Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 6965–6969
Photobiocatalysis
Chromoselective Photocatalysis Enables Stereocomplementary Bio-
catalytic Pathways**
Rodriguez, Lee J. Edwards, Selin Kara, Tamara Mielke, Jared Cartwright, Gideon Grogan,
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
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
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)
Angew. Chem. Int. Ed. 2021, 60, 6965 –6969
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
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