Benzoid-quinoid tautomerism of schiff bases and their structural analogs: LI. Photoacylotropic enaminoketones with a fluorophoric migrant

Article abstract of DOI:10.1023/A:1013143924110

The acylation of ortho-substituted (R = OH, OMe, NHTs) 2-arylaminomethylene-2,3-dihydrobenzo[b]thiophen-3-ones with (4-methyl-2-oxobenzo[b]pyran-7-yloxy)acetyl chloride results in formation of the corresponding N-acyl enaminoketones. The same reaction wit

Full text of DOI:10.1023/A:1013143924110

Russian Journal of Organic Chemistry, Vol. 37, No. 9, 2001, pp. 1318 1322. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 9, 2001,
pp. 1384 1388.
Original Russian Text Copyright
2001 by Rybalkin, Popova, Dubonosov, Shepelenko, Revinskii, Bren’, Minkin.
Benzoid Quinoid Tautomerism of Schiff Bases
and Their Structural Analogs: LI.* Photoacylotropic
Enaminoketones with a Fluorophoric Migrant**
V. P. Rybalkin, L. L. Popova, A. D. Dubonosov, E. N. Shepelenko,
Yu. V. Revinskii, V. A. Bren’, and V. I. Minkin
Research Institute of Physical and Organic Chemistry, Rostov State University,
pr. Stachki 194/2, Rostov-on-Don, 344090 Russia
Received December 22, 2000
Abstract The acylation of ortho-substituted (R = OH, OMe, NHTs) 2-arylaminomethylene-2,3-dihydro-
benzo[b]thiophen-3-ones with (4-methyl-2-oxobenzo[b]pyran-7-yloxy)acetyl chloride results in formation of
the corresponding N-acyl enaminoketones. The same reaction with 2-(2-hydroxyphenylaminomethylene)-2,3-
dihydrobenzo[b]thiophen-3-one gives rise to a tautomeric mixture of acyloxyphenyl and N-acyl enamino-
ketone isomers. Irradiation at a wavelength corresponding to the absorption region of the N-acyl enamino-
ketone isomer leads to N O acyl group transfer to afford O-acylated isomer. The complex formation ability
of all tautomeric forms with respect to Zn2+ and Ni2+ cations and their fluorescence properties were studied.
The ability of N-acylated enaminoketones to
undergo photoinduced acylotropic rearrangements [2]
provides the possibility for creation of photochromic
fluorescent sensors for metal cations if the molecule
contains fluorescence and complexing centers [3, 4].
The present communication reports on the synthesis
and properties of ortho-substituted N-aryl Schiff bases
derived from 3-hydroxybenzo[b]thiophene-2-carbalde-
hyde and modified through introduction of a fluoro-
phoric acyl migrant, (4-methyl-2-oxobenzo[b]pyran-7-
Scheme 1.
R =
I, II, R = MeO (a), TsNH (b), HO (c).
____________
*
For communication L, see [1].
**
This study was financially supported by the Foundation of
CRDF and the Ministry of Education of the Russian Federa-
tion (grant no. REC-004) and by the Russian Foundation for
Basic Research (project no. 00-15-97320).
1070-4280/01/3709-1318$25.00 2001 MAIK Nauka/Interperiodica

BENZOID QUINOID TAUTOMERISM OF SCHIFF BASES . . . LI.
1319
Scheme 2.
R =
; R = MeO (a), TsNH (b).
yloxy)acetic acid residue. Enaminoketones Ia Ic were
obtained by condensation of 3-hydroxybenzo[b]thio-
phene-2-carbaldehyde [5] with ortho-substituted
anilines, following the procedure described in [6]. The
The structure of compounds IIa and IIb as
Z-enaminoketones is confirmed by the presence in
the IR spectra of absorption bands of the amide
1
(1710 cm ) and endocyclic carbonyl groups
1
1
(1650 cm ), as well as by a characteristic chemical
structure of Ia Ic was confirmed by the IR and H
1
1
shift of the methine proton ( 9.2 ppm) [2] in the H
NMR spectra. The IR band at 1650 cm corresponds
NMR spectrum. The enaminoketone structure of com-
pound III in crystal was derived from the IR spectrum
which contained the following absorption bands: 1760
(ester carbonyl), 1640 (exocyclic carbonyl group),
1630 (lactone carbonyl in the pyran ring), and
3350 cm (strongly broadened N H band).
Scheme 2 illustrates transformations of N-acylated
enaminoketones IIa and IIb on irradiation in solution
to the ring carbonyl involved in a strong intramole-
cular hydrogen bond. However, the set of signals
1
observed in the H NMR spectra includes two pairs
of doublets from the amine and methine protons,
indicating the presence of both E and Z isomers [7].
By the action on compounds Ia Ic of (4-methyl-2-
oxobenzo[b]pyran-7-yloxy)acetyl chloride we obtained
acylated enaminoketones IIa, IIb, and III (Scheme 1).
1
Scheme 3.
M2+ = Zn2+, Ni2+; R =
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 9 2001

