Synthesis and spectral properties of 3-(2-aryl-5-methyl-1,3-oxazol- 4-yl)-2-(2,5-dimethylthiophen-3-yl)cyclopent-2-en-1-ones

Article abstract of DOI:10.1016/j.mencom.2014.09.010

Alkylation of ethyl 4-(2,5-dimethylthiophen-3-yl)-3-oxobutanoate with 2-aryl-4-bromoacetyl-5-methyl-1,3-oxazole followed by decar-boxylation-cyclization of the intermediate diketo ester affords the title photochromic compounds. Relationship between the fl

Full text of DOI:10.1016/j.mencom.2014.09.010

Mendeleev
Communications
Mendeleev Commun., 2014, 24, 277–279
Synthesis and spectral properties of 3-(2-aryl-5-methyl-1,3-oxazol-
4-yl)-2-(2,5-dimethylthiophen-3-yl)cyclopent-2-en-1-ones
Andrey G. Lvov,a Valerii Z. Shirinian,*a Alexey M. Kavunb and Mikhail M. Krayushkina
a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian
Federation. Fax: +7 499 135 5328; e-mail: shir@ioc.ac.ru
b Higher Chemical College, Russian Academy of Sciences, 125047 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2014.09.010
Alkylation of ethyl 4-(2,5-dimethylthiophen-3-yl)-3-oxobutanoate with 2-aryl-4-bromoacetyl-5-methyl-1,3-oxazole followed by decar-
boxylation–cyclization of the intermediate diketo ester affords the title photochromic compounds. Relationship between the fluorescent
characteristics, thermal stability of photoinduced form as well as quantum yields of photochemical reactions and the structure of these
compounds was evaluated.
Photochromic diarylethenes have been intensely studied to develop
Br
O
a new photoswitching materials and molecular devices.1–4 For
the practical application of diarylethenes it is necessary to improve
some of their physical and chemical properties, such as thermal
stability of photoinduced form and quantum yields of both photo-
reactions.1 To reach this goal, relationship between the properties
of photochromic molecules and their structures is to be derived.
Typically, the photochromic reactivity of diarylethenes mainly
depends on the type of heteroaryl groups and their electron
donor/acceptor substituents.5–8 Furthermore, much attention was
paid to establishing a relationship between these properties and
the substituents in the ethene bridge.9–13 However, each class
of photochromic compounds is specified by its own distinctive
correlation between the structure and spectral characteristics.
Moreover, small modifications in the structure can induce con-
siderable changes in these properties. For these reasons, target syn-
thesis of photoswitching compounds with predefined parameters
remains quite difficult issue.
Recently, we proposed a new unique class of photochromic
diarylethenes of cyclopentenone series comprising derivatives
of benzene, thiophene, benzothiophene and naphthalene as aryl
residues.11 The presence of a carbonyl group in the ethene bridge
contributes to various chemical modifications. The effect of
substituents in the cyclopentenone ring on various photochromic
characteristics has been studied.11,14–16 Furthermore, the first
unsymmetrical 2,3-diarylcyclopent-2-en-1-ones containing thio-
phene and oxazole derivatives as aryl substituents were syn-
thesized and the dependence of the photoswitching properties on
the asymmetry of molecule has been reported.17 It was found that
such isomeric photochromic diarylethenes show different thermal
stability, absorption maxima, and quantum yields of cyclorever-
sion reaction. The purpose of this work was to study the effect of
substituents at the oxazole ring on the photoswitching charac-
teristics of unsymmetrical diarylethenes bearing two different
heterocyclic substituents, namely, thiophene and oxazole.
The target diarylethenes 4 were obtained according to the
previously developed protocol† by alkylation of keto ester 118
with bromo ketones 2 followed by cyclization in a water–alcohol
alkaline solution (Scheme 1). Oxazoles 2a–e with various substi-
tuents at the benzene ring have been synthesized by condensation
N
CO2Et
Me
R
O
2a–e
O
Na, benzene
Me
Me
S
1
CO2Et
O
O
KOH
EtOH/
H2O Me
N
N
O
O
Me
Me
Me
R
R
S
O
S
Me
Me
3a–e
4a–e
a R = Ph
b R = 4-FC6H4
c R = 4-CF3C6H4
d R = 4-(piperidin-1-yl)phenyl
e R = 4-MeOC6H4
Scheme 1
of the corresponding benzaldehydes with acetylacetone oxime
followed by reduction of the N-oxides and bromination of the
resulting ketones.
