A Fluorescence-Based High-Throughput Screening Method for Olefin Metathesis Using a Ratiometric Fluorescent Probe

Article abstract of DOI:10.1021/acs.orglett.9b04462

(Z)-1,8-Di(pyren-1-yl)oct-4-ene (1) was prepared as a probe for olefin metathesis. The conversions of substrate by olefin metathesis under various conditions were calculated using the ratiometric fluorescence intensity change of 1. The conversions calcula

Full text of DOI:10.1021/acs.orglett.9b04462

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
A Fluorescence-Based High-Throughput Screening Method for
Olefin Metathesis Using a Ratiometric Fluorescent Probe
Hyeongju Noh,† Taeho Lim,† Byoung Yong Park, and Min Su Han*
Department of Chemistry, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea
S
* Supporting Information
ABSTRACT: (Z)-1,8-Di(pyren-1-yl)oct-4-ene (1) was pre-
pared as a probe for olefin metathesis. The conversions of
substrate by olefin metathesis under various conditions were
calculated using the ratiometric fluorescence intensity change
of 1. The conversions calculated by 1 and gas chromatography
were consistent. These results show that conversions of olefin
metathesis can be simply obtained from the fluorescence
change of 1 and this method can be applied to the high-
throughput screening (HTS) method for various olefin
metathesis.
ver the past decades, olefin metathesis, rearranging two
carbon−carbon double bonds,1 has emerged as one of
methods using colorimetric11 and fluorometric assays12 have
been developed. The instruments required for these assays are
relatively inexpensive, with short analysis times. Fluorescence-
based HTS methods have attracted considerable attention
because of their high sensitivity and easy sample preparation.
Despite its importance of olefin metathesis, only one
fluorescence-based HTS method for olefin metathesis has
been developed thus far. In 2015, Reuter et al. reported a
profluorescent substrate based HTS method13 that can be used
to measure the catalytic efficiency of only ring-closing
metathesis (RCM), one class of olefin metathesis, because
the fluorescence of the profluoroscent substrate was induced
through RCM. Therefore, this method has a critical limitation
because it cannot be applied to other olefin metatheses or
other substrates. Previously, our group developed some of
HTS methods for catalytic organic reactions using an optical
chemosensor system to screen various reaction conditions
including additives, substrates, solvents, and temperature.14
The activity of the catalyst was easily analyzed based on optical
changes, and the values obtained correlated well with those
obtained by GC analysis. Therefore, we thought that these
strategies would be a solution for the limitations found in the
previous HTS method.
O
the most attractive and powerful tools in various synthetic
fields, involving polymers, natural products, pharmaceuticals,
and organic synthesis.2 However, olefin metathesis catalysts
developed earlier had various limitations including sensitivity
to air and moisture.3 To overcome these limitations, a number
of catalysts have been developed using complexes of ligands
and transition metals.4 Among them, commercially available
ruthenium-based catalysts like Grubbs catalyst I (G-I), Grubbs
catalyst II (G-II), Grubbs catalyst III (G-III), Hoveyda−
Grubbs catalyst I (HG-I), and Hoveyda−Grubbs catalyst II
(HG-II) are representative olefin metathesis catalysts.
Although these catalysts exhibit outstanding catalytic reactivity
and stability, there are still demands for the development of
eco-friendly, cost-effective, and air- and moisture-stable olefin
metathesis catalysts. Therefore, there have been several
attempts to develop more efficient olefin metathesis catalysts
and reaction conditions.5
Development of an efficient catalytic system consists of
numerous elements, including ligands, metals, temperature,
solvents, and additives. These numerous considerations require
many development processes and lead to a waste of human and
material resources. To address these issues, high-throughput
screening (HTS) methods, which were originally used for
enhancing the efficiency of drug discovery in the pharmaceut-
ical industry,6 have been applied to the development of an
efficient catalytic system. These methods facilitated analysis to
optimize the catalytic system. Because of the advantage of the
HTS method, various technologies, such as high-performance
liquid chromatography (HPLC),7 gas chromatography (GC),8
nuclear magnetic resonance (NMR),9 and mass spectrometry
(MS),10 have been used as HTS methods for developing a
catalytic system. However, these protocols have some draw-
backs, primarily the high cost of the instrument and the long
analysis time per sample. To overcome these drawbacks, HTS
In this study, a fluorescence-based HTS method for olefin
metathesis was developed using fluorescent probe (Z)-1,8-
di(pyren-1-yl)oct-4-ene (1), which contained a Z-olefin as the
reaction site and two pyrenes as the fluorophore. Pyrene has a
dual fluorescence emission, depending on the distance between
the two pyrenes, the monomer emission ranging from 380 to
410 nm, and the excimer emission ranging from 450 to 500
nm.15 In addition, pyrene has high chemical stability toward
various reaction conditions because it consists of only carbon
Received: December 12, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04462
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Letter
and hydrogen. The Z-olefin not only is the reaction site but
also can induce pyrene excimer emission by conformation
restriction in 1. If the internal olefin of 1 participates in olefin
metathesis, the pyrenes may be located far away from each
other. Therefore, the fluorescence intensity of the excimer will
decrease as olefin metathesis progresses, and the fluorescence
intensity of the monomer will increase. These changes in
fluorescence intensity can be used for measuring the
conversions of various olefin metathesis reactions (Scheme 1).
