Anthraquinones as Photoredox Catalysts for the Reductive Activation of Aryl Halides

Article abstract of DOI:10.1002/ejoc.201701461

Quinones are ubiquitous in nature as structural motifs in natural products and redox mediators in biological electron-transfer processes. Although their oxidation properties have already been used widely in chemical and photochemical reactions, the applications of quinones in reductive photoredox catalysis are less explored. We report the visible-light photoreduction of aryl halides (Ar–X; X = Cl, Br, I) by 1,8-dihydroxyanthraquinone. The resulting aryl radical anions fragment into halide anions and aryl radicals, which react through hydrogen abstraction or C–C bond-forming reactions. The active photocatalyst is generated from 1,8-dihydroxyanthraquinone by photoinduced single-electron reduction to the radical anion or subsequent protonation and further reduction (or vice versa) to the semiquinone anion. Subsequent visible-light excitation of the anthraquinone radical anion or semiquinone anion converts them into very potent single-electron donors. A plausible mechanism for the reaction is proposed on the basis of control experiments and spectroscopic investigations.

Full text of DOI:10.1002/ejoc.201701461

A Journal of
Accepted Article
Title: Anthraquinones as photoredox catalysts for the reductive
activation of aryl halides
Authors: Javier Bardagi, Indrajit Ghosh, Matthias Schmalzbauer, Tamal
Ghosh, and Burkhard König
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
Anthraquinones as photoredox catalysts for the reductive activation of aryl halides
Javier I. Bardagi, Indrajit Ghosh, Matthias Schmalzbauer, Tamal Ghosh, and Burkhard König*
Institute of Organic Chemistry, University of Regensburg, D-93040 Regensburg, Germany
*Correspondence to: burkhard.koenig@ur.de
Graphical abstract
Abstract
Quinones are ubiquitous in nature, as structural motif of natural products or redox mediator in
biological electron transfer processes. While their oxidation properties have already been widely
used in chemical and photochemical reactions, applications of quinones in reductive photoredox
catalysis are less explored. We report the visible light photoreduction of aryl halides (Ar–X;
where, X = Cl, Br, I) by 1,8-dihydroxyanthraquinone. The resulting aryl radical anions fragment
into halide anions and aryl radicals, which react in hydrogen abstraction or C–C bond forming
reactions. The active photocatalyst is generated from 1,8-dihydroxyanthraquinone by
photoinduced single electron reduction to the radical anion or subsequent protonation and
further reduction (or vice versa) to the semiquinone anion. Subsequent visible light excitation of
the anthraquinone radical anion or semiquinone anion converts them into very potent single
electron donors. A plausible mechanism of the reaction is proposed based on control
experiments and spectroscopic observations.
Keywords: Quinone, radical anion, electron transfer, C–H arylation, visible light
T
he use of visible light for the generation of reactive intermediates in organic transformations
has many advantages, and the growing interest over the last decade on visible light mediated
photoredox catalytic synthetic transformations demonstrates the feasibility of the approach.1-9
Ru- or Ir-based coordination complexes have gained enormous attention because of their
excellent visible light harvesting properties, modest to extremely high oxidation and reduction
potentials, relatively long excited state lifetimes, and reasonably good chemical and photo
stability under synthetic oxidative and reductive conditions.2 In addition, considerable efforts
have been made in developing metal-free photoredox catalytic methods using organic dyes,8, 10-
14
such as eosin Y10-11 or rhodamine derivatives,13-14 or organic heterogeneous photocatalysts15
for synthetic transformations.
Quinones8, 16-17 occur in nature and are known for their excellent electron accepting and
hydrogen-abstracting abilities.16, 18 Not surprisingly, different quinone derivatives have been
used for photooxidative transformations using visible light.19-22 Fukuzumi reported the
photoredox catalytic conversion of benzene to phenol using 2,3-dichloro-5,6-dicyano-p-
benzoquinone (DDQ) under visible light irradiation.23 Itoh and subsequently Yuan reported
trifluoromethylations of arenes and heteroarenes using anthraquinone-2-carboxylic acid24 and
DDQ25, respectively. Itoh reported the synthesis of carboxylic acids from their respective
methyl aromatics using 2-chloroanthraquinone.26 Recently, Brasholz disclosed cascade
1
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
dehydrogenation/6π-cyclization/oxidation
reactions
of
1-(nitromethyl)-2-
aryltetrahydroisoquinolines in the presence of 1-aminoanthraquinone in order to obtain
tetracyclic 12-nitroindolo[2,1-a]isoquinoline products.19 Similarly, the same group also reported
an intermolecular version using 1,5-diaminoanthraquinone as a photocatalyst for synthetic
transformations of N-aryl-tetrahydroisoquinolines and nitromethane to prepare the same
product.
