Azo-dimethylaminopyridine-functionalized Ni(II)-porphyrin as a photoswitchable nucleophilic catalyst

Article abstract of DOI:10.3762/bjoc.16.179

We present the synthesis and the photochemical and catalytic switching properties of an azopyridine as a photoswitchable ligand, covalently attached to a Ni(II)-porphyrin. Upon irradiation with 530 nm (green light), the azopyridine switches to the cis con

Full text of DOI:10.3762/bjoc.16.179

Azo-dimethylaminopyridine-functionalized Ni(II)-porphyrin as
a photoswitchable nucleophilic catalyst
Jannis Ludwig1, Julian Helberg2, Hendrik Zipse2 and Rainer Herges*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 2119–2126.
1Otto Diels Institute of Organic Chemistry, University of Kiel,
Otto-Hahn-Platz 3-4, Kiel D-24098, Germany, and 2Department of
Chemistry, Ludwig-Maximilians-Universitaet Muenchen,
Butenandtstrasse 5–13, 81377 Muenchen, Germany
https://doi.org/10.3762/bjoc.16.179
Received: 08 June 2020
Accepted: 21 August 2020
Published: 31 August 2020
Email:
Rainer Herges* - rherges@oc.uni-kiel.de
Associate Editor: J. A. Murphy
* Corresponding author
© 2020 Ludwig et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
catalysis; Ni(II)-porphyrins; nucleophilic catalyst; photoswitch; record
player molecules; spin switch
Abstract
We present the synthesis and the photochemical and catalytic switching properties of an azopyridine as a photoswitchable ligand,
covalently attached to a Ni(II)-porphyrin. Upon irradiation with 530 nm (green light), the azopyridine switches to the cis configura-
tion and coordinates with the Ni2+ ion. Light of 435 nm (violet) isomerizes the ligand back to the trans configuration, which deco-
ordinates for steric reasons. This so-called record player design has been used previously to switch the spin state of Ni2+ between
singlet and triplet. We now use the coordination/decoordination process to switch the catalytic activity of the dimethylamino-
pyridine (DMAP) unit. DMAP is a known catalyst in the nitroaldol (Henry) reaction. Upon coordination to the Ni2+ ion, the
basicity of the pyridine lone pair is attenuated and hence the catalytic activity is reduced. Decoordination restores the catalytic ac-
tivity. The rate constants in the two switching states differ by a factor of 2.2, and the catalytic switching is reversible.
Introduction
Photoswitchable catalysis has been realized following several modulation in acid–base-controlled processes [4]. Molecular
approaches using a variety of photochromic systems. Feringa et motors for stereodivergent anion binding catalysis [5], azopep-
al. recently published a review including systems based on tides for the acetylation of sugars [6], enzyme mimics [7], and
double bond isomerizations [1]. An earlier review from the the utilization of intermolecular cooperative effects [8] are
same group summarized light and redox responsive catalytic further applications of photoswitchable catalysis.
systems including azobenzenes, diarylethenes, spiropyranes,
and stilbenes [2].
Particularly interesting and close to our approach is an early
work by Inoue et al. who achieved control of the transformat-
Diarylethenes were reported in the context of photoswitchable ion of CO2 and 1,2-epoxypropane to propylene carbonate using
catalysis as inhibitors of the Karstedt´s catalyst [3] and for pKa an aluminum porphyrin and a photoresponsive ligand. The cata-
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lytic activity of the metal porphyrin depended on the axial coor-
dination of an azostilbene and coordination of the latter ligand
was controlled by photoisomerization of the stilbene unit [9].
Hecht et al. reported on the photoswitching of the basicity of a
piperidine nitrogen by reversible steric shielding of the nitrogen
lone pair. The photoswitchable base was applied as a nucleo-
philic catalyst in the nitroaldol (Henry) reaction. Attached to a
sterically demanding azobenzene unit, the lone pair was
shielded in the cis configuration and therefore catalytically inac-
tive. The basicity was restored by isomerization to the trans
state leading to a rapid conversion to the β-nitroalcohol, the
product of the Henry reaction [10-12].
