Online monitoring of microwave-enhanced reactions by UV/Vis spectroscopy

Article abstract of DOI:10.1002/ejoc.201000441

Microwave-enhanced reactions are very fast in comparison to thermal reactions. They are often finished within a few minutes. Thus, the determination of the end point often fails, because conventional analytical methods are too slow. We developed a fast method using a UV/Vis sensor, which allows a direct online monitoring of the reaction process. The potency of the method is demonstrated by means of five representative reactions.

Full text of DOI:10.1002/ejoc.201000441

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000441
Online Monitoring of Microwave-Enhanced Reactions by UV/Vis Spectroscopy
[a]
[a]
[b]
[a]
Eberhard Heller,* Jessica Klöckner, Werner Lautenschläger, and Ulrike Holzgrabe*
Keywords: Online monitoring / Microwave chemistry / UV/Vis spectroscopy / Sensors
Microwave-enhanced reactions are very fast in comparison
to thermal reactions. They are often finished within a few
minutes. Thus, the determination of the end point often fails,
because conventional analytical methods are too slow. We
developed a fast method using a UV/Vis sensor, which allows
a direct online monitoring of the reaction process. The po-
tency of the method is demonstrated by means of five repre-
sentative reactions.
Introduction
welded together for the incoming and the outgoing beam
supported by a quartz glass cover (see Supporting Infor-
mation).
Microwave-enhanced reactions in organic synthesis are
well established for a large variety of reactions.[
1–3]
The re-
action time can be shortened in comparison to thermal
heating from days or hours to minutes. The problem of such
fast reactions is to determine the optimal end point of the
conversion. Typical analytical methods controlling the pro-
gress of the reaction such as NMR spectroscopy, TLC or
HPLC take often longer than the microwave irradiation
time. The time range can vary from several minutes to half
an hour or even more depending on the treatment of the
sample prior to the measurement. Within this time the reac-
tion progresses and by- and decomposition products may
develop. This decreases the yield of the major product and
increases the efforts to purify the main product.
Results and Discussion
Typically, in order to obtain a valid UV/Vis spectrum,
solutions containing 10–4 mol/L of an analyte are used. In
contrast, chemical reactions are performed in solutions of
higher concentrations (0.1–1.0 mol/L). Classical UV/Vis
measurements of these solutions are incapable of measure-
ment due to the facts that the absorbance is not linear, and
the obtained spectra are complex. However, the spectro-
scopic sensor used here is able to measure the absorbance
in the concentrated solutions of the chemical reaction. Nev-
ertheless, for the monitoring of the ongoing reaction the
UV wavelength has to be carefully chosen. The reactions
can be carried out in an open flask or in a special quartz-
glass vessel, which allows working beyond the boiling point
at atmospheric pressure or under pressure. This is exem-
plified by the following reactions.
In principle, UV/Vis[ and Raman spectroscopy are
suitable to monitor the reaction progress in a microwave
reactor. However, the UV/Vis method cannot be carried out
in the reaction solution itself. A sample needs to be taken
from the solution, diluted and measured. Recently, we de-
veloped such a real-time observation of reactions by fast
FTIR spectroscopy.[ Unfortunately, this method is not ap-
plicable to high-boiling solvents (e.g. DMF, methanesul-
fonic acid), because the solvent has to be evaporated before
the FTIR measurement can take place. Herein, we report a
4]
[5]
6]
Examples 1 and 2
Fluorinated fluorescein-type dyes can be used as reporter
[
7]
method, which allows direct online observation of the pro- molecules in biological systems. They can form stable
gress of the reaction by using a UV/Vis sensor (in the reac- complexes with tetracysteine motifs engineered into the tar-
tion mixture) and computer software. The experimental get proteins of interest. Here, we report a microwave-
setup of the UV/Vis sensor consists of two fiber-optic cables driven syntheses of the fluorogenic dyes 3 and 6 (Scheme 1),
which are carried out in methanesulfonic acid in an open
[
8]
[
7]
flask. The conversion of the corresponding phenols and
anhydrides were monitored between λ = 600 and 700 nm.
The conversion to give the dyes 3 and 6 were completed
after 15 and 30 min, respectively. Longer reactions times led
to decomposition of the products and decrease of the yields.
Whereas conventional approaches yielded 85% (3) and
[
a] Institute of Pharmacy and Food Chemistry, University of
Würzburg,
Am Hubland, 97074 Würzburg, Germany
Fax: +49-931-3185494
E-mail: eheller@pharmazie.uni-wuerzburg.de
u.holzgrabe@pharmazie.uni-wuerzburg.de
b] Mikrowellenlaborsysteme GmbH,
[
Auenweg 37, 88299 Leutkirch/Allgäu, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000441.
7
4% (6), respectively, with reaction times between 36 and
[
7]
48 h, the yields of the microwave-supported syntheses are
Eur. J. Org. Chem. 2010, 3569–3573
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3569

