Article
pubs.acs.org/Macromolecules
Photoinduced Free Radical Promoted Copper(I)-Catalyzed Click
Chemistry for Macromolecular Syntheses
Mehmet Atilla Tasdelen,†,‡ Gorkem Yilmaz,† Birol Iskin,† and Yusuf Yagci†,*
†
Department of Chemistry, Istanbul Technical University, Maslak, TR-34469, Istanbul, Turkey
Department of Polymer Engineering, Faculty of Engineering, Yalova University, TR-77100 Yalova, Turkey
‡
S Supporting Information
*
ABSTRACT: Photoinduced copper(I)-catalyzed Huisgen 1,3dipolar cycloaddition (CuAAC) via photoinduced electron
transfer using free radical photoinitiators has been developed as
a new platform to serve as orthogonal click system. Photoinitiators acting at near UV and visible range, namely 2, 2dimethoxy-2-phenyl acetophenone, 2-benzyl-2-dimethylamino-4′morpholino butyrophenone, 2,4,6-trimethylbenzoyl)diphenylphosphine oxide, dicyclopentadienyl bis[2,6-difluoro-3(1-pyrrolyl)phenyl] titanium) and camphorquinone/benzyl alcohol were tested with copper(II) chloride/N,N,N′,N″,N″-pentamethyldiethylenetriamine complex to catalyze the CuAAC via photoinduced electron transfer reaction. This strategy has been
applied in construction of various macromolecular architectures including telechelic polymers and block copolymers.
Spectroscopic and chromatographic investigations revealed that successful macromolecular syntheses have been achieved by this
technique.
■
INTRODUCTION
Click chemistry describes a class of chemical reactions that are
easy to perform, give rise to their intended products in very
high yields with little or no byproducts, work well under many
conditions, and are unaffected by the nature of the groups
being connected to each other. The most prominent example of
click chemistry reaction is based on the well-established
copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition
(CuAAC) reaction between azides and terminal alkynes,
discovered by the groups of Sharpless1 and Meldal.2 The
CuAAC click reaction has received considerable attention as
powerful modular synthesis approach,3 which has found
numerous applications in organic chemistry, supramolecular
chemistry, polymer chemistry, drug discovery, bioconjugation,
and materials science.1,4 However, it has some limitations
including the need for a metal catalyst5 and an inability to
control the reaction by external stimulation or to conduct the
reaction in the absence of solvent.6 To further develop this
fundamentally important aspect of synthetic chemistry, the
discovery of alternative robust, efficient, and orthogonal click
reactions or the modification of well-known click reactions is
particularly relevant. Light-induced reactions offer the possibility of both spatial and temporal control over the reaction,
which are not available in thermal conjugation-reactions.7 In
recent years, photoinduced thiol−ene/thiol−yne coupling,8
photoinduced 1,3-dipolar cycloaddition reaction of alkenes and
nitrile imines,9 “strain-promoted” cycloaddition reaction of the
photochemically generated cycloalkynes and azides10 photoinduced ester formation reaction of benzodioxinones with
alcohols11 and photoinduced Diels−Alder reactions12 have
© 2011 American Chemical Society
been developed. Although thiol−ene/thiol−yne reactions can
proceed extremely rapidly yielding products quantitatively
under facile conditions (i.e., at ambient temperature and
humidity under an air atmosphere), the simultaneous reactions
of the photochemically formed primary radicals with enes
reduce the click efficiency.13 In the other methods such as
photoinduced acylation reactions using benzodioxinones, the
preparation and purification of click reagents cumbersome and
result in low product yields.11a
Light induced reduction of metal ions such as silver,14 gold,15
and copper16 complexes can be successfully carried out by
either direct photolysis or indirect photolysis. In direct
photolysis, desired metal ions are generated upon irradiation
of its higher oxidation state species at appropriate wavelengths.
Typically, the absorption of light by copper ligand promotes the
intramolecular electron transfer from the π-system of the ligand
to the central ion resulting in the transformation of Cu(II) ion
into Cu(I) and the ligand into the radical complex.17 However,
this method involving UV exposure suffers from the need for
long irradiation times. For indirect approach, the reduction of
the Cu(II) ion is dependent on a photoactivator. Thus, a
photoinitiator absorbs light in the UV−visible region, where
copper complex is transparent and forms reactive intermediates
such as free radicals, which strongly promote the photoreduction of the Cu(II) into Cu(I). The nature of the
photochemically generated radicals and the redox properties
Received: November 4, 2011
Revised: December 2, 2011
Published: December 9, 2011
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Synthesis of Telechelic Polymers via Click Chemistry.
