Sunflow: Sunlight Drives Fast and Green Photochemical Flow Reactions in Simple Microcapillary Reactors – Application to Photoredox and H-Atom-Transfer Chemistry

Article abstract of DOI:10.1002/ejoc.201601394

“Sunflow“ – The combination of a microcapillary reactor in continuous flow mode with sunlight as the most sustainable energy source imaginable was applied to a range of photoredox and H-atom-transfer reactions making them both fast and green.

Full text of DOI:10.1002/ejoc.201601394

A Journal of
Accepted Article
Title: Sunflow: Sunlight Drives Fast and Green Photochemical Flow
Reactions in Simple Micro Capillary Reactors — Application to
Photoredox- and H-Atom Transfer Chemistry
Authors: Alexander Matthias Nauth, Alexander Lipp, Benjamin Lipp,
and Till Opatz
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601394
Link to VoR: http://dx.doi.org/10.1002/ejoc.201601394
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Sunflow: Sunlight Drives Fast and Green Photochemical Flow
Reactions in Simple Micro Capillary Reactors — Application to
Photoredox- and H-Atom Transfer Chemistry
Alexander M. Nauth,[a] Alexander Lipp,†[a] Benjamin Lipp,†[a] and Till Opatz*[a]
Abstract: “Sunflow” — The combination of a micro capillary reactor
in continuous flow mode with sunlight as the most sustainable energy
source imaginable was applied to a range of photoredox and H-atom
transfer reactions turning them both fast and green.
spectrum which overcomes the hazards of working with mercury
lamps or other UV-light sources, the emissions of which may not
induce a blink reflex.
Since the pioneering works of Giacomo Luigi Ciamician, Max
Dennstedt and Günther Otto Schenk, much progress has been
made in optimizing reaction setups to achieve more efficient
sunlight-driven reactions.[18] Special photo macro reactors like the
SOLFIN,[19] the CPC[14a] or flatbed reactors[20] were designed. The
hearts of the SOLFIN and the CPC are relatively big tubes in
parabolic mirrors which focus the solar light and increase the light
flux in the tubes. These reactors are cyclic flow reactors, in which
the reaction medium is circulated repeatedly between the reaction
tubes and the reservoirs leading to a constant mixing of irradiated
and non-irradiated solution. This is particularly troublesome if the
products of a photoreaction are unstable upon prolonged
exposure to light or to components of the reaction mixture.
Moreover, such macro reactors are not easy to build and are
therefore not suitable for most laboratories. Furthermore, these
setups require relatively long irradiation times. Due to the
day/night cycle, the efficient use of day time is essential when
using sunlight as sole energy source. An efficient and fast photon
absorption by the photoredox catalyst or by the photosensitizer is
crucial for achieving a rapid reactant turnover. According to the
Lambert-Beer law, a higher irradiated surface to volume ratio
correlates with a higher average photon number in the solution
and therefore with a higher turnover with the same light intensity.
This higher ratio can be accomplished by using micro capillary
reactors in a continuous flow mode.[21] The significant acceleration
of photoredox reactions by using micro capillary flow reactors with
artificial light sources has been demonstrated repeatedly.[22] In
2015, Kim et al. reported a sunlight-induced radical bromination
of toluene derivatives in an FEP capillary flow reactor within short
reaction times.[23]
Introduction
Photoredox catalysis has attracted tremendous interest within the
scientific community and is currently one of the most active
research areas in the field of organic chemistry.[1] After the
absorption of a photon, the electronically excited photoredox
catalyst is converted back to its ground state via two successive
redox reactions. By engaging in these electron transfer reactions,
substrates of the reaction mixture can be oxidized or reduced
depending on the conditions. This opens up the possibility to
perform net redox neutral transformations involving unique bond-
forming events in a one-pot fashion using light as a sustainable
energy source.[2] A large variety of photoredox catalysts including
transition metal complexes (based on Ru,[3] Ir[4] or Au[5])[6], organic
[10] [11]
dyes (rose bengal,[7] eosin Y,[8] TPP,[9] DAP2+
)
or
semiconductors (TiO2[12]) have been employed for this purpose.
