Chromoselective Photocatalysis Enables Stereocomplementary Biocatalytic Pathways**

Article abstract of DOI:10.1002/anie.202100164

Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (S)- or the (R)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from Agrocybe aegerita, green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (R)-1-phenylethanol (99 % ee). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from Rhodococcus ruber to form (S)-1-phenylethanol (93 % ee).

Full text of DOI:10.1002/anie.202100164

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 6965–6969
International Edition: doi.org/10.1002/anie.202100164
German Edition: doi.org/10.1002/ange.202100164
Photobiocatalysis
Chromoselective Photocatalysis Enables Stereocomplementary Bio-
catalytic Pathways**
Luca Schmermund, Susanne Reischauer, Sarah Bierbaumer, Christoph K. Winkler, Alba Diaz-
Rodriguez, Lee J. Edwards, Selin Kara, Tamara Mielke, Jared Cartwright, Gideon Grogan,
Bartholomꢀus Pieber,* and Wolfgang Kroutil*
Abstract: Controlling the selectivity of a chemical reaction
with external stimuli is common in thermal processes, but rare
in visible-light photocatalysis. Here we show that the redox
potential of a carbon nitride photocatalyst (CN-OA-m) can be
tuned by changing the irradiation wavelength to generate
electron holes with different oxidation potentials. This tuning
was the key to realizing photo-chemo-enzymatic cascades that
give either the (S)- or the (R)-enantiomer of phenylethanol. In
combination with an unspecific peroxygenase from Agrocybe
aegerita, green light irradiation of CN-OA-m led to the
enantioselective hydroxylation of ethylbenzene to (R)-1-phe-
nylethanol (99% ee). In contrast, blue light irradiation
triggered the photocatalytic oxidation of ethylbenzene to
acetophenone, which in turn was enantioselectively reduced
with an alcohol dehydrogenase from Rhodococcus ruber to
form (S)-1-phenylethanol (93% ee).
light irradiation results in a common photoredox cycle and the
expected mono-substituted product. In the case of blue light,
the Rh-6G radical anion, which is formed after quenching of
Rh-6G* with a sacrificial electron donor, can absorb a second
photon, resulting in the highly reducing Rh-6GCÀ* species that
enables the formation of the di-substituted product.[5a]
Here we show that electron holes with different oxidation
potentials can be generated by using a heterogeneous carbon
nitride (CN) catalyst by changing the incident photon energy.
The combination of this strategy with two enantioselective
biocatalysts[6] allowed us to selectively produce the (S)- or
(R)-enantiomer of a chiral alcohol in photo-chemo-enzymatic
reaction sequences (Scheme 1C).
We recently realized that the choice of the wavelength is
crucial for high selectivities in metallaphotocatalytic cross
couplings using a heterogeneous carbon nitride material,
which is made from urea and oxamide in molten salt (CN-
OA-m).[5b,c,7] While this can be rationalized by a purely kinetic
effect, there is also evidence that a wavelength-controlled
generation of excited species with different oxidation poten-
tials might be responsible for this phenomenon. CN-OA-m
has a strong absorption up to ꢀ 460 nm and a comparably
weaker absorption band up to ꢀ 700 nm, which were ascribed
as the p–p* and n–p* electron transitions, respectively
(Figure 1A).[8] The selective induction of the n–p* electron
transition using long wavelengths (525 nm) should result in
electron holes with a lower oxidation potential compared to
irradiation using blue light (440 nm). The choice of the
wavelength should not affect the reduction potential of the
electron that is promoted into the valence band. Although
M
any parameters influence the selectivity of a chemical
reaction.[1] For instance, catalytic reactions can be controlled
by varying the catalyst/coordinated ligands, directing groups[2]
or by tuning external parameters (Scheme 1A).[1a,3] The
selectivity of photochemical reactions varies with different
wavelengths,[4] but examples that use this for visible-light
photocatalysis are rare.[5]
In one example, selective control between either a one- or
two-fold substitution of 1,3,5-tribromobenzene with N-meth-
ylpyrrole using Rhodamin 6G (Rh-6G) as photocatalyst was
demonstrated (Scheme 1B).[5a] This selectivity switch is
explained by the chromoselective generation of two photo-
catalytic species that differ in their reduction potential. Green
[*] L. Schmermund, S. Bierbaumer, Dr. C. K. Winkler, Prof. W. Kroutil
Institute of Chemistry, Department of Organic and Bioorganic
Chemistry, University of Graz, NAWI Graz, BioTechMed Graz
Heinrichstrasse 28, 8010 Graz (Austria)
T. Mielke, Dr. J. Cartwright, Prof. G. Grogan
Department of Chemistry, University of York
Heslington, York, YO10 5DD (UK)
Prof. W. Kroutil
Field of Excellence BioHealth-University of Graz
8010 Graz (Austria)
E-mail: wolfgang.kroutil@uni-graz.at
S. Reischauer, Dr. B. Pieber
Department of Biomolecular Systems, Max Planck Institute of
Colloids and Interfaces
Am Mꢀhlenberg1, 14476 Potsdam (Germany)
E-mail: Bartholomaeus.Pieber@mpikg.mpg.de
[**] A previous version of this manuscript has been deposited on
a preprint server (https://doi.org/10.26434/chemrxiv.13521527).
