The regioselective [2 + 2] photocycloaddition reaction of 2-(3,4-dimethoxystyryl)quinoxaline in solution

Article abstract of DOI:10.1039/c9pp00028c

Herein, the [2 + 2] photocycloaddition between two molecules of (E)-2-(3,4-dimethoxystyryl)-quinoxaline (1) in an acetonitrile solution to form only one cyclobutane isomer out of eleven possible isomers is described. The observed photocycloaddition reaction is reversible; thus, the studied photocycloaddition reaction can be considered as a photoreversible photochromic process. The removal of two methoxy groups from the (E)-2-(3,4-dimethoxystyryl)quinoxaline (1) structure produces compound 2, which participates only in the photoisomerization reaction. The change of the quinoxaline residue in 1 to quinoline results in the formation of compound 3, which demonstrates the regioselective oxidization electrocyclic transformation through the formation of a novel C-N bond.

Full text of DOI:10.1039/c9pp00028c

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Saifutyarova, E. N. Gulakova, E. O. Guskova, T. M. Aliyeu, N. Shepel and Y. V. Fedorov, Photochem.
Photobiol. Sci., 2019, DOI: 10.1039/C9PP00028C.
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DOI: 10.1039/C9PP00028C
Journal Name
ARTICLE
Regioselective [2+2] photocycloaddition reaction of 2-(3,4-
dimethoxystyryl)quinoxaline in solution
Received 00th January 20xx,
Accepted 00th January 20xx
Olga A. Fedorovaa,b, Alina E. Saifutyarova,a,b Elena N. Gulakova,a Elena O. Guskova,b
Tseimur M. Aliyeu,a Nikolai E. Shepel,a Yuri V. Fedorov a
DOI: 10.1039/x0xx00000x
www.rsc.org/
The [2+2] photocycloaddition between two molecules of (E)-2-(3,4-dimethoxystyryl)-quinoxaline (1) in acetonitrile solution
to form only one cyclobutane isomer out of eleven possible was described. The observed photocycloaddition reaction is
reversible, thus, the studied photocycloaddition reaction could be considered as photoreversible photochromic process.
The removing of two methoxy group in (E)-2-(3,4-dimethoxystyryl)quinoxaline (1) structure produces compound 2 which is
able to participate in photoisomerization reaction only. The change of quinoxaline residue in 1 to quinoline one results in
the compound (3) demonstrated the regioselective oxidized electrocyclic transformation through the formation of novel C-
N bond.
in photoelectrocyclic transformation by presence of additional N-
atom in heterocyclic part (Scheme 1). This small structural change
Introduction
While the photochemistry of stilbenes and its derivatives has been causes the remarkable difference in photochemical behavior. In this
well studied,1 there have been relatively less reports on the paper we present the results of the investigation of the
photolysis of stilbene derivatives with nitrogen-bearing rings. photochemical behavior of quinoxaline derivatives 1 and 2 aimed to
Nonetheless, it has been found that hetarylphenylethenes understand the influence of the heterostilbene structure on its
(heterostilbenes) like stilbenes exhibit a diverse photochemical photochemical transformation.
behavior in solution such as reversible trans-cis isomerization,2-4
OMe
1) hv, O2
N+
OMe
OMe
cyclization of cis-heterostilbene to dihydrophenanthrene derivarive
and further oxidation to the phenanthrene heteroanalogue,5-7 and
dimerization of trans-heterostilbene to yield cyclobutane
products.8-10
2) HClO4
N
OMe
ClO4
N
,
=
,
,
N
N
,
,
N
N
N N
N
N
Recently we have demonstrated that ortho-styryl-substituted N-
heterocycles comprising one and two nitrogen atoms generally
undergo the regioselective C−N bond formation during
photocyclization, resulting in formation of the family of
(aza)benzo[c]quinolizinium derivatives.11-13 The photocyclization
reactions of styryl derivatives (Scheme 1) were carried out in air-
saturated solutions in acetonitrile or water upon irradiation with
unfiltered or filtered light of a high-pressure Hg vapor lamp. The
observation showed that under photolysis conditions the
compounds also demonstrated trans-cis isomerization, but did not
undergo a [2+2] photocycloaddition reaction resulting in the
formation of the intermolecular dimers.
