Pyrimidopteridine-Catalyzed Hydroamination of Stilbenes with Primary Amines: A Dual Photoredox and Hydrogen Atom Transfer Catalyst

Article abstract of DOI:10.1021/acscatal.0c05540

The applicability of a heteroaromatic photoredox catalyst in an additive-free photo-mediated hydroamination of stilbenes is described. Initiated by the excitation of a highly potent organic pyrimidopteridine photoredox catalyst (E?[PrPPT?/PrPPT·-] = +2.10 V vs SCE in MeCN), the photo-mediated hydroamination of stilbenes was enabled using unprotected, primary aliphatic, allylic, benzylic amines for the synthesis of various α-phenyl phenethylamine derivatives. Notably, the stereogenic center of α-chiral amines was fully preserved. Both starting materials serve as competent quenching partners. Fluorescence- and competitive fluorescence-quenching experiments as well as electron paramagnetic resonance spectroscopic analysis and density functional theory calculations allowed a plausible reaction mechanism to be deduced.

Full text of DOI:10.1021/acscatal.0c05540

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Research Article
Pyrimidopteridine-Catalyzed Hydroamination of Stilbenes with
Primary Amines: A Dual Photoredox and Hydrogen Atom Transfer
Catalyst
Tobias Taeufer, Richy Hauptmann,‡ Firas El-Hage,‡ Thea S. Mayer, Haijun Jiao, Jabor Rabeah,
and Jola Pospech*
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ABSTRACT: The applicability of a heteroaromatic photoredox
catalyst in an additive-free photo-mediated hydroamination of
stilbenes is described. Initiated by the excitation of a highly potent
organic pyrimidopteridine photoredox catalyst (E*[PrPPT*/
PrPPT·−] = +2.10 V vs SCE in MeCN), the photo-mediated
hydroamination of stilbenes was enabled using unprotected,
primary aliphatic, allylic, benzylic amines for the synthesis of
various α-phenyl phenethylamine derivatives. Notably, the stereo-
genic center of α-chiral amines was fully preserved. Both starting
materials serve as competent quenching partners. Fluorescence-
and competitive fluorescence-quenching experiments as well as
electron paramagnetic resonance spectroscopic analysis and density
functional theory calculations allowed a plausible reaction mechanism to be deduced.
KEYWORDS: organic photoredox catalysis, pyrimidopteridines, hydroamination, stilbenes, primary amines
oxidation potentials (Figure 1b). The amine scope comprises
carbamates, sulfonamides, and N-heterocycles that do not
competitively quench the single-electron transfer between
alkene and the photoredox catalyst. Via either of the reaction
pathways, a β-amino radical is formed, requiring additional
hydrogen atom transfer (HAT) reagents to ensure the
conversion of the amine−alkene adduct and the turn-over of
the photoredox catalyst.21−23 The sole examples of the photo-
mediated hydrofunctionalization of stilbenes (Eo1x/2> +1.50 V vs
SCE in MeCN)24 were reported by Yasuda and co-workers via
direct excitation of the alkene using high-energy light.25−28
Knowles et al. have recently disclosed a protocol on the
photo-mediated deracemization of imidazolidinones via a
sequence of stereo discriminating steps applying three distinct
molecular catalysts.29,30 However, the use of chiral aliphatic
amines in light-driven hydroaminations is absent in literature.
