Sunlight-Driven Copper-Catalyst Activation Applied to Photolatent Click Chemistry

Article abstract of DOI:10.1002/chem.201404056

The synthesis, full characterization, photoreduction properties, and catalytic activity for the copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reaction of a copper(II)–DMEDA (N,N′-dimethylethylendiamine) complex is reported. Spectroscopic studies (UV/Vis, EPR) demonstrated that under daylight illumination highly effective copper(II) to copper(I) reduction occurs in this complex. These findings are in agreement with a high photoreduction quantum yield value of 0.22 in MeOH, and a value approaching unity as determined in THF. The reduction process, which can also be conducted by irradiation at 365 nm by using a standard TLC (thin layer chromatography) lamp, is ascribed to a highly efficient photoinduced electron transfer (PET) process mediated by the benzophenone photosensitizer present in the carboxylate counterion. Having deaerated the reaction mixture, the photogenerated copper(I) species proved to be highly active for the CuAAC reaction, demonstrated by reactions conducted with low catalyst loading (0.5 mol %) on a range of clickable protected and non-protected mono- and disaccharides. Once initiated, the reaction can be stopped at any time on introducing air into the reaction medium. Deoxygenation followed by irradiation restores the activity, making the copper(II)–DMEDA complex a switchable catalyst of practical value.

Full text of DOI:10.1002/chem.201404056

DOI: 10.1002/chem.201404056
Full Paper
&
Click Chemistry
Sunlight-Driven Copper-Catalyst Activation Applied to
Photolatent Click Chemistry
Rꢀdouane Beniazza,[a] Romain Lambert,[a] Lydie Harmand,[a] Florian Molton,[b]
Carole Duboc,[b] Sergey Denisov,[a, c] Gedeminas Jonusauskas,[c] Nathan D. McClenaghan,[a]
Dominique Lastꢀcouꢁres,[a] and Jean-Marc Vincent*[a]
Abstract: The synthesis, full characterization, photoreduction
properties, and catalytic activity for the copper(I)-catalyzed
alkyne-azide cycloaddition (CuAAC) reaction of a copper(II)–
DMEDA (N,N’-dimethylethylendiamine) complex is reported.
Spectroscopic studies (UV/Vis, EPR) demonstrated that under
daylight illumination highly effective copper(II) to copper(I)
reduction occurs in this complex. These findings are in
agreement with a high photoreduction quantum yield value
of 0.22 in MeOH, and a value approaching unity as deter-
mined in THF. The reduction process, which can also be con-
ducted by irradiation at 365 nm by using a standard TLC
(thin layer chromatography) lamp, is ascribed to a highly ef-
ficient photoinduced electron transfer (PET) process mediat-
ed by the benzophenone photosensitizer present in the car-
boxylate counterion. Having deaerated the reaction mixture,
the photogenerated copper(I) species proved to be highly
active for the CuAAC reaction, demonstrated by reactions
conducted with low catalyst loading (0.5 mol%) on a range
of clickable protected and non-protected mono- and disac-
charides. Once initiated, the reaction can be stopped at any
time on introducing air into the reaction medium. Deoxyge-
nation followed by irradiation restores the activity, making
the copper(II)–DMEDA complex a switchable catalyst of prac-
tical value.
Introduction
lytic systems continuous light irradiation is employed to gener-
ate, from an inactive copper(I) complex, a catalytically active
photoexcited copper(I) species, which is able to reduce alkyl or
aryl halides through single-electron-transfer processes to gen-
erate alkyl or aryl radicals. For one of these catalytic systems, it
was also shown that sunlight can be used instead of lamp irra-
diation.[6]
The search for catalysts that can be activated by using day-
light, such as copper(II)–DMEDA (N,N’-dimethylethylendiamine)
complex 1 described herein (Scheme 1), attracts considerable
interest, with solar photons being considered as ideal “re-
agents”.[1,2] Besides the economical and practical advantages
that the use of daylight as an energy source represents, ex-
ploiting light as a stimulus to activate a catalyst offers other
key benefits, including potential spatial and temporal control
over the catalytic event and convenient handling of stable pre-
catalysts under ambient conditions, as well as the possibility to
control the onset of the generated catalytic species by varying
the irradiation parameters, typically the irradiation time, power,
and excitation wavelength.[3,4] Recently, photoredox copper(I)
catalysis has emerged as a powerful tool for the formation of
CÀC,[5–7] CÀN,[8–11] CÀS,[12] or CÀO[13] bonds. In such photocata-
Scheme 1. Structures of the photoreducible complexes 1 and 2.
