Selective Synthesis of Z-Cinnamyl Ethers and Cinnamyl Alcohols through Visible Light-Promoted Photocatalytic E to Z Isomerization

Article abstract of DOI:10.1002/asia.201901778

A photocatalytic E to Z isomerization of alkenes using an iridium photosensitizer under mild reaction conditions is disclosed. This method provides scalable and efficient access to Z-cinnamyl ether and allylic alcohol derivatives in high yields with excellent stereoselectivity. Importantly, this method also provides a powerful strategy for the selective synthesis of Z-magnolol and honokiol derivatives possessing potential biological activity.

Full text of DOI:10.1002/asia.201901778

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Selective Synthesis of Z-Cinnamyl Ethers and Cinnamyl Alcohols
via Visible Light Promoted Photocatalytic E to Z Isomerization
Authors: Hengchao Li, Hang Chen, Yang Zhou, Jin Huang, Jundan Yi,
Hongcai Zhao, Wei Wang, and Linhai Jing
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To be cited as: Chem. Asian J. 10.1002/asia.201901778
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10.1002/asia.201901778
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Selective Synthesis of Z-Cinnamyl Ethers and Cinnamyl Alcohols
via Visible Light Promoted Photocatalytic E to Z Isomerization
Hengchao Li,†,[a] Hang Chen,†,[a] Yang Zhou,†,[a] Jin Huang,[a] Jundan Yi,[a] Hongcai Zhao,[a] Wei
Wang,*[a] and Linhai Jing*[a]
†
These authors contributed equally to this work
Abstract: A photocatalytic E to Z isomerization of alkenes using an
iridium photosensitizer under mild reaction conditions is disclosed.
This method provides scalable and efficient access to Z-cinnamyl
ether and allylic alcohol derivatives in high yields with excellent
stereoselectivity. Importantly, this method also provides a powerful
strategy for the selective synthesis of Z-magnolol and honokiol
derivatives possessing potential biological activity.
Alkenes are versatile building blocks in biologically active
entities and serve as universal precursors for numerous chemical
transformations to generate important organic compounds in
chemical, natural product, material, and pharmaceutical fields.[1]
Cinnamyl ethers and derivatives have analogous fundamental
properties as important alkenes. Furthermore, they are also
important starting materials for a variety of organic reactions,
including the allylic substitution reactions,[2] aziridoarylation,
bromocarbocyclization,[3] and Claisen rearrangement (Scheme
1a).[4] To date, the common ways to access cinnamyl ethers are
transition-metal-catalyzed
etherification[5]
and
palladium-
catalyzed Mizoroki-Heck reaction (Scheme 1a).[6] However, all of
these strategies have been established to obtain the
thermodynamically favorable E-cinnamyl ethers. By contrast, the
stereoselective generation of Z-cinnamyl ethers are quite limited,
which has been restricted to a limited number of available
synthetic methods. Hence, there is a high demand for developing
methods for the facile synthesis of stereochallenging Z-
alkenes.[2a,7]
Scheme 1. Realization of a photocatalyzed isomerization for the synthesis of
cis-cinnamyl ethers.
utilizing either its electron-transfer or energy-transfer reactivity.[13]
Of which, visible-light-mediated E
Z isomerization of simple
Indeed, a number of conceptually diverse strategies have been
introduced to address this issue, including Wittig reactions,[8]
alkenes can be achieved by using photosensitizers as a safe and
clean energy source (Scheme 1b).[14] This approach has
significant appeal both in terms of atom economy and
environmentally benign nature. In recent years, photocatalysts
such as iridium-base triplet photosensitisers,[15] organic dyes such
as riboflavin,[16] aromatic keto compounds such as 2-iodo-9-
fluorenone,[17] and covalent organic frameworks (COF)[18] have
been employed to promote the E to Z isomerization of alkenes
(Scheme 1b). Detailed investigations by Weaver, Rueping,
Gilmour, Wang, Balaraman and others, carefully delineated the
sensitized isomerization mechanisms and delivered the E to Z
cross-coupling
of
alkenyl
halides,[9]
stereoselective
semihydrogenation of alkynes,[10] Z-selective metathesis[11] and
Peterson olefinations.[12] However, the highlighted reactions lack
high Z-configuration selectivity and often require high energy
reagents and expensive transition-metal based catalysts.
Consequently, the alternative solution for this problem would
allow preparation of the Z-isomer from the more readily available
E-isomer and permit divergence from a common and easily
accessed intermediate.
