Highly efficient and regiospecific photocyclization of 2,2′-diacyl bixanthenylidenes

Article abstract of DOI:10.1039/c1ob05072a

In contrast to the reversible photochemistry of the 2,2′-substituted bixanthenylidenes (1a-f), the photocyclization of 2,2′-diacyl bixanthenylidenes (1g-j) reveals an irreversible process where the initial cyclic intermediate C(E) can undergo a rapid [1,1

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PAPER
Highly efficient and regiospecific photocyclization of 2,2¢-diacyl
bixanthenylidenes†
Mao Mao, Qing-Qing Wu, Ming-Guang Ren and Qin-Hua Song*
Received 13th January 2011, Accepted 14th February 2011
DOI: 10.1039/c1ob05072a
In contrast to the reversible photochemistry of the 2,2¢-substituted bixanthenylidenes (1a–f), the
photocyclization of 2,2¢-diacyl bixanthenylidenes (1g–j) reveals an irreversible process where the initial
cyclic intermediate C(E) can undergo a rapid [1,11] hydrogen shift to form stable isomer C¢(E) in a
degassed solution, which cannot revert to the starting compound, so giving a highly efficient and
regiospecific photocyclization.
been well established by means of flash photolysis over a wide
Introduction
temperature range and steady-state methods in low temperature,
and these conformers are unstable in room temperature.6b,7 Z, E
isomerization, with a low activation energy of 2,2¢-disubstituted
bixanthenylidenes, occurs easily at room temperature, and the ratio
of Z/E in solution depends on the bulkiness of the 2 and 2¢
substituents.8 However, few reports on photochemical reactions
such as the photocyclization of bixanthenylidenes are found.7d,9 In
the presence of iodine and air, bixanthenylidene can be converted
photochemically to helixanthen (2) undergoing a photocyclization
to generate a labile photoisomer C. The dihydro-intermediate can
either revert thermally or photochemically to 1, or be oxidized to
2.7d,9 Herein, we report the conclusive mechanism for rationalizing
the regioselectivity and efficiency in the photocyclization of
synthesized 2,2¢-substituted bixanthenylidenes (1a–j).
Bistricyclic aromatic enes (BAEs) are attractive substrates for
the study of the conformational behavior and dynamic stereo-
chemistry of overcrowded polycyclic aromatic enes and for the
interplay of strain and delocalization effects.1 Thermochromic2
and photochromic2a,3 BAEs have been extensively investigated,
and serve as candidates for chiroptical molecular switches and
molecular motors.4
The phenomenon of thermochromism in BAEs has been
attributed to a unimolecular equilibrium between two distinct
and interconvertible conformers:5 a colorless or yellow ambient-
temperature form A, and a deep-blue or deep-green high-
temperature form B, with enthalpy differences between them of
4–30 kJ mol-1. The B isomer is formed from the triplet state A.6 The
form A was identified as the ground state, anti-folded or unevenly
anti-folded conformer, and the form B was the thermochromic, a
low energy local minimum twisted conformer.
Results and discussion
The bixanthenylidenes in aerated THF solutions were irradiated
with a 500 W Xe lamp to generate the corresponding helixanthens
2, which were one or two regioisomeric helixanthens, 2(E) and 2(Z)
(Scheme 1), but another possible kind of regioisomeric product
2(Z¢), from cyclization between the 1- and 1¢-positions, was not
found (vide infra).
To investigate the regioselectivity, the crude product mixtures
from the photolysis of 1 were analyzed with HPLC chromatog-
raphy. Fig. 1 clearly shows that the concentration of 1b decreases
and 2b(E, Z) increases with photolysis time, and consumption of
1b is equal to increment of 2b(E, Z), and the ratio of 2b(E) : 2b(Z)
is a constant (44 : 56) during photolysis. These indicate that the
photoconversion is a very clean reaction with no byproducts and
the photoproducts 2b(E, Z) are stable to irradiation light.
