Electrochemical Radical Borylation of Aryl Iodides

Article abstract of DOI:10.1002/cjoc.201900001

Herein, we report the first electrochemical strategy for the borylation of aryl iodides via a radical pathway using current as a driving force. A mild reaction condition allows an assorted range of readily available aryl iodides to be proficiently converted into synthetically valuable arylboronic esters under transition metal catalyst-free conditions. Moreover, this method also shows good functional group tolerance. Initial control mechanistic experiments reveal the formation of aryl radical as a key intermediate and the current plays an important role in the generation of radical intermediate.

Full text of DOI:10.1002/cjoc.201900001

Accepted Article
Title: Electrochemical Radical Borylation of Aryl Iodides
Authors: Junting Hong, Qianyi Liu, Feng Li, Guangcan Bai, Guoquan Liu, Man Li,
Onkar S. Nayal, Xuefeng Fu and Fanyang Mo*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2019, 37, 10.1002/cjoc.201900001.
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published online in Early View: http://dx.doi.org/10.1002/cjoc.201900001.
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Electrochemical Radical Borylation of Aryl Iodides†
Junting Hong,‡a Qianyi Liu,‡a Feng Li,b Guangcan Bai,b Guoquan Liu,b Man Li,a Onkar S. Nayala , Xuefeng Fuc and Fanyang
Mo*,a,d
a Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, China
b State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, Beijing 100871, China
c Department of Biomedical Engineering, College of Engineering, Peking University, Beijing 100871, China
d Jiangsu Weiming Environmental Protection S&T Co., ltd
Cite this paper: Chin. J. Chem. 2019, 37, XXX—XXX. DOI: 10.1002/cjoc.201900XXX
Summary of main observation and conclusion Herein, we report the first electrochemical strategy for the borylation of aryl iodides via a radical
pathway using current as a driving force. A mild reaction condition allows an assorted range of readily available aryl iodides to be proficiently
converted into synthetically valuable arylboronic esters under transition metal catalyst-free conditions. Moreover, this method also shows good
functional group tolerance. Initial control mechanistic experiments reveal the formation of aryl radical as a key intermediate and the current plays
important role in the generation of radical intermediate.
numerous transition-metal-free aromatic borylation protocols
Background and Originality Content
have been developed using anilines,46-49 aryl (pseudo)halides,50-56
and aryl carboxylates57 as the coupling substrates (Scheme 1b).
Arylboronic acids and their derivatives have found widespread
applications in organic synthesis.1-3 Consequently, the methods of
Inspired from these strategies, we thought to develop an
their preparation have been intensively developed in the past
electrochemical radical borylation of aryl halides via cathodic
decades. Conventional approaches to the synthesis of arylboronic
reduction under transition-metal-free condition (Scheme 1c).
acids or esters from aryl halides require either prior preparation of
We present herein mild radical borylation of aryl halides with
highly reactive organometallic reagents,4-5 or the procedures6
commercially available bis(pinacolato)diboron (B2pin2), as the
required transition-metal such as Pd,7-10 Ni,11-13 Cu,14-16 Zn17 and
boron source. After the comparison of the present study with the
Fe18 as a catalyst. These methods are widely used in laboratories
known borylation methods, we found that the electrochemical
and pharmaceutical companies. However, the limited functional
radical borylation reaction has many advantages over the known
group compatibility and the use of expensive metals and/or
protocols. Primarily, the overall process is sustainable in which
ligands or strong bases are the obvious drawbacks in these
electron serves as the reductant with solvents being oxidized at
methods. Nevertheless, there is a strong need to the development
the anode, therefore no sacrificing anode is needed. Furthermore,
of efficient, mild, and transition-metal-free synthetic approach for
this reaction can be performed under air atmosphere. Moreover,
the borylation of aryl halides.
