Picomole-Scale Real-Time Photoreaction Screening: Discovery of the Visible-Light-Promoted Dehydrogenation of Tetrahydroquinolines under Ambient Conditions

Article abstract of DOI:10.1002/anie.201603530

The identification of new photocatalytic pathways expands our knowledge of chemical reactivity and enables new environmentally friendly synthetic applications. However, the development of miniaturized screening procedures/platforms to expedite the discovery of photochemical reactions remains challenging. Herein, we describe a picomole-scale, real-time photoreaction screening platform in which a handheld laser source is coupled with nano-electrospray ionization mass spectrometry. By using this method, we discovered an accelerated dehydrogenation pathway for the conversion of tetrahydroquinolines into the corresponding quinolines. This transformation is readily promoted by an off-the-shelf [Ru(bpy)₃]Cl₂?6 H₂O complex in air at ambient temperature in direct sunlight, or with the aid of an energy-saving lamp. Moreover, radical cations and trans-dihydride intermediates captured by the screening platform provided direct evidence for the mechanism of the photoredox reaction.

Full text of DOI:10.1002/anie.201603530

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201603530
German Edition: DOI: 10.1002/ange.201603530
Photoredox Reactions
Picomole-Scale Real-Time Photoreaction Screening: Discovery of the
Visible-Light-Promoted Dehydrogenation of Tetrahydroquinolines
under Ambient Conditions
Suming Chen+, Qiongqiong Wan+, and Abraham K. Badu-Tawiah*
Abstract: The identification of new photocatalytic pathways
expands our knowledge of chemical reactivity and enables new
environmentally friendly synthetic applications. However, the
development of miniaturized screening procedures/platforms
to expedite the discovery of photochemical reactions remains
challenging. Herein, we describe a picomole-scale, real-time
photoreaction screening platform in which a handheld laser
source is coupled with nano-electrospray ionization mass
spectrometry. By using this method, we discovered an accel-
erated dehydrogenation pathway for the conversion of tetra-
hydroquinolines into the corresponding quinolines. This trans-
formation is readily promoted by an off-the-shelf [Ru-
(bpy)3]Cl2·6H2O complex in air at ambient temperature in
direct sunlight, or with the aid of an energy-saving lamp.
Moreover, radical cations and trans-dihydride intermediates
captured by the screening platform provided direct evidence
for the mechanism of the photoredox reaction.
approach has been developed for sub-nanomole-scale photo-
reaction screening.
Catalytic dehydrogenation is one of the most common
reactions for the large-scale manufacturing of commodity
chemicals.[4] In particular, the catalytic dehydrogenation of
tetrahydroquinolines to quinolines has attracted broad inter-
est.[5] Much attention has been focused on the development of
mild reaction conditions and efficient catalytic systems for
high-yielding reactions. However, most reaction systems
involving transition-metal-based catalysts require long reac-
tion times and moderately high temperatures (typically
> 1008C). Recently, a [Ru(phd)3]2+/Co(salophen) cocatalyst
system (phd = 1,10-phenanthroline-5,6-dione) was synthe-
sized efficiently and found to promote the dehydrogenation
of some tetrahydroquinolines to quinolines in relatively short
reaction times (5–6 h) under ambient conditions.[5d] Photo-
catalytic reactions are seldom reported for this transforma-
tion. Mesoporous graphite carbon nitride was recently
developed that was capable of the catalytic dehydrogenation
of 1,2,3,4-tetrahydroquinoline to quinoline upon illumination
with visible light for 4.5 h at 1008C.[5e] The development of
a mild catalytic system involving readily available catalysts for
this dehydrogenation reaction is still desirable and will
facilitate chemical synthesis.
