Photocatalyst-free visible light promoted: E → Z isomerization of alkenes

Article abstract of DOI:10.1039/c9gc04303a

A simple and green method of visible light driven photocatalytic E to Z isomerization of alkenes has been developed. A variety of (Z)-alkenes can be prepared in the presence of visible light, without any additional photocatalyst. This protocol features photocatalyst-free conditions, which are mild, tolerant, and operationally simple, and is easy to implement.

Full text of DOI:10.1039/c9gc04303a

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DOI: 10.1039/C9GC04303A
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Photocatalyst-Free Visible Light Promoted E → Z Isomerization of
Alkenes
Received 00th January 20xx,
Accepted 00th January 20xx
Jianbin Xu,a,§ Na Liu,a,§ Haiping Lv,b Chixian He,a Zining Liu,a Xianfu Shen,a Feixiang Cheng*a
and Baomin Fan*b
DOI: 10.1039/x0xx00000x
a. previous work: photocatalyst-catalyzed E to Z isomerization of alkenes
A simple and green method of visible light driven photocatalytic E
to Z isomerization of alkenes has been developed. A variety of (Z)-
alkenes can be prepared in the presence of visible light, without
any additional photocatalyst. This protocol features
photocatalyst-free conditions, which are mild, tolerant, and
operationally simple, and is easy to implement.
[Ir] or (-)-riboflavin
R
R
Visible light or UV light
E
Z
Olefins are an important structural unit that is widely present in
materials and bioactive molecules. Olefins are also one of the most
commonly used groups in organic synthesis, since they can be easily
functionalized, acting as starting materials for the synthesis of a
variety of chemical intermediates and functional backbones[1]. Since
the Z/E configuration of an olefin has a large influence on its
chemical, physical and physiological properties and functions, the
highly stereo-selective construction of olefins is a major research
topic in organic synthesis[2]. In general, the methods for selectively
synthesizing thermodynamically stable E-form olefins are relatively
mature, but there are still significant challenges for the synthesis of
thermodynamically unstable Z-olefins. The synthesis of Z-form
olefins often depends on kinetic control of the reaction, which
b. this work: photocatalyst-free photocatalytic E to Z isomerization of alkenes
NaOH
R
R
Visible Light
E
yield up to 98%;
Z/E up to 97:3
Z
Figure 1. Photocatalytic E to Z isomerization
In recent years, organic photochemical and photocatalytic
methods, especially visible light-promoted chemical reactions, have
been greatly developed, and more and more scientists have used
these methods to attempt to solve long-term synthetic
challenges[5]. Not surprisingly, visible light-promoted reactions have
been employed to synthesize Z-olefins[6]. Several photocatalysts
have been used to promote photocatalytic E-to-Z isomerization.
Metal photocatalysts such as Ir(ppy)3 and [Ir(ppy)2(bpy)]PF6 are
widely used in the E/Z isomerization of olefins. Weaver et al. have
used [Ir] catalysts to achieve the photocatalytic E→Z isomerization
of arylallylamines, hydrocarbons, alcohols, esters, stilbenes and
alkenyl boronates with high selectivity[7]. The metal-free catalysts (-
)-Riboflavin[8], condensed nucleus compounds[9] and ketones[10] can
also promote photocatalytic E→Z isomerization of olefins. Herein,
we report a simple reaction for the visible light-promoted E→Z
isomerization of (E)-stilbene, which requires only an added base
NaOH.
usually requires high energy reagents to promote the synthesis[3]
.
However, it is also very easy to accidentally isomerize
thermodynamically unstable Z-olefins to thermodynamically more
stable E-form olefins during the reaction and separation process.
Thus, satisfactory stereoselective results are not obtained in most
cases. In addition, the reaction conditions used for these strategies
are harsh (requiring, for example, strong bases and extremely low
temperatures) and expensive transition metal catalysts are also
required[4]. Although significant progress has been made on the
synthesis of olefins, there is still an urgent need to develop a simple
and efficient method for the green and stereoselective synthesis of
olefins, especially the synthesis of Z-olefins.
a. Yunnan Guizhou Plateau Chemical Functional Materials Pollution Control
Research Center, Qujing Normal University，Qujing, 655011, China
b. YMU-HKBU Joint Laboratory of Traditional Natural Medicine, Yunnan Minzu
University, Kunming, 650500, China.
Results and discussion
Fax: (+86)-874-8998658; E-mail addresses: chengfx2019@163.com (Feixiang
Cheng), FanBM@ynni.edu.cn (Baomin Fan).
§ These authors contributed equally to this work.
