Halogenase-Inspired Oxidative Chlorination Using Flavin Photocatalysis

Article abstract of DOI:10.1002/anie.201600783

Chlorine gas or electropositive chlorine reagents are used to prepare chlorinated aromatic compounds, which are found in pharmaceuticals, agrochemicals, and polymers, and serve as synthetic precursors for metal-catalyzed cross-couplings. Nature chlorinates with chloride anions, FAD-dependent halogenases, and O₂ as the oxidant. A photocatalytic oxidative chlorination is described based on the organic dye riboflavin tetraacetate mimicking the enzymatic process. The chemical process allows within the suitable arene redox potential window a broader substrate scope compared to the specific activation in the enzymatic binding pocket. Chlorination of arenes with chloride anions: The photochemical analogue of the enzymatic chlorination of Flavin-adenine dinucleotide (FAD)-dependent halogenases is possible in the presence of riboflavin, air, acetic acid, and blue light (see scheme; RFT=riboflavin tetraacetate).

Full text of DOI:10.1002/anie.201600783

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201600783
German Edition: DOI: 10.1002/ange.201600783
Oxidative Chlorination
Halogenase-Inspired Oxidative Chlorination Using Flavin
Photocatalysis
Thea Hering, Bernd Mꢀhldorf, Robert Wolf,* and Burkhard Kçnig*
Abstract: Chlorine gas or electropositive chlorine reagents are
used to prepare chlorinated aromatic compounds, which are
found in pharmaceuticals, agrochemicals, and polymers, and
serve as synthetic precursors for metal-catalyzed cross-cou-
plings. Nature chlorinates with chloride anions, FAD-depen-
dent halogenases, and O2 as the oxidant. A photocatalytic
oxidative chlorination is described based on the organic dye
riboflavin tetraacetate mimicking the enzymatic process. The
chemical process allows within the suitable arene redox
potential window a broader substrate scope compared to the
specific activation in the enzymatic binding pocket.
challenging. The few examples known suffer from drastic
conditions and low selectivity[2,4] or rely on stronger or metal-
based stoichiometric oxidants.[5] Over the last years, halogen-
ases have been successfully isolated and used for the
halogenation (mostly bromination) of aromatic compounds.[6]
These reactions show high selectivity and have also been
scaled up to gram amounts,[6b] but as the enzymes are
naturally substrate specific the scope of accessible products
is limited, and the isolation and handling of the enzymes is
difficult.
Chlorinated aromatic compounds are ubiq-
uitous in organic chemistry. They serve as key
precursors for metal-catalyzed cross-cou-
plings and are widely employed in natural
products, pharmaceuticals, and materials sci-
ence to tune biological or electronic proper-
ties.[1] While traditional chemistry mostly
relies on the use of hazardous and toxic
chlorine gas or synthetic equivalents such as
NCS and tBuOCl as the source of electro-
philic chlorine, nature has developed a more
elegant strategy based on the enzymatically
catalyzed oxidation of abundant and non-
toxic chloride ions in an oxidative chlorina-
tion.[2] Halogenases efficiently yield aryl hal-
ides from halide ions and aromatic com-
pounds using either O2 or hydrogen peroxide
(haloperoxidases) as the oxidant.[3] With
respect to environmental factors, these are
the ideal oxidants as only water is produced
as a by-product. For this reason a variety of
Scheme 1. Analogy of the mechanistic model of chloride oxidation by FAD-dependent
chemical oxidative halogenations have been halogenases (top) and the proposed photocatalytic halogenase mimetic system (bottom);
developed.[2] However, while great progress
R’=CH2(CHOAc)3CH2OAc.
has been made in the area of oxidative
bromination, oxidative chlorination remains
We aimed to develop a biomimetic system inspired by
flavin adenine dinucleotide (FAD)-dependent halogenases,
which is one of the main families of this enzyme group.[3a] The
FAD dependent system combines several advantages: O2 is
used as oxidant avoiding the separate addition of H2O2 as
required for heme and vanadate dependent haloperoxidases.
The cofactor FAD is a purely organic, metal-free catalyst, and
simple flavin derivatives are known to act as oxidative
photocatalysts.[7] The enzymatic mechanism (Scheme 1)
involves the reduction of FAD by NADH2 to yield a reduced
FADH2, which reacts with oxygen to form a peroxo species
FAD-OOH that is subsequently attacked by chloride ions to
form the “Cl+” equivalent HOCl.[8] Our system replaces FAD
[*] T. Hering, B. Mꢀhldorf, R. Wolf, B. Kçnig
Universitꢁt Regensburg, Fakultꢁt fꢀr Chemie und Pharmazie 93040
Regensburg (Germany)
E-mail: robert.wolf@ur.de
burkhard.koenig@ur.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201600783.
ꢂ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial License, which permits use,
distribution and reproduction in any medium, provided the original
work is properly cited, and is not used for commercial purposes.