In the present study, we aimed to 1) develop a picomolar
photoreaction screening platform to support rapid photo-
reaction discovery, 2) study the underlying mechanism of
discovered reaction pathways, and 3) use this mechanism to
develop the reaction into a preparative-scale synthetic
method. By coupling a portable laser source with nano-
electrospray ionization (nESI) mass spectrometry (MS), we
were able to establish the first MS-based[6] picomole-scale
real-time photoreaction screening platform. This platform
can be used for the direct and rapid screening of chemical
transformations, and results are available within seconds of
reaction initiation. With this screening platform, we discov-
ered an effective photocatalytic pathway involving the
dehydrogenation of 1,2,3,4-tetrahydroquinolines to the cor-
responding quinolines. Surprisingly, the reaction was cata-
lyzed by the common visible-light-harvesting complex [Ru-
(bpy)3]Cl2 (bpy = 2,2’-bipyridine) under ambient conditions.
The corresponding scaled up dehydrogenation reactions
afforded the desired products in excellent yield in 2–4 h at
ambient temperature, either under irradiation with an energy-
saving lamp or by exposure to sunlight. Both sets of
conditions show significant advantages over current methods.
When combined with tandem MS, this method enabled the
characterization of the structure of the ruthenium complexes.
Furthermore, this real-time MS method[7] enabled the capture
R
ecently, photochemical synthesis driven by safe solar
photons has attracted significant attention owing to a global
drive toward renewable, clean, and sustainable energy
technologies.[1] Therefore, the discovery of new photocatalytic
reactions, which expand synthetic chemistry through the
utilization of solar energy, has become increasingly important.
Screening approaches[2] have become the mainstay of discov-
ery processes to identify new chemical reactions and have
increased the efficiency of reaction development. The use of
microchannel reactors[3] is one of the most common
approaches and provides an effective means of photoreaction
screening. However, such studies usually require at least
milligram (micromole) quantities of substrate per reaction,
which may be a prohibitively large amount in preliminary
reaction screening. Furthermore, the extra analytical proce-
dures needed to evaluate the reaction add further complica-
tions and provide limited insight into the reaction mechanism.
The miniaturization of reactions to enable the use of reagents
in nanomole-scale quantities or less, combined with real-time
product detection that offers access to fleeting intermediates,
is a potential solution to this problem. However, no such
[*] Dr. S. M. Chen,[+] Dr. Q. Q. Wan,[+] Prof. Dr. A. K. Badu-Tawiah
Department of Chemistry and Biochemistry
The Ohio State University
Columbus, OH 43210 (USA)
E-mail: badu-tawiah.1@osu.edu
[+] These authors contributed equally.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603530.
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of radical cations and trans-dihydride intermediates involved
in the photoredox reaction and provided direct evidence for
the reaction mechanism.
[Ru(bpy)3]Cl2·6H2O (5; Figure 1b). In the absence of light,
nESI MS analysis of a mixture of dibenzylamine (1a; MW=
197) and 5 (5 mol%) produced the protonated derivative of
1a at m/z 198 (Figure 1c). However, in the presence of laser
light, a high abundance of ions corresponding to the expected
dehydrogenation product N-benzylidenebenzylamine (m/z
196) was observed within seconds (Figure 1d); this peak
corresponds to a loss of 2 Da as compared to that of reactant
at m/z 198. The product yield was calculated (see Figure S1 in
the Supporting Information for the correction of ionization
efficiencies) as 60% (real time; see Table S1), as compared to
a 6% yield for the control experiment (100 mm reactant and
5 mm catalyst 5 in CH3CN under ambient conditions without
laser irradiation). Interestingly, the photoredox reaction
ceased immediately after the photon irradiation was turned
off. Application of the DC voltage continuously, while the
laser source was turned off, enabled products to be removed
and ensured that the small glass reactor tip was filled with
fresh reagent (see Figure S2). This favorable recovery allowed
different experimental conditions to be continuously moni-
tored in a single capillary. For example, while the DC voltage
was turned off, the reaction mixture could be continuously
exposed to light. Results showed an increased yield of
83% when irradiation was continued for 1 min (see
Figure S3c), as opposed to 60% for real-time anal-
ysis. A further increase in the light-exposure time to
2 min led to a yield of 92% (see Figure S3d). Similar
results were recorded for the dehydrogenation reac-
tion of N-phenylbenzylamine (1b; MW= 183) to
generate N-benzylideneaniline (m/z 182; see Fig-
ure S4). These results indicate that by using the new
MS-based photoreaction screening approach, the
reactivity of reactants can be judged in seconds, in
contrast to traditional photoreaction experiments
that require a reaction time of several hours. The
high efficiency of the current platform may be
associated with confinement effects typical of micro-
reactor systems[8] and the high irradiance of the laser
source.