The oxidation of primary amines is particularly challenging due
to the increased oxidation potential (Eo1x/2= +1.51 V vs SCE in
MeCN for n-butylamine) as compared to secondary and
INTRODUCTION
■
Photo-mediated hydroamination reactions have become a
valuable tool for the construction of omnipresent C−N
bonds.1−4 Both nitrogen- and carbon-centered radicals have
precedence to serve as versatile synthetic intermediates with
broad applicability in organic synthesis.5−12 Particularly,
photocatalytic hydroamination reactions of electron-rich
alkenes were recently reported by the groups of Knowles12−15
and Nicewicz.6,16−18 Two variants for photo-mediated C−N
bond forming strategies are applied. Knowles and co-workers
have investigated aminium radical cation and amidyl radical
addition to olefins. For instance, electron-deprived aminium
radicals are prone to react with electron-rich olefins (Figure
1a). Noteworthily, less electron-rich styrene derivatives were
solely converted in intramolecular reactions but constituted the
restrictive boundary in intermolecular scenarios as a result of
an electrophilicity−nucleophilicity mismatch.13 On the other
hand, the concept of proton-coupled electron transfer (PCET)
enabled the generation of less electrophilic neutral N-centered
radicals engaging less electron-rich terminal aliphatic olefins in
hydroamidation reactions.19 Nicewicz and co-workers have
studied alkene single-electron oxidation.17,20 This approach
benefits from converting the alkene to a more electrophilic
species that can be trapped by a nucleophile. However, this
methodology is restricted to electron-rich alkenes and
methoxy-substituted styrene derivatives with relatively low
Received: December 18, 2020
Revised: March 10, 2021
Published: April 6, 2021
https://doi.org/10.1021/acscatal.0c05540
© 2021 American Chemical Society
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RESULTS AND DISCUSSION
■
We have recently reported the optimized synthesis and
characterization of pyrimidopteridine-N-oxide (PPTNO)
heterocycles.40 Photo- and electrochemical analysis revealed
that these compounds have high excited-state reduction
potentials (E*[PrPPTNO*/PrPPTNO·−] = +2.29 V vs SCE
in MeCN). This constitutes a strong thermodynamic driving
force for the oxidation of a broad range of organic substrates.
We observed a decent reactivity of all PPTNO catalysts in the
photo-mediated hydroamination of stilbenes (Eo1x/2= +1.53 V
for (E)-stilbene vs SCE in MeCN) with n-butylamine and
benzylamine (Eo1x/2= +1.29 V for benzylamine vs SCE in
MeCN) (Table 1). The reaction was optimized using (E)-1-
fluoro-4-(4-methylstyryl)benzene (2a) and benzylamine (1a)
and a variety of pyrimidopteridine-based catalysts upon
irradiation at 396 nm. In the presence of tetrapropylpyr-
imidopteridine-N-oxide (PrPPTNO), the hydroamination
product (3a/b) was obtained in 73% isolated yield in a
61:39 regio-isomeric ratio (entry 1). The corresponding
methyl-, butyl-, and phenyl-substituted PPTNO catalysts
(entries 2−4) resulted in a lower product yield, presumably
due to diminished solubility (entry 2) and catalyst decom-
position (entry 4). The deoxygenated tetrapropylpyrimidop-
teridine PrPPT (E*[PrPPT*/PrPPT·−] = +2.10 V vs SCE in
MeCN) resulted in a comparable yield and regioselectivity,
suggesting that the corresponding N-oxide functions as a
catalyst precursor (entry 5). In fact, the corresponding stilbene
oxide was found as a side product during the onset of the
reaction.
Notably, the reaction proceeds in the absence of any
additives. The addition of protic solvents like water (entry 6)
and other additives impaired the product yield.41 Other
conventionally used photoredox hydroamination catalysts did
not yield any product under these reaction conditions (entries
7−9).
Figure 1. Photo-mediated hydroamination of (a) aliphatic olefins via
amine oxidation15 and (b) styrenes via alkene oxidation16 and (c)
cocatalyst-free photo-mediated hydroamination of stilbenes with
primary amines.
With the optimized reaction conditions in hand, we set out
to evaluate the scope of the hydroamination reaction. Linear
tertiary amines (Eo1x/2 = +1.13 V and +0.82 V vs SCE in MeCN
for Bu2NH and Bu3N, respectively).31,32 Thus, only a limited
number of photoredox catalysts are able to oxidize primary
amines.15,33 The difficulty in sustaining the stereoinformation
of α-chiral amines is associated with the drastic acidification of
the α-amino C−H bond (ΔpKa(α-C−H) ≈ −30). The
decreased α-C−H bond dissociation energy (ΔBDFE(α-C−
H) ≈ −45 kcal.mol−1) upon single-electron oxidation of an
amine to the aminium radical leads to α-deprotonation and
racemization of the stereogenic center.9,34−37 Furthermore,
delocalization of the unpaired spin can form a C-centered
radical that undergoes C−C instead of C−N bond
formation.38,39
We have identified a potent organic photoredox catalyst that
is capable of oxidizing stilbene derivatives in the presence of
primary amines with similar or even lower oxidation potentials
while fully preserving the stereoinformation of α-chiral centers.
