Selective electrocatalytic hydroboration of aryl alkenes

Article abstract of DOI:10.1039/d0gc03890c

Organoboron compounds are powerful precursors of value-added organic compounds in synthetic chemistry, and transition metal-catalysed borylation has always been dominant. To avoid toxic reagents and costs associated with metal catalysts, simpler, more eco

Full text of DOI:10.1039/d0gc03890c

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Selective electrocatalytic hydroboration of aryl
alkenes†
Cite this: Green Chem., 2021, 23,
1691
Yahui Zhang,a Xiangyu Zhao,b Ce Bi,a,c Wenqi Lu,a Mengyuan Song,a
Dongdong Wanga and Guangyan Qing
*
a,c
Organoboron compounds are powerful precursors of value-added organic compounds in synthetic
chemistry, and transition metal-catalysed borylation has always been dominant. To avoid toxic reagents
and costs associated with metal catalysts, simpler, more economical and effective approaches for deliver-
ing organoborons are highly desirable. Here, without the use of any metal catalysts, a CH3CN-involved
electrochemical borylation reaction is reported with alkenes and HBpin as substrates. The site-selectivity
is realised to achieve mono- or di-functional borylation of an unsaturated bond by regulating the pro-
portion of HBpin. In addition, the success of gram-scale experiments and versatile conversions confirms
the potential applications of this strategy in industrial synthesis. The vital auxiliary role of N,N-diisopropyl-
ethylamine (DIEA) in the process of acetonitrile electrolysis is disclosed in the mechanism study. The pro-
posed new strategy provides a generic platform for the discovery of additional challenging electro-
chemical systems for hydrogenation reactions.
Received 17th November 2020,
Accepted 29th January 2021
DOI: 10.1039/d0gc03890c
rsc.li/greenchem
alkenes and pinacolborane (HBpin) to produce the alkylboro-
nate derivatives, which exhibit high chemoselectivity, regio-
Introduction
Organoboron derivatives are indispensable multifunctional selectivity, and stereoselectivity in some cases.
building blocks that have been widely applied in organic syn-
Despite the significant advances of transition-metal-cata-
thesis, drug discovery and materials science,1–3 owing to their lysed hydroboration reactions to access the corresponding
broad availability in fine chemistry, high stability in air, and alkylboronate esters, the necessity of metal catalysts with
efficient conversion into various functional groups (Fig. 1a).4–6 ligands severely limits their application in industrial synthesis
In particular, the widely employed Suzuki–Miyaura cross-coup- as the generation of hazardous waste in these transformations
ling reaction can effectively utilise the C(sp3) organoboron is still inevitable.19,20 Particularly in large-scale synthesis, the
species and aryl or alkyl halides for the construction of new C– preparation, storage, removal or recovery of metal catalysts
C bonds.7 In the past decades, transition-metal-catalysed imposes significant limitations.21 Alternatively, the evolution
alkene hydroboration, which involves the addition of a B–H from transition-metal to metal-free catalysts for directed bory-
bond across the π-system of an unsaturated double bond,8 has lation is highly desirable.22 In recent years, power generation
been extensively researched in synthetic chemistry. After the from various renewable energy sources, such as wind and
emergence of Wilkinson’s catalyst, (Ph3P)3RhCl,9 a variety of solar, has surged drastically.23,24 This shift in green source of
transition-metal catalysts containing precious metals10–12 such energy accounted for almost two-thirds of net new power
as rhodium, iridium and ruthenium and Earth-abundant capacity25 and drives a transition from thermochemical to
metals13–18 such as iron, cobalt and nickel have been devel- electrochemical processes.26 Correspondingly, organic electro-
oped to effectively catalyse the hydroboration of terminal chemical synthesis research has attracted considerable atten-
tion. As a sustainable and eco-friendly synthetic tool,27–30
electrochemistry exhibits flexible control in the synthesis of
aKey Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of
value-added organic compounds by modulating the magnitude
Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian
of current or voltage. Thereupon, various core skeletons, such
116023, P. R. China. E-mail: qinggy@dicp.ac.cn
bSixth Laboratory, Sinopec Dalian (Fushun) Research Institute of Petroleum and
Petrochemicals, 96 Nankai Road, Dalian 116045, P. R. China
cCollege of Chemistry and Chemical Engineering, Wuhan Textile University, 1
Sunshine Road, Wuhan 430200, P. R. China
as the formation of C–S, C–N, C–C, C–Cl, C–P and other
bonds,31–35 can be constructed successfully through electro-
chemical synthesis. Although boron chemistry has been
acknowledged by two Nobel Prizes, the development of C–B
formation by electrochemical synthesis has rarely been
reported.36,37 Although electrophotocatalytic borylation of aryl
†Electronic supplementary information (ESI) available. CCDC 2022111. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0gc03890c
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(3) a detailed mechanism was fully determined. The designed
strategy not only conforms to the concept of atom-economic
chemistry, but also provides a new idea for the development of
electrochemical hydrogenation.
