Acylation of Arenes with Aldehydes through Dual C-H Activations by Merging Photocatalysis and Palladium Catalysis

Article abstract of DOI:10.1021/acs.orglett.1c01184

An acylation of arenes with aldehydes through dual C-H activations at room temperature is reported. The acylation was initiated by phenanthraquinone-catalyzed hydrogen atom transfer from aldehyde under visible light irradiation. The aldehyde-derived acyl

Full text of DOI:10.1021/acs.orglett.1c01184

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Letter
Acylation of Arenes with Aldehydes through Dual C−H Activations
by Merging Photocatalysis and Palladium Catalysis
Haiyang Wang,† Tao Li,† Dongyan Hu, Xiaogang Tong, Liyan Zheng, and Chengfeng Xia*
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ABSTRACT: An acylation of arenes with aldehydes through dual C−
H activations at room temperature is reported. The acylation was
initiated by phenanthraquinone-catalyzed hydrogen atom transfer
from aldehyde under visible light irradiation. The aldehyde-derived
acyl radical merged with palladium-catalyzed activation of arenes to
afford the cross coupling products.
significance, the use of an excess of oxidant (such as TBHP or
ryl ketones are important structural motifs widely found
Ain natural products, pharmaceuticals, and functional PIDA) to generate acyl radicals and the requirement of a high
materials.1 Because of their importance, many synthetic
reaction temperature limit their applications.
Because the chemical reactivity of electronically excited
molecules differs fundamentally from that in the ground state,
the photochemistry induced by visible light provides fresh
opportunities to expand the potential of aldehydes.6 It was
determined that the formyl hydrogen of aldehyde could be
abstracted by photoexcited tetrabutylammonium decatungstate
(TBADT)7 or anthraquinone (AQ)8 to acyl radical through a
hydrogen atom transfer (HAT) process. By merging the
process with palladium catalysis, Zhang et al. recently realized
the acylation of aryl halides under mild conditions (Scheme
1b).9 Here we reported the photochemical regioselective
arylation through direct double C−H activations of aldehydes
and arenes under photocatalysis in conjunction with palladium
catalysis.
Our initial investigations focused on the model photo-
chemical acylation between 4-methylbenzaldehyde 1 and 2-
phenylpyridine 2. As shown in entry 1 of Table 1, when AQ
was exploited as the photocatalyst for the generation of acyl
radical and Pd(OAc)2 as the transition metal catalyst, no
coupling product was detected. Screening the possible
photocatalysts identified that phenanthraquinone (PQ)10
efficiently abstracted formyl hydrogen and the corresponding
acylation product 3 was afforded in 82% yield (entry 2). In
addition to Ag2O, other oxidants [such as MnO2, Mn(OAc)2,
O2, K2S2O8, etc.] for the recycling photocatalysis were also
evaluated. However, these experiments resulted in much lower
yields because aldehydes were very sensitive to these strong
protocols for aromatic ketones have been achieved. The
Lewis acid-promoted Friedel−Crafts acylation of aromatics is
one of the most classical methods for synthesis of aryl ketones,
although the reaction suffers from poor regioselectivity and
harsh conditions.2 Beyond typical cross coupling methods, the
C−H activation strategy provides a unique opportunity to
construct carbonyl compounds without prefunctionalization
steps.3 Over the past several decades, transition metal-
catalyzed oxidative C−H functionalization has been signifi-
cantly developed for the preparation of complex molecules
from simple starting materials with high atom efficiency.4 The
abundant aldehydes are commercially available and inex-
pensive and could serve as ideal acylation reagents for the
synthesis of aryl ketones in the presence of ortho-chelating
groups and strong oxidants (Scheme 1a).5 Although of great
Scheme 1. Generation of Aryl Ketone via Acylation from
Aldehydes
Received: April 7, 2021
Published: April 19, 2021
https://doi.org/10.1021/acs.orglett.1c01184
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3772−3776
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a
Table 1. Screening for the Optimal Conditions
Scheme 2. Substrate Scope of Photochemical Acylation
b
entry
photocatalyst
oxidant
Ag2O
Ag2O
MnO2
Mn(OAc)2
air
O2 (1 atm)
K2S2O8
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
solvent
yield (%)
1
2
3
4
5
6
7
8
AQ
PQ
PQ
PQ
PQ
PQ
PQ
PQ
PQ
PQ
PQ
PQ
PQ
PQ
none
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
DCE
CHCl3
acetone
DMF
THF
CH3CN
CH3CN
0
82
69
58
35
26
25
49
55
52
65
0
9
10
11
12
13
Ag2O
Ag2O
Ag2O
0
0
0
c
14
15
a
Reaction conditions: 1 (0.5 mmol, 2.5 equiv), 2 (0.2 mmol, 1 equiv),
Pd(OAc)2 (0.04 mmol, 20 mol %), photocatalyst (0.02 mmol, 10 mol
%), and oxidant (0.4 mmol, 2.0 equiv) in the solvent (4 mL)
irradiated by blue LEDs under an argon at room temperature for 12 h.
