Copper-Photocatalyzed Contra-Thermodynamic Isomerization of Polarized Alkenes

Article abstract of DOI:10.1021/acs.orglett.0c02894

The contra-thermodynamic isomerization of α- and β-substituted cinnamate derivatives catalyzed by the Cu(OAc)2/rac-BINAP complex under blue light irradiation is reported. The use of an oxazolidinone template, which favored the complexation of the copper catalyst to the substrate, allowed the E → Z isomerization of the catalytically formed chromophore under simple and robust reaction conditions in good to excellent ratios. The mechanism of this process based on the transient formation of a chromophore was also studied.

Full text of DOI:10.1021/acs.orglett.0c02894

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Letter
Copper-Photocatalyzed Contra-Thermodynamic Isomerization of
Polarized Alkenes
́
Thibaud Bregent, Jean-Philippe Bouillon, and Thomas Poisson*
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ABSTRACT: The contra-thermodynamic isomerization of α- and
β-substituted cinnamate derivatives catalyzed by the Cu(OAc)2/
rac-BINAP complex under blue light irradiation is reported. The
use of an oxazolidinone template, which favored the complexation
of the copper catalyst to the substrate, allowed the E → Z
isomerization of the catalytically formed chromophore under
simple and robust reaction conditions in good to excellent ratios.
The mechanism of this process based on the transient formation of a chromophore was also studied.
Scheme 1. State of the Art and Present Work
lkenes are feedstocks and important molecules in the
arsenal of organic chemists to build up complex
A
molecules. Moreover, olefins are ubiquitous in Nature and
present in a myriad of natural products. Therefore, the control
of the alkene geometry is of paramount interest and stimulated
the organic practitioners to develop efficient methods.1 This
endeavor gave birth to efficient olefination reactions, as the
Horner−Wadsworth−Emmons, Julia−Kocienski, Peterson, or
Wittig olefination reactions, for example,2 or to stereo-
controlled olefin metathesis reactions among others.3 As an
alternative to control the stereochemistry of an alkene, the
geometrical isomerization of a preexisting olefin4 is a promising
approach that fits with the principle of atom economy.5 While
the thermodynamically favored Z → E isomerization is well-
known, the opposite contra-thermodynamic E → Z isomer-
ization remains more challenging, and practical synthetic
methods to achieve this goal still remain highly desirable. With
that respect, photochemical activation modes relying either on
the direct excitation of the olefin with UV light (λ < 300 nm)
or the use of triplet sensitizers are the predominant pathways
to permit this endergonic process. Although tremendous
advances were made since the middle of the twentieth century
in the development and the understanding of this process,
most of the methods relied on the use of UV irradiation and
present a limited substrate scope (Scheme 1, eq 1).6 From
2014, a new impetus was given to the field by Weaver, who
demonstrated the possible E → Z isomerization of allylic
amines thanks to an energy transfer (EnT) between an Ir
photosensitizer and the substrate under visible-light irradiation
(Scheme 1, eq 2).7 Subsequently, Gilmour brought significant
milestones to the field and developed several EnT-based
isomerization processes to access various Z-olefins using
organic and metal-based photosensitizers from the E-isomer
(Scheme 1, eqs 2 and 3).8
Received: August 28, 2020
These important contributions demonstrated the possibility
of carrying out the geometrical interconversion of the
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Scheme 2. (A) Design Plan. (B) Optimization of the Structure of the Template. (C) Optimization of the Reaction Conditions
and Sensitivity Tests
thermodynamically favored E-isomer into the contra-thermo-
dynamic Z-isomer using an EnT manifold under simple
reaction conditions. In addition, the use of copper salt in
photochemistry has a long history, particularly in [2 + 2]
cycloaddition under UV irradiation.9 Surprisingly, copper has
been scarcely used in alkene isomerization.10 In that respect,
we should mention the work from Deyrup, who studied the
Cu-photocatalyzed cis → trans isomerization of cyclooctene
using CuCl2, albeit with a moderate efficiency (Scheme 1, eq
4).10b As part of our program to develop an efficient copper-
catalyzed and photocatalyzed process,11 we sought to develop
a practical method for the E → Z isomerization of polarized
alkenes under visible-light irradiation. In the early 1980s, Lewis
brought an alternative concept to permit the E → Z
isomerization of polarized alkenes using a Lewis acid catalyst
(Scheme 1, eq 5).12 The addition of boron- or aluminum-
based Lewis acid favored the contra-thermodynamic con-
version of E-cinnamates into the Z-isomer under UV
irradiation (λ = ca. 300 nm). This facilitated interconversion
results from a change of the spectroscopic properties and
therefore of the photochemical behavior of the substrate.
