Copper catalyst activation driven by photoinduced electron transfer: A prototype photolatent click catalyst

Article abstract of DOI:10.1002/anie.201203014

PET cat. While the copper(II) tren ketoprofenate precatalyst 1 (see picture) is inactive at room temperature in methanol, it is quantitatively and rapidly reduced to its cuprous state upon light irradiation to provide a highly reactive click catalyst. By simply introducing air into the reaction medium the catalysis can be switched off and then switched on again by bubbling argon followed by irradiation. Copyright

Full text of DOI:10.1002/anie.201203014

Angewandte
Chemie
DOI: 10.1002/anie.201203014
Switchable Catalyst
Copper Catalyst Activation Driven by Photoinduced Electron
Transfer: A Prototype Photolatent Click Catalyst**
Lydie Harmand, Sarah Cadet, Brice Kauffmann, Luca Scarpantonio, Pinar Batat,
Gediminas Jonusauskas, Nathan D. McClenaghan, Dominique Lastꢀcouꢁres, and
Jean-Marc Vincent*
The development of catalysts with light-controlled activity is
a challenging area of current research, with a strong potential
for development.[1] Considering transition metal catalysts, the
generation of a highly active catalytic species on demand by
light irradiation of an inactive precatalyst presents several key
benefits: 1) metal species that are stable for handling under
ambient conditions; 2) switching on catalysis at a given time
and place; 3) tuning the onset of the generated active catalyst
by controlling the photoactivation process, which would be
valuable for highly exothermic reactions conducted on a large
scale. To date, most examples of photocontrolled transition
metal catalysts are the so-called photocaged catalysts, which
rely on the photodissociation or photodecomposition of
ligands in coordinatively saturated metal complexes to
generate vacant active sites.[1,2] Inoue and co-workers
reported a rare example of a photoswitchable catalyst based
on the use of a bulky photoisomerizable styryl pyridine ligand
which, in its Z form, activates an aluminium porphyrin
catalyst through efficient coordination to the metal center.[3]
Photoinduced electron transfer to a metal cation might
represent a valuable strategy to switch on the activity of
metal-based catalysts that are inactive in their high-valent
oxidized form, but active in their low-valent reduced form.
This oxidation-state dependent activity is typically observed
for the copper(I)-catalyzed alkyne-azide cycloaddition
triazole, which is released by protonation. Among all the
catalysts described so far, the only reported latent catalyst is
the thermally activated [(NHC)CuCl] complex (NHC =
N-heterocyclic carbene) developed by Nolan and co-work-
ers.[8] This complex is inactive under ambient conditions at
room temperature (DMSO is used as solvent) but only
becomes active if the temperature is raised to 608C and water
is added to the reaction mixture.
Chow and Buono-Core discovered that certain aromatic
ketones, benzophenones in particular, could sensitize the
photoreduction of transition-metal ML2 complexes such as
[Cu(acac)2] (acac = acetylacetonate),[9] [Ni(acac)2],[10] or
[Cu(Bp)2] (Bp = dihydrobis(1-pyrazolyl)borate) in hydro-
gen-donating solvents.[11] Extensive studies of the sensitized
photochemistry showed that the photoreduction involves
quenching of the triplet aromatic ketone, most likely by
a charge-transfer complex intermediate, which performs
a monoelectronic oxidation of the ligand (L) to generate
a ligand-centered radical that abstracts a H atom from the
solvent (RCH2OH). Overall the process generates the
À
reduced complex ML, the protonated ligand L H, and an
a-hydroxy radical RCCHOH.
Herein, we report the synthesis, characterization, prelimi-
nary photochemical studies, and catalytic activity of the
[Cu(tBuBz3tren)ketoprofenate]ketoprofenate
complex
(CuAAC) click reaction,[4]
a
major reaction currently
1 (Figure 1). This species, which possesses a benzophenone-
like chromophore, is the first example of a photolatent click
catalyst.
exploited in all fields of the chemical sciences.[5,6] Within the
last ten years a large number of catalysts have been developed
for this reaction.[7] Their common feature is that copper(I) has
to be generated, the reaction proceeding first through the
formation of a copper(I)-acetylide, followed by cycloaddition
with a copper(I)-bound azide to generate a copper(II)-
[*] L. Harmand, S. Cadet, Dr. L. Scarpantonio, Dr. N. D. McClenaghan,
Dr. D. Lastꢀcouꢁres, Dr. J.-M. Vincent
Universitꢀ Bordeaux—CNRS UMR 5255
351 Crs de la Libꢀration, 33405 Talence (France)
E-mail: jm.vincent@ism.u-bordeaux1.fr
Dr. P. Batat, Dr. G. Jonusauskas
Universitꢀ Bordeaux—CNRS UMR 5798 (France)
Dr. B. Kauffmann
European Institute of Chemistry and Biology, Pessac (France)
Figure 1. Structural formulas of complexes 1 and 2 (left) and molecular
structure of the [Cu(tBuBz3tren)CH3CO2]+ cation (right).
