Electrochemical Intramolecular C—H/O—H Cross-Coupling of 2-Arylbenzoic Acids

Article abstract of DOI:10.1002/cjoc.201800031

A synthetic protocol to lactones by electro-oxidative induced C—H activation of 2-arylbenzoic acids has been developed. By using Na₂SO₄ aqueous solution as a cheap and green supporting electrolyte, different 2-arylbenzoic acids could provide the corresponding lactones in 30%—90% yields. This reaction could be conducted on a gram scale with a good efficiency as well as a high utility for natural product synthesis.

Full text of DOI:10.1002/cjoc.201800031

Electrochemical Intramolecular C-H/O-H Cross-Coupling of 2-Arylbenzoic
Acids
a
b
a
a
a
a
Ailong Shao, Yong Gao, Na Li, Jirui Zhan, Chien-Wei Chiang* and Aiwen Lei*
Dedicated to Professor Xiyan Lu on the occasion of his 90th birthday
ABSTRACT A synthetic protocol to lactones by electro-oxidative induced C-H activation process of 2-arylbenzoic acids has been developed. By using a
2 4
cheap and green Na SO aqueous as a supporting electrolyte, different 2-arylbenzoic acids could provide the corresponding lactones in 30% - 90% yields.
This reaction could conduct a good reaction efficiency on the reaction of gram scale as well as a high utility for natural product synthesis.
KEYWORDS electrochemical oxidation, C-H activation, hydrogen evolution, green electrolyte , lactones
Introduction
Organic electro-synthesis is recognized as a versatile and
environmentally friendly synthetic tool, for example, the Kolbe
Scheme 1 Intramolecular dehydrogenative C–H/O–H cross-coupling
for the synthesis of benzo-3,4-coumarins.
[
1]
[2]
electrolytic synthesis.
Over the past decades, introducing
transition metals catalysis and oxidants to accomplish oxidative
cross-coupling reaction has emerged as a powerful platform for
[3]
the direct oxidative C–H functionalization. Recently, utilizing
electrochemical anodic oxidation to replace chemical oxidants in
dehydrogenative cross-coupling reactions has been attracting
growing attention. Only few examples of oxidant-free
[7]
[4]
[5]
[6]
dehydrogenative cross-coupling for C-C, C-N, N-N, C-S,
bond formations have been reported.
In the neutral and organic solvent system, the solubility of
inorganic electrolytes such as NaCl or Na SO is limited, hence
2
4
organic electrolytes such as tetrabutylammonium salt is usually
expolited referring to its good solubility and conductivity. However,
excess of organic electrolyte is usually used in these reactions
which might cause pollution and uneconomic. It is well worth to
note that compared with some commercial available organic
electrolytes, the usage of cheap inorganic salts as electrolytes in
organic synthesis is apparently appeal the perspective of
sustainable and green chemistry. Thus, decreasing the usage of
electrolytes and replacing the expensive organic salts is an
important issue in electro-synthesis which we need to overcome.
C-O bond formation is an important way to achieve the
Results and Discussion
Therefore, 2-phenylbenzoic acid 1a was tried as a model
substrate first to investigate the reaction conditions. Initially, the
desired benzo-3,4-benzocoumarin 2a was observed in 70% yield
n
with 0.3 M of Bu NBF as the electrolyte in acetonitrile under a
4
4
constant current in 6 mA (Table 1, entry 1). With this evidence,
the influence of electrolytes was explored. When
iodine-containing reagents, such as tetrabutylammonium iodide
and KI were used as electrolytes, the reaction could not be
promoted (Table 1, entry 2, 3). Inorganic salts, for example,
[
8]
Na CO , NaOAc and Na SO in the CH CN/H O system were also
production of ethers and esters. Only few examples about C-O
bond formation under electrochemical conditions by utilizing
2
3
2
4
3
2
tested. Interestingly, only Na SO can provide a better result
2
4
[9]
(
Table 1, entry 4, 5 and 6). A possible explanation is that the
alcohols as substrates has been reported previously. The
introduction of carboxylic acids as weak nucleophiles into the
reaction system to form C-O bond has a profound impact.
