Continuous process for click reactions using glass micro-reactor functionalized with β-cyclodextrin

Article abstract of DOI:10.1016/j.tetlet.2013.04.042

A micro-reactor based on glass micro-chip functionalized with β-cyclodextrin was prepared and used to develop a flow process for click reactions of aromatic azides and acetylenes. Products with 97-99% yield were obtained within a few minutes. Modified surface remained stable and almost no degradation was observed after 50 h of continuous use.

Full text of DOI:10.1016/j.tetlet.2013.04.042

Tetrahedron Letters xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Continuous process for click reactions using glass micro-reactor
functionalized with b-cyclodextrin
Muhammad Nazir Tahir a, Riaz-ul Qamar b, Ahmad Adnan c, Eunae Cho a, Seunho Jung a,
⇑
a Department of Bioscience and Biotechnology, Bio/Molecular Informatics Center & Institute for Ubiquitous Information Technology and Application (CBRU), Konkuk University,
Seoul 143-701, Republic of Korea
b University of Würzburg, Department of Organic Chemistry, Am Hubland, 97074 Würzburg, Germany
c Department of Chemistry, GC University, Lahore 54000, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history:
A micro-reactor based on glass micro-chip functionalized with b-cyclodextrin was prepared and used to
develop a flow process for click reactions of aromatic azides and acetylenes. Products with 97–99% yield
were obtained within a few minutes. Modified surface remained stable and almost no degradation was
observed after 50 h of continuous use.
Received 2 March 2013
Revised 6 April 2013
Accepted 11 April 2013
Available online xxxx
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Glass micro-chip
Click reaction
Continuous process
b-Cyclodextrin
Introduction
b-Cyclodextrin (b-CD) is a cyclic oligosaccharide composed of
seven a-1,4-linked glucopyranosyl units. In its hydrophobic cavity
The trend to apply surface-modified materials is increasing rap-
idly in modern science and technology. Applications of such mod-
ified surfaces for example, in catalytic process have the potential of
several advantages over traditional chemical methods for example,
low amount of reagents needed, less or no waste production along
with financial savings.1 Different materials for example, silicon,
glass, quartz, and organic polymers are used for manufacturing
such functional surfaces. Among these materials, glass is more
attractive because it is optically transparent at wavelengths
>320 nm and has a low intrinsic fluorescence. Moreover, it is read-
ily available at low cost, has high mechanical stability, and easy
surface modification techniques are known for this material.2,3
The click reaction of azides and acetylenes has gained a high
popularity since its discovery in 2001.4 The resulting 1,2,3-tria-
zoles are interesting heterocyclic compounds and have found
numerous applications in organic, pharmaceutical, and biological
chemistry. Traditionally, click reactions are conducted in batch
process but some examples of micro-level flow process have also
been reported in last few years.5 Flow process has many advanta-
ges over batch process for example, large surface area-to-volume
ratios, small dimensions leading to good mixing, easy heat transfer,
easy to scale up etc.
it can include various molecules or parts of molecules of appropri-
ate size and chemistry by reversible formation of a host–guest
complex. While there are many reports on the application of natu-
ral6,7 or modified8,9 b-CD (powder) as a catalyst for enhancing the
efficiency10,11 in different chemical reactions applications of b-CD
as a catalyst is not common in click reactions. Shin et al.11 and
Kaboudin et al.10 used b-CD and Cu2–b-CD in click reactions and
reaction time was decreased significantly. These results encour-
aged us to prepare a micro-reactor based on glass micro-chip
(GMC) with its micro channels functionalized with b-CD. This mod-
ified GMC was used to develop a flow process for copper catalyzed
azide–alkyne cycloaddition (CuAAC) click reactions of aromatic
azides with aromatic or aliphatic acetylenes. To the best of our
knowledge, it is the first report on a flow process for click reactions
at room temperature using GMC.
Results and discussion
Functionalization of glass micro-chip with b-CD
GMC surface functionalization was started by formation of ter-
minated epoxide mono-layer (Scheme 1) and characterized by ele-
mental analysis and X-ray photoelectron spectroscopy (XPS). The
narrow-scan XPS spectrum (Fig. 1a) of C1s region from epoxide ter-
minated surface displayed a signal at 285.0 electron volt (eV) cor-
responding to C–C and C–Si bonds and a signal at 286.15 eV
⇑
Corresponding author. Tel.: +82 2 450 3520; fax: +82 2 452 3611.