Notably, the reaction proceeds in the absence of additional
HAT cocatalysts. Mechanistic analysis implying luminescence
quenching and electron paramagnetic resonance (EPR)
experiments supported by DFT calculations suggest a scenario
in which the heteroaromatic photoredox catalysts are engaged
in proton and hydrogen atom transfer events. This discovery
extends the repertoire of photo-mediated hydroamination
reactions toward the synthesis of α-phenyl phenethylamine
derivatives under mild reaction conditions.
a
Table 1. Optimization of Reaction Conditions
regioselectivity
entry
alteration of reaction conditions
none
MePPTNO instead of PrPPTNO
BuPPTNO instead of PrPPTNO
PhPPTNO instead of PrPPTNO
PrPPT instead of PrPPTNO
MeCN:H2O (9:1) as solvent
Mes-Acr-Me-BF4 instead of PrPPTNO
[Ir{dFCF3ppy}2(bpy)]PF6 instead of
PrPPTNO
3a:3b
yield
1
2
3
4
5
6
7
8
61:39
65:35
68:32
78:22
65:35
64:36
73%
43%
49%
14%
73%
30%
<5%
<5%
9
Ru(bpy)3(PF6)2 instead of PrPPTNO
<5%
a
Optimization reactions were performed on a 0.5 mmol scale with 5
mol % catalyst loading. Isolated yields are shown. The ratio of regio-
isomers was determined by 19F NMR spectroscopy.
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a
Table 2. Scope of the Organic Photoredox-Catalyzed Hydroamination
a
Reactions were run at a 0.5 mmol scale using 1 (3.0 equiv), 2 (1.0 equiv), and PrPPTNO (5.0 mol %). Yields refer to the isolated material after
purification and are the average of at least two experiments. Irradiation from two 30 W ONFURO LED panels; 30 °C. Regio-isomeric ratios were
b
c
d
1
determined by H NMR. Enantiomeric ratios were determined by chiral HPLC. 5.0 equiv of amine was used. Isolated yield after 24 h. 1 (1.0
e
equiv) and 2 (2.0 equiv) were used. Isolated yield after 24 h. Substrate was the corresponding acetate.
12, and 13) in a 1:1 mixture of diastereomers with no erosion
in enantiomeric excess of the α-stereogenic center. Methyl
phenylalaninate gave the corresponding α-phenyl phenethyl-
amine 11 in 34% yield (1:1 d.r.), while both (S)-(−)-1-
phenylethylamine and (R)-(+)-1-phenylethylamine furnished
the products 12 and 13 in 68% and 63%, respectively. The
diastereomers were readily separable by column chromatog-
raphy in enantiomeric ratios of 99:1. Next, we turned our
attention to the scope of the alkene component by testing
various unsymmetrically substituted stilbenes (Table 2). We
chose benzylamine 2a as the representative amine. Notably,
benzylamine is scarcely used in other photo-mediated
hydroamination protocols and has, to the best of our
knowledge, mostly been demonstrated to deliver hydro-
amination products in intramolecular reactions.14
Scheme 1. Deprotection of N-Benzyl-1,2-diphenylethan-1-
amine (14)
Scheme 2. Hydroamination of (Z)-Stilbene ((Z)-2b)
In the present case, we observed good and reliable yields
with various stilbene derivatives. As discussed, vide infra, the
best results were achieved with nonpolarized stilbenes (40−
73% isolated yield for substrates 3 and 19−22). The presence
and branched primary aliphatic amines were viable substrates
for the hydroamination of (E)-stilbene 2b (Table 2, 4−9). In
addition, silyl protected alcohols were well tolerated.
Furthermore, we were pleased to find that various benzylic
amines furnished the desired products 14−18 in moderate to
good yields. Notably, chiral amines could be added to the
prochiral stilbene, yielding α,α′-chiral secondary amines (11,
of electron-rich arenes (E ≤ +1.20 V vs SCE in MeCN)24
ox
1/2
generally impaired the desired C−N bond formation, likely
due to competitive arene oxidation (25 and 26), leading to
catalyst deactivation in the absence of a sacrificial oxidant.42,43
Likewise, electron-deficient stilbenes (Eo1x/2> +1.60 V vs SCE in
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Figure 2. Effect of stilbene geometry and polarity on the rate of product formation and isomerization.
substrates bearing a more polarized CC double bond (25
and 26). The deprotection of the benzylic group in product 14
was readily achieved through hydrogenation in the presence of
Pd/C, providing the free α-phenyl phenethylamine 31 in 84%
isolated yield (Scheme 1). To provide a mechanistic rational
for the reaction, we resorted to a series of control experiments.