Anthraquinone (Aq) and its derivatives (Figure 1A) form coloured radical anions upon
single electron reduction (see Figure 1B for the absorption spectra of electrochemically
generated radical anion of 1,8-dihydroxyanthraquinone (Aq-OH)). The radical anions of
anthraquinones possess moderate reduction potentials in their ground state (e.g., E0(Aq/Aq•–)
and E0(Aq-OH/Aq-OH•–) are –0.82 V and –0.55 V vs SCE, respectively)27, and can be accessed
electrochemically, chemically, or via photoinduced single electron transfer processes in the
presence of a suitable electron donor under inert atmosphere (see Figure 3A). Eggins28 as well
as Lund and Eriksen29 showed that the excited states of coloured radical anions of
anthraquinone (Aq) (or in general quinone derivatives, see ref. 28) are powerful reductants and
are capable of transferring electrons to organic substrates that possess extremely negative
reduction potentials (e.g., 1,2-dibromobenzene, an aryl halide substrate with a reduction
potential of –1.88 V vs SCE28). Although the radical anions of quinone derivatives are known as
common intermediates in natural and synthetic photochemical energy storage,16, 30 they have not
been applied in photoredox catalytic reductive transformations so far. Knowing that the excited
28-29, 31
states of radical anions act as powerful reductants,12-13,
we envisioned employing
anthraquinone derivatives for the photoredox catalytic reduction of aryl halide substrates,
including aryl chlorides, to obtain aryl radicals either for metal-free dehalogenation reactions12,
or for synthetically important carbon–carbon12,
bond formation reactions. Such
32-35
32, 36-38
photoredox catalytic methods12-13, 32-39 are valuable alternatives to well-established transition
metal based activation methods. Herein, we report the use of 1,8-dihydroxyanthraquinone as
photocatalyst for synthetically important C–C bond formation reactions using aryl halides as
bench–stable starting materials and visible light.
2
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
Figure 1. (A) Chemical structures of anthraquinone derivatives investigated herein. (B)
Spectroelectrochemistry of Aq-OH in deaerated DMF. (C) Formation of 1,8-
dihydroxyanthraquinone semiquinone anion (Aq-OH-H–) in the presence of Na2S2O4.40 In the
inset, the luminescence spectrum (Ex = 427 nm) of Aq-OH-H– is shown. For the formation of
the radical anion and the semiquinone anion of Aq-OH upon photoirradiation in the presence of
Et3N see Figure 3.
We began our investigations with methyl 2-bromobenzoate (1a) as a test substrate and
1,8-dihydroxyanthraquinone as a photoredox catalyst. A mixture of 1a, Aq-OH (10 mol%), and
triethylamine (Et3N, 1.0 equiv, as a sacrificial electron donor and hydrogen atom donor, see
below) was irradiated in DMF with a blue LED (Ex = 455  15 nm) for 24h. Methyl benzoate
2a was obtained in 61% yield (see entry 1 in Table 1).41 Continuous irradiation of the same
reaction mixture for 48h gave 2a in 74% yield for ca. 90% substrate conversion. Control
experiments, i.e., omitting each of the individual catalytic components, confirmed that the
photocatalyst, electron donor, and most importantly light irradiation are necessary for the
photoredox catalytic dehalogenation reactions to take place (see entries 1–6 in Table 1). Among
other derivatives of anthraquinone, including Unisol Blue, used for microscopy, Aq-OH was
selected for further experiments due to the better chemical product yield (c.f., entries 1 and 11–
15 in Table 1). Natural occurring Frangulin A/B, derivatives of 1,8-dihydroxyanthraquinone,
perform as good as Aq-OH (entry 16) indicating a superior catalytic performance of this core
structure.
