Visible-light-promoted oxidative decarboxylation of arylacetic acids in air: Metal-free synthesis of aldehydes and ketones at room temperature

Article abstract of DOI:10.1016/j.cclet.2019.12.031

A metal-free photocatalytic oxidative decarboxylation reaction at room temperature was developed for the synthesis of aromatic aldehydes and ketones from the corresponding arylacetic acids. The reaction was realized under blue-light irradiation by adding 1 molpercent of 4CzIPN as photocatalyst and air as oxidant. This reaction represents a novel decarboxylation of a sp³-hybridized carboxylic acids without traditional heating, additional oxidants, and metal reagents under mild conditions.

Full text of DOI:10.1016/j.cclet.2019.12.031

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CCLET 5389 No. of Pages 5
Chinese Chemical Letters xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Visible-light-promoted oxidative decarboxylation of arylacetic acids in
air: Metal-free synthesis of aldehydes and ketones at room
temperature
a
a
c
a
Shuaiqi Hea, Xiaolan Chena, , Fanlin Zeng , Peipei Lu , Yuyu Peng , Lingbo Qu ,
*
Bing Yua,b,
*
a
College of Chemistry, Zhengzhou University, Zhengzhou, 450001 China
b
Henan Nonferrous Metals Geological Exploration Institute, Zhengzhou 450052, China
Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, Changsha University of Science and Technology, Changsha
c
410114, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A metal-free photocatalytic oxidative decarboxylation reaction at room temperature was developed for
the synthesis of aromatic aldehydes and ketones from the corresponding arylacetic acids. The reaction
was realized under blue-light irradiation by adding 1 mol% of 4CzIPN as photocatalyst and air as oxidant.
This reaction represents a novel decarboxylation of a sp3-hybridized carboxylic acids without traditional
heating, additional oxidants, and metal reagents under mild conditions.
Received 7 November 2019
Received in revised form 15 December 2019
Accepted 24 December 2019
Available online xxx
© 2019 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Decarboxylation
Metal-free
Photocatalysis
4CzIPN
Arylacetic acids
Due to their easy availability, carboxylic acids have been widely
applied as inexpensive starting materials and building blocks for
organic synthesis. Decarboxylation reaction is one of the most
useful transformations of carboxylic acids [1–5]. Over the past few
reported a series of transition-metal-catalyzed procedures (e.g.,
Mn [26] and Fe [27]) for the oxidative decarboxylation of arylacetic
acids in the presence of complex ligands and additional strong
oxidants. In 2014, the Song’s group developed an elegant Cu(OAc)2-
catalyzed aerobic oxidative decarboxylation of phenylacetic acid to
synthesize the aldehydes and ketones in sealed tubes under 120 ꢀC
(Scheme 1a) [25]. Recently, Bhat and co-workers achieved the
same oxidative decarboxylation reaction of arylacetic acid with
K2S2O8 as oxidant. Notably, the excess amount of K2S2O8 (2 equiv.)
and high temperature (90 ꢀC) were required (Scheme 1b) [21].
Although remarkable advances have been made, previous methods
required stoichiometric oxidants and high temperature, which
may limit their applications. From the point view of green
chemistry, the exploration of a metal-free catalytic system with
green oxidants (ideally atmosphere pressure of oxygen or air)
under mild conditions is highly desired [28–30].
decades,
a myriad of strategies have been developed for
decarboxylation of carboxylic acids to synthesize new compounds
[6–14]. Among them, transition-metal-catalyzed decarboxylation
of sp-hybridized and sp2-hybridized carboxylic acids has been
extensively explored in previous reports [15–17]. However, the
decarboxylation of saturated sp3-hybridized carboxylic acids is still
a daunting challenge due to the inert nature of the CÀÀC
s bond
[18–20]. Moreover, the requirements of transition-metal catalysts,
stoichiometric toxic oxidant, and high temperature in those
previous reported systems have limited their applications.
Aromatic aldehydes and ketones are important compounds,
displaying a wide range of applications in organic synthesis. One of
the most straightforward strategies for the synthesis of aromatic
aldehydes and ketones is oxidative decarboxylation of correspond-
ing arylacetic acids [21–25]. For example, Mirkhani and co-workers
As a rapidly developing field, visible-light-promoted reactions
have drawn huge attention in recent years [31–51]. Particularly,
1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) has
proven to be an efficient metal-free photocatalyst for the trans-
formations of carboxylic acids in organic synthesis, medicine and
materials [52–57]. In the course of our continuing efforts on green
* Corresponding authors at: College of Chemistry, Zhengzhou University,
Zhengzhou 450001, China.
chemistry [58–61], we herein disclosed
a 4CzIPN-catalyzed
transition-metal-free procedure to achieve the oxidative
E-mail addresses: chenxl@zzu.edu.cn (X. Chen), bingyu@zzu.edu.cn (B. Yu).
https://doi.org/10.1016/j.cclet.2019.12.031
1001-8417/© 2019 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: S. He, et al., Visible-light-promoted oxidative decarboxylation of arylacetic acids in air: Metal-free synthesis
of aldehydes and ketones at room temperature, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.cclet.2019.12.031

