The Dual Role of Benzophenone in Visible-Light/Nickel Photoredox-Catalyzed C?H Arylations: Hydrogen-Atom Transfer and Energy Transfer

Article abstract of DOI:10.1002/anie.201901327

A dual catalytic protocol for the direct arylation of non-activated C(sp³)?H bonds has been developed. Upon photochemical excitation, the excited triplet state of a diaryl ketone photosensitizer abstracts a hydrogen atom from an aliphatic C?H bond. This inherent reactivity was exploited for the generation of benzylic radicals which subsequently enter a nickel catalytic cycle, accomplishing the benzylic arylation.

Full text of DOI:10.1002/anie.201901327

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International Edition: DOI: 10.1002/anie.201901327
German Edition: DOI: 10.1002/ange.201901327
Photochemistry
The Dual Role of Benzophenone in Visible-Light/Nickel Photoredox-
À
Catalyzed C H Arylations: Hydrogen-Atom Transfer and Energy
Transfer
Abhishek Dewanji+, Patricia E. Krach+, and Magnus Rueping*
Abstract: A dual catalytic protocol for the direct arylation of
a photosensitizer itself for the H-atom abstraction. Organic
compounds,[4a,6] as well as inorganic complex structures,[4d,5f,7]
are known for their ability to abstract H-atoms upon photo-
chemical excitation. Among the organic compounds applied,
diaryl ketones have long served this purpose, owing to their
accessible triplet excited states. Therefore, we decided to
engage this class of compounds in catalytic amounts as
3
À
non-activated C(sp ) H bonds has been developed. Upon
photochemical excitation, the excited triplet state of a diaryl
ketone photosensitizer abstracts a hydrogen atom from an
À
aliphatic C H bond. This inherent reactivity was exploited for
the generation of benzylic radicals which subsequently enter
a nickel catalytic cycle, accomplishing the benzylic arylation.
3
[8]
À
organic photosensitizers (PS) for the C(sp ) H arylation.
À
T
he direct functionalization of inert C H bonds facilitates
the access to complex molecular structures in organic syn-
Diarylmethane moieties are important structural motifs in
bio-active compounds, drug molecules, and organic materi-
als.[9] Hence, methods for their facile synthesis are in great
demand. We therefore aimed at developing a general protocol
for the benzylic arylation using visible-light catalysis and at
the same time expanding the application of diaryl ketones in
dual photoredox/transition-metal catalysis.[10,11] Our mecha-
nistic hypothesis is depicted in Scheme 1.
À
thesis. Hence, the direct modification of C H bonds has been
extensively investigated in the last two decades. Whereas
transition metal-catalyzed processes, with the assistance of
directing groups, have been widely used for the activation of
2
[1]
3
À
À
C(sp ) H bonds, applying this strategy to C(sp ) H bonds is
more challenging.[2] Nevertheless, organic free-radical
chemistry has been utilized as an alternative tool for the
[3]
À
functionalization of inert aliphatic C H bonds.
Conventionally, harsh reaction conditions employing
strong acids, peroxides or high-energy UV light are necessary
for the generation of C(sp3) radicals. The recent progress in
the field of visible light-mediated photoredox-catalyzed
reactions allows easier access to C(sp3)-centered radical
intermediates and their subsequent functionalization.[4] Fur-
thermore, the combination of photocatalysis and nickel
catalysis has emerged as an elegant approach for the direct
3
[5]
À
arylation of unactivated C(sp ) H bonds. For example,
MacMillan and co-workers reported a protocol for the a-
arylation of cyclic and acyclic amines, lactams, and ureas.[5a]
The groups of Doyle[5b] and Molander[5c] described the a-
arylation of ethers. Recently, our group successfully achieved
3
[5g]
À
the arylation of allylic C(sp ) H bonds.
The H-atom abstraction agent, in these reports, has been
a transient radical species generated either through electron
transfer or via energy transfer by the photocatalyst. We
considered simplifying this step by using the excited state of
Scheme 1. Mechanistic hypothesis.