E-mail address: shjung@konkuk.ac.kr (S. Jung).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.04.042
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M. N. Tahir et al. / Tetrahedron Letters xxx (2013) xxx–xxx
Scheme 1. Surface modification of glass micro-chip.
corresponding to C–O bonds.12 The measured (C–C + C–Si)/(C–O)
ratio of 0.49 is in agreement with the theoretical stoichiometric
values of 0.50 indicating successful modification of glass surface.
Changes in elemental composition (Table 1) also support XPS re-
sults. The thickness of the formed epoxide layer, calculated using
Eq. 1, was 0.91 nm and is slightly higher than the length of 3-glyci-
doxypropyl) trimethoxysilane (GOPTMS) molecule (0.88 nm, as
calculated with Chem3D), confirming that the formed layer was
still a monolayer:
linkages while another signal at 286.51 eV corresponds to over-
lapped signal for C–O and C–N linkages. Theoretical value of (C–
C + C–Si)/(C–O + C–N) is 1.38 (calculated according to the structure
shown in 4) and is in good agreement with experimental value of
1.56 corresponding to 89% conversion of amine (3) into alkynyl ter-
minated monolayer (4). Appearance of additional signals in N1s re-
gion at 402.70 eV along with usual nitrogen signal at 400 eV
(Fig. 1d) indicate that terminal amine was converted into quater-
nary ammonium cation.14 Relative ratio of two N1s signals was
0.92 (theoretical value 1.0) and shows that 92% of the terminal
amine (–NH2) were converted into quaternary ammonium cations
ðBNþꢀÞ. All these results confirm successful reaction of pentynyl
chloride with amine terminated monolayer.
At the end of this reaction, acetylene terminated surface (4) was
converted into b-CD (5) and phenyl (6) terminated surface by click
reactions. C1s narrow-scan XPS spectrum of b-CD functionalized
GMP (5, Fig. 1e) showed two signals corresponding to (C–C + C–
Si) at 285 eV and (C–O + C–N) at 286.63 eV. Ratio of (C–O + C–N)/
(C–C + C–Si) (theoretical 7.13, experimental 4.53) indicate that
63.55% alkynyl groups were clicked with b-CD-N3. Relatively low
conversion was probably due to steric hindrance of bigger sized
b-CD-N3. Moreover, Increase in C content from 43.23% to 52.63%
and N from 2.24% to 5.53% along with decrease in Si content from
29.56% to 20.37% after click reaction of alkynyl terminated mono
layer (4) with b-CD-N3 (Table 1) also indicates the successful
immobilization of b-CD on GMC.
ꢀ
ꢁ
ꢀd
Isi ¼ I1si exp
ð1Þ
ksi;ccosh
Variables, Isi (the absolute silicon peak intensity), I1si (the absolute
silicon peak intensity of unmodified glass), d (the thickness of the
adsorbed layer), ksi;c (the attenuation length of Si2p electrons in
the hydrocarbon layer), and h (the electron takeoff angle), were cal-
culated from XPS data as described in.13
Subsequently, the epoxide coated GMC was converted into
amine terminated surface by reacting with ethylenediamine. In
XPS spectrum (Fig. 1b), three signals corresponding to C–C + C–Si
(285.0 eV), C–O (285.68 eV), and C–N (286.87 eV) appeared. Excel-
lent agreement between measured value (0.32) for (C–C + C–Si)/
(C–O + C-N) and theoretical (0.33) indicates a successful reaction
corresponding to 95% conversion of the epoxides (2) to amine
groups (3) on the surface. In addition, appearance of a signal of or-
ganic nitrogen at 400 eV from the resultant monolayer (not shown
here, see electronic Supplementary data) and elemental composi-
tion (Table 1) also indicate that reaction was very successful.