3
( )
N+
OCH3
OCH3
N
N+
OCH3
OCH3
N+
N
OCH3
OCH3
39%
57%
78%
N+
N+
OCH3
N+
OCH3
OCH3
OCH3
OCH3
OCH3
50%
3a
, 70%
45%
N
OMe
OMe
N
OMe
OMe
N
N
At the same time, the study of the photolysis of (E)-2-(3,4-
dimethoxystyryl)quinoxaline (compound 1 in Scheme 1) showed
that photocyclization reaction has not been observed in this case.
Compound 1 differs from compound 3 which effectively participates
N
2
3
1
Scheme 1 Photocyclization reaction of the series of hetarylphenylethenes and
structures of the compounds 1 and 2 studied in the present paper.
a. A. N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of
Sciences, 119991, Vavilova str. 28, Moscow, Russia, E-mail: fedorova@ineos.ac.ru
D. Mendeleev University of Chemical Technology of Russia, 125047, Miusskaya sqr.
9, Moscow, Russia† Footnotes relating to the title and/or authors should appear
here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Results and discussion
Synthesis of the compounds 1 and 2
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R
14
N
Synthesis of the compound 1 has been described earlier in
.
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DOI: 10.1039/C9PP00028C
h(365 nm)
h(365 nm)
N
Compound 2 has been prepared by using the similar method (see
R
R
N
N
Experimental part).
R
Z-1a,2a
1,2
E-
Optical properties
R =OMe
R =OMe
1a
2a
)
R =OMe ( ), H (
The absorbance and fluorescence of both studied compounds 1 and
2 were measured. The absorption shift in acetonitrile caused by the
presence of MeO substitutions can be seen in Fig. 1. The λmax varies
from 365 nm to 380 nm when going from 2 to 1. Optical properties
of the dyes are presented in Table 1. The stilbenes 1 and 2 show
intensive fluorescence (Figs. S1, S2 in ESI). Fluorescence quantum
yields of the compounds are 50% and 30% accordingly. Compound
1 demonstrates the large Stock’s shift (128 nm), dye 2 in opposite
has got closed position of absorption and fluorescence spectra.
h(365 nm,
h(313 nm)
full light)
OMe
MeO
N
N
N
N
fluorescence (515nm)
OMe
1b
1.0
E-1
OMe
Scheme 2 Photochemical transformations of compounds 1 and 2
0.8
E-2
0.6
0.4
0.2
0.0
E-1
140
120
100
80
250
300
350
400
450
500
60
Wavelength, nm
Fig. 1 Absorption spectra of E-1 and E-2, МеCN, 20°С, СE-1 =CE-2 = 410-5 М.
Z-1
40
20
0
4.0
4.5
Time, min
Table 1 Optical and photochemical characteristics of compounds 1 and 2, МеCN, 20°С
Fig. 2 HPLC of 1 solution after irradiation with light 365 nm, separation on HPLC,
isolated E-1 and Z-1a; acetonitrile: water, 85:15
abs
flmax (),
365
styryl-
Compound
φ(E-Z) / φ(Z-E)
max
ex=365nm
/
cyclobutane
313
cyclobut
-
ane styryl
1
2
380
365
508(0.5)
423 (0.3)
0.1/0.45
0.03/
0.013
-
1.0
0.26/0.33
0.8
0.6
0.4
0.2
0.0
E-1
Photochemical study of the compounds 1 and 2
Irradiation of the acetonitrile solutions of stilbene derivatives 1 and
2 with filtered light (λ = 365 nm, 405nm or 313nm) of high pressure
mercury lamp resulted in the decrease of the absorption intensities
along with a slight blue shift of the absorption maxima that is
indicative for the formation of photostationary mixtures of E- and Z-
isomers of 1 and 2 (Scheme 2, Figures S3 and S4, ESI). More
effective formation of Z-isomer has been found upon irradiation by
365 nm light. For compound 2, the NMR observation indicates that
photostationary mixture contains E-isomer with coupling constant
of C=C double bond equal 3J = 16.53 Hz and Z-isomers whose
coupling constant of C=C double bond is 3J = 12.08 Hz (Figs. S5,S6 in
ESI). In case of dye 1 even short irradiation with light at 365nm
shows the formation of two photoproducts, namely Z-isomer 1a
and cyclobutane 1b (Scheme 2, Figs. S7, S8 in ESI). The isolation of
Z-isomer was done by using HPLC (Fig. 2) what allows the obtaining
of the absorption spectrum of pure Z-1a isomer (Fig. 3). The
quantum yields of photoisomerization reactions were determined
by analyzing the kinetics of changes in the absorption of solutions of
dyes 1 and 2 upon irradiation with 365 nm light (see Experimental
part), their magnitudes are listed in Table 1.