[*] Dr. A. Dewanji,[+] P. E. Krach,[+] Prof. Dr. M. Rueping
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
The triplet PS*, formed upon photochemical excitation of
the ground state PS, initiates the reaction via H-atom
abstraction from toluene. The resulting benzyl radical is
trapped by an in situ-generated Ni0 species A to form B. The
NiI species B oxidatively adds to the haloarene substrate and
the resulting NiIII complex C subsequently produces the
desired coupling product via reductive elimination. This
step leads also to the formation of D which, upon base-
assisted reduction by the reduced PS (PS-H), gives A, closing
E-mail: magnus.rueping@rwth-aachen.de
Prof. Dr. M. Rueping
KAUST Catalysis Center (KCC), King Abdullah University of Science
and Technology (KAUST)
Thuwal, 23955-6900 (Saudi Arabia)
E-mail: magnus.rueping@kaust.edu.sa
[+] These authors contributed equally to this work.
the catalytic cycle (Ered [NiI/Ni0] = À1.10 V vs. Ag/AgCl in
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.201901327.
red
DMF;[12] E1/2
[PhCOPh] = À2.20 V vs. Ag/AgNO3 in
MeCN[13]). An alternative mechanism in which the oxidative
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addition of Ni0 species to the aryl halide precedes the radical
trapping step might also be operating.[14]
For the initial experiments, we selected 4,4’-dichloroben-
zophenone (3) (E1/2red [3] = À2.09 V vs. Ag/AgNO3 in MeCN,
lmax = 344 nm) (see the Supporting Information) as photo-
sensitizer, as it was recently successfully used by Guin and co-
workers, in combination with household CFL lamps, in the
3
[6h]
À
C(sp ) H alkenylation/alkynylation reaction.
Initially, 4-
bromobenzonitrile (2a) was irradiated, in the presence of 3
(25 mol%), NiCl2·6H2O (5 mol%), 4,4’-di-tert-butyl-2,2’-
dipyridyl (dtbbpy) as ligand (5 mol%), and K2HPO4 as base
(2 equiv), with two 23 W CFL lamps for 18 hours, in toluene
as solvent. The reaction setup was cooled with a fan from the
top to keep the reaction temperature constant at approx-
imately 358C (see the Supporting Information). Pleasingly,
the desired coupling product 4a was formed in 51% yield
(Table 1, entry 1). The reaction reached full conversion with
further dilution of the reaction mixture from 0.35m to 0.2m
Table 1: Optimization of the reaction conditions.[a]
Entry
2a (X mmol)
3 (Y mol%)
Conc. [m]
Yield [%][b]
Figure 1. Substrate scope of aryl bromides. [a] Reaction conditions: 2
(0.2 mmol), 3 (0.05 mmol, 0.25 equiv), NiCl2·6H2O (0.01 mmol,
0.05 equiv), dtbbpy (0.01 mmol, 0.05 equiv), K2HPO4 (0.4 mmol,
2.0 equiv), 1 (6 mL), under irradiation with two 23 W CFL lamps,
under Ar, 48 h; yields after purification. [b] 4 mL of toluene were used.
[c] 18 h. [d] 24 h. [e] Amount of inseparable bibenzyl impurity is given
in parenthesis.
1
2
3
4
5
0.35
0.2
0.2
0.2
0.2
25
25
20
10
5
0.12
0.05
0.05
0.05
0.05
51
94[c]
100
48
37
[a] The reaction mixture in toluene was irradiated with two household
CFL lamps from two sides for 18 h under Ar. [b] Calculated, with CH2Br2
as internal standard, from NMR spectra. [c] Yield after purification.
took place only at the C–Br bond (4e, 62% yield). Fluorine as
substituent was tolerated as well and the corresponding
product 4 f was isolated in 71% yield. However, the yield
dropped to 41% when the substituent at the para position was
changed to a phenyl group (4g). To our delight, electron-rich
substrates were also suitable for this reaction. 4-Bromoto-
luene gave the desired product 4h as an inseparable mixture
with trace amount of bibenzyl, which was formed as a result of
the homocoupling of the benzyl radicals (Scheme 2b). 1-
Bromo-4-(tert-butyl)benzene as substrate showed similar
reactivity (68% yield, 4i). Compound 4j was obtained in
excellent yield (92%) when 4-bromoanisole reacted with
toluene. The reaction between bromobenzene and toluene
furnished diphenylmethane (4k) in 55% yield. Pleasingly,
increased steric hindrance posed by meta and ortho substitu-
ents did not inhibit the reaction (4l–4n). To be noted, radical
species were never formed at the benzylic position of the
bromotoluenes (2h, 2l, and 2n). This chemoselectivity can be
attributed to the statistical leverage of 1 over the limiting
reagents.
and gave 4a in 94% yield after purification (Table 1, entry 2).
Furthermore, 20 mol% of 3 was also sufficient to achieve full
reaction efficiency (Table 1, entry 3). A further decrease in
the amount of photosensitizer led to diminished yields
(Table 1, entries 4 and 5).
