Electrochemical esterification reaction of alkynes with diols: Via cleavage of carbon-carbon triple bonds without catalyst and oxidant

Article abstract of DOI:10.1039/d0gc02193h

A novel electrochemical esterification of alkynes for the synthesis of esters was developed in which diols and their derivatives were used as the partners. This method is green as it is catalyst-free, oxidant-free, and additive-free and shows atom-economy. This is the first example of an electrochemical reaction via cleavage of carbon-carbon triple bonds. Meanwhile, this is also the first example of a carbon-carbon triple bond cleavage reaction of alkynes with diols. This journal is

Full text of DOI:10.1039/d0gc02193h

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Page 1 of 8
Green Chemistry
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DOI: 10.1039/D0GC02193H
ARTICLE
Electrochemical Esterification Reaction of Alkynes with Diols via
Cleavage of Carbon-Carbon Triple Bonds Without Catalyst and
Oxidant
Received 00th January 20xx,
Accepted 00th January 20xx
Pei-Long Wang, a,b Hui-Zhi Shen, a Hui-Hui Cheng, a Hui Gao *a and Pin-Hua Li *a
DOI: 10.1039/x0xx00000x
A novel electrochemical esterification of alkynes for the synthesis of esters was developed in which diols and their
derivatives were used as the partent. This method is green for its characteristic of catalyst-free, oxidant-free, additive-free
and atom-economy. This is the first example of electrochemical reaction via cleavage of carbon-carbon triple bonds.
Meanwhile, this is also the first example of carbon-carbon triple bonds cleavage reaction of alkynes with diols.
plenty amount of oxidants.8 Developing green and sustainable
carbon-carbon triple bonds cleavage reactions is till challenging.
As for esterification reaction of alkynes via cleavage of carbon-
Introduction
Cleavage reaction of carbon-carbon bond which is a usful
synthetic tool for the construction of many valuable compounds has
received widespread attention and been aplied in the field of
agrochemicals, pharmaceuticals, and materials.1 In the past decades,
cleavage reactions of carbon-carbon sigle bond and carbon-carbon
double bond have been developed intensively.2,3 However, owing
to the large bond energy of carbon-carbon triple bond, the cleavage
reaction of it was developed slowly and it is still one of the most
challenging subjects in organic synthesis. One of the example of
cleavage reactions of carbon-carbon triple bond was metathesis of
alkynes.4 However, transiton-metal catalysts were needed, for
example, Mo or W were commonly used. Beside metathesis
reaction, examples of cleavage reactions of carbon-carbon triple
bond were rare, and most of them involved the use of transition-
metal catalysts and oxidants. Transition-metal catalysis played a
important role in the carbon-carbon triple bond cleavage reaction.
In 2001, Jun reported a rhodium-catalyzed hydroiminoacylation
reaction via cleavage of carbon-carbon triple bonds.5 After that, a
serious of transition-metal catalyzed cleavage of carbon-carbon
triple bonds were developed.6 However, the use of expensive or
toxic transiton-metal added the cost of product and the difficulty of
purification. In addition, most of these reactions need harsh
reaction conditions. A few examples of cleavage reaction of carbon-
carbon triple bonds which can avoid the use of transiton-metal
catalyst have been developed.7 However, many of them still need
carbon triple bonds, only two methods have been reported. In
2008, Jiang reported a palladium-catalyzed cleavage reaction of
carbon-carbon triple bond with molecular oxygen promoted by
lewis acid (scheme 1a).9 However, there are many drawbacks of this
reaction: (1) This reaction used the expensive transition-metal-
catalyst Pd(OAc)2; (2) the reaction conditions are harsh, which need
high pressure and temperature; (3) the reaction need high pressure
O2 as the oxidant and ZnCl2·2H2O as an additive; (4) there was no
example of diols as substrate. In 2014, Guo reported PIFA-mediated
Previous work:
a) Pd-catalyzed cleavage reaction of carbon-carbon triple bonds .