Specifically, the functionalization of 2-phenylbenzoic acid thereof
has performed the development of C-O bond forming reactions to
generate benzo-3,4-coumarins (Scheme 1a). Developing the
synthetic method for benzo-3,4-coumarin derivatives could
engage to the applications of natural product synthesis,
alkaline electrolytes will consume the hydrogen ions in the solvent,
which leads to the hydrogen evolution become slow on the
cathode and decreases the reaction yield. Therefore, further
optimization was carried out with anhydrous Na SO as electrolyte.
2
4
Notably, when acetic acid was added, the neutralized system
could improve the yield of 2a to 90% (Table 1, entry 7). Other
acids, such as sulfuric acid and hydrochloric acid also gave
relatively good results in this reaction (Table 1, entry 8 and 9).
Additionally, reduce the electrolyte loading of anhydrous Na SO
to 0.125 mmol and 0.05 mmol respectively could provide the
same yields respectively (Table 1, entry 10 and 11). Other sulfates
[
10]
pharmaceutical research and material science.
Previously,
[11]
[8e, 12]
transition metals catalysis, photocatalysis,
in-situ generated hypervalent iodine compounds as catalyst,
and utilizing an
2
4
[13]
have been reported as useful methods for developing the C-O
bond construction. Herein, with the aid of anode oxidation and
cathode hydrogen evolution in a simple undivided cell, C-H/O-H
cross-coupling reaction of 2-phenylbenzoic acid can be
accomplished under mild conditions without the usage of metal
additives and excess of oxidants (Scheme 1b).
such as Li SO , K SO , Cs SO as electrolytes could also give a good
2
4
2
4
2
4
result, but a slight decrease compared to Na SO .
2
4
a
The Institute for Advanced Studies (IAS), College of Chemistry and Molecular
Nanchang 330022, Jiangxi, People’s Republic of China.
Sciences, Wuhan University, Wuhan 430072, Hubei, People’s Republic of China.
College of Chemistry and Chemical Engineering, Jiangxi Normal University,
E-mail: aiwenlei@whu.edu.cn; cwchiang@whu.edu.cn
b
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1002/cjoc.201800031
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a
Table 1 Optimization of conditions.
Table 2 Electrochemical intramolecular dehydrogenative lactonization of
a
2
-arylbenzoic acids to the synthesis of benzo-3,4-coumarins.
b
Entry Electrolytes
Solvents
Yield (%)
70
1
2
3
4
5
6
7
8
9
nBu
nBu
KI
Na CO
NaOAc
4
NBF
4
CH
CH
3
CN
CN
4
NI
3
n.d.
n.d.
38
c
c
c
c
CH
CH
CH
CH
3
CN/H
3
CN/H
3
CN/H
3
CN/H
2
O
2
O
2
O
2
O
2
3
45
Na
2
Na
2
Na
2
Na
2
SO
4
SO
4
SO
4
SO
4
75
d
CH
3
CN/H
2
O/CH
3
COOH
90
e
CH
3
CN/H
2
O/H
2
SO
4
85
f
CH
3
CN/H
2
O/HCl
85
g
d
d
d
d
d
1
0
1
2
3
4
Na
Na
Li
2
SO
4
CH
CH
CH
CH
CH
3
CN/H
3
CN/H
3
CN/H
3
CN/H
3
CN/H
2
2
2
2
2
O/CH
O/CH
O/CH
O/CH
3
COOH
3
COOH
3
COOH
3
COOH
3
COOH
89
h
1
1
1
1
2
SO
4
90
h
2
SO
4
88
h
K
2
SO
4
85
h
Cs
2
SO
4
O/CH
84
a
Reactions were performed on a scale of 0.25 mmol of 1a, 0.3 M
b
c
electrolyte 10 mL solvent, room temperature, N
7
mL CH CN, 3.0 mL H
HCl. 0.125mmol Na
2
, 3.5 h. Isolated yield.
d
e
mL CH
3
CN, 3 mL H
2
O. 6.5 mL CH
3
CN, 3.0 mL CH
3
COOH, 0.5 mL H
CN, 3.0 mL H
4
. n.d. = not detected.
