Heteroleptic Copper-Based Complexes for Energy-Transfer Processes: E → Z Isomerization and Tandem Photocatalytic Sequences

Article abstract of DOI:10.1021/acscatal.1c01983

Energy-transfer processes involving copper complexes are rare. Using an optimized heteroleptic copper complex, Cu(bphen)(XantPhos)BF4, photosensitized E → Z isomerization of olefins is demonstrated. The XantPhos ligand afforded sensitizers with improved catalyst stability, while the bphen ligand lengthened the excited-state lifetime. A series of 25 di- and trisubstituted alkenes underwent photoisomerization, including macrocycles and 1,3-enynes. Cu(bphen)(XantPhos)BF4 could also be employed in a tandem ATRA/photoisomerization process employing arylsulfonyl chlorides, an example of photoisomerization with halide-substituted olefins.

Full text of DOI:10.1021/acscatal.1c01983

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Research Article
Heteroleptic Copper-Based Complexes for Energy-Transfer
Processes: E → Z Isomerization and Tandem Photocatalytic
Sequences
Corentin Cruché, William Neiderer, and Shawn K. Collins*
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ABSTRACT: Energy-transfer processes involving copper complexes are rare. Using an optimized heteroleptic copper complex,
Cu(bphen)(XantPhos)BF4, photosensitized E → Z isomerization of olefins is demonstrated. The XantPhos ligand afforded
sensitizers with improved catalyst stability, while the bphen ligand lengthened the excited-state lifetime. A series of 25 di- and
trisubstituted alkenes underwent photoisomerization, including macrocycles and 1,3-enynes. Cu(bphen)(XantPhos)BF4 could also
be employed in a tandem ATRA/photoisomerization process employing arylsulfonyl chlorides, an example of photoisomerization
with halide-substituted olefins.
KEYWORDS: copperphotocatalysis, isomerization, tandem processes, olefins
state lifetimes. As mentioned above, an emerging ET process in
organic synthesis is photocatalytic E ↔ Z isomerization.
Control of alkene geometry is known to be critical to biological
function,8−10 but it is equally important in the context of
organic synthesis. Alkenes remain one of the most utilized
building blocks, and olefination methods such as phosphorous
ylide chemistry11,12 and catalytic olefin cross metathesis13 have
identified control of the resulting olefin geometry as a key goal.
In 1964, Hammond and co-workers reported the E ↔ Z
isomerization of stilbenes through sensitized excitation,14
where the extended conjugation of the arene and alkene π-
systems in the E-isomer can be preferentially excited over the
Z-isomer. The resulting Z-isomer is relatively deconjugated
through the twisting of the arene due to allylic strain. The
strategy proposed by Hammond represents an interested shift
in thinking on how to control olefin geometry in synthesis, as
the Z-isomer could become accessible from any E/Z mixture
1. INTRODUCTION
Heteroleptic copper-based complexes possessing one diimine
ligand (NN) and bisphosphine (PP) have exhibited consid-
erable potential in photocatalysis.1 The potential for broad
tuning of photophysical properties has been exploited for a
variety of photoredox processes.2 Recently, our group has
demonstrated that control of the ligand framework can also
fine-tune complexes for other photocatalytic processes (Figure
1).3 For example, judicious choice of ligands resulted in the
first proton-coupled electron-transfer (PCET) reactions
catalyzed by Cu(NN)(PP)X complexes.3,4 The use of
copper-based catalysts for energy-transfer processes is rare. A
recent example was reported by Poisson and co-workers, where
a Cu(OAc)2/BINAP system was exploited for photoisomeri-
zation of polarized alkenes.5 Surprisingly, only a single report
of Cu(NN)(PP)X complexes for synthetic energy-transfer
(ET) processes has been reported, where a screening afforded
a Cu(dmp)(BINAP)BF4 complex as optimum for vinyl azide
sensitization.3 In 2002, McMillin and co-workers reported that
complexes of the type Cu(NN)(PP)X can have excited-state
lifetimes as high as ∼16 μs,6 which would be beneficial for
transformations occurring via ET.7 In addition, optimizing the
diimine or the bisphosphine ligand could fine-tune important
photophysical parameters such as triplet energies and excited-
Received: May 1, 2021
Revised: June 23, 2021
Published: July 4, 2021
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© 2021 American Chemical Society
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(MeCN, Purple LEDs, 400 nm, shown as dark green bars).