Having the optimized reaction conditions (see entries 1–2 and 8–10 in Table 1) in hand,
which requires simple mixing of aryl halides, commercially available inexpensive Aq-OH,
Et3N, and photoirradiation with blue LEDs under nitrogen, the scope of the method was
explored with different aryl halide substrates. Good to excellent photo-reduction yields (up to
87%) were obtained with aryl bromide substrates possessing different substituents, for example,
–CN, –CO2Me (–o, –m, and –p), –CHO, –COMe, and –CONH2 (see Figure 2), demonstrating
the mild nature of the photoredox catalytic reaction condition. Bromobenzene gave a poor
product yield due to its very high reduction potential. The yield increased with iodobenzene as
substrate (25%); 4-iodotoluene was reduced in 27% yield and 4′-iodoacetophenone in 60%
yield. Aryl chloride substrates with suitable reduction potentials (e.g., 2-chlorobenzonitrile, see
Figure 2 for reductive C–H arylation reactions using 2-chlorobenzonitrile) could also be
activated using this catalytic method. However, aryl halide substrates with very high reduction
potentials (e.g., 4-bromoanisole), could not be activated effectively for synthetically useful
transformations and describe the limit of the substrate scope of the reaction.
Table 1. Control reactions and optimization of the photoredox catalytic reduction condition
using methyl 2-bromobenzoate (1a) as a test substrate under visible light irradiation.[a]
Yield in %
Entry
Donor (equiv)
Catalyst (mol%)
Conditions
(% conv) [b]
Control reactions
1
2
3
4
5
6
7
Et3N (1.0)
Et3N (1.0)
–
Et3N (1.0)
–
Et3N (1.0)
Et3N (1.0)
Aq-OH (10)
Aq-OH (10)
–
455 nm, 24 h
455 nm, 48 h
455 nm, 48 h
455 nm, 48 h
455 nm, 48 h
Dark, 48 h
61 (80)
74 (90)
0 (1) [c]
–
≥ 1 (2) [c]
13 (24)
0 (0) [c]
Aq-OH (10)
Aq-OH (10)
Aq-OH (10)
455 nm, 48 h
9 (11) [d]
Optimization of the reaction condition
8
9
10
Et3N (1.0)
Et3N (1.0)
Et3N (0.7)
Aq-OH (2)
Aq-OH (5)
Aq-OH (10)
455 nm, 24 h
455 nm, 24 h
455 nm, 24 h
47 (58)
44 (54)
47 (64)
3
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
Different anthraquinone derivatives (see Figure 1A for chemical structures)
11
12
13
14
15
16
Et3N (1.0)
Et3N (1.0)
Et3N (1.0)
Et3N (1.0)
Et3N (1.0)
Et3N (1.0)
Aq (10)
2-EtAq (10)
2-NH2-Aq (10)
Aq-NH2 (10)
Unisol Blue (10)
Frangulin (10)
455 nm, 24 h
455 nm, 24 h
455 nm, 24 h
455 nm, 24 h
455 nm, 24 h
455 nm, 24 h
35 (62)
34 (62)
48 (76)
6 (18)
3 (16)
58 (74)
[a] Reactions were performed under nitrogen using 0.1 mmol methyl 2-bromobenzoate (1a)
in 1.0 mL of DMF with a blue LED (Ex = 455  15 nm. See supporting information for the
reaction setup). [b] Calculated from GC analysis (error ca. ± 5%) using the internal standard
method (see supporting information for details). In the parenthesis, substrate conversion is
[c]
[d]
shown. The yield is below the GC quantification limit. The reaction was performed
with 4-bromoanisole as substrate.
The photoredox catalytic single electron transfer (SET) reduction reactions proceed via
a radical mechanism as strongly supported by the experimental results and recent literature
42
reports.32,
Additionally, when a reaction mixture containing 4′-bromoacetophenone or 2-
bromobenzonitrile was irradiated in the presence of 1,1-diphenylethylene, a radical scavenger,43-
44
the coupling products 3n and 3o+3p were obtained in 42% and 58% isolated yields (see
Figure 2 and supporting information for further details), respectively, providing further evidence
for the radical nature of the reaction.
