Oxidative C-C Bond Cleavage for the Synthesis of Aryl Carboxylic Acids from Aryl Alkyl Ketones

Article abstract of DOI:10.1055/s-0037-1609751

A metal-free and one-pot two-step synthesis of aryl carboxylic acids from aryl alkyl ketones has been achieved. The reactions were performed with iodine as the catalyst, DMSO and TBHP as the oxidants. Under the optimal reaction conditions, a number of aryl alkyl ketones could be converted into their corresponding aryl carboxylic acids in good to excellent yields (up to 94%).

Full text of DOI:10.1055/s-0037-1609751

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© Georg Thieme Verlag Stuttgart · New York
2018, 29, A–E
letter
A
en
L. Xu et al.
Letter
Synlett
Oxidative C–C Bond Cleavage for the Synthesis of Aryl Carboxylic
Acids from Aryl Alkyl Ketones
Liang Xua
Shengpeng Wangb
(1) I2 (10 mol%), DMSO (6 equiv)
(2) TBHP (2 equiv)
O
O
R
Ar
OH
Ar
Bajin Chenb
Meichao Li*a
Xinquan Hua
Baoxiang Hua
Liqun Jina
PhCl, 130 °C
Ar = aryl, heteroaryl
= H, alkyl, phenoxy
26 examples
66–94% isolated yields
R
Nan Suna
Zhenlu Shen*a
a College of Chemical Engineering, Zhejiang University of Technology,
Hangzhou 310014, P. R. of China
limc@zjut.edu.cn
zhenlushen@zjut.edu.cn
b Transfar Zhilian Co., Ltd., Xiaoshan Economy & Technology Develop-
ment Zone, Hangzhou 311215, P. R. of China
Received: 05.03.2018
Accepted after revision: 11.04.2018
Published online: 16.05.2018
et al. recently established a new palladium-catalyzed carbo-
nylation procedure for synthesis of aryl carboxylic acids
from aryl halides and formic acid.5
DOI: 10.1055/s-0037-1609751; Art ID: st-2018-k0137-l
Aryl alkyl ketones are a stable and readily available class
of compounds. Carbon–carbon bond-cleavage reactions be-
tween the carbonyl carbon and the α-carbon of aryl alkyl
ketones have been reported to successfully produce aryl
carboxylic acids. A number of transition-metal-catalyzed
C–C bond-cleavage reactions are available for the synthesis
of aryl carboxylic acids, including Cu, Hg, W, Mn, and Ru.6
However, these methods were hampered by the need of
transition-metal catalysts, which were not the best choice
because of metal contamination. Only a few methods were
reported for the synthesis of aryl carboxylic acids from aryl
alkyl ketones with transition-metal-free processes.7 Never-
theless, just moderate yields of aryl carboxylic acids were
obtained in most cases. Bjørsvik and co-workers7a have pro-
vided a simple method for aerobic oxidation of methyl aryl
ketones into the corresponding aryl carboxylic acids using
1,3-dinitrobenzene as the catalyst in the presence of stoi-
chiometric amounts of t-BuOH. Seven substrates were test-
ed, but one of them, 3-nitroacetophenone, could not give
the corresponding product. Hypervalent iodine reagents
[hydroxyl(2,4-dinitrobenzensulfonyloxy)iodo]benzene and
pentafluoroiodobenzene bis(trifluoroacetate) have been
used as stoichiometric oxidants in the oxidative conversion
of aryl ketones into carboxylic acids.7e,f Therefore, it is high-
ly desirable to develop an efficient and metal-free catalytic
oxidization protocol for the synthesis of aryl carboxylic
acids from aryl alkyl ketones.
Abstract A metal-free and one-pot two-step synthesis of aryl carbox-
ylic acids from aryl alkyl ketones has been achieved. The reactions were
performed with iodine as the catalyst, DMSO and TBHP as the oxidants.