[a] R. Beniazza, R. Lambert, Dr. L. Harmand, S. Denisov, N. D. McClenaghan,
Dr. D. Lastꢀcouꢁres, Dr. J.-M. Vincent
Inspired by the early work of Chow and Buono-Core,[14–17] we
recently proposed a complementary strategy for copper-cata-
lyst activation mediated by light. Photoinduced electron trans-
fer (PET) to a metal cation was introduced as a way to gener-
ate a catalytically active low-valent copper(I) species from the
corresponding inactive high-valent copper(II) precatalyst.[18,19]
We reported that a copper(II)-tren (tren = tris(2-aminoethyl)-
amine) precatalyst 2 (Scheme 1), in which a benzophenone-like
photosensitizer is introduced in the guise of the ketoprofenate
Universitꢀ de Bordeaux, Institut des Sciences Molꢀculaires UMR-CNRS 5255,
351, Crs de la Libꢀration, 33405 Talence Cedex (France)
E-mail: jm.vincent@ism.u-bordeaux1.fr
[b] F. Molton, Dr. C. Duboc
Univ. Grenoble Alpes, DCM UMR-CNRS 5250, 38000 (Grenoble)
[c] S. Denisov, Dr. G. Jonusauskas
Universitꢀ de Bordeaux, Laboratoire Ondes et Matiꢁre d’Aquitaine,
UMR-CNRS 5798, 351 Crs de la Libꢀration, 33405 Talence (France)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404056.
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carboxylate counterion, can be efficiently photoreduced upon
irradiation at 365 nm. This photoreduction process was exploit-
ed to photogenerate, on-demand, a highly reactive catalyst for
link the benzophenone chromophore to the copper complex.
Instead of the ketoprofenate anion, present in 2 (a chiral anti-
inflammatory drug, for which photostability issues have been
identified),[42] the 3-benzoylbenzoate counterion is favored in
1. The complex, [Cu(DMEDA)2(H2O)2](3-benzoylbenzoate)2 1,
was prepared by reacting CuOTf2 (OTf=trifluoromethanesulfo-
nate) with two equivalents of the sodium salt of the commer-
cially available 3-benzoylbenzoic acid in water, leading to
quantitative precipitation of [Cu(3-benzoylbenzoate)2(H2O)] as
a blue/green powder. When [Cu(3-benzoylbenzoate)2(H2O)] re-
acted with two equivalents of DMEDA in THF/H2O, the solution
turned from light blue to deep blue. Slow diffusion of diethyl
ether vapor led to the formation of purple crystals of 1, which
were isolated in 94% yield (Scheme 2). Single-crystal X-ray dif-
the
copper(I)-catalyzed
alkyne-azide
cycloaddition
(CuAAC),[20,21] a click reaction now recognized as one of the
most useful chemical transformations employed in all fields of
the chemical sciences.[22,24] The irradiation, from a standard 4 W
thin-layer chromatography (TLC) lamp, was shown to mediate
a highly efficient photoinduced electron-transfer process, lead-
ing to fast copper(II) to copper(I) reduction (Fred =0.17 in
MeOH), the final electron source being the solvent, typically
MeOH, THF, or toluene.