Pd(OAc)2 (2 mol %)
O2 (7.5 atm)
O
O
R3-OH (as solvent)
+
R1
R2
+
R1
O
R3
R2
O
R3
ZnCl2•2H2O (20 mol %)
100 °C, 24 h
R3 = Me, Et, n-Pr
no example of diols
b) Oxidant-mediated cleavage reaction of carbon-carbon triple bonds.
O
O
PhI(OCOCF3)2 (3.5 equiv.)
60 °C, 15 h
R3-OH (as solvent)
R1
R2
+
+
R1
O
R3
R2
O
R3
R3 = Me, Et, n-Pr, i-Pr, n-Bu
no example of diols
c) Electrochemical reaction of alkenes with diols.
RVC (+) I Pt (-), I = 12.5 mA
HO
R2
R1
R3
R2
O
O
(2,4-Br2C6H3)3N (10 mol%)
R1
+
iPrCO2H (2 equiv)
R3
HO
Et4NPF6 (1 equiv), MeCN, 80 °C
This work:
d) Electrochemical cleavage reaction of carbon-carbon triple bonds without catalyst and oxidant.
a. Key Laboratory of Green and Precise Synthetic Chemistry and Applications,
Ministry of Education; Department of Chemistry, Huaibei Normal University, Huaibei,
Anhui 235000, P.R. China.
HO
O
O
R1
R2
+
+
R1
O
R2
O
OR
OR
undivided cell
H/RO
E-mail: gaohui20032@163.com
catalyst free
oxidant free
additive free
b. Information College, Huaibei Normal University, Huaibei 235000, China.
E-mail: wangpeilong1987@163.com
R1 = Ar
R2 = Ar, alkyl, H
not suitble for mono-alcohol
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 1 Methods for the construction of α,β-dialkoxy-stilbene
and electrochemical reaction with diols.
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Green Chemistry
Page 2 of 8
ARTICLE
Journal Name
esterification reaction of alkynes with alcohols via oxidative Table
1
Electrochemical dialkoxylation of diaryl acetylenes:
View Article Online
DOI: 10.1039/D0GC02193H
cleavage of carbon triple bonds (scheme 1b).10 However, plenty optimization of conditionsa,b
amount of oxidant (3.5 equivalent) was used and no example of
C(+) I Pt (-), I = 18 mA
Ph3N (10 mol %)
O
diols was reported.
OH
OH
+
O
In recent years, electrochemical synthesis has been developed
into one of the most fascinating synthetic methods because of its
green and economical character.9 Electrochemistry provides an
economical way to transfer electrons directly at the electrode.
Therefore, many reactions can be conducted by anode oxidation
directly without use of a catalyst, redox medium or oxidant. In
2018, Xu’s group realized an electrochemical coupling reaction with
alkenes and diols (scheme 1c).10 However, the electrochemical
reaction of alkynes and diols has never been reported. It is worth
mentioning that the cyclic product was obtained in Xu’s work but
our target product is esters. Therefore, how to avoid the formation
of cyclic product is challenging. In addition, Xu’s work needs amine
as catalyst and acid as additive. Herein, we report a novel
electrochemical esterification of diaryl acetylene with diols and
their derivatives for the synthesis of esters in the absence of
catalyst, oxidant and additive (scheme 1d). This is the first example
of electrochemical reaction via cleavage of carbon-carbon triple
bonds. Meanwhile, This is also the first example of carbon-carbon
triple bonds cleavage reaction of alkynes with diols. Fortunately,
the desired esters were formed with a yield of up to 80%.
HO
n-Bu4NPF6 (1 equiv.)
EtCOOH (2 equiv.)