Under the optimal reaction conditions, a number of aryl alkyl ketones
could be converted into their corresponding aryl carboxylic acids in
good to excellent yields (up to 94%).
Key words aryl alkyl ketones, aryl carboxylic acids, iodine, dimethyl
sulfoxide, tert-butyl hydroperoxide
As a useful intermediate, the carboxylic acids moiety is
one of the most important functional groups in organic
chemistry and can be easily converted into corresponding
esters, acid halides, amides, and anhydrides. Furthermore,
the aryl carboxylic acids play an important role as versatile
building blocks in the synthesis of natural products, phar-
maceuticals, agricultural chemicals, polymers, and dyes.1
Consequently, the development of efficient methods for the
synthesis of aryl carboxylic acids has stimulated consider-
able interest.
There are many reports on the synthesis of aryl carbox-
ylic acids. The most widely used method involves direct ox-
idation of methylarenes to aryl carboxylic acids. Stoichio-
metric amounts of metal oxidants2 such as Na2Cr2O7,
KMnO4, and CrO3 and catalytic amounts of transition-metal
catalysts3 have been successfully employed for direct oxida-
tion of methylarenes. Aryl carboxylic acids could also be
prepared from various starting materials, such as aromatic
aldehydes, benzylic alcohols, 2-oxo-2-arylacetaldehyde,
aryl halides, arylolefins, arylalkynes, etc.4 For example, Wu
On the other hand, molecular iodine has attracted much
attention as a non-metal, inexpensive, non-toxic, and readi-
ly available catalyst or mediator for various organic trans-
formations.8 It also has been utilized for the synthesis of
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

B
L. Xu et al.
Letter
Synlett
aryl carboxylic acids. Kalmode and Chaskar et al. have de-
veloped a viable protocol for the oxidation of arylacetic acid
through cleavage of C–C bond to aryl carboxylic acid in 24–
28 h in the presence of stoichiometric amounts of I2 with
DMSO as oxidant and solvent.9 An iodine-catalyzed C–C
bond cleavage of aryl alkyl ketones for the synthesis of aryl
carboxylic acids with DMSO as oxidant has been reported
by Bathula and coworkers.10 However, 4 equiv of hydroxyl-
amine hydrochloride should be used as promoter and in
some cases the yields of aryl carboxylic acids are not satis-
factory.
Inspired by the aforementioned work, in continuation of
our work on the development of iodine-catalyzed oxidation
reactions,11 we attempted to develop a one-pot two-step
oxidation of aryl alkyl ketones to the corresponding aryl
carboxylic acids with iodine as the catalyst and DMSO and
tert-butyl hydroperoxide (TBHP) as the oxidants (Scheme 1).
Table 1 Optimization of Reaction Conditionsa
O
O
O
Catalyst
DMSO
O
oxidant
OH
solvent
1a
1aa
2a
Entry Cat. (mol%)
Oxidant
(equiv)
Solvent
Temp (°C) Time (h)b Yield
(%)c
1
2
I2 (10)
TBHP (2)
TBHP (2)
TBHP (2)
PhCl
120
120
120
120
reflux
reflux
reflux
reflux
120
110
130
130
130
130
130
130
130
130
130
130
13 (7)
(7)
85
NR
NR
NR
69
74
59
54
67
20
92
65
91
92
ND
ND
86
84
KI (10)
PhCl
3
TBAI (10)
PhCl
(7)
4
PhI(OAC)2 (10) TBHP (2)
PhCl
(7)
5
I2 (10)
I2(10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (5)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
DTBP (2)
Oxone (2)
H2O2 (2)
TBHP (1.5)
TBHP (3)
TBHP (2)
CH3CN
CH2Cl2
toluene
DCE
18 (13)
13 (8)
20 (14)
10 (6)
12 (6)
30 (24)
6 (3)
6
7
8
Previous works
9
DMSO
PhCl
I2 (1.5 equiv)
DMSO (oxidant and solvent)
O
OH
10
11
12
13
14
15
16
17
18
19
20d
Ar
120 °C, 24–28 h
Ar
OH
O
PhCl
ref. 9
59–90% yield
PhCl
20 (12)
6 (3)
I2 (10 mol%)
NH2OH HCl (4 equiv)
I2 (15)
I2 (20)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
PhCl
O
O
Ar
OH
PhCl
6 (3)
Ar
R
DMSO (oxidant and solvent)
100 °C
ref. 10
41–93% yield
PhCl
6 (3)
This work
PhCl
6 (3)
(1) I2 (10 mol%), DMSO (6 equiv)
(2) TBHP (2 equiv)
O
PhCl
6 (3)
O
R
Ar
OH
Ar
PhCl
6 (3)
PhCl, 130 °C
66–94% yield
PhCl
4.5 (3) 90
6 (3) 58
Scheme 1 The improved method for the synthesis of aryl carboxylic
PhCl
acids via oxidative C–C bond cleavage
a Reaction conditions: 1a (1 mmol), DMSO (6 equiv), solvent (2 mL).