Figure 1: Basicity and nucleophilicity switching of the 4-(N,N-dimethyl-
amino)pyridine “record player” molecule 1.
in a number of reactions, for example the Baylis–Hillman reac-
We now present a photoswitchable catalyst whose basicity is tion [27] and the Steglich esterification [28,29]. To achieve
controlled by coordination/decoordination to the Ni2+ ion in a control of the catalytic activity of DMAP, dimethylamino
Ni-porphyrin. So-called record player molecules, including a record player 1 was investigated as a photoswitchable DMAP
Ni(II)-porphyrin as the square planar base complex and azopy- catalyst in the nitroaldol reaction of 4-nitrobenzaldehyde and
ridines as photoswitchable axial ligands, were previously inves- nitroethane (Henry reaction) as a model reaction.
tigated for spin switching applications [13-26]. In the present
Results and Discussion
study, the basicity change linked to the coordination/decoordi-
nation process is explored. If the azo substituent is in cis config- Synthesis
uration, the lone pair of the pyridine unit coordinates to the For the synthesis of 4-N,N-dimethylamino record player mole-
central Ni2+ ion, reducing the nucleophilic power and the cule 1, a modular approach described by Heitmann et al. was
basicity of the pyridine (Figure 1). In the trans configuration chosen [30] since the alternative mixed aldehyde route failed
intramolecular coordination is prohibited and nucleophilicity for related porphyrin derivatives [17]. Ni-porphyrin precursor 3
and basicity are restored. Reversible switching between the two and 4-(N,N-dimethylaminoazo)pyridine 4 were synthesized ac-
states is achieved by irradiation with green (trans→cis) and cording to literature procedures [17,30].
violet (cis→trans) light.
Following the conditions for the Suzuki cross-coupling reaction
The title compound of this study is 4-(N,N-dimethylamino)pyri- published for a similar system [30], decomposition products and
dine derivative 1 (Figure 1). The parent 4-(N,N-dimethyl- only 25% yield were obtained. Optimization by lowering the
amino)pyridine (DMAP, 2) is known as a nucleophilic catalyst reaction temperature increased the yield to 91% (Scheme 1).
Scheme 1: Synthesis of 4-N,N-dimethylamino record player molecule 1 by Suzuki reaction between Ni-porphyrin precursor 3 and bromo-substituted
4-(N,N-dimethylaminoazo)pyridine 4.
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Switching properties of porphyrin 1
expectable because DMAP is a very strong ligand with respect
Preliminary investigations on the photochemical switching to axial coordination to Ni-porphyrins [25]. According to
properties of porphyrin 1 were performed to optimize the reac- 1H NMR studies, however, the trans isomer exhibits concentra-
tion conditions for the catalytic process. Among the different tion-dependent coordination, which clearly indicates intermo-
switching wavelengths investigated, 530 nm (trans→cis) and lecular coordination. Again, this is due to the very strong coor-
435 nm (cis→trans) provided the largest conversion rates dination power of the DMAP ligand. At a total concentration of
(Figure S1 in Supporting Information File 1). The influence of 4 mM of the Ni-porphyrin, after irradiation with 435 nm
the solvent on the switching efficiency was evaluated by (THF-d8, 25 °C) 46% of the molecules are in cis and 54% are in
UV–vis (Figure S2, Supporting Information File 1) and NMR trans configuration. Whereas the cis isomers are almost com-
experiments (Figure 2). It is important to note that there are two pletely coordinated, the trans porphyrins still exhibit 7% coor-
ways to define the switching efficiency of record player mole- dination, which is due to a fast intermolecular ligand exchange
cules: 1. the cis–trans isomerization of the azopyridine unit, and [14]. At a total porphyrin concentration of 40 mM, the intermo-
2. the coordination–decoordination process. Both switching pro- lecular coordination of the trans species increases to 25%.
cesses are connected by the molecular design; however, they are Taking the incomplete photochemical conversion to the trans
not necessarily equally efficient because the cis isomer might configuration into account (60%), only 45% of the Ni-porphy-
not coordinate completely and there might be coordination in rin species should be catalytically active, if a solution of 40 mM
the trans state due to intermolecular coordination. On this total porphyrin concentration is applied.