For instance, employing (Z)-stilbene (2b) in the hydro-
amination reaction resulted in a diminished yield of 22% of
phenethylamine 14 after 16 h (Scheme 2).
Following the course of the reaction of (E)- and (Z)-stilbene
((E)-2b and (Z)-2b) by calibrated GC revealed significant
differences in the initial reaction rates of the reaction. Starting
from (E)-2b, the hydroamination product was formed 3.6
times faster (initial rate for (E)-2b is 1.62 × 10−6 M·s−1) as
compared to product formation from (Z)-2b (4.45 × 10−7 M·
s−1) (Figure 2). In the absence of an amine, a dynamic E/Z-
interconversion is apparent. E/Z-isomerization is strongly
suppressed in the presence of an amine, accounting for up to
6% after 26 h. Electronically, E- and Z-stilbenes behave
virtually identically.44 In the case of E-stilbene, putative single-
electron oxidation by the excited-state photocatalyst yields a
planar radical cation, stabilized by hyperconjugation and
accessible for nucleophilic attack. In contrast, a planar
arrangement of the phenyl rings is not possible in the case
of Z-stilbenes due to the steric repulsion of the ortho-C−H
bonds. Thus, nucleophilic attack is impaired and back-electron
transfer to the catalyst is competitive, which is accountable for
the diminished reaction rates. Polarized stilbenes tend to
undergo more facile E/Z-isomerization due to an elongated
double bond and a decreased bond order, resulting in a lower
activation barrier for the rotation around the central carbon−
Figure 3. Results of classical and competitive Stern−Volmer
experiments with stilbene ((E)-2b) and benzylamine (1a).
MeCN) resulted in diminished yields (22−24). In these cases,
an increased amount of aminium radical homocoupling
product was observed. Product yields could be increased by
inverting the substrate ratio from 5:1 to 1:2. An inverted
substrate ratio did not benefit the product yield in other cases.
To our delight, natural product-derived stilbenes featuring
estrone 29 and tetrahydrocannabinol 30 motifs were adequate
substrates for the photo-mediated hydroamination. The
corresponding products were isolated in 59% and 69% yield,
1
respectively. The ratio of regio-isomers was determined by H
NMR spectroscopy. In general, the C−N bond formation
occurs in proximity to the more electron-poor arene with
selectivities averaging a ratio of 70:30 up to ratios of 95:5 for
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Figure 5. Benzylamine (1a) and PrPPT: EPR spectra simulation with
two non-equivalent N nuclei (AN1 = 8.25 G, AN2 = 3.25 G) (top);
radical accumulation through irradiation via operando EPR spectros-
copy (bottom).
Figure 4. Stilbene ((E)-2b) and PrPPT: EPR spectra simulation with
two non-equivalent N and one H nuclei (g = 2.005; AN1 = 7.84 G, AN2
= 3.72 G; AH1 = 7.45 G) (top); radical accumulation through
irradiation via operando EPR spectroscopy (bottom).
carbon bond.45−47 We surmised that this circumstance could
explain the inferior yields of polarized stilbenes. Monitoring
the reaction of ethyl (E)-4-styrylbenzoate ((E)-2c) showed
that E/Z-isomerization is indeed twice as fast as compared to
unpolarized E-stilbene (1.93 × 10−7 M·s−1 vs 1.06 × 10−7 M·
s−1). In addition, the rate of the product formation from (E)-
2c is decreased by a factor of 2.3 (8.08 × 10−7 M·s−1).