1b). Comparison of the two graphs revealed that the
ratiometric fluorescence intensity change of 1 was highly
correlated with the GC conversion of 5 at each time interval.
Based on the correlation between the fluorescence intensity
ratio and the GC conversion of 5 at each time interval, the
standard curve of the conversion of 5 versus log (I378/I462
)
graph was obtained (Figure 1c). The conversion, which was
calculated from this standard curve, was denoted as
fluorescence conversion.
To expand the application of 1 as a chemosensor for the
analysis of other CM reactions, we analyzed CM reactions
between 5 and other internal olefins (cis-4-octene (7) or
methyl oleate (8)) in the presence of 0.1 mol % of 1. The
ratiometric fluorescence intensity change of 1 of these CM
reactions correlated well with their corresponding GC
conversions of 5. The standard curves for other combinations
of CM reaction were in the Supporting Information (Figure S3
for 5 and 7 and S4 for 5 and 8). In addition, to examine the
influence of 1 on olefin metathesis, CM was carried out in the
presence and absence of 1. 5 was used as a terminal olefin, and
three kinds of internal olefin (6, 7, 8) were used as counter
olefins. The conversion of 5 was monitored at differing time
points using GC. There was no difference in conversion of 5 in
the presence or absence of 1. These results show that a small
amount of 1 does not affect olefin metathesis reactions (Figure
S5 in the Supporting Information). Therefore, these results
supported the fact that fluorescent probe 1 can determine the
conversion of olefin substrates in olefin metathesis through
fluorescence changes.
Scheme 1. Schematic Representation of the Fluorescent
Probe 1 Based High-Throughput Screening Method for
Olefin Metathesis
The activities of metathesis catalysts using 5 as the terminal
olefin and three kinds of internal olefins (6, 7, 8) were then
screened. The commercially available metathesis catalysts, G-I,
G-II, G-III, HG-I, and HG-II, were used for these CM
reactions. Once the reactions were completed, the conversion
of 5 was analyzed by fluorescence and GC separately. As
shown in Figure 2, the fluorescence conversion of 5 exhibited
high correlation with the GC conversion of 5.18 These results
indicate the following: (1) The ratiometric fluorescence
intensity change of 1 in CM reactions can effectively be used
to screen the activities of the catalysts. (2) The method could
screen the CM reactions effectively with only 0.1 mol % of 1.
(3) The method provided information on the conversion of
substrates in olefin metathesis. These important findings in our
study suggest a method to overcome the drawbacks of the
profluorescent substrate-based HTS method for olefin meta-
thesis.13
Recently, there have been several attempts to use additives
in olefin metathesis to improve the reactivity of the catalyst or
to suppress the side reaction.19 Therefore, the effects of various
additives on CM reactions using the fluorescence-based HTS
method were evaluated. CM was carried out using 5 and three
kinds of internal olefins (6, 7, 8) as a substrate, G-II as catalyst,
and additives in the presence of 0.1 mol % of 1. Fourteen kinds
of additives were selected, including polymers, organic bases,
organic acids, carbonyls, and Lewis acids. In our experiments,18
the efficiency of G-II decreased in the presence of pyridine or
triethylamine in all substrates. These results were similar to the
previous research that confirm a deactivation effect of amine-
containing additives.19c,f,20 On the other hand, tin(II) chloride
(SnCl2) increased the efficiency of G-II in all substrates. SnCl2
is known to increase the reactivity of catalysts.21 These results
correlated well with previous results. As shown in Figure 3b,
fluorescence conversion correlated well with the GC
To develop the fluorescence-based HTS method for olefin
metathesis using fluorescent probe 1, we studied the
correlation between the analysis based on the fluorescence of
1 and GC analysis in olefin metathesis. The cross-metathesis
(CM) reaction of allylbenzene (5) and cis-1,4-diacetoxy-2-
butene (6) was chosen as a model reaction16 and was carried
out using G-II in the presence of 0.1 mol % of 1 (Figure 1).