Reduction of aryl halides
X; X = Br, I
H
Aq-OH, Et3N
R
R
DMF, 40 0C
455 nm
Aryl bromides
Aryl iodides
H
H
H
H
O
H
H
H
H
H
O
H
O
NH2
O
2a, –o, 74 (90)
2b, –m, 87 (99)
2c, –p, 51(71)
O
O
N
2e, –o, 56(95)
2d, 80 (95) 2f, –p, 85 (94)
*
2g, 40 (52)
2h, 63 (95)
2i, 4 (32)
2j, 60
2k, 27 (53)
2l, 25 (59)
substrate conversion was not determined
*
C–H arylation reactions using aryl halides
Trap
X; X = Br, Cl, I
Aq-OH, DIPEA
Trap
+
R
R
DMSO, 25 0
455 nm
C
O
N
N
N
N
N
N
N
R
O
CN
NH2
N
O
O
O
O
CN
F3C
O
O
3e, R= Me, 60% (65% *)
3f, R= H, 66% *
3a, 56%
3b, 51%
3c, 36%
3d, 36%
3h, 35%
3i, 31%**
3g, 60% *
N
O
O
O
S
NC
NC
NC
CN
3o
3p
O
3j, 44%
3k, 19%
3l, 21%
3m, 14%
3n, 42%
58%, 3o:3p = 3:1
*
**
The reaction was performed with 2-chlorobenzonitrile
The reaction was performed with 4-Iodotoluene
Figure 2. Photoredox catalytic reduction yields and the yields of C–C bond forming reactions
using aryl halides as substrates (Ar–X; where, X = I, Br, Cl). For photoreduction reactions,
substrate conversions after 48h are shown in the parenthesis (see also supporting information).
4
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
With these results, we explored the photoredox catalytic generation of aryl radicals from
their respective halides for C–C bond forming reactions using arenes, heteroarenes, and
unsaturated double bonds. The main challenge in this case, which was also observed for other
photoredox catalytic C–H arylation reactions using aryl halides,12-13, 32, 36-38 is the competing
hydrogen atom abstraction by the reactive aryl radicals either from the radical cation of the
amine or from the solvent leading to the reduction product. However, when a reaction mixture
containing 2-bromobenzonitrile, Aq-OH, DIPEA, and benzene was irradiated with blue LEDs,
the desired product 3m was obtained in 14% yields.45 Similarly, when furan and thiophene were
introduced as the radical trapping reagent, the desired products 3k and 3l were obtained in
useful yields. Note that thiophene and furan efficiently trap aryl radicals under photoredox
conditions in redox neutral reactions, in which the excited state of the photocatalyst transfers an
electron to the aryl radical precursor and no sacrificial electron donor is necessary (e.g.,
photoredox catalytic generation of aryl radicals from their respective diazonium salts11, 32).
However, they consistently fail to trap aryl radicals generated from the respective aryl halides
under other reported photoredox catalytic conditions. For instance, it has recently been reported
that thiophene and furan fail to trap 2-benzothiazole radicals generated via single electron
transfer from the respective bromides under photoredox catalytic conditions in the presence of
fac-Ir(ppy)3 and R3N as an electron donor.36 The reason for this is still unclear and needs further
detailed investigations. The Aq-OH–based photoredox catalytic system allows the C–H
arylations of thiophene and furan, although in low ca. 20% isolated yields. When 1,3,5-
trimethoxybenzene, a substituted arene, was used as a trapping reagent for the 2-cyanophenyl
radical, the expected product was obtained in 60% isolated yield. With pyrrole derivatives as
trapping reagents, the C–H arylated products were isolated in moderate to good yields. Notably,
unprotected pyrrole reacts cleanly with the aryl radicals. All investigated heterocycles (i.e.,
furan, thiophene, and pyrrole derivatives) were selectively arylated at the 2-position (see Figure
2). C–H functionalization reaction of 1,1-diphenylethylene with 2-bromobenzonitrile gave 3n in
42% yield. 4'-Bromoacetophenone gave a mixture of 3p and 3q in 58% isolated yield
confirming that both hydrogen atom abstraction and the oxidation reaction take place
subsequent to the aryl radical addition step (see Figure S3). The C–H arylation reactions are
also effective when aryl chlorides were used as the precursors of aryl radicals. Using 2-
chlorobenzonitrile as substrate compounds 3e–3g were obtained in good isolated yield.