E. Heller, J. Klöckner, W. Lautenschläger, U. Holzgrabe
SHORT COMMUNICATION
Scheme 1. Syntheses of the fluorinated dyes 3 and 6.
higher than 90%. Figure 1 shows the overlaid transmission
Figure 2 shows the monitoring of the synthesis of com-
spectra of the reaction solution for the synthesis of com- pound 6. After 30 min, no change of the spectrum was ob-
pound 3. The signal of the starting compound (red) at served.
656 nm disappeared after 3 min. During the reaction, the
transmission increased (blue – violet – green). The reaction
was finished after 15 min.
Figure 2. Transmission spectra of the reaction solution for the syn-
thesis of 6: 0 min (red), 10 min (blue), 20 min (violet) and 30 min
(
green).
Example 3
In Scheme 2, the preparation of compound 9 is shown.
This substance is an intermediate in the synthesis of thieno-
pyridine being allosteric modulators of the muscarinic re-
Figure 1. Transmission spectra of the reaction solution for the syn-
thesis of 3: 0 min (red), 3 min (blue), 10 min (violet) and 15 min
(green).
Scheme 2. Synthesis of 9.
570
3
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 3569–3573

Microwave-Enhanced Reactions
ceptors.[9] Even though the same starting materials were
used for the thermal-driven reaction (Figure 3), the yields
and the reaction times ranged widely between 0.5 h/
[
10a]
[10b]
[10c]
18%,
6 h/65%
and 8 h/65%
in the literature.
Scheme 3. Synthesis of 12.
before the irradiation. The blue curve gives the situation
after 3 min and the green one after 5 min of irradiation.
There was no significant change of the spectrum after that
time. We isolated the product with an 85% yield. This com-
pound is very sensitive. Longer reaction times decreased the
yield and made the workup procedure more complicated.
Figure 3. Absorption spectra of the reaction solution for the syn-
thesis of 9: 0 min (red), 3 min (blue), 5 min (violet) and 10 min
(green).
For comparison, we carried out the reaction thermally in
d. In the microwave reactor, equipped with a pressure and
3
a UV/Vis sensor, the reaction time was found to be 10 min
only. Thereafter, the spectrum did not show any change at
all.
Example 4
The pyrylium dye 12 obtained by conversion of the alde- Figure 4. Absorption spectra of the reaction solution for the syn-
thesis of 12: 0 min (red), 3 min (blue) and 5 min (green).
hyde 10 with the pyrylium salt 11 is used as a fluorescence
[11]
marker for proteins. In the classical way, 12 was synthe-
sized according to ref.[ in 10 min and 65% yield (see
Scheme 3).
12]
Example 5
In the microwave reactor, the reaction was finished in
Compound 14 (Scheme 4) is used as a fluorescence
5
min (Figure 4). The red spectrum shows the absorbance marker for allosteric modulators of the muscarinic recep-
Scheme 4. Synthesis of 14.
Eur. J. Org. Chem. 2010, 3569–3573
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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E. Heller, J. Klöckner, W. Lautenschläger, U. Holzgrabe
SHORT COMMUNICATION
tors as an alternative to radioactive labeling.[13] The reac- do not change, the microwave irradiation was stopped and the
workup of the reaction mixture started. An MLS Ethos 1600 mi-
tion was carried out in the classical way, but the yields fluc-
crowave system (MLS GmbH, Leutkirch, Germany) was used, and
tuated strongly, and the workup was complicated (Fig-
the reactions were performed in an open flask/reaction vessel (Ex-
amples 1 and 2) and with a special quartz batch reactor (Exam-
ure 5). In the microwave reactor, the reaction was finished
within 8 min with an 82% yield. The by-products could be
ples 3–5).
easily removed by column chromatography.
General Procedure for the Syntheses of Fluorinated Fluoresceins:
20 mmol of the resorcin and 10 mmol of the phthalic anhydride
were dissolved in 50 mL of methanesulfonic acid in a flask
equipped with a temperature and a UV/Vis sensor and heated in
the microwave oven (3 min to 120 °C). The reaction temperature
was kept constant until no significant change of the UV/Vis spec-
trum was observed. The mixture was poured into 400 mL of ice/
water and stirred for 1 h. The precipitate was filtered off, washed
neutral and dried in vacuo. The residue was dissolved in acetic
anhydride (50 mL) and pyridine (4 mL) and heated up to 65 °C
within 2 min. The temperature was kept constant for 5 min, poured
into 400 mL of ice/water and stirred for 30 min. The aqueous layer