With the aim of providing a straightforward protocol to
perform E → Z isomerization, we conjectured that the use of a
copper salt coordinated with an appropriate ligand, in a
catalytic amount, should allow this endergonic process under
visible-light irradiation by forming a transient chromophore
species (Scheme 1, eq 6).13 The coordination, by changing the
HOMO and LUMO levels of the substrate, will induce a
bathochromic shift in the absorption (π → π* transition) of
the complexed substrate and therefore a lowest energy
transition from the ground state (S0) to the existing state
(S1). Importantly, apart the compulsion to proceed under
visible-light irradiation, the success of the approach has several
prerequisites. (1) The E → Z quantum yield (ΦE→Z) should be
enhanced upon complexation and higher than the Z → E
quantum yield (ΦZ→E) of both complexed and noncomplexed
olefins, (2) the complexed E substrate should have a stronger
light absorption of than the Z-one, and (3) the E-isomer
should have a larger equilibrium constant for complexation
than the Z-isomer (Scheme 2A). To address these challenges,
we sought to take benefit from the ability of Cu complexes to
coordinate a cinnamate derivative bearing an appropriate
achiral template to carry out this E → Z isomerization
(Scheme 2A).14 These templates have been widely used in
Lewis acid catalysis and photocatalytic [2 + 2] cycloaddition to
perform catalytic enantioselective transformations.12e−m,14 At
the outset of the project, we studied the α-methyl cinnamate
backbone as a model. Indeed, to our knowledge, no efficient
and general isomerization protocol of these substrates was
reported yet.15 The contra-thermodynamic isomerization of the
E-methyl α-methylcinnamate 1 into the Z-isomer was observed
in the presence of 5 mol % of Cu(OAc)2 and rac-BINAP in
71% yield after irradiation with blue LEDs. Importantly, no
significant isomerization occurred when (E)-1 was irradiated in
the absence of Cu(OAc)2 and/or rac-BINAP, demonstrating
the ability of this complex to promote the E → Z geometrical
inversion. Encouraged by this promising proof of concept, we
evaluated various achiral templates that would enhance the
coordination of the Cu-complex (Cu(OAc)2/rac-BINAP) and
favor the isomerization. Amide (E)-2 was less efficient than the
ester, while the introduction of a phosphate coordination site
increased the isomerization into (Z)-3 to 72%. Pleasingly,
oxazolidinone proved to be an efficient template and allowed
the isomerization of (E)-4 into (Z)-4 in 85% yield. Other
templates, N-methyl-2-aminopyridine, azaindoline, and pyr-
azole, were slightly less efficient than the oxazolidinone.16
Then the reaction conditions were optimized, and the use of
EtOAc instead of THF permitted the isomerization of (E)-4
into (Z)-4 in 89% yield (Scheme 2C).16 It is worth mentioning
that the Cu(OAc)2/rac-BINAP was the best combination as all
other ligands tested were less efficient in this isomerization
process.16 The replacement of Cu(OAc)2 by other metal salts,
Pd, Zn, Ni, and Mn, did not afford any isomerization
reaction.16 These results clearly demonstrated the real added
value of using copper in terms of efficiency, cost, and
sustainability. Finally, control experiments demonstrated that
the reaction was inefficient in the absence of light, while the
use of thermal conditions (80 °C) in the dark was
unproductive as well. To demonstrate the versatility and to
ensure the reproducibility of our protocol, the sensitivity
assessment regarding the reaction parameters was carried out
(Scheme 2C).17 Interestingly, this contra-thermodynamic
isomerization protocol is robust and insensitive toward
temperature, light intensity, scale, moisture, and oxygen.