MeCN, 80 °C, 12 h
1a
2a
3a
Variation from the above
conditions
Yield
(%)
Entry
1
2
3
4
5
6
none
66
I = 12 mA
I = 15 mA
I = 20 mA
C (+)∣C (-)
Pt (+)∣C (-)
52
56
63
61
0
7
Pt (+)∣Pt (-)
0
8
DMF instead of MeCN
DMSO instead of MeCN
H2O instead of MeCN
50 °C
trace
0
9
10
11
12
13
14
15
0
55
38
54
0
Room temperature
n-Bu4NBF4 instead of n-Bu4NPF6
n-Bu4NOAc instead of n-Bu4NPF6
no electric current
0
Results and discussion
double the amount of all reagents
except for solvent
triple the amount of all reagents
except for solvent
16c
17d
72
76
We initiated our study by using diphenyl acetylene (1a) and glycol
(2a) as model substrates under N2 atmosphere in the presence of C
as the anode, Pt as the cathode, 10 mol% of Ph3N as a catalyst, 1
equivalent of n-Bu4NPF6 as an electrolyte, EtCOOH as an additive,
and MeCN as a solvent at 80 °C. To our delight, the desired product
ester 3a was obtained in good yield (66%, entry 1, Table 1). Next,
we assessed the effect of current intensity on the conversion and
found that on increasing or decreasing the current intensity, the
conversion was reduced (entries 2-4). Then, we assessed the effect
of the electrode on the conversion and found that the anode
material was crucial for this reaction. When a C anode was used,
the different material of the cathode had a slight effect on the yield
(entry 5). When a Pt anode was used, despite the cathode being C
or Pt, no product was obtained (entries 6-7). The choice of solvent
is also crucial for this reaction. Other solvents that are usually used
in electrochemical synthesis such as DMF, DMSO, H2O were tested,
but almost no product was obtained (entries 8-10). The yield was
reduced as the temperature was decreased (entries 11-12).
Additionally, we surveyed electrolytes and found that the use of n-
Bu4NBF4 reduced the yield to 54% and that n-Bu4NOAc even led to
no product formation (entries 13-14). As a control experiment, we
conducted the reaction without electricity and found that none of
the desired product 3a was afforded (entry 15). To improve the
yield further, we increased the concentration. When doubling the
amount of all reagents except solvent, the yield was raised to 72%
(entry 16). When tripling the amount of all reagents except for the
solvent, the yield was raised to 76% (entry 17). Furthermore, we
tested the necessity of the catalyst Ph3N and additive EtCOOH. It is
worth mentioning that without a catalyst and additive, the
18d, e
19d,e,f
20d, e, g
21d, e, h
22d, e, i
without EtCOOH and Ph3N
10 h
76
67
66
55
73
1 mL of 2a
0.5 mL of 2a
under air
a
Reaction conditions: C anode, Pt Cathode, constant current = 18
mA, undivided cell, 1a (0.2 mmol), 2a (0.5 mL), Ph3N (0.02 mmol, 10
mol%), n-Bu4NPF6 (0.2 mmol, 1 equiv.), EtCOOH (0.4 mmol, 2
equiv.), MeCN (5 mL), 80 °C, 12 h, N2. Isolated yield. 1a (0.4
mmol), 2a (1 mL), Ph3N (0.04 mmol, 10 mol%), n-Bu4NPF6 (0.4
mmol, 1 equiv.), EtCOOH (0.8 mmol, 2 equiv.), MeCN (5 mL). 1a
(0.6 mmol), 2a (1.5 mL), Ph3N (0.06 mmol, 10 mol%), n-Bu4NPF6 (0.6
mmol, 1 equiv.), EtCOOH (1.2 mmol, 2 equiv.), MeCN (5 mL).
without EtCOOH and Ph3N. f 10 h. g 1 mL of 2a. h 0.5 mL of 2a. i under
air.
b
c
d
e
transformation proceeded as well as before, affording the desired
product with a yield of 76% (entry 18). Shortening the reation time
to 10 h lead to decrement of the yield (entry 19). The yield was
reduced as the amount of 2a was decreased (entries 20-21). Finally,
we conducted this reaction under air in place of N2, and the yield
was found to be slightly lower (entry 22). Thus, through
optimization of the conditions, we established the optimized
reaction conditions to be as follows: C anode, Pt cathode, 18 mA of
constant current, 0.6 mmol of 1a, 1.5 mL of glycol, 1 equivalent of
Table 2 Substrate scopea,b
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Green Chemistry
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DOI: 10.1039/D0GC02193H
ARTICLE
O
O
HO
RO
C(+) I Pt (-), I = 18 mA
+
+
OR
OR
O
O
R1
R1
R1
R2
n-BuN4PF6 (1 equiv.)