was extracted with chloroform (3ϫ150 mL), and the combined or-
2 4
ganic layers were dried with Na SO . After evaporation, the residue
was dissolved in 100 mL of methanol and 100 mL of THF; 25%
ammonia (10 mL) was added, the solution was stirred for 2 h, and
the mixture was poured into water (500 mL). The solution was
acidified with 10% HCl until pH = 2 was reached. The residue was
filtered, washed and dried in vacuo.
Figure 5. Absorption spectra for the reaction solution for the syn-
thesis of 14: 0 min (red), 3 min (blue), 5 min (violet) and 8 min
2,7-Difluorofluoresceine (3): Yield 91%.
(green).
3
,4,5,6-Tetrafluorofluoresceine (6): Yield 93%.
Analytical data were in agreement with those of ref.[7]
-Cyano-2,6-dihydroxy-4-methylpyridine (9): Cyanoacetamide
59 mmol, 5.0 g) was dissolved in 50 mL of methanol. Ethyl acet-
3
(
8
Conclusions
acetate 7 (59 mmol, 7.5 mL) and piperidine (60 mmol, 6.0 mL)
were added, and the mixture was heated (Methods A and B). The
mixture was allowed to cool down to 25 °C, the solid was filtered
off and then dissolved in hot water. Afterwards, the solution was
acidified slowly until pH = 5. The white solid was filtered off again,
washed with ice-cold water and dried in vacuo. Method A: 3 d of
reflux, yield 4.57 g (52%). Method B: Microwave, 3 min to 100 °C,
The examples clearly demonstrate that monitoring of fast
microwave-supported reactions is possible by online mea-
surement of the UV/Vis spectra in solution. However, pre-
treatment of the reaction samples for NMR or HPLC mea-
surements can take up to half an hour in addition of the
measurement itself. TLC does not need a sample prepara-
tion in principle, but the development time is mostly at least 10 min at 100 °C, yield 3.25 g (37%). Analytical data were in agree-
1
0 min. Thus, the UV/Vis method presented here is optimal ment with those of ref.[10]
for the identification of the endpoint of fast microwave-en-
hanced reactions. Overreactions occurring during NMR,
2
,6-Dimethyl-4-[(E)-2-(2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]-
quinolin-9-yl)vinyl]pyranylium Tetrafluoroborate (12): Julolidin-9-
HPLC or TLC monitoring can be avoided. It is also suitable carbaldehyde (10) (30 mmol, 6.04 g), 2,4,6-trimethylpyrylium tetra-
for high-boiling solvents (e.g. DMF, DMSO), where the fluoroborate (33 mmol, 6.93 g) and 1 drop of HBF were dissolved
FTIR method fails, because it needs the complete removal in 60 mL of acetonitrile and heated up in a microwave oven to
4
[
6]
100 °C within 3 min. The temperature was kept constant at 100 °C.
After 5 min, the reaction was finished, and the solvent was re-
moved. The crude product was purified by column chromatography
of the solvent. However, the starting materials and the
products should not absorb near the maximum absorption
of the solvent, and the products and the reactants must
have different UV/Vis spectra in some areas. These require-
ments can be easily fulfilled.
on silica gel. The by-products were eluted with CHCl
25:1) and the main product with CHCl /MeOH (9:1). Yield
0.04 g (85%). Analytical data were in agreement with those of
3
/MeOH
(
3
1
ref.
[12]
(
1
E)-1-(6-Hydroxyhexyl)-2,6-dimethyl-4-[2-(2,3,6,7-tetrahydro-
H,5H-benzo[ij]quinolizidin-9-yl)ethenyl]pyridinium Tetrafluorobo-
Experimental Section
General Procedure for the Online Monitoring: The spectrum analy-
ses were done with a modified UV/Vis spectrometer (Ocean Optics,
Ostfildern, Germany) with a special cell for direct control (made
by MLS GmbH) for quantitative control of the reactions. The spec-
trometer was connected to a PC with the Spectral Suite software.
The UV/Vis spectra were measured continuously with 2 s integra-
tion time and saved either every 2 min or in shorter intervals de-
pending on the length of the reaction time. As soon as the spectra
rate (14): Compound 12 (10 mmol, 3.93 g) and the amino alcohol
13 (10 mmol, 1.17 g) were dissolved in 60 mL of acetonitrile and
heated to 100 °C within 3 min. The temperature was kept constant
at 100 °C, and the reaction was finished in 8 min. The solvent was
evaporated, and the residue was purified by column chromatog-
raphy on silica gel. The by-products were eluted with CHCl
the main product with CHCl /MeOH (25:1). Yield: 4.05 g (82%)
of a dark-red solid, m.p. 147 °C (dec.). R = 0.41 (CHCl /MeOH,
3
and
3
f
3
3572
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Eur. J. Org. Chem. 2010, 3569–3573