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202100164.
Dr. A. Diaz-Rodriguez, Dr. L. J. Edwards
Chemical Development, Medicinal Science and Technology, Pharma
R&D, GlaxoSmithKline Medicines Research Centre
Gunnels Wood Road, Stevenage SG1 2NY (UK)
ꢁ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
Prof. S. Kara
Department of Engineering, Biological and Chemical Engineering,
Biocatalysis and Bioprocessing Group, Aarhus University
Gustav Wieds Vej 10, 8000 Aarhus (Denmark)
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Figure 1. Chromoselective generation of excited CN-OA-m species with
different oxidation potentials. A) Switching between p–p* and n–p*
electron transitions using different wavelengths. B) The oxidation of
ethylbenzene 1a to acetophenone 3a is only possible using blue light.
Scheme 1. A) General approaches to control of the outcome of a chem-
À
ical reaction. B) Chromoselective control in photocatalytic C H-aryla-
tions.[5a] C) This study: Chromoselective control of the stereochemical
outcome of photo-chemo-enzymatic reactions.
H2O2 in the presence of ethylbenzene (1) and the AaeUPO,
which in turn catalyses the asymmetric hydroxylation of
1 (Figure 2). Performing the reaction in tricine buffer using
528 nm LEDs indeed resulted in a high selectivity towards
(R)-1-phenylethanol formation [(R)-2a, up to 3.8 mM, 98%
ee] with low amounts (3%) of acetophenone (3a). When the
same reaction was carried out using shorter wavelengths, 3a
became the main product, thus supporting our hypothesis.
Ketone (3a) formation was also the preferred reaction in the
presence of blue light in phosphate buffer. It is worth to note,
that the type of buffer had a significant influence on the
outcome on the reaction, whereby the molecular reason needs
to be clarified.
It was previously shown that UPOs are deactivated in the
presence of blue light, a photocatalyst and O2 due to the
generation of reactive oxygen species (ROS) that harm the
enzyme.[15] Consequently, one might expect that green light
might be less harmful to the UPO and lead to higher
conversions in comparison to blue light. To investigate this
aspect, UPO and CN-OA-m were incubated for one hour in
the presence of oxygen and green or blue light, before 1a was
added (Figure S48). The mixture incubated at longer wave-
lengths indeed led to a higher conversion for the asymmetric
hydroxylation after addition of 1a.
such a behavior was previously suggested,[8] there is, to the
best of our knowledge, no report that applies this concept for
controlling the selectivity of chemical reactions.
We hypothesized that such a strategy would allow us to
induce a photocatalytic reaction of a substrate with green
light selectively in the presence of a second compound that is
only photo-oxidized when shorter wavelengths are used. The
3
À
photocatalytic aerobic oxidation of benzylic sp C H bonds,
which is feasible with other members of the carbon nitride
family and blue light irradiation,[9] served as a model reaction
for our initial studies. In a series of experiments, we were
indeed able to show that only blue light results in the desired
carbonyl products and no reaction occurs at longer wave-
lengths (Figure 1B).
Carbon nitrides are used to catalyse the formation of O2
and H2 via water oxidation[10] and the production of hydrogen
peroxide from oxygen and alcohols, which requires the
reduction of O2.[11] Hydrogen peroxide can then be used as
stoichiometric oxidant in the enantioselective hydroxylation
of ethylbenzene derivatives catalysed by the unspecific
peroxygenase (UPO)[12] from A. aegerita[13] (AaeUPO)
acting as chiral catalyst.[14]
We hypothesized that a chromoselective activation of CN-
OA-m with green light enables the selective formation of
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been transformed with AaeUPO using an in situ H2O2
generation system before.
Ethylbenzenes bearing a methyl substituent in the ortho-
or meta-position were hydroxylated with 99% regioselectivity
at the ethyl group to give the desired chiral alcohols (R)-2b,c.