b Values in parentheses referred to reaction time of the first step.
c Determined by GC using an internal standard method using biphenyl as the
internal standard substance.
Initially, we started to optimize the reaction conditions
with acetophenone (1a) as the model substrate (Table 1).
The first-step reaction was carried out with 10 mol% of I2
and 6 equiv of DMSO in chlorobenzene at 120 °C. After 1a
was converted into intermediate 1aa completely, 2 equiv of
TBHP was added into the mixture. Benzoic acid (2a) could
be obtained with 85% GC yield in 13 h of total reaction time
(Table 1, entry 1). As outlined in Table 1, the influence of
different catalysts was studied. To our surprise, KI, tetrabu-
tylammonium iodide (TBAI), and PhI(OAC)2were ineffective
for the formation of the intermediate 1aa (Table 1, entries
2–4). Results in Table 1 showed the solvent can affect this
transformation, and chlorobenzene was found to be the
most suitable solvent (Table 1, entries 1, 5–9). Then the ef-
fect of reaction temperature was further examined. At the
beginning, we attempted to reduce the reaction tempera-
ture to 110 °C, while the yield of 2a was dramatically
dropped into 20% (Table 1, entry 10). When the reaction
temperature was increased from 120 °C to 130 °C, the yield
of 2a was increased to 92% and the reaction time was short-
ened to 6 h (Table 1, entry 11).
d DMSO (4 equiv).
Later on, the loading of I2 was further tested. When the
loading of I2was reduced to 5 mol%, 2a was obtained in 65%
(Table 1, entry 12). However, the yield of 2a remained al-
most unchanged by increasing the loadings of I2 from 10
mol% to 15 mol% or 20 mol% (Table 1, entries 13 and 14).
Then the different oxidants which were used in the second-
step reaction were studied. The results showed that other
oxidants like di-tert-butyl peroxide (DTBP) and Oxone were
ineffective for the conversion 1aa into 2a. No desired prod-
uct could be detected in 6 h (Table 1, entries 15 and 16).
Hydrogen peroxide showed good performance and 86%
yield of 2a could be achieved (Table 1, entry 17). Further
studies have found that the dosage of 2 equiv of TBHP was
appropriate (Table 1, entries 18 and 19). The loading of
DMSO has also been tried to reduce. However, 1a could not
be transformed into 1aa completely in 6 h, and the yield of
2a was only 58% (Table 1, entry 20). After detailed explora-
tion of the reaction conditions with 1a as the substrate, I2
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

C
L. Xu et al.
Letter
Synlett
(10 mol%) with DMSO (6 equiv) in the first step, TBHP
(2 equiv) in the second-step and chlorobenzene as the
solvent at 130 °C were found to be the optimal reaction
conditions.