† 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
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Table 1. The screening of reaction conditions for the isomerization
reaction[a]
Li2CO3, Cs2CO3, NH2PF6 and Na2PO4 were also tested, but the results
did not show better results (Table 1, entries 5-9), NEt3 was also less
View Article Online
DOI: 10.1039/C9GC04303A
effective. Next, a broad range of solvents was screened, including
THF, DMF, DMSO, dioxane, toluene and i-PrOH. None of these were
effective solvents, resulting in poor E/Z ratios (Table 1, entries 11-
16). No E→Z isomerization was observed in the absence of light
(Table 1, entry 17), and in the absence of an inorganic base, the E/Z
ratio was only 3:97 (Table 1, entry 18).
Base
MeCN/H2O, Blue LED
E-1a
Z-1a
Table 2. Substrate scope for disubstituted stilbenes [a]
.
Entry
1
Base
NaOH
Na2CO3
K2CO3
KHCO3
Li2CO3
Cs2CO3
NH2PF6
K3PO4
Na2PO4
NEt3
Solvent
MeCN/H2O
MeCN/H2O
MeCN/H2O
MeCN/H2O
MeCN/H2O
MeCN/H2O
MeCN/H2O
MeCN/H2O
MeCN/H2O
MeCN/H2O
THF/H2O
Time Yield(%)[b]
Z/E[c]
97:3
R2
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
98
91
93
91
86
81
88
94
84
92
93
86
78
83
84
92
96
99
R1
NaOH
R1
MeCN/H2O(10:1), Blue LED
2
90:10
62:38
66:34
41:59
32:68
85:15
42:58
28:72
36:64
65:35
53:47
44:56
49:51
55:45
28:72
/
R2
E-1a-1i
Z-1a-1i
3
Me
4
F
Me
5
F
Me
, 94%, Z/E=93:7
Me
, 98%, Z/E=92:8
6
Z-1d
, 96%, Z/E=91:9
Z-1c
Z-1b
7
Cl
Br
Cl
8
Br
Cl
, 94%, Z/E=90:10
Cl
, 95%, Z/E=92:8
9
Z-1e
Z-1f
Z-1g
, 92%, Z/E=85:15
10
11
12
13
14
15
16
17[d]
18
MeO
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
/
F3C
DMF/H2O
CF3
, 94%, Z/E=72:28
Cl
, 91%, Z/E=88:12
Z-1h
DMSO/H2O
Dioxane/H2O
Toluene /H2O
i-PrOH /H2O
MeCN/H2O
MeCN/H2O
Z-1i
[a]
Reaction conditions: E-1 (0.3 mmol), NaOH (0.3 mmol ), MeCN
(2mL), H2O (0.2mL), 30W blue led, air, rt.
With these optimized conditions in hand, we evaluated the
substrate scope of the reaction. First, we tested symmetric
stilbenes that contained identical groups. As shown in Table 3,
stilbenes with two electron-donating substituents, such as 4,4-
dimethylstilbene and 3,3-dimethylstilbene, were converted with
high yields (98% and 94%) and Z/E ratios (92:8 and 93:7), similar to
the unsubstituted stilbene (Z-1b, Z-1c). When the two methyl
groups were replaced with halogens, 4,4-difluorostilbene, 4,4-
dichlorostilbene and 4,4-dibromostilbene substrates were still
capable of undergoing conformational conversions with good
results, as was 3,3-dichlorostilbene (Z-1d, Z-1e, Z-1f, Z-1g), the yield
of Z-alkenes from 92%-96%, and the Z/E ratio from 85:15 to 92:8.
When the substituent was the electron-withdrawing -CF3 group, the
yield and Z/E ratio of the product were somewhat decreased,
resulting in a yield of 82% and a Z/E configuration ratio of 72:28 (Z-
1h). Stilbene substrates with two different substituents were also
evaluated in the reaction, 3-chloro-3'-methoxystilbene yielded a Z-
form product with the ratio of 92:8 and 91% yield (Z-1i).
3:97
[a] Reaction conditions: E-1a (0.3 mmol), base (0.3 mmol ), solvent
(2mL), H2O (0.2mL), 30W blue led, air, rt.;
determined by 1H NMR of crude product; [d] No light.
[b]
[c]
isolated yield;
Our initial studies began with performing the visible light-driven
isomerization of E-stilbene (E-1a) to Z-stilbene (Z-1a) in CH3CN and
H2O under a 30 W blue LED light, excellent performance was
obtained when sodium hydroxide was added to the reaction, with a
99% yield and an E/Z ratio of 97:3 (Table 1, entry 1). When Na2CO3
as catalyst, this resulted in a good yield of 91% and an Z/E ratio of
90:10 after 24 h (Table 1, entry 2). Then, a series of inorganic bases
were used in the reaction. High yields could be obtained using the
bases K2CO3 and KHCO3 (93% and 91% yield, respectively), but the
E/Z ratio was lower (Table 1, entries 3-4). Other inorganic bases
2 | J. Name., 2012, 00, 1-3
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Table 3. Substrate scope for monosubstituted olefins[a]
.