General procedure for the synthesis of telechelic polymers is as
follows: polymers bearing clickable functionalities, PSt−N3 or PCL−
Alkyne (1.0 equiv) is dissolved in a glass bottle containing DMSO-d6
(1 mL). Then CuIICl2 (1.5 equiv), PMDETA (1.5 equiv), TMDPO
(2.5 equiv), and the corresponding low molar mass acetylene or azido
click compounds (1.5 equiv) were added to the solution. After
dissolution, the reaction mixture is added to an NMR tube with a
syringe. The reaction tube was irradiated by a Ker-Vis blue
photoreactor equipped with a circle of six lamps (Philips TL-D
18W) emitting light nominally at 400−500 nm. 1H NMR spectra were
periodically recorded, with each measurement involving 16 scans.
Conversions were determined by integration of the signal from a
proton in one of the starting molecules and the corresponding proton
in the product.
Synthesis of Poly(Styrene-b-(ε-caprolactone)) (PSt-b-PCL)
via Click Chemistry. PSt-N3 (1.0 equiv), PCL-Alkyne (1 equiv),
CuIICl2 (1.5 equiv), PMDETA (1.5 equiv), and TMDPO (2.5 equiv)
were dissolved in DMSO-d6 (1 mL) in a glass bottle and added into an
NMR tube with a syringe. The reaction tube was irradiated by a KerVis blue photoreactor equipped with a circle of six lamps (Philips TLD 18W) emitting light nominally at 400−500 nm. 1H NMR spectra
were periodically recorded, with each measurement involving 16 scans.
Conversions were determined by integration of the signal from a
proton in one of the starting molecules and the corresponding proton
in the product. After complition of the reaction, the reaction solvent is
directly transferred into a beaker containing excess methanol/water
(10:1, vol/vol) to precipitate the polymer. The obtained polymer is
further characterized by FT-IR and GPC measurements.
of the copper complex are quite crucial for the success of the
process. Recently, our group has developed a new photochemical protocol to catalyze the CuAAC reaction between
azides and alkynes by in situ generation of Cu(I) from Cu(II)
complex with UV-light.18 Later on, Bowman and co-workers
demonstrated the comprehensive spatial and temporal control
of the CuAAC reaction using standard photolithographic
techniques by photochemical reduction of Cu(II).6c
Herein, we report the wavelength tunability of light-induced
CuAAC click reaction using near UV and visible light
photoinitiators. In the context of macromolecular engineering,
this reaction is very valuable and permitting the preparation of
highly defined telechelic polymers and block copolymers. This
process gives the spatial control of the click reaction by
adjusting the copper concentration of the system and
consequently, coupling efficiency by choosing appropriate
light intensities.
■
EXPERIMENTAL PART
Materials. Phenylacetylene (98%, Aldrich), benzyl bromide (98%,
Aldrich), sodium azide (99%, Aldrich), dimethyl-d6 sulfoxide (99.96
atom % D, Aldrich), benzyl alcohol (99%, Aldrich) and copper(II)
chloride (98%, Aldrich) were used as received. N,N,N′,N″,N″Pentamethyldiethylenetriamine (99%, Aldrich) was distilled before
use. Benzyl azide was synthesized according to literature procedure.18
1
H NMR (250 MHz, CDCl3): δ (ppm) = 4.4 (Ph−CH2−N3), 7.4−7.5
(aromatic protons). 2, 2-Dimethoxy-2-phenyl acetophenone (DMPA,
Ciba Specialty Chemicals), 2-benzyl-2-dimethylamino-4′-morpholinobutyrophenone (DBMP, Ciba Specialty Chemicals), 2,4,6trimethylbenzoyl)diphenylphosphine oxide (TMDPO, Ciba Specialty
Chemicals), and dicyclopentadienylbis[2,6-difluoro-3-(1-pyrrolyl)phenyl] titanium (Titanocene, Ciba Specialty Chemicals) and
(camphorquinone (CQ, Ciba Specialty Chemicals) were used as
received.