1320
RYBALKIN et al.
Fig. 1. Electron absorption spectra of enaminoketone IIa
in toluene (1) before irradiation and after irradiation for
(2) 10 s, (3) 20 s, (4) 1 min, and (5) 2 min ( exc 436 nm).
Fig. 2. Electron absorption spectra of enaminoketone III
(1) in toluene, in toluene containing (2) 2.4, (3) 4.8, and
(4) 7% of DMSO, and (5) in DMSO.
[2, 8]. Structures Z-IIa and Z-IIb show a characteristic
maximum at 418 428 nm in the electron absorption
IIb with subsequent thermal N O transfer of the
acyl group and formation of O-acyl isomers IVa and
IVb which absorb at 360 370 nm. However, unlike
most previously studied analogs, the photochemical
transformation goes until equilibrium (Scheme 2)
establishes between the N- and O-acyl isomers
(Fig. 1). The state of the equilibrium strongly depends
on the solvent nature and compound structure: polar
solvents stabilize O-acyl form IVa or N-acyl form
IIb. The equilibrium constants K0 = [IV]/[II] and
Gibbs energies G0294 are given in table. In the pres-
ence of CCl3CO2H the equilibrium is displaced com-
pletely toward initial isomer Z-IIa and Z-IIb.
spectrum. Irradiation at that band (
436 nm)
exc
induces fast Z,E isomerization of compounds IIa and
Thermodynamic parameters of the equilibria involving
compounds IIa and IIb
K0
G0294, kJ/mol
Solvent
IIa
IIb
IIa
IIb
Toluene
1.04
7.3
0.001
4.7
Compound III in solution gives rise to a dynamic
equilibrium between the enaminoketone (E-III) and
N-acyl forms (Z-V) in the ground state (Scheme 3).
In nonpolar solvents, such as hexane, benzene, and
toluene, the equilibrium is displaced toward isomer
Acetonitrile
DMSO
1.63
1.03
0.07
1.15
0.001
6.3
a
a
a
Irradiation of compound IIa in DMSO at
both acylotropic N O migration and partial decomposition.
436 nm induces
exc
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 9 2001

BENZOID QUINOID TAUTOMERISM OF SCHIFF BASES . . . LI.
E-III which is stabilized by intramolecular hydrogen
1321
(a)
bond; it absorbs in the region of 470 nm (Fig. 2, 1),
which is typical of enaminoketones of the benzo[b]-
thiophene series [6]. In polar solvents like DMSO
the equilibrium shifts almost completely toward the
N-acylated form (Z-V) whose absorption maximum
is located at 432 nm (Fig. 2, 5). Assuming that the
molar absorption coefficient at
470 nm corre-
sponds to the pure tautomer E-III, mabaxout 15% of E-III
remains in the solution and 85% of Z-V appears.
The presence of N-acyl isomer Z-V in the mixture
makes it possible to effect photoinduced rearrange-
ment of Z-V into VI. In fact, irradiation of solutions
of III in polar solvents at
tion of O-acyl isomer VI which absorbs at
436 nm leads to forma-
exc
440
460 nm. However, as with compounds IIa and IIb,
the fraction of VI does not reach 100% but a multi-
component equilibrium involving all possible isomers
establishes (Scheme 3). It is difficult to estimate the
fractions of each particular isomer on the basis of
electron absorption spectra, for bands of three of them
strongly overlap. Study of the fluorescence spectra of
compound III in toluene and in DMSO, as well as
in the equilibrium mixture obtained after irradiation,
shows that only one isomer (E-III) fluoresces, in
agreement with the data of [9]. The shapes and posi-
tions of the fluorescence bands in different solvents
(b)
fl
are similar (
490 550 nm; Fig. 3), but they
strongly differ mianx intensity. Insofar as only isomer
E-III is present in toluene, the fluorescence excitation
spectrum almost coincides with the absorption spec-
trum (Fig. 3a, 3). In dimethyl sulfoxide, fluorescence
is induced by excitation in the region of 470 nm,
which corresponds (see above) to a 15% fraction of
E-III (Fig. 3b, 5).
Addition of salts M(OAc)2 to the equilibrium mix-
ture (Scheme 3) could give rise to formation of two
kinds of complexes, VII and VIII, whose absorption
and fluorescence properties should differ considerably.
However, regardless of the solvent polarity, addition
of Zn2+ or Ni2+ ions before or after irradiation resulted
in complete displacement of the equilibrium toward
complex VII. This compound is characterized by
a broadened long-wave absorption band in the region
Fig. 3. Electron absorption and emission spectra of
enaminoketone III in (a) toluene and (b) DMSO: (1, 4) ab-
sorption, (2, 5) fluorescence, (3, 6) fluorescence excitation.
fl
440 510 nm and fluorescence band at max 510 nm.
Thus we have demonstrated the possibility, in
principle, for creation of photochromic enaminoketone
systems involving a fluorophoric migrant, which are
capable of forming complexes with metal cations.
Search for fluorescent sensors in this series of photo-
chromic compounds will be continued through varia-
tion of the migrating group and creation of new
complexing centers.
EXPERIMENTAL
The electron absorption spectra were measured on
a Specord M-40 spectrophotometer. The fluorescence
spectra were obtained on a Hitachi 650-60 spectro-
fluorimeter. Solutions of compounds IIa, IIb, and III
were irradiated with a DRSh-250 mercury lamp using
a set of removable light filters. The IR spectra were
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 9 2001