account, isomerization and coordination were investigated sepa-
rately by 1H NMR spectroscopy. The protons meta to the azo The photostationary states after irradiation with 435 and 530 nm
group in the azopyridine unit (H-11) are responsive to the con- are strongly solvent dependent (Figure 2). THF turned out to
figuration of the azo group (cis or trans) and the chemical shifts provide the largest conversion rate (96%) to the coordinated
of the pyrrole protons of the porphyrin ring are strongly depend- (cis) state [31]. The back-reaction with 56% decoordination
ent on the axial coordination at the central Ni2+ ion (Figure 3, (trans isomer) is less efficient. At the other end of the scale, in
and for details see Tables S1 and S2 in Supporting Information acetone only 69% coordination (530 nm) and 75% decoordina-
File 1). The analysis of the NMR data revealed that 98.8–99.9% tion (435 nm) is achieved. It should be noted that a high conver-
of the cis isomer is coordinated in all solvents, which is sion to the inactive state (in photocatalysis as well as photophar-
Figure 2: Composition of the different states of porphyrin 1 (1 mM) in the PSS at 530 nm and 435 nm, determined by 1H NMR spectroscopy
(600 MHz; for details, see Supporting Information File 1).
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Figure 3: a) UV–vis cuvette with a solution of porphyrin 1 (13.1 µM in THF) and the corresponding UV–vis spectra at 25 °C in the PSS at 530 nm
(green) and 435 nm (blue). b) Signals of proton H-11 (1H NMR, 600 MHz, THF-d8, 318 K), which are an indicator for the cis–trans isomerization of the
azo unit of porphyrin 1, in the PSS at 530 nm (green) and 435 nm (violet-blue). The signal at 6.54 ppm belongs to the cis species while the signal at
6.41 represents the trans isomer. c) 1H NMR signals (1H NMR, 600 MHz, THF-d8, 300 K) of the pyrrole protons in the PSS at 530 nm and 435 nm.
Coordination/decoordination is fast on the NMR timescale and signal averages for the pyrrole protons are observed. 50.40 ppm corresponds to 100%
coordinated (paramagnetic) Ni-porphyrin and 10.83 ppm is the chemical shift of the pure decoordinated (diamagnetic) system. The signals are broad
because of paramagnetic line broadening of the coordinated species and fast ligand exchange.
macology) is more important than achieving a high conversion mined (Figure S5 in Supporting Information File 1). Thus, the
to the active state because the latter problem can be compen- thermal relaxation of the coordinated cis state is insignificant
sated by increasing the concentration, whereas incomplete deac- for our catalytic studies.
tivation is a problem, which cannot be circumvented. Hence,
THF was chosen as the solvent of choice for the catalysis exper- Experiments using porphyrin 1 as a catalyst
iments.
The Henry reaction (Scheme 2) was chosen as a model reaction
to investigate the catalytic activity of porphyrin 1 in the two dif-
A decreasing switching efficiency of 1 at 530 nm from trans to ferent switching states because the rate of the background reac-
cis was observed at higher concentrations (83% cis-1 at 40 mM tion without catalyst is low, and the influence of the solvent on
total porphyrin), whereas the switching to the trans state is the reaction rate is small [10,32].
almost unaffected (60%, Table S3 in Supporting Information
File 1). The reduced switching efficiency from the trans to the
cis isomer at higher concentrations can be explained by the
intermolecular coordination, which stabilizes the trans isomer.
Long-term switching experiments were performed to investi-
gate the fatigue resistance of 1. In contrast to other record
player molecules, which are stable up to 100 000 switching
cycles, the switching efficiency slowly decreased over
100 switching cycles to ≈80% of the original value (see Sup-
porting Information File 1, Figure S4). A thermal decomposi-
tion at 40 °C in THF was observed by a color change from red
to green. Hence, the catalytic experiments were limited to
25 °C.
Scheme 2: General scheme of the nitroaldol (Henry) reaction that was
used to investigate photoswitchable catalysis. The following reaction
conditions were applied: (0.4 M 4-nitrobenzaldehyde (6), 12 equiva-
lents of nitroethane (5), and 4 mM or 40 mM of catalyst (see
Scheme 3) at 25 °C in THF-d8) to yield 7. For details see Tables S4
and S5 in Supporting Information File 1.