5
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
Figure 3. (A) Changes in the absorption spectra of Aq-OH upon photoirradiation (Ex = 455 
15 nm) in the presence of Et3N under nitrogen in DMF. The green and blue arrows indicate the
absorption maxima of Aq-OH•– and Aq-OH-H–, respectively. For the photoirradiation
experiment in DMSO see Figure S2. (B) Changes in the fluorescence spectra of 1,8-
dihydroxyanthraquinone semiquinone anion (Aq-OH-H–) upon successive addition of methyl 2-
bromobenzoate (1a, test substrate). In the inset, changes in the absorption spectra of Aq-OH-H–
upon successive addition of 1a are shown. (C) Alternative luminescence quenching titration plot
of Aq-OH-H– with 4-bromoanisole (green curves) and 2-bromo-4-(trifluoromethyl)pyridine
(blue curves) is shown (see the Stern-Volmer quenching plot in Figure (D)). In the inset changes
in the absorption spectra of Aq-OH-H– upon addition of 2-bromo-4-(trifluoromethyl)pyridine
and 4-bromoanisole are shown. (D) Stern-Volmer quenching plot of Aq-OH-H– with methyl 2-
bromobenzoate, 2-bromo-4-(trifluoromethyl)pyridine, and the fluorescence response (in this
case, intensity) of Aq-OH-H– upon alternating addition of 2-bromo-4-(trifluoromethyl)pyridine
and 4-bromoanisole are shown (see the mechanism section and supporting information for more
details). (E) Chemical structures along with their respective reported absorption maxima27 of
photo- and redox-active species of Aq-OH are depicted.
The presence of different photo- and redox-active species (including all intermediates in
the reaction mixture) under the synthetic reaction conditions, and the challenges of performing
spectroscopic and electrochemical experiments under non-idealized reaction conditions do not
allow us to report a complete mechanism. However, based on the literature reports28-29, 32, 42 and
spectroscopic investigations (see Figure 1 and 3 and supporting information) a proposed
mechanism is depicted in Figure 4. The reaction did not yield any photoreduction product (see
entry 6 in Table 1) or C–H arylated products in the dark. The absence of an effective
6
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
luminescence quenching of Aq-OH in the presences of methyl 2-bromobenzoate demonstrates
that the excited state of Aq-OH is not responsible for initiating the single electron transfer to
aryl halides (Figure S1). Upon visible light photoexcitation, the excited state of Aq-OH is
quenched by Et3N generating the radical anion Aq-OH•– (and the radical cation of Et3N, i.e.,
Et3N •+) that upon protonation and successive reduction (or vice versa via a consecutive or a
coupled step) forms the semiquinone anion, Aq-OH-H–. The presence of both Aq-OH•– and
Aq-OH-H– species was confirmed by irradiating a mixture of Aq-OH and Et3N under nitrogen
in DMF and in DMSO (see Figure 3A and Figure S2). Notably, the electron transfer from the
ground state radical anion of anthraquinone or the semiquinone anion to the aryl halide
substrates is thermodynamically unlikely due to their intrinsic low ground state reduction
potentials (c.f., E0(Aq-OH/Aq-OH•– = ca. – 0.5 V vs SCE and ref. 46). The semiquinone anion
Aq-OH-H– could be generated in the presence of Na2S2O4 under nitrogen (see Figure 1) and is
luminescent.40, 47 The absorption spectra of Aq-OH-H– did not change in the presence of methyl
2-bromobenzoate, but the Aq-OH-H– luminescence was significantly quenched (Figure 3). The
luminescence of Aq-OH-H– is also quenched in the presence of 2-bromo-4-
(trifluoromethyl)pyridine (another investigated substrate), but was not effected significantly by
the presence of 4-bromoanisole (a substrate that is difficult to activate under the photoredox
catalytic condition (see Figure 3). The alternating quenching experiment (see Figure 3), in
which 2-bromo-4-(trifluoromethyl)pyridine and 4-bromoanisole were added alternatively, gave
the expected luminescence responses (i.e., when 4-bromoanisole was added the luminescence
was unaltered, but upon addition of 2-bromo-4-(trifluoromethyl)pyridine in the same solution
the luminescence was quenched). Upon photoexcitation, the radical anion of Aq-OH is capable
of transferring an electron to the investigated aryl halide substrate. Previous work28-29 on the
luminescence quenching of the excited state of Aq•– by aryl halides supports this proposal.