Similar analysis was performed, and the ratio of two regioiso-
mers (2(E) : 2(Z)) was obtained from HPLC chromatograms of
the crude photolysis products in aerated solution, and are listed
in Table 1. With increasing the size of 2,2¢ substituents, the ratio
of 2(E) : 2(Z) increases from 44 : 56 for 1b (R = F) to 79 : 21 for
Photochromism resulting from conformational isomerizations
of 9,9¢-bi-9H-xanthen-9-ylidene (X = O, bixanthenylidenes, 1) has
Department of Chemistry, Joint Laboratory of Green Synthetic Chemistry,
University of Science and Technology of China, Hefei, 230026, China.
E-mail: qhsong@ustc.edu.cn; Fax: + 86 551 3601592; Tel: + 86 551 3601592
† Electronic supplementary information (ESI) available: Synthesis and
characterization data of 4b, 4d–j, 1b, 1d–j, 2b–e(E, Z) and 2f–j(E), UV–Vis
spectrum changes of photolysis of 1 recorded at different irradiation times,
HPLC chromatograms for photolysis of 1b–j, and copies of NMR spectra
of new compounds. See DOI: 10.1039/c1ob05072a
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Table 1 The regioselectivity (2(E) : 2(Z)) and quantum yields of photo-
conversion of 1
b
Entry
1: substituents
2(E) : 2(Z)a
U1→2
1
2
3
4
5
6
7
8
9
1a: R = H
0.027
0.0052
0.0042
0.0025
0.0034
0.018
0.36
0.26
0.080
0.24
1b: R = F
44 : 56
61 : 39
71 : 29
79 : 21
100 : 0
100 : 0
100 : 0
100 : 0
100 : 0
1c: R = CH3
1d: R = OCH3
1e: R = Et
1f: R = CN
1g: R = COCH3
1h: R = COOH
1i: R = COOMe
1j: R = CONEt2
10
a From HPLC chromatograms of photolysis crude in CH3CN–THF(v/v,
9 : 1) solutions. b Measurement in THF solutions, error within 5%.
The photoconversion of 1 to 2 can be observed by a UV–Vis
spectrometer. The spectral changes with an isobestic point at about
390 nm clearly show the conversion of 1 to 2 under 370 nm light: the
concurrent disappearance of the absorption band (340–390 nm) of
1 and the appearance of the 400–500 nm band of 2 (Fig. 2). Hence,
the conversion extent of 1 could be estimated by monitoring the
increase in absorbance of 2 at 440 nm. Furthermore, because the
absorbance of the two regioisomeric products are very similar in
the visible region, the photoconversion efficiency (U1→2) could be
measured by monitoring the UV–Vis absorption change of the
photolysis systems of 1 under the irradiation of a monochromatic
light (370 nm) from a fluorescence spectrometer, and data are listed
in Table 1.
Scheme 1 Photoconversion of 2,2¢-substituted bixanthenylidenes (1) to
helixanthens (2) in aerated solutions.
Fig. 1 HPLC chromatograms of the crude products in the photolysis of
1b in aerated acetonitrile–THF (v/v, 9 : 1) irradiated for different times
under 365 nm light. Insert: percentages of 1b, and 2b(E, Z).
1e (R = Et), and photolysis of 1f and 2,2¢-diacylbixanthenylidenes
(1g–j) give one kind of regioisomeric product, 2(E). Hence, the
steric hindrance of the R substituents seems to be responsible
for the regioselectivity. The order of the steric strain in the three
possible photoisomers would be C(Z¢) > C(Z) > C(E), which
result in the expected oxidation products 2(Z¢), 2(Z) and 2(E),
respectively (Scheme 1). No detectable 2(Z¢) may be because of a
too large a strain between the substituent groups to form C(Z¢) or
its formation cannot compete with the more rapid ring opening.
The high proportion of 2b(Z) (56%) in the photolysis of 1b may
be because of the small size of fluorine.
The regioselectivity and the efficiency in the photoconversion
is determined by the formation rates of the two C photoisomers
and the relative rates of ring opening and oxidation by O2. If the
difference in the oxidation rate for the C intermediates is neglected,
the photoconversion rates are determined by the relative rates
between the photocyclization rates of 1 and the ring-opening rates
of C. The rates should be controlled by the steric strains of the C
intermediates, which depend on the size of the substituent groups
R, finally giving different efficiencies and regioslectivity.