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by the cheap dye riboflavin tetraacetate (RFT), which is
known to form reduced RFTH2 upon excitation with visible
light in the presence of benzyl alcohols (Scheme 1).[7] This
allows us to replace the biomolecules FAD and NADH2 and
to perform the reactions in organic solvents using a stable and
inexpensive catalyst.
as the described mediator and enables the chlorination via the
following reaction cycle. In the first step, the photocatalyst
RFT is excited by visible light irradiation (lmax = 455 nm) to
RFT* and reduced to RFTH2 by oxidation of the benzylic
alcohol (pMBA). RFTH2 is re-oxidized by air forming H2O2,
which does not directly oxidize chloride, but forms peracetic
acid (HOOAc) in an equilibrium with acetic acid (HOAc).
The hereby in situ generated HOOAc subsequently reacts
with chloride to form the electrophilic chlorine species HOCl,
which attacks anisole (1) in an electrophilic aromatic sub-
stitution reaction. However, we cannot exclude other electro-
philic chlorine species in equilibrium with HOCl, for example,
Cl2O, ClOAc, Cl2, and H2OCl+, being involved.[4b,12]
With this mechanistic model in hand, we optimized the
reaction conditions for the highest formation of peracetic acid
(see the Supporting Information). The equilibrium of H2O2
and acetic acid is known to be shifted towards the side of
peracetic acid by strong acids.[11a] Therefore, hydrochloric acid
proved to be the ideal chloride source as it dissolved well in
acetonitrile and is a strong acid at the same time. The reaction
with triethylammonium chloride (TEACl) and 20 mol%
H2SO4 also led to product formation, but with a slightly
lower yield. No chlorination was observed with any of the
tested chloride salts (TEACl, NaCl, KCl, and NH4Cl) in the
absence of added acid. Furthermore, elevated temperatures
are known to be beneficial for peracetic acid formation.[11b]
An increase of the reaction temperature from 258C to 458C
improved the yield of chloroanisole (2) from 28% to 66%
(p:o 5:1); a further increase to 608C led to decomposition of
the photocatalyst (Supporting Information, Table S4). We
also varied the peracid and replaced acetic acid by the
stronger acids formic acid and triflic acid (Supporting
Information, Table S3). Formic acid showed significantly
lower yields than acetic acid, while triflic acid with 5 equiv
TEACl and 5 equiv HCl gave a comparable yield of the
chlorinated anisole. Alternative reagents for the generation of
peracetic acid such as acetic anhydride or acetyl chloride
enabled product formation, but were less efficient than acetic
acid.
A key challenge in developing a photocatalytic halogen-
ase mimetic system is the efficient generation of electrophilic
hypochlorite. In analogy to the enzymatic system, RFTH2
forms a short-lived flavin-peroxo species RFT-OOH, which
should oxidize chloride ions to OClÀ (Scheme 1). However, in
the enzyme the reaction of the flavin peroxide to form
hypochlorite and the subsequent chlorination of the substrate
are catalyzed by the complex enzyme environment. For
enzymes such as RebH the mediation by a lysine residue in
the active center is crucial for the reactivity and selectivity of
the reaction. Moreover, X-ray studies of halogenases have
shown that the substrate and the flavin peroxide (FAD-OOH)
are brought in very close proximity (ca. 10 ꢀ) before
a reaction takes place.[3a,9] This is also the reason why the
simple chemical system, using anisole (1) as the substrate,
10 mol% RFT as the photocatalyst under aerobic conditions
and irradiation with blue light (lmax = 455 nm) in the presence
of HCl as the chloride source and p-methoxy benzyl alcohol
(pMBA) as a replacement for NADH2 in 2 mL acetonitrile,
did not yield any chlorination product of anisole (Scheme 2).
Scheme 2. Test reaction for the chlorination of anisole (1) with the
photocatalytic system using 20 mmol of 1 in 2 mL acetonitrile.
To chemically mimic the enzymatic system, a mediator is
needed, which is sufficiently long lived in order to enable the
formation of perchloric acid. During the course of our
investigations we discovered that peracetic acid can oxidize
chloride ions and is able to perform oxidative chlorination of
aromatic compounds (Supporting Information, Table S2).[10]
Peracetic acid is highly explosive when isolated, but it can be
formed in equilibrium with acetic acid and H2O2.[11] As it is
known that RFT-OOH formed in the photocatalytic oxida-
tion quickly releases one equivalent of H2O2,[7a] we added
10 equiv of acetic acid to the system described above and, to
our delight, observed the chlorination of anisole (1).
The optimized conditions depicted in Scheme 3 were used
to investigate the substrate scope. While an enzyme usually
has a highly specific binding pocket and thus a narrow
substrate scope, but high selectivity, our system does not bind
the substrate and should allow a broader substrate scope. The
results are summarized in Table 1. The system works excel-
lently for arenes with nitrogen + M substituents such as N,N-
dimethylaniline (entry 1) or amides (entries 2,3). Substrates
with an alkoxy group, such as anisole (entry 4) or dipheny-
lether (entry 5), can also be successfully chlorinated in good
to moderate yields. When the arene is too electron-rich, as for
Control reactions showed that all reaction components
are essential to observe the chlorination reaction (Supporting
Information, Table S1). Based on this we propose an in situ
formation of peracetic acid as depicted in Figure 1, which acts
Figure 1. Proposed mechanism of the peracetic acid mediated oxida-
Scheme 3. Oxidative chlorination of anisole (1) with the photocatalytic
tion of chloride by flavin photocatalysis.
halogenase mimetic system.
