General and Efficient Intermolecular [2+2] Photodimerization of Chalcones and Cinnamic Acid Derivatives in Solution through Visible-Light Catalysis

Article abstract of DOI:10.1002/anie.201708559

[2+2] Photocycloaddition, for example, the dimerization of chalcones and cinnamic acid derivatives, is a unique strategy to construct cyclobutanes, which are building blocks for a variety of biologically active molecules and natural products. However, most attempts at the above [2+2] addition have focused on solid-state, molten-state, or host–guest systems under ultraviolet-light irradiation in order to overcome the competition of facile geometric isomerization of nonrigid olefins. We report a general and simple method to realize the intermolecular [2+2] dimerization reaction of these acyclic olefins to construct cyclobutanes in a highly regio- and diastereoselective manner in solution under visible light, which provides an efficient solution to a long-standing problem.

Full text of DOI:10.1002/anie.201708559

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
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Accepted Article
Title: General and Efficient Intermolecular [2+2] Photodimerization of
Chalcones and Cinnamic Acid Derivatives in Solution through
Visible Light Catalysis
Authors: Li-Zhu Wu, Tao Lei, Chao Zhou, Mao-Yong Huang, Lei-Min
Zhao, Bing Yang, Chen Ye, Hongyan Xiao, Qing-Yuan Meng,
Vaidhyanathan Ramamurthy, and Chen-Ho Tung
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708559
Angew. Chem. 10.1002/ange.201708559
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http://dx.doi.org/10.1002/ange.201708559

10.1002/anie.201708559
Angewandte Chemie International Edition
COMMUNICATION
General and Efficient Intermolecular [2+2] Photodimerization of
Chalcones and Cinnamic Acid Derivatives in Solution through
Visible Light Catalysis
Tao Lei, Chao Zhou, Mao-Yong Huang, Lei-Min Zhao, Bing Yang, Chen Ye, Hongyan Xiao, Qing-Yuan
Meng, Vaidhyanathan Ramamurthy, Chen-Ho Tung, and Li-Zhu Wu*
Abstract: Represented by dimerization of chalcones and cinnamic
phenylpyridinato-C2,N) iridium) leading to the formation of their
cyclobutanes in good to excellent yields. Although the
intermolecular [2+2] photodimerization of olefins[10] (for example,
cyclic molecules) under visible light (via redox process) has
been recently highlighted, a general method for the dimerization
of classical nonrigid olefins in solution is still missing. In this
communication, we highlight the results of our investigations on
the visible light induced [2+2] dimerization of several acyclic
olefins derived from cinnamic acid and chalcone. The method
reported here is fairly general and highly stereo- and
diastereoselective. Although intramolecular cycloaddition of
acyclic 1,n-dienes is exploited extensively in organic synthesis,
as far as we are aware, the intermolecular cycloaddition of
acyclic olefins have not received the same level of attention due
to the lack of a universal strategy. We believe the current report
fulfills the above gap.
acid derivatives, photo [2+2] cycloaddition is a unique strategy to
construct cyclobutanes, the building blocks for
a variety of
biologically active molecules existing in nature. However, most
attempts for the above [2+2] addition focused on solid-state, molten-
state or host-guest systems under ultraviolet light irradiation in order
to overcome the competition of facile geometric isomerization of
nonrigid olefins. Here, we report a general and simple method to
realize the intermolecular [2+2] dimerization reaction of these acyclic
olefins to construct cyclobutanes in
a
highly stereo- and
diastereoselective manner in solution under visible light, which
provides an efficient solution to the long pending problem.
