Aerobic oxidative coupling of resveratrol and its analogues by visible light using mesoporous graphitic carbon nitride (mpg-C3N4) as a bioinspired catalyst

Article abstract of DOI:10.1002/chem.201303587

The first aerobic oxidative coupling of resveratrol and its analogues by mesoporous graphitic carbon nitride as a bioinspired catalyst with visible light has been developed. With this method, δ-viniferin and its analogues were synthesized in moderate to high yield. The metal-free conditions, visible-light irradiation, and the ideal oxidant, molecular oxygen, make this coupling reaction environmental friendly and practical. Copyright

Full text of DOI:10.1002/chem.201303587

DOI: 10.1002/chem.201303587
Communication
&
Green Chemistry
Aerobic Oxidative Coupling of Resveratrol and its Analogues by
Visible Light Using Mesoporous Graphitic Carbon Nitride (mpg-
C3N4) as a Bioinspired Catalyst
Tao Song,[a] Bo Zhou,[a] Guang-Wei Peng,[a] Qing-Bao Zhang,[a] Li-Zhu Wu,[c] Qiang Liu,*[a] and
Yong Wang*[b]
with horseradish peroxidase/H2O2.[3a] Although this stilbenoid
dimer widely existed in a variety of species, such as, gnetum
hainanense,[5b] rheum maximuwicizii,[5c] and stressed grapevine
cultures,[5d–5 g] its low abundance impeded the studies of their
biological properties.
Abstract: The first aerobic oxidative coupling of resvera-
trol and its analogues by mesoporous graphitic carbon ni-
tride as a bioinspired catalyst with visible light has been
developed. With this method, d-viniferin and its analogues
were synthesized in moderate to high yield. The metal-
free conditions, visible-light irradiation, and the ideal oxi-
dant, molecular oxygen, make this coupling reaction envi-
ronmental friendly and practical.
To achieve biomimetic synthesis of d-viniferin and its ana-
logues, both chemically and enzymatically catalytic methods
have been employed. The traditional chemical methods gener-
ally need use of stoichiometric metallic oxidants[2] (silver salt,
manganese salts, copper salt, ferric salt, thallium salt, cerous
salt, and their metallic oxides), and the enzymatic methods
usually employ different oxidative enzymes,[3] for example, per-
oxidase/H2O2 and laccases botrytis cinerea, under strictly con-
trolled conditions. Despite the great achievements in the syn-
thesis of d-viniferin and its analogues, the chemoselectivity of
the reactions in vitro were often poor due to the complex cou-
pling pathways of different radicals generated from the oxida-
tion of resveratrol or its analogues (Scheme 1). In this sense,
the development of a facile protocol for highly selective prepa-
ration of d-viniferin and its analogues is still strongly demand-
ed.
Resveratrol and its analogues, a class of vital compounds in
drug development, have received increasing attention in
recent years, including studies into their diverse bioactivities,
development of novel potent analogues for pharmaceutical
purposes,[1] and exploration of their potential applications as
building blocks for the construction of more complicated olig-
omers by means of bioinspired synthesis.[2–4]
Among its oligomers, d-viniferin, a dimeric resveratrol with
a dihydrobenzofuran skeleton, exhibited excellent bioactivi-
ties,[3c,e,5f] such as inhibition of COX-I (cyclooxygenase-I) and
COX-II (cyclooxygenase-II), and modest cytotoxicity against
CEM (human lymphoblastoid cells) proliferation. d-Viniferin
was initially isolated from grapevines as a phytoalexin in
1977.[5a] The first biomimetic synthesis of d-viniferin was ach-
ieved at the same year by oxidative dimerization of resveratrol
Molecular oxygen is not only a cheap, environmentally
benign and most readily available oxidant in vitro, but also
a major source of reactive oxygen species (ROS) in vivo.[6] It is
believed that molecular oxygen and ROS are responsible for
the biosynthesis of d-viniferin by a metabolic sequence in-