O
O
(1) I2 (10 mol%), DMSO (6 equiv)
(2) TBHP (2 equiv)
OH
R
R
PhCl, 130 °C
2
1
COOH
COOH
In order to explore the generality of this protocol for the
synthesis of aryl carboxylic acids, various aryl methyl ke-
tones were examined under the optimized reaction condi-
tions.12 The results in Scheme 2 demonstrated that most of
the aryl methyl ketones underwent smooth transforma-
tions to afford the corresponding aryl carboxylic acids in
good to excellent yields (2a–v). The protocol could tolerate
various aryl methyl ketones bearing electron-donating and
electron-withdrawing groups. The reactions of acetophe-
nones with electron-donating substituents such as 4-meth-
yl, 4-isopropyl, 4-tert-butyl, and 4-methoxy proceeded
smoothly to afford the corresponding products 2b–e in 81–
94% isolated yields. 3-Methyl- and 2-methylacetophenones
were selectively converted into their corresponding benzoic
acids 2f and 2g in 82% and 81% isolated yields, respectively.
Acetophenones bearing electron-withdrawing groups such
as 4-NO2, 4-CN, and 4-COOCH3 could also give the expected
products (2h, 2i and 2j) with good yields in 6 h. 3-Nitroace-
tophenone (1k), which could not converted into 3-nitro-
benzoic acid (2k) in Bjørsvik’s work,7a could give 2k in 84%
isolated yield with this protocol. The substrate scope was
further extended to various halogen-substituted acetophe-
nones. As expected, 4-fluoro-, 4-chloro-, 4-bromo-, and 3-
chloroacetophenones were smoothly oxidized into their
corresponding benzoic acids (2l–o) efficiently in 6–8 h.
However, 2-chloroacetophenones showed lower reactivity,
only 73% isolated yield of 2p was achieved in 6 h. When 4-
acetylbiphenyl was chosen as the substrate, 71% isolated
yield of 4-biphenylcarboxylic acid (2q) could be obtained.
Under the optimized reaction conditions 1-acetonaphthone
and 2-acetonaphthone could afford 1-naphthoic acid (2r)
and 2-naphthoic acid (2s) in 84% and 82% isolated yield, re-
spectively. Having established the scope with respect to the
aryl methyl ketones, we turned our attention to various he-
teroaryl methyl ketones such as 2-acetylthiophene and 2-
acetylfuran. When 1t and 1u were submitted to the oxida-
tion reactions, the isolated yields of thiophene-2-carboxylic
acid (2t) and furan-2-carboxylic acid (2u) were higher than
65%. It should be noted that the reaction temperature was
lowered to 110 °C to reduce side reactions. The substrate
1,1′-(1,4-phenylene)diethanone with two acetyl groups
was also subjected to this protocol. Terephthalic acid (2v)
could be obtained in 75% isolated yield after the reaction
time was prolonged to 17 h.
COOH
COOH
2a, 6 h, 87%
2b, 6.5 h, 92%
2c, 6 h, 81%
2d, 7 h, 83%
COOH
COOH
COOH
COOH
O2N
O
2f 7 h, 82% (92%)b
2e, 6 h, 94%
2g 5 h, 81%
COOH
2h, 6 h, 81%
COOH
COOH
COOH
O
NC
F
NO2
O
2i, 6 h, 84%
COOH
2j, 6 h, 89%
COOH
2k, 6 h, 84%
2l, 6 h, 93%
COOH
COOH
Cl
Br
Cl
Cl
2m, 6 h, 88%
2n, 8 h, 89%
2o, 6 h, 81%
2p, 6 h, 73%
COOH
COOH
S
COOH
COOH
2t,c 11 h, 72% (85%)b
2q, 14 h, 71%
2r, 6 h, 84%
2s, 6h, 82%
COOH
O
COOH
HOOC
2v, 17 h, 75%
2u,c 13 h, 66% (74%)b
Scheme 2 Scope of aryl methyl ketone derivatives. a The reactions
were carried out with 1 mmol of 1, 10 mol% of I2, 6 equiv DMSO, and
2 equiv of TBHP in 2 mL of chlorobenzene at 130 °C. Isolated yields are
given. b Values in parentheses referred to the yield determined by GC
using an internal standard method. cReaction temperature: 110 °C.