At last, the thermal relaxation time for the cis→trans isomeriza-
tion in THF was determined by 1H NMR spectroscopy. Similar No catalytic effect for the N=N functional group of azoben-
to previous record player systems a long half-life of the meta- zenes was previously observed in similar systems [10]. Ni-por-
stable cis isomer of about 505 d (25 °C, THF-d8) was deter- phyrin Ni-TPPF20 8 (without azopyridine substitution) does not
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Beilstein J. Org. Chem. 2020, 16, 2119–2126.
catalyze the reaction either (Figure S6, Supporting Information All catalysts and reference compounds were used in two con-
File 1). Thus, it should be obvious that any catalytic effect, if centrations 4 mM (1 mol % with respect to 4-nitrobenzalde-
detected, must be due to the DMAP unit.
hyde (6, Figure 4a) and 40 mM (10 mol %, Figure 4b). As
stated above, intermolecular coordination of trans-1 increases at
In our kinetic experiments, the following compounds were used higher concentrations (7% at 4 mM and 25% at 40 mM) and the
as catalysts or reference compounds: record player 1 in the PSS trans→cis conversion drops from 93 to 83% at 40 mM concen-
at 530 nm (cis/trans ratio: 93:7 at 4 mM and 83:17 at 40 mM tration of 1. So one can expect that the catalytic switching effi-
total concentration of 1) and 435 nm (cis/trans ratio: 46:54 for ciency of 1 should drop at higher concentrations.
4 mM and 40:60 at 40 mM), DMAP 2, azopyridine 4, and
Ni-porphyrin 8 (Scheme 3 and Figures S8 and S9, Supporting As expected, DMAP 2 exhibits the highest catalytic activity.
Information File 1).
The azo-substituted DMAP trans-4 is slightly less effective
Scheme 3: DMAP (2), azopyridine trans-4, record player trans- and cis-1 and Ni-porphyrin 8 were used in kinetic experiments.
Figure 4: Conversion of 4-nitrobenzaldehyde (6) in the Henry reaction with nitroethane (5) as a function of time, using (a, left) 4 mM concentration of
catalyst or reference compound (1 mol % compared to 4-nitrobenzaldehyde) or 40 mM catalyst (10 mol % compared to 4-nitrobenzaldehyde) (b,
right). Rate constants (ktot) are given for DMAP (2) (black dots), azopyridine trans-4 (orange), photoswitchable porphyrin 1 at PSS435nm (predominant-
ly trans-1, blue), and 1 at PSS530nm (predominantly cis-1, green). The background reaction was determined with porphyrin 8 (cyan dots) and for a
sample without catalyst or reference compounds (red stars).
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because of the electron-withdrawing effect of the azo group. (7% trans, 93% cis isomer) by a factor of 1.3. At a concentra-
Surprisingly, however, the catalytic effect drops by a factor of tion of 1 of 40 mM the photoswitchable catalyst at PSS435nm
54 (4 mM pure trans-1 vs trans-4) or by a factor of 18 (40 mM (60% trans, 40% cis) is 2.25 times more efficient as the cata-
pure trans-1 vs trans-4), if the azopyridine unit of trans-4 is at- lyst at PSS530nm (17% trans, 83% cis) (Figure 4a and b). The
tached to the porphyrin core as realized in trans-1. This is unex- theoretical reaction rate k0 of the nitroaldol reaction with 100%
pected because any electron-withdrawing effect of the electron uncoordinated trans-1 (40 mM) is 8.6 times higher than the
poor porphyrin should be weak. There are eight bonds between reaction rate with pure cis-1. Thus, incomplete conversion and
the porphyrin β-position and the pyridine ring including one intermolecular coordination are further reasons for the relative-
meta-phenyl connection attenuating the through-bond conjuga- ly low switching efficiency.