Small amounts of the reaction mixtures were collected each
time during a 5 h, and ethyl vinyl ether was added to each
sample to terminate the metathesis. The reaction mixtures
were analyzed by both fluorescence spectrophotometer and
GC. As expected, the excimer fluorescence intensity at 462 nm
decreased and the monomer fluorescence intensity at 378 nm
increased during the reaction (Figure 1a). These fluorescence
changes, excimer to monomer, were converted to the
fluorescence intensity ratio (log (I378/I462)).17 Using these
ratiometric fluorescence intensity changes at different time
intervals, the kinetic profile of fluorescence intensity ratio of
the reaction mixtures versus time was obtained. The initial
slope of the log (I378/I462) versus time graph, which is
considered the reaction rate, increased with increasing amount
of G-II. Similar to Figure 1a, GC conversion of 5 increased
during the reaction and the slope of the conversion of 5 versus
time graph increased with increasing amount of G-II (Figure
B
DOI: 10.1021/acs.orglett.9b04462
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Figure 1. Schematic of the CM reaction. Reaction conditions: 5 (0.3 mmol), 6 (0.6 mmol), G-II (0.5−2 mol %), 1 (0.1 mol %), dodecane (0.3
mmol, internal standard), and toluene (3.0 mL) at room temperature. (a) Kinetic profile (0−300 min) of the ratiometric fluorescence intensity
changes of 1 in the CM reaction at different concentrations of G-II (x = 0.5, 1, 2). Inset: Fluorescence spectra of 1 in the CM reaction (0−300 min,
G-II 0.5 mol %). (b) Kinetic profile (0−300 min) of the conversion of 5 in the CM with 1 using GC at different concentrations of G-II (x = 0.5, 1,
2). (c) Standard curve A: GC conversion of 5 versus log (I378/I462).
Figure 2. (a) Schematic of catalyst screening for cross-metathesis. (b)
Figure 3. (a) Schematic of an additive screening for cross-metathesis.
Correlation graph between the conversion of 5 by 1 and GC.
(b) Correlation graph between the conversion of 5 by 1 and GC.
C
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Figure 4. Reaction conditions: 12 (0.3 mmol), G-II (0.5−2 mol %), 1 (0.1 mol %), and toluene (3.0 mL) at room temperature. (a) Kinetic profile
(0 to 600 min) of the ratiometric fluorescence intensity changes of 1 in the RCM reaction at different concentrations of G-II (x = 0.5, 1, 2). (b)
Kinetic profile (0 to 600 min) of the conversion of 12 in the RCM with 1 using GC at different concentrations of G-II (x = 0.5, 1, 2). (c) Standard
curve D: GC conversion of 12 versus log (I378/I462).
Figure 5. (a) Schematic of catalyst screening for ring-closing metathesis. (b) Correlation graph between the conversion of 12 by 1 and GC. (c)
Schematic of additive screening for ring-closing metathesis. (d) Correlation graph between the conversion of 12 by 1 and GC.
conversion of 5, with errors in the range below 7%. These
results indicate that this fluorescence-based HTS method is an
efficient tool for screening additives of olefin metathesis.
To examine the general application of the fluoresce-based
HTS method, 1 was used in RCM, the other class of olefin
metathesis. Diethyl diallylmalonate (12) was used as a model
substrate,16 and experiments similar to the CM reactions were
carried out. Model RCM reactions were carried out using G-II,
and the standard curve for calculating the fluorescence
conversion of 12 was obtained (Figure 4).
D
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04462.
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Experimental procedures (preparation of 1 and screen-
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S1−S4, and spectral data for products (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: happyhan@gist.ac.kr.
ORCID
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125, 6977. (e) Angelovski, G.; Keranen, M. D.; Linnepe, P.;
Grudzielanek, S.; Eilbracht, P. Adv. Synth. Catal. 2006, 348, 1193.
(f) Guo, H.-M.; Tanaka, F. J. Org. Chem. 2009, 74, 2417. (g) Xia, B.;
Gerard, B.; Solano, D. M.; Wan, J.; Jones, G., II; Porco, J. A., Jr. Org.
Lett. 2011, 13, 1346.
Min Su Han: 0000-0001-9588-6980
Author Contributions
†H.N. and T.L. contributed equally.
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Lee, S. Angew. Chem., Int. Ed. 2011, 50, 4386. (b) Kim, S.; Jung, E.;
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIT) (NRF-2017M1A2A2049102 and NRF-
2018R1A4A1024963).
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