Note that a slight decrease of the amount of (Z)-4 was
observed under diluted conditions (35% drop).16
Having these optimized conditions in hand, we evaluated the
scope and limitations of this contra-thermodynamic isomer-
ization protocol (Scheme 3A). The introduction of an ethyl
substituent at the α-position reduced the efficiency of the
isomerization process as the obtained Z/E ratio dropped from
89:11 to 78:22. The substitution pattern was evaluated with
the introduction of a methyl substituent at the para, meta, and
ortho positions of the aromatic ring (9−11). The Z/E ratio
B
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Scheme 3. Photoinduced Isomerization Catalyzed by Cu(OAc)2/rac-BINAP under Blue LEDs Irradiation*
*
The Z/E isomerization ratio was determined on the crude reaction mixture using a GC−FID analysis. a Z:E Isomerization ratio determined on the
1
b
c
crude reaction mixture by H NMR. 24 h reaction time instead of 17 h. Ethyl ester derivative was used as the substrate.
decreased from 93:7 for the para-substituted substrate to 85:15
for the meta one, while the ortho substitution gave a lower
71:29 Z/E ratio.
Various electron-donating groups were introduced on the
aromatic ring, and the Z/E ratio ranged from 93:7 to 73:27 for
13, while the naphthyl derivative 15 gave a moderate 77:23 Z/
E ratio. Halogen atoms and a CF3 group were well tolerated
and provided a fairly decent Z/E ratio (16−20, up to 95:5).
Then the presence of other functional groups on the aromatic
ring was evaluated.
The synthetically useful Bpin derivative 21 was tested and
gave an excellent 95:5 ratio, the methylsulfone substituted
C
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Scheme 4. Mechanistic Study of the E → Z Isomerization Catalyzed by Cu(OAc)2/rac-BINAP
substrate (22) provided a 98:2 isomerization ratio, while the
presence of a keto substituent (23) provided a lower
isomerization ratio. The introduction of five-membered
heterocycles (24 and 25) was deleterious as the isomerization
ratio dropped to 60:40 and 76:24, respectively, probably due to
a minimization of the 1,3-allylic strain (A1,3) in the Z-isomers.
Interestingly, we demonstrated that the presence of the
oxazolidinone was mandatory for this transformation with
the substrate 26 bearing a O-cinnamyl substituent. Indeed,
only the isomerization of the olefin bearing the oxazolidinone
was observed, albeit with a moderate 63:37 Z/E ratio.
Unfortunately, some substrates were reluctant in our hand,
highlighting the limitation of the process. The sterically
hindered mesityl derivative 27 remained unreactive probably
due to of geometrical distortion that hamper the conjugation
with the olefin residue. In the same vein, the presence of an
alkyl substituent (28) shut down the isomerization, which
highlighted the importance of a conjugation between the
aromatic derivative and the olefin part.
Then this isomerization protocol was extended to the β-
substituted cinnamate derivatives having an oxazolidinone or
ester template (Scheme 3B). Interestingly, our isomerization
protocol was suitable for the ester derivatives, as the reaction
with 30 gave a 97:3 Z/E ratio, while the E-isomer of 29 bearing
the oxazolidinone template gave a slightly lower 91:9 Z/E
ratio. The introduction of an ethyl substituent at the β-position
(31−32) required the introduction of the oxazolidinone
template to afford a decent 97:3 Z/E ratio. Then, the
substitution on the aromatic ring was studied. The presence
of electron donating group was well tolerated and our
isomerization process was efficient on ester or oxazolidinone
derivatives in most of the cases, except for 35, 37, and 41, for
which the presence of the oxazolidinone template was
beneficial. Halogen-substituted cinnamates provided an ex-
cellent Z/E ratio for both esters and oxazolidinone (43, 45−
48), except for 44, which required the presence of the
oxazolidinone template to reach a decent ratio. Interestingly,
the nitro substituent gave lower isomerization ratio (49 and
50), probably due to its ability to act as a singlet quencher
(vide infra).18 Subsequently, to evaluate the efficiency of our
protocol, we investigated other substrates (Scheme 3C). The
ethyl β-trifluoromethylcinnamate (51) gave a low Z/E ratio,
while the addition of the oxazolidinone template provided a
slight increase to 33:67 (52). Then the nitrile, phosphonate,
and sulfone analogues were evaluated (53−55). The
acrylonitrile derivative (53) gave an interesting 79:21 Z/E
ratio, while 54 and 55 were poorly reactive under our
isomerization conditions (Z/E ratio <15:85).