Fig. 2 UV–Vis absorption spectra of the photolysis of 1g in the aerated
THF recorded after each irradiation for 5 s under 365 nm light from a UV
lamp.
Data in Table 1 show that 2,2¢-diacylbixanthenylidenes (1g–j)
give much higher values of U1→2 than other bixanthenylidenes
(1a–f) (vide infra), and the values of 1b–f are lower than that of
1a. This indicates that the steric hinder of the 2,2¢ substituents
(R) plays a dominating role in the efficiency of photocyclization
of 1b–f. A highly inefficient photocyclization of 2,2¢-di-tert-
butylbixanthenylidene provided further support.10
According to the electronic effects of R (CN, F, CH3, Et, OCH3),
the values of U1→2 gradually decrease from electron-withdrawing
groups (EWGs) to electron-donating groups (EDGs) for 1b–f.
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These results show that the C1-C8¢ coupling reactivity of the
reagent with EWGs (a positive C1d +) is higher than that with
EDGs (a negative C1d-) in the photocyclization. In other words, the
intermediate C(E) with EWGs is more stable than that with EDGs.
Therefore, besides the steric effects of R, electronic effects also play
an important role on the efficiency of the photocyclization.
To understand the highly efficient and regiospecific photocy-
clization of 2,2¢-diacylbixanthenylidenes (1g–j), the photolysis of
1g in degassed benzene-d6 with a sealed Pyrex tube was monitored
by 1H NMR spectroscopy, under irradiation with a hand-held UV
lamp (Fig. 3). As shown in Fig. 3, the disappearance of some peaks
of 1g is accompanied by the appearance of a series of new peaks
a–f , which are assigned to an enol isomer C¢g(E) shown in Chart
1. The concentration of C¢g(E) increases with irradiation time, and
the c, d peaks shift to low field and the b peak shifts to high field
in NMR spectra. The changing chemical shifts with the amount
of C¢g(E) indicate the existence of an interaction between C¢g(E)
molecules, such as a hydrogen bond. Comparison of the structures
of C¢g(E) and Cg(E) shows that Cg(E) undergoes a [1,11] hydrogen
shift to generate C¢g(E).
solution, and cannot revert to 1g under the irradiation conditions.
However, C¢g(E) could be rapidly oxidized to 2g upon exposure
of the degassed solution to air, shown in Fig. 3.
The photoisomer C(E) was undetectable by steady-state meth-
ods (NMR and UV spectrometry) due to its too short half-life at
room temperature, and thus the conversion of Cg(E) to C¢g(E)
must be more rapid than its formation. Therefore, the formation
of C¢g(E), preventing ring opening of Cg(E) to revert to 1g, results
in a highly efficient and regiospecific photocyclization. Analogue
phenomena have been observed from photocyclization of cis-
stilbenes in both the absence and presence of catalysts11 and 3-
styrylpyridines in anaerobic conditions.12
In addition, relative to the oxidation of C(E, Z), the hydrogen
migration in C(E) would compete more effectively with the ring
opening, leading tothe regiospecific product 2(E), in the photolysis
of the diacylbixanthenylidenes. When the hydrogen migration is a
relative slow process, such as the photolysis of 1i, we observed
a minor product 2i(Z) in THF–methanol (v/v 1 : 9) solution
(2i(E) : 2i(Z) = 95 : 5), see Fig. S1 in the ESI.†
Conclusions
In summary, we have demonstrated that the efficiency and regiose-
lectivity in the photocyclization of 2,2¢-disubstituted bixanthenyli-
denes depend on both the steric hindrance effects and electronic
effects of the substituent groups. Moreover, we have observed an
irreversible photocyclization of 2,2¢-diacylbixanthenylidenes (1g–
j), that is, the intermediate C(E) generated initially can undergo a
rapid thermal [1,11] hydrogen shift to form a stable enol C¢(E)
in a degassed solution, which cannot revert to 1, but can be
rapidly oxidized to 2(E) in the presence of oxygen. This implies
that the photoconversion efficiency is independent of the oxygen
concentration during the irradiation period. It can be expected
that the clean photoconversion has a potential application as a
convenient chemical actinometry on the radiation measurement
in UV wavelength region of 250–400 nm.