Results
Reaction design
Initially, we began the study with the reaction of styrene (1a)
and HBpin (2) as the model substrates. Notably, no hydrobora-
tion product was observed without an electric current (Table 1,
entry 1), which illustrates the indispensability of the electro-
chemical reaction. Controlled experiments also revealed that a
nitrogen atmosphere was essential to this transformation
(Table 1, entry 2). By using tetrabutylammonium hexafluoro-
phosphate (nBu4NBF4, 20 mol%) as a supporting electrolyte
and CH3CN as a solvent, the reaction was performed under a
constant current (15 mA) in an undivided cell under an inert
atmosphere, equipped with two Pt plates as an anode and a
cathode. We found that the anode surface was covered with a
thick layer of an unknown yellow compound, and the target
product 4,4′,5,5′-tetramethyl-2-(phenylethyl)-1,3,2-dioxaboro-
lane (3a) was obtained in a yield of 35% (calculated by gas
chromatography (GC), Table 1, entry 3), accompanied by the
generation of by-products, such as the Markovnikov-selective
product 4,4′,5,5′-tetramethyl-2-(1-phenylethyl)-1,3,2-dioxaboro-
lane (4a), phenylethane (5) and B2pin2 (6) (see GC-MS in
Fig. S4 in the ESI†). In addition, no product was detected if
acetone or DMF was used as the solvent (Table 1, entries 4 and
Fig. 1 Reaction design. (a) Significance of the organoboron derivatives
in functional group transformations. (b) Chemical structure of alkylboro-
nate prepared by electrochemical synthesis, fulfilling the requirements
of green chemistry. (c) Illustration of the electrochemical synthesis strat-
egy of organoboron esters and its remarkable characteristics.
halides has been realised, research on the selective bonding
between carbon and boron atoms through an atom-economical
electrosynthesis to produce alkylboronate esters remains
lacking (Fig. 1b).
Table 1 Optimisation of the reaction conditionsa
Electrochemical oxidative cross-coupling reactions have
been proved to be a powerful strategy for forming chemical
bonds,31 and are accompanied by the formation of hydrogen
(H2) on the cathode surface. These distinctive features pose an
opportunity and challenge to the feasibility of electrochemical
hydroboration. The opportunity is the direct generation of
active boron radical intermediates by a single electron transfer
between HBpin and the electrode surface, and a boron-free
radical coupling reaction catalysed by transition-metal cata-
lysts has been proven to be a powerful way to achieve the
alkene hydroboration reaction.38,39 The challenge is that
hydrogen evolution competes with the hydrogenation conver-
sion reaction. Therefore, effective preparation and utilization
of hydrogen ions to synthesise the target product while ensur-
ing a high reaction rate on the cathode under electrochemical
conditions is challenging.
Here, we implemented an attractive protocol for the selec-
tive electrochemical synthesis of alkene hydroboration pro-
ducts (Fig. 1c): (1) N,N-diisopropylethylamine (DIEA) was
found to play a key role in the electrolysis of the solvent
CH3CN to provide sufficient hydrogen ions to participate in
the conversion of hydroboration products effectively; (2) gram-
scale experiments were successfully performed smoothly; and
Variation from the
reaction conditions
Yield of
Entry
3ab (%)
3a/4a/5/6c
1
2
3
4
5
6
7
8
No electric current
Air instead of N2
CH3CN
Acetone
DMF
N.D.
N.D.
35
N.D.
N.D.
56
53
42
31
51
—
—
83 : 1 : 5 : 11
—
—
62 : 9 : 17 : 12
61 : 10 : 17 : 12
60 : 8 : 22 : 10
63 : 0 : 30 : 7
57 : 9 : 24 : 10
82 : 5 : 11 : 2
None
0 °C instead of rt
40 °C instead of rt
C(+)|Pt(−) instead of Pt(+)|Pt(−)
Pt(+)|Ni(−) instead of Pt(+)|Pt(−)
DIEA (0.8 eq.)