this electrochemical radical borylation of aryl iodides approach got
In this context, Organic electrochemistry offers a green and
remarkable increase in reaction rate over the reported work on
sustainable alternative tool to conventional chemical approaches
borylation of aryl iodides by Zhang’s group under the similar
for organic transformations.19-35 Various versatile reactive
intermediate (e. g., radical cation, radical anion, and radical) can
be efficiently generated by anodic oxidation, cathodic reduction,
as well as paired electrolysis. Our laboratory has been engaged in
the development of organic electrochemistry to overcome unmet
synthetic challenges.36-38 With this background, we thought to the
synthesis of aryl boronates from aryl halides through
electrochemical transformation (Scheme 1c).
reaction conditions.54
In the context of electro-reduction of aryl halides, it’s well
known that aryl halide easily transforms to aryl radical via a
reversible, dissociative electron transfer (DET) process. Ensuing,
this radical is widely used in various organic transformations
(Scheme 1a).39 Except this strategy, transition-metal-free
borylation approaches are also well known for the synthesis of a
diverse range of organoboron compounds via radical pathway.40-42
Since the pioneering work of the Wang group in 2010,43-45
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Junting Hong et al.
Concise Report
Scheme 1 Electrochemical borylation of aryl halides
the presence of cesium carbonate (entry 1). Notably, either
increasing or decreasing the current resulted in lower yields
(entries 2 and 3). After that, other bases such as CsF, CsOAc,
t-BuOK and MeONa were also investigated under similar reaction
conditions. However, all these bases were less effective. (entries
4-5 and 7-8). To our delight, the significant change was observed
in the yield of the desired product after the elevation of reaction
temperature in 2h (entry 6). Moreover, other solvents, such as
EtOH, THF and MeCN were also tested to optimize the best
reaction solvent. Regrettably, no better yields of 3a were obtained
(entries 9-11). In the cases of THF and MeCN solvents, the
reaction media became less conductive (30 V, 1-3 mA). Therefore,
electrolyte Bu4NClO4 was added in the reaction media to succeed
the reaction (entries 10-11). After the screening of additional
reaction times, it is revealed that 3 h is the optimum time for the
completion of the reaction (entry 12). Moreover, drastically
changes were observed in the yield of the desired product after
increasing the amount of B2pin2 from 3.0 and 4.0 equivalent (entry
1 and entries 12-13). Finally, in the absence of current, deleterious
impact was observed in the yield of 3a (entry 14), which clearly
indicates the significant role of current in the generation of aryl
radical. Moreover, the “on and off” experiment also indicated the
requirement of continuous current (entry 15).
Well-known electroreduction of
halide
process
(a)
aryl
cathode
reduction
+
e
+
X
Ar
X
Ar
Ar
X
Well-established
via radical mechanism
(b)
borylation
ArN2X
ArX
ArCO2NHPI
BPO
...
hv
etc.
heat,
,
base,
Ar
+
B
Bpin
2pin2
radical
processes
Electrochemical radical
of
iodides this work
(c)
borylation
aryl
(
)
R
R
cathode reduction
I
Bpin
no
B
base
,
2pin2
Sustainable
Soft conditions
(not
Simple operation (undivided cell)
process
anode,
or
(no sacrificing
sensitive
to air
electrolyte)
moisture)
Results and Discussion
Results (sample title, optional)
In this investigation, we chose phenyl iodide 1a as model
substrate and pinacol diboron (B2pin2) 2 as a borylating agent
(Table 1). Interestingly, 54% yield of 3a was obtained using
platinum meshes as the electrodes with 13 mA constant current in
Table 1 Reaction optimizationa
+ -
/
Pt
,
cc
O
O
O
O
O
O
Pt
( ) ( )
I
B
+
B B
ase
.
e u v
m
B
S l
2 0
(
i
q
(
)
o ven
t 5
L
)
a
1
.