A study of spectral properties of the obtained compounds
revealed that all diarylethenes display photochromic properties.
Irradiation of their acetonitrile solution by UV light results in
isomerization to give coloured form, whereas irradiation with
visible light returns them to the initial colourless isomer. The
spectral changes during photochromic reactions of all diaryl-
ethenes are characterized by the presence of isosbestic points at
330–350 nm.‡ The typical absorption spectra of the initial form
and the photoinduced form at the photostationary state (PSS) of
diarylethenes 4a,c,d in acetonitrile are shown in Figure 1.
The results of spectral and kinetic studies of the photochromic
compounds obtained are summarized in Table 1.§ Synthesis
of diarylethenes with various substituents at the benzene ring
‡
The absorption spectra of diarylethenes 4a–e before and after UV
irradiation are provided in Online Supplementary Materials.
In order to evaluate a correlation between the structure and spectral
†
§
Experimental procedures and characterization of the target and inter-
mediate compounds as well as their H and 13C NMR spectra can be
found in Online Supplementary Materials.
properties, we have used the spectral characteristics of compound 4a
1
reported previously.17
© 2014 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2014, 24, 277–279
Table 1 Spectral and kinetic properties of diarylcyclopentenones 4a–e in acetonitrile (C = 2×10–5 mol dm–3) at 298 K.
lAmax/nma
lBmax/nma
Compound
j
A–B (313 nm)b
j
B–A (517 nm)c
tBth®erAm1/2/hd
(e/dm3 mol–1 cm–1)a
(e/dm3 mol–1 cm–1)a
4a
4b
4c
4d
4e
298 (2.60×104)
293 (1.85×104)
308 (2.11×104)
326 (3.00×104)
293 (2.86×104)
523 (0.76×104)
517 (0.83×104)
522 (0.76×104)
528 (0.93×104)
515 (0.86×104)
0.22
0.23
0.22
0.15
0.23
0.166
0.109
0.138
0.084
0.098
3200
2500
500
4700
700
aAbsorption maxima (extinction coefficients) of open-ring (A) and closed-ring (B) isomers. b Quantum yields of photocyclization under irradiation at 313 nm.
c Quantum yields of cycloreversion under irradiation at 517 nm. d Half-life times of the ring-closed isomers in the dark.
O
benzene ring on the chemical shifts of protons is insignificant
in both forms.¶ The chemical shifts of the aromatic proton of the
thiophene ring of diarylethenes 4a–e in open form are observed
at d 6.54 ppm, whereas those of the cyclic isomer are observed
at d 6.82–6.86 ppm. While the effect of electron-donating and
electron-withdrawing substituents on the chemical shifts of thio-
phene ring and methyl group protons is very small, the downfield
shift by 0.28–0.32 ppm of the thiophene protons in cyclic form
relative to that of the open form is quite unusual and is not typical
of diarylethenes. At the same time, the signals of methyl group
protons in the thiophene ring in cyclic form are shifted upfield
relative to those of the open form, which is logical and explainable.
Such shift of the signal of the proton of the thiophene ring in
cyclic form can be only explained by a deshielding effect of closely
spaced carbonyl group,17 as a thiophene ring with formation of a
cyclic form loses its aromaticity, and this in turn, on the contrary,
should result in an upfield shift.