During the past decade, visible-light photoredox catalysis in
organic synthesis has witnessed a surge in research activity by
isomerization
of
stilbenes,[15b,17,18]
allylamines,[15a]
α,β-
unsaturated esters,[15d,16a] cinnamonitriles,[16c] allylic alcohols,[19]
styrenyl boron species[15f] and vinyl silanes.[20] Inspired by these
seminal investigations, we aim to realize stereoselective
synthesis of cis-cinnamyl ethers via photocatalytic isomerization
of the easily accessed trans-isomers with high fidelity (Scheme
1c). Due to the planar starting material (E-cinnamyl ether) and
1,3-allylic strain[21] of the product (Z-cinnamyl ether), it was
expected that the excited state species generated by an initial
[a]
H.-C. Li, H. Chen, Y. Zhou, J. Huang, J.-D. Yi, H.-C. Zhao, Dr. W.
Wang, Dr. L.-H. Jing
Chemical Synthesis and Pollution Control Key Laboratory of Sichuan
Province/College of Chemistry & Chemical Engineering, China West
Normal University
No. 1 Shi Da Road, Nanchong, 637009 (China)
E-mail: wangwei1987@cwnu.edu.cn; jlhhxg@cwnu.edu.cn
Supporting information for this article is given via a link at the end of the
document.
energy transfer process would contribute to generate
a
1
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10.1002/asia.201901778
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delocalized biradical intermediate[15a,16a,16c] where C-C bond
rotation is possible.
LED light did not lead to the product Z-1a, which again suggests
the visible-light driven photosensitization pathway (Table 1, entry
16). Notably, the better result was obtained in the presence of 1
mol % of fac-Ir(ppy)3 with the concentration of substrate at 0.2 M.
(Table 1, entry 17, Z/E = 90:10).
Inspired by our recent work on a highly efficient palladium-
catalyzed one-pot reaction for building up E-cinnamyl ethers from
simple and readily available aryl halides, phenols and allyl
chloride (Scheme 1a),[6b] our initial investigation was conducted
using E-cinnamyl phenyl ether 1a as a model substrate for the
synthesis of Z-isomer through photocatalytic isomerization. We
were pleased to find that E-1a could be converted to the expected
Z-1a in the presence of fac-Ir(ppy)3 (2 mol%) with a good
selectivity of Z/E 84:16 under irradiation by blue LED (Table 1,
entry 1). Various photocatalysts were then tested in this process,
and the triplet sensitizer fac-Ir(ppy)3 still proved to be highly
effective (Table 1, entries 1-5 and Table S1, entries 1-3). With fac-
Ir(ppy)3 as the photocatalyst, the solvent screening (Table 1 and
Table S1) revealed that THF served as the best solvent for this
reaction, providing the best efficiency and Z/E ratio of the product.
Remarkably, the photochemical isomerization worked well in a
number of solvents (1,4-dioxane, toluene, DMA, or CH3CN) with
the exception of methanol which may be due to the solubility of
the cinnamyl ether or the photosensitizer in the corresponding
solvent (Table 1, entries 6-10). Although there was no obvious
influence noticed on the prolonged reaction time (Table1, entry
11), lowering the reaction temperature had a positive effect on the
reaction performance (Table1, entries 1, 12 and 13, Z/E = 84:16
87:13). Control experiments were conducted and the
results showed that both visible light and the photosensitizer are
essential for this reaction (Table1, entries 14 and 15).
Furthermore, performing the reaction at 70 ℃ in absence of blue
Having established the reaction conditions to achieve the
efficient E to Z isomerization of cinnamyl ethers, we proceeded to
explore the scope of this transformation (Scheme 2). It is
noteworthy that the isolated yields of the Z-isomers were reported
for the reactions with the use of the pure E-isomers as the
substrates. To our delight, the reaction works well for a number of
cinnamyl ether analogues (Scheme 2). For aryl cinnamyl ether
substrates with electron-rich (E-1b, E-1c), electron-deficient (E-
1d, E-1h, E-1i), and halogenated (E-1e–1g) substituents at the
para-position of the phenol ether, the reactions proceeded well
with high Z-alkene conversion (Z-1b–1i, 74-93%). Good results
were also obtained for the substrates bearing the substituted
groups at the ortho position (Z-1j–1o, 76-85%). While an amino
at ortho-position led to the isomerization futile, exchanging NH2
with NHBoc provided a good yield (Z-1l, 76%). Furthermore,
naphthalene substrates (Z-1q, 65% and Z-1r, 60%) as well as
heterocyclic aromatic substrates (Z-1s, 82% and Z-1t, 71%) were
well tolerated with good to excellent selectivities albeit with
increasing the amount of fac-Ir(ppy)3. The structure of Z-1r was
confirmed by X-ray crystallographic analysis,[22] which clearly
Table 1. The screening of reaction conditions for the photocatalytic
isomerization[a]
.