mm
a
o
2
3
0 5
l
)
me
T
ti
,
v
e ce
(
un
di id d ll
Time (h) Solvent
Conv. of 1a (%)b
Yield (%)b
44
Entry
1
Base
1a / 2 / Base
Current (mA)
10
T (oC)
Cs2CO3
1:3:2
50
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
69
61
79
71
58
100
51
59
85
40
64
98
97
19
52
Cs2CO3
Cs2CO3
CsF
1:3:2
1:3:2
1:3:2
1:3:2
1:3:2
1:3:2
1:3:2
1:3:2
1:3:2
1:3:2
1:4:2
1:3.5:2
1:4:2
1:4:2
13
20
50
50
50
50
65
65
65
78
65
65
65
65
65
65
54
2
52
3
13
33
4
CsOAc
Cs2CO3
tBuOK
MeONa
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
13
16
5
13
64
6
13
40
7
13
44
8
13
30
9
13
THFc
34
10
11
12
13
14
15d
13
MeCNc
MeOH
MeOH
MeOH
MeOH
11
13
76
13
67
0
14
0 – 13
33
a Reaction conditions: Platinum mesh electrodes (1 cm × 1 cm), 1a (0.5 mmol), 2, base (2.0 equiv.), solvent (5 mL), heating, undivided cell, sealed tube. cc,
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Electrochemical Radical Borylation of Aryl Iodides
Chin. J. Chem.
b
c
d
constant current. Determined by GC-FID with n-decane as an internal standard. Bu4NClO4 as the electrolyte was added. “On and off” control
experiment. During a total time of 3 hours, the current continued for 3 min and then interrupted for 27 min, cycled for 6 times.
cc
13 mA
With the optimal reaction conditions in hand, we explored the
substrate scope of aryl iodide (Table 2). Notably, a wide range of
aryl iodide derivatives bearing both electron-withdrawing and
electron-donating groups could be borylated in moderate to good
yields. Aryl iodides with an electron-donating group furnished the
corresponding borylation product in good yields (3a-3e). A
sterically hindered aryl iodide such as 2e and 2f smoothly engaged
in the reaction and afforded a moderate yield of the respective
product. In addition, aryl iodides with an electron-withdrawing
group, such as an amide and an ester, produced borylation
products in moderate yields (3g−j). Notably, for substrate 1j’, the
ester group was converted to phenolic hydroxyl and furnished
product 3j in 60% yield.
Pt(+)/Pt(-),
I
Bpin
Cs2CO
3 (2 equiv)
65 o
+
B
2pin2
R
R
MeOH
3
h
C,
(5 mL),
undivided cell
2
3
1
Bpin
Bpin
Bpin
MeO
3a
, 76%
3c
3b
,
62%
, 63%
Bpin
Bpin
Bpin
To study the mechanism, we designed a series of electron
paramagnetic resonance (EPR) experiments. In this context,
dimethylpyridine N-oxide (DMPO) was introduced as a spin
trapping agent to convert the short-living active radical into EPR
detectable nitroxide radicals.58 The EPR spectra were simulated
using the least-squares fitting method with EasySpin’s function
garlic (for details, see Supporting Information). The parameters,
isotropic g values and hyperfine constants A were carefully
compared with literature data and used as the unique fingerprint
of the trapped radicals.
3e
, 60%
3f
, 45%
3d
, 60%
Bpin
O
Bpin
Bpin
Ph
N
H
EtOOC
3i
Me2N
, 48%
, 49%
3h b
,
56%
3g
Interestingly, when the current is passed through in a reaction
medium for 5 minutes at room temperature, the strong EPR signal
is appeared (see Figure 1a). Besides, in the absence of current, the
intensity of EPR signal is decreased under similar conditions. This
EPR signal is generally assigned to be DMPO•Ph, which is already
known in various reported literature.59-60 However, on the basis of
previous reports, we found that the signals of some other species
such as DMPO•CH3,60-61 DMPO•CH2OH,61-62 appear very close or
similar to the EPR signal of DMPO•Ph. To verify an
electrochemically generating radical species by the reduction of
halides on cathode process, we carried out the reaction with
n-C4F9I in the presence of 45 mA current. As expected, a very
Bpin
I
Bpin
O
Ph
O
HO
3k
, 30%
1j'
,
c 61%
3j
a
Reaction conditions: 1 (0.5 mmol), 2 (2.0 mmol, 4.0 equiv.),
Cs2CO3 (1.0 mmol, 2.0 equiv), MeOH (5 mL), platinum mesh
o
electrodes (1 cm × 1 cm), constant current 13 mA, 65 C, 3 h,
b
undivided cell, sealed tube. Using reticulated vitreous carbon
(RVC) electrodes. c From 1j’.