O
UV
N
N
VIS
Me
Me
Me
Ar
Me
Ar
S
O
S
O
Me Me
Closed form
Open form
0.7
0.6
0.5
0.4
0.3
0.2
4a (initial form)
4a (PSS)
4c (initial form)
4c (PSS)
4d (initial form)
4d (PSS)
0.1
0.0
The efficiency of light energy utilization measured as the
quantum yield of the photochemical reaction is an important
characteristic of photochromic diarylethenes.19–21 Measurement
of the quantum yield of the cyclization reaction of the compounds
obtained indicates that replacement of a hydrogen atom by
fluorine one or trifluoromethyl or methoxy group nearly does not
affectthisvalue. Ontheotherhand, introductionofadialkylamino
group results in a considerable decrease in cyclization efficiency
(by 7%). The unexpected result was obtained for the quantum
yield of the cycloreversion (ring-opening) reaction. Introduction
of any substituent leads to a decrease of its value relative to that
of unsubstituted photochrome. Some interesting patterns can be
observed by comparing the values of cycloreversion quantum
yields and extinction coefficients, in particular, it was identified
that an increase in the extinction coefficient of cyclic form results
in a decrease in the quantum yield of the reverse photochromic
reaction. In fact, diarylethene 4d that has the largest extinction
coefficient (9300 dm3 mol–1 cm–1) exhibits also the smallest
cycloreversion efficiency (8.4%), whereas diarylethenes 4a and
4c with extinction values of 7600 dm3 mol–1 cm–1 manifest high
yields, 16.6 and 13.8%, respectively.
250 300 350 400 450 500 550 600 650 700
l/nm
Figure 1 Absorption spectra of diarylethenes 4a,c,d (C ~ 2.5×10–5 mol dm–3
in MeCN) before and after UV irradiation (l = 313 nm) at 293 K.
allowed us to examine in detail their effect on the absorption
maxima and the extinction coefficients of the open and cyclic
forms. The introduction of a fluorine atom (compound 4b) or
methoxy group (compound 4e) at the para position of the benzene
ring results in a slight hypsochromic shift (by 5 nm) of the
absorption maxima of the open form relative to that of diaryl-
ethene bearing an unsubstituted benzene substituent. However, the
extinction coefficient decreases in the former case and increases
in the latter case. A decrease in extinction is a result of the
introduction of an electron-withdrawing trifluoromethyl group
(compound 4c), but in this case the absorption maximum shifts
by about 10 nm to the long wavelength side. The largest batho-
chromic shift (by 28 nm) is caused by the introduction of an
electron-donating dialkylamino group (compound 4d) that also
accompanied by an increase in absorption band intensity.
The absorption maxima of the photoinduced form of the
diarylethenes 4 lie in the green region of the optical spectrum
(515–528 nm). The effect of the trifluoromethyl group (com-
pound 4c depriving of electron pair conjugated with the benzene
ring) on the absorption maximum of the photoinduced form is
very weak, whereas the presence of fluorine substituent (com-
pound 4b) leads to a hypsochromic shift by 6 nm. In the case
of compounds with electron-donating substituents, a shift of absorp-
tion maximum to the blue spectral region is observed for methoxy
derivative 4e (hypsochromic shift by 6 nm), whereas for piperidino
derivative 4d this shift is bathochromic. The presence of substi-
tuents with two unshared electron pairs leads to increasing the extinc-
tion coefficient of the photoinduced form (compounds 4b,d,e),
whereas similar parameters of diarylethenes 4a and 4c do not differ.
An 1H NMR study of the open and cyclic forms of the diaryl-
ethenes obtained has shown that the effect of substituents at the
The thermal stability of the photoinduced form is yet another
important photoswitching property of diarylethenes. We have
found that replacement of a hydrogen atom in 4a by a dialkyl-
amino group (4d) provides a considerable increase in half-life
period (from 3200 to 4700 h, Table 1). On the other hand, the
introduction of miscellaneous substituents reduces thermal stability,
down to 500 h in the case of diarylethene 4c.
Among all obtained compounds, only diarylethene 4d with a
strong electron-donating dialkylamino group possesses fluorescent
properties. This characteristic of the photochromic diarylethenes
is important for the development of non-destructive readout
¶
The characteristic signals in the 1H NMR spectra of forms A and B of
diarylethenes 4a–e, as well as the full NMR spectra of compound 4e
before and after UV irradiation are available in Online Supplementary
Materials.
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Mendeleev Commun., 2014, 24, 277–279
6 S. Kobatake and Y. Terakawa, Chem. Commun., 2007, 1698.
memory elements.1,4 The open form of compound 4d emits light
at 570 nm under UV irradiation at the absorption maximum of
initial form A (326 nm).†† The Stokes shift is rather large and
amounts to 13131 cm–1, apparently due to competition between
the emission and photoswitching processes. UV irradiation leads
to the formation of a photoinduced isomer and an almost three-
fold decrease in emitted light intensity. Irradiation with visible
light results in formation of the initial form of the diarylethene
and recovery of starting fluorescence; this process can be repeated
several times.