Z-1a
200
250
300
350
400
450
Wavelength, nm
Fig. 3 Absorption spectra of E-1 and Z-1a, obtained by HPLC, МеCN, 20°С, С1
=
410-5
М
More prolonged photolysis of E-1 under the unfiltered light
or light at 365nm led to more pronounced and more
complicated spectral changes. At first, a fast decrease of the
absorption intensities of the long wavelength absorption
bands and a slight blue shift of the absorption maxima were
observed as it was found for E-Z-photoisomerization (Fig. 4). In
case of E-2 the irradiation by unfiltered light causes only E-Z-
photoisomerization, no other photoprocesses were found (Fig.
2 | J. Name., 2012, 00, 1-3
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S5 in ESI). Prolonged irradiation of E-1 resulted in substantial
ARTICLE
According to the NMR observation, the main photochemical
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decrease of the long wavelength absorption band in the process was the formation of the only one cyclobutane derivative
DOI: 10.1039/C9PP00028C
spectral region at 380 nm indicating destruction of the 1b, as revealed by the presence of two doublets in a characteristic
conjugated chromophoric system (Fig. 4). At the same time, region of the cyclobutane ring protons (5.0-5.3 ppm) (13C, COSY
new significantly blue-shifted band appeared at 320 nm that is spectra of 1b are presented in ESI, Figs. S13 and S14). Only side
the characteristic absorption region of non-conjugated process detected was the E-Z-isomerization of the initial
aromatic and heteroaromatic fragments (Fig. 4). The photolyt compounds that is in agreement with the steady-state optical
was analyzed by HPLC and it was shown that the process was spectroscopy data (Fig. 6).
completed in 7 min (Fig. S10 in ESI). To explain this
observation, we assumed that E-1 undergoes the [2+2]
OMe
OMe
0.100
MeO
3
0.095
photocycloaddition reaction of the ethylene double bonds
3
0.090
b'
a'
N
N
a
6
resulting in the formation of cyclobutane derivatives 1b, in
which conjugation between aromatic rings is disrupted by the
cyclobutane ring closure (Scheme 2). The photolysis product
was purified by HPLC, its absorption spectrum is presented in
Fig. S11 in ESI, NMR spectrum in Fig. 6b in comparison with
those of initial E-1 (Fig. 6a). The quantum yield of direct
cycloaddition reaction was determined upon irradiation of
solution containing 10-3M of E-1 by 365nm light. The
cycloreversion proceeds at irradiation with 313nm light.
Kinetic studies allowed us to determine the value of the
quantum yield of reverse photocycloaddition reactions (Table
1). The kinetic curve of reversible reaction is shown in Fig. 5.
HPLC confirming that the main products formed during
photolysis of 1b are E-1 and Z-1a is presented in Fig. S12 in ESI.