MeCN, 80 °C, 12 h, N2
1
2
3
3
entry
1
alkyne
diol
yield (%) entry
alkyne
diol
product
yield (%)
67
product
O
O
O
OH
OH
OH
76
HO
HO
O
1a
1b
2a
2a
3a
3i
Cl
O
OH
17
O
O
HO
OH
OH
Cl
2
50
1q
2a
OH
65
O
O
3b
OH
3c
3a
O
O
OH
OH
OH
3
4
61
57
HO
HO
75
60
43
O
1c
1d
2a
2a
3k
OH
NC
18
NC
HO
O
O
O
OH
1r
2a
OH
O
O
3d
3a
O
OH
OH
OH
OH
OH
OH
OH
5
6
7
8
9
64
74
67
48
53
t-Bu
TMS
F
t-Bu
TMS
F
O
O
O
O
HO
HO
HO
HO
HO
1e
1f
2a
2a
2a
2a
3l
OH
3e
3f
OH
OH
t-Bu
19
20
HO
HO
O
O
OH
1s
2a
2a
51
68
52
O
O
3b
TMS
F
O
O
OH
OH
OH
O
3k
1g
1h
1i
3g
NC
NC
NC
O
O
OH
1t
OH
Cl
Cl
O
O
3h
3b
Cl
O
O
OH
44
O
2a
3k
3i
Cl
Cl
Cl
NC
OH
O
21
O
HO
OH
OH
OH
10
11
47
50
51
MeOOC
COOMe
1u
2a
O
O
O
HO
39
63
51
1j
2a
3c
3j
MeOOC
O
OH
OH
O
O
3b
OH
3k
NC
HO
OH
OH
NC
22
23
O
O
HO
HO
O
1k
2a
2a
OH
1v
2a
2a
O
O
OH
O
O
3a
3d
OH
OH
56
56
58
61
O
3c
OH
OH
OH
3k
OH
12
13
14
NC
F
NC
F
HO
HO
HO
O
O
O
1l
1w
OH
O
O
O
3g
3k
3h
3a
O
O
OH
OH
OH
63
70
71
63
O
O
O
3e
NC
Cl
OH
t-Bu
t-Bu
24
25
NC
Cl
HO
OH
1x
2a
1m
2a
O
O
3a
O
OH
80
60
O
64
HO
OH
O
OH
3f
TMS
1a
1a
2b
2c
TMS
3m
O
O
OH
1n
2a
O
OH
OH
26
27
51
80
O
HO
3a
3n
O
O
OH
64
62
76
71
O
HO
OH
O
OH
3g
OH
OH
F
15
16
F
1a
1a
2d
3o
HO
HO
O
O
O
1o
1p
2a
2a
OH
O
O
O
O
28
29
31
40
O
O
OH
HO
OH
O
3a
2c
3p
OH
3h
O
O
O
HO
Cl
O
Cl
O
1a
2d
3q
OH
O
3a
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Green Chemistry
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DOI: 10.1039/D0GC02193H
ARTICLE
a Reaction conditions: C anode, Pt cathode (1 cm x 1 cm), I = 18 mA, undivided cell, 1 (0.6 mol), 2 (45 equiv. 27 mmol), n-Bu4NPF6 (1 equiv.
0.6 mmol), MeCN (5 mL), 80 °C, 12 h, N2. b Isolated yield.
n-Bu4NPF6 as electrolyte, and 5.0 mL of MeCN as solvent under N2 diethylene glycol and diethylene glycol monomethyl ether. To our
at 80 °C for 12 h.
delight, the desired products 3p and 3q were obtained smoothly,
With the optimized conditions in hand, we moved on to explore albeit in relatively low yields.