The photoreaction screening platform is as shown in
Figure 1a (see Scheme S1 in the Supporting Information for
a photograph of the setup). We used a readily available
handheld laser source (laser pointer), which allowed the
generation of high lighting power density and coherent blue
visible light. This light was directed toward miniscule volumes
of reaction mixture contained in the transparent glass
capillary of the nESI emitter. Charged microdroplets con-
taining reactants/products were generated when a direct
current (DC) voltage of 1.0 kV was applied to the solution.
Synchronized application of the laser source and the DC
voltage enabled analysis of the photoreaction in real time.
Furthermore, the MS detection method allowed both the
qualitative (on the basis of the MS/MS fragmentation pattern)
and quantitative assessment (by measuring ion intensity) of
reaction progress.
To validate the utility of this method, we first analyzed the
photocatalyzed dehydrogenation (removal of two hydrogen
atoms) from the linear secondary amines 1a and 1b with
The high photocatalytic activity of off-the-shelf
photocatalyst 5 towards the dehydrogenation of
linear secondary amines was surprising. We further
tested its efficiency on the more difficult transforma-
tion of removing four hydrogen atoms from 1,2,3,4-
tetrahydroquinolines; to the best of our knowledge,
this transformation has not been reported before. We
selected 1,2,3,4-tetrahydroquinoline (3a), 6-methoxy-
1,2,3,4-tetrahydroquinoline (3b), and 8-methyl-
1,2,3,4-tetrahydroquinoline (3c) as model com-
pounds and evaluated their susceptibility towards
the ruthenium-based dehydrogenation reaction. The
catalyst 5 was sensitive and effective during the
photocatalytic conversion of 3 into quinolines 4
(Figure 2). Protonated species derived from 3a
Figure 1. a) Schematic illustration of the real-time photoreaction screening plat-
form. The electrospray voltage is 1.0 kV. b) Photocatalyzed dehydrogenation of
1,2,3,4-tetrahydroquinolines and linear secondary amines. c,d) This approach
was used to monitor the progress of the photocatalytic conversion of 1a
(m/z 198, 100 mm) in the presence of 5 (5 mm) in CH3CN in positive-ion mode
without (c, control) or with laser irradiation (d, real time).
(m/z 134) were observed in the absence of light
(Figure 2a). Only trace amounts of the intermediate
(3,4-dihydroquinoline, m/z 132) and product 4a (m/z
130) were detected. Upon application of the laser, the
intensity of the peaks for the intermediate at m/z 132
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this ruthenium-based photocatalyzed dehydrogenation
reaction.
To investigate whether the dehydrogenation reaction
occurring in the screening platform is unique to the
electrochemical environment of the electrospray process,
we developed a non-electrical gas-driven spray MS ana-
lytical apparatus (see Figure S8). All intermediates, includ-
ing radical cations (m/z 132, 162, 163, 146, and 147), from
the conversion of 3a, 3b, and 3c could also be observed by
using this non-electrical apparatus under laser irradiation
(see Figure S9). This result indicates that the production of
these intermediates, especially radical cations, can be
ascribed to a photocatalytic and not an electrochemical
process. In contrast, no conversion of these reactants was
observed in the nano-electrospray capillary, even at an
increased voltage of 4 kV, with the laser light source turned
off (see Figure S10). Moreover, the photocatalytic dehy-
drogenation of linear secondary amines (see Figure S11)
and 1,2,3,4-tetrahydroquinolines (see Figure S12) pro-
ceeded with ease in an off-line experiment in centrifuge
tubes under laser irradiation. This observation further
demonstrates that these reactions do not rely on the
electrical environment of the nESI MS detection method.