61
To further clarify the reduction process of iodides, we designed
cyclic voltammetry (CV) measurements. Initially, when we mixed
phenyl iodide 1a with LiClO4 solution, a single reduction peak was
appeared which is shown in Figure 2a. Noteworthy, after the
adding of B2pin2 and base Cs2CO3 in the model reaction, the single
reduction peak appeared with a sharp band. While this peak was
completely disappeared in the absence of phenyl iodide 1a as
shown in Figure 2b. Thus, these results indicate towards the single
electron reduction process of aryl iodides on the cathode. In
addition, we conducted differential pulsed voltammetry (DPV)
experiments to further indicate toward the single electron
reduction of aryl iodide shown in Supporting Information.
special “quasi-quartet” signal of DMPO•C4F9 appears within 30
minutes shown in Figure 1b due to the inductive effect caused by
the adjacent fluorine substituents. On the basis of results of
control experiments presented in Supporting Information, the
mentioned literatures and the cyclic voltammetry studies (vide
infra), we concluded that the strong EPR signal appears due to the
formation of DMPO•Ph species in Figure 1a. Thus, these results
indicate toward the generation of aryl radicals and current
enhance the rate of formation of aryl radical.
Table 2 Substrate scopea
According to our mechanistic investigations and recent relevant
literatures,63-64 a plausible mechanism for this electrochemical
borylation of aryl iodides is proposed (Scheme 2). Initially, at the
cathode, aryl iodide 1 undergoes reduction to afford the
corresponding radical anion C, which is further transformed into
the aryl radical D via carbon-halogen bond cleavage. Further, aryl
radical D interacts with B2pin2 in the presence of a base (RO–) to
generate the desired product 3 along with the formation of G
Chin. J. Chem. 2019, 37, XXX－XXX
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Junting Hong et al.
Concise Report
through intermediate E and F. Meanwhile, the reaction becomes
redox neutral due to the anodic oxidation of the solvent to give
oxides which balance the overall transformation. Consequently,
intermediate G quenched by solvent, oxides or other aryl iodides
via SET to afford E. However, the possibility of the formation of
desired product 3 via the generation of boron-centered radical I
cannot be ruled out at this stage.
(b) With B2pin2 and base
0.5
B2pin2 + Cs2CO3
PhI + B2pin2 + Cs2CO3
-2.40 V
0.4
0.3
0.2
0.1
0.0
-0.1
(a)
O
N
O
N
=
2.0063
=
=
g
current
rt
H
Pt(+)/Pt(-),
Cs CO ,
H
AN 14.9 G
AH 21.7 G
+
+
PhI
B
2pin2
Ph
MeOH,
3
2
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
Ref. 18
A
DMPO
Potential (V) vs SCE
•
DMPO Ph
0 mA Exp
0 mA Sim
15 mA Exp
15 mA Sim
Figure 2 Cyclic voltammetry (CV) experiments. Reaction conditions:
1a (3 mmol), LiClO4o(3 mmol), B2pin2 (6 mmol), Cs2CO3 (3 mmol),
MeOH (30 mL), 20 C. L-type glassy carbon as working electrode,
Pt wire as counter electrode and saturated calomel electrode (SCE)
as reference. Scan rate: 0.02 V/s.