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CCLET 5389 No. of Pages 5
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S. He et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
visible-light (Table 1). To our delight, the reaction with 4CzIPN
(5 mol%) as photocatalyst and Cs2CO3 (50 mol%) as additive in air
under the irradiation of blue light gave the desired aldehyde 2a in
66% yield (entry 1). While the same reaction under nitrogen
atmosphere only delivered the product 2a in 9% yield (entry 2).
Such a finding suggested that the oxidative decarboxylation of 1a
with 4CzIPN as photocatalyst and air as the sole oxidant is possible.
Subsequently, various solvents including DMSO, CH3CN, THF, H2O,
1,4-dioxane, toluene, acetone and PEG400 were surveyed in the
presence of 4CzIPN/Cs2CO3 (entries 3–10). It was found that the
reaction in CH3CN gave a higher yield of 2a up to 75%, indicating
CH3CN is the optimal solvent for this transformation (entry 4).
Next, the effects of different bases on the reaction were
investigated. The inorganic bases like K2CO3, K3PO4, KOH, and
the organic bases such as DABCO (1,4-diazabicyclo[2.2.2]octane),
DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), TMEDA (tetramethyle-
thylenediamine), Et3N, DMAP (dimethylaminopyridine), TMG
(1,1,3,3-tetramethylguanidine), 2,6-lutidine, and TBD (1,5,7-tria-
zabicyclo[4.4.0]dec-5-ene) were screened respectively with
4CzIPN (5 mol%) as photocatalyst in CH3CN under blue light
irradiation (entries 11–21). The results indicated that TMG was
superior to the others and the yield was as high as 91% (entry 19).
Then, some organic photocatalysts such as Eosin B, Eosin Y, Rose
Bengal were applied as catalysts in the presence of TMG (50 mol%)
in CH3CN for 6 h under air at room temperature, respectively
(entries 22–24). However, the oxidative decarboxylation reactions
almost did not work. Furthermore, it is delighted to find that even
the catalyst loading of 4CzIPN was reduced to 1 mol%, the reaction
performance was almost maintained giving the product 2a in 92%
Scheme 1. Oxidative decarboxylation of phenylacetic acids.
decarboxylation of arylacetic acids toward the corresponding
aromatic aldehydes and ketones at room temperature under
visible-light illumination (Scheme 1c). Importantly, in this
procedure air is the sole oxidant, which eliminate the need for
large quantities of oxidants. This strategy circumvents the use of
transition-metal catalysts and traditional heating.
We started our study using 4-methoxyphenylacetic acid (1a) as
the model substrate in the presence of 4CzIPN and Cs2CO3 under
Table 1
Optimization of reaction conditions.a
Entry
Photocatalyst
Solvent
Base
Yield (%)b
1
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
Eosin B
Eosin Y
Rose Bengal
4CzIPN
–
DMF
DMF
DMSO
CH3CN
THF
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
K3PO4
KOH
DABCO
DBU
TMEDA
Et3N
DMAP
TMG
2,6-Lutidine
TBD
66
9
47
75
28
6
34
trace
54
15
69
70
70
23
77
30
31
56
91
18
58
trace
trace
trace
98 (92)
trace
2c
3
4
5
6
7
8
9
H2O
1,4-Dioxane
Toluene
Acetone
PEG400
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25d
26
TMG
TMG
TMG
TMG
TMG
a
Reaction conditions: p-Methoxyphenylacetic acid (0.2 mmol), 4CzIPN (5 mol%), base (50 mol%), in CH3CN (1.5 mL) under air at room temperature for 6 h with the
irradiation of 25 W blue LEDs.
b
Yields were determined by 1H NMR using 1,1,2,2-tetrachloroethane as the internal standard (isolated yield in parentheses).
Reaction performed under a nitrogen atmosphere.
1 mol% of 4CzIPN as photocatalyst.
c
d
Please cite this article in press as: S. He, et al., Visible-light-promoted oxidative decarboxylation of arylacetic acids in air: Metal-free synthesis
of aldehydes and ketones at room temperature, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.cclet.2019.12.031