2
O. 6.5
f
3
2
O, 0.5 mL H
2
SO
4
. 6.5 mL CH
SO
3
2
O, 0.5 mL
a
Standard condition: 1 (0.25 mmol), Na
COOH (3 mL), H O (0.5 mL), room temperature, N
yields are shown. * = Minor regioisomer.
2
SO
4
(20 mol%), CH
3
CN (6.5 mL),
g
h
2
SO
4
. 0.05mmol Na
2
CH
3
2
2
, 3.5 h. Isolated
Thus, with the optimized condition in hand, we turned to
screen the functional group tolerances for the construction of
benzo-3,4-coumarins (Table 2). Firstly, the substitution group
effect of aryl ring 1 will be scoped. The reaction could be
furnished the corresponding coumarins in moderate to good
Furthermore, the reaction on a gram scale was performed, a 8
mmol portion of 1a could be cyclized to 2a in 80% yield (Scheme
2
a).
yields. For example, electron-neutral substituents, such as –H,
Table 3 Electrochemical intramolecular dehydrogenative lactonization of
t
4
-CH3, 3-CH3, 4- Bu, possessed good reactivity in this
transformation (Table 2, 2a - 2d). In addition, both electron-rich
a
2
-arylbenzoic acids to the synthesis of benzo-3,4-coumarins.
and electron-poor functional group could achieve good yields
(
Table 2, 2e, 2f and 2g). Halogen substituted groups, such as
fluoro, chloro could also be tolerated in this reaction and showed
moderate results. (Table 2, 2h, 2i). However, when 1c and 1f were
used as substrates, two isomers were observed with the
1
regio-selectivity ratio of 10:1 and 15:1 by H-NMR, respectively.
On the other hand, di-substituted substrates were introduced to
this reaction, the corresponding products could be afforded in
good yields as well (Table 2, 2j, 2k). Notably, single regioisomer
product of 2k could be obtained in this reaction, it means that
electron density on aryl ring would play an important role on the
site-selectivity. Changing the aryl ring 1 to a naphthyl group could
also give the product with 55% yield (Table 2, 2l). Moreover,
heterocycles such as thienyl, pyridinyl, indolyl were abided under
the electrochemical conditions, providing corresponding products
with moderate yields (Table 2, 2m, 2n, 2o).
In addition, either electron-rich or electron-poor substituents
2
of
Ar
could
provide
good
reactivity
to
build
benzo-3,4-benzocoumarins (Table 3, 2p
-
2s). Meanwhile,
substrates with halogen groups, such as fluoro, chloro, were
demonstrated a moderate yield (Table 3, 2t, 2u), and naphthalene
was showed a yield of 45% (Table 3, 2v).
a
Standard condition: 1 (0.25 mmol), Na
COOH (3 mL), H O (0.5 mL), room temperature, N
yields are shown.
2
SO
4
(20 mol%), CH
3
CN (6.5 mL),
CH
3
2
2
, 3.5 h. Isolated
The result indicated that this electrochemical synthesis had great
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potential in a practical synthesis. Meanwhile, by way of
demonstration, benzo-3,4-coumarins, as synthesized by exploiting
the electrochemical method reported herein, was utilized in the
formal synthesis of sabilactone (2w), which is a bioactive natural
product only found in the seeds or skin of Sabina vulgaris Antoine
in mechanism B: first, the dehydrogenation of carboxylic acid on
-
the cathode or deprotonation via electrogenerated CH COO gives
3
-
1a , which is then oxidized to the carboxyl radical III through the
anode oxidation. Subsequently, through the intramolecular attack
by carboxyl radical, the cyclized intermediate IV, which is rapidly
aromatized through a single electron transfer and deprotonation
by anode oxidation to generate the desired product 2a. Both
mechanism A and B, the concomitant cathodic reaction was
reducing H+ to release hydrogen gas.
(
2
Scheme 2b). The natural product synthesis started from
,4,6-trihydroxybenzoic acid (S-3), which was converted into the
electrolysis substrate 1w through seven steps synthesis (see the
Supporting Information (SI) for details). By using this
electrochemical protocol, the C-O cross-coupling product of
sabilactone
(2w)
can
be
produced
directly
from
Scheme 4 Proposed mechanism.