X; X=Br, Cl, I
*
OH O OH
R
*
OH O OH
O
OH
R
h
*
X; X=Br, Cl, I
R
OH O OH
h

OH O OH
O
Et3N
– X
+H+
+e–
OH O OH
Et3N
O
Aq-OH
H
–
+ H
OH
–
R
R
h
–e–
–H+
Aq-OH
Trap
Aq-OH
OH O OH
OH O OH
Trap
H
R
OH
O
Aq-OH
Figure 4. Proposed mechanism of the photoredox catalytic cycle for the photoreduction of aryl
halides and C–C bond formation reactions using suitable trapping reagents.
Upon single electron transfer to aryl halides (Ar–X, where X = Cl, Br, and I), the
generated Ar–X•– subsequently fragments to yield aryl radicals that either abstract a hydrogen
atom from the radical cation of R3N (i.e., Et3N•+ or DIPEA•+) or from the solvent molecules (in
this case, DMF or DMSO) to yield the reduction product, or reacts with (hetero)aryls or
unsaturated double bonds yielding the corresponding C–C coupling products after oxidation and
release of a proton (Figure 4). Gas chromatography–mass spectrometry (GC-MS) analysis of the
crude product mixture confirmed the formation of diethylamine (see Figure S3 in SI). During
the course of the photoreaction, Aq-OH degrades. Such bleaching is often observed in
7
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
photoredox catalytic reactions using organic dyes44 or transition metal photocatalysts.48 In
addition, the formation of substituted Aq-OH from a reaction with the aryl radicals was
observed.49 These photoproducts can continue to participate in the photoredox transformations.50
Radical anions and semiquinone anions of anthraquinones, a class of quinoid
compounds, gain sufficient redox energy under the visible light irradiation to perform
synthetically challenging reductive transformations. 1,8-Dihydroxyanthraquinone (Aq-OH)
forms its coloured radical anion and semiquinone anion upon visible light irradiation in the
presence of R3N under nitrogen. The radical anion and the semiquinone anion are excited by
visible light and transfer a single electron to aryl halides, which react in dehalogenation or C–C
bond forming reactions. We believe that the described results will pave the way to a broader use
of commercially available and inexpensive anthraquinone derivatives in synthetic photoredox
catalytic reductive transformations.
Experimental Section
General procedure for the photoreduction of aryl halides (Method A): In a 5 mL snap vial,
equipped with a magnetic stirring bar, the respective aryl halide (0.1 mmol, 1.0 equiv) and
catalyst (0.01 mmol, 0.10 equiv) were dissolved in dry DMF (total volume of the solution 1.0
mL) and the resulting mixture was degassed ( 2) via a syringe needle. Triethylamine (Et3N,
0.10 mmol, 1.0 equiv) was added under N2, and the reaction mixture was irradiated through the
plane bottom side of the snap vial at 40 C using a 455 nm LED. The reaction progress was
monitored by GC analysis. The yields of the reduction products were determined from GC
measurements using appropriate internal standards.
General procedure for C–H arylation reactions (Method B): In a 5 mL snap vial, equipped
with a magnetic stirring bar, the aryl halide (0.2 mmol, 1.0 equiv) and 1,8-dihydroxy-9,10-
anthraquinone (Aq-OH, 0.02 mmol, 0.1 equiv) were dissolved in dry DMSO (total volume of
the solution 1.0 mL), and the resulting mixture was degassed by syringe needle. N,N-
diisopropylethylamine (DIPEA) (0.20–0.40 mmol, 1–2 equiv) and the corresponding trapping
agent (4.0–6.0 mmol, 20–30 equiv) were added under N2 and the reaction mixture was
irradiated through the plane bottom side of the snap vial at 25 C using a 455 nm LED for the
time reported below. The reaction progress was monitored by GC analysis until full conversion
was obtained or the reactions slow down significantly. For work-up, one of the two following
procedures were used: (i) the reaction mixture was transferred into a separating funnel and 10
mL of distilled water and 2 mL of brine were added. The resulting mixture was extracted with
ethyl acetate (3  10 mL). The combined organic layers were dried over MgSO4, filtered and
concentrated in vacuum. Silica gel (ca. 1.0 g) was added to prepare the dry load for column
chromatography. (ii) The reaction mixture was transferred into a flask and DMSO was partially
evaporated (approx. 0.5 mL) under reduced pressure, ethyl acetate (10 mL) and silica gel (ca.