Characterization. UV spectra were recorded on a Shimadzu UV1601 spectrometer. 1H NMR spectra in DMSO-d6 with Si(CH3)4 as an
internal standard were recorded at room temperature at 250 MHz
using a Bruker DPX 250 spectrometer. FT-IR spectra were recorded
on a Perkin-Elmer FT-IR Spectrum One-B spectrometer. Molecular
weights were determined by gel permeation chromatography (GPC)
instrument, Viscotek GPCmax Autosampler system, consisting of a
pump, three ViscoGEL GPC columns (G2000HHR, G3000HHR and
G4000HHR), and a Viscotek differential refractive index (RI) detector
with a THF flow rate of 1.0 mL min−1 at 30 °C. The RI detector was
calibrated with PS standards having narrow molecular weight
distribution. Data were analyzed using Viscotek OmniSEC Omni−
01 software.
Spectroscopic Investigations of Click Reactions. General
experimental procedure for the light-induced copper(I)-catalyzed
click reaction: DMSO-d6 (0.5 mL) and benzyl azide (11 μL, 0.2
mM) were added to an NMR tube containing Cu(II)Cl2 (1.4 mg, 0.02
mM), PMDETA (2.2 μL, 0.02 nM) and a photoinitiator (0.02 mM).
After 1−2 min, the phenylacetylene (13 μL, 0.2 mM) was added via
syringe. The reaction tube was irradiated by a Ker-Vis blue
photoreactor equipped with a circle of 6 lamps (Philips TL-D 18W)
emitting light nominally at 400−500 nm. 1H NMR spectra were
periodically recorded, with each measurement involving 16 scans.
Conversions were determined by integration of the signal from a
proton in one of the starting molecules and the corresponding proton
in the product (e.g., protons a and d in Figure S1, Supporting
Information).
Synthesis of Polymers with Clickable Functionalities. ωAzido functional polystyrene19 (PSt-N3) (Mn,NMR: 1910, Mn,GPC: 2000,
Mw/Mn:1.12) and α-alkyne functional poly(ε-caprolactone)20 (PCLAlkyne) (Mn,NMR: 2900, Mn,GPC: 4400, Mw/Mn:1.12) were synthesized
according to literature.
■
RESULTS AND DISCUSSION
The use of electron transfer reactions of free radicals in
synthetic polymer chemistry dates back to more than 3 decades
ago.21 Photochemically generated electron donor radicals can
efficiently be oxidized by appropriate salts to the corresponding
cations capable of initiating cationic polymerization of
appropriate monomers (Scheme 1).22
Scheme 1. Oxidation of Photochemically Formed Radicals
by Onium Salts
Many UV and visible light free radical photoinitiators were
reported to be powerful promoters not only for cationic
polymerization but also in situ formation of polymer/metal
nanocomposites based on both epoxy and (meth)acrylates.14,15,23 In order to demonstrate further value of this
simple redox process, the CuAAC click reactions between
benzyl azide and phenylacetylene as simple model click
components were performed in the presence of either type I
(2, 2-dimethoxy-2-phenyl acetophenone (DMPA), 2-benzyl-2dimethylamino-4′-morpholino butyrophenone (DBMP), 2,4,6trimethylbenzoyl)diphenylphosphine oxide (TMDPO) and
dicyclopentadienyl bis[2,6-difluoro-3-(1-pyrrolyl)phenyl] titanium (titanocene)) or type II (camphorquinone (CQ)) visible
light photoinitiators with copper(II) chloride/N,N,N′,N″,N″pentamethyldiethylenetriamine complex. The reactions were
performed under irradiation at 400−500 nm, where the copper
complexes were transparent and the light was absorbed
exclusively by the visible light photoinitiators (Figure 1).
First, a control experiment in the absence of a photoinitiator
under identical experimental conditions (benzyl azide and
phenylacetylene, CuIICl2 and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) ligand) was carried out. In this case,
only a little amount of coupled product was formed (Table 1,
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For DBMP, the 2-ethyl-2-benzyl substitution extended its
spectral sensitivity to above 400 nm, although the absorption
was quite weak at this wavelength. Under visible light
irradiation, DBMP generated an inert benzoyl radical25 and
an active aminoalkyl radical, which readily reduced Cu(II) into
Cu(I) ion and readily catalyzed the CuAAC click reaction
(Scheme 2). After 60 min, the reaction reached up to 93%
Scheme 2. Photochemical Generation of CuICl by Type I
Photoinitiator (DBMP)
Figure 1. UV−vis spectra of various photoinitiators and CuIICl2 in
DMSO. Concentration in all cases was 2 mM.