tion. A closer look at the kinetic data in Table 1 (sample I, II,
and VII) might give an explanation for the surprisingly low cat- Conclusion
alytic activity of trans-1. While the background reaction of the A photoswitchable base catalyst for the nitroaldol (Henry) reac-
Henry reaction is very slow but still measurable, the addition of tion was synthesized and investigated. Principle of function is
Ni-porphyrin 8 inhibits this reaction completely (see also Sup- the control of the basicity of a DMAP derivative by light-in-
porting Information File 1, Figure S8). We propose that the duced coordination/decoordination of an azo-DMAP unit cova-
actual nucleophile in the Henry reaction, the nitroethane anion lently bound to a Ni-porphyrin. Upon irradiation with 530 nm
intermediate, coordinates to the Ni-porphyrin, which slows (green) light and 435 nm (violet) light, the nitroaldol reaction
down nucleophilic addition to the nitrobenzaldehyde. Hence, was accelerated/decelerated by a factor of 2.25. One of the
the switchable catalyst 1 contains both, a catalytic center factors reducing the catalytic activity and switching efficiency,
(nitrogen lone pair) and an inhibitor (Ni2+ ion). If the nitrogen obviously, was the deactivation of the nitroalkyl anion interme-
lone pair coordinates to the Ni2+ ion in the cis configuration of diate by coordination to the Ni-porphyrin. The DMAP-cata-
1, both reactive centers keep each other at bay. The isomeriza- lyzed acetylation of alcohols should not suffer from this draw-
tion to the trans configuration unleashes the catalytic as well as back because the reactive intermediate (acylpyridinium cation)
the inhibitory properties of 1.
is positively charged and does not coordinate to the Ni2+ ion
[33]. Another promising approach towards coordination-based,
Nevertheless, the reaction was faster after irradiation of the photoswitchable catalysts are photodissociable ligands. These
catalyst 1 (4 mM in THF-d8) with 435 nm (54% trans, 46% cis ligands coordinate in their trans form and decoordinate in the
isomer) as compared to the catalyst 1 irradiated with 530 nm cis configuration driven by steric hindrance [14,34,35].
Table 1: Pseudo-first order rate constants (k) of the Henry reaction (Scheme 2) in the presence of different catalysts and reference compounds.
Calculated rate constants are based on the assumption of complete isomerization and no intermolecular coordination. The concentration of the cata-
lyst is given in mol % (in parentheses) with respect to the initial nitrobenzaldehyde (6) concentration.
sample
catalyst (mol %)
cis/trans ratio
k [s−1]
I
blank (0%)
–
3.32 × 10−9
–a
II
Ni-TPPF20 8 (1%)
–
III
DMAP 2 (1%)
–
6.33 × 10−6
2.32 × 10−6
2.57 × 10−8
3.38 × 10−8
(2.45 × 10−8)b
(4.30 × 10−8)b
–a
IV
trans-NMe2-azopyridine 4 (1%)
530 nm PSS: NMe2-record player 1 (1%)
435 nm PSS: NMe2-record player 1 (1%)
cis-NMe2-record player 1 (1%)
trans-NMe2-record player 1 (1%)
Ni-TPPF20 8 (10%)
0:100
93:7
46:54
100:0
0:100
–
V
VI
calc.b
calc.b
VII
VIII
IX
DMAP 2 (10%)
–
1.09 × 10−4
3.08 × 10−5
3.84 × 10−7
8.62 × 10−7
(1.95 × 10−7)b
(1.68 × 10−6)b
trans-NMe2-azopyridine 4 (10%)
530 nm PSS: NMe2-record player 1 (10%)
435 nm PSS: NMe2-record player 1 (10%)
cis-NMe2-record player 1 (10%)
trans-NMe2-record player 1 (10%)
0:100
83:17
40:60
100:0
0:100
X
XI
calc.b
calc.b
aVery slow, not measurable; bassuming complete isomerization and no intermolecular coordination. For details see Supporting Information File 1,
section IV.
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12.Stoll, R. S.; Peters, M. V.; Kuhn, A.; Heiles, S.; Goddard, R.; Bühl, M.;
Thiele, C. M.; Hecht, S. J. Am. Chem. Soc. 2009, 131, 357–367.
doi:10.1021/ja807694s
Supporting Information
13.Venkataramani, S.; Jana, U.; Dommaschk, M.; Sönnichsen, F. D.;
Tuczek, F.; Herges, R. Science 2011, 331, 445–448.
doi:10.1126/science.1201180
Supporting Information File 1
General information, synthetic and photophysical
procedures, and copies of NMR spectra.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-179-S1.pdf]
14.Dommaschk, M.; Schütt, C.; Venkataramani, S.; Jana, U.; Näther, C.;
Sönnichsen, F. D.; Herges, R. Dalton Trans. 2014, 43, 17395–17405.