0.085
0.080
0.075
0.070
0.065
0.060
0.055
0.050
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0
N
N
7
b
12
16
15
12
OMe
8
9
OMe
15
3.85
3.80
3.75
3.70
3.65
3.60
3.55
3.50
3.45
3.40
3.35
3.30
3.25
3.20
Chemical Shift (ppm)
6
16
7
8
9
a, a'
b, b'
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
Chemical Shift (ppm)
0.24
3
OMe
0.23
0.22
0.21
0.20
0.19
0.18
0.17
0.16
0.15
0.14
0.13
0.12
0.11
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
6
N
3
7
8
b
12
OMe
OMe
N
a
0.45
9
16
15
0.40
0 min
a, 12
0.35
15
0.30
0.25
0.20
0.15
b
3.95
3.90
3.85
3.80
Chemical Shift (ppm)
7, 8
6, 9
16
0.10
9 min
0.05
0.00
-0.01
250
300
350
400
450
500
9.1
9.0
8.9
8.8
8.7
8.6
8.5
8.4
8.3
8.2
8.1
8.0
7.9
7.8
7.7
7.6
7.5
7.4
7.3
7.2
7.1
7.0
6.9
Chemical Shift (ppm)
Wavelength, nm
Fig. 6 1Н NMR of E-1 (a) and cyclobutane 1b (b) in CD3CN, Bruker, 400MHz
Fig. 4 Spectral changes during photolysis of E-1 in acetonitrile under irradiation
with 365nm light during 9 min, С1 = 410-5
М
Jab =9.61Hz
Jab' =7.55Hz
NOESY:
Ha'
B
Hb'
A
0.8
0.6
0.4
0.2
0.0
1.0
A
Hb
Ha
B
1b + E-1 + Z-1
0.8
0.6
0.4
0.2
0.0
r-ctt
5.30
5.25
5.20
5.15
5.10
5.05
5.00
4.95
Chemical Shift (ppm)
1b
0
50
100
150
200
Fig. 7 1H NMR spectra (aliphatic part) of cyclobutane 1b in CD3CN, Bruker,
600MHz (a) and interactions of nucleus observed in NOESY spectrum are shown
in the square
Time, min
The relative positions of the protons in the cyclobutane
structure can be identified with the aid of the two-dimensional
NMR NOESY spectroscopy, in which the cross-peaks in 2D spectra
depends on the distance between the corresponding nuclei (Fig. 7
300
400
Wavelength, nm
500
Fig. 5 Spectra of 1b and photostationary mixture (1b+E-1+Z-1a) formed after and Fig. S15 in ESI).15 In the obtained structure cyclobutane isomer
photolysis of 1b during 200 min; inserted picture shows spectral changes during
photolysis of 1b in acetonitrile at λ=313 nm, С1 = 4*10-5
М
has strong through-space interactions between protons H-12, H-15
and H-3 with protons of cyclobutane what means that these
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protons are close to each other, as they are located on the same one molecule with acceptor quinoxaline residue of tVhieewoAtrthicelerOonnlinee.
side of the cyclobutane ring (Fig. 7). Moreover, the signals for the Such structure could play important role in the proper arrangement
DOI: 10.1039/C9PP00028C
protons of the cyclobutane fragment of the resulting photoadduct of molecules in dimer assembly allowing the occurrence of
appear as a symmetric AA′BB′ spin system with the vicinal coupling photocycloaddition reaction. Aimed to find the dimerization in
constants 3Jab = 3Ja′b′ = 9.61 and 3Jab′ = 3Ja′b = 7.55 Hz (Fig. 7). It is also solution we analyzed the influence of molecular concentration on
indicative of a transoid geometry of Ha and Hb protons in 1b.16 Thus, fluorescence spectra. In case of the formation of styryl eximers,
we can conclude about formation of r-ctt cyclobutane dimer shoulder in long-wavelength region of fluorescence should be
(Scheme 3). Similar structure of cyclobutane derivative with head- observed. Unfortunately, there were no remarkable changes in
to-tail arrangement of heterocyclic and phenyl fragments formed fluorescence spectra upon increasing of concentration of styryl 1
upon photolysis of crown-ether styryl dye was found to (Fig. S1 in ESI). Also no cross-peaks between aromatic and
demonstrate closed values of vicinal coupling constants.17
heteroaromatic parts of molecules in NOESY spectrum of 1
For a better understanding of the remarkable regioselectivity of confirming the preorganization of 1 in dimers were found (Fig. S9 in
the studied reaction, quantum chemical calculations were carried ESI).
out. Thus, geometry optimization (PM6, MOPAC) of cyclobutane
1
2
3
4
and electrocyclic poducts showed that electrocyclic product is 55
kJ/mol less stable in comparison with cyclobutane which completely
matches with the experimental regioselectivity of this reaction (see
Fig. S17 in ESI).