the substrate scope by first varying the substituents on the diaryl
acetylene, as shown in Table 2. For the symmetric diaryl acetylenes,
substituents on the para-, meta- and ortho- position were well
tolerated (3b-3d, entries 2-4). In addition to methyl, bulky groups
substituent tert-butyl was also tolerated (3e, entry 5). In addition,
the trimethylsily group substituted diaryl acetylene was found to be
well tolerated in this esterification reaction, affording the
corresponding product in good yield (3e-3f, entries 5-6). Moreover,
a diverse range of halogen substituents on the various positions of
the phenyl rings were also found to be compatible with this method
Figure 1 X-ray structure of 3j
(3g-3i, entries 7-9). The strong electron-withdrawing ester group
was well tolerated in our catalytic system, affording the
corresponding product 3j in moderate yield (entry 10). The
structure of 3j was conformed by the X-ray (Figure 2). The electron
poor system (3j) working as well as some of the more electron-rich
systems is interesting, since the strong electron-withdrawing group
would decrease the electron density of aromatic ring and its
conjugate system, making the electron hard to lose and the
substrate hardly be oxidized. To explain this phenomenon, a series
of CV studies were carried out (Figure 2) and showed that the
oxidation potentials of the 1a (2.1 V) and ester group substituted
substrate 1j (2.2 V) were not so different. For the asymmetric diaryl
acetylenes, we first assessed diaryl acetylenes that contained only
one substituent (1k-1r, entries 16-24). In general, substituents on
the para-, meta- and ortho- position were well tolerated. Alkyl
groups were suitable for this reaction (1k-1m, entries 11-13). The
trimethylsilyl group substituted diaryl acetylene proceeded
smoothly to afford the corresponding product in good yield (1n,
entry 14). Halogen substituents on the various positions of the
phenyl rings were also found to be compatible with this method
(1o-1q, entries 15-17). The strong electron-withdrawing cyano
group substituted diaryl acetylene proceeded smoothly to afford
the corresponding product in good yield (1r, entry 18). The
asymmetric diaryl acetylenes, which contains different substituents
on its two phenyl rings, were well tolerated (1s-1x, entries 19-24).
After screening the substrate scope for diaryl acetylenes, we next
moved on to explore the substrate scope for alcohols. Linear diols,
propane-1,3-diol and butane-1,4-diol were all found to be
compatible with our newly developed protocol, affording the
corresponding products in moderate yields (3m-3n). It is worth
mentioning that with the extension of the chain length of the linear
diols, the yield was reduced (3a, 3m and 3n). Interestingly, non-
linear diol 2,2-dimethylpropane-1,3-diol was fully applicable to this
catalytic system with excellent yield (80%, 3o). Finally, we tested
Table 3 Substrate scopea,b
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Page 5 of 8
Green Chemistry
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Journal Name
ARTICLE
O
O
HO
HO
HO
HO
C(+) I Pt (-), I = 18 mA
