A Mechanochemically Triggered "click" Catalyst

Article abstract of DOI:10.1002/anie.201505678

"Click" chemistry represents one of the most powerful approaches for linking molecules in chemistry and materials science. Triggering this reaction by mechanical force would enable site- and stress-specific "click" reactions - a hitherto unreported observation. We introduce the design and realization of a homogeneous Cu catalyst able to activate through mechanical force when attached to suitable polymer chains, acting as a lever to transmit the force to the central catalytic system. Activation of the subsequent copper-catalyzed "click" reaction (CuAAC) is achieved either by ultrasonication or mechanical pressing of a polymeric material, using a fluorogenic dye to detect the activation of the catalyst. Based on an N-heterocyclic copper(I) carbene with attached polymeric chains of different flexibility, the force is transmitted to the central catalyst, thereby activating a CuAAC in solution and in the solid state.

Full text of DOI:10.1002/anie.201505678

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201505678
Click Chemistry
German Edition:
DOI: 10.1002/ange.201505678
A Mechanochemically Triggered “Click” Catalyst
Philipp Michael and Wolfgang H. Binder*
Abstract: “Click” chemistry represents one of the most
powerful approaches for linking molecules in chemistry and
materials science. Triggering this reaction by mechanical force
would enable site- and stress-specific “click” reactions—
a hitherto unreported observation. We introduce the design
and realization of a homogeneous Cu catalyst able to activate
through mechanical force when attached to suitable polymer
chains, acting as a lever to transmit the force to the central
catalytic system. Activation of the subsequent copper-catalyzed
“
click” reaction (CuAAC) is achieved either by ultrasonication
or mechanical pressing of a polymeric material, using a fluo-
rogenic dye to detect the activation of the catalyst. Based on an
N-heterocyclic copper(I) carbene with attached polymeric
chains of different flexibility, the force is transmitted to the
central catalyst, thereby activating a CuAAC in solution and in
the solid state.
W
ith the discovery of the copper-catalyzed “click” chemis-
[
1]
try (CuAAC) by the research groups of Meldal and
[2]
Sharpless, the efficiency and quality of this reaction has
been truly probed and proven in many areas of chemistry.
Not only the fields of polymer, biological sciences, and
drug discovery have profited, but materials and surface
science have also taken significant advantage from the overall
valuable properties of this reaction. Thus, for example, cross-
linking reactions (such as network formation), surface-
modification methods (such as self-assembled monolayers
[3]
Figure 1. a) Mechanochemical activation of latent copper(I) N-hetero-
cyclic carbene (NHC) catalysts (I, II) by ultrasound or compression.
b) Fluorogenic “click” reaction of nonfluorescent coumarin dye 1 with
2 catalyzed by the activated mechanocatalysts I or II, which result in
the formation of the highly fluorescent dye 3. c) Mechanochemical
activation of the latent polymeric NHC-copper(I) catalysts (I, II) within
the crystalline poly(tetrahydrofuran) (PTHF) matrix by compression.
[
4]
[5]
[
6]
[7]
[8]
[9]
or polymeric surfaces), or the generation of self-healing
[
10]
I
materials have been enabled as a consequence of its high
substrate tolerance as well as its solvent insensitivity usually
under low temperatures. Besides the known large number of
a Cu -based catalytic system is needed, which is not active
under ambient conditions, but can solely be activated by
mechanical force. Second, a catalytic system able to transmit
mechanical force to the mechanochemical labile bond
through affixed polymer chains is required. To fulfill both
criteria, we designed an N-heterocyclic carbene (NHC)
I
homogeneous and heterogeneous Cu -based catalysts, espe-
cially latent catalytic “click” systems have been designed, for
example, those which can be triggered by a specific stimulus.
Thus, a site-specific activation of the “click” reaction by
[15]
complex
bearing attached polymer chains on the NHC
[
11]
[8a,12]
triggers such as light, pressure,
or a simple heteroge-
ligands. This system is projected to be not catalytically active
[13]
[16]
neous ball-milling procedure
have been achieved. How-
for the “click” reaction at room temperature.
Thus, the
ever, a homogeneous mechanochemical “click” system has
not yet been developed to the best of our knowledge
catalytic systems I and II were designed, where the “click”
[16,17]
activity was known to be poor or absent
in the bis(NHC)-
I
(
Figure 1).