doi:10.1039/c4dt03048f
15.Dommaschk, M.; Näther, C.; Herges, R. J. Org. Chem. 2015, 80,
8496–8500. doi:10.1021/acs.joc.5b01524
16.Dommaschk, M.; Thoms, V.; Schütt, C.; Näther, C.; Puttreddy, R.;
Rissanen, K.; Herges, R. Inorg. Chem. 2015, 54, 9390–9392.
doi:10.1021/acs.inorgchem.5b01756
Acknowledgements
We thank Dr. Florian Gutzeit for providing the substance for
reference measurements (maximum shift of a fivefold coordi-
nated Ni complex in different solvents).
17.Dommaschk, M.; Peters, M.; Gutzeit, F.; Schütt, C.; Näther, C.;
Sönnichsen, F. D.; Tiwari, S.; Riedel, C.; Boretius, S.; Herges, R.
J. Am. Chem. Soc. 2015, 137, 7552–7555. doi:10.1021/jacs.5b00929
18.Ludwig, J.; Gröbner, J.; Dommaschk, M.; Huber, L. M.; Peters, M. K.;
Hövener, J.-B.; Herges, R. J. Porphyrins Phthalocyanines 2020, 24,
480–488. doi:10.1142/s1088424619501803
Funding
This work was funded by the Deutsche Forschungs-
gemeinschaft (DFG) within the collaborative research center
677 (SFB 677) “Function by Switching”.
19.Herges, R.; Dommaschk, M.; Heitmann, G.; Gröbner, J. Neuartiges
MRT-Kontrastmittel, Verwendung zur Temperaturmessung und
Verfahren zur Temperaturmessung. German Pat. Appl.
DE102017119482A1, Feb 28, 2019.
ORCID® iDs
Hendrik Zipse - https://orcid.org/0000-0002-0534-3585
20.Herges, R.; Dommaschk, M.; Heitmann, G.; Gröbner, J.; Thoms, V.
Neuartiges MRT-Kontrastmittel, Verwendung zur Temperaturmessung
und Verfahren zur Temperaturmessung. WO Pat. Appl.
WO2019/038428A2, Feb 28, 2019.
Rainer Herges - https://orcid.org/0000-0002-6396-6991
21.Dommaschk, M.; Gröbner, J.; Wellm, V.; Hövener, J.-B.; Riedel, C.;
Herges, R. Phys. Chem. Chem. Phys. 2019, 21, 24296–24299.
doi:10.1039/c9cp04156g
Preprint
A non-peer-reviewed version of this article has been previously published
as a preprint: https://doi.org/10.3762/bxiv.2020.70.v1
22.Gutzeit, F.; Dommaschk, M.; Levin, N.; Buchholz, A.; Schaub, E.;
Plass, W.; Näther, C.; Herges, R. Inorg. Chem. 2019, 58,
12542–12546. doi:10.1021/acs.inorgchem.9b00348
23.Matino, F.; Schull, G.; Jana, U.; Köhler, F.; Berndt, R.; Herges, R.
Chem. Commun. 2010, 46, 6780–6782. doi:10.1039/c0cc00959h
24.Thies, S.; Bornholdt, C.; Köhler, F.; Sönnichsen, F. D.; Näther, C.;
Tuczek, F.; Herges, R. Chem. – Eur. J. 2010, 16, 10074–10083.
doi:10.1002/chem.201000603
References
1. Dorel, R.; Feringa, B. L. Chem. Commun. 2019, 55, 6477–6486.
doi:10.1039/c9cc01891c
2. Vlatković, M.; Collins, B. S. L.; Feringa, B. L. Chem. – Eur. J. 2016, 22,
17080–17111. doi:10.1002/chem.201602453
3. Kaur, B.; Raza, R.; Stashick, M. J.; Branda, N. R. Org. Chem. Front.
2019, 6, 1253–1256. doi:10.1039/c9qo00221a
25.Heitmann, G.; Schütt, C.; Gröbner, J.; Huber, L.; Herges, R.