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3
of isolated yield (entry 25). Additionally, the control experiment
without any photocatalyst only gave trace amount of product 2a (
entry 26). These results suggested the application of 4CzIPN as a
photocatalyst and the irradiation of blue light were critical for such
an efficient transformation. Finally, the optimal conditions were
established as follows: 1a (0.2 mmol), 4CzIPN (1 mol%), TMG
(50 mol%), CH3CN (1.5 mL) under air at room temperature for 6 h
with the irradiation of 25 W blue LEDs.
Scheme 3. The gram-scale synthesis of 2a.
With the optimum conditions in hand, we further explored the
scope of this visible-light-induced 4CzIPN-catalyzed oxidative
decarboxylation reaction via the reaction of some commercially
available arylacetic acids 1 (Scheme 2). When the phenylacetic
acids bearing methoxy group at ortho- or para-positions, 92 % and
80 % yields of corresponding products 2a or 2b were obtained,
respectively. However, the phenylacetic acid with meta-substitut-
ed methoxy group gave relatively poor yield of the aldehydes 2c.
These results suggested that the position of the substituents on the
aromatic rings had a significant effect on the reactivity. When the
phenylacetic acids with methyl or tert-butyl group at para-
position, the oxidative decarboxylation reaction also showed good
reactivities, giving the corresponding aldehydes 2d and 2e in
moderate yields (40% and 55%). Unfortunately, only 25% yield of 2f
was observed when the -Cl containing phenylacetic acid was
employed as substrate. In the cases of 2c and 2f, no obvious
improvements were observed even with higher catalyst loading
(2 mol%) and longer reaction time (12 h). Notably, no byproducts
were observed in those cases, suggesting that the low yields were
probably due to the low reactivity of those substrates. Interesting-
ly, 1-naphthylacetic acid and 2-naphthylacetic acid were also
applicable in this procedure, affording the desired aldehydes 2g
and 2 h in 45% and 60% yields. Moreover, the heteroaromatic acid
such as 2-(thiophen-2-yl)acetic acid was also suitable in this
reaction delivering the thiophene-2-carbaldehyde 2i in 63% yield
under standard conditions.
aromatic acetic acids as substates under optimal reaction
conditions (Table 2). To our delight, when α-hydroxy-p-methox-
yphenylacetic acid 1j was employed as reactant, the corresponding
aldehyde 2j was obtained in excellent yield of 93%. Next, we used
2-hydroxy-2,2-diphenylacetic acid 1k and 2-cyclohexyl-2-hy-
droxy-2-phenylacetic acid 1l as substrates, the corresponding
products, benzophenone 2k and cyclohexyl(phenyl)methanone 2l
were obtained in good yields of 70% and 82%, respectively. On this
basis, a series of ketones (2m-2q) were synthesized from the α-
substituted phenyl acetic acids (1m-1q) under the standard
reaction conditions in moderate to good yields (51%–78%).
Moreover, the aliphatic substrate, e.g., cyclohexanecarboxylic acid
did not give any product, indicating that aliphatic carboxylic acids
are not suitable in this protocol.
To gain a deeper insight into the mechanism of this photo-
catalysis reaction, the control experiments were conducted as
described. When the model reaction was treated with 5 equiv. of
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), the reaction was
totally suppressed and no product 2a was detected (Scheme 4).
This result revealed that a radical pathway was probably involved
in the reaction process.
Furthermore, the Stern-Volmer quenching experiments was
carried out by separately adding various components of the
reaction system to the degassed anhydrous CH3CN solution
containing 4CzIPN. However, none of the reactants had obvious
fluorescence quenching effect on the photocatalyst (Supporting
information). Considering that it is an aerobic reaction, the
experiment of oxygen quenching was carried out later. As
expected, molecular oxygen showed a signifcant fluorescence
quenching effect on the photocatalyst. The linear relationship
between I0/I (I0 and I are the fluorescence intensities before and
after the O2 was bubbled into the solution, respectively) and the
Additionally, a scale-up reaction (10 mmol-scale) for the model
reaction was conducted under the identical conditions. Delight-
fully, a great yield (84%) of the desired product 2a was isolated after
prolonging the reaction time to 15 h, demonstrating this simple
photocatalytic system is practical and potentially useful
(Scheme 3).
In order to expand the scope of this reaction under standard
conditions, we then explored the reaction with α-substituted
time for oxygen bubbling indicates that O2 may quench 4CzIPN
ꢁÀ
(Supporting information). When the superoxide radical (O2
)
scavenger 4-benzoquinone (BQ) was added to the model reaction
[62], the yield of 2a was reduced to 18%, which suggesting the
ꢁ
generation of superoxide radical (O2 À) in the oxidative decarbox-
ylation reaction, which was further confirmed by EPR spectroscopy
[63] (Supporting Information).
Based on the above results and previous reports [64], a
plausible reaction mechanismwas proposed as shown in Scheme 5.
Under blue-light irradiation, 4CzIPN was excited to produce the
corresponding excited-state 4CzIPN*, which is sufficiently reduc-
tive (E1/2(Pꢁ+/P*) = -1.04 V vs. SCE [52]) to donate an electron to O2
ꢁ
generating the superoxide radical O2 À (Ered = -0.86 V vs. SCE [65])
along with the release highly oxidative 4CzIPNꢁ+. In the presence of
base, carboxylic acid 1 was deprotonated to give the carboxylate
anion A. The highly oxidative 4CzIPNꢁ+ (Eox(Pꢁ+/P) = +1.52 V vs. SCE
[52]) was able to oxidize the carboxylate anion A (Eox = +1 ꢂ +1.25 V
vs. SCE [66]) producing the radical B and regenerating the
photocatalyst 4CzIPN. Subsequently, the elimination of CO2 from
radical B gave the benzylic radical C, which then combined with
ꢁ
superoxide radical (O2 À) to give the anion D. Finally, the anion D
was protonated to deliver the intermediate E, followed by
dehydration to produce the desired product 2.
In conclusion, we have developed a photocatalytic protocol for
the oxidative decarboxylation of phenylacetic acid derivatives to
Scheme 2. Oxidative decarboxylation of arylacetic acids to aldehydes. Reaction
conditions: Arylacetic acid (0.2 mmol), 4CzIPN (1 mol%), TMG (50 mol%), in CH3CN
(1.5 mL) in air at room temperature for 6 h with the irradiation of 25 W blue LEDs.
Isolated yields were given.
Please cite this article in press as: S. He, et al., Visible-light-promoted oxidative decarboxylation of arylacetic acids in air: Metal-free synthesis
of aldehydes and ketones at room temperature, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.cclet.2019.12.031