B
B
A
B
B
A
B
A
B
B
A
A
A
A
A
A
A
B
A
B
B
Intermolecular PCA reactions in solution are usually inefficient,
due to short excited state lifetimes, and afford mixtures of
cyclobutane isomers.18 For the hetarylphenylethene derivaives it
has been demonstrated that the regioselectivity and
stereospecificity of [2+2] photocycloaddition can be controlled in
solution and in solid state in the different ways.19 Thus, the [2+2]
photocycloaddition of the crown-derivatized styrylheterocycles
takes place only in their supramolecular dimeric complexes, in
which a favorable arrangement of the reactive C=C double bonds is
provided.20 A catalytic amount of HCl plays a key role in enhancing
the [2+2] photocyclization reactions between (Z)- and (E)-4-
styrylpyridines to give r-cct and r-ctc cyclobutane dimers through a
cation– interaction.21 Similar approach has been applied for
dimerization of (E)-styrylthiazoles.8 The combination of HCl and
cucurbituril template was used to decrease the amount of
cyclobutane products in photocyclization of styrylpyridine
derivatives.22 Another templates for photocycloaddition reaction
have been described in23. Chemists from Japan explored the
viability of a new method for the synthesis of silicon-containing
A
A
5
6
7
8
B
B
B
B
B
A
A
A
B
10
B
9
11
A
B
B
A
A
A
A
B
A
B
B
Scheme 3 Possible isomeric cyclobutane derivatives that can be formed from 1,2-
disubstituted ethenes upon [2+2] cycloaddition; in a frame the isolated isomer is
presented
The analysis of the photolysis showed that the observed
photocycloaddition reaction depends on the concentration of E-1 in
acetonitrile solution. Thus, at concentration of E-1 lower then 10-5M
the photocyloaddition did not observe after 13 min of irradiation,
whereas, upon the irradiation of solution with concentration of E-1
10-3M or higher, the photocycloaddition was fully completed in 7
min. The high concentration of ethylene components can provide
easer their interaction or formation of dimers with appropriate to
photocycloaddition mutual arrangement of stilbene molecules.
Photolysis reaction of E-1 in the solid state did not produce any
cyclobutane derivatives. While X-ray crystal structure of E-1 has
been obtained earlier (Fig. S16 in ESI)14, the reason of the failed
photodimerization in solid state is clear. According to well-known
“topochemical postulate”,28 the double bonds of crystalline
reactants must be parallel to each other, and the center-to-center
distance of the reacting alkenes must be less than 4.2 Å apart.
These two essential criteria have not been realized in crystal
structure of E-1 (Fig. S16 in ESI).
macrocyclic
compounds,
which
utilizes
intramolecular
photocycloaddition reactions of substrates containing styrene and
stilbene
reaction
centers
tethered
by
bis
(dimethylsilylmethyl)benzene chains.24 The [2+2] cycloaddition-
cycloreversion has been realized in solid state25, for instance, by
using
benzoxazolyl)ethane.26 Also in solid state the quantitative [2+2]
photocycloaddition of crystalline trans-2,4-dichloro-6-
of
1-(N,N-dimethyl-4-benzenamino)-2-(2-
styrylpyrimidine to produce the corresponding r-ctt cyclobutane
dimer has been described in 27. In two last cases photocycloaddition
produces selectively the regio- and stereoisomer of cyclobutane
dictated by the molecular packing of the alkenes in the crystal. At
the same time, authors mentioned that in solution, styrylpyridines,
both as free bases and as various pyridinium salts, produce low
yields of mixture dimers upon irradiation.
Earlier it was shown that the irradiation of hetarylphenylethenes
containing two substituents in 3rd and 4th positions of aromatic ring
can lead to the formation of 11 possible isomers of
cyclobutanes.20,28 The similar situation is possible in our case
(Scheme 3). But the photolysis experiments demonstrated the
formation of only one cyclobutane isomer out of eleven possible.
This fact points on the stereospecific formation of cyclobutane
derivative. To explain this fact, we can propose the head-to tail
dimer organization of 2-styrylquinoxaline 1 in solution by dipole
interaction between the donor phenyl ring with methoxy groups of
Experimental
Materials and methods
All reagents and solvents were obtained from commercial sources
and used as received. For the spectroscopic (UV/Vis and
fluorescence) studies, HPLC-grade MeCN and water from a Milli-Q
ultrapure water system were used.