These results collectively suggested the possibility of
extending the screened small-scale reactions to a larger-
scale synthesis.
We began a detailed investigation of the new ruthe-
nium-based photoreaction by performing the reaction in
bulk solution under the ambient loading conditions of
photocatalyst 5; however, the laser light source used in the
screening platform was replaced with a common energy-
saving lamp (23 W) and sunlight. The results of these bulk-
phase reactions are in agreement with those obtained from
the photoreaction screening platform. The linear amines 1a
and 1b were converted into the corresponding imines in high
yields of 81 and 96%, respectively, when irradiated with the
lamp for 1.5 h (Table 1). In the case of 3a, the fully
dehydrogenated quinoline 4a was formed in 86% yield
under sunlight for 2 h at 358C and in 73% yield under lamp
light for 2 h at 258C. Furthermore, 3b was transformed into 6-
methoxyquinoline (4b) in 46% yield when exposed to
Figure 2. Use of the photoreaction screening platform to monitor the
photocatalytic conversion of 1,2,3,4-tetrahydroquinolines 3 (3a at m/z
134, 3b at m/z 164, and 3c at m/z 148) into quinolines 4 (4a at m/z 130,
4b at m/z 160, and 4c at m/z 144) in positive-ion mode. Control: MS
spectra for 3 (100 mm) and 5 (5 mm) in CH3CN without laser irradiation.
Real time: MS spectra recorded after the simultaneous application of DC
spray voltage and laser irradiation. The third and fourth horizontal rows
show mass spectra recorded after continuous irradiation of the reaction
mixture for 1 and 2 min, respectively. I is the relative abundance of
detected ions.
and final product 4a at m/z 130 significantly increased in real
time (Figure 2d), with the corresponding relative ion intensity
(see photoreaction screening method and the calculation in
the Supporting Information); the intensity increased from 7
and 1% (control; see Table S1) to 44 and 17% (real time; see
Table S1), respectively. The relative distribution of intermedi-
ate and product ions had changed after continuous light
exposure for just 1 min (Figure 2g), during which the amount
of intermediate (m/z 132) decreased to 13% (see Table S1),
thus enabling the conversion into 4a (72%; see Table S1).
Laser exposure for 2 min resulted in the depletion of the
intermediate (2%; see Table S1), thus generating quinoline
with 93% relative ion intensity (Figure 2j). Similar reaction
progression and product distributions were observed for the
photocatalyzed dehydrogenation of 3b (Figure 2b,e,h,k) and
3c (Figure 2c,f,i,l). Under the same conditions, however, no
products were observed in the absence of catalyst 5 (see
Figures S5 and S6), thus demonstrating the key role of this
photocatalyst. Furthermore, the correlated increase in rela-
tive ion intensity with irradiation time implies that the light-
triggered catalytic effect in the capillary is the predominant
process, as compared with contributing effects from the
droplet environment.[6b] No evident dehydrogenation prod-
ucts were observed when the rapidly moving charged droplets
were illuminated (see Figure S7). Interestingly, radical cations
formed from the initial interaction of the amine with the
excited Ru complex were detected for reactants 3b and 3c at
m/z 163 (Figure 2e) and m/z 147 (Figure 2 f), respectively.