2
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Table 1: Scope of the flavin-catalyzed oxidative chlorination and results obtained by direct addition of
formation of an N-chloramine. This
selectivity is not observed with
amides (entries 2, 3).
H2O2.[a]
For comparison, Table 1 also
shows the yields of chlorination
obtained by adding 6 equiv of
H2O2 directly to the reaction mix-
ture instead of being generated by
the photocatalytic process (reaction
contained no RFT and pMBA).
Even though the direct addition of
H2O2 always gave full conversion of
the substrate, the yields were con-
siderably lower for most substrates
than in the photocatalytic system.
The slow generation of peroxide by
the flavin-catalyzed process is ben-
eficial for the reaction as it circum-
vents the problem of unselective
side reactions and over-chlorination
often observed for H2O2-based sys-
tems. The same observation was
made for haloperoxidase-catalyzed
reactions.[6e]
[d]
Entry
Substrate
Product
Conv
[%][b]
Yield
H2O2
[%][b,c]
96
14
1
100
100
96
(o:di 2:1)
(o:di 1:0)[g]
97
(p:o 3:1)
37
2[e]
(p:o 1:0)
98
(p:o 1:1)
3[e]
24
66
(p:o 1:0)
4
5
100
79
17
55
80
40
–
6
7
100
0
23
–
–
In
conclusion,
visible-light
flavin photocatalysis allows the oxi-
dative chlorination of arenes
inspired by FAD-dependent halo-
genases. The biomolecules FAD
and NADH2 were replaced by the
cheap organic dye riboflavin tetraa-
cetate and methoxy benzyl alcohol
as the reducing agent. As a result,
the reaction can be performed in
organic media. Acetic acid was
added to the system forming per-
acetic acid in situ, which acts as
a mediator to activate the peroxide
for chloride oxidation. Compared
to the specific binding pocket of an
enzyme, the activation by peracetic
acid is a more general strategy and
64
(p:o 1:3)
68
(p:o 1:5)
8
70
9[f]
76
49
63
64
11
84
10[f]
[a] Reactions were performed with 0.02 mmol of the substrate, 10 equiv HCl, 10 equiv HOAc, 6 equiv
pMBA and 10 mol% RFT in 2.0 mL MeCN. The reaction mixtures were irradiated for 2.5 h at 458C.
[b] Determined by GC-FID using an internal standard. [c] Based on conversion. [d] 6 equiv H2O2,
10 equiv HOAc, and 10 equiv HCl in 2 mL MeCN. [e] With KCl addition. [f] With TFA. [g] di=dichlori-
nation additionally at the para position.
example in dimethoxybenzene carrying two + M-substituents,
the yield decreases due to the unselective direct oxidation of
the substrate by the photocatalyst (entry 6). The acidic
conditions lead to a protonation of RFT observable by UV/
VIS measurements (Supporting Information, Tables S4, S5).
In its protonated form, RFT is known to have a high oxidative
power.[13] Substrates, which are too electron poor, for
example, trifluoromethoxybenzene (entry 7), are not
attacked by hypochlorite and do not give chlorination
products neither in the photocatalytic system nor when
peracetic acid is added directly (Supporting Information,
Table S2). Acetophenones (entries 9, 10) are mono-chlori-
nated in the a-position. The reaction proceeds via the enol
form and therefore works better when the stronger triflic acid
is used instead of acetic acid.[14] It is worth noting that
aromatic amines (entries 1, 8) show ortho selectivity for the
chlorination. This may be explained by the intermediate
thus allows a broader substrate scope. The developed system
allows the chlorination of electron rich arenes, for example,
anisole, methylanilines, diphenyl ether, and amides, as well as
the a-chlorination of acetophenones.
Acknowledgements
We gratefully acknowledge financial support by the Deutsche
Forschungsgemeinschaft (DFG), GRK 1626. T.H. thanks the
Fonds der Chemischen Industrie for a fellowship.
Keywords: enzyme models · flavins · oxidative ·
chlorination photocatalysis
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Received: January 23, 2016
Revised: January 23, 2016
Published online: January 23, 2016
4
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Angewandte
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Communications
Oxidative Chlorination
T. Hering, B. Mꢁhldorf, R. Wolf,*
B. Kçnig*
&&&& — &&&&
Halogenase-Inspired Oxidative
Chlorination Using Flavin Photocatalysis
Chlorination of arenes with chloride
anions: The photochemical analogue of
the enzymatic chlorination of Flavin-ade-
nine dinucleotide (FAD)-dependent halo-
genases is possible in the presence of
riboflavin, air, acetic acid, and blue light
(see scheme: RFT=riboflavin tetraace-
tate).
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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