[**] The University of Bordeaux 1, the CNRS, the Rꢀgion Aquitaine, and
the European Research Council (FP7/2008-2013, ERC grant agree-
ment no. 208702) are gratefully acknowledged for their financial
support.
Precatalyst 1 was designed to be easily prepared while
possessing two key structural/functional features: 1) A tetra-
dentate tren-derived ligand (tren = tris(2-aminoethyl)amine),
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203014.
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which should favor the stabilization of the CuI reduced
species, thus preventing the formation of colloidal Cu0, which
may occur during the photoreduction process. Moreover
we,[12] and others,[13] have previously shown that the cop-
per(I)-tren complexes are excellent click catalysts. 2) A
ketoprofenate-ligating counter-anion as a photosensitizing
group. Ketoprofen (2-(3-benzoylphenyl)-propionic acid) is
a well-known, commercially available non-steroidal anti-
inflammatory drug containing a benzophenone chromophore
and a carboxylic acid group.
from a d–d transition. As shown in Figure 2, when a deoxy-
genated blue methanolic solution of 1 (1.9 mm) is irradiated at
365 nm, the complete disappearance of the 828 nm absorption
band is observed within approximately 30 min and a limpid
colorless solution, consistent with the formation of a cuprous
[Cu(tBuBz3tren)]+ cationic species, is observed.
The quantum yield for this copper reduction process
(Fred) in methanol was determined to be relatively high at
0.17, in comparison with a ferrioxylate salt actinometer
irradiating at 365 nm. This assumes that the photoprocess is
the only factor responsible for the changing absorption.[15]
Direct evidence for the clean formation of a cuprous
[Cu(tBuBz3tren)]+ cationic species comes from 1H NMR
The tBuBz3tren ligand (Bz = benzyl) was prepared in
40% yield of isolated product through a reductive amination
reaction (see the Supporting Information for details). The
1
copper(II)
complex
[Cu(tBuBz3tren)ketoprofenate]-
experiments. The H NMR spectrum of 1 is characteristic of
ketoprofenate (1) was isolated as a blue powder in 90%
yield by reacting the tBuBz3tren ligand with CuOTf2 (OTf =
trifluoromethanesulfonate) and the sodium salt of ketoprofen
in methanol. Complex 1 was characterized by UV–Vis and
FT-IR spectroscopy, elemental analysis, and ESI-MS. In
particular, the ESI-MS spectrum (given in the Supporting
Information) shows a major peak (100%) at m/z = 900.49,
which is assigned to the [Cu(tBuBz3tren)ketoprofenate]+
cation, suggesting that one ketoprofenate is bound to the
copper(II) ion, as represented in Figure 1. This is further
supported by single crystal X-ray diffraction studies of the
structural analogue [Cu(tBuBz3tren)CH3CO2](CH3CO2)
(2),[23] which reveal a copper(II) ion in a distorted trigonal
bipyramidal geometry with an acetate occupying the axial
coordination site in a syn monodentate mode (Figure 1). As
with complex 1, the ESI-MS spectrum of 2 (given in the
Supporting Information) shows an intense peak at m/z =
706.42 (100%), which is attributed to the [Cu(t-
BuBz3tren)CH3CO2]+ cation.
a paramagnetic complex (S = 1= ), affording very broad peaks
2
for the aromatic protons of the ketoprofenate anions and tren
ligand (between 7.30 and 8.80 ppm), and the methyl protons
of the tert-butyl groups (at 1.49 ppm), even if they are quite
far from the copper(II) ion (Figure 3). Neither the methylenic
Complex 1 is fully soluble in most organic solvents, in
particular alcohols such as MeOH, which seemed to be an
appealing solvent for the photoreduction process.[9–11] More-
over, as MeOH is considered a green solvent,[14] it is a good
solvent candidate for click reactions. The copper(II) to
copper(I) photoreduction process was first evidenced by
following the disappearance of the characteristic visible
absorption band of 1, which is centered at 828 nm and arises
Figure 3. Evolution of the 1H NMR spectrum of a CD3OD solution of
1 (0.5 mL, 5 mm, deoxygenated by freeze-pump-thaw cycles, sealed
tube) upon irradiation of the NMR tube at 365 nm.
protons of the benzylic positions and the tren scaffold, nor the
aliphatic protons of the ketoprofenate ions are observed.