The design of our mechanochemical “click” catalyst starts
with two principal considerations based on considerations
Cu complexes, but present in the corresponding mono-
I
(NHC)-Cu X complexes. Attaching polymer chains of differ-
ent flexibility which are able to act as a lever to transmit the
[14]
I
generally accepted in the mechanochemical world:
first
mechanical force onto the central Cu complex leads to the
initial formation of biscarbene [Cu(polymer-NHC) ]X com-
2
plexes I and II (Figure 2). The selected polymers are
poly(isobutylene) (PIB), which displays a low glass-transition
[
*] P. Michael, Prof. Dr. W. H. Binder
Institute of Chemistry, Chair of Macromolecular Chemistry
Faculty of Natural Sciences II
Martin-Luther University Halle-Wittenberg
von Danckelmann-Platz 4, 06120 Halle (Saale) (Germany)
E-mail: wolfgang.binder@chemie.uni-halle.de
temperature (T ca. À808C) and is highly flexible at RT, and
g
the rigid poly(styrene) (PS; T ca. 1008C). The subsequently
g
mechanically activated catalysis is detected through a fluoro-
genic “click” reaction of 3-azido-7-hydroxycoumarin (1) with
phenylacetylene (2) that is able to detect traces of a fluo-
rescently active dye generated by the “click” reaction.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505678.
1
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Figure 2. a) Synthetic route to the biscarbene complexes [Cu(PIB-NHC )]X (I) and [Cu(PS-NHC )]X (II), and the monocarbene complex [Cu(PIB-
2
2
À
À
À
NHC)]X (III) (X=Br ; I ; PF ). i) N-methylimidazolium telechelic polymers 5 and 7 generated by a quaternization reaction using NaI/15-crown-5-
6
ether and N-methylimidazole. Transformation to the bis- (I, II) and monocarbene (III) complexes for PIB by ii) reaction of 5 with KHMDS (11) and
1
[
Cu(CH CN) ]PF (10); for PS by iii) reaction of 7 with NaOtBu (12) and 10 (for synthetic details see the Supporting Information). b) H NMR
3
4
6
spectra of N-methylimidazolium-telechelic PIB (5b) and the corresponding mono- and biscarbene copper(I) complexes (Ib, IIIb). c) Structure of
À1
À1
model complex [Cu(NHC) ]X (IV). d) GPC traces for PIB-Br (4a, M
=2200 gmol ), IIIa (Mn(GPC) =2900 gmol ), and Ia
2
n(GPC)
À1
(Mn(GPC) =4750 gmol ) indicating the doubling of the molecular weight.
As known for various mechanochemically triggered
systems (e.g. mechanochemically triggered Ru-NHC com-
spectra as well as MALDI-TOF-MS spectra prove the
attachment of the N-methylimidazolium moiety (for synthetic
details as well as analytical data see the Supporting Informa-
tion). The final bis(N-methylimidazol-2-ylidene-telechelic
[
18]
[19]
plexes or cyclobutane-1,3-diones) the positioning of the
weak link” in the middle of the polymer chains leads to
“
significantly better transmittance of force—therefore, our
mechanochemical Cu -NHC complex is placed within two
polymer) complexes [Cu(polymer-NHC) ]X I and II were
2
I
obtained by reaction of 5 with tetrakis(acetonitrile)copper(I)
hexafluorophosphate ([Cu(CH CN) ]PF ; 10) in the presence
polymer chains of equal chain length, thereby allowing either
3
4
6
À1
the chain length of the polymers (from M = 4750 gmol to
1
of potassium hexamethyldisilazide (KHMDS; 11), or of 7
with 10 and NaOtBu (12; see the Supporting Information).
Additionally, the pure PIB monocarbene ([Cu(PIB-NHC)]X
complex (III) was obtained cleanly by column chromatog-
raphy (see the Supporting Information) as a mechanochemical
system with only one polymer handle attached to the copper
complex.
n
À1
7200 gmol ) or its rigidity (PIB versus PS) to be tuned.