(12–17 h) was required for efficient transformation. β-O-4
(3e) is an important fragment of lignin. To our delight, it
could transformed smoothly and afford 4-methoxybenzoic
acid (2e) in 79% isolated yield. When a branched substrate,
phenyl isopropyl ketone, was submitted to the oxidation re-
action, it could not be converted into the intermediate 1aa,
and no product 2a could be observed.
Carbinols can be converted into their corresponding ke-
tones under oxidative conditions, thus 1-phenylethanol was
also subjected to this protocol. However, no desired product
2a could be obtained.
To understand the mechanism of this transformation,
several control experiments were then carried out (Scheme
4). Acetophenone (1a) could easily be converted into
phenylglyoxal (1aa) in high yields in the presence of iodine
and DMSO (Scheme 4, eq. 1). When 1aa was treated with 2
equiv of TBHP, 64% GC yield of benzoic acid (2a) and 32% GC
yield of tert-butyl benzoate (2aa) were obtained (Scheme 4,
eq. 2). We had tried the conversion of 2aa into 2a in the
The scope of this reaction was subsequently extended to
several aryl alkyl ketones with varying alkyl chain lengths
(Scheme 3). The reactions with aryl alkyl ketones 3a–d pro-
ceeded smoothly to give the benzoic acids 2a, 2b, 2m, and
2a in good yields (71–84%). However, in contrast to the re-
action of aryl methyl ketones 1, much longer reaction time
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

D
L. Xu et al.
Letter
Synlett
firmed by GC-MS and releases HI by a subsequent Korn-
blum oxidation.15 Homolytic cleavage of TBHP gives tert-bu-
toxyl radical and hydroxyl radical, which attacks 1aa to
form radical 1ac. Radical 1ac can be converted into radical
1ad and formic acid. Formic acid is decomposed under
130 °C, and CO2 is released. Radical 1ad reacts with hydrox-
yl radical to form 2a directly. Otherwise, 1ad is turned to
2aa in the presence of tert-butoxyl radical. Subsequently,
the HI-catalyzed hydrolysis of 2aa gives the product 2a.13
O
O
(1) I2 (10 mol%), DMSO (6 equiv)
(2) TBHP (2 equiv)
R
OH
R1
R1
PhCl, 130 °C
3
2
Substrate
Product
Substrate
Product
(time, isolated yield)
(time, isolated yield)
O
O
COOH
COOH
3a
2a, 14 h, 81%
3d
2a, 16 h, 71%
O
O
O
COOH
COOH
O
•
•
O
HOOtBu
OH + OtBu
O
O
OtBu
2aa
O
3b
2b, 14 h, 80%
3e
2e, 17 h, 79%
O
I2
O
DMS
•
OtBu
COOH
1a
OH
Cl
O
2a
Cl
•
O
3c
2m, 16 h, 84%
HI
I
DMSO
Scheme 3 Synthesis of carboxylic acids from aryl alkyl ketones.
Reagents and conditions: 3 (1 mmol), I2 (10 mol%), DMSO (6 equiv),
TBHP (2 equiv), PhCl (2 mL), 130 °C.
1ad
•
DMSO
DMS
OH
HCOOH
heat
1ab
O
O
O
•
CO2
O
OH
1aa
1ac
presence of 10 mol% of iodine and 6 equiv of DMSO. As ex-
pected, 2a was observed in 92% GC yield (Scheme 4, eq. 3).
This result is consistent with the literature reports that tert-
butyl esters can be hydrolyzed into acids in the presence of
iodine.13 To further confirm the mechanism, an isotope
trace experiment was conducted with dimethyl sulfoxide
labeled with 18O (Scheme 4, eq. 4). 18O-Labeled carbon diox-
ide (CO18O) could be detected with GC-MS after 1a was oxi-
dized with I2/DMS18O/TBHP.