1.0 g) was added to prepare the dry load for column chromatography. In both methods,
purification was achieved by flash column chromatography using petrol ether/ethyl acetate as
eluents on silica gel.
For the additional spectroscopic investigations and characterization of C–H arylated products
see supporting information.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (GRK 1626) for financial support. JIB thanks
the Alexander von Humboldt foundation for a scholarship. JIB thanks Consejo Nacional de
Investigaciones Científicas y Técnicas (CONICET) and Agencia Nacional de Promoción
Científica y Técnica (ANPCyT) for financial support. MS thanks BAYLAT and the DAAD for
8
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
financial support. We thank Dr. R. Vasold and Ms. Regina Hoheisel for GC–MS and CV
measurements respectively.
Notes and References
1.
2.
3.
4.
5.
6.
7.
8.
9.
D. A. Nicewicz and D. W. C. MacMillan, Science, 2008, 322, 77-80.
C. K. Prier, D. A. Rankic and D. W. C. MacMillan, Chem. Rev., 2013, 113, 5322-5363.
C. Stephenson and T. Yoon, Acc. Chem. Res., 2016, 49, 2059-2060.
B. König, Eur. J. Org. Chem., 2017, DOI: 10.1002/ejoc.201700420, 1979-1981.
T. P. Yoon, M. A. Ischay and J. N. Du, Nat. Chem., 2010, 2, 527-532.
M. H. Shaw, J. Twilton and D. W. C. MacMillan, J. Org. Chem., 2016, 81, 6898-6926.
J. M. R. Narayanam and C. R. J. Stephenson, Chem. Soc. Rev., 2011, 40, 102-113.
N. A. Romero and D. A. Nicewicz, Chem. Rev., 2016, 116, 10075-10166.
For an example of using quantum dots in visible light photoredox transformations see A.
Pal, I. Ghosh, S. Sapra and B. König, Chem. Mater., 2017, 29, 5225−5231.
10. M. Neumann, S. Fuldner, B. König and K. Zeitler, Angew. Chem. Int. Ed., 2011, 50, 951-
954.
11. D. P. Hari and B. König, Chem. Commun., 2014, 50, 6688-6699.
12. I. Ghosh, T. Ghosh, J. I. Bardagi and B. König, Science, 2014, 346, 725-728.
13. I. Ghosh and B. König, Angew. Chem. Int. Ed., 2016, 55, 7676-7679.
14. A. Das, I. Ghosh and B. König, Chem. Commun., 2016, 52, 8695-8698.
15. F. Z. Su, S. C. Mathew, L. Mohlmann, M. Antonietti, X. C. Wang and S. Blechert, Angew.
Chem. Int. Ed., 2011, 50, 657-660.
16. N. El-Najjar, H. Gali-Muhtasib, R. A. Ketola, P. Vuorela, A. Urtti and H. Vuorela,
Phytochem. Rev., 2011, 10, 353-370.
17. H. Nohl, W. Jordan and R. J. Youngman, Adv. Free Radical Bio., 1986, 2, 211-279.
18. J. M. Lu, S. V. Rosokha, I. S. Neretin and J. K. Kochi, J. Am. Chem. Soc., 2006, 128,
16708-16719.
19. F. Rusch, L. N. Unkel, D. Alpers, F. Hoffmann and M. Brasholz, Chem. Eur. J., 2015, 21,
8336-8340.
20. S. Lerch, L. N. Unkel and M. Brasholz, Angew. Chem. Int. Ed., 2014, 53, 6558-6562.
21. N. Tada, Y. Ikebata, T. Nobuta, S. I. Hirashima, T. Miura and A. Itoh, Photochem.
Photobiol. Sci., 2012, 11, 616-619.
22. C. W. Kee, K. F. Chin, M. W. Wong and C. H. Tan, Chem. Commun., 2014, 50, 8211-
8214.
23. K. Ohkubo, A. Fujimoto and S. Fukuzumi, J. Am. Chem. Soc., 2013, 135, 5368-5371.
24. L. Cui, Y. Matusaki, N. Tada, T. Miura, B. Uno and A. Itoh, Adv. Synth. Catal., 2013,
355, 2203-2207.