A related strategy developed by Bowman and co-workers,
consists of exploiting photoinitiators, which upon irradiation
generate radicals that reduce copper(II) ions to copper(I).[25–27]
The potential of such reactivity was demonstrated in the pho-
topatterning of surfaces[25] and polymer synthesis.[26,27] Very re-
cently, the same group reported a copper(II) precatalyst with
an acylphosphinate radical photoinitiator counteranion, which
upon activation by light irradiation proved to be an effective
catalyst for atom-transfer radical-polymerization (ATRP) and
CuAAC reactions.[28] The irradiations were conducted by using
visible light (typically 5–10 mWcmÀ2 in the 400–500 nm wave-
length region), a possible limitation of such a system is being
that the reduction efficiency is moderate, with a maximum of
ꢀ50% generation of the copper(I) species. Examples of
“direct” copper(II) to copper(I) photoreductions applied to pho-
toinduced CuAAC reactions have also been reported.[29–31]
Beyond the copper-catalyzed systems it should be emphasized
that a range of light-induced click reactions have been devel-
oped.[32] It is noteworthy that the copper(II) ions of a laccase
protein could be effectively reduced through a Zn–porphyrin
photosensitized process, the generated copper(I) species could
then mediate the four-electron reduction of O2 into water.[33]
By exploiting extremely efficient photochemical reactions,
one can envisage the development of a photosensitized cop-
per(II) to copper(I) reduction process occurring effectively by
using sunlight. We describe herein the synthesis, full character-
ization, photoreduction properties, and catalytic activity for the
CuAAC reaction of the copper(II)–DMEDA complex
1 (Scheme 1). Copper–DMEDA complexes are well-known and
versatile coordination compounds that have been shown to
exhibit excellent catalytic properties for important chemical
transformations,[34] in particular for reactions catalyzed by cop-
per(I) species.[34–38] The DMEDA ligand is structurally simple,
commercially available, and inexpensive, making it valuable
from a practical point of view. Moreover, considering the
prominent role of copper(I)-catalyzed processes in modern
chemistry,[34,39–41] the design of a photoreducible copper(II)–
DMEDA precatalyst emerged as a target of choice.
Scheme 2. Synthesis of complex 1.
fraction analysis revealed that 1 has a monomeric structure
(Figure 1) with a distorted octahedral geometry with the four
nitrogen atoms in the equatorial plane (average dCu···N 2.045 ꢃ),
while two elongated CuÀOH2 bonds are present in the axial
positions (average dCu···O 2.603 ꢃ). Remarkably, the carboxylate
anions are not directly bound to the copper(II) ion, but interact
with the [CuDMEDA2(OH2)2]2+ dication through an extended
network of hydrogen bonds, with each oxygen atom of the
carboxylates participating in two hydrogen bonds to the hy-
drogen atoms of water (dCO···HO 1.833 ꢃ) and the secondary
amine (dCO···HN 2.006 ꢃ) ligands (Figure 1).
Figure 1. Molecular structures of the [CuDMEDA2(OH2)2]2+ dication in 1, and
view showing the hydrogen-bond interactions between the dication and
the four surrounding benzoates. Cu, green; C, grey; H, white; N, blue; O,
red.
Photoreduction and photophysical studies
Results and Discussion
Figure 2a shows the UV/Vis spectra recorded when a deoxy-
genated (by multiple freeze-pump-thaw cycles in a sealed cell)
solution of 1 (1.9 mm) in THF (3 mL) was irradiated at 365 nm
by using a 4 W TLC lamp (the reductions were conducted in
a room in which the only source of light was the TLC lamp). A
Synthesis and characterization of precatalyst 1
The photosensitizer in 1 was introduced through the copper
carboxylate counterion, a synthetically convenient strategy to
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very fast reduction occurs, with a ꢀ80% decrease of the ab-
sorption intensity at 640 nm being observed after 5 min irradi-
ation. The disappearance of this low-energy absorption band,
which is ascribed to a d–d transition, is conducive with reduc-
tion of the initial copper(II) ion. Complete reduction was ach-
ieved in ꢀ15 min, affording a colorless THF solution.
The reduction was confirmed by EPR studies, which showed
the fast disappearance of the copper(II) signal when an EPR
tube containing a deaerated THF solution of 1 was irradiated
(Figure 3).
Figure 3. Evolution of the EPR spectra (X band, 100 K) of deaerated (Ar bub-
bling) 1 mm THF solution of complex 1 when irradiated at 365 nm by using
a TLC lamp (gx: 2.216, gy =2.055, gz =2.045, Ax (Cu)=565 MHz).