Fig. 3 1H NMR spectrum change of 1g in degassed benzene-d6 before
and after irradiation, for times shown (1–5), under a 365 nm light at room
temperature; (6) standing at room temperature for 5 h; (7) after opening
the sealed tube; (8) neat 2g(E) in benzene-d6.
Experimental
General methods
Solvents of chemical quality were distilled prior to use. Melting
points are uncorrected. 1H NMR spectra were recorded on Bruker
AV spectrometers (300 or 400 MHz). 13C NMR spectra were
recorded on Bruker AV spectrometers (75 or 100 MHz). Mass spec-
tra were measured on a Bucker BIFLEXTM III Mass spectrometer.
FTIR spectra were acquired on a BRUKER VECTOR22 infrared
spectrometer. Fluorescence–emission spectra were carried out on
a Perkin–Elmer Instruments LS55 Luminescence Spectrometer.
UV–Vis absorption spectra were recorded with a Shimadzu UV–
Vis 2450 spectrometer at room temperature. HPLC analysis was
performed on an Agilent HPLC system (1200 series) with a Agilent
C-18 reverse phase column or a Agilent SIL normal phase column
(5 mm ¥ 4.6 mm ¥ 150 mm).
Chart 1 Two intermediates, Cg(E) and C¢g(E), in the photoconversion
of 1g to 2g(E)
In contrast to light yellow and fluorescent 1g and 2g(E), C¢g(E)
is a dark red, non-fluorescent compound with a good stability
under room-temperature irradiation conditions in the degassed
1
solution. H NMR spectra show no other detectable products
Synthesis of 2,2¢-disubstituted dixanthylenes (1)
besides C¢g(E) during the 70 min irradiation and no change in
the irradiated sample after standing at room temperature for
5 h. Furthermore, 1g could convert completely to C¢g(E) after a
sufficient irradiation. Hence, C¢g(E) is very stable in the degassed
The 2-substituted xanthones (4) were synthesized according to
procedures shown in Scheme 2. The substituted o-phenoxybenzoic
acids (3a–e, a: R = H, b: R = F, c: R = CH3, d: OCH3, e:
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Scheme 3 Reagents and conditions: Method A for 1a–e: (1) (COCl)2,
reflux, 10 h; (2) Cu powder, p-xylene, reflux, 7 h; method B for 1f–j: Zn,
HCl, CH3COOH, reflux, 1 h.
irradiated with a 500 W Xe lamp for a certain time. The solvent
in the photolysis mixture was removed by rotary evaporation. The
residue was subjected to column chromatography on silica gel,
and the corresponding helixanthen 2(E) were obtained. 2(Z) was
obtained from HPLC of crude photolysis products of 1.
Measurements for conversion quantum yields of 1 to 2
3 mL samples in quartz cuvettes were irradiated with 370 nm light
from a fluorescence spectrometer with a 20 nm slit for 1b–e and
a 5 nm slit for other compounds. The conversion extent of the
2,2¢-disubstituted dixanthylenes (1) was monitored by the increase
in absorbance at 440 nm of the helixanthens (2). After certain time
intervals, the absorbance of the irradiated solutions was recorded
by a UV–Vis spectrometer. The photoconversion rates of 1a–f
depended on the concentration of oxygen in the solutions. Hence,
solvents were saturated sufficiently with air before preparation of
the sample solutions in all measurements. Since the photoproducts
2 absorb 370 nm light, in order to avoid the competition of
absorbing the irradiated light between reactants 1 and products 2,
the conversion extent of reactants 1 was controlled within 5% in
the measurements.