duced in response to biotic or abiotic stress factors. Unfortu-
nately, due to the relatively unreactive properties of molecular
oxygen in the triplet ground state, there is no report on the
production of d-viniferin by aerobic oxidative coupling of re-
sveratrol at room temperature in vitro as far as we know. Re-
cently, polymeric graphitic carbon nitride materials have at-
tracted considerable attention because of their potential appli-
cations in sustainable chemistry as metal-free heterogeneous
catalysts. g-C3N4 (graphitic carbon nitride), with a semiconduc-
tor band gap of 2.7 eV (conduction band (CB) at À1.3 V (pH 7)
versus the normal hydrogen electrode (NHE) (E8)), in principle,
[a] T. Song, Prof. Dr. B. Zhou, G.-W. Peng, Q.-B. Zhang, Prof. Dr. Q. Liu
State Key Laboratory of Applied Organic Chemistry
College of Chemistry and Chemical Engineering
Lanzhou University
Lanzhou 730000 (P. R. China)
E-mail: liuqiang@lzu.edu.cn
[b] Prof. Dr. Y. Wang
ZJU-NHU United R&D Center
Key Lab of Applied Chemistry of Zhejiang Province
Department of Chemistry
Zhejiang University
Hangzhou 310028 (P. R. China)
ÀC
ÀC
can reduce O2 to O2 (E8(O2/O2 )=À0.16 V vs. NHE) under visi-
ble-light illumination, then trigger the aerobic oxidative reac-
tion.[7] In the present work, we report a visible-light photocata-
lytic protocol for the facile preparation of d-viniferin and its an-
alogues by using mesoporous graphitic carbon nitride (mpg-
C3N4) and molecular oxygen at room temperature. Herein,
mpg-C3N4 was selected as the photocatalyst in view of its large
surface area, N-containing surface and framework, high stabili-
[c] Prof. Dr. L.-Z. Wu
Key Laboratory of Photochemical Conversion
and Optoelectronic Materials
Technical Institute of Physics and Chemistry
University of Chinese Academy of Sciences
the Chinese Academy of Sciences
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303587.
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Communication
After the optimal reaction
conditions were established,
a series of resveratrol analogues
were subjected to this reaction
system and the results of the
aerobic oxidative coupling are
shown in Table 2. We were
pleased to observe that pteros-
tilbene
(4-hydroxy-3’,5’-dime-
thoxy-trans-stilbene (2a)), a nat-
ural methoxylated analogue of
resveratrol, which showed the
antihyperglycemic
activity,[1b]
antioxidant activity,[1e] and mod-
erate inhibitory activity of COX-
I, COX-II,[1d] smoothly produced
the dehydrodimers with the d-
Scheme 1. Proposed biosynthetic pathways of resveratrol dimers.
viniferin skeleton in 85% yield
and 100% conversion. In addi-
ty, and unique properties of a semiconductor (redox poten-
tial).[8] With this method, 4-hydroxy-trans-stilbenes including re-
sveratrol were smoothly transformed to d-viniferin and its ana-
logues in moderate to high yields.
tion, an acetylated analogue of resveratrol (4-hydroxy-3’,5’-di-
acetyl-trans-stilbene (3a)), which exhibited cancer cell-growth
inhibition activity,[1g] was also tolerated in this oxidative cou-
Initial success was obtained upon subjecting trans-
resveratrol to irradiation by a purple LED (3 W,
Table 1. Optimization of the reaction conditions.[a]
410 nm) with mpg-C3N4 (5% w/w) as the photocata-
lyst in acetonitrile bubbled with air, and the d-vinifer-
in was generated in 34% yield (19% conversion)
after 24 hous irradiation (Table 1, entry 1). Because
the reactivity of resveratrol could be influenced by
environmental pH in a great manner (see Figure S2 in
the Supporting Information),[9] a series of bases were
selected to improve the efficiency of the oxidative
Entry
Solvent
Base
Oxidant
Conv. [%]
Yield [%][b]
1
2
3
4
5
6
7
8
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
MeOH
toluene
TFT[e]
–
air
air
air
air
air
air
air
air
air
air
air
air
air
19
0
0
0
0
34
0
0
0
0
81
coupling.