entry
catalyst
solvent
scale
(mmol)
0.1
Z/E
temp (℃)
ratio[b]
84:16
1
2
3
fac-Ir(ppy)3
THF
THF
THF
50
50
50
Ir(ppy)2(bpy)PF6
0.1
0.1
11:89
17:83
Ir(ppy)2(dtbbpy)
PF6
4
5
6
Ru(bpy)3(PF6)2
THF
THF
50
50
50
0.1
0.1
0.1
2:98
3:97
83:17
Rose bengal
fac-Ir(ppy)3
1,4-
dioxane
toulene
7
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
none
50
50
50
50
50
35
25
35
35
70
35
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
80:20
78:22
83:17
13:87
84:16
87:13
87:13
0:100
0:100
0:100
90:10
8
CH3CN
DMA
CH3OH
THF
9
10
11[c]
12
THF
13[c]
14
15[d]
16[d]
17[e]
THF
THF
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
THF
Scheme 2. Photocatalytic E-to-Z isomerization of aryl cinnamyl ethers.
[a] Standard reaction conditions: Reactions were performed with 0.2 mmol of
substrate E-1 (E/Z > 20:1) at 35 ℃ in THF (1.5 mL) using fac-Ir(ppy)3 (1 mol%)
under 24 h of blue LED irradiation, unless otherwise noted. [b] Isolated yields of
the Z-isomer was reported. [c]Determined by 1H NMR spectroscopy of the crude
product. [d] fac-Ir(ppy)3 (2 mol%) and DMA (1.5 mL) were required for
isomerization. [e] fac-Ir(ppy)3 (2 mol%) was used. [f] E/Z ratio of the substrate
1z was 83:17.
THF
THF
[a] Reaction conditions: E-1a (0.1 mmol) and photocatalysis (2 mol%) in the
indicated degassed solvent (1.0 mL) were irradiated by blue LED bulb for 24 h
under argon, unless otherwise noted. [b] Z/E selectivity was determined by 1H
NMR spectroscopy of the crude product. [c] 48 h. [d] Reaction was performed
in darkness. [e] E-1a (0.2 mmol) and fac-Ir(ppy)3 (1 mol%) in THF (1.5 mL).
2
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showed the cis-configuration of the double bond for 1r. Meanwhile, and highly twisted conformation inducing by the Z-isomers
the photochemical isomerization of E-aryl cinnamyl ethers was
applicable to substrates with electron-rich and electron-deficient
substituents on the cinnamyl chain (Z-1u–1x, 75-91%). Moreover,
the same seems to be applicable for the unsaturated bonds such
as allyl and propenyl in ortho-position of the phenol ether, which
resulted in very good yields of 90% (Z-1y) and 90% (Z,Z-1z),
respectively. It is of particular note that the isomerization of the
double bonds in two positions of the substrate E,E-1z was
observed, which indicating the photochemical isomerization
reaction is also wellsuited to alkyl-substituted alkenes. Finally, the
magnolol and honokiol derivatives (E,E-2 and E,E-3) which
possess antitumoral, antiangiogenic, and anxyolitic activities[23]
were also suitable substrates to deliver Z,Z-isomers in high yields
(Z,Z-2, 80% and Z,Z-3 60%).
Encouraged by these results obtained for cinnamyl ethers, a
variety of allylic alcohol substrates were successively modified at
the aryl ring (R1) and the double bond (R2) and investigated under
the standard conditions, as shown in Scheme 3. Unsubstituted Z-
cinnamyl alcohol 4a was obtained in 78% yield. Gratifyingly,
substituting R2 = H by Me or CH2Cl led to a significant
improvement in the geometric ratios (Z-4b, 91% and E-4c, 92%).