Since the first report of ultraviolet light induced photo-
dimerization of thymoquinone by Liebermann in 1877, photo
[2+2] cycloadditions of olefins (i.e. Cu(I) catalysis, direct
excitation and sensitization, photoredox catalysis) has become a
sought after synthetic tool to construct the important building
blocks of cyclobutanes from olefins.[1-3] However, geometric
isomerization that accompanies bimolecular cycloaddition and
low stereo and regio selectivity of cyclobutanes make the
intermolecular photo [2+2] cycloaddition of acyclic olefins difficult
to be employed as a synthetic tool in solution. Although the first
geometric isomerization in solution and cycloaddition of
cinnamic acid in solid state was reported in 1881 and 1895,
respectively,[4-6] it is rather challenging to acquire a general
easily adoptable method for the solution dimerization of
cinnamic acids and related acyclic olefins so far. Given the
important role of substituted cinnamic acids and chalcones
derivatives have historically played in uncovering the hidden
secrets of photodimerization of olefins in crystals and organized
assemblies,[7-9] we initiated to explore the possibility of the
intermolecular [2+2] addition of the nonrigid classical olefins in
solution.
The feasibility of the proposed methodology and optimization
of the conditions were initially established with chalcone as the
substrate and Ir(ppy)3 as the photocatalyst. A degassed 1,4-
dioxane solution containing 0.2 mM of olefin and 1 mol % of
Ir(ppy)3 was irradiated with blue LEDs (λ = 450 nm) at room
temperature. The structure and yield of cyclobutanes obtained in
the solution are provided in Table 1. Following results are
noteworthy: (a) Parent chalcone, which dimerized inefficiently
upon direct excitation by ultraviolet light (0.1 M, λ > 300 nm, 6%
yield), gave major anti-head-to-head dimer in 73% yield under
the above conditions. (b) Both electron-withdrawing and
electron-donating substituted chalcones gave good yields of
dimers 3-17. (c) Replacement of phenyl substituent with furan
also worked well to afford dimer 13 under the reaction condition.
(d) Amongst the chalcones, (E)-1-(4-methoxyphenyl)-3-(p-
tolyl)prop-2-en-1-one gave the highest yield of the dimer. (e) In
addition to chalcone, under the above conditions cinnamic acid
derivatives also dimerized with high diastereoselectivities (18-26,
Table 1). (f) Interestingly, even cinnamides showing behaviour
similar to chalcones and cinnamates dimerized to main
diastereomeric cyclobutanes 27-31. (g) Perusal of Table 1
revealed that in all examples, the cyclobutane products are
mainly in an anti-head-to-head configuration and the yield of
these dimers varied between 38 and 93% that are far higher
than in the literature.[7b]
To our delight, the intermolecular photo [2+2] cycloaddition
and cross photocyclo [2+2] addition of chalcones and cinnamic
acids, respectively, occurred efficiently upon visible light
irradiation of
a
catalytic amount of Ir(ppy)3 (fac-tris(2-
[*]
Dr. T. Lei, C. Zhou, M.-Y. Huang, L.-M. Zhao, Dr. B. Yang, C. Ye,
Dr. H. Xiao, Dr. Q.-Y. Meng, Prof. Dr. C.-H. Tung, 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, Beijing 100190 (P. R. China)
E-mail: lzwu@mail.ipc.ac.cn
The generality of intermolecular [2+2] photodimerization was
established by irradiating two-component systems consisting of
1,1-diphenylethylene with three different olefins, chalcone,
cinnamate and cinnamamide. For these experiments, the
solvent used was 1,2-dichloroethane, and the concentrations of
the two olefins were maintained at 0.2 mM (chalcone or
cinnamates or cinnamamide) and 1.0 mM (1,1-diphenylethylene).
As illustrated in Table 2, with 1 mol % of Ir(ppy)3 cross [2+2]
photodimerization showed good tolerance towards substitution
on the carbons of C=O and C=C of the chalcones, cinnamates
Homepage: http://lsp.ipc.ac.cn/
Prof. Dr.V.Ramamurthy
Department of Chemistry,University of Miami
Miami, FL 33124-0431 (USA)
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201708559
Angewandte Chemie International Edition
COMMUNICATION
Table 1. Scope of Dimerization Illustrated with Variously Substituted Olefins.[a]
O
O
O
O
O
hv 1 mol
R
R
,
%
R
R
Ir(ppy)3
r.t.