O
View Article Online
Na2CO3
NH2
DOI: 10.1039/C9GC04303A
NH2
NH2
R2
O
R
NaOH
R1
MeCN/H2O(10:1), Blue LED
R1
R2
R
MeCN/H2O(10:1), Blue LED
E-3a-3i
Z-3a-3i
E-2a-2j
Z-2a-2j
NH2
NH2
O
O
O
MeO
H3CO
Z-3c,
Me
H3C
Me
Z-3a,
95%, Z/E=56:44
95%, Z/E=66:34
Z-3b,
93%, Z/E=62:38
Z-2c,
Z-2a
92%, Z/E=92:8
, 96%, Z/E=93:7
Z
-2b,
93%, Z/E=91:9
H3CO
NH2
O
NH2
NH2
O
MeO
H3C
O
Cl
F
O2N
MeO
Z-3f,
95%, Z/E=5:95
OMe
, 91%, Z/E = 85:15
Z-3e,
94%, Z/E=61:39
Z-3d,
92%, Z/E=57:43
Z-2f
, 93%, Z/E=91:9
Z-2e
, 93%, Z/E=82:18
Z-2d
NH2
O
NH2
O
NH2
O
F3C
H2N
Br
Cl
O2N
F
Z-3i,
88%, Z/E=41:59
Z-3g,
Z-3h,
91%, Z/E=53:47
92%, Z/E=46:54
Z-2i
, 90%, Z/E= 88:12
Z-2h
, 78%, Z/E=65:35
Z-2g,
80%, Z/E=74:26
91%, Z/E= 81:19
[a] Reaction conditions: E-3 (0.3 mmol), Na2CO3 (0.3 mmol ), MeCN
(2mL), H2O (0.2mL), 30W blue led, air, rt.
OHC
N
, 72%, Z/E=77:23
Our success in the isomerization of stilbenes motivated us to
explore other polarized, unsymmetrical alkenes, such as chalcone,
Z-2k
Z-2j,
[a]
Reaction conditions: E-2 (0.3 mmol), NaOH (0.3 mmol), MeCN
cinnamaldehyde,
cinnamate
and
cinnamamide.
protocol.
(2mL), H2O (0.2mL), 30W blue led, air, rt.
However, only cinnamamides were amenable to this
When use Na2CO3 instead of NaOH as catalyst, cinnamamide can
achieved 95% yield and Z/E configuration ratio of 52:48 (Z-3a).
Under this condition, substrates with electron-donating groups can
achieved good yields and moderate Z / E ratios (Z-3b, Z-3c, Z-3d, Z-
3e), the yield of Z-cinnamamides from 92%-95%, and the Z/E ratio
from 57:43 to 66:34. Cinnamamide substrates with halogen groups
also give good results, the yield from 88% to 92% and the Z/E ratio
from 41:59 to 53:47. However, strongly electron-withdrawing group
such as 4-nitrocinnamamide can only got a Z/E ratio of 5:95 were
lower.
Monosubstituted stilbene substrates were equally suitable for
use in the photocatalytic system. Substrates that contained electron
donating groups in the mtea- or para- position,
the reactions proceeded well with high Z-alkenes conversion (Z-2a,
Z-2b, Z-2c), the yield from 92% to 96% and the Z/E ratio from 91:9
to 93:7. When one benzene ring contained three methoxy group
substitutents, the system also shows good catalytic activity, gaving
the 93% yield and a Z/E configuration ratio of 85:15 (Z-2d). The
halogen-substituted substrate 4-fluorostilbene gave a yield of 83%
and a Z/E ratio of 82:18 (Z-2e), 4-chlorostilbene gave a similar result
(Z-2f). When the substrate contained strong electron withdrawing
groups like the trifluoromethyl and nitro groups, the yield and the
Z/E ratio were decreased (Z-2g, Z-2h). Furthermore, the para-
position has an amino or aldehyde group which could smoothly
obtained the desired Z-alkenes (Z-2i, Z-2j). In addition, good result
was also obtained of styrene pyridine (Z-2k), the reaction worked
well to deliver the desired product in 72% yield and 77:23 ratio of
Z/E form.