Finally, the mechanism of this isomerization process was
studied. The UV−vis measurements of the mixture of the E
and Z isomers of 31 with 10 mol % of the Cu(OAc)2/rac-
BINAP complex were carried out (Scheme 4A). Interestingly,
an increase of the absorption of the E-isomer with the catalyst
was observed (green), while the mixture with the Z-isomer
(yellow) provides a similar absorption spectrum as the one of
the Cu(OAc)2/rac-BINAP mixture. These results support a
better coordination of the copper catalyst with the E-isomer
than with the Z-one along with a better absorption of the light,
in agreement with our initial mechanistic analysis (Scheme
2A). Then, the isomerization of (E)-31 under an atmosphere
of O2 and 1,3-cyclohexadiene (both triplet quenchers) and in
the presence of azulene (a singlet and triplet quencher) were
carried out (Scheme 4B). The presence of an oxygen
atmosphere and 1,3-cyclohexadiene did not change the
outcome of the reaction, while the addition of azulene strongly
hampered the efficiency of the reaction. These results
suggested the involvement of a singlet state mechanism for
this isomerization, in agreement with the observation made for
the isomerization of ethyl cinnamate19 and cinnamides.12c
Finally, a quantum yield of 0.9 was determined for the E → Z
isomerization (ΦE→Z = 0.9), while the Z → E isomerization
was found as an inoperative process under our reaction
conditions (ΦZ→E = 0). These results clearly explained the
efficiency of our developed protocol for the E → Z
isomerization of α- and β-substituted cinnamate derivatives
bearing the oxazolidinone template.
In conclusion, we developed herein a novel concept for the
E → Z contra-thermodynamic isomerization under visible light
irradiation using the combination of Cu(OAc)2/rac-BINAP as
a catalyst. This robust catalytic system, insensitive to moisture
and air, was efficient for the underexplored isomerization of α-
methylcinnamate derivatives. Likewise, this reaction manifold
was extended to the isomerization of β-substituted cinnamate
derivatives with excellent and similar efficiency as those already
reported. The efficiency of the reaction was explained thanks to
a better coordination of the Cu(OAc)2/rac-BINAP complex to
the E-isomer along with a higher quantum yield for the E → Z
isomerization. Finally, control experiments with quenchers
suggested a singlet-state mechanism.
D
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2020, 59, 330−334. (b) Faßbender, S. I.; Molloy, J. J.; Muck-
̈
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02894.
■
Lichtenfeld, C.; Gilmour, R. Geometric E→ Z Isomerisation of
Alkenyl Silanes by Selective Energy Transfer Catalysis: Stereo-
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Metternich, J. B.; Daniliuc, C. G.; Watson, A. J. B.; Gilmour, R.
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sı
Experimental procedures, compound characterization
1
data, and H and 13C spectra of the products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Thomas Poisson − Normandie Univ, INSA Rouen,
UNIROUEN, CNRS, 76000 Rouen, France; Institut
Universitaire de France, 75231 Paris, France; orcid.org/
0000-0002-0503-9620; Email: thomas.poisson@insa-
rouen.fr
Authors
́
Thibaud Bregent − Normandie Univ, INSA Rouen,
Bremen-Kuhne; von, M.; Neugebauer, J.; Gilmour, R. Photocatalytic
̈
E→ Z Isomerization of Polarized Alkenes Inspired by the Visual
Cycle: Mechanistic Dichotomy and Origin of Selectivity. J. Org. Chem.
2017, 82, 9955−9977. (h) Metternich, J. B.; Gilmour, R. A Bio-
Inspired, Catalytic E→ Z Isomerization of Activated Olefins. J. Am.