Table 1. Photoinduced Free Radical Promoted CuAAC Click
Reactiona between Benzyl Azide and Phenylacetylene Using
Free Radical Photoinitiators
conversion (Table 1, run 3, Figure S1 in the Supporting
Information).
Upon photolysis, TMDPO underwent α-scission and
produced a benzoyl and phosphinoyl radicals. While both
were reactive in typical free radical reactions (addition to
olefinic monomers and oxygen, and atom abstractions), the
phosphonyl radical displays higher efficiency due to the higher
electron density on the phosphorus atom and the more
favorable steric conditions arising from its pyramidal structure.21d,22d,26 However, their participation in electron transfer
reactions strongly depended on the substitution with electron
donating groups and the reduction potential of the oxidizing
salt used. Although unknown, the rate constant for the reaction
of phosphinoyl radicals with Cu(II) (E1/2red = 0.16 V (SCE))27
was expected to be larger than for iodonium salt (E1/2red = −0.2
V (SCE)),28 which was ket = 1.6 × 107 M−1 s−1.22d Analogous
to the reaction with iodonium salts, phosphinoyl radicals readily
reduced Cu(II); i.e. quantitative click reaction was attained in
20 min (Table 1, run 4). With the absorption of visible light,
titanocene photoinitiator underwent the generation of carboncentered radicals resulting from a Ti−C bond cleavage. The
titanium-centered diradical can readily reduce Cu(II) into
Cu(I) as well as promote the CuAAC click reaction. The radical
photoinitiator, CQ was also active in the range of visible light
and capable of generating radicals via Norrish type II reaction
in the presence of hydrogen donor, such as benzyl alcohol. The
reducing species involved in Cu(II) ion reduction was
frequently benzyl radical, which was well-established as
powerful reducing agent (Scheme 3).17c,29 The Cu(I) ion
produced was surely responsible for the initiation of the
CuAAC click reaction (Table 1, run 6).
It is clear that both type I and type II visible light
photoinitiators were capable of initiating the CuAAC click
reaction. The efficiency of the process depends on both the
absorbency and quantum yield particular photoinitiator
involved (Table 1). As can be seen from Figure 2, where
reaction yields were plotted vs time, type I photoinitiators were
more efficient in the process. This behavior was due to the fact
that, with type II photoinitiators, reactive radicals were
produced by a bimolecular reaction with a relatively lower
quantum yield. When using type I photoinitiators, the transient
a
All reactions were carried out under irradiation at 400−500 nm with a
light intensity of 45 mW cm−2 at room temperature, in DMSO-d6. The
initial concentrations of azide and alkyne were 0.2 mM and the
photoinitiator, CuIICl2 and PMDETA were 0.02 mM. bQuantum
yield.24 cYield determined by 1H NMR spectroscopy. dThe ratio of
CQ/BzOH = 1/3.
run 1) indicating negligible contribution from self-absorption
by the copper complex. The radical photoinitiators including
DMPA, DBMP, TMDPO and Titanocene are good sources of
electron donor radicals via Norrish type I reaction (Table 1,
runs 2−5). However, DMPA was essentially inactive and the
click reaction did not proceed even after prolonged irradiation
times (90 min) due to its low absorption behavior in the visible
light range (Table 1, run 2).
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Scheme 3. Photochemical Generation of CuICl by Type II
Photoinitiators (CQ/BzOH)
signal at 7.9 ppm indicate the formation of triazole (Figure S5
in the Supporting Information). The efficiency of the click
reaction, as determined from the ratio of the characteristic
peaks of polymers to that of the functional groups are listed in
Table 2.
Table 2. Telechelic Polymers via Photoinduced Free Radical
Promoted CuAAC clicka Reaction
a
All reactions were carried out under irradiation at 400−500 nm with a
light intensity of 45 mW cm−2 at room temperature, in DMSO-d6. The
initial concentrations of azide and alkyne were 0.5 mM, the
photoinitiator was 1.3 mM, and CuIICl2 and PMDETA were 0.08
mM; time = 120 min.
To test the suitability of the visible light-induced CuAAC
process in the macromolecular synthesis, a block copolymer
formation was attempted between an alkyne end-functionalized
poly(ε-caprolactone) (PCL−alkyne) (Mn = 4400, polydispersity index (PDI) = 1.12) and an azide end-functionalized
polystyrene (PSt−N3) (Mn = 2000, PDI = 1.12). The reaction
was performed under similar experimental conditions as with
the small molecular weight precursor. The formation of block
copolymer was evidenced by spectral analyses. The 1H NMR
spectrum of block copolymer obtained by click reaction
indicated characteristic peaks of both precursors (Figure 3).