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studied in a 10 mm quartz cell with stirring or in a 1 mm and 0.1
mm quartz cells without stirring. The radiatiDoOnIi:n1t0e.1n0s3iVt9iie/eCws9AwPrtPeic0rlee0O04n2.l86inC6e
Physical measurements
1H-NMR spectra were recorded in CD3CN and CD3COCD3 on a Bruker 10–6 Einstein s–1 L–1 for λ = 365 nm and 9.38 10–7 Einstein s–1 L–1 for λ
400, 500 or 600 MHz, and 13C NMR spectra were recorded at 101 or = 313 nm. For calculation of the spectra of the Z-isomers of dyes 1
151 MHz at ambient temperature using 5 mm tubes. Chemical shifts and 2 by Fischer's method the solutions were irradiated at λ = 313,
were determined with accuracy of 0.01 and 0.1 ppm for H and 13C 365 and 405 nm until the photostationary states were attained.29
1
spectra, respectively, and are given relative to the residual signal of
the solvent that was used as internal reference. Spin−spin coupling
constants for the proton spectra were determined with accuracy of
0.1 Hz. The 1H NMR signal assignments were performed using COSY
and NOESY 2D NMR techniques. The 13C NMR signal assignments
were performed by means of HSQC and HMBC 2D NMR techniques.
The reaction course and purity of the final products was
followed by TLC on silica gel (DC-Fertigfolien ALUGRAM Xtra SIL60
G/UV254, MACHEREY-NAGEL). TLC was performed on silica gel on
DC-Fertigfolien, eluent: EtOAc, Hexane. Column chromatography
was conducted over silica gel (Kieselgel, particle size 40-60 µm, 60
Å, Acros Organics) on a flash chromatograph Biotage Isolera Prime
and HPLC UltiMate 3000.
To determine the quantum yields of the forward and backward
reactions of E—Z photoisomerization of dyes 1 and 2 the kinetics of
changes in the absorption of 4.010-5 M solutions of dyes upon
irradiation with 365 nm light were analyzed. For this purpose, the
program Sa3.3 (Simulation–adjustment) allowing the numerical
simulation of the concentration of the various species versus time
and the optimization of the parameter values until a good fit is
obtained was used36,37
.
The absorption spectra of the
corresponding Z-isomers were preliminarily obtained using the
Fisher's method. The quantum yield of the forward
photocycloaddition reaction of dye 1 was determined by analyzing
the kinetics of spectral changes upon irradiation of 1.010-3
M
Preparation and handling of the solutions were carried out
under red light. Photochemical reactions were carried out with a
high-pressure Hg vapor lamp (120 W) and an immersed Hg
photoreactor (125 W) with a borosilicate glass filter.
Elemental analyses were carried out in the Microanalysis
Laboratory of the A.N. Nesmeyanov Institute of Organoelement
Compounds.
solution of dye 1 with 365 nm light. The quantum yield of the
backward photocycloaddition reaction was determined by analyzing
the kinetics of spectral changes upon irradiation of 1.010-5
M
solution of cyclobutane 1b with 313 nm light. In both cases, during
the fitting procedure using the program Sa3.3, the quantum yields
of the forward and backward reaction of E—Z photoisomerization
of dye 1 were fixed as known.
Melting points were measured on Melt-temp melting point
electrothermal apparatus and were uncorrected.
Synthesis of compounds
Synthesis of (E)-2-(3,4-dimethoxystyryl)quinoxaline (1) has beens
described in14.
Steady-state optical measurements
Synthesis
of
2,2'-((1R,2R,3S,4S)-2,4-bis(3,4-
The absorption spectra were obtained on a fiber-optic AvaSpec-
2048-USB2, Avantes BV spectrophotometer or Varian-Cary 300
spectrophotometer. Fluorescence spectra were measured on a
FluoroLog-3-221 spectrofluorometer. Spectral measurements were
carried out in air-saturated acetonitrile solutions (acetonitrile of
spectrophotometric grade, water content 0.005%, Aldrich) at 20 ± 1
C°; the concentrations of the studied compounds were about from
4.010-6 M to 110-3 M. All measured fluorescence spectra were
corrected for the nonuniformity of detector spectral sensitivity.