Although
K2CO3,
1,8-diazabicyclo-
K2CO3
DBU
DBN
DABCO
[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-
5-ene (DBN), and 1,4-diazabicyclo [2.2.2] octane
(DABCO) failed to produce d-viniferin (entries 2–5),
addition of pyridine, 2,6-lutidine, or 2,4,6-collidine sig-
nificantly improved both the yield and conversion
(entries 6–8). The best result was obtained when the
reaction was performed with 2,6-lutidine (86% yield,
79% conversion). The attempts to further increase
the yield by prolonging irradiation time (entry 7, foot-
note [d]) and increasing the amount of the photoca-
talyst mpg-C3N4 (entry 7, footnote [c]) were unsuc-
cessful. Subsequently, a rapid solvent screening was
performed (entries 9–13), and the results revealed
the superiority of acetonitrile as the reaction
medium. To further optimize the conditions, pure
oxygen instead of air was bubbled into the solution,
but the yield of d-viniferin decreased slightly
(entry 14). Other oxidants including H2O2, tBuOOH
(TBHP), and CCl3Br were also less efficient (entries 15–
17). Control experiments showed that the presence
of photocatalyst, oxidant, and light source is necessa-
ry for the desired reaction to proceed (entries 18–20).
pyridine
32
79
72
38
19
34
23
37
78
37
64
55
0
2,6-lutidine
2,4,6-collidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
86[c]/80[d]/86
78
60
55
74
61
45
76
62
64
51
0
9
10
11
12
13
14
15
16
17
18[h]
19[i]
20[j]
THF
acetone
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
[f]
O2
[g]
H2O2
TBHP[e,g]
CCl3Br[g]
air
–
air
0
0
0
0
[a] Reaction conditions: trans-resveratrol (1a) (20 mg), mpg-C3N4 (4 mg, 5% w/w),
bases (5 equiv), solvents (5 mL), the bubbling rate of air is one bubble per second,
purple LEDs (3 W, 410 nm), RT, 24 h. [b] Isolated yield after purification by chromatog-
raphy on polyamide. [c] mpg-C3N4 (8 mg) was used, reaction time: 24 h. [d] Reaction
time: 48 h. [e] TFT=trifluorotoluene, TBHP=tetr-butyl hydroperoxide (5-6m in
decane), [f] The bubbling rate of the oxygen is one bubble per second. [g] Oxidants
(5 equiv), the reaction was carried out under argon. [h] No photocatalyst. [i] The reac-
tion was carried out under argon without oxidants. [j] The reaction was carried out in
the dark.
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pling reaction, affording the desired product 3b in 51% yield
and 100% conversion. Then the 4-hydroxy-trans-stilbenes with
electron-neutral (4a) and -donating groups (9a, 10a) were ap-
plied to the reaction system giving the desired products with
typical dihydrofuran structure in good yields. To our delight,
the 4-hydroxy-trans-stilbenes bearing electron-withdrawing
groups, such as fluorine (5a), chlorine (6a), bromine (7a), and
trifluoromethyl (8a), could be effectively converted into the
corresponding dihydrobenzofuran-based stilbene dimers in
similar yields except for fluorine-substituted substrate (5a,
40% yield, 100% conversion). The oxidation potentials of these
substrates measured by cyclic voltammetry under our mimetic
reaction conditions (pH 7.55) were similar, which was in ac-
cordance with the experimental fact that the effects of func-
tional groups were negligible (see Figure S4 in the Supporting
Information). Moreover, incorporation of one or two methoxyl
substituents at different positions on the B ring of the sub-
strates (10a–13a) revealed that steric modification of the sub-
strate can be accomplished, and the reaction gave the corre-
sponding products in 73–83% yields. Besides the 4-hydroxy-
trans-stilbenes, we also employed 3,4-dihydroxy-trans-stilbenes
(14a, 15a), which showed excellent radical-scavenging activi-
ties.[1 h] However, benzodioxane-based stilbenes dimers (14b,
15b) instead of dihydrobenzofurans were formed dominantly.