The synthesis of the catalysts I and II (Figure 2a) starts
either from the appropriately bromo-telechelic PIB (4),
[20]
prepared by living carbocationic polymerization (LCCP)
or from the bromo-telechelic PS (6) synthesized by atom-
[21]
transfer radical polymerization (ATRP). In both cases, the
N-methylimidazolium-functionalized polymers (5 and 7) were
synthesized by in situ transformation through nucleophilic
substitution to the iodo-telechelic moieties using sodium
iodide/15-crown-5-ether. Quaternization reactions with
N-methylimidazole results in the formation of the N-methyl-
imidazolium telechelic polymers (5, 7), which are now able to
Structural proof of the synthesized complexes I, II, and III
1
was accomplished by H NMR spectroscopy as well as
1
MALDI-TOF-MS analysis. Comparing the H NMR spectra
of the biscarbene [Cu(PIB-NHC) ]X (I) as well as the
2
monocarbene [Cu(PIB-NHC)]X complex (III) to that of the
initial N-methylimidazolium-telechelic PIB (5) clearly shows
1
act as macroligands for catalysts I and II. The H NMR
the disappearance of the imidazolium proton H at d =
i
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1
0.42 ppm and the shift of the signals corresponding to the
II to a temperature of 608C (see Table 1, entries 3 and 9)
CH groups of the respective NHC moieties (see Figure 2b).
MALDI-TOF mass spectra of the mechanocatalyst I also
proves the prospected structure, fitting well with its calculated
induces only a poor activity (less than 3% conversion as
determined by H NMR spectroscopy). However, triggering
1
the reaction by ultrasound, which is a generally accepted tool
+
[18,22]
isotope patterns (e.g. Ia, C130H242Cu N O , [M] : simulated
for applying mechanochemical force in solution,
leads to
1
4
À1
2
À1
1
956.829 gmol , found: 1956.846 gmol ; for detailed infor-
a significant catalytic activity (Table 1, entries 5—7, and 10. In
[18]
mation see the Supporting Information). Moreover, the
increase in the molecular weight upon formation of the
accordance with different mechanochemical systems,
an
increase in the chain length of the attached polymers, acting
À1
biscarbene [Cu(PIB-NHC) ]X (I) complex was proven by
as a mechanical handle, from M (GPC) = 4750 gmol (Ia) to
2
n
À1
comparing the molecular weights by gel-permeation chroma-
tography (GPC; see Figure 2d as well as the Supporting
Information). A doubling of the molecular weight in the case
17200 gmol (Ic)) leads to an increase in the catalytic activity
from 10 to 27% in the conversion of the “click” reaction,
congruent to an increased mechanical activation through the
À1
of the biscarbene complex Ia (4750 gmol ) compared to the
applied ultrasound. Moreover, a model [Cu(NHC) ]X cata-
2
À1
starting polymer 4a (2200 gmol ) and to the monocarbene
lyst (IV; see Figure 2c) without attached polymer chains
shows only a poor catalytic activity for the “click” reaction
after 24 h at room temperature (Table 1 entry 11). As
a consequence of the expected higher rigidity of the poly-
(styrene)-based catalyst II, its catalytic activity after activa-
tion by ultrasound is 44% (Table 1 entry 10) and thus higher
than those of catalyst Ic, which contains PIB with an even
higher molecular weight (Table 1 entry 7). Furthermore, the
PIB-based monocarbene complex (IIIa) is active, even with-
out applied ultrasound, as no shielding second ligand blocks
the catalytic active site of the copper center.
À1
complex IIIa (2900 gmol ) clearly indicated the desired
chemical structures (see Table S1 in the Supporting Informa-
tion). Similar observations were made for the [Cu(PS-
NHC) ]X (II) complex, thereby proving the final biscarbene
2
complex structure (see the Supporting Information).
The reactivity of the catalysts towards “click” reactions
was probed first in solution (see Table 1) by using a conven-
tional “click” reaction of phenylaceylene (2) and benzylazide
1
(
8), monitored by H NMR spectroscopy. The native catalysts
I and II are inactive at room temperature (see Table 1
entries 2 and 8), irrespective of the molecular weight of the
attached polymer chain, thereby proving that the initial
Finally, and most importantly, the mechanochemical
activation was probed within a real solid polymer matrix,
devoid of any solvent. As it is known that an optimal
transmittance of mechanochemical force is achieved when the
structural polymer contains crystalline regions, high-molec-
biscarbene [Cu(polymer-NHC) ]X complexes I and II are
2
inactive in their “latent” state. Even heating the catalysts Ia or
[
23]
ular-weight poly(tetrahydrofurane) (PTHF)
1
(M (GPC) =
n
À1
12 000 gmol ; crystallinity 68%) was chosen as the struc-
Table 1: Ultrasound triggered “click” reaction of phenylacetylene (2) and
tural polymer, into which the mechanocatalyst I was embed-
ded to check the pressure-induced activation. Critical for the
detection of the so-generated, mechanically induced catalytic
activity is a reliable detection system for monitoring the
progress of the “click” reaction directly within a solid polymer
matrix. Based on previous knowledge of the fluorogenic
benzylazide (8) using mechanocatalysts [Cu(PIB-NHC) ]X (I) or [Cu(PS-
2
[
a]
NHC) ]X (II) in solution.