1,4-dioxane, Ar,
2 Ar1
R
Ar1
Ar1
Ar1
Ar1
(minor)
(major)
Chalcone derivatives
(a)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
-BrPh
-BrPh
60%
p
p
-MePh
-MePh
p
o
o
-MePh
Ph
Ph
-MePh
p
p
-ClPh
-ClPh
o
o
-FPh
-FPh
-FPh
p
p
-MeOPh
-MeOPh
-FPh
p
p
p
10
,
36h,
5
8
9
,
64%
24h,
,
78%
,
67%
7
,
4
24h,
6
,
78%
38%
24h,
3
,
73%
,
76%
O
24h,
24h,
O
24h,
24h,
O
O
O
O
O
O
O
O
O
O
O
O
-ClPh
p
-ClPh
p
-MeOPh
p
-MeOPh
p
-MePh
-MePh
p
p
-MePh
p
Ph
Ph
Ph
Ph
-MePh
p
Ph
Ph
-MePh
-MePh
67%
p
p
-MePh
-MePh
86%
-
Ph
Ph
-MePh
-MePh
72%
p
p
-CF Ph
-CF Ph
3
CNPh
-CNPh
76%
p
p
p
p
p
Furan- 2
13
2 -Furan
52%
p
3
15
16
17
,
24h,
12
14
,
,
,
,
,
71%
24h,
11
24h,
24h,
,
49%
24h,
24h,
24h,
Cinnamic acid derivatives
(b)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Et
Et
Bn
Bn
Et
Et
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
-
p
-
o-
BrPh
-BrPh
p
o
-MePh
-MePh MeOPh
-MeOPh
p
p
ClPh
-ClPh
p
p
Ph
Ph
Ph
Ph
Ph
21
Ph
Ph
Ph
22
25
55%
,
88%
23
,
90%
24
36h,
20
36h,
,
,
82%
36h,
,
88%
36h,
18
,
78%
36h,
,
81%
24h,
36h,
19
,
88%
36h,
7:1 d.r.
5:1 d.r.
>9:1 d.r.
7:1 d.r.
10:1 d.r.
8:1 d.r.
19:1 d.r.
8:1 d.r.
O
O
O
O
O
O
O
O
O
O
O
O
O
O
nBu
N
nBu
Bn
N
Bn
Ph
Ph
N
H2N
NH2
O
O
N
N
N
N
N
H
H
H
H
H
H
-
Ph
Ph
73%
p HOPh
-HOPh
p
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
26
12:1 d.r.
,
93%
27
36h,
,
28
29
30
31
32
79%
,
80%
,
73%
,
86%
,
74%
24h,
36h,
,
36h,
36h,
16:1 d.r.
36h,
19:1 d.r.
36h,
14:1 d.r.
>12:1 d.r.
>19:1 d.r.
6:1 d.r.
[a] Yield of isolated major product, d.r. (major: minor) determined by 1H NMR analysis of the unpurified reaction mixture. The products of chalcone derivatives
had no minor configuration and their byproducts were hardly to be detacted or analyzed.
and cinnamamides, yielding cross photodimers with high
stereospecificity. Even substrates (refer to the olefin precursors
in 52, 53, 55, 56) that showed low reactivity during
homodimerization exhibited high reactivity towards 1,1-
diphenylethylene during cross photodimerization. Generality of
the cross [2+2] photodimerization was further established with
the use of α-methylstyrene and various substituted styrenes as
the second olefin (59-62). From the data presented, it is clear
that all obtained cyclobutanes products are in anti-configuration.
Notably, a sharp decreasing yield of dimer to 20% about the
template in presence of triethylamine or catalyzed by
Ru(bpy)3Cl2 suggested the negative effect of electron transfer on
this transformation albeit the reduction potential of the Ir(III)
complexes is sufficient to reduce chalcone (Scheme 1). In this
situation,
we
speculated
that
the
Ir(ppy)3-catalyzed
intermolecular [2+2] photodimerization proceeds via energy
transfer pathway rather than electron transfer one. One would
expect isomerization to compete with dimerization because
triplet acyclic olefins would lead to isomerization.[11] Indeed,
under our irradiation conditions, geometric isomerization
competed with dimerization at early stage, but the cyclobutane
dimer was the only isolated product at completion of irradiation.