Scheme 2 Proposed mechanism
3470
3480
3490
3500
3510
3520
3530
3540
3550
Magnetic Field (G)
O
B
O
Ar
Bpin
(b)
+
=
g
AN 13.8 G
AH 15.4 G
2.0067
=
=
O
N
O
F
N
3
F
C3F7
current
Pt(+)/Pt(-),
I
H
+
+
At the cathode
+ e
C4F9I
B
2pin2
rt
Cs CO ,
MeOH,
3
2
H
B
•
less
O
=
AF 0.57 G
possible
DMPO
Ref. 18
B
DMPO C4F9
2pin2
2
-
I
O
Ar
Ar
I
Ar
I
0 mA Exp
45 mA Exp
45 mA Sim
B
1
D
C
B
O
O
Ar
E
RO
to
oxides
iodides
SET
or
solvent,
RO
Bpin
H
RO
Bpin
G
other
aryl
O
O
B
B
O
3470
3480
3490
3500
3510
3520
3530
3540
3550
O
Ar
Ar
Bpin
Magnetic Field (G)
RO
F
3
At the anode
Figure 1 EPR experiments. Reaction conditions: iodides (0.3 mmol),
B2Pin2 (1.2 mmol, 4.0 equiv.), Cs2CO3 (0.6 mmol, 2.0 equiv), DMPO
(0.3 mmol, 30 μL), MeOH (3 mL), platinum mesh electrodes (1 cm
× 1 cm), stirred at room temperature with given current and time.
(a) Without B2pin2 and base
+
-
-
H
e,
Solvent
Oxides
0.20
Only LiClO4
Conclusions
PhI + LiClO4
0.16
0.12
0.08
0.04
0.00
-0.04
-2.50 V
In summary, we have developed a sustainable and practical
electrochemical borylation of aryl iodides, providing a very useful
approach to arylboronic esters. EPR, CV and PVD experiments
verified the formation of aryl radicals in this electrochemical
borylation reaction, which supports
a radical borylation
mechanism. This electrochemical borylation reaction is
complementary to other borylation methods and might inspire
further investigations in the radical borylation strategy.
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
Potential (V) vs SCE
Experimental
An undivided cell was equipped with two platinum mesh
electrodes and connected to a DC regulated power supply. Aryl
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Electrochemical Radical Borylation of Aryl Iodides
Chin. J. Chem.
iodide 1 (0.5 mmol), B2pin2 (2 mmol, 508 mg, 4 equiv), Cs2CO3 (1.0
mmol, 326 mg, 2 equiv) and 5 mL of anhydrous MeOH was added
to the cell. The mixture was electrolyzed using constant current
conditions (13 mA) at 65 oC under magnetic stirring. After almost 3
hours, the TLC analysis indicated that the electrolysis was
complete (witnessed by the disappearance of the aryl iodide).
Upon cooling to room temperature, the reaction mixture was
transferred to a 100 mL flask by methanol, and then a little silica
gel was added into it. After removal of the solvent in vacuo, the
residue was poured onto a silica gel column and purified by
column chromatography to give the desired product 3.
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3841-3844.
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T.;
Murata,
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Lipshutz, B. H.; Moser, R.; Voigtritter, K. R., Miyaura
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The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Acknowledgement (optional)
The project was supported by the Natural Science Foundation
of China (Grant 21772003). We also thank the “1000-Youth
Talents Plan” and Peking University for start-up funds. This work
was also partially funded by the Peking University carbon capture,
utilization and storage project supported by BHP Billiton.
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Concise Report
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(The following will be filled in by the editorial staff)
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Manuscript revised: XXXX, 2019
Manuscript accepted: XXXX, 2019
Accepted manuscript online: XXXX, 2019
Version of record online: XXXX, 2019
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Electrochemical Radical Borylation of Aryl Iodides
Chin. J. Chem.
Entry for the Table of Contents
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Junting Hong, Qianyi Liu, Feng Li, Guangcan Bai,
Guoquan Liu, Man Li, Onkar S. Nayal and
Fanyang Mo*
The first electrochemical strategy for the radical borylation of aryl iodides is reported in this paper.
The significant promoting effect of current in the aryl radical generation revealed by EPR studies.
Chin. J. Chem. 2019, 37, XXX－XXX
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
This article is protected by copyright. All rights reserved.