Control experiments proved that all reagents, as well as
light-irradiation, are indispensable for the reaction to proceed
(see the Supporting Information). Furthermore, a wide range
of diaryl ketones and similar compounds were evaluated as
photosensitizers (see the Supporting Information). Interest-
ingly, among the diaryl ketones tested, 3 turned out to be the
most efficient candidate for our reaction (see the Supporting
Information).
With the optimized conditions in hand, we proceeded to
explore the scope of the reaction. First, the reactivity of
different bromoarenes was tested in the reaction with toluene
(Figure 1). Aryl bromides bearing electron-withdrawing
groups (CF3, Ac, or CO2Me) in the para position reacted
efficiently to give the corresponding desired products 4b–4d
in good yields (53–79%). Notably, chemoselectivity was
achieved in the case of 4-chlorobromobenzene as the reaction
Substrates 2p–2r with fused rings were also tolerated and
the products 4p–4r were obtained, respectively, in good to
excellent yields. Furthermore, a heteroaromatic bromide
2
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Figure 2. Substrate scope of aryl iodides and chlorides. [a] Reaction
conditions: 5/6 (0.2 mmol), 3 (0.05 mmol, 0.25 equiv), NiCl2·6H2O
(0.01 mmol, 0.05 equiv), dtbbpy (0.01 mmol, 0.05 equiv), K2HPO4
(0.4 mmol, 2.0 equiv), 1 (6 mL), under irradiation with two 23 W CFL
lamps, under Ar, 48 h; yields after purification. [b] In the case of aryl
iodides, TBAB (0.2 mmol, 1.0 equiv) was added. [c] Amount of
inseparable bibenzyl impurity in parenthesis. [d] Calculated, with
CH2Br2 as internal standard, from NMR spectra.
with our assumption, the yield of 4a increased to 72%. The
same strategy was applied to other iodoarenes 5 and all of
them worked quite well, furnishing the desired products in
good yields.
When 4-chlorobenzonitrile was reacted with toluene for
48 hours, the desired product was isolated in 83% yield.
However, substrates bearing acyl and carboxymethyl groups
showed moderate reactivity affording the products 4c and 4d
with 61% and 36% NMR yield. A di-substituted aryl chloride
was also tested and gave the corresponding product 4u in
66% yield. Similar efficiency (68%) was observed when 2-
chloro-6-methoxypyridine was reacted with toluene under the
standard conditions. Finally, 2-chloro-5-fluoropyridine also
reacted in moderate yield. It is worth mentioning that while
exploring the chloro-substrate scope, sometimes the photo-
sensitizer 3 was also partially benzylated at the C–Cl bond,
leading to a mixture of monobenzyl and dibenzyl benzophe-
nones. A control experiment was conducted replacing 3 with
4-chloro-4’-benzyl benzophenone, which gave a decreased
yield of 57% (Supporting Information, Table S1, entry 15).
This observation provides a rationale for diminished yields in
Scheme 2. Scale-up and mechanistic insight.
substrate, 2-bromo-6-cyanopyridine 2s, furnished the prod-
uct 4s in almost quantitative yield (99%).
After exploring the scope of the reaction with respect to
the structure of the bromo-containing substrates, we turned
our attention to the iodo and chloro analogues (Figure 2).
Unexpectedly, 4-iodobenzonitrile (5a) exhibited much lower
reactivity (39% NMR yield) compared to its bromo ana-
logue 2a. This was surprising because the aryl–I bond should
be more reactive towards oxidative addition compared to the
aryl–Br bond.[15] This observation led us to consider an
underlying parallel propagation mechanism where 3 acts as
a triplet sensitizer (Scheme 2c).[16]
Recently, our group and the Molander group proposed an
energy-transfer step from the photocatalyst to a NiII species
leading to an excited NiII species, which undergoes subse-
quent homolysis to give NiI and a halide radical.[5c,g] The
halide radical can then abstract a H-atom from the H-donor.
To find support for this hypothesis, and in pursuit of an
improved scope of iodoarenes, the reaction between 5a and
toluene (1) was conducted in the presence of 1 equivalent of
tetrabutyl ammonium bromide (TBAB) as an additive. In line
À
the case of chloroarenes. Next, other C H bond-donating
coupling partners were explored in reaction with bromo-
derivative 2a (Figure 3). Para-xylene reacted smoothly to
produce diarylmethane 12a in 89% yield in 24 hours. Meta
and ortho isomers of xylene gave similar yields as well (82%
and 87%, respectively). Following the same trend, mesitylene
showed excellent reactivity with 85% yield for product 12d.