Control experiments were conducted using chalcone as the
model substrate. These experiments established that there was
no dimerization without blue light and Ir(ppy)3. As tabulated in
Scheme 1, Ir(ppy)3 absorbs up to 500 nm, and has triplet energy
of ~58 kcal/mol and photoredox potentials of 0.77 (oxidation)
and -1.73 V (reduction) (vs SCE). To probe the value of Ir(ppy)3
and to ascertain whether photoinduced energy or electron
transfer is involved in the dimerization process, the reactions
were conducted in presence of Ir(ppy)2(dtbbpy)(PF6) and
Ir[dF(CF3)ppy]2(dtbbpy)(PF6). These two, having triplet energies
similar to Ir(ppy)3, possess reduction potentials lower than
Ir(ppy)3. When the solution of chalcone and Ir(ppy)2(dtbbpy)(PF6)
or Ir[dF(CF3)ppy]2(dtbbpy)(PF6) was irradiated with blue light, the
expected cyclobutane dimers were obtained in moderate yields.
To make sure the feasibility of energy transfer from Ir(ppy)3
to chalcone, we recorded the absorption and emission spectra to
determine the singlet and triplet energies of Ir(ppy)3 and
chalcone, respectively (see Figure S3). Excitation of Ir(ppy)3 at
450 nm led to strong emission with a maximum at 510 nm in 1,4-
dioxane, which is the typical 3MLCT emission of Ir(III)
complex.[12] Addition of chalcone to this solution resulted in
emission quenching of Ir(ppy)3 at room temperature. Strikingly,
when Ir(ppy)3 was excited at 77 K, a new emission with λmax at
590 nm appeared in addition to Ir(ppy)3 emission. The new
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10.1002/anie.201708559
Angewandte Chemie International Edition
COMMUNICATION
Table 2. Scope of Cross [2+2] Reaction Illustrated with Variously Substituted Olefins.[a]
R1
R4
R2
R3
hv 1 mol
,
%
Ir(ppy)3
r.t.
R2
+
R1
R4
ClCH2CH
2Cl, Ar,
R3
O
O
O
O
O
O
O
O
Furan- 2
Ph
o
Ph
Ph
-MePh
Ph
-
p
CNPh
-MePh
Ph
p
-MePh
Ph
Ph
Ph
p
-MeOPh
Ph
-ClPh
Ph
-MeOPh
p
p
p
Ph
Ph
Ph
Ph
Ph
Ph
Ph
79%
Ph
Ph
Ph
Ph
86%
Ph
Ph
Ph
84%
40
33
34
35
36
73%
O
,
,
,
80%
,
84%
37
,5h,
5h,
5h,
10h,
5h,
,
39
5h,
38
,
50%
O
82%
5h,
,10h,
O
O
O
O
O
O
Ph
o
Ph
Ph
Ph
-ClPh
-MePh
O
p
O
O
Bn
-BrPh
p
2-Py
-ClPh
p
O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
85%
41
42
43
44
45
46
,
5h,
,
78%
,
5h 63%
,
92%
,
91%
,
77%
88%
47, 5h,
5h,
5h,
5h,
5h,
O
O
O
O
O
O
O
Ph
O
O
Ph
Ph
NH
Ph
Ph
S
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
NH2
NH
Ph
OH
N
Ph
Ph
Ph
Ph
Ph
92%
Ph
Ph
Ph
Ph
Ph
Ph
49
50
,
10h,
48
,
83%
67%
,
71%
CN
55
,
10h,
85%
O
10h,
51
52
,
24h,
,
62%
92%
24h,
O
53 24h
,
54
10h,
92%
,
24h,
O
O
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
O
Ph
Ph
O
O
O
Ph
Ph
-MeOPh
p
-ClPh
p
-ClPh
-MeOPh
p
p
Ph
Ph
60
-ClPh
74%
-MeOPh
60%
p
p
62
56
,
80%
,
91%
5h,
3:1 d.r.