Finally, an attempt was made to evaluate the photostability of
the complexes. Solutions of the complexes were irradiated for
24 h, and the %catalyst remaining was calculated by 1H
NMR25 (indicated as %Cu in light green bars, Figure 2).
Successful E ↔ Z photoisomerization relies on selectively
activating one geometric isomer over the other. Three
complexes were shown to promote >96% E → Z isomer-
ization.26 Although the Cu(dmp)(BINAP)BF4 complex
possessed promising photophysical properties and promoted
isomerization (83%Z), stability studies showed that the
complex had completely degraded in solution during the
time of irradiation (indicated as %Cu in light green bars in
Figure 2) and other complexes displaying greater stability were
prioritized. Neither the corresponding homoleptic complex
(Cu(dmp)2BF4) nor the Cu(dmp)(dppf)BF4 complex re-
sulted in any conversion of E- to Z-1 (Figure 2). Note that
complexes containing the dppf ligand have been reported to
possess competing photoinduced energy transfer from the
Cu(I) complex to the ferrocene moiety with little observable
emission at room temperature.25b When evaluating the four
criteria (ET, τ, %Z, and %Cu), both the Cu(dmp)(XantPhos)-
BF4 and Cu(dmp)(DPEPhos)BF4 complexes were identified
as most promising since they ranked consistently high in τ, %Z,
and %Cu. To distinguish between the complexes, isomer-
ization processes were evaluated at only 1 h of reaction time
(see insert in Figure 2). The %Z for the copper complex based
on DPEPhos dropped to 64%, while the %Z for the XantPhos
complex decreased to only 74%. As such, a slight edge was
given to the Cu-based complex bearing XantPhos, and further
optimization was conducted to investigate the effect of diimine.
Given the promising results with Cu-based complexes bearing
XantPhos as a bisphosphine, another screening was performed
using 17 different diimines with XantPhos as bisphosphine
(Figure 2b). Diimines included phenanthroline- and bipyr-
idine-based scaffolds and possessed different degrees of
substitution that influenced both steric and electronic
properties. Once again, all complexes were evaluated against
four criteria (ET, τ, %Z, and %Cu). A total of six sensitizers
demonstrated >90% E → Z isomerization (Figure 2b, dark
green bars). The three complexes showing the largest %Z were
based on the ligands dmp, bphen, and bathocup. The
bathocup-containing complex had low stability (24%Cu), so
all three complexes were evaluated in the same model
photoisomerization but for only 1 h of irradiation time.
Indeed, the Cu(bphen)(XantPhos)BF4 complex was the only
candidate to maintain the high %Z. As such, when considering
both high %Z and high %Cu, two complexes displayed
excellent photoisomerization and stability: Cu(bphen)-
(XantPhos)BF4 and Cu(quintri)(XantPhos)BF4. The former
was selected for further study, given the bphen diimine
afforded a longer τ (1798 ns) than the quintri complex. The
analogous triazole-based ligand complexes underwent signifi-
cant degradation in solution during irradiation (Cu(pytri)-
(XantPhos)BF4:0%Cu; Cu(iquintri)(XantPhos)BF4:54%Cu).
However, further study is warranted to understand the
structure/activity relationships in complexes bearing triazole-
based ligands.
Figure 1. Photocatalysis using Cu(NN)(PP)X complexes and
possible application to E ↔ Z isomerization.
through photochemistry. Not surprisingly, the interest in
visible-light-mediated photocatalysis has helped advance the
field considerably.15 Weaver and co-workers have studied the
use of Ir(ppy)3 for the isomerization of allylic amines,16 while
Gilmour and co-workers have exploited (−)-riboflavin for a
host of isomerization processes.17
The value of isomerization can be illustrated by tandem
processes utilizing the Z-isomer, which consist of isomerization
followed by C−C cross-coupling18,19 C−O cross-coupling,20
selective reduction,21 condensation/cyclization,22 and photo-
chemical [2 + 2] cycloaddition.23 Herein, we report on the
design of a heteroleptic copper-based photosensitizer for E ↔
Z isomerization via energy transfer. In addition, the first
isomerization of 1,3-enynes and macrocyclic alkenes, a new
tandem ATRA/photoisomerization process employing arylsul-
fonyl chlorides, and a rare example of photoisomerization with
halide-substituted olefins are reported.
2. RESULTS AND DISCUSSION
2.1. Optimization of Catalyst Structure. As previous
work has shown that diimine dmp affords complexes with
higher excited-state lifetimes,6 which would have the potential
as energy-transfer catalysts,17,24 initial investigations involved
screening Cu-based complexes bearing dmp and eight other
bisphosphines. Efficiency was judged against four criteria
(Figure 2a).