Microwave-Enhanced Reactions
1
2
4
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3
CN): δ = 1.41–1.52 (m, 7 H, 3Ј-H,
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), 2.72 (t, J = 6.0 Hz, 4 H,
ЈЈЈ-H, 7ЈЈЈ-H), 3.52 (q, J = 5.0 Hz, 2 H, 6Ј-H), 4.21 (t, J = 8.5 Hz,
H, 2ЈЈЈ-H, 6ЈЈЈ-H), 2.68 (s, 6 H, 2ϫ CH
3
1
2
1
H, 1Ј-H), 6.77 (d, J = 16.0 Hz, 1 H, 2ЈЈ-H), 7.09 (s, 2 H, 8ЈЈЈ-H,
0ЈЈЈ-H), 7.53 (s, 2 H, 3-H, 5-H), 7.54 (d, J = 6.0 Hz, 1 H, 1ЈЈ-H)
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3
3
CN): δ = 21.3 (2ϫ CH ), 22.2 (C-
2
2
1
(
(
ЈЈЈ, C-6ЈЈЈ), 26.0 (C-3Ј), 27.1 (C-4Ј), 28.1 (C-1ЈЈЈ, C-7ЈЈЈ), 29.3 (C-
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C-1ЈЈ), 146.6 (10bЈЈЈ), 154.4 (C-2, C-6) ppm. FTIR (ATR): 3547
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[8]
[9]
This work was partially supported by the Deutsche Forschungs-
gemeinschaft (No. HO1368/12-1) and by Prof. M. J. Lohse of the
Institute of Pharmacology and Toxicology, University of
Würzburg.
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Eur. J. Org. Chem. 2010, 3569–3573
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