This ability to distinguish between a methyl and an ethyl
group has not been reported before. A possible explanation
for this selectivity might be a preferred formation of the
secondary intermediate radical over the primary radical.
Acetophenone substituted with ethyl in the para-position (1i)
allowed one to access a bi-functionalised chiral hydroxyke-
tone 2i, which is otherwise difficult to make. The same is true
for 2j.
Recycling experiments further showed that CN-OA-m
can be reused by centrifugation and one washing step with
water. CN-OA-m was reused three times after drying at room
temperature (Figure S49–S51). Transferring the photo-
chemo-enzymatic hydroxylation from a total volume of
1 mL in 1.5 mL glass vials successfully to a larger scale
(7 mL volume, 10 mL tubes) in another photoreactor (pro-
vided by GlaxoSmithKline, S5),[16] showed the robustness and
reproducibility of the approach. The hydroxylation of 1a
worked equally well giving up to 7.5 mM of (R)-2a.
Recently, photo-chemo-biocatalytic cascades were
reported combining a photoredox oxidation of ethylbenzene
with an enzymatic reduction.[17] In a related approach a photo-
chemo-biocatalytic cascade that yields the corresponding (S)-
enantiomers was set up by taking advantage of the chromo-
selective activation of CN-OA-m (Scheme 3). The blue-light
mediated oxidation of 1a to 3a proceeded smoothly in KPi
buffer. The resulting ketone (3a) was stereoselectively
reduced using an alcohol dehydrogenase (ADH-A) from
Rhodococcus ruber in the presence of NAD+ as cofactor.[18]
The optimized two-step one-pot procedure led to 2.5 mM (S)-
2a with an ee of 93%. The lower ee obtained in the
photochemo-enzymatic cascade compared to previous
reports by ADH-A (ee 99%),[19] can be explained by the
formation of a small amount of rac-1-phenylethanol during
the photocatalytic reaction under blue light irradiation
(Table S3). This cascade represents a stereocomplementary
pathway compared to the pathway with AaeUPO using the
same photocatalyst. Interestingly, it was noticed that MeOH
was not required for the reaction to hydroxylate ethylbenzene
with AaeUPO. Without MeOH the same concentration of
Figure 2. Influence of different wavelengths and buffers on the photo-
chemo-enzymatic hydroxylation of ethylbenzene; reaction conditions:
AaeUPO (25 nM), ethylbenzene (10 mM), CN-OA-m (2 mgmLÀ1),
MeOH (250 mM), KPi (100 mM, pH 7.5) or tricine (100 mM, pH 7.5),
455 nm (1440 mmol photons mÀ2 sÀ1) or 528 nm (1330 mmol photons
mÀ2 sÀ1), 308C, 8 h.
Scheme 2. Substrate scope of AaeUPO using H2O2 generated by CN-
OA-m under green light irradiation. Absolute configurations were
determined by reference material except otherwise stated. [a] Based on
external calibration curves of 2i. [b] (R)-Enantiomer determined by
measurement of the specific rotation (208C, c=1.00, CHCl3) and
comparison to literature.
The milder conditions subsequently allowed an extension
of the substrate scope for AaeUPO (Scheme 2). Nine addi-
tional substrates were converted with high stereoselectivity
(> 98% ee) to the corresponding alcohols with concentrations
of 1.0–6.0 mM. None of these ethylbenzene derivatives has
Scheme 3. Light-driven enantioselective oxyfunctionalizations of 1a by
using chromoselective CN-OA-m and AaeUPO or ADH-A.
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product was detected. Thus, the reaction is possible without
Conflict of interest
a sacrificial electron donor like MeOH or formate, which is in
contrast to some examples reported in literature.[14a,20] For
practical reasons, MeOH was still used since it simplified the
preparation of stock solutions of the hydrophobic substrates.
To test whether the cascade can also be transferred to other
substrates, para- and ortho-bromo-substituted ethylbenzene
(1g, 1h) were investigated: Using the blue-light pathway, (S)-
2g was obtained with an ee of > 99% (1 mM) and (S)-2h with
an ee of 94% (1.4 mM) (Figure S32 and S37).
The authors declare no conflict of interest.
À
Keywords: carbon nitrides · C H activation · chromoselectivity ·
photobiocatalysis · unspecific peroxygenases
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Marie Skłodowska-Curie grant agreement No 764920. S.B.
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Manuscript received: January 5, 2021
Accepted manuscript online: February 2, 2021
Version of record online: February 26, 2021
Angew. Chem. Int. Ed. 2021, 60, 6965 –6969
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org 6969