b
c
Isolated yields by silica gel column chromatography. The reaction
was conducted in the dark while the mixture was being refluxed.
oxidants (entries 3−7). Further screening the solvents revealed
that MeCN was optimal, lower yields were obtained in
CH2Cl2, DCE, acetone, etc. (entries 8−11), and no products
were harvested in DMF and THF (entries 12 and 13,
respectively). The photochemical nature of this acylation was
incontestably confirmed as essentially no product was observed
when the controlled experiments were performed in the dark
while refluxing or without a photocatalyst (entries 14 and 15).
With the optimized reaction conditions established, the
scope of this photochemical acylation was investigated
(Scheme 2). It was discovered that the acylation tolerated a
wide range of substituents on the aromatic ring of
benzaldehydes. The acylation of 2-phenylpyridine with non-
substituted benzaldehyde (4) and aldehydes bearing electron-
donating substituents on different positions, such as Me, iPr,
and OMe (5−9), proceeded well, and 70−85% yields were
obtained. The halogen substituents (10−12) were also suitable
for the acylation. In addition to the electron-donating groups,
the substrates containing an electron-withdrawing group, such
as 4-CO2Me (13), 4-Ph (14), and 4-CN (15) groups, that
were also examined for the acylation all afforded the product in
good yields (64−88%). We next evaluated polycyclic aromatic
aldehydes and found that both α- and β-naphthaldehyde (16
and 17, respectively) were efficient substrates. In addition, this
acylation was also extended to heterocyclic aldehydes.
Reactions with 3-thiophenaldehyde, 2-furanaldehyde, and 2-
benzofuranaldehyde afforded products in high yields (18−20,
respectively), especially for 2-benzothiophenaldehyde (21,
96% yield).
Moreover, we also found that this acylation was not limited
to aryl aldehydes; aliphatic aldehydes were also reactive, and
the corresponding products (22−25) were afforded with
moderate yields. The substituted 2-phenylpyridine analogues
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were also exploited for the coupling reaction, and moderate
yields were obtained (26−29). Finally, we investigated the
reactivity of benzo[h]quinoline as the arene substrate with aryl
aldehydes. It is worth noting that benzaldehyde or the para-
substituted benzaldehydes containing different phenyl elec-
tron-donating or electron-withdrawing groups were also
suitable for the PQ/palladium dual catalyzed acylation (30−
33), albeit lower yields were obtained. Moreover, the acylation
of 2-phenylpyridine (2) with 4-methylbenzaldehyde (1) was
also conducted on a 2 mmol scale, and a 73% yield of product
3 was harvested after 24 h.
Scheme 3. Proposed Mechanism of Acylation through the
Dual Catalysis Process
To improve our understanding of the reaction mechanism,
UV−vis experiments with PQ as the HAT photocatalyst were
conducted (Figure 1a). Obvious absorption peaks of the buff
Figure 1. Mechanistic studies. (a) UV−vis absorption spectra of
acetonitrile solutions (1 × 10−4 M) of PQ. (b) Quenching of the PQ
emission (1 × 10−5 M in CH3CN) in the presence of increasing
amounts of 1. (c) Radical trapping experiments with TEMPO.
catalyst. Meanwhile, 2-phenylpyridine 2 reacted with Pd-
(OAc)2 through C−H activation to afford a five-membered
palladacycle intermediate B.11 Photogenerated acyl radical A
was then trapped by palladacycle B followed by further
oxidation with Ag2O to form Pd(IV)12 intermediates C.