C(+) I Pt (-), I = 18 mA
View Article Online
(1 equiv.)DOI:R110.1039/D0GC02193H
alkyl
+
+
OH
OH
O
O
R1
R1
R1
n-BuN4PF6
MeCN, 80 °C, 12 h, N2
n-BuN4PF6 (1 equiv.)
MeCN, 80 °C, 12 h, N2
5
2
3
4
2
3
entry
1
alkyne
diol
product
yield (%)
29
entry
alkyne
diol
product
yield (%)
49
O
O
O
O
OH
3a
OH
OH
OH
OH
1
2
CH3
HO
HO
HO
5a
5b
5c
5d
2a
2a
2a
2a
4a
4b
4c
4d
2a
2a
2a
2a
3a
O
O
O
O
OH
OH
2
46
OH
58
CH3
HO
3b
OH
3c
3b
OH
3c
O
O
O
O
OH
OH
CH3
CH3
3
4
31
42
OH
OH
HO
HO
3
4
50
52
HO
HO
O
O
O
O
OH
OH
3d
3d
O
O
O
OH
OH
OH
OH
OH
5
6
7
8
43
63
35
32
OH
HO
HO
HO
HO
OH
OH
OH
OH
5
6
7
8
63
72
42
40
t-Bu
Cl
CH3
O
HO
HO
HO
HO
5e
5f
2a
2a
3l
4e
4f
2a
2a
3e
3h
O
O
t-Bu
Cl
O
OH
OH
t-Bu
TMS
F
OH
CH3
CH3
O
3e
t-Bu
TMS
F
O
O
O
OH
O
O
5g
5h
2a
2a
3f
4g
2a
3r
3j
O
O
O
O
OH
3g
OH
MeOOC
CH3
O
O
O
MeOOC
4h
4i
2a
2a
O
O
OH
OH
OH
9
41
Cl
OH
3a
OH
3a
HO
HO
OH
OH
OH
9
52
59
CH2CH3
O
O
HO
HO
HO
5i
2a
3h
Cl
O
O
O
OH
10
11
31
52
O
10
CH2CH2CH3
5j
5f
5f
2a
2b
2c
3r
3s
3t
4j
2a
2a
O
O
O
t-Bu
OH
HO
OH
OH
3a
11
12
13
14
48
54
44
70
CH2CH2CH2CH3
O
t-Bu
O
O
4k
OH
O
OH
12
13
58
48
t-Bu
t-Bu
HO
CH3
HO
OH
O
OH
t-Bu
O
O
4a
2b
2c
3m
O
HO
OH
OH
OH
OH
CH3
5f
2d
3u
O
HO
t-Bu
4a
3n
O
HO
OH
a Reaction conditions: C anode, Pt cathode (1 cm x 1 cm), I = 18 mA,
undivided cell, 5 (0.6 mol), 2 (45 equiv. 27 mmol), n-Bu4NPF6 (1
equiv. 0.6 mmol), MeCN (5 mL), 80 °C, 12 h, N2. b Isolated yield.
CH3
O
OH
3o
4a
2d
a Reaction conditions: C anode, Pt cathode (1 cm x 1 cm), I = 18 mA,
undivided cell, 4 (0.6 mol), 2 (45 equiv. 27 mmol), n-Bu4NPF6 (1
equiv. 0.6 mmol), MeCN (5 mL), 80 °C, 12 h, N2. b Isolated yield.
Table 5 Substrate scopea
Beside diaryl acetylenes, arylalkylalkynes were also suitable for
this reaction (Table 3). We screened the substituents on the phenyl
ring firstly. Substituents on the para-, meta- and ortho- position of
the phenyl ring were well tolerated (4b-4d, entries 2-4). Alkyl and
halogen substituents were found to be compatible with this method
(4b-4f, entries 2-6). To our delight, the strong electron-donating
substituents working as well as the strong electron-withdrawing
substituents (4g-4h, entries 7-8). Next, we moved on to explore the
alkyl group of the arylalkylalkynes and found that a serious of alkyl
groups were compatible with this method (4l-4k, entries 9-11). At
last, we explored the substrate scope for alcohols. Linear and non-
linear diols were applicable to this catalytic system (3m-3o, entries
12-14).
O
C(+) I Pt (-), I = 18 mA
+
R
OH
O
R
n-BuN4PF6 (1 equiv.)
MeCN, 80 °C, 12 h, N2
1a
2
6
O
O
O
O
O
O
b
c
6a
, complex
6b
, complex
6c
, complex
a Reaction conditions: C anode, Pt cathode (1 cm x 1 cm), I = 18 mA,
undivided cell, 5 (0.6 mol), 2 (45 equiv. 27 mmol), n-Bu4NPF6 (1
equiv. 0.6 mmol), MeCN (5 mL), 80 °C, 12 h, N2. b 60°C. c 75°C.
Table 4 Substrate scopea,b
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ARTICLE
Journal Name
Anode
Ph
Cathode
View Article Online
DOI: 10.1039/D0GC02193H
Ph
1a
A
Ph
Ph
H2O
e-
O
OH
7
O
O
O
OH
O
8
Ph
3
O
e
HO
OH
HO
C
Figure 2 Cyclic voltammograms recorded in MeCN with 0.1 M n-
Bu4NPF6 as the supporting electrolyte. 1a (1 mM). 1j (1 mM). 2a (1
mM).