Figure 2. Photoinduced free radical promoted click coupling reaction
of benzyl azide and phenylacetylene as a function of time using
CuIICl2/PMDETA/photoinitiator as the catalyst system. Alkyne/azide
ratio: 1. Azide/CuIICl2/photoinitiator ratio: 1:0.1:0.1. Yield determined by 1H NMR spectroscopy.
excited states (singlet and triplet) have very short lifetimes, thus
preventing any quenching by the metal ions. Notably,
titanocene-based photoinitiator was found to be most efficient
for the CuAAC click reaction. In all cases, the reaction yields
increased with the irradiation time since the oxidizable radicals
were continuously produced.
To evaluate the versatility and efficiency of visible lightinduced CuAAC process as a new general method in synthetic
polymer chemistry, we have conducted selected experiments in
polymer functionalization and block copolymer formation. For
the telechelic polymer30 preparation, both azide or alkyne endfunctionalized polymers in conjunction with low molecular
weight counterparts having various functionalities, namely,
propargyl alcohol, 4-pentynoic acid, propargyl pyrene, and
benzyl azide were used in the photoinduced click process. In
polystyrene functionalization, the successful transformations of
azide end groups into functional triazoles were confirmed by
the disappearance of the methine proton neighboring the azido
group (4.1 ppm) and the appearance of the methine proton
neighboring the triazole (5.1 ppm) in the 1H NMR spectra.
The additional signal at 8.1 ppm assigned to the proton of the
triazole rings was not clearly distinguishable in the NMR
spectra as it was partly masked by the broad aromatic regions of
polystyrene (Figure S2−4 in the Supporting Information). The
integration of aforementioned signals at 4.1 and 5.1 ppm
confirmed the successful formation of the triazole chain ends
with functional groups. In the poly(ε-caprolactone) case, the
Figure 3. 1H NMR spectra of the mixture of precursor polymers (PStN3 and PCL-Alkyne) and block copolymer (PSt-b-PCL) prepared by
photoinduced free radical promoted CuAAC click reaction.
While the methylene−oxy protons at 4.6 ppm from the PCL−
alkyne completely disappeared, a new signal corresponding to
the methyl and methylene protons linked to the triazole ring
was observed at around 5.1 ppm (b and d protons in Figure 3).
The efficiency of the click reaction for the formation of block
copolymer was found to be 86% as determined from
integration of the aforementioned signals.
The block copolymer formation was also followed by FT-IR
spectroscopy. The IR spectrum associated with block
copolymer showed the presence of both components (PSt
and PCL) in the structure (Figure S6 in the Supporting
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Information). Peaks resonating at 1730, 1360, 1290, 1180,
1100, and 960 cm−1 were due to PCL chain whereas transitions
from 1595, 1490, and 700 cm−1 and overtones between 1945
and 1805 cm−1 were specific to the PSt component. As
anticipated, the azide and alkyne stretching bands of precursor
polymers disappeared completely, confirming that quantitative
block copolymer formation by click reaction.
In Figure 4 the normalized GPC curves of the individual
precursor polymers as well as of the resulting block copolymer
Figure 4. Overlay of GPC traces showing the modular formation of
PSt-b-PCL from PSt−N3 and PCL−alkyne.
were shown. In the elution curve of block copolymer, a clear
shift toward lower retention times was observed, as well as the
disappearance of the chromatographic peak corresponding to
precursor polymers. These observations confirm that the
described approach is also useful for block copolymer
formation.
In summary, a mild and highly versatile visible light-induced
CuAAC click reaction via photoinduced electron transfer using
free radical photoinitiators has been developed. The
applications of this method in the construction of various
macromolecular architectures such as the telechelic polymers
and block copolymer were also described. The use of visible
light to induce the click reaction makes this protocol easy to
implement in the biological and material sciences.
■
ASSOCIATED CONTENT
* Supporting Information
S
1
H NMR and FT-IR spectra of various products. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Fax: +90−212−2856386. Telephone: +90−212−2853241.
■
ACKNOWLEDGMENTS
The authors thank Istanbul Technical University Research
Fund.
■
REFERENCES
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