The capture of the radical cations in our method provides key
information regarding the mechanism (as discussed later) of
Table 1: Scaled-up dehydrogenation of 1 and 3 by the Ru complex.[a]
Compound
Light
Catalyst loading
[mol%]
t
[h]
Yield[b]
[%]
1a
1b
3a
3a
3b
3b
3b
3c
3c
lamp
lamp
sun light
lamp
sun light
lamp
lamp
sun light
lamp
5
5
5
5
1
1
3
5
5
1.5
1.5
2
2
2
2
2
2
4
81 (2b)
96 (2a)
86 (4a)
73 (4a)
46 (4b)
40 (4b)
71 (4b)
53 (4c)
65 (4c)
[a] Reaction conditions: A solution (3 mL) of 1 or 3 (10 mm) and 5 was
exposed to a 23 W energy-saving lamp at 258C or sun light (outdoor
1
temperature: 358C). [b] The yield was determined by H NMR analysis
(400 MHz) of the crude reaction mixture with (1Z,5Z)-cycloocta-1,5-
diene as an internal standard.
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sunlight and in 40% yield under the lamp with a catalyst
loading of only 1%. Among the substrates examined, only
sterically demanding 3c required a longer reaction time of
4 h, after which a 65% yield of 8-methyl-quinoline 4c was
obtained under lamp light. This is due to the inefficient
conversion from the imine intermediate. The production of 4a
under sun light was also monitored by 1H NMR spectroscopy
with the standard reagents 3a and 4a as controls (see
Figure S13). We found that 3a was fully dehydrogenated to
give 4a under ambient conditions (see details of the
characterization of the reaction products in Figures S14–
S22). We believe this finding is significant because the
catalytic approach described herein is a simple and effective
method for obtaining quinolines under visible light with an
off-the-shelf catalyst under ambient conditions. This MS-
based screening approach for structural analysis was also used
to characterize the occurrence of ruthenium-based complexes
(see Figures S23–S28 and related discussion in the Supporting
Information).
H2O2.[10] The presence of H2O2 in the reaction mixture was
detected in a titration reaction with KI (see Figure S34).[11]
Tautomerization of the dihydroquinoline intermediate B into
C (Scheme 1; these intermediates are almost energetically
equal)[12] allows the generation of the final quinoline product
through a second dehydrogenation step.
In conclusion, by coupling a handheld laser source with
nESI MS, we have established a picomole-scale real-time
photoreaction screening platform. The platform exhibits high
efficiency and rapid photoreaction screening capabilities. A
new chemical transformation involving the conversion of
tetrahydroquinolines into quinolines with [Ru(bpy)3]-
Cl2·6H2O was discovered, and the corresponding radical-
cation intermediates were captured on the photoreaction
platform in real time. More importantly, the larger-scale
reactions confirm that the new dehydrogenation pathway can
proceed under ambient conditions with visible-light sources,
such as sunlight. We believe that this platform may find
application in the discovery of other novel photoreactions.
The above experimental evidence and the strong depend-
ence of this photocatalyzed dehydrogenation on oxygen (see
Figure S29–S33) led us to propose an aerobic photoredox
oxidation mechanism (Scheme 1). First, the active [Ru-
(bpy)3]2+ species is converted into the excited-state complex
[Ru*(bpy)3]2+ upon irradiation with visible light. Subse-
Acknowledgements
This research was supported by start-up funds of the Ohio
State University.
Keywords: dehydrogenation · mass spectrometry ·
reaction screening · quinolines · visible light
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4
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Received: April 11, 2016
Revised: May 4, 2016
Published online: && &&, &&&&
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Communications
Photoredox Reactions
The here and now of it: A picomole-scale,
real-time platform based on mass spec-
trometry for the screening of photoredox
reactions enabled the discovery of an
accelerated photocatalytic transforma-
tion of tetrahydroquinolines into quino-
lines under ambient conditions (see
scheme). The screening platform also has
the advantage that key mechanistic
information can be derived from captured
intermediates.
S. M. Chen, Q. Q. Wan,
A. K. Badu-Tawiah*
&&&& — &&&&
Picomole-Scale Real-Time Photoreaction
Screening: Discovery of the Visible-Light-
Promoted Dehydrogenation of
Tetrahydroquinolines under Ambient
Conditions
6
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