The complex Cu(bphen)(XantPhos)BF4 was crystallized
via vapor diffusion of Et2O into a CH2Cl2 solution. X-ray
crystallographic analysis (Figure 3) confirmed the tetrahedral
geometry about the copper center in the ground state. The
shielding provided by n-Bu groups (proposed to extend the
The first and second criteria included photophysical
properties such as triplet state energy (ET, shown as light
blue bars) and excited-state lifetime (shown as dark blue bars).
The third criterion was the efficiency to promote the
photochemical E ↔ Z isomerization of alkenyl ester E-1
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a
Figure 2. Evaluation of Cu-based sensitizers in a model E ↔ Z isomerization processa. Tabular data found in the Supporting Information. For
details on obtaining τ and ET, please see the Supporting Information. The %Z is derived from the Z/E ratios determined by crude 1H NMR for the
photochemical E ↔ Z isomerization of alkenyl ester E - 1 (MeCN, Purple LEDs, 400 nm, shown as dark green bars). Note that the E/Z ratio
remains identical following chromatography. The %Cu represents complex photostability: solutions of the complexes were irradiated for 24 h, and
1
the %catalyst remaining was calculated by H NMR.
excited lifetime through restricting relaxation via isomerization
to a square planar geometry) is also visible. The photo-
isomerization was also evaluated for possible scale up. As such,
the reaction mixture was injected into a continuous flow
apparatus.27 The process was performed at scales of 200 mg,
500 mg, 1 g, and 5 g and all provided the desired product Z-1
in reproducible %Z (91−92) and yield (95−97%) (Figure 4).
Further investigation of the Cu(bphen)(XantPhos)BF4
sensitizer showed that the efficiency of the isomerization of
E-1 decreased when carried out in the presence of ambient air
(97 → 70%Z), or TEMPO (1 equiv, 97 → 44%Z), suggesting
a mechanism for photoisomerization involving energy trans-
fer28 and formation of a diradical intermediate.7 When the
isomerization of E-1 was performed with Cu(bphen)-
(XantPhos)BF4 under blue LEDs, the %Z decreased to 91%;
however, performing the reaction with 5 mol % of complex
restored the %Z to 97%. The isomerization process was equally
efficient in dichloromethane with 1 h reaction time (97%Z-1).
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bicyclic systems were also isomerized with high selectivities.
The cycloheptyl-derivative 9 provided the Z-isomer (95:5 Z/
E) in 98% yield following irradiation in the presence of
Cu(bphen)(XantPhos)BF4. In addition, indalone-substituted
olefin 10 underwent smooth isomerization (99% yield, 99:1 Z/
E). In contrast, indenyl−olefin 11 was only isolated in 56%Z,
demonstrating the importance of the additional endocyclic
carbonyl in 10 for achieving high selectivity in the isomer-
ization. The ethyl- and methyl-substituted allylic alcohols 1520
and 1632,33 could be photoisomerized using the copper-based
photocatalyst at 400 nm to afford the Z-isomer in 94 and 97%
Z, respectively. Phosphonate ester 17 was isomerized to a 92:8
Z/E ratio using the optimized copper catalyst under blue LEDs
(previously reported under UV-light conditions).20 In
a
addition, recent reports of using other copper-based systems
(Cu(OAc)2/BINAP, Blue LEDs) for photoisomerization
reported only a 14:86 Z/E ratio.34 In addition, a p-MeO-
substituted phosphonate ester 18 whose isomerization had
previously been challenging under photocatalysis with
anthracene at 365 nm (58:42 Z/E)21a underwent isomerization
in 94% and 90:10 Z/E (at 450 nm). The boronate ester 20 and
the trisubstituted olefinic nitrile (21) underwent photo-
iosmerization efficiently. Previously reported isomerization of
diester 22 used riboflavin (5 mol %) as a photocatalyst,35 and
comparable results could be obtained with the optimized
copper complex (1 mol %, 9:91 Z/E, 93% yield 22). A
macrocyclic trisubstituted olefin 23 was also treated to the
optimized conditions and could be converted to the Z-isomer
in 97% yield (96:4 Z/E). The photoisomerization of 1,3-
enynes was also explored with the Cu(bphen)(XantPhos)BF4
complex. The TMS-substituted enyne 24 afforded a good yield
(92%) but only a 47:53 Z/E ratio of stereoisomers. A
trisubstituted enyne 25 was not more successful, and
isomerization resulted in only 50% conversion to the
corresponding Z-isomer under the optimized conditions.