24h,
57
,
58
59
89%
86%
61
2:1 d.r.
88%
5h,
,
,
,
,
24h,
5h,
2:1 d.r.
24h,
2h,
3:1 d.r.
[a] Yield of isolated product, d.r. (major: minor) determined by 1H NMR analysis of the unpurified reaction mixture.
O
O
The above observations support our contention that in our
system Ir(ppy)3 acts as the triplet sensitizer.
hv 1 mol
,
%
photocatalyst
O
Ph
Ph
24h
r.t.,
2 Ph
Ph
1,4-dioxane, Ar,
=
49.2 kcal/mmol
Ph
Ph
As outlined above, the major isomer of products in all cases
was found to be anti-head-to-head derived from two substrates
(Table 1 and Table 2). However, dimerization of unsymmetric
olefins in general can give four dimers, syn-head-to-head, anti-
head-to-head, syn-head-to-tail and anti-head-to-tail. Furthermore,
geometric isomerization accompanies the dimerization, thus
these dimers could come from two trans, a cis and a trans or two
cis substrates. To gain an understanding of the high selectivity
of anti-configuration in our work and the preferred pathway to
the cyclobutanes, DFT calculation was performed using the
homodimerization of chalcone as the model (for complete details,
see Supporting Information). Highlights of the results provided
below are consistent with the mechanism proposed in Scheme 2:
(a) In the ground state, the trans-chalcone is more stable than
the cis-chalcone while the geometric isomerization is easy in the
triplet state. (b) Beginning from the triplet-state complex
composed of an excited trans-chalcone with a ground state
trans-chalcone in the trans-head-to-head geometry, formation of
a diradical intermediate, entrance to the ground state surface
through a conical intersection (crossing point) between the triplet
and ground state surfaces and closing of the diradical to anti-
head-to-head dimer are thermodynamically feasible. Importantly,
anti-head-to-head dimer is slightly more stable than the
accretion of two individual olefins. (c) Of the two possibilities of
initial bond formation (α-α and β-β in Scheme 2), C–C bond
formation between α carbons of the olefins is found to be
chalcone
cyclobutane
anti
(
-head-to-head)
red
=
E1/2
E
T
-1.48V,
E
*/M-
)
1/2 (M+
/M*
)
ET
Yield
E
(kcal/mmol)
1/2 (M
photocatalyst
Ir(ppy)3
73%
57%
50%
20%
57.8
+0.31V
+0.66V
-1.73V
-0.96V
49.2
Ir(ppy)
2
(dtbbpy)(PF6)
(dtbbpy)(PF6)
60.8
+1.21V
+0.77V
-0.89V
-0.81V
Ir[dF(CF3)ppy]
2
46.5
O
Cl2
Ru(bpy)3
O
hv 1 mol
,
%
O
Ir(ppy)3
24h
Ar, r.t.,
Ph
Ph
2 Ph
Ph
1,4-dioxane,
20 mol %TEA
Ph
Ph
chalcone
cyclobutane
anti
(
-head-to-head)
=
Yield
21%
Scheme 1. List of Control Experiments Performed (All the Potentials are
relative to SCE).
emission was found to have a long lifetime (80 µs) consistent
with its assignment to be the triplet of chalcone (see Figure S3).
Because of the non-overlapping absorption of chalcone and
emission of Ir(ppy)3 at room temperature, the singlet-singlet
energy transfer from excited Ir(ppy)3* to chalcone is unlikely.
However, the occurrence of overlap between 3MLCT emission of
Ir(ppy)3 and phosphorescence of chalcone suggested that the
triplet-triplet energy transfer from Ir(ppy)3* to chalcone is likely.