9
10
11
72
a Reaction conditions: 1a (1.0 mmol),
2
(1.1 mmol), nBu4NBF4
(20 mol%), Pt(+)|Pt(−), constant current (I) = 15 mA, CH3CN : THF =
4 : 1 (v/v), the total volume of the solvent was 10 mL, N2, rt, 3 h. b The
yields of products were determined using GC analysis using naphtha-
lene as an internal standard. c Selectivity was determined using GC
analysis. N.D. = not detected.
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5), which demonstrates the necessity of CH3CN in this electro-
chemical hydroboration conversion. If a mixed system CH3CN/
THF (v : v = 4 : 1) was selected as the reaction solvent, we found
that a 1.0 equivalent of precursor 1a led to full conversion in
the presence of 1.1 equivalents of 2 at room temperature for
3 h and the yield of product 3a was increased to 56% (Table 1,
entry 6), with a 3a/4a/5/6 ratio of 62 : 9 : 17 : 12. Furthermore,
an appropriate ratio of CH3CN to THF in the mixed solvent
was required. Other mixed solvent systems, such as CH3CN/
Et2O and CH3CN/1,4-dioxane, were unsuitable for this reaction
(Table S1 in the ESI†).
Subsequently, to further optimise the yield and selectivity
of the target product, several factors affecting the reaction
results were investigated, including temperature, reaction
time, amount of current and electrodes. The selected results
are listed in Table 1 and Tables S2 and 3 in the ESI.† If the
reaction temperature was decreased to 0 °C or increased to
40 °C and 60 °C, 3a was obtained in 53%, 42% and 38%
yields, respectively (Table 1, entries 7 and 8, Table S2 in the
ESI†). Moreover, the reaction efficiency was susceptible to the
reaction time and the amount of current, and a detailed con-
dition screening is depicted in Table S3 in the ESI.† In
addition, the electrode material had a significant effect on the
reaction results; replacing the anode platinum sheet with a
graphite rod led to a decreased yield of 3a, and if a nickel
sheet was used as the cathode, a slight decrease in the target
product was detected (Table 1, entries 9 and 10).
Scheme 1 Electrochemical hydroboration of different aryl alkenes.
Reaction conditions: 1 (1.0 mmol), 2 (1.1 mmol), DIEA (80 mol%),
nBu4NBF4 (20 mol%), Pt(+)|Pt(−), constant current (I)
=
15 mA,
CH3CN : THF = 4 : 1 (v/v), the total volume of the solvent was 10 mL, N2,
rt,
h. Isolated yields are shown. a The constant current was
3
We observed that the surface of the anode had a thin layer
of yellow substance after the electrochemical reaction, which
may have resulted due to the accumulation of compounds on
the electrode surface after the loss of electrons and protons.
According to the literature,40 DIEA can act as a sacrificial elec-
increased to 20 mA and the reaction time was prolonged to 4 h. N.D. =
no detect.
tron donor to promote the turnover of the catalytic cycle. electron-donating groups (–tBu, –OMe and –Me) on the
Therefore, DIEA is thought to participate in this system. benzene ring of styrene were tolerated to deliver the corres-
Further explorations indicated that the addition of DIEA could ponding terminal alkene hydroboration products (3e–3g) in
promote the formation of product 3a (Table 1, entry 11) and a satisfactory yields. In addition, the internal trans-/cis-
0.8 equivalent of DIEA was the optimal choice (Table S4 in the β-methylstyrene was also compatible with our electrochemical
ESI†), as increasing or decreasing the dosage of DIEA led to a method, and the product 3h was successfully afforded in a
lower yield of 3a. In addition, no unknown yellow compound 71% yield. Obviously, the steric hindrance and multi-substi-
was observed on the anode surface. Hence, using nBu4NBF4 tuted modes of the aryl ring had a strong influence on the
(20 mol%) as the supporting electrolyte and CH3CN : THF (v : v hydroboration transformation of the terminal alkenes. To
= 4 : 1) as the mixed solvent, with the addition of 0.8 equivalent obtain the target product (3i–3n) in higher yields, the constant
of DIEA, and 15 mA constant current at room temperature for current was increased from 15 mA to 20 mA and the reaction
3 h under an inert atmosphere, were considered as the optimal time was extended from 3 h to 4 h. Similarly, the other internal
reaction conditions, with a 3a/4a/5/6 ratio of 82 : 5 : 11 : 2. alkenes possessing a larger steric hindrance were proven to be
Under these conditions, 3a was obtained in 72% isolated yield. amenable substrates (3o–3q). Interestingly, when 2-vinyl-
naphthalene was chosen as the substrate, the mono-borylation
Substrate scope with the methodology
product (3r) and the di-borylation product (7r) were all
With the optimal reaction conditions established, we set out to detected with the yields of 43% and 22%, respectively. This
explore the substrate scope by varying the peripheral substi- interesting result will be discussed later. Proton, carbon,
tutes (R1 and R2) of skeleton 1. As depicted in Scheme 1, boron nuclear magnetic resonance (1H, 13C, 11B NMR) spectro-
various alkylboronate derivatives were synthesised in satisfac- scopic data of all the products are smoothly obtained and
tory yields. Electron-withdrawing fluorinated substitution pat- shown in the ESI;† it is worth noting that the resonances for
terns (ortho, meta or para) on the reacting aryl rings of the carbons attached to the borons are not observed in the carbon
styrene derivatives (3b–3d) all gave moderate yields without spectra due to the quadrupolar moment of 11B nuclei.41
any dehalogenated byproducts. The para-substitutes with the Unfortunately, some alkyl alkenes, such as phenyl vinyl ether,
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could not be transformed to the final product and remained in
the reaction solution.