Terminal alkyne groups were attached by reaction of amine ter-
minated surface (3) with pentynyl chloride. In C1s narrow-scan XPS
spectrum (Fig. 1c), the signal at 285 eV corresponds to C–C + C–Si
C1s XPS narrow-scan spectrum of phenyl terminated GMC sur-
face (6) also shows two signals (Fig. 1f). Signals at 285 eV and
286.60 eV correspond to C–C + C–Si and C–O + C–N linkages
respectively. Theoretical (1.44) and experimental (1.06) values of
(C–O + C–N)/(C–C + C–Si) show that 78.26% terminal acetylene
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M. N. Tahir et al. / Tetrahedron Letters xxx (2013) xxx–xxx
3
Figure 1. XPS spectra for different steps of glass micro-chip functionalization: narrow-scan C1s spectrum of epoxide (a, 2) amine (b, 3) and alkynyl terminated monolayer (C,
4), N1s narrow-scan XPS spectrum of alkynyl terminated layer (d, 4), C1s narrow-scan spectrum after immobilization of b-CD on glass micro-chip (e, 5) and C1s narrow-scan
XPS spectrum of phenyl terminated mono-layer on glass micro-chip (f, 6).
Table 1
Elemental composition and relative number of different bond linkages at different stages of glass micro-chip functionalization with b-cyclodextrin and phenyl terminated mono-
layer and after 50 h use in a continuous process for click reaction of benzyl azide and phenyl acetylene
Substrate
Elemental comp. (atomic %)
C–C + C–Si bonds
Ther. Exp.
C–O + C–N bonds
Ther. Exp.
Si
O
C
N
Unmodified glass micro-chip (1)
With epoxide terminated layer (2)
With amine terminated layer (3)
With acetylene terminated layer (4)
With b-CD terminated layer (5)
With phenyl terminated layer (6)
5, After 50 h use
41.19
31.47
28.61
29.56
20.37
17.06
20.46
58.81
31.41
29.56
24.98
21.47
27.20
21.51
37.12
39.04
43.22
52.63
51.33
52.52
0.00
2.79
2.24
5.53
4.41
5.51
2
2
14
8
26
8
1.97
4
6
9
57
18
57
4.03
6.07
9.64
53.24
21.40
52.06
1.93
13.36
11.76
22.60
12.94
groups (4) were converted into phenyl groups (6). Changes in ele-
mental composition (Table 1) also support these results.
jected into the reactor from one inlet while CuSO4ꢁ5H2O and so-
dium ascorbate (in water) from the other inlet. Product (7) was
collected at the receiving end, extracted with ethyl acetate, and
analyzed by NMR spectroscopy. Conditions for concentration of re-
agents and injection rate were optimized using different concen-
trations (0.1–1.5 M) of benzyl azide and phenyl acetylene (data
not shown here) and found that 1 M solution of benzyl azide and
Application of modified GMC as a micro-reactor for click
reactions
A continuous process for click reaction of benzyl azide and phe-
nyl acetylene was established using micro-reactor based on GMC
(Table 2). Benzyl azide and phenyl acetylene (in t-BuOH) were in-
phenyl acetylene injected at the rate of 5
SO4ꢁ5H2O (5 mol % per acetylene group) and 0.2 M sodium
l
L minꢀ1 with 0.05 M Cu-
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M. N. Tahir et al. / Tetrahedron Letters xxx (2013) xxx–xxx
Table 2
zyl azide and phenyl acetylene in water. A downfield variation in b-
CD protons was observed. This deviation was more prominent for
Application of unmodified and coated with epoxide, amine, acetylene, b-CD, and
phenyl terminated monolayer glass chip as a control in click reaction of benzyl azide
and phenyl acetylene
H-5 proton of b-CD (
D
d = ꢀ0.099 with benzyl azide and ꢀ0.107
with phenyl acetylene), which is located inside the cavity at the
narrow side of b-CD. These observations indicate that b-CD acts
as a phase transfer catalyst and speeds up the reaction by making
a host–guest complex with the aromatic ring of azide or acetylene
compounds.