Finally, intermediates C underwent reductive elimination to
release coupling product 3 and regenerate the Pd(II) catalyst.
In summary, we have developed an acylation protocol
between aldehydes and arenes via merging photocatalysis and
palladium catalysis. The photoexcited PQ abstracted a
hydrogen atom from aldehydes to generate the acyl radical
through a HAT process. Then it merged with a palladium-
catalyzed C−H activation to afford the cross coupling
products. In addition, the method utilized both aromatic and
aliphatic aldehydes as an abundant acyl source to afford aryl
ketones under mild reaction conditions, as well as tolerance for
a broad range of functional groups.
solution of PQ in the visible light region as well as in the UV
region were observed (Figure 1a). Because the fluorescence
signal excited by visible light is very weak, the alternative
emission at 395 nm excited by UV was measured. Stern−
Volmer quenching studies revealed that the excited state of PQ
was effectively quenched by cumaldehyde (Figure 1b).
Moreover, the Stern−Volmer analysis revealed a linear
correlation indicating a dynamic quenching of the excited
PQ by cumaldehyde (Figure S3). These results indicated that
the excited PQ was responsible for triggering the formation of
the acyl radical from aldehyde. To verify that the acyl radical
participated in the acylation reaction, the controlled experi-
ment with radical scavenger TEMPO was conducted under the
standard conditions. The reaction was completely suppressed,
and no coupling product was formed. Instead, TEMPO-
quenched product 34 was isolated in 74% yield (Figure 1c).
These results suggested that an acyl radical was generated
through a HAT process under the photochemical conditions
and participated in the downstream palladium-catalyzed
acylation.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01184.
Experimental procedures, 1H and 13C NMR spectra, and
characterization data of compounds (PDF)
On the basis of literature reports and experimental results, a
plausible mechanism for the merging of the photocatalytic
cycle and the palladium catalytic cycle is outlined in Scheme 3.
The photocatalyst PQ was initially irradiated by visible light to
reach an electronically excited state (PQ*). Then it abstracted
a hydrogen from aldehyde 1 to generate an acyl radical A and
PQ-H. PQ-H was oxidized by Ag2O to recycle the photo-
AUTHOR INFORMATION
Corresponding Author
■
Chengfeng Xia − Key Laboratory of Medicinal Chemistry for
Natural Resource, Ministry of Education, Yunnan Provincial
Center for Research & Development of Natural Products,
School of Chemical Science and Technology, Yunnan
3774
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University, Kunming 650091, China; orcid.org/0000-
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0001-5617-5430; Email: xiacf@ynu.edu.cn
Authors
Haiyang Wang − Key Laboratory of Medicinal Chemistry for
Natural Resource, Ministry of Education, Yunnan Provincial
Center for Research & Development of Natural Products,
School of Chemical Science and Technology, Yunnan
University, Kunming 650091, China
Tao Li − Key Laboratory of Medicinal Chemistry for Natural
Resource, Ministry of Education, Yunnan Provincial Center
for Research & Development of Natural Products, School of
Chemical Science and Technology, Yunnan University,
Kunming 650091, China
Dongyan Hu − Key Laboratory of Medicinal Chemistry for
Natural Resource, Ministry of Education, Yunnan Provincial
Center for Research & Development of Natural Products,
School of Chemical Science and Technology, Yunnan
University, Kunming 650091, China
Xiaogang Tong − Key Laboratory of Medicinal Chemistry for
Natural Resource, Ministry of Education, Yunnan Provincial
Center for Research & Development of Natural Products,
School of Chemical Science and Technology, Yunnan
University, Kunming 650091, China
Liyan Zheng − Key Laboratory of Medicinal Chemistry for
Natural Resource, Ministry of Education, Yunnan Provincial
Center for Research & Development of Natural Products,
School of Chemical Science and Technology, Yunnan
University, Kunming 650091, China; orcid.org/0000-
0002-3726-1568
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Complete contact information is available at:
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Author Contributions
†H.W. and T.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (21871228 and 22061044) and
the Program for Changjiang Scholars and Innovative Research
Team in University (IRT_17R94). The authors thank Dr.
Rong Huang of the Advance Analysis and Measurement
Center of Yunnan University for his help with NMR
experiments.
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