Nowadays, the aspects of eco-friendliness and environmental
sustainability are becoming increasingly important. The growing
field of green chemistry strives for reactions that combine high
yields with a good atom economy, are devoid of heavy metals or
other toxic agents, use renewable starting materials and are not
energy intensive.[13] Even though many photoreactions fulfill these
requirements, the ultimate approach to green chemistry would be
the use of sunlight as sole energy source for a chemical
transformation. It is therefore not surprising that this idea has
been appealing to chemists for a long time, leading to the
development of a large variety of sunlight driven reactions
cycloadditions,[14]
and
The beneficial influence of a flow setup on the speed of
photo(redox) transformations as well as the potential use of
sunlight as an entirely eco-friendly light source are two well-known
principles of photochemistry. However, the application of a micro
capillary continuous “sunflow” system to photoredox and H-atom
transfer reactions has, to the best of our knowledge, not been
described. This might be due to the widespread belief that
expensive equipment is required to obtain satisfactory results.
including
for
example
[2+2]
photoalkylations[16]
photooxygenations,[15]
dehydratisations.[17] The solar spectrum covers all wavelengths
needed for photoredox catalysis down to about 330 nm.
Furthermore, the human eye is adapted to the solar emission
[a]
Prof. Dr. Till Opatz
Institute of Organic Chemistry, Johannes Gutenberg University
Mainz, 55128 Mainz, Germany; fax: (+49)-6131-39-22338; phone:
(+49)-6131-39-22272; e-mail: opatz@uni-mainz.de
http://www.chemie.uni-mainz.de/OC/AK-Opatz/index.php
Both authors contributed equally to this work.
Results and Discussion
†
Therefore, the aim of this study was to combine the advantages
of a simple and inexpensive micro capillary reactor with the use
Supporting information for this article is given via a link at the end of
the document
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10.1002/ejoc.201601394
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of sunlight to accomplish green and fast photoredox and H-atom
transfer reactions in a continuous flow mode. To exemplify the
utility of this “sunflow” approach, we chose
a range of
photoreactions which have recently been investigated by our
group, including a benzophenone mediated C–C coupling of 2-
chlorobenzazoles with alcohols, ethers and carbamates, an
oxidative cyanation of tertiary amines as well as a phenanthrene
catalyzed Minisci type cross coupling reaction (scheme 1).[7a, 24]
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
Reaction time (min)
Reaction
time (min)
0
1
2
5
10
20
30
59%
60
64%
Yield[a]
0%
22%
31%
44%
57%
61%
Figure 1. Kinetic investigation of the benzophenone mediated C–C coupling of
tetrahydrofuran with 2-chlorobenzoxazole in the “sunflow” reactor. For each
data point, 5 mL of the stock solution (described in the SI) were injected into the
“sunflow” reactor and withdrawn after the indicated time period. [a] Yields were
calculated by 1H-NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene as
internal standard.
In order to demonstrate the applicability of these “sunflow”
conditions for various substrates such as alcohols, ethers and
carbamates, the reaction protocol was applied to ethanol, diethyl
ether as well as Boc-azepane (table 1). In all cases, the use of the
micro capillary flow reactor led to a significant acceleration of the
reaction as judged by the direct comparison with the analogous
batch reactions. Moderate to good yields could be obtained which
were in most cases close to those reported in the literature under
batch conditions (UV-A).[24a] Individual kinetic studies and
optimization of each of these systems might have allowed a
further increase of the yields.
Scheme 1. Overview of reactions used to exemplify the utility of the presented
“sunflow” approach.
As a first example, the benzophenone mediated C–C coupling
protocol developed recently by our group was investigated (A,
scheme 1) which is an example of a UV-A-driven photoreaction.
The combination of tetrahydrofuran and 2-chlorobenzoxazole was
chosen as the benchmark system.[24a] Performing the reaction
under typical batch conditions yielded 21% of the coupled product
after three hours of exposure to sunlight (60 mL stock solution in
a 100 mL round bottom flask). In order to confirm the anticipated
acceleration of the reaction by increasing the surface-to-volume-
ratio, the same reaction was performed in an NMR tube, resulting
in an almost doubled yield (38.8%) in the same time period. In
order to further accelerate the reaction, a capillary reactor was
constructed for harvesting sunlight. This reactor was built out of
an aviary fence, a wooden plank and a 25 m FEP tube (OD:
1.59 mm / ID: 1.0 mm) with fittings resulting in an acquisition value
below 80 € / 90 US-$ (see SI). The first kinetic studies showed a
remarkably fast reaction as highlighted in figure 1. After only
20 minutes, high yields (61%) comparable to those achieved by
irradiation with an UV-A spotlight for 24 h could be obtained.[24a]
Since visible light represents a large part of the solar emission,
we next focused on catalyst systems with absorption maxima in
this wavelength region. The oxidative α-cyanation of tertiary
amines to the corresponding α-aminonitriles was chosen as
model reaction (B, scheme 1). A kinetic study using several
organic (rose bengal and eosin Y) and transition metal
([Ru(bpy)3]2+) based catalysts in combination with oxygen as
terminal oxidant was performed (figure 2). Furthermore, a
catalyst-free approach using bromotrichloromethane as reported
[25]
by the group of Kirsten Zeitler was investigated (figure 3).