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10.1002/anie.201708559
Angewandte Chemie International Edition
COMMUNICATION
favourable. This is consistent with the intuitive expectation of
formation of a more stable diradical C. (d) The most exciting
results relate to the guidance concerning the absence of syn-
head-to-head dimer. This is because the syn-head-to-head
dimer is higher in energy than the accretion of two individual
olefins. The diradical intermediate formed in this pathway prefers
to cleave to two olefins rather than close to form the less stable
dimer. (e) Addition of excited trans-chalcone to a cis one or
excited cis-chalcone to a cis one was found to involve higher
energies. Hence, the computational results favour a pathway
that involves addition of excited trans-chalcone triplet with a
ground state trans-chalcone to give the stable diradical C
(Scheme 2) that closes to yield the isolated anti-head-to-head
dimer as a sole cyclobutane product.
Keywords: visible light catalysis • [2+2] addition • cyclobutanes
• chalcones and cinnamic acid derivatives
[1]
[2]
C. Liebermann, Ber. Dtsch. Chem. Ges. 1877, 10, 2177-2179.
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[5]
V. Ramamurthy, J. Sivaguru, Chem. Rev. 2016, 116, 9914-9993.
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D. Roth, Angew. Chem. Int. Ed. 1989, 28, 1193-1207.
[6]
[7]
J. Bertram, R. Kürsten, J. Prakt. Chem. 1895, 51, 316-325.
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Trans. 1, 1998, 1315-1318.
In conclusion, we report an efficient cost-effective method for
the synthesis of highly energy rich and strained cyclobutanes
from acyclic C=C bonds. Photophysical studies and DFT
calculations revealed that the triplet chalcones and cinnamic
acid derivatives produced by energy transfer from the excited
Ir(ppy)3* are responsible for the formation of cyclobutanes in
highly stereo- and diastereoselective manner, without any extra
additives and directing groups in solution. Adopting to visible
light catalysis methodology, we have overcome the perennial
problem of low yield and low-selectivity of intermolecular [2+2]
dimerization of chalcones and cinnamic acid derivatives in
solution. Observed results provide confidence that our approach
is likely to be general for additional applications.
[8]
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O
Ph
O
(a)
ν
h
,
Ir(ppy)3
Ph
Ph
Ph
Z-chalcone
E-chalcone
(b)
T
O
O
β
transfer
Energy
Ph
Ph
Ph
Ph
α
E-chalcone
*
Ir(ppy)3
Ir(ppy)3
[11] J. B. Metternich, R. Gilmour, J. Am. Chem. Soc. 2015, 137, 11254-
11257.
E-chalcone
ν
h
[12] T. Hofbeck, H. Yersin, Inorg. Chem. 2010, 49, 9290-9299.
O
O
D3
D1
O
O
O
O
α α
Ph
Ph
Ph
Ph
Ph
Ph
β
Ph
β
Ph
Ph
Ph
Ph
Ph
D
C
cyclobutane
anti
(
-head-to-head)
Scheme 2. Proposed Mechanism.
Acknowledgements
Financial support from the Ministry of Science and Technology
of China (2013CB834804, 2014CB239402, 2017YFA0206903),
the National Natural Science Foundation of China (21390404,
91427303, 21473227), the Strategic Priority Research Program
of the Chinese Academy of Science (XDB17030400) is gratefully
acknowledged. V. Ramamurthy acknowledges the Chinese
Academy of Sciences for a fellowship and the US National
Science Foundation (CHE-1411458) for its support.
This article is protected by copyright. All rights reserved.

10.1002/anie.201708559
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Tao Lei, Chao Zhou, Mao-Yong Huang,
Lei-Min Zhao, Bing Yang, Chen Ye,
Hongyan Xiao, Qing-Yuan Meng,
Vaidhyanathan Ramamurthy, Chen-Ho
Tung, and Li-Zhu Wu*
Page No. – Page No.
General and Efficient Intermolecular
[2+2] Photodimerization of Chalcones
and Cinnamic Acid Derivatives in
Solution through Visible Light
Catalysis
Represented by dimerization of chalcones and cinnamic acid derivatives, we take
advantage of visible light induced energy transfer to construct cyclobutanes of
nonrigid olefins in a highly stereo- and diastereoselective way in solution, which
provides an efficient way toward the long pending issue.
This article is protected by copyright. All rights reserved.