Coumarin 481 in ethanol (φfl = 0.78) was used as a reference for the
fluorescence quantum yield measurements. The fluorescence
dimethoxyphenyl)cyclobutane-1,3-diyl)diquinoxaline (1b). Solution
of 1 in acetonitrile (10-3M) was irradiated with unfiltered light of
high pressure mercury lamp for 4 minutes. The product was
1
isolated by HPLC, eluent: H2O:CH3CN, 35:65, m.p. 79-81°C, H NMR
(MeCN-d3, 500.13 MHz, 25 °C) d: 3.53 (s, 6Н, OСН3), 3.62 (s, 6Н,
ОСН3), 4.99 (m, 2Н, H-a, H-d), 5.28 (m, 2Н, H-b, H-c), 6.65-6.67 (d,
2Н, Н-15, Н-15', 4J = 8.0), 6.81 (s, 2Н, H-12, Н-12'), 6.87-6.89 (d, 2Н,
Н-16, Н-16', 3J = 8.0), 7.74 (m, 2Н, Н-7, H-7'), 7.80 (m, 2Н, Н-7, H-7'),
7.97-7.99 (d, 2Н, Н-6, H-6', 3J = 8.0), 8.05-8.07 (d, 2Н, Н-9, H-9', 3J =
8.0), 8.62 (s, 2Н, H-3, Н-3'); 13С NMR (MeCN-d3, 500.13 MHz, 25 °C)
δ: 43.8 (2C, C-a, C-d), 46.9 (2C, C-b, C-c), 55.1 (4C, ОСН3), 111.1 (2C,
C-15, C-15'), 112.0 (2C, C-12, C-12'), 120.3 (2C, C-16, C-16'), 128.8
(2C, C-9, C-9'), 129.1 (2C, C-6, C-6'), 129.3 (2C, C-7, C-7'), 129.9 (2C,
C-8, C-8'), 132.4 (2C, C-2, C-2'), 140.8 (2C, C-10, C-10'), 141.8 (2C, C-
quantum yields were calculated by eqn (1),
1 ― 10 ― 퐴
)
∙ 푛2
푅
푆 ∙
(
∙
휑푓푙 = 휑푓푅푙
,
(1)
―퐴
2
(
)
푆푅
1 ― 10
∙ 푛푅
wherein φfl and φfl are the fluorescence quantum yields of the 5, C-5'), 146.7 (2C, C-3, C-3'), 147.6 (2C, C-13, C-13'), 148.6 (2C, C-
R
studied solution and the standard compound, respectively; A and AR 14, C-14'), 156.2 (2C, C-11, C-11'). Elemental analysis: calculated (%)
are the absorption of the studied solution and the standard for C36H32N4O4 (MW 568.44): C, 73.20; H, 5.63; N, 9.30; found C,
respectively; S and SR are the areas underneath the curves of the 73.95; H, 5.52; N, 9.58.
fluorescence spectra of the studied solution and the standard
Synthesis of (E)-2-styrylquinoxaline (2). A solution of 2.8 mmol
respectively; and n and nR are the refraction indices of the solvents of 2-methylquinoxaline, 2.10 mmol of benzaldehyde, 1.03 mmol of
for the substance under study and the standard compound. The piperidine, and 1.50 mmol of acetic acid in toluene was kept in an
quantum yields were calculated using corrected fluorescence inert atmosphere at 115°C for 48 h, and then the reaction mixture
spectra.