Upon irradiation of the NMR tube, a fast and dramatic
change in the spectrum is observed, with the complete
disappearance of the broad resonances of 1 within ca.
45 min while, concomitantly, sharp resonances assigned to
the diamagnetic (S = 0) cuprous [Cu(tBuBz3tren)]+ cation
appear. In particular, all of the proton resonances of the
tBuBz3tren ligand of the cuprous cation can be clearly
identified at 7.28 and 7.06 ppm for the phenyl rings, and at
3.56, 2.78, and 1.27 ppm for the benzylic CH2, CH2N, and tBu
groups, respectively. The complicated resonance pattern
observed for the aromatic protons of ketoprofen is ascribed
to the coexistence, after reduction, of several types of
ketoprofen species: one ketoprofenate interacting with the
copper(I) cation, and one equivalent of ketoprofen (acid
form) with respect to copper released during the reduction
Figure 2. Evolution of the visible absorption spectrum of a MeOH
solution of 1 (3 mL, 1.85 mm, deoxygenated by freeze-pump-thaw
cycles, sealed cell) upon irradiation at 365 nm.
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process (see the photochemical study below and the proposed
mechanism).
(Supporting Information, Figure S1). This is in agreement
with a radical process, which necessitates H-atom abstraction
[Eq. (3)].
To gain insight into the photoreduction mechanism,
preliminary transient absorption studies were performed in
degassed MeOH solution. On irradiating 1 at 355 nm,
a positive transient absorption band is observed in the visible
region at 530 nm (Supporting Information, Figure S4), which
is characteristic of the triplet ketoprofen, the formation of
which is a key primary step in the photoinduced sequence of
events.[16,17] Although a similar signature is noted for the
ketoprofen alone (acid form), it is clear that the decay kinetics
are very different (Supporting Information, Figure S5). The
kinetics of the deexcitation of the ketoprofen (acid form)
exhibit a typical, relatively slow decay (t = 370 ns) of the
visible transient absorption band,[16,17] which normally
involves reaction with methanol, a rather efficient hydrogen
donor.[17] For complex 1, the decay of the absorption band has
a time constant of 9.4 ns. Thus, the long decay time constant is
dramatically reduced in the presence of the copper complex.
As in previous reports by Chow, Buono-Core, and co-workers
on the related benzophenone-sensitized photoreduction of
[Ni(acac)2], [Cu(acac)2], or [Cu(Bp)2] complexes,[9–11] the
observed triplet decay in 1 is thus much faster (k = 1.0 ꢀ
108 sÀ1) than the triplet decay measured in MeOH for the
acid form of ketoprofen (k = 2.7 ꢀ 106 sÀ1), which shows the
intervention of a fast additional quenching pathway for this
excited state in the presence of the copper complex. From
these results, and by analogy with previous studies, a tentative
mechanism for the reduction process is proposed, according
to equations 1–4. Irradiation of 1 produces the ketoprofen
(ket) triplet [Eq. (1)] which can be quenched by electron
The driving force and hence the feasibility for the initial
photoinduced electron transfer [PET; Eq. (2)] can be esti-
mated by considering the difference in potential for oxidation
of the ligated CuII center and the potential for reduction of the
excited ketoprofen moiety.[18] The reduction of ketoprofen at
À1.36 V (V vs. SCE; SCE = saturated calomel electrode) and
the triplet energy of related benzophenones at 3.0 eV have
previously been determined.[19,20] Cyclic voltammetry was
performed in both methanol and in acetonitrile (wherein the
reversibility of the redox processes at the copper center was
improved, thus allowing half-wave potential determination,
see Figure S6 in the Supporting Information). A fully
reversible wave attributed to the CuI/CuII couple (À0.34 V
vs. SCE) as well as a quasi-reversible wave corresponding to
CuII/CuIII (+ 1.29 V vs. SCE) were observed. The tren-like
ligand clearly plays an important role in facilitating these two
one-electron oxidation processes. The thermodynamic driving
force (DG0) for the PET process from CuII to the ketoprofen
triplet is ca. À0.35 eV, showing it to be exothermic and
favorable. Further studies will be conducted to confirm the
proposed mechanism, in particular the intermediacy of
a ligand-centered radical, as unambiguously evidenced for
the photoreduction of [Cu(acac)2] and [Ni(acac)2].[9,10]
The latent behavior and click reactivity of 1 was probed
with the reaction between the unprotected sugar b-d-galac-