25. B. Chang, H. G. Shao, P. Yan, W. Z. Qiu, Z. G. Weng and R. S. Yuan, ACS Sustain.
Chem. Eng., 2017, 5, 334-341.
26. N. Tada, K. Hattori, T. Nobuta, T. Miura and A. Itoh, Green Chem., 2011, 13, 1669-1671.
27. R. M. Wightman, J. R. Cockrell, R. W. Murray, J. N. Burnett and S. B. Jones, J. Am.
Chem. Soc., 1976, 98, 2562-2570.
28. B. R. Eggins and P. K. J. Robertson, J. Chem. Soc. Faraday Trans., 1994, 90, 2249-2256.
29. P. Nelleborg, H. Lund and J. Eriksen, Tetrahedron Lett., 1985, 26, 1773-1776.
30. A. R. Cook, L. A. Curtiss and J. R. Miller, J. Am. Chem. Soc., 1997, 119, 5729-5734.
31. D. Gosztola, M. P. Niemczyk, W. Svec, A. S. Lukas and M. R. Wasielewski, J. Phys.
Chem. A, 2000, 104, 6545-6551.
32. I. Ghosh, L. Marzo, A. Das, R. Shaikh and B. König, Acc. Chem. Res., 2016, 49, 1566-
1577.
33. J. D. Nguyen, E. M. D'Amato, J. M. R. Narayanam and C. R. J. Stephenson, Nat. Chem.,
2012, 4, 854-859.
34. H. Kim and C. Lee, Angew. Chem. Int. Ed., 2012, 51, 12303-12306.
9
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10.1002/ejoc.201701461
European Journal of Organic Chemistry
35. E. H. Discekici, N. J. Treat, S. O. Poelma, K. M. Mattson, Z. M. Hudson, Y. D. Luo, C. J.
Hawker and J. R. de Alaniz, Chem. Commun., 2015, 51, 11705-11708.
36. A. Arora and J. D. Weaver, Org. Lett., 2016, 18, 3996-3999.
37. S. O. Poelma, G. L. Burnett, E. H. Discekici, K. M. Mattson, N. J. Treat, Y. D. Luo, Z. M.
Hudson, S. L. Shankel, P. G. Clark, J. W. Kramer, C. J. Hawker and J. R. de Alaniz, J.
Org. Chem., 2016, 81, 7155-7160.
38. I. Ghosh, R. S. Shaikh and B. König, Angew. Chem. Int. Ed., 2017, 56, 8544–8549.
39. M. Majek, U. Faltermeier, B. Dick, R. Perez-Ruiz and A. Jacobi von Wangelin, Chem.
Eur. J., 2015, 21, 15496-15501.
40. For the chemical generation of the semiquinone anion we recommend preparing a fresh
Na2S2O4 solution under inert atmosphere. See SI for details.
41. See supporting information for the photochemical reaction setup and general procedures.
42. C. Costentin, M. Robert and J. M. Saveant, J. Am. Chem. Soc., 2004, 126, 16051-16057.
43. W. Liu, H. Cao, H. Zhang, H. Zhang, K. H. Chung, C. A. He, H. B. Wang, F. Y. Kwong
and A. W. Lei, J. Am. Chem. Soc., 2010, 132, 16737-16740.
44. A. Graml, I. Ghosh and B. König, J. Org. Chem., 2017, 82, 3552-3560.
45. For the C–H arylation reactions, the use of DMSO as solvent and N,N-diisopropyl-
ethylamine (DIPEA) as the sacrificial electron donor provided better chemical yields.
46. Electrochemical experiments in ref. J. Am. Chem. Soc. 1976, 98, 2562 indicate that
E0(Aq-OH-H•/Aq-OH-H– < – 0.5 V vs SCE).
47. Note that the absorption maxima of Aq-OH-H–, generated in the presence of Na2S2O4, is
consistant with the reported values. See ref. 27.
48. J. J. Devery, J. J. Douglas, J. D. Nguyen, K. P. Cole, R. A. Flowers and C. R. J.
Stephenson, Chemical Science, 2015, 6, 537-541.
49. Under the photoredox catalytic reaction conditions, substitution products were confirmed
by HPLC and HPLC/MS analysis.
50. At present the role of such photoproduct/s in determining the kinetics or chemical yield is
unclear.
10
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