Furthermore, 12e was formed in a good yield (67%) within
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the photocatalyst plays an additional role of energy-transfer
agent. The newly designed transformation tolerates a broad
range of aryl halides and different arenes containing methyl
groups, leading to diarylmethanes in good yields. Notably, the
reaction could be scaled up with ease without the need of
specialized equipment.
Acknowledgements
P.E.K. thanks the Deutsche Bundesstiftung Umwelt for
financial support. We thank Marc Jentsch for helping us
with CV measurements. The research leading to these results
has received funding from the European Research Council
under the European Unionꢀs Seventh Framework Pro-
gramme (FP/2007-2013)/ERC Grant Agreement no. 617044
(SunCatChem).
À
Figure 3. C H bond-donating coupling partners. [a] Reaction condi-
tions: 2a (0.2 mmol), 3 (0.05 mmol, 0.25 equiv), NiCl2·6H2O
(0.01 mmol, 0.05 equiv), dtbbpy (0.01 mmol, 0.05 equiv), K2HPO4
(0.4 mmol, 2.0 equiv), 7–11 (6 mL) under irradiation with two 23 W
CFL lamps, under Ar, 48 h; yields after purification. [b] 24 h. [c] o-
Xylene (4 mL) was used. [d] Mesitylene (5 mL) and benzene as co-
solvent (1 mL) were used. [e] 3 (20 mol%), 4-chlorotoluene (4 mL),
20 h.
Conflict of interest
The authors declare no conflict of interest.
20 hours when relatively electron-deficient 4-chlorotoluene
was reacted with 2a.
À
Keywords: atom transfer · benzophenone · C
H functionalization · energy transfer · metallaphotoredox
To examine the scalability of the reaction, the trans-
formation with substrate 2a was performed on a larger scale
(1.0 mmol) with the same reaction setup (Scheme 2a). Pleas-
ingly, the yield of 4a did not decrease significantly with the
scale-up. It is noteworthy that a flow-reactor, which is often
used for scaling up photoredox-catalyzed transformations,
was not necessary in this case. Finally, further experiments
were performed to gain a deeper insight into the mechanism.
The involvement of the benzyl radical in the reaction was
confirmed by the formation of bibenzyl (13) as a side-product.
Regarding the generation of the benzyl radicals, this process
can be accomplished either by the photocatalyst with the
formation of PS-H (Scheme 1), which is subsequently
involved in the generation of Ni0, or by a chlorine radical,
which results from the homolysis of the excited state of the
ligated NiCl2 complex by the light-source.[17]
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To address these questions, an equimolar mixture of
NiCl2·6H2O and dtbbpy (0.05 mmol) in toluene (6 mL) was
irradiated for 24 hours (Scheme 2d). Formation of bibenzyl
was not detected in the NMR spectra after the reaction. When
the same experiment was performed with 3, 0.022 mmol of
bibenzyl (arising from the homocoupling of benzyl radicals)
and 0.017 mmol of benzopinacol 14 were detected by NMR
spectroscopy after the reaction. Therefore, we conclude that
diaryl ketone 3 serves as initiator as well as catalyst for this
reaction. This, together with the improved yields for iodoar-
enes after the addition of TBAB, leads to the conclusion that 3
acts as both hydrogen-atom-transfer agent and energy-trans-
fer agent.
In summary, we developed a protocol for the benzylic
arylation of toluene, and derivatives, with aryl halides as
arylating agents using a dual metal/ photocatalytic system.
Benzophenone was applied as photocatalyst, serving both
purposes of hydrogen-atom-transfer and electron-transfer
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À
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Manuscript received: October 1, 2018
Version of record online: && &&, &&&&
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Photochemistry
A. Dewanji, P. E. Krach,
M. Rueping*
&&&& — &&&&
The Dual Role of Benzophenone in
Visible-Light/Nickel Photoredox-
Benzophenone resurrected: The inherent
reactivity of benzophenone upon photo-
chemical excitation is used for the ali-
functional groups. Mechanistic experi-
ments shed light on the dual role of the
new photocatalyst of the metallaphotor-
edox family.
À
Catalyzed C H Arylations: Hydrogen-
Atom Transfer and Energy Transfer
À
phatic C H arylation reaction. The con-
ditions are tolerant to a broad range of
6
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
These are not the final page numbers!