While photoisomerization (PI) has been used in tandem
with other thermally promoted transformations, sequential
photocatalytic processes are rare.17a,36 As such, the Cu-
(bphen)(XantPhos)BF4 complex was investigated for a
tandem atom-transfer radical addition (ATRA)2f−i of TsCl37
across alkynes and subsequent photoisomerization. Photo-
isomerization of trisubstituted olefins bearing halogen
substituents is rare,38 but it should prove useful given the
precedent for cross-coupling. Gratifyingly, ATRA of TsCl
across phenylacetylene under purple light-emitting diode
(LED) irradiation afforded a 55:45 E/Z ratio of 26 and
upon addition of an additional portion of complex,39
subsequent isomerization afforded a 95:5 E/Z ratio of 26 in
an overall yield of 83% for the two steps (Table 2). The
tandem ATRA/PI process was applicable to other alkynes such
as 4-Me, 4-MeO, 4-Br, and 4-F-phenylacetylenes, which
afforded the corresponding chloroalkenes in good yields and
selectivities (27 → 30, 65−74%, >90% E/Z). Different
sulfonylchlorides could also be used, as 4-chlorophenylsulfo-
nylchloride and 4-fluorophenylsulfonylchloride provided the
corresponding products 31 and 32 following ATRA/PI with
phenylacetylene (71−81%, 93−94%E). Finally, although tetra-
substituted alkenes are rare products for both ATRA and PI
processes, submitting phenylpropyne to the optimized
conditions afforded olefin 33 in good E-selectivity (34%,
89:11 E/Z).
Figure 3. X-crystallographic analysis of Cu(bphen)(XantPhos)BF4.
Atoms represented as ellipsoids. Hydrogens omitted for clarity.
Carbon = gray, phosphorus = orange, oxygen = red, copper = bronze,
nitrogen = purple, boron = pink, and fluorine = bright green.
Figure 4. Scale-up of the photochemical E ↔ Z isomerization using
continuous flow methods.
Note that no significant isomerization of E-1 (<3%) was
observed when irradiation occurred with the ligands bphen or
XantPhos alone, neither when in the presence of the
homoleptic complex Cu(bphen)2BF4.29
2.2. Evaluation of Scope. Following identification of the
heteroleptic Cu(bphen)(XantPhos)BF4 complex, it was
evaluated in the isomerization of various olefins (Table 1).
In addition to the optimized conditions (MeCN, Purple
LEDs), substrates with problematic solubility could undergo
photoisomerization in CH2Cl2 (Blue LEDs). First, the
photoisomerization of various di-substituted olefins, such as
cinnamates 3 and 5,30 allylic alcohol 4, and the stilbenes31
6
and 7, were studied and afforded good selectivities (73−77%Z,
entries 1 → 5). Next trisubstituted cinnamates and allylic
alcohols were investigated (entries 6 → 22). When ester 1 was
irradiated for only 1 h, a 95:5 Z/E ratio was obtained, while 24
h irradiation was reported for riboflavin to achieve the same
selectivities.17a Various other cinnamates possessing hetero-
cycles (12), electron-withdrawing groups (13), or olefin
substituents (14) all generally provided high yields (>90%)
and selectivities (>90%Z). Trisubstituted olefins appended to
The chloroalkene 26 could be engaged in sequential
functionalization reactions (Scheme 1). When subjected to
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a
Table 1. Evaluation of Cu(I)-Based Photocatalysts in E ↔ Z Isomerization Processes
a
Yields following chromatography (single experiment). All isomerizations used pure E-isomers as starting materials unless otherwise noted. Z/E
b
c
1
1
ratios determined by crude H NMR. Starting material was a 80:20 E/Z mixture. Yield determined by H NMR.
standard palladium catalysis with phenylacetyelene, the enyne
34 was isolated in 86% yield. Subsequent stereoinvertive cross-
coupling via Ni catalysis afforded the trisubstituted alkene 35
in 71% yield (unoptimized).