This is likely a consequence of augmenting 1,3-allylic strain in the
product. Testing the scope of electronic modulation revealed
various cinnamyl alcohol substrates with electron-withdrawing
and electron-donating groups at the para- or meta-position were
all amenable to generate Z-isomer products in good to high yields
(Z-4e–4j, 4o, 4p, 68-90%). When the substituted group was
introduced at the ortho position, a significant increase in the
reaction efficiency was observed (Z-4k–4n, 93-99%, Z/E = 78:22–
93:7). This observation may be attributable to severely congested
bearing ortho-substituted groups.[16] However, a reduction of
selectivity was observed with ortho-substituents allyl alcohols (R2
= Me, Z-4q, Z/E = 26:74 and Z-4r, Z/E = 48:52). As reported by
Gilmour in their seminal work,[16] this is presumably caused by the
fact that the deconjugation of the starting material as it
compromises the efficiency of the initial excitation of the trans-allyl
alcohols. The Z/E selectivity was slightly increased with the rising
of the temperature and the extension of the photoisomerization
time (Z-4q and Z-4r). The replacement of the benzene ring with a
larger naphthalene ring led to a slight decrease in the geometric
ratio (Z-4s, 80%), possibly due to resonance forms partially
facilitating bond rotation and relaxation to the starting E-
isomer.[16e] What’s more, heterocyclic aromatic substrates were
also tolerated with good to excellent selectivities (Z-4t, 91% and
Z-4u, 72%). However, an obvious decrease in the geometric ratio
was noted for substrate E-4u bearing a thiophene ring. Although
the 5-membered ring retains the requisite aromatic character, 1,3-
allylic strain in both isomers of the thiophene (E-4u and Z-4u) is
significantly reduced compared to the 6-membered ring
analogs.[16,21] These results indicate that distortion in the Z-isomer
product and planarity in the E-isomer substrate are key to
ensuring high levels of stereocontrol.
To demonstrate the synthetic utility of this method, the
photochemical isomerization reaction also proved amenable to
scale-up with the Z-isomer of 1a having been isolated in 82% yield
on a 1 mmol scale using only 0.2 mol% of fac-Ir(ppy)3 (Scheme
4a). Finally, several control experiments to confirm the origin of
the Z-selectivity are shown in scheme 4. In the presence of 1
equiv of a radical scavenger TEMPO in the reaction medium, the
yield of Z-configured product decreases significantly under the
Scheme 3. Scope of photocatalytic isomerization of allylic alcohol derivatives.
[a] Standard reaction conditions: Reactions were performed with 0.2 mmol of
substrate E-4 (E/Z > 20:1) at 35 ℃ in THF (1.5 mL) using fac-Ir(ppy)3 (1 mol%)
under 24 h of blue LED irradiation. [b] Isolated yields of the Z-isomer was
reported. [c] Determined by 1H NMR spectroscopy of the crude product. [d]
Isolated yields of Z/E mixtures and the Z/E ratio were determined by 1H NMR
spectroscopy. [e] at 50 ℃ for 36 h.
Scheme 4. Mechanistic investigations of this reaction.
3
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standard conditions (Z/E = 90:10
33:67). Substituting methyl
by cyclopropyl on the double bond led only to a very minor erosion
of selectivity (Z-4b and Z-4v, Z/E = 95:5 and 93:7, respectively).
However, the installation of cyclopentyl or cyclohexyl led to a
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Figure S1-S3). According to Hammond’s mechanistic analyses,[24]
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4b 91%, Z-1a 87%, Z-4a 78%), shows that selectivity correlate
with quenching efficiency. Additional support for the biradical
intermediate is reflected in the increasing Z/E ratio when
improving the stabilizing auxiliary groups.
In conclusion, we have established an efficient and
operationally simple protocol to access thermodynamically less
stable cis-alkenes through a photochemical isomerization. Many
cis-cinnamyl ethers and cinnamyl alcohols, which are not readily
accessible via classic methods, can thus be achieved in
synthetically useful ratios and yields. This reaction is remarkably
clean and tolerates a wide range of functional groups with regard
to both the cinnamyl ethers and cinnamyl alcohols. In addition, Z-
configured magnolol and honokiol derivatives, which possess
potential biological activity, could be easily obtained in good yields
and selectivity by this straightforward photocatalysis
isomerization.
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Acknowledgements
We acknowledge financial support from the National Natural
Science Foundation of China (21602144), and the Science and
Technology Program of Sichuan Province (2018JY0485).
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Keywords: Allylic alcohols • Cinnamyl ethers • Isomerization •
Iridium • Photocatalysis
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Entry for the Table of Contents
An efficient geometrical E Z isomerization of cinnamyl ethers and cinnamyl alcohols is disclosed via energy transfer using an
iridium photosensitizer. A series of contra-thermodynamic cis-cinnamyl ether and alcohol derivatives were achived through a
photoisomerization of the readily available trans-isomer. This simple reaction system shows satisfactory Z-selectivity, broad substrate
scope, and functional group tolerance.
Hengchao Li,† Hang Chen,† Yang Zhou,† Jin Huang, Jundan Yi, Hongcai Zhao, Wei Wang,* and Linhai Jin*
Selective Synthesis of Z-Cinnamyl Ethers and Cinnamyl Alcohols via Visible Light Promoted Photocatalytic E to Z
Isomerization
6
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