1322
RYBALKIN et al.
obtained from samples dispersed in mineral oil using
a Specord 75IR spectrometer. The H NMR spectra
mp 243 C. Found, %: C 67.12; H 4.22; N 2.60.
C28H21NO6S. Calculated, %: C 67.32; H 4.24; N 2.80.
1
were recorded on a Varian Unity-300 instrument
(300 MHz) in DMSO-d6 with TMS as reference.
2-(2-Methoxyphenylaminomethylene)-2,3-di-
hydrobenzo[b]tiophen-3-one (Ia) was synthesized by
the procedure described in [6] from 2-methoxyaniline
2-[N-(4-Methyl-2-oxobenzo[b]pyran-7-yloxy-
acetyl)-N-(2-p-tolylsulfonylaminophenyl)amino-
methylene]-2,3-dihydrobenzo[b]thiophen-3-one
(IIb). Yield 79%, mp 194 C. Found, %: C 63.78;
H 4.07; N 4.16. C34H26N2O7S2. Calculated, %:
and 3-hydroxybenzo[b]thiophene-2-carbaldehyde. The C 63.93; H 4.10; N 4.39.
product was recrystallized from toluene. Yield 92%,
yellowish brown crystalline substance, mp 163 C.
Electron absorption spectrum (toluene), max, nm
2-[2-(4-Methyl-2-oxobenzo[b]pyran-7-yloxy-
acetyloxy)phenylaminomethylene]-2,3-dihydro-
benzo[b]thiophen-3-one (III). Yield 61%, mp 221 C.
Found, %: C 66.67; H 3.84; N 2.70. C27H19NO6S.
Calculated, %: C 66.79; H 3.94; N 2.89.
3
1
(
10 , l cm 1 mol ): 310 (32.7). 428 (11.6).
Found, %: C 67.64; H 4.52; N 4.76. C16H13NO2S.
Calculated, %: C 67.82; H 4.62; N 4.94.
REFERENCES
2-[2-(p-Tolylsulfonylamino)phenylamino-
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from 2-(p-tolylsulfonylamino)aniline and 3-hydroxy-
benzo[b]thiophene-2-carbaldehyde. The product was
recrystallized from 1-butanol DMF (1:1). Yield 83%,
yellow powder, mp 256 C. Electron absorption spec-
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3
1
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(4-Methyl-2-oxobenzo[b]pyran-7-yloxy)acetic
acid was synthesized by the procedure described in
[10], by alkylation of 7-hydroxy-4-methylbenzo[b]-
pyran-2-one with chloroacetic acid. Yield 67%, color-
less crystalline substance, mp 212 C. IR spectrum, ,
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1
1
cm : 1750, 1630. H NMR spectrum, , ppm: 2.4 s
(3H, CH3), 4.72 s (2H, CH2), 6.12 s (1H, 3-H), 6.88 s
(1H, 8-H), 6.94 d (1H, 6-H), 7.63 d (1H, 5-H, J =
6 Hz). Found, %: C 61.31; H 4.19. C12H10O5. Cal-
culated, %: C 61.54; H 4.30. The acid was converted
into the corresponding chloride by the procedure
described in [11]. (4-Methyl-2-oxobenzo[b]pyran-7-yl-
oxy)acetyl chloride was isolated as a colorless crys-
talline substance. Yield 78%, mp 142 C.
7. Rybalkin, V.P., Bren’, V.A., Minkin, V.I.,
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Acylated enaminoketones IIa, IIb, and III. A so-
lution of 1.2 mmol of (4-methyl-2-oxobenzo[b]pyran-
7-yloxy)acetyl chloride in a minimal amount of
anhydrous acetonitrile was added to a warm solution
of 1 mmol of enaminoketone Ia Ic in a minimal
amount of anhydrous acetonitrile in the presence of
triethylamine. The yellow precipitate was separated
and recrystallized from appropriate solvent.
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2-[N-(4-Methyl-2-oxobenzo[b]pyran-7-yloxy-
acetyl)-2-methoxyphenylaminomethylene]-2,3-di-
hydrobenzo[b]thiophen-3-one (IIa). Yield 66%,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 9 2001