2
[
24]
“
click” reaction,
the fluorogenic 7-hydroxy-3-(4-phenyl-
I
[c]
[d]
[25]
Ultra-
sound
T
t
[h]
Cu
M (GPC)
Conversion
[%]
1H-[1,2,3]triazole-1-yl)-coumarin dye (3)
(lem = 427 nm,
n
[
b]
À1
catalyst [gmol
]
lex = 260 nm as well as 360 nm) was used to probe and
quantify the catalyst activity in the solid state.
1
2
3
4
5
6
7
8
9
1
1
on
off
off
off
on
on
on
off
off
on
off
RT
RT
72
168 Ia
without
–
0
0
4750
4750
2900
4750
8900
17200
13100
13100
13100
–
The biscarbene [Cu(PIB-NHC) ]X complex Ic (concen-
2
À6
À1
60 8C 72
Ia
IIIa
Ia
<2
11
10
17
27
0
<3
44
0
tration 5.17 ” 10 mmolCu mgsample ) was embedded into the
PTHF matrix (65% crystallinity after embedding; deter-
mined by DSC), together with the two reactive components
phenylacetylene (2) and the initial nonfluorescent 3-azido-7-
[
e]
RT
RT
RT
RT
RT
24
24
24
24
Ib
Ic
À4
À1
hydroxycoumarin dye (1; 1.56 ” 10 mmoldye mgsample ). Ap-
plication of 10 tons pressure by a hydraulic press (3 ” 30 min)
causes a clear induction of the catalytic activity as detected by
144 II
60 8C 72
RT
RT
II
II
IV
0
1
24
24
the increased fluorescence at 427 nm (l = 360 nm) of the
ex
transformed fluorogenic dye 3, thus proving the mechano-
chemical activation of the latent copper catalyst by compres-
sion (see Figure 3b, B and C). The measured fluorescence
after compression corresponds to a dye concentration of
[
a] For all reactions 1 equiv azide 8, 1 equiv alkyne 2, and 0.01 equiv of
I
the Cu catalyst (0.75 mm; I, II, III) were used in 30:1 THF/MeOH. All
experiments were repeated at least twice and display the same results
with maximum deviation of Æ1%. [b] 90 min pulsing at 20 kHz with
3
1
0% of maximal amplitude of 125 mm with a pulse sequence of 5 s pulse,
0 s break, 60 min without pulsing, repeated three times. [c] For I and III
À6
À1
about 6.66 ” 10 mmoldye mgsample and correlates to a con-
version of approximately (4.3 Æ 1)% of the “click” reaction
a PIB standard and for II a PS standard were used. [d] In [D ]THF,
detected through the increasing triazole resonance at d=8.12 ppm as
8
(see the Supporting Information), thereby proving that no
significant fluorescence is observed before compression (Fig-
ure 3a). Control experiments were conducted to exclude the
well as the shift of the CH resonance from d=4.34 to 5.58 ppm.
2
[
e] 1.1 equiv Na/ascorbate were additionally used for reaction of IIIa.
1
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Acknowledgements
This research was supported by the German Science Foun-
dation, DFG-project BI 1337/7-1, the projects BI1337/8-1 and
1
3373/8-2 within the SPP 1568 (“Design and Generic Princi-
ples of Self-Healing Materials”), and the project A3 within
the SFB TRR 102.
Keywords: click chemistry · copper · mechanochemistry ·
self-healing materials
How to cite: Angew. Chem. Int. Ed. 2015, 54, 13918–13922
Angew. Chem. 2015, 127, 14124–14128
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6
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À4
À1
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À1
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2
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Received: June 19, 2015
Published online: September 30, 2015
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