Importantly, a photoreduction quantum yield approaching
unity was determined for a degassed 1.9 mm solution of 1 in
THF (lex =365 nm, ferrioxylate actinometry using a microversion
of Hatchard and Parker’s method, based on six separate experi-
ments), suggesting that essentially one copper(II) atom is re-
duced for every photon absorbed.[43] The inherently high
values and consequent difficulty of sample preparation and
manipulation hampers more precise determination of this
value. In MeOH, a good H-donating solvent, the reduction is
very fast with typically a 70% decrease of the absorption band
at 610 nm observed within 5 min of irradiation (spectra provid-
ed in the Supporting Information). However, it should be
noted that in MeOH the appearance of colloidal copper nano-
particles was rapidly observed (after ꢀ5–7 min irradiation) as
evidenced through the diagnostic plasmon resonance absorp-
tion bands in the visible spectral region. This finding means
that in the absence of reactants that trap the copper(I) species,
copper(0) is rapidly generated in MeOH, through a copper(I)
photoreduction process or through copper(I) disproportiona-
tion. In MeCN and CH2Cl2, two much less efficient H-donating
solvents, the reduction process is still observed, with complete
reduction in ꢀ40 min and 70 min, respectively (see the Sup-
porting Information for spectra).
Figure 2. Evolution of the UV/Vis spectra of deaerated (freeze-pump-thaw
degassed, sealed quartz cuvettes) 1.9 mm THF solutions of complexes 1 and
2 under different illumination conditions (A=absorbance): a) Complex 1 irra-
diated at 365 nm by using a TLC lamp; b) Complex 1 under direct sunlight
illumination (sunny day); c) Complex 1 under daylight illumination (rainy
day); d) Complex 2 under daylight illumination (rainy day).
Quantum yields of copper reduction in 1 of 0.22 and 0.03
were established in MeOH and MeCN, respectively.
In light of the remarkably high photoreduction quantum
yield measured for 1, it was anticipated that effective reduc-
tion could proceed under daylight illumination. As shown in
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Figure 2b, when a deoxygenated THF solution of 1 in a sealed
cell is positioned behind a window and exposed to direct sun-
light, ꢀ85% of the copper(II) species was reduced in 30 min.
As expected for a photochemical process driven by daylight,
the weather has a significant influence on the reduction kinet-
ics, a reduction of ꢀ80% in 60 min typically being observed
for 1 when exploiting the light of a rainy day, under otherwise
analogous conditions. Under the same conditions (experiments
conducted in parallel) the daylight photoreduction of complex
2 proceeded well, despite a lowered efficiency. After 20 min of
illumination, reduction of 63 and 37% of 1 and 2 were mea-
sured, respectively. A colorless, fully reduced solution of 2 was
obtained after ꢀ80 min.
photochemical side reaction, which is more clearly visible in
THF, and reverts in minutes to the initial product.
Information on the interaction of the excited anionic moiety,
and the ground-state inorganic moiety in solution, may also be
revealed by the lifetime of the triplet absorption band. The in-
organic cation and benzophenone anion pairing may be ex-
pected to be dependent on both the concentration of 1 and
the solvent nature. Figure 4 shows the benzophenone-like trip-
Similar to the observations with complex 2, photoreduced
complex 1 is extremely sensitive to O2; instantaneous recolora-
tion was observed when air was bubbled into a colorless solu-
tion of reduced 1. The UV/Vis spectrum of the reoxidized solu-
tion is similar to the initial spectrum, nonetheless with a signifi-
cant increase of the absorption observed between 400 and
500 nm (Figure S2). When the reoxidized solution is deoxygen-
ated (Ar bubbling) and then irradiated, reduction occurs at
a rate comparable to that of the initial reduction. EPR experi-
ments (Figure S3) confirmed such behavior, showing that after
air reoxidation a copper(II) complex with slightly different EPR
signal is formed, affording a ꢀ60:40 ratio between the starting
complex 1 and the new species. Importantly, both species are
reduced to copper(I) species upon irradiation, as shown by the
disappearance of both signals. These results show that the
Figure 4. Benzophenone-like triplet state lifetime as a function of concentra-
tion of 1 in degassed methanol (dots) and degassed THF (squares). Excita-
tion at 355 nm.
