710 (αꢀpyrone), 1600 (—CH=N). EAS, λmax/nm (logε): 460
(3.48). MS, m/z (Irel (%)): 370 (66).
5ꢀ(9HꢀFluorenꢀ2ꢀylimino)methylꢀ6ꢀhydroxyꢀ4ꢀmethylꢀ5ꢀ2Hꢀ
1ꢀbenzopyranꢀ2ꢀone (2i). The yield was 68%, m.p. 276—277 °C,
Rf 0.61. Found (%): C, 78.49; H, 4.72; N, 3.76. C24H17NO3.
Calculated (%): C, 78.47; H, 4.63; N, 3.81. 1H NMR, δ: 6.39 (s, 1 H,
H(3)); 2.72 (s, 3 H, 4ꢀMe); 14.93 (s, 1 H, 6ꢀOH); 7.24 (d, 1 H,
H(7), J7,8 = 7.9 Hz); 7.57 (d, 1 H, H(8), J8,7 = 7.9 Hz); 9.43 (s, 1 H,
—CH=N); 7.30—7.86 (m, 7 H, H(1´), H(3´), H(4´), H(5´),
H(6´), H(7´), H(8´)); 3.96 (s, 1 H, —CH2—). IR, ν/cm–1: 1700
(αꢀpyrone), 1540 (—CH=N). EAS, λmax/nm (logε): 440 (3.45).
MS, m/z (Irel (%)): 367 (55).
6ꢀHydroxyꢀ4ꢀmethylꢀ5ꢀ([1,1´;4´,1″]terphenylꢀ4ꢀylimiꢀ
no)methylꢀ2Hꢀ1ꢀbenzopyranꢀ2ꢀone (2j). The yield was 81%, m.p.

E/ZꢀIsomerization of imines
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
1995
274—275 °C, Rf 0.61. Found (%): C, 80.84; H, 4.93; N, 3.02.
C29H21NO3. Calculated (%): C, 80.74; H, 4.87; N, 3.25. 1H NMR,
δ: 6.51 (s, 1 H, H(3)); 2.75 (s, 3 H, 4ꢀMe); 14.65 (s, 1 H, 6ꢀOH);
7.25—7.88 (m, 15 H, H(7), H(8), H(2´), H(3´), H(5´), H(6´),
H(2″), H(3″), H(5″), H(6″), H(1´´´), H(2´´´), H(3´´´), H(5´´´),
H(6´´´)), 9.57 (s, 1 H, —CH=N). IR, ν/cm–1: 1750 (αꢀpyrone),
1550 (—CH=N). EAS, λmax/nm (logε): 450 (3.29). MS, m/z
(Irel (%)): 431 (100).
4. V. A. Chernoivanov, A. D. Dubonosov, V. I. Minkin, V. A.
Bren´, A. E. Lyubarskaya, Zh. Org. Khim., 1989, 25, 443
[J. Org. Chem. USSR, 1989, 25 (Engl. Transl.)].
5. O. A. Osipov, V. I. Minkin, A. D. Garnovskii, V. A. Kogan,
Azometiny [Azomethines], Ed. Yu. A. Zhdanov, RGU Publ.,
RostovꢀonꢀDon, 1967, 293 pp. (in Russian).
6. E. N. Shepelenko, V. A. Bren´, A. D. Dubonosov, A. E.
Lyubarskaya, V. I. Minkin, Khim. Geterotsikl. Soedin., 1989,
591 [Chem. Heterocycl. Compd., 1989, 24 (Engl. Transl.)].
7. A. D. Garnovskii, I. S. Vasil´chenko, Usp. Khim., 2002, 71,
1064 [Russ. Chem. Rev., 2002, 71 (Engl. Transl.)].
8. J. Griffiths, Dyes and Pigments, 1982, 3, 211.
9. J. Berthet, S. Delbaere, L. M. Carvalho, G. Vermeersch,
P. J. Coelho, Tetrahedron Lett., 2006, 47, 4903.
10. W. G. Herkstroeter, J. Am. Chem. Soc., 1973, 95, 8686.
11. A. Padwa, F. Albrecht, J. Am. Chem. Soc., 1974, 96, 4849.
12. M. J. S. Dewar, E. G. Zoebisch, E. F. Healy, J. P. Stewart,
J. Am. Chem. Soc., 1985, 107, 3902.
Authors are grateful to V. A. Ananikov for the consulꢀ
ting on the NOESY experiments performing and discusꢀ
sion of the results.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 08ꢀ03ꢀ
90016_Bel_a).
References
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Received November 16, 2006;
in revised form July 7, 2008