Next, different conceivable mechanistic scenarios were
examined. As noted above, PrPPTNO serves as a precatalyst
and is active by photo-induced oxygen atom transfer to
stilbene (2) to form stilbene oxide and the catalytically active
species PrPPT.48 The reductive quenching of the excited-state
photoredox catalyst PrPPT* (E*[PrPPT*/PrPPT·−] = +2.10
V vs SCE in MeCN) upon oxidation of benzylamine (1a)
(Eo1x/2[1a·+/1a] = +1.29 V vs SCE in MeCN) is exergonic by
−18.5 kcal·mol−1. The oxidation of (E)-stilbene ((E)-2b)
(Eo1x/2[(E)-2b·+/(E)-2b] = +1.53 V vs SCE in MeCN) is
exergonic by −12.9 kcal·mol−1. Thus, both reaction trajectories
are feasible on thermodynamic grounds. Next, we conducted
classical and competitive Stern−Volmer quenching experi-
ments of the excited-state catalyst PrPPT* (Figure 3). (E)-
Stilbene ((E)-2b, KSV1 = 65 M−1) proved to be a twice as
potent quencher of the excited-state catalyst as compared to
benzylamine (1a, KSV3 = 33 M−1).49
Figure 6. Light on−off experiment. Yields were determined by
calibrated GC.
benzylamine (1a) showed a distinct first-order dependence on
the concentration of stilbene (KSV2 = 43
M
−1). These
experiments demonstrate that the electron transfer (ET)
from stilbene to the excited-state photocatalyst PrPPT* is
kinetically favored over the ET from benzylamine.
EPR spectra of a solution containing a 5:1 ratio of (E)-
stilbene ((E)-2b) and the photo-active heterocycle PrPPT
showed a nine-line signal at g = 2.005 upon irradiation at 396
nm (Figure 4). The spectra can be simulated by a spin half (S
= 1/2) center interacting with two non-equivalent nitrogen
nuclei (I = 1; AN1 = 7.84 G and AN2 = 3.72 G) and a hydrogen
nucleus (I = 1/2; AN1 = 7.45 G) that is thus located in the core
of the heterocycle. The hydrogen nucleus most likely originates
from acetonitrile or the amine side chain through deprotona-
tion by the catalyst radical anion PrPPT·−. The signal of the
The successive addition of benzylamine (1a) to a solution
containing a constant concentration of the photocatalyst
PrPPT and (E)-stilbene ((E)-2b) only had a marginal effect
on the luminescence quenching (KSV4 = 5.1 M−1). In contrast,
adding stilbene to a solution of the photocatalyst PrPPT and
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non-equivalent nitrogen nuclei (I = 1; AN1 = 7.84 G and AN2
=
3.72 G), equally indicating the expected electron transfer
between the excited-state catalyst and benzylamine (1a).
However, no coupling with a hydrogen nucleus was observed.
The accumulated radical species possesses a long life time,
persisting for several hours. The same split pattern was
observed upon irradiation of EPR samples mimicking reaction
conditions and in samples containing the catalyst, benzylamine
(1a), and (E)-stilbene (2b) in a 1:1:1 ratio.50 Based on the
results in an aprotic environment (Figure 4), we hypothesize
that an initial proton transfer from the radical cation of the
amine takes place, thus generating an aminyl radical and the
neutral catalyst radical PPTH·. This assumption is further
backed up by density functional theory (DFT) calculations on
a B3PW91 level of theory. However, the proton transfer from
the neutral catalyst radical PPTH· to excess benzylamine (1a)
is exergonic by −5.76 kcal.mol−1, generating the radical anion
PPT·− and benzylammonium as a counterion. The aminyl
radical and benzylammonium form the condensation product
of benzylamine as a minor pathway as observed in some
experiments.
A basic light on−off experiment revealed that the product is
only formed during irradiation (Figure 6). Considering the
longevity of the radical species formed from benzylamine and
the photoredox catalyst, product formation in the absence of
light would be conceivable. Thus, we reasoned that the short-
lived radical cation formed upon activation of (E)-stilbene
((E)-2b) by the excited-state PrPPT* is the key species that
yields the desired product.
Figure 7. Plausible catalytic cycle for the photo-mediated hydro-
amination via stilbene activation.