was evaporated in a vacuum. The residue was purified with column
The light intensity was measured by a Nova P/N 7Z01500 chromatography (SiO2, eluent hexane–ethyl acetate, 2:1). The
power meter equipped with 3A-FS P/N 7Z02628 thermal product was recrystallized from methanol to yield 0.10 g (20%),
power/energy measurement sensor . The photochemical m.p. 102-105°C, lit. 106-107.5oC.30 1H NMR (acetone-d6, 400.13
transformations were induced by irradiation of acetonitrile MHz, 25 °C): 7.39 (t, 1Н, Н-4`), 7.47 (t, 2H, H-3`, H-5`), 7.55-7.59 (d,
solutions of compounds 1 and 2 with a high pressure mercury lamp 1H, H-а, J=16.0), 7.78 (t, 1H, H-8), 7.78-7.80 (d, 2H, H-2`, H-6`,
(DRK-120, 120 W). Particular lines of the mercury lamp spectrum J=8.0), 7.84 (t, 1H, H-7), 8.03-8.05 (d, 1H, H-6, J=8.0), 8.04-8.08 (d,
with λ = 313, 365 and 405 nm were isolated by glass filters from the 1H, H-b, J=16.0), 8.05-8.07 (d, 1H, H-9, J=8.0), 9.19 (s, 1H, H-3); 13С
standard set of color optical glasses. The photoprocesses were NMR (acetone-d6, 400.13 MHz, 25 °C) δ: 125.4 (C-a), 127.4 (3C, C-
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
2', C-6', C-8), 127.9 (C-10), 128.9 (2C, C-3', C-5'), 129.1 (C-4'),
129.2 (2C, C-6, C-9), 130.2 (C-7'), 136.1 (C-b), 136.3 (C-1'),
145.1 (C-3), 150.8 (2C, C-2, C-5). Elemental analysis: calculated (%)
for C16H12N2 (MW 232.29): C, 82.73; H, 5.21; N, 12.06; found C,
82.85; H, 5.30; N, 12.03.
Acknowledgements
Yu.V. Fedorov thanks Russian Science Foundation (grant 19-43-
04127).
View Article Online
DOI: 10.1039/C9PP00028C
Notes and references
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Conclusions
Here, we reported the [2 + 2] photocycloaddition between two
molecules of (E)-2-(3,4-dimethoxystyryl)quinoxaline (1) to
form the associated r-ctt cyclobutane dimer which was
isolated and identified. The observed photocycloaddition
reaction is reversible. The direct photodimerization occurs
upon irradiation with 365nm light, the reversible
phototransformation into the initial styryl derivative proceeds
upon the irradiation with 313 nm light. From our knowledge
the described reaction of the photolysis of E-1 is the first
example of the regioselective and stereospecific formation of
cyclobutane derivative without applying of any methods of
preorganization of molecules in dilute solution (metal ions,
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(Scheme 3) showed the effect of structural changes on the way
of phototransformation. Thus, removing of two methoxy
group in (E)-2-(3,4-dimethoxystyryl)quinoxaline (1) structure
produces compound
2 which is able to participate in
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N
OMe
OMe
N
OMe
OMe
N
N
N
2
3
1
h(313nm)
h(full light)
h(full light)
h(313nm)
h(full light)
regio- and steroselective
photocycloaddition
E-Z photoisomerization
selective C-N electrocyclization
Scheme 3 The illustration of dependence of photochemical
way of transformation on heterostilbene structure.
The observed photocycloaddition reaction of E-1 could be
considered as photoreversible photochromic process.31 Also it
provides the easy way for obtaining of cyclobutane derivative.
Cyclobutanes are important synthetic intermediates providing
atom-economic one-step transitions from simple to complex
structures that is especially important in the total synthesis of
natural products and other intricate molecules.32,33 The
majority of naturally occurring cyclobutane derivatives
demonstrates remarkable biological activity making them
promising lead structures for the creation of novel anticancer,
antibacterial and fungicidal drugs.34, 35
Conflicts of interest
There are no conflicts to declare.
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Regioselective [2+2] photocycloaddition reaction of 2-(3,4-
View Article Online
DOI: 10.1039/C9PP00028C
dimethoxystyryl)quinoxaline in solution
Olga A. Fedorova, Alina E. Saifutyarova, Elena N. Gulakova, Elena O. Guskova,
Tseimur M. Aliyeu, Nikolai E. Shepel, Yuri V. Fedorov
OMe
MeO
h(313 nm)
N
N
N
N
OMe
OMe
h(365 nm,
full light)
N
N
E-
OMe
OMe
1.0
2-(3,4-dimethoxystyryl)quinoxaline
0.8
0.6
0.4
0.2
0.0
cyclobutane
300
400
500
wavelength, nm
The [2+2] photocycloaddition between two molecules of (E)-2-(3,4-dimethoxystyryl)-
quinoxaline (1) in acetonitrile solution is the first example of the regioselective and
stereospecific formation of cyclobutane derivative without applying of any methods of
preorganization of molecules in dilute solution.