To prove that high yield (99%) in click reaction of benzyl azide
and phenyl acetylene in few minutes was obtained due to catalytic
effect of b-CD immobilized on GMC and not due to any other func-
tional group or due to length of micro-channel itself, un-modified
GMC (1) and GMC functionalized with epoxide (2), amine (3), acet-
ylene (4), and phenyl (6, having triazole but not CD) terminal
mono-layer was also used for click reaction of benzyl azide and
phenyl acetylene. Results (Table 2) show that there was some back
ground reaction (22–24%) in case of un-modified GMC or in case of
epoxide (2) and amine (3) terminated layer. This back ground reac-
tion is not surprising because all reagents were supplied in soluble
form (water/t-BuOH). Little bit higher yield (36% and 31%) in case
of GMC functionalized with acetylene (4) and phenyl (5) termi-
nated layer was probably due to Cu ions bounded by the triazole
and terminal acetylene units thus providing catalytically active
coatings.
Surface
Yield (%)
Unmodified GMC (1)
24
22
24
36
99
31
With epoxide terminated layer (2)
With amine terminated layer (3)
With acetylene terminated layer (4)
With b-CD terminated layer (5)
With phenyl terminated layer (6)
ascorbate (20 mol % per acetylene group) gave product with 98–
100% yield.
The role of b-CD as a phase transfer catalyst in click reactions is
already known.10,11 To prove that b-CD cavity forms inclusion com-
plex with aromatic azides or acetylenes that enhances the click
reactions, Shin et al.11 recorded 1H NMR of b-CD mixture with ben-
To confirm that b-CD was acting as a phase transfer catalyst by
making a host–guest complex with the aromatic ring of azide/acet-
ylene compounds, a click reaction using non-aromatic compounds
Table 3
Click reactions of azides and acetylenes conducted in a continuous process using micro reactor based on glass micro-chip functionalized with b-cyclodextrin (5)
Entry
1
Azide
Alkyne
Product
Yield (%)
99
2
3
37
99
4
99
5
6
98
99
7
99
8
9
98
97
10
11
98
97
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M. N. Tahir et al. / Tetrahedron Letters xxx (2013) xxx–xxx
5
azole (>98%) and ethylenediamine (>98%) were from Tokyo Chem-
ical Industry, Sodium azide was from Junsei Chemical Co., Ltd.
Korea while glass micro chip (15 ꢂ 45 mm, channel length
332 mm, internal vol, 6 lL) and chip holders (Fluidic Connect
4515) were purchased from Micronit, The Netherlands.
Water was purified using Direct-Q Millipore water purification
system from SAM WOO S&T Co., Ltd. Korea. Solvents used for col-
umn chromatography were distilled while other chemicals were
used without further purification. NMR spectra were recorded on
a Bruker AMX spectrometer at 500 MHz. Thin layer chromatogra-
phy (TLC) was carried out on Merck Kieselgel 60 F254 plates. XPS
spectra were recorded by using a Sigma Probe (ThermoVG, UK)
photoelectron spectrometer. High-resolution spectra were ob-
tained using monochromatic Al-K X-ray radiation at 15 kV and
a
100 W and an analyzer pass energy of 50 eV (1.0 eV step size) for
wide-scan and 20 eV (0.1 eV step size) for narrow-scan. All high-
resolution spectra were corrected with a linear background before
fitting.
Figure 2. Narrow-scan C1s XPS spectrum of b-CD functionalized glass micro-chip
(5) after 50 h use in a flow process for click reaction of benzyl azide and phenyl
acetylene.
Mono-6-(p-toluenesulfonyl)-6-deoxy-cyclodextrin (b-CD-OTs)
was synthesized from b-CD as described in17 which in next step
was converted into mono-6-azido-b-cyclodextrin (b-CD-N3)
according to the procedure described in.18 1H NMR data of prod-
ucts was in accordance with17,18 (detailed procedure is given in
electronic Supplementary data).
was also conducted. For this purpose, 1-octyl azide was reacted
with propargyl alcohol under the same conditions used for benzyl
azide and phenyl acetylene click reaction but only 37% yield (8)
was obtained. These results indicate that the higher yield in short
time in case of click reaction of benzyl azide and phenyl acetylene
was obtained mainly due to host–guest inclusion complexation of
b-CD and aromatic compounds and not due to any possible com-
plexation between b-CD and Cu(1).10 This assumption is also sup-
ported from the results of Shin et al.11 who did not observe any
shift in b-CD protons after mixing it with Cu(1) in water (D2O)
while complexation of metals with terminal acetylenes is already
known.15,16
To demonstrate the scope and generality of the reaction, differ-
ent aromatic azides were reacted with a variety of aromatic and
aliphatic acetylenes using the same micro-reactor in a continuous
flow process (Table 3). All reactions show excellent yields (95–99%)
indicating that the methodology can be applied for a range of
azides and acetylenes.