topyranosyl azide (3) and propargyl alcohol (4), both of which
are soluble in methanol. Figure 4 shows the conversion
profiles of click reactions run at room temperature in
deoxygenated CD3OD (Ar purged) with 0.5 mol% 1 (reac-
tions followed by 1H NMR spectroscopy, see Figure S2 in the
Supporting Information). It is noteworthy that under ambient
transfer from copper(II), see below, leading to a formal
conditions 1 exhibits no residual background activity
[trenCuIIIketCÀ]+ intermediate, followed by a one-electron
(Figure 4, ), a crucial parameter when considering effective
*
+
oxidation of the ligand to generate [trenC CuIIketCÀ]+
photoswitchable catalysts. Irradiating the NMR tube at
[Eq. (2)]. The resulting ligand-centered radical may then
abstract an H atom from the H-donating MeOH solvent
[Eq. (3)]. A back electron transfer affords the copper(I)
species and regenerates the ketoprofenate counteranion
[Eq. (4)]. It is noteworthy that the photoinduced reduction
does not occur in 2 (where acetate replaces the ketoprofen),
which shows the fundamental photostability of the inorganic
moiety when irradiating at 365 nm. Moreover, when the
methanol solvent is replaced by dichloromethane, which is
a poor H-donating solvent, the reduction process is much less
efficient as ca. 75% of 1 is reduced after 3 h irradiation
Figure 4. Reaction profiles of CD3OD solutions (0.33m, 0.75 mL) of 3
and 4 with 1 (0.5 mol%) followed by 1H NMR spectroscopy and
applying external stimuli.
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365 nm for 15 min, for example, after a latency period of 2 h,
catalysts should prove very useful. Indeed, copper(I)-poly-
amino/imino complexes possessing accessible coordination
sites are well-known to be extremely reactive towards oxygen,
thus complicating the preparation, handling, and storage of
such compounds. Moreover, when conducting highly exo-
thermic reactions, which is the case with CuAAC, particularly
on large scale and/or in high concentration, the use of
a catalytic system that can be switched on upon irradiation
and switched off by simply introducing air into the reaction
medium, should be very attractive. The fact that the onset of
the active species can be finely controlled depending on the
irradiation time is an additional interesting feature for such
applications. Finally, we believe that the reported photo-
induced reduction process could be extended, not only to
other important copper(I)-catalyzed reactions, but also to
other transition-metal-catalyzed processes. Studies along
these lines are currently in progress.
^
switches the catalysis on (Figure 4, ). The resulting copper(I)
catalyst is highly reactive, affording an almost quantitative
formation of the triazole 5 in ca. 3 h.
Once started, the reaction can be stopped completely at
any time by simply bubbling air through the reaction mixture,
because of the almost instantaneous oxidation of the catalyst
&
by molecular oxygen (Figure 4, ). Indeed, when exposed to
air the solution immediately turned green, indicating that the
photogenerated copper(I) complex is readily reoxidized by
oxygen. Interestingly, the inhibition process is fully reversible.
Bubbling Ar through the reaction mixture followed by 15 min
irradiation restores the catalytic activity, with a reaction rate
similar to that observed before oxygen inhibition (Figure 4).
For comparison, conversions of 10–25% were determined by
1H NMR spectroscopy after 3 h reaction time using 0.5 mol%
of CuSO4/sodium ascorbate,[4a] CuPPh3CN,[21] or CuOAc[22] in
solvents that provided the best activity for these catalysts in
our hands: D2O for CuSO4 and CuPPh3CN, and CD3OD for
CuOAc (24 h reaction profiles are given in the Figure S3 of
the Supporting Information).
Received: April 19, 2012
Keywords: click chemistry · copper · Huisgen cycloaddition ·
.
photocontrolled catalysts · photolatent catalysts
Finally, click reactions were conducted for a range of
alkynes with aromatic, aliphatic, electron-withdrawing, or
bulky substituents (round-bottom flask, 20 min Ar bubbling,
30 min of irradiation). The catalyst loading was 0.5 mol%,
except for the ethyl propiolate (1 mol%). The reactions were
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In conclusion, by exploiting the benzophenone-derived
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generate a highly reactive copper(I)-tren click catalyst on
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