2a
O
H2
+
HO
B
To our excitement, no only internal alkynes, but also terminal
alkynes were suitable for this reaction (Table 4). Alkyl groups on the
para-, meta- and ortho- position were well tolerated (5b-5f, entries
2-6). The trimethylsilyl group substituted phenylacetylene
proceeded smoothly to afford the corresponding product (5g, entry
7). Halogen substituents were also found to be compatible with this
method (5h-5i, entries 8-9). Strong electron-donating methoxy
group was suitable for this reaction (5j, etries 10). Linear and non-
linear diols were applicable to this catalytic system (3s-3u, entries
11-13).
It is interesting that this reaction is suitable only for diols, not for
mono alcohols. we tried to use the methanol, ethanol, propanol as
he partners of the diphenylacetylene, but they were all failed. The
reaction system was complex (Table 5).
To gain insight into the reaction mechanism, a series of CV
studies were carried out (Figure 2). The oxidation peak for 1a was
found to occur at 2.1 V, The oxidation peak for 2a was not obvious,
but the curve of 2a was not the same as the black curve. It seemed
that there was a peek at about 2.8V, which illustrated the fact that
the glycol is electroactive. 1a was oxidized more easily than 2a.
To further probe the mechanism, a series of control experiments
were carried out (scheme 2). Firstly, when TEMPO was added to the
model reation, no desired product was obtained, which indicated
Scheme 3 Plausible reaction mechanism.
that the reaction went through a radical pathway (scheme 2a).
Furthermore, the plausible intermediate 2-hydroxy-1,2-
diphenylethanone (7) and benzil (8) were subjected to the standard
reaction conditions, and the desired product 3a was obtained in 43%
and 42% yields respectively.
Based on the CV experiments, the control experiments and
previous reports,10-12 a plausible mechanism for the electrochemical
esterification of alkynes is proposed in scheme 3. First, diaryl
acetylene is transformed to the radical cation A by anode oxidation,
which undergoes deprotonation with water from the reagents to
generate the radical intermediate 7. This is followed by further
anode oxidation, where the cation intermediate 8 is formed. At the
cathode, glycol (2b) was transformed to anion B and H2. Then B was
oxidized by anode to form the radical C. Finally, Further
fragmentation of with radical C to generate the desired ester
product 3.
Conclusions
In conclusion, we developed the first electrochemical
carbon-carbon triple bonds cleavage reaction. It provided a
facile strategy for the synthesis of esters from alkynes. This is
also the first example of carbon-carbon triple bonds cleavage
reaction of alkynes with diols. No catalyst, oxidant and additive
was used. The substrate scope was wide. Internal and terminal
alkynes were all suitable for this reaction; Linear diols, non-
linear diols, diol derivatives were also suitable for this reaction.
C (+) I Pt (-), I = 18 mA
O
TEMPO (2 equiv.)
OH
OH
(a)
+
O
HO
n-Bu4NPF6 (1 equiv.)
MeCN, 80 °C, 12 h, N2
1a
2a
3a
, yield: 0%
O
O
O
C (+) I Pt (-), I = 18 mA
OH
OH
OH
OH
+
+
(b)
(c)
HO
HO
n-Bu4NPF6 (1 equiv.)
MeCN, 80 °C, 12 h, N2
OH
7
2a
2a
3a
3a
, yield: 43%
O
O
O
C (+) I Pt (-), I = 18 mA
Conflicts of interest
The authors declare no competing financial interest.
n-Bu4NPF6 (1 equiv.)
MeCN, 80 °C, 12 h, N2
O
8
, yield: 42%
Scheme 2 Control experiment
6 | J. Name., 2012, 00, 1-3
Acknowledgements
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ARTICLE
2018, 83, 10589. (v) S. Wu, S. Luo, W. Guo, T. Wang, Q. Xie, J.
We gratefully acknowledge the Anhui Provincial Natural Science
Foundation (No. 1808085QB29) and Key Project of Provincial
Natural Science Research Foundation of Anhui Universities, China
(No. KJ2018A0675, KJ2018A0389) for financial support.
Wang and G. Liu, Organometallics, 2018, 37,V2ie3w3A5rti.cle(wOn)linSe.
DOI: 10.1039/D0GC02193H
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Zhou, J.-Y. Wang, T.-S. Zhang, G. Li, W.-J. Hao, S.-J. Tu and B.
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