Chem. Soc. 2015, 137, 11254−11257. (i) During the preparation of
this manuscript, Gilmour and co-workers reported the photocatalytic
UNIROUEN, CNRS, 76000 Rouen, France
Jean-Philippe Bouillon − Normandie Univ, INSA Rouen,
UNIROUEN, CNRS, 76000 Rouen, France
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02894
̈
isomerization of β-borylated alkenes; see: Molloy, J. J.; Schafer, M.;
Notes
Wienhold, M.; Morack, T.; Daniliuc, C. G.; Gilmour, R. Boron-
Enabled Geometric Isomerization of Alkenes via Selective Energy-
Transfer Catalysis. Science 2020, 369, 302−306.
The authors declare no competing financial interest.
(9) (a) Moggi, L.; Juris, A.; Sandrini, D.; Manfrin, M. F.
Photocatalysis by Transition-Metal Coordination Compounds in
Homogeneous Phase. Part II: Photochemical Steps Involving the
Organic Substrate. Rev. Chem. Intermed. 1984, 5, 107−155.
(b) Salomon, R. G. Homogeneous Metal-Catalysis In Organic
Photochemistry. Tetrahedron 1983, 39, 485−575.
(10) (a) Onishi, M.; Hiraki, K. Phosphine Steric Effects on the
Catalytic Activities of a Few Chloro(tertiary phosphine)copper(I)
Oligomers in Photoisomerizations of Norbornadiene and trans-
Stilbene. Inorg. Chim. Acta 1992, 202, 27−30. (b) Deyrup, J. A.;
Betkouski, M. F. Alkene Isomerization. An Improved One-Step
Synthesis of trans-Cyclooctene. J. Org. Chem. 1972, 37, 3561−3562.
(c) Evers, J. T. M.; Mackor, A. Photocatalysis III Photochemical
Isomerization of Cyclohexenes and Cycloheptene in the Presence of
Copper(I) Trifluoromethanesulfonate. Identification and Acid-Cata-
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2321−2324.
(11) (a) Poutrel, P.; Ivanova, M. V.; Pannecoucke, X.; Jubault, P.;
Poisson, T. Copper-Catalyzed Enantioselective Formation of C-CF3
Centers From β-CF3 -Substituted Acrylates and Acrylonitriles. Chem. -
Eur. J. 2019, 25, 15262−15266. (b) Nitelet, A.; Thevenet, D.; Schiavi,
B.; Hardouin, C.; Fournier, J.; Tamion, R.; Pannecoucke, X.; Jubault,
P.; Poisson, T. Copper-Photocatalyzed Borylation of Organic Halides
Under Batch and Continuous-Flow Conditions. Chem. - Eur. J. 2019,
25, 3262−3266. (c) Ivanova, M. V.; Bayle, A.; Besset, T.;
Pannecoucke, X.; Poisson, T. Copper-Mediated Introduction of the
CF2PO(OEt)2 Motif: Scope and Limitations. Chem. - Eur. J. 2017, 23,
17318−17338. (d) Ivanova, M. V.; Bayle, A.; Besset, T.;
Pannecoucke, X.; Poisson, T. Copper Salt-Controlled Divergent
Reactivity of [Cu]CF2PO(OEt)2 with α-Diazocarbonyl Derivatives.
Angew. Chem., Int. Ed. 2016, 55, 14141−14145. (e) Ivanova, M. V.;
Bayle, A.; Besset, T.; Poisson, T.; Pannecoucke, X. Copper-Mediated
Formation of Aryl, Heteroaryl, Vinyl and Alkynyl Difluoromethyl-
phosphonates: a General Approach to Fluorinated Phosphate Mimics.
Angew. Chem., Int. Ed. 2015, 54, 13406−13410. (f) Bayle, A.; Cocaud,
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Mediated Synthesis of Aryldifluoromethylphosphonates: a Sandmeyer
Approach. Eur. J. Org. Chem. 2015, 2015, 3787−3792.
ACKNOWLEDGMENTS
This work was partially supported by Normandie Universite
■
́
́
(NU), the Region Normandie, the Centre National de la
́
Recherche Scientifique (CNRS), Universite de Rouen
Normandie (URN), INSA Rouen Normandie, Labex SynOrg
(ANR-11-LABX-0029), and Innovation Chimie Carnot (I2C).
́
T.B. thanks the Region Normandie for a doctoral fellowship
(RIN 50% program). T.P. thanks the Institut Universitaire de
France (IUF) for support and the Agence National pour la
Recherche (ANR-CE07-0004-1) for funding.
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