Additional figures and tables for turnovers as well as yield to
turnover ratios are provided in the SI. In all cases, high to
excellent yields could be obtained after very short reaction times.
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Table 1. Comparison of the investigated reaction type A under “sunflow” and
batch conditions with sunlight.
batch with
sunlight
Literature
‘‘Sunflow“
(UV-lamp)24a
Compound
100
90
80
70
60
50
40
30
20
10
0
Yield[a] Time Yield[b] Time
Yield
Time
21%[c]
/
1440
min
68%
59%
54%
38%
20 min
180 min 70%
39%[d]
1440
min
30 min 24%[c] 30 min
30 min 8%[c] 30 min
30 min 15%[c] 30 min
68%
59%
74%
1440
min
2160
min
0
10
20
30
40
50
60
Reaction time (min)
For each substrate, the stock solution (described in the SI) was either pumped
through the “sunflow” reactor or placed in a round bottom flask in the sun. [a]
Isolated yield, [b] yields were calculated based on 1H-NMR spectroscopy using
1,4-bis(trimethylsilyl)benzene as internal standard, [c] 60 mL stock solution in a
100 mL flask, [d] 1.1 mL stock solution in a NMR tube.
Reaction
time (min)
0
1
2
5
10
20
30
12%
60
6%
Yield[a]
0%
31%
49%
70%
100% 40%
Figure 3. Kinetic investigation of the photocyanation with BrCCl3 in the “sunflow”
reactor. For each data point, 5 mL of the stock solution (described in the SI)
were injected into the reactor and withdrawn after a specific time period. [a]
Yields were calculated based on 1H-NMR spectroscopy using 1,4-
bis(trimethylsilyl)benzene as internal standard.
To further demonstrate the utility of this “sunflow” approach for the
α-cyanation of aliphatic as well as benzylic substrates, a series of
amines, comprising tri-n-butylamine, atropine as well as 6,7-
100
90
80
70
60
50
40
30
20
10
0
dimethoxy-2-methyl-1,2,3,4-tetrahydroisoquinoline
was
investigated (figure 4). The decrease of the yield for elevated
reaction times can be explained by decomposition of the formed
α-aminonitriles.
100
90
80
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
16
18
20
Reaction time (min)
[Ru(bpy)3]
Rose Bengal
Eosin Y
Reaction time
(min)
0
2
3
5
7
10
20
Yield[a]
0%
0%
0%
34%
77%
27%
43%
87%
34%
57%
97%
44%
70%
93%
53%
74%
89%
65%
85%
84%
70%
[Ru(bpy)3]2+
0
5
10
15
20
Yield[a]
Rose Bengal
Reaction time (min)
6,7-Dimethoxy-2-methyl-1,2,3,4-tetrahydroisoquinoline
Yield[a]
Eosin Y
Tributylamine
Atropine
Figure 2. Kinetic investigation of the photocyanation with [Ru(bpy)3]Cl2 (blue),
rose bengal (orange) and eosin Y (gray) in the “sunflow” reactor. For each data
point, 20 mL of the stock solution (described in the SI) and oxygen were pumped
through the reactor in the stated residence time. [a] Yields were calculated by
1H-NMR spectroscopy using pyridine as internal standard.
Figure 4. Kinetic investigation of the rose bengal catalyzed photocyanation of
three exemplary amines in the “sunflow” reactor. For each substrate, the stock
solution (described in the SI) and oxygen were pumped through the “sunflow”
reactor in the stated residence time. Yields were calculated by 1H-NMR
spectroscopy using pyridine as internal standard. For the data points marked in
red, isolated yields were determined as well. Tabulated data is provided in the
SI.