•
OH
Scheme 5 Plausible reaction pathways
In summary, we have successfully developed a one-pot
two-step oxidation of aryl alkyl ketones to the correspond-
ing aryl carboxylic acids with iodine as the catalyst and
DMSO and TBHP as the oxidants. The reaction tolerated var-
ious aryl alkyl ketones bearing electron-donating and elec-
tron-withdrawing groups in ortho, meta, and para positions
of the aromatic ring. This catalytic oxidation system dis-
played high reaction efficiency, and numerous aryl carbox-
ylic acids could be obtained in good to excellent yields. A
noteworthy feature of this work is that the protocol is metal
free and easy to operate.
O
O
I2 (10 mol%), DMSO (6 equiv)
PhCl, 130 °C, 3 h
(1)
1a
1aa
GC yield: 95%
O
TBHP (2 equiv)
O
(2)
2a
1aa
Funding Information
PhCl, 130 °C, 3 h
2aa
This project was supported by the National Natural Science Founda-
tion of China (21776260, 21773211 and 21376224), Natural Science
Foundation of Zhejiang Province (LY17B060007) and Hangzhou
GC yield: 64%
I2 (10 mol%), DMSO (6 equiv)
PhCl, 130 °C, 2 h
32%
(3)
(4)
2a
GC yield: 92%
2aa
1a
Qianjiang Distinguished Experts Project.
N
oaitn
a
l
N
a
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F
o
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n
d
oaitn
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n
i
a
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7
7
6
2
6
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oaitn
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hanei
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orvn
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ce
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7
B
0
6
0
0
0
7)
(1) I2 (10 mol%), DMS18O (6 equiv)
(2) TBHP (2 equiv)
Supporting Information
CO18O
2a
+
PhCl, 130 °C, 6 h
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1609751.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
f
rmoaitn
Scheme 4 Control experiments for mechanism studies
A plausible reaction mechanism for the transformation
of aryl methyl ketones into aryl carboxylic acids is shown in
Scheme 5. Initially, substrate 1a reacts with molecular io-
dine, and α-iodoketone 1ab is formed.14 Then α-iodoketone
1ab is converted into phenylglyoxal (1aa) which was con-
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(12) Typical Procedure for the Synthesis of Carboxylic Acid (2a)
A mixture of acetophenone (1a, 120 mg, 1 mmol), DMSO (47
mg, 6 mmol), and iodine (25 mg, 0.1 mmol) in chlorobenzene (2
mL) was stirred at 130 °C for 3 h until 1a was disappeared com-
pletely (monitored by thin layer chromatography). After being
cooled to room temperature, TBHP (0.26 mL, ca. 2 mmol) was
added to the above reaction mixture, and the reaction solution
was stirred at 130 °C for another 3 h. Then the reaction was
quenched with water, and the pH of the aqueous phase was
adjusted to 11 with 0.1 mol/L NaOH. The aqueous phase was
washed with diethyl ether 3 times with a total ether volume of
10 mL. Then the pH of the aqueous phase was adjusted to 2
with 0.1 mol/L HCl and extracted with diethyl ether 3 times
with a total ether volume of 20 mL. The combined ether phase
was dried over anhydrous sodium sulfate and evaporated in
vacuo to obtain the crude product. The crude product was puri-
fied by column chromatography on silica gel with ethyl ace-
tate/petroleum ether as the eluent to afford the desired product
2a as a white solid (87% yield). 1H NMR (500 MHz, DMSO-d6): δ
= 12.93 (s, 1 H), 7.96–7.94 (m, 2 H), 7.63–7.60 (m, 1 H), 7.51–
7.48 (m, 2 H). 13C NMR (125 MHz, DMSO-d6): δ = 167.2, 132.8,
130.8, 129.2, 128.5.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E