Dalton Trans. 2016, 45, 11407–11412. doi:10.1039/c6dt01727d
26.Köbke, A.; Gutzeit, F.; Röhricht, F.; Schlimm, A.; Grunwald, J.;
Tuczek, F.; Studniarek, M.; Longo, D.; Choueikani, F.; Otero, E.;
Ohresser, P.; Rohlf, S.; Johannsen, S.; Diekmann, F.; Rossnagel, K.;
Weismann, A.; Jasper-Toennies, T.; Näther, C.; Herges, R.; Berndt, R.;
Gruber, M. Nat. Nanotechnol. 2020, 15, 18–21.
4. Gurke, J.; Budzák, Š.; Schmidt, B. M.; Jacquemin, D.; Hecht, S.
Angew. Chem., Int. Ed. 2018, 57, 4797–4801.
doi:10.1002/anie.201801270
5. Dorel, R.; Feringa, B. L. Angew. Chem., Int. Ed. 2020, 59, 785–789.
doi:10.1002/anie.201913054
6. Niedek, D.; Erb, F. R.; Topp, C.; Seitz, A.; Wende, R. C.;
Eckhardt, A. K.; Kind, J.; Herold, D.; Thiele, C. M.; Schreiner, P. R.
J. Org. Chem. 2020, 85, 1835–1846. doi:10.1021/acs.joc.9b01913
7. Saha, M.; Bandyopadhyay, S. Chem. Commun. 2019, 55, 3294–3297.
doi:10.1039/c9cc00018f
doi:10.1038/s41565-019-0594-8
27.Octavio, R.; de Souza, M. A.; Vasconcellos, M. L. A. A.
Synth. Commun. 2003, 33, 1383–1389. doi:10.1081/scc-120018699
28.Steglich, W.; Höfle, G. Angew. Chem., Int. Ed. Engl. 1969, 8, 981.
doi:10.1002/anie.196909811
8. Ren, C. Z.-J.; Solís Muñana, P.; Dupont, J.; Zhou, S. S.; Chen, J. L.-Y.
Angew. Chem., Int. Ed. 2019, 58, 15254–15258.
29.Neises, B.; Steglich, W. Org. Synth. 1985, 63, 183.
doi:10.15227/orgsyn.063.0183
doi:10.1002/anie.201907078
30.Heitmann, G.; Dommaschk, M.; Löw, R.; Herges, R. Org. Lett. 2016,
18, 5228–5231. doi:10.1021/acs.orglett.6b02507
9. Sugimoto, H.; Kimura, T.; Inoue, S. J. Am. Chem. Soc. 1999, 121,
2325–2326. doi:10.1021/ja983960i
31.Dommaschk, M. Intramolekularer
10.Peters, M. V.; Stoll, R. S.; Kühn, A.; Hecht, S. Angew. Chem. 2008,
120, 6056–6060. doi:10.1002/ange.200802050
Light-Driven-Coordination-Induced-Spin-State-Switch (LD-CISSS) in
Nickel(II)porphyrinen zur Entwicklung photoschaltbarer
MRT-Kontrastmittel”; Diplomarbeit, Christian-Abrechts-Universität zu
Kiel, 2011; pp 45f.
11.Hecht, S.; Peters, M. V.; Stoll, R. S. Photoschaltbare Katalysatoren.
German Patent DE102006057612.8, Dec 5, 2006.
2125

Beilstein J. Org. Chem. 2020, 16, 2119–2126.
32.Kürti, L.; Czako, B. Strategic Applications of Named Reactions in
Organic Synthesis; Academic Press: London, U.K., 2009; pp 202–203.
33.Xu, S.; Held, I.; Kempf, B.; Mayr, H.; Steglich, W.; Zipse, H.
Chem. – Eur. J. 2005, 11, 4751–4757. doi:10.1002/chem.200500398
34.Thies, S.; Sell, H.; Bornholdt, C.; Schütt, C.; Köhler, F.; Tuczek, F.;
Herges, R. Chem. – Eur. J. 2012, 18, 16358–16368.
doi:10.1002/chem.201201698
35.Peters, M. K.; Hamer, S.; Jäkel, T.; Röhricht, F.; Sönnichsen, F. D.;
von Essen, C.; Lahtinen, M.; Naether, C.; Rissanen, K.; Herges, R.
Inorg. Chem. 2019, 58, 5265–5272.
doi:10.1021/acs.inorgchem.9b00349
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