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Table 2 shows a comparison of the isolated yields for various α-
aminonitriles prepared under “sunflow” conditions with the ones
reported in the literature.[7a] In all cases, the green “sunflow”
protocol afforded higher yields within shorter reaction times
compared to batch photoreactions using a household CFL bulb or
a blue LED.
100
90
80
70
60
50
40
30
20
10
0
Table 2. Comparison of investigated reaction type B under “sunflow” and batch
[26]
conditions.
“Sunflow”
Literature
Compound
Yield[a]
Time
Yield
Time
[b]
[c]
[b]
[b]
93%
99%
97%
5 min
80%7a
180 min
0
10
20
30
40
50
60
Reaction time (min)
85%[d]26
,
10 min
5 min
180 min
180 min
120
Reaction
time (min)
98%[e]25
0
1
2
5
10
20
30
60
[b]
Yield[a]
0%
0%
2%
6%
20% 26% 39% 83% 83%
80%7a
Figure 5. Kinetic investigation of the phenanthrene catalyzed Minisci type
reaction of Boc-L-valine with 4-cyanopyridine in the “sunflow” reactor. For each
data point, 5 mL of the stock solution (described in the SI) were injected into the
“sunflow” reactor and withdrawn after a specific time period. [a] Yields were
calculated based on 1H-NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene
94%
73%
2 min
3 min
89%7a
180 min
960 min
[b]
Table 3. Comparison of the investigated reaction type C under “sunflow” and
batch conditions.
65%7a
“Sunflow”
Literature
Compound
Yield[a]
Time
Yield24b
Time
79%
62%
60 min
79%
80%
1440 min
For each substrate, the stock solution (described in the SI) and oxygen were
pumped through the “sunflow” reactor. [a] Isolated yield, [b] with rose bengal as
catalyst, [c] with bromotrichloromethane, [d] literature reaction uses additional 1
mol% [Ru(bpy)3]Cl2, [e] literature yield and reaction time only for oxidation to the
iminium ion with BrCCl3.
60 min
60 min
1440 min
1440 min
Finally, the “sunflow” approach was extended to the mostly UV-B
driven phenanthrene-catalyzed Minisci type reaction of carboxylic
acids with aromatic nitriles (C, scheme 1).[24b] Boc-L-valine and 4-
cyanopyridine were chosen as a model system for kinetic
investigations (figure 5). Since sunlight comprises only a small
fraction of UV-B, this reaction is generally much slower than the
visible light-driven cyanations mentioned above (compare
reaction times in figure 2–5). Again, high yields could be obtained
in relatively short reaction times compared to the literature.[24b]
The small fraction of solar UV-B makes the photon absorption and
excitation of phenanthrene the rate determining step. Assumption
of a constant photon flux results in a zero order kinetics leading
to a linear increase of yield and turnover with time.
45%
55%
For each substrate, the stock solution (described in the SI) was pumped through
the “sunflow” reactor. [a] Isolated yield.
Conclusions
The concept of “sunflow” could also be used to prepare an
alkaloid-like structure as well as a C-arylated dipeptide derivative
in reasonable yields and within short reaction times using the
same photoredox cross-coupling reaction (Table 3).
In the present study, sunlight as the most sustainable light source
was combined with a micro capillary reactor to achieve fast and
environmentally benign photoreactions in continuous flow. To
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F. Rominger, A. S. K. Hashmi, Angewandte Chemie International Edition
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behavior. The optimized conditions were applied to a range of
exemplary reactions, highlighting the applicability of this method
for various transformations. The used “sunflow” reactor is easy to
build and readily affordable for every research group interested in
eco-friendly flow photochemistry.
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Keywords: Sunlight • Flow chemistry • Photocatalysis • C-C
coupling • Green chemistry
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“Sunflow” ― The combination of a
micro capillary reactor and sunlight
enables fast and green photoredox and
Key Topic*
Alexander M. Nauth, Alexander Lipp,†
Benjamin Lipp,† and Till Opatz*
H-atom
transfer
reactions
in
continuous flow. Kinetic studies reveal
high reaction rates with a simple and
inexpensive reactor set-up (less than
90 US-$ acquisition value).
Page No. – Page No.
Title: Sunflow: Sunlight Drives Fast
and Green Photochemical Flow
Reactions in Simple Micro Capillary
Reactors — Application to
Photoredox- and H-Atom Transfer
Chemistry
*one or two words that highlight the emphasis of the paper or the field of the study
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