2
,4,6-trihydroxybenzoic acid (S-3), in 40% yield.
Scheme
2
Application of the electro-oxidative dehydrogenative
cross-coupling.
Moreover, to gain more insight of the reaction system, we
explored the presence of radical intermediates. 3 equivalent of
radical trapping agents such as TEMPO, BHT were added to the
reaction mixture. As the results, the reaction could be completely
inhibited (Scheme 3a), which showed the same phenomenon as
ref 8h and 8g. So we suspected that the reaction might proceed
through a radical intermediate. Additionally, we also explored the
intermolecular kinetic isotopic effect by competition experiments
(
K /K = 1.14) (Scheme 3b). The result suggest that the C-H bond
H D
cleavage might not be the turnover-determining step in this
process.
Scheme 3 Intermolecular competition experiment.
Conclusions
In conclusion, a new protocol of directly electro-oxidative
intramolecular dehydrogenative lactonization of 2-arylbenzoic
acids has been developed, accomplished the formation of
lactones under catalyst- and oxidant-free conditions. Besides, a
wide range of substrate scope of 2-arylbenzoic acids are well
transformed. Importantly, this electro-oxidative method can apply
to a gram scale experiment and the synthesis of a natural product.
Meanwhile, based on the results of CV study and DFT calculation,
we could provide a new vision to the reaction mechanism.
In terms of above results, cyclic voltammetry (CV)
experiments (for details, see the Supporting Information) and a
[14]
previous study,
two plausible reaction mechanisms A and B
were described in Scheme 4. The mechanism A is only for some
substrates with methoxy group, for example 1f. Firstly,
single-electron-transfer (SET) oxidation of 1f by the anode leads to
the formation of the radical cation I. Subsequently, the generated
aromatic ring radical cation is attacked by carboxyl group to form
the intermediate II. The radical adduct II is rapidly aromatized
through a single electron transfer and deprotonation by anode
oxidation to generate the desired product 2f. On the other hand,
Experimental
General procedure for electro-oxidative induced intramolecular
dehydrogenative lactonization of 2-arylbenzoic acids
In an oven-dried undivided three-necked bottle (10 mL)
equipped with a stir bar, a carbon electrode and a platinum
electrode (1.5 × 1.5 cm ) as the anode and cathode, 2-arylbenzoic
2
acid (0.25 mmol) was added. After flushing the bottle three times
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with N , CH CN (6 mL) was injected in the bottle through a syringe.
2
3
Morofuji, T.; Shimizu, A.; Yoshida, J., J. Am. Chem. Soc. 2013, 135,
And anhydrous sodium sulfate (20 mol%) was added in a tube (5
mL) with 0.5 mL water and 3 mL acetic acid. After dissolved
completely, the solution was injected in the bottle. The reaction
mixture was stirred and electrolyzed at a constant current of 6
mA under room temperature for 3.5 h (3.2 F). After completion of
the reaction, as indicated by TLC and GC-MS, the solvent was
removed with a rotary evaporator. And the mixture was purified
by column chromatography on silica gel using petroleum ether
and ethyl acetate (PE/EA = 10/1 - 3/1) as eluent to afford pure
product.
5
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Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
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Acknowledgement
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This work was supported by the National Natural Science
Foundation of China (21390402, 21520102003). The Program of
Introducing Talents of Discipline to Universities of China (111
Program) is also appreciated.
1
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Manuscript received: XXXX, 2017
Revised manuscript received: XXXX, 2017
Accepted manuscript online: XXXX, 2017
Version of record online: XXXX, 2017
2
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Entry for the Table of Contents
Page No.
Electrochemical Intramolecular C-H / O-H
Cross-Coupling of 2‑Arylbenzoic Acids
a
b
a
a
Ailong Shao, Yong Gao, Na Li, Jirui Zhan,
a
a,b
Chien-Wei Chiang* and Aiwen Lei*
This article is protected by copyright. All rights reserved.