let lifetime of 1 as a function of concentration in both MeOH
and THF, with solubility and detectable signal dictating the
higher and lower concentration values, respectively. Triplet
quenching can depend on a number of solvent parameters,
making direct comparison of MeOH and THF difficult. However,
it is clear that a greater concentration dependence is observed
in MeOH than in THF. This can be rationalized in terms of the
higher capacity of more polar MeOH to solvate the charged
species compared with that of THF (relative polarities of 0.762
and 0.207).[45] This would limit the contact of the photosensitiz-
er with the inorganic moiety during the lifetime of the excited
state and hence quenching, which is significantly enhanced in
MeOH on increasing the concentration of 1. On the other
hand, in THF the triplet is strongly quenched at 2 mm and is
only slightly affected on increasing concentration. This is con-
sistent with either a direct THF solvent reaction or a stronger
association between the photoactive anion and copper-con-
taining cation, and shows that for this type of system attention
should be paid to the absolute catalyst concentration in
a given solvent.
photoreduction/oxidation process exhibits
a
satisfactory
degree of reversibility, necessary to achieve switching ON/OFF
for multiple cycles during catalysis.
In terms of the photophysical properties of 1 in MeCN,
MeOH, and THF, transient absorption spectroscopy provides
a convenient tool to follow the temporal evolution of the non-
emissive initially formed benzophenone triplet state (lmax
=
533, 526, and 520 nm in MeCN, MeOH, and THF, respectively),
which is formed on the sub-nanosecond timescale upon pho-
toexcitation of this photosensitizer.[18,44] Studies of the change
of this excited-state absorption gives information about the
first steps in the generation of the catalytically active copper(I)
species, which is unambiguously evidenced by steady-state
spectrophotometry and EPR studies, as well as the intercompo-
nent communication. At similar concentrations of 1, the rate of
triplet transient absorption change in MeCN is faster than that
in MeOH (t=84 ns, 2.4 mm, MeCN; t=140 ns, 2.5 mm, MeOH),
despite an approximately ten-fold higher quantum yield of re-
duction for the latter, which may suggest a non-direct monoe-
lectronic reduction of copper from the benzophenone-like trip-
let state, and hence an intervening state in the reduction pro-
cess (Figure S6–S8). In THF (Figure S5 and S8), a faster initial
rate of triplet change was obtained (t=9 ns, 2.8 mm), and was
similarly observed with the 3-benzoylbenzoic acid itself (t=
7 ns, 2.9 mm), suggesting a benzophenone-localized process or
specific interaction with the THF solvent as key initial steps. On
longer timescales, some yellow photoproduct formation (time
constant=53 ms, see Figure S9) is observed on irradiating com-
plex 1 or the 3-benzoylbenzoic acid alone, denoting a minor
Catalytic properties
To assess the switching properties of precatalyst 1 and the re-
activity of the photogenerated copper(I) species, protected
and non-protected mono- and disaccharides were chosen as
they represent a prominent class of clickable substrates. Cata-
lytic reactions were typically conducted in NMR tubes, at
0.5 mol% precatalyst loading and 0.33m reactant concentra-
tion, affording a convenient way to follow the reaction kinetics.
The reaction mixtures were deoxygenated by Ar bubbling
(15 min) keeping the tube protected from light (by using alu-
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the reaction can be stopped at any time by bubbling air into
the reaction medium because of an extremely fast copper(I) to
copper(II) oxidation by molecular oxygen. Deoxygenation (Ar
bubbling), followed by irradiation (10 min) fully restores the
catalytic activity. The robustness of this reversible process is
satisfactory, the reaction having been switched ON/OFF three
times in the same run, without significantly changing the ki-
netics in the ON state. It should be noted that when the reac-
tions are conducted in CD3OD, the triazoles were obtained as
a mixture of hydrogenated and deuterated compounds, owing
to partial terminal alkyne H/D exchange during catalysis (see
1H and 13C NMR provided in the Supporting Information).