In conclusion, we have prepared novel unsymmetrical photo-
chromic diarylethenes of the cyclopentenone series bearing
oxazole moieties with various aromatic substituents and identified
relationships between their spectral characteristics and structural
features.
7 H. Liu, S. Pu, G. Liu and B. Chen, Dyes Pigments, 2014, 102, 159.
8 S. L. Gilat, S. H. Kawai and J.-M. Lehn, Chem. Eur. J., 1995, 1, 275.
9 Z. Li, J. Xia, J. Liang, J. Yuan, G. Jin, J. Yin, G.-A. Yu and S. H. Liu,
Dyes Pigments, 2011, 90, 290.
10 H. Ogawa, K. Takagi, T. Ubukata,A. Okamoto, N.Yonezawa, S. Delbaere
and Y. Yokoyama, Chem. Commun., 2012, 48, 11838.
11 V. Z. Shirinian, A. A. Shimkin, D. V. Lonshakov, A. G. Lvov and M. M.
Krayushkin, J. Photochem. Photobiol. A, 2012, 233, 1.
12 R. Göstl, B. Kobin, L. Grubert, M. Pätzel and S. Hecht, Chem. Eur. J.,
2012, 18, 14282.
13 N. I. Makarova, P. V. Levchenko, E. N. Chepelenko, A. V. Metelitsa, V. S.
Kozyrev, V. P. Rybalkin, V. A. Bren’and V. I. Minkin, Russ. Chem. Bull.,
Int. Ed., 2011, 60, 1899 (Izv. Akad. Nauk, Ser. Khim., 2011, 1866).
14 V. Z. Shirinian, D. V. Lonshakov, A. G. Lvov, A. A. Shimkin and M. M.
Krayushkin, Photochem. Photobiol. Sci., 2013, 12, 1717.
15 V. Z. Shirinian, D. V. Lonshakov, V. V. Kachala, I. V. Zavarzin, A. A.
Shimkin, A. G. Lvov and M. M. Krayushkin, J. Org. Chem., 2012, 77,
8112.
This work was supported by the Russian Foundation for Basic
Research (grant no. 14-03-31871).
16 D. V. Lonshakov, V. Z. Shirinian, A. G. Lvov and M. M. Krayushkin,
Mendeleev Commun., 2013, 23, 268.
17 V. Z. Shirinian, A. G. Lvov, M. M. Krayushkin, E. D. Lubuzh and B. V.
Nabatov, J. Org. Chem., 2014, 79, 3440.
Online Supplementary Materials
18 A. A. Shimkin, V. Z. Shirinian, A. K. Mailian, D. V. Lonshakov, V. V.
Gorokhov and M. M. Krayushkin, Russ. Chem. Bull., Int. Ed., 2011, 60,
139 (Izv. Akad. Nauk, Ser. Khim., 2011, 134).
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2014.09.010.
19 S. Nakamura, K. Uchida and M. Hatakeyama, Molecules, 2013, 18, 5091.
20 S. Fukumoto, T. Nakashima and T. Kawai, Angew. Chem. Int. Ed., 2011,
50, 1565.
References
1 M. Irie, Chem. Rev., 2000, 100, 1685.
2 Molecular Switches, ed. B. L. Feringa and W. R. Brown, Wiley-VCH,
Weinheim, 2011.
21 S. Lee, Y. You, K. Ohkubo, S. Fukuzumi and W. Nam, Angew. Chem.
Int. Ed., 2012, 51, 13154.
3 New Frontiers in Photochromism, eds. M. Irie, Y. Yokoyama and T. Seki,
Springer, Tokyo, 2013.
4 V. Z. Shirinian, D. V. Lonshakov, A. G. Lvov and M. M. Krayushkin,
Russ. Chem. Rev., 2013, 82, 511.
5 M. Irie, K. Sakemura, M. Okinaka and K. Uchida, J. Org. Chem., 1995,
60, 8305.
†† For the modulation plot of the fluorescent properties of compound 4d,
see Online Supplementary Materials.
Received: 25th March 2014; Com. 14/4332
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