D
D
NaOH
+
MeCN/D2O, Blue LED
E-1a
Z-1a
nd
92% yield, Z/E=92:8
Scheme 1. Deuteration experiment
To understand the mechanistic pathway and the possible
intermediates, a number of control experiments were carried out.
First, we replaced the water with deuterated water in order to
verify whether the water is involved in the reaction. After reaction
24 hours, we did not detect any deuterated product (Scheme 1).
Next, the reaction was carried out under the N2 atmosphere，and
added the free-radical scavenger TEMPO led 8:92 ratio. The
involvement of superoxide anion (O2•−) was investigated by carrying
out the model reaction in the presence of radical scavenger, 2,2-
diphenyl-1-picrylhydrazyl (DPPH). Notably, no Z-form stilbene was
Table 4. Substrate scope for cinnamamide [a]
.
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formed (Scheme 3b). It clearly indicates the suppression of
superoxide anion (O2•−) formation by the radical scavenger DDPH
(Scheme 2).
NaOH
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Ph
DOI: 10.1039/C9GC04303A
Ph
Ph
Ph
MeCN / H2O, Blue LED
Z-1a
E-1a
air (O2)
NaOH
hv
base
N
MeCN / H2O,
2
O2
Ph
94% yield, Z/E = 11:89
93% yield, Z/E = 8:92
NR
Ph
Ph
Ph
standard conditions
1c
1b
2.0 equiv TEMPO
Figure 3. Proposed mechanism of the isomerization reaction.
Based on the control experiments and EPR experiments,
proposed mechanism for this photocatalyst-free photocatalytic E-
to-Z isomerization of olefins is given in Figure 3. Initially, oxygen will
produce superoxide radical under visible light irradiation,
accompanied by the interaction of stilbene and sodium hydroxide,
stilbene transforms into radical cation intermediate 1b.
Subsequently, a double radical intermediate 1c was generated
under the action of superoxide radical with 1b, and finally inverted
to form Z-stilbene.
standard conditions
2.0 equiv DPPH
Scheme 2. Control experiments
NaOH
MeCN/H2O, LED
Ph
a)
Ph
Conclusions
In summary, a simple and clean method has been developed for
the E → Z isomerization of stilbenes and cinnamamides under
visible light. This system uses only the inexpensive and readily
available base NaOH or Na2CO3 as a catalyst without requiring
metal catalysts or organic dyes as photocatalyst. This visible light-
promoted system yields good catalytic results for both symmetric
stilbene substrates and asymmetric olefins, cinnamamide
compounds can also achieve good conversion.
NaOH
Ph
b)
Ph
MeCN, LED
Ph
c)
d)
Ph
MeCN/H2O, LED
MeCN/H2O, LED
Conflicts of interest
There are no conflicts to declare.
NaOH
Acknowledgements
Field / G
Figure 2. EPR spectra (X band, 9.4 GHz, rt) of (a) (E)-1a and NaOH in
the mixture of MeCN and water, under blue LED irritation 2h; (b)
(E)-1a and NaOH in MeCN, under blue LED irritation 2h; (c) (E)-1a in
the mixture of MeCN and water, under blue LED irritation 2h; (d)
NaOH in the mixture of MeCN and water, under blue LED irritation
We are very grateful to the Progaram for Innovative Research Team
(in Science and Technology) in University of Yunnan Province and
the Applied Basic Research Project of Yunnan Province
(2018FD077) for financial support.
Notes and references
In addition, various EPR experiments were done afterward. As
shown in Figure 2, strong superoxide anion radical signals (O2•−) are
observed in the mixture of (E)-1a and NaOH in MeCN and water
under visible light after the addition of DMPO. Moreover, when the
reaction mixture without water, similar strong radical signals are
detected. However, no signal is found when without (E)-1a or NaOH
in the reaction mixture after the addition of DMPO, indicating that
1a and NaOH were necessaries to formation of the radical.
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J. Name., 2013, 00, 1-3 | 5
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Green Chemistry
Page 6 of 6
COMMUNICATION
Journal Name
View Article Online
DOI: 10.1039/C9GC04303A
Photocatalyst-Free Visible Light Promoted
Isomerization of Alkenes
E Z
Jianbin Xu,# a Na Liu,# a Haiping Lv,b Chixian He,a Zining Liu,a Xianfu Shen,a Feixiang Cheng*a and Baomin Fan*b
Blue LED
R
R
NaOH or Na2CO3
E
Z
photocatalyst-free
29 examples
yield up to 98%;
Z/E up to 97:3
A photocatalyst-free, simple and green method of visible light driven photocatalytic E to Z isomerization of
alkenes has been developed.
6 | J. Name., 2012, 00, 1-3
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