To further confirm the necessity of the 4-hydroxy group in
stilbenes for the aerobic oxidative coupling, unsubstituted
trans-stilbene (16a) and trimethylated resveratrol (17a) were
applied to the photoreaction system. As shown in Scheme 2A,
when the two compounds were subjected to the standard
conditions for 24 hours, most of substrates were recovered
and only a small amount of compounds were formed. Based
on TLC and GCMS analysis, no dehydrodimers but correspond-
ing aldehydes were detected. The formation of aldehydes
could be attributed to the cleavage of [2+2] cycloaddition
products generated by the reaction of stilbenes with singlet
oxygen.[10] The sharp contrast between stilbenes without 4-hy-
droxy benzene moieties (16a, 17a) and 4-hydroxy-substituted
stilbenes (1a, 4a) in the photoreaction indicated that a qui-
none methide radical should be included during the formation
of the dehydrodimer.[3f]
Table 2. The scope of aerobic oxidative coupling of resveratrol and its
analogues.[a]
Entry
Substrate
Product
Conv. [%]
Yield [%]
1
2
3
1a R=H
2a R=Me
3a R=Ac
1b
2b
3b
79
100
100
86
85
51
4
5
6
7
8
9
10
4a R=H
5a R=F
4b
5b
6b
7b
8b
9b
10b
100
100
100
100
100
90
71
40
77
71
70
88
77
6a R=Cl
7a R=Br
8a R=CF3
9a R=OH
10a R=OMe
100
11
12
13
11 a R=3-OMe
12a R=2-OMe
13a R=3,4-OMe
11 b
12b
13b
100
100
100
78
73
84
14
15
14a R=OH
15a R=OMe
14b
15b
86
90
81
85
[a] Reaction conditions: substrates (20 mg), mpg-C3N4 (4 mg, 5% w/w),
2,6-lutidine (5 equiv), CH3CN (5 mL), the bubbling rate of air is one
bubble per second, purple LEDs (3 W, 410 nm), RT, 24 h. [b] Isolated yield
after purification by chromatography on polyamide or silica gel.
It has been reported that the formation of a d-viniferin skel-
eton could proceed by the coupling of two quinone methide
radicals[2b,3d,e] or the addition of a quinone methide radical to
another double bond of stilbenes.[4g] If the latter pathway is
possible, a hybrid of resveratrol (1a) and unreactive trans-stil-
bene (16a) should be produced when the two compounds
were mixed together under the standard conditions. However,
only d-viniferin was formed in 79% yield (based on 76% con-
version of 1a). In contrast, the reaction between resveratrol
(1a) and five equivalents of 4-hydroxy-trans-stilbene (4a) af-
forded not only the dimers of self-coupling (1b, 4b), but also
the hybrid (18) (Scheme 2B). The results of these two experi-
ments strongly indicate that the reaction was performed
through a radical–radical coupling pathway.
glet oxygen that reacted with resveratrol resulting in the for-
mation of cycloaddition products rather than dehydrodimers.
The highly selective generation of d-viniferin in the present
system might be attributed to the unique properties of mpg-
C3N4. The large surface area (ca. 200 m2 gÀ1) and the basic resi-
due groups of mpg-C3N4 could absorb molecular oxygen on
the surface of the catalyst efficiently, and the low potential of
the CB (À1.3 V versus NHE) under irradiation provided a favora-
ble driving force in reducing molecular oxygen to produce su-
ÀC
peroxide radical anion (O2 ), thereby suppressing the genera-
tion of singlet oxygen.[7a] As a result, d-viniferin was produced
Up to now, there is the only one previous report on the
photochemistry between resveratrol and molecular oxygen.[10]
This involves energy transfer from a sensitizer to generate sin-
with high chemoselectivity.
ÀC
To verify the generation of the O2 in the initial step of the
reaction, potassium superoxide was used as another source of
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the holes generated in the va-
lence band of mpg-C3N4 are in-
jected by electrons available
from 1aÀ, a phenolate anion
generated by the reaction of 1a
ÀC
with O2 radical anion or the
ionization of resveratrol in the
presence of 2,6-lutidine, thus
leading to the formation of
C
phenoxy radical 1a . In turn, the
C
delocalization of radical 1a
could afford a quinone methide
radical (M5) or a benzyl radical
(M10). The coupling of one radi-
cal M5 and another radical M10,
followed by tautomeric rear-
rangement and intramolecular
nucleophilic attack to the meth-
ylene of the intermediate semi-
Scheme 2. A) The reaction of resveratrol analogues without 4-hydroxyl substituents under standard conditions;
B) The hybrid reaction of stilbenes.
superoxide anion instead of the photocatalytic aerobic condi-
tions and resveratrol 1a was transformed to d-viniferin in 44%
yield with 65% conversion.[11] Moreover, addition of 5,5-di-
methyl-1-pyrroline N-oxide (DMPO), a superoxide anion and
phenoxy radical quencher, to the photocatalytic aerobic oxida-
tive coupling system entirely suppressed the reaction, no d-
viniferin was detected by TLC analysis even after 24 hours of ir-
quinone, gave d-viniferin as shown in Scheme 3.