Scheme 2 Reagents and conditions: (a) K2CO3, Cu, CuI, anisole, reflux,
5 h; (b) 98% H2SO4, rt, 1 h; (c) CuCN, DMF, reflux, 8 h; (d) NBS, CCl4,
reflux, 6 h; (e) NaOH, THF–H2O, reflux, overnight; (f) MnO2, CH2Cl2,
reflux, 2 d; (g) KMnO4, H2O, reflux, 8 h; (h) polyphosphoric acid, 180 ◦C,
4 h; (i) SOCl2, CH3OH, reflux, 1 h; (j) SOCl2, reflux, 0.5 h; (k) diethylamine,
CHCl3, reflux, 2 h.
R = Et) were prepared by Ullmann condensations of the ap-
propriately para-substituted phenols with o-chlorobenzoic acid.13
Adding concentrated H2SO4, cyclization of 3a–e gave the 2-
substituted xanthones 4a–e in high yields. 4f could be obtained
by refluxing 2-bromoxanthone and CuCN in DMF. Oxidation of
2-(1-hydroxyethyl)xanthone with manganese dioxide in refluxing
CH2Cl2 for about 48 h afforded 2-acetylxanthone (4g) in 90% yield.
Xanthone-2-carboxylic acid (4h) was obtained by intramolecular
Friedel–Crafts acylation of polyphosphoric acid with 3h, which
is from the oxidation of the ethyl group of 3e with potassium
permanganate in an alkaline solution. Reaction of 4h with SOCl2
in CH3OH solution leads to xanthone-2-carboxylic acid methyl
ester (4i) in 95% yield. The carboxamide 4j could be obtained by
first refluxing 4h with SOCl2 for 0.5 h, removing the SOCl2, and
then adding diethylamine and freshly distilled CHCl3 to reflux for
2 h.
The xanthones 4 are converted to 2,2¢-disubstituted dixan-
thylenes 1 according to two conventional methods as follows
(Scheme 3). Method A: the xanthones 4 are converted into
unstable gem-dichlorides by refluxing in oxalyl dichloride to
facilitate the subsequent intermolecular coupling by the action of
activated Cu powder in refluxing p-xylene. Method B is a classical
method for the preparation of dixanthylenes, converting by a
bimolecular Zn–HCl reduction in glacial acetic acid. Compounds
1a–e were prepared according to method A in high yields (70–
80%). Compounds 1f–j were obtained following method B in
higher yields (60–80%) over method A (only 10%).
To obtain the quantum yields for photoconversion of com-
pounds 1a–j, [U1→2 = (rate of dixanthylene electrocycliza-
tion)/(rate of photons absorbed)], the absorbances at 440 nm
(A440, absorption peaks of 2) and 370 nm (A370) were measured at
certain irradiated time intervals. The A440 of the irradiated solution
depends on the extent of photoconversion of 1 to 2. The molar
extinction coefficient at 440 nm (e440) was obtained from the UV–
Vis absorption spectrum of 2, and the concentration of 2 in the
photolysis system was obtained from A440/e440. The plot of the
concentration against the irradiation time (t/ min) is well fitted as
a straight line. The slope of the straight line, B, is a conversion rate
of 1 to 2. The intensity of the incident light I0 (unit: einstein min-1)
was measured using ferrioxalate actinometry.14 The intensity of
light absorbed (Ia) by the solution was calculated in term of Beer’s
law, Ia = I0 (1–10-A ). These values allowed the calculation of
370
the quantum yield, U = BV0/Ia, wherein V0 was the volume of
irradiation solution, 3 ¥ 10-3 L.
1H NMR spectra of photolysis of 1 g in degassed benzene-d6
To obtain information about the mechanism of photocyclization
of 1g, the progress of its irradiation in degassed solution was
monitored by NMR spectroscopy. 1g was dissolved in benzene-
d6 (20 mM) and placed in a Pyrex NMR tube. The solution was
degassed by three freeze–pump–thaw cycles, and then sealed under
vacuum. The degassed solution of 1g was irradiated under 365 nm
light from a hand-held UV lamp, and the 1H NMR spectra were
recorded after several incremental irradiation periods.
Synthesis and characterization of helixanthens 2
Dixanthylene was dissolved in CHCl3 with saturated concen-
tration in a flask. Under an air atmosphere, the solution was
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Acknowledgements
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This work was supported by the National Natural Science
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