In summary, photoisomerization of olefins has been
demonstrated with heteroleptic copper complexes. Using
Cu(bphen)(XantPhos)BF4, 25 di- and trisubstituted alkenes
underwent photoisomerization, including the first attempts at
photoisomerization of macrocyclic alkenes and challenging 1,3-
enynes. Cu(NN)(PP)X-type complexes bearing different
bisphosphines and diimines afford sensitizers with a range of
triplet energies and excited-state lifetimes, which can
theoretically allow optimization of a catalyst structure to
selectively excite one geometric isomer of a compound over
another, as well as be applicable to the design of other energy-
transfer processes. The optimization of the Cu(bphen)-
(XantPhos)BF4 complex also highlighted the importance of
photostability in photocatalysis using the complexes. Even if
structure/activity relationships for photostability are still not
clear, the above investigations highlight the need for further in-
depth studies. The Cu(bphen)(XantPhos)BF4 complex could
also be employed in a new tandem ATRA/photoisomerization
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a
Table 2. Sequential ATRA/Photoisomerization Processes Employing the Cu(bphen)(XantPhos)BF4 Catalyst
a
Yields represent the pure E-isomers following chromatography. Control reactions investigated during the synthesis of 26: with no catalyst: 0% 26;
(ii) in the dark with catalyst: 19% 26, 58:42 E/Z.
Scheme 1. Diversification of Haloakenes
Experimental procedures, tabular data, absorbance,
emission and excited lifetime data, and spectral data
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
Shawn K. Collins − Département de Chimie, Centre for Green
Chemistry and Catalysis, Université de Montréal, Montréal,
QC H2V 0B3, Canada; orcid.org/0000-0001-9206-
5538; Email: shawn.collins@umontreal.ca
Authors
a
PhCCH (3 equiv), Pd(PPh3)2Cl2 (10 mol %), CuI (15 mol %),
b
Corentin Cruché − Département de Chimie, Centre for Green
DIPEA (4 equiv), CH2Cl2, 21 °C, and 18 h. MeMgBr (3 equiv),
Chemistry and Catalysis, Université de Montréal, Montréal,
Ni(acac)2 (10 mol %), tetrahydrofuran (THF), −78 °C to rt, and 2 h.
QC H2V 0B3, Canada
William Neiderer − Département de Chimie, Centre for Green
process employing arylsulfonyl chlorides. The above exem-
Chemistry and Catalysis, Université de Montréal, Montréal,
plifies the potential utility of the complexes for new energy-
QC H2V 0B3, Canada
transfer processes in synthetic photochemistry.40
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c01983
ASSOCIATED CONTENT
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* Supporting Information
Notes
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c01983.
The authors declare no competing financial interest.
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(5) Brégent, T.; Bouillon, J.-P.; Poisson, T. Copper-Photocatalyzed
contra-Thermodynamic Isomerization of Polarized Alkenes. Org. Lett.
2020, 22, 7688−7693.
ACKNOWLEDGMENTS
■
The authors acknowledge the Natural Sciences and Engineer-
ing Research Council of Canada (NSERC, Discovery 1043344
and RGPIN-2020-05006), Université de Montréal and the
Fonds de recherche Nature et technologie via the Centre in
Green Chemistry and Catalysis (FRQNT-2020-RS4-265155-
CCVC) for generous funding. The Canadian Foundation for
Innovation is thanked for financial support of continuous flow
infrastructure. Vanessa Kairouz is thanked for her guidance and
support of continuous flow experiments. The authors thank D.
(6) Cuttell, D. G.; Kuang, S.-M.; Fanwick, P. E.; McMillin, D. R.;
Walton, R. A. Simple Cu(I) Complexes with Unprecedented Excited-
State Lifetimes. J. Am. Chem. Soc. 2002, 124, 6−7.
(7) Zhou, Q. -Q.; Zou, Y. -Q.; Lu, L. -Q.; Xiao, W. -J. Visible-Light-
Induced Organic Photochemical Reactions through Energy-Transfer
Pathways. Angew. Chem., Int. Ed. 2019, 58, 1586−1604.
(8) Fisch, F.; Fleites, C. M.; Delenne, M.; Baudendistel, N.; Hauer,
B.; Turkenburg, J. P.; Hart, S.; Bruce, N. C.; Grogan, G. A Covalent
Succinylcysteine-like Intermediate in the Enzyme-Catalyzed Trans-
formation of Maleate to Fumarate by Maleate Isomerase. J. Am. Chem.
Soc. 2010, 132, 11455−11457.
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Chartrand and Emilie Morin for help obtaining excited-state
lifetime data.
(9) Alder, A.; Jamil, M.; Marzorati, M.; Bruno, M.; Vermathen, M.;
Bigler, P.; Ghisla, S.; Bouwmeester, H.; Beyer, P.; Al-Babili, S. The
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