Modification of glass surface with b-CD
To modify inner micro-channels, GMC (1) was washed and
etched as described in.19 Immediately after cleaning, GMC was
transferred into degassed (3-glycidoxypropyl)trimethoxysilane in
a glass reaction cell. To remove trace amounts of oxygen that might
enter during sample transfer, the reaction cell was filled with argon
followed by vacuum and this cycle was repeated 3 times. Finally,
the glass cell was refilled with argon and heated in an oil bath
for 16 h at 130 °C (Scheme 1). The samples were removed from
the glass cell and sonicated in acetone for 5 min, rinsed several
times with acetone and n-hexane and dried in a stream of argon.
Subsequently, the epoxide coated GMC (2) was dipped in the neat
ethylenediamine as described in.14 Reaction was carried out at
40 °C for 24 h followed by washing and drying steps. To attach ter-
minal alkyne group, GMC functionalized with amine terminated
monolayer (3) was dipped in a mixture of 5-chloro-1-pentyne
(5 mL) and triethylamine (0.5 mL) in dry dichloromethane (5 mL)
at room temperature for 24 h.
In the last step of this reaction, mono-6-azido-b-cyclodextrin
(b-CD-N3) was prepared according to the procedure described
in17,18 and clicked with acetylene terminated mono layer on GMC
(4). For this purpose, b-CD-N3 (500 mg, 0.43 mmol) was dissolved
in DMSO/H2O (8:1, 10 mL) followed by addition of N,N-diisopro-
pylethylamine (167 mg, 3.35 g, 1.3 mmol, 3 equiv/azide group)
and CuI (20.5 mg, 0.1 mmol, 0.25 equiv/azide group). GMC func-
tionalized with terminal alkynyl group (4) was incubated in the
reaction mixture at 40 °C for 24 h under N2 followed by cleaning
steps.
Stability of modified glass surface
Stability of the modified surface is very important in flow pro-
cess at industrial scale. To follow the changes that occurred on
glass surface during continuous process; XPS spectrum of the mod-
ified surface was recorded after 50 h (Fig. 2, Table 1). There was al-
most no change in elemental composition or relative proportions
of C–O or C–N/C–C bond linkages. These results indicate that mod-
ified surface remained stable and can be used for a longer time
without losing efficiency.
Conclusions
GMC functionalized with b-CD can be used to develop a flow
process for CuAAC click reactions. GMC surface remained stable
and can be used for several cycles without any degradation or loss
in efficiency. The process can be scaled up to pilot plant level using
multiple GMC in parallel.
To prepare the modified surface having the triazole ring (6) but
not b-CD, benzyl azide was reacted with acetylene terminated sur-
face (4) following the procedure described for click reaction of b-
CD-N3.
Experimental
Acknowledgments
Materials and methods
Financial support of Konkuk University (KU Brain Pool) for MNT
is gratefully acknowledged. This work was supported by Priority
Research Centers Program through National Research Foundation
of Korea Grant funded by the Korean Government Ministry of Edu-
cation, Science and Technology (NRF 2012-0006686) and was
b-CD (P97%), N,N-diisopropylethylamine (99.5%), (3-glycidoxy-
propyl) trimethoxysilane (98%), 5-chloro-1-pentyne (98%), di-
methyl sulfoxide (DMSO, 99.9%), and copper(1) iodide (99.5%)
were purchased from Sigma–Aldrich. 1-(p-toluenesulfonyl)-imid-
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6
M. N. Tahir et al. / Tetrahedron Letters xxx (2013) xxx–xxx
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partly supported by the National Research Foundation of Korea
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Supplementary data
Supplementary data (Synthesis of mono-6-(p-toluenesulfonyl)-
6-deoxy-cyclodextrin (b-CD-OTs), mono-6-azido-b-cyclodextrin
(b-CD-N3), NMR data for compounds 7–17 and XPS spectra) associ-
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