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S. He et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
Table 2
Oxidative decarboxylation of α-substituted aromatic acetic acid to aldehydes and ketones.a
Entry
1
Substrate
Product
Yield (%)b
93
1j
2j
2
3
4
70
82
64
1k
2k
2l
1l
1m
2m
2n
2o
5
6
7
56
51
78
1n
1o
1p
2p
8
72
1q
2q
a
Reaction conditions: α-Substituted aromatic acetic acid (0.2 mmol), 4CzIPN (1 mol%), TMG (50 mol%), in CH3CN (1.5 mL) in air at room temperature for 6 h with the
irradiation of 25 W blue LEDs.
b
Isolated yields were given.
synthesize corresponding aromatic aldehydes or ketones. This
reaction can be carried out in air at room temperature by using
1 mol% of 4CzIPN as photocatalyst. This transition-metal-free
procedure avoids the use of excess amount of inorganic/organic
oxidants and high temperature. The employment of air as green
oxidant eliminate the generation of waste from traditional
oxidants. Further research for the application of such a oxidative
carboxylation strategy is underway in our research group.
Scheme 4. Control experiments.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge the financial support from the National
Natural Science Foundation of China (Nos. 21501010, 21971224),
Scheme 5. The Proposed mechanism.
Innovation and Entrepreneurship Training Program of Zhengzhou
Please cite this article in press as: S. He, et al., Visible-light-promoted oxidative decarboxylation of arylacetic acids in air: Metal-free synthesis
of aldehydes and ketones at room temperature, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.cclet.2019.12.031

G Model
CCLET 5389 No. of Pages 5
S. He et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
5
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Appendix A. Supplementary data
Supplementarymaterialrelatedtothisarticlecanbefound, inthe
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Please cite this article in press as: S. He, et al., Visible-light-promoted oxidative decarboxylation of arylacetic acids in air: Metal-free synthesis
of aldehydes and ketones at room temperature, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.cclet.2019.12.031