Scheme 3. 5-exo-trig vs HAT to the Benzylic Position
A postulated mechanism for the photo-mediated pyrimi-
dopteridine (PPT)-catalyzed hydroamination of stilbenes is
shown in Figure 7. The reaction sequence commences with the
activation of the catalyst precursor PrPPTNO (ε396 = 1066
M
−1·cm−1) through oxygen atom transfer to stilbene to form
stilbene oxide and the photoredox-active hydroamination
catalyst PrPPT. The catalytic cycle starts with the excitation
of PrPPT (ε396 = 40 M−1·cm−1) to PrPPT* (E*[PrPPT*/
PrPPT·−] = +2.10 V vs SCE in MeCN).21 The subsequent
oxidation of the stilbene (Eo1x/2= +1.53 V vs SCE in MeCN for
(E)-2b) proceeds via single-electron transfer. The cationic
stilbene radical reacts with the primary amine to form the
corresponding ammonium adduct bearing a C-centered radical
in the beta position. Based on EPR experiments, the catalyst
radical anion PrPPT·− can act as a base. Thus, a proton
transfer from the amine−stilbene adduct to the catalyst is
postulated.51 The subsequent hydrogen atom transfer from the
protonated catalyst PrPPTH· to the benzylic radical to form
the desired product was calculated to be exergonic by −47.4
kcal.mol−1. The major driving force is arguably the rear-
omatization of the heteroaromatic photoredox catalyst. The
assumption of a rapid HAT is further supported by the EPR
results showing a highly reactive short-lived species and the
absence of a 5-exo-trig cyclization of the inevitable radical
intermediate 9· formed in the reaction of allylamine and
stilbene ((E)-2b) (Scheme 3).52
Scheme 4. Stability of α-Stereogenic Centers in (a) the
Presence and (b) in the Absence of Stilbene
Noteworthily, although an electron transfer from the amine
to the excited-state photocatalyst is evident, no racemization of
α-chiral amines was observed neither in the presence nor in the
absence of stilbene ((E)-2b) (Scheme 4). This evidence
supports the intermediacy of a neutral aminyl radical that is
stabilized by hyperconjugation and is not susceptible to
racemization.37
protonated radical was only observed in the absence of
benzylamine and rapidly vanishes in the absence of light.
The EPR spectra of an analogous sample containing
benzylamine (1a) also showed a nine-line signal at g = 2.005
upon irradiation at 396 nm (Figure 5). The spectra can be
simulated by a spin half (S = 1/2) center interacting with two
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European Union (ESF/14-BM-A55-0049/16). We thank Dr.
A. Spannenberg for X-ray measurements, Dr. Reni Grauke and
Dr. Dirk Holmann for the assistance with EPR spectroscopy,
Everaldo Ferreira Krake for assistance with NMR spectroscopy,
and the staff of the analytic department for assistance with the
data acquisition.
CONCLUSIONS
■
In conclusion, we have demonstrated the applicability of a
heteroaromatic photoredox catalyst in an additive-free photo-
mediated hydroamination of stilbenes. A wide range of primary
amines is compatible in this reaction, including linear,
branched, functionalized, benzylic, and α-chiral amines, and
multiple substituted and natural product-derived stilbenes were
competent coupling partners. The utilized pyrimidopteridine-
derived organic photoredox catalyst presumably plays a
multifunctional role. On the one hand, the heterocycle acts
as a strong excited-state oxidant, capable of oxidizing a variety
of stilbenes. On the other hand, the reduced catalyst can act as
a proton acceptor and hydrogen atom transfer reagent, thus
enabling shuffling the hydrogen atom that is required for the
turn-over of the amine−stilbene radical adduct and the
regeneration of the photocatalyst. Mechanistic aspects of the
transformation were examined using various analytical tools,
including luminescence quenching, EPR measurements, and
DFT calculations.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c05540.
■
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AUTHOR INFORMATION
Corresponding Author
Jola Pospech − Leibniz Institute for Catalysis, 18059 Rostock,
Germany; orcid.org/0000-0001-7794-2600;
Email: Jola.Pospech@catalysis.de
■
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light promoted regioselective amination and alkylation of remote
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Authors
Tobias Taeufer − Leibniz Institute for Catalysis, 18059
Rostock, Germany
Richy Hauptmann − Leibniz Institute for Catalysis, 18059
Rostock, Germany
Firas El-Hage − Leibniz Institute for Catalysis, 18059 Rostock,
Germany
Thea S. Mayer − Leibniz Institute for Catalysis, 18059
Rostock, Germany
Haijun Jiao − Leibniz Institute for Catalysis, 18059 Rostock,
Germany; orcid.org/0000-0002-2947-5937
Jabor Rabeah − Leibniz Institute for Catalysis, 18059 Rostock,
Germany; orcid.org/0000-0003-2162-0981
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c05540
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Author Contributions
‡R.H. and F.E. contributed equally. All authors have given
approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support was provided by the German Research
Foundation (DFG, grant number: 401007518) and by the
■
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