The high reactivity of the photogenerated copper(I) species
was exploited to prepare a range of triazoles 6–19 (Scheme 3)
All reactions were conducted in NMR tubes at low catalyst
loading (0.5 mol%) in CD3OD, CD3OD/D2O, or [D8]THF to
ensure complete solubility of the reactants (0.25 mmol). The re-
Figure 5. Reaction profiles for the formation of triazoles 5 and 15 followed
by H NMR spectroscopy under different illumination conditions (see experi-
1
mental conditions in the text): Profiles for compound 15, obtained by using
TLC lamp irradiation (green), daylight illumination (red) or from an NMR
tube kept in the dark (black); profiles for compound 5 obtained by using
TLC lamp irradiation (orange), daylight illumination (blue) or from an NMR
tube kept in the dark (purple)
1
actions reached full conversion (followed by H NMR spectros-
copy) at the time indicated in Scheme 3. The triazoles 6–19
were obtained in good to excellent isolated yields with reac-
tion times ranging from 10 min to 8 h.
minum foil) and, after a given time, exposed to daylight
behind a window (direct sunlight), or 10 min of the 365 nm UV
light of a TLC lamp (4 W) positioned ꢀ1 cm from the NMR
tube. Figure 5 shows the reaction profiles for the formation of
compounds 15 and 5 under various illumination conditions.
When 1 was activated by TLC lamp irradiation, the reaction
proceeded very rapidly with complete conversion obtained in
ꢀ10 min and 40 min for 15 and 5, respectively. Interestingly,
no latency period was observed, in agreement with the fast
generation of a highly reactive copper(I) species. In the ab-
sence of light, the background activity of 1 was very low, with
typically less than 5% of triazoles formed within 2–4 h. Impor-
tantly, under daylight illumination the reaction proceeded very
well, with complete conversion reached within ꢀ40 min and
100 min for 15 and 5, respectively.
Finally, the synthesis of the unprotected disaccharide 19 was
conducted on a 5 mmol scale in a round-bottom flask (30 mL
of CH3OH) at 0.5 mol% precatalyst loading under daylight irra-
diation (sunny day). Under these conditions, 1.94 g of the dis-
accaharide was isolated after 3 h of reaction (92% yield). When
the same reaction was conducted on the same scale in the
dark, <5% conversion was measured after 3 h (Scheme 4).
Conclusion
Taking advantage of an extremely efficient benzophenone-sen-
sitized PET process, the copper(II) to copper(I) photoreduction
of complex 1 proceeds under daylight illumination with unpre-
cedented efficiency. The photogenerated copper(I) species is
extremely reactive for the CuAAC reaction, as demonstrated
through the preparation of mono-, di- and tetrasaccharides,
full conversion being achieved at low catalyst loading with
short to moderate reaction times, typically in THF or MeOH sol-
vents. Either ambient light or TLC lamp irradiation at 365 nm
can be conveniently used to activate the copper(II) precatalyst,
due to the extremely high photoreduction quantum yields.
Deaerated solutions (Ar bubbling) are required for the reac-
tions to proceed because the photogenerated copper(I) spe-
cies is extremely sensitive to dioxygen. Such reactivity can be
exploited to turn OFF/ON the catalyst activity on-demand.
Once initiated by light, the reaction can be stopped by intro-
ducing air into the reaction mixture, and deoxygenation fol-
lowed by irradiation enables reinitiation of the reaction with
similar initial rates. Considering the ease of preparation, the re-
markable efficiency of its photoreduction under ambient light,
and the reactivity of the photogenerated copper(I) species,
precatalyst 1 has strong potential for applications, not only in
CuAAC reactions, but also in other important copper(I)-cata-
lyzed processes. Studies along these lines are currently in prog-
ress.
The switching properties of 1 were then evaluated for the
peracetylated b-d-galactopyranosyl azide 4 and propargyl alco-
hol 3, in [D8]THF. As shown in Figure 6, the activity of 1 is rap-
idly turned on upon irradiation at 365 nm. Once switched on
1
Figure 6. Reaction profile for the formation of triazole 5 followed by H NMR
spectroscopy and applying external stimuli.
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cil (FP7/2008-2013) ERC grant
agreement no. 208702 and IR-
RPE CNRS 3443 are gratefully ac-
knowledged for their financial
support.
Keywords: benzophenone
·
click
chemistry
·
copper
catalysts
·
photoactivation
·
photoinduced electron transfer
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Received: June 20, 2014
Published online on August 29, 2014
Chem. Eur. J. 2014, 20, 13181 – 13187
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