In summary, we have developed a facile and efficient photo-
ÀC
chemical approach for specifically generating ROS (O2 ) to
achieve the oxidative coupling of resveratrol and its analogues
in vitro by mpg-C3N4, a metal-free heterogeneous catalyst. The
photoreaction of resveratrol and molecular oxygen catalyzed
by mpg-C3N4 provided an ideal model to mimic biotransforma-
tion of resveratrol to d-viniferin. With this method, 4-hydroxy-
trans-stilbenes, including resveratrol, were smoothly trans-
formed to d-viniferin and its analogues in moderate to high
yields. Because the successful applications of mpg-C3N4 in the
photoreaction are still rare,[7,8] the work described here is also
ÀC
radiation.[12] The results indicated that the O2 generated by
the electron-transfer pathway between mpg-C3N4 and molecu-
lar oxygen was the vital ROS in the initial step of the radical
coupling pathway.
We have shown in Table 1 that the base 2,6-lutidine is crucial
for the high efficiency of the oxidative dimerization. The pH
value of the acetonitrile solution containing resveratrol (1.75ꢁ
10À2 molLÀ1) is about 7.10, whereas the value approximately
increased to 7.58 when 2,6-lutidine (8.75ꢁ10À2 molLÀ1) was in-
troduced to the reaction system. Meanwhile, in the buffered
aqueous solution, the voltammograms showed the first oxida-
tion peak of resveratrol was shifted to more negative poten-
tials with increasing pH values (see Figure S2 in the Supporting
Information). Because the first oxidation peak is due to the oxi-
dation of the phenol moiety,[13] the decrease of the oxidation
potential indicated the generation of the corresponding phe-
nolate anion, a much stronger electron donor. Therefore, it is
believed that the acceleration of the reaction by 2,6-lutidine
should be attributed to the increased phenolate anion gener-
ated by the ionization of resveratrol and its analogues.
On the basis of the above results, the photocatalytic aerobic
oxidative coupling reaction can be rationalized by the follow-
ing mechanism (Scheme 3). When mpg-C3N4 was irradiated
with visible light, excitation of electrons from the valence band
into the conduction band generates electron and hole pairs.
Subsequently, electron transfer from the conduction band of
mpg-C3N4 to molecular oxygen produced a superoxide radical
ÀC
anion (O2 ), which can further react with resveratrol by hydro-
gen abstraction following proton transfer or proton transfer
À
C
following hydrogen abstraction to afford 1a radical and 1a
anion along with the formation of H2O2. On the other hand,
Scheme 3. Proposed mechanism.
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and mpg-C3N4 as the photocatalyst.
Experimental Section
General procedure for photocatalytic oxidative coupling of
resveratrol and its analogues
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Substrates 1–15a, 5% w/w mpg-C3N4, CH3CN (5 mL), and base
(5 equiv) were added to a 10 mL reaction tube equipped with
a magnetic stir bar. The solution was bubbled into air in the rate
of one bubble per second. After this, the solution was stirred at
a distance of ~2 cm from a 3 W 410 nm purple LEDs at room tem-
perature for 24 h. Upon the completion of the reaction, the solvent
was removed under reduced pressure. The crude product was puri-
fied by flash chromatography.
Acknowledgements
We are grateful for financial support from the National Science
Foundation of China (21172102 and 21090343), the Ministry of
Science and Technology of China (2013CB834804), and the
Fundamental Research Funds for the Central Universities.
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Keywords: mpg-C3N4
·
oxidative coupling
·
oxygen
·
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Received: September 10, 2013
Published online on December 4, 2013
Chem. Eur. J. 2014, 20, 678 – 682
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