Gram-scale experiments
To further test the potential application of the established
electrochemical protocol, a series of gram-scale reactions at
10 mmol was conducted. Initially, styrene was selected as the
model substrate (Fig. 2a). To shorten the reaction time to
some extent, a detailed current screening was performed
(shown in Table S5 in the ESI†). Ultimately, if the constant
current was increased to 45 mA and the reaction time was
extended to 12 h, a high conversion of styrene and a high yield
1
of 3a were realised. Meanwhile, in situ H NMR of the scale-up
test was successfully performed (Fig. 3a). The detailed sample
collection and processing information are described in the
note of Fig. 3. The treated data displayed a linear decrease in
Fig. 3 In situ 1H NMR data of the electrochemical hydroboration of
styrene (10 mmol) with HBpin (11 mmol). (a) 1H NMR spectra were col-
lected every 0.5 h. The test samples were made up of 0.3 mL of original
samples and 0.2 mL of CD3CN. The total volume of the reaction solvent
was 100 mL. (b) Decreased trends of styrene and HBpin under electroly-
sis, determined by the analysis of the in situ 1H NMR data. Initial peak
areas of the CH group at the benzyl site of styrene and the CH3 group of
HBpin were defined as 100%, respectively. (c) An increased trend of
product 3a under electrolysis, determined by the analysis of the in situ
1H NMR data. Final peak area of CH2 at the benzyl site of 3a was defined
as 100%.
both styrene and HBpin substrates (Fig. 3b) and a linear
increase in 3a within 8 h (Fig. 3c), which reveal the high
control of the electrochemical borylation reaction. After 8 h,
Fig. 2 Gram scale experiments and comparisons of our work with the
three metal systems in various aspects. (a) Gram scale experiments the entire reaction tended to be stable, which guides large-
under electrolysis. Reaction conditions: 1 (10 mmol), 2 (11 mmol), DIEA
scale experiments in the future. Styrene derivatives (terminal
alkenes with the electron-withdrawing and electron-donating
groups, and internal alkenes, 10 mmol) were added to react
(80 mol%), nBu4NBF4 (20 mol%), Pt(+)|Pt(−), constant current (I) =
45 mA, CH3CN : THF = 4 : 1 (v/v), the total volume of the solvent was
100 mL, N2, rt, 12 h. Isolated yields are shown. (b–d) Comparison of
with HBpin (11 mmol); satisfactory yields of various organo-
catalyst loading, reaction time and reaction steps of our work with three
metal catalysts. These conclusions are based on 1 mmol styrene, and the boron esters were obtained (Fig. 2a). The simple and easy-to-
synthetic time and steps of ligands are included. (e) Comparison of
material costs of our work with the three metal catalyst systems. Cost
accounting was based on the gram yield of 3a (6.1 mmol) in our work,
and the gram yields of 3a in the metal catalysts were same as the yields
operate characteristics of the electrochemical synthesis are
desirable for industrial application.42 In this electrochemical
hydroboration reaction, the excellent site-selectivity and yields
obtained in the gram scale highlight the high potential for
scale-up.
when 1 mmol styrene was selected as the substrate. All materials were
purchased from Alfa Aesar.
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In addition, to further evaluate the gram-scale application indicated that THF did not participate in the hydroboration
2
of this electrochemical hydroboration reaction, three tran- reaction directly. D NMR data further confirmed that no deu-
sition-metal catalysts12,14,18 (including precious and Earth- terium atom was added to the target product. However, if
abundant metals) for the hydroboration of alkenes were CD3CN/THF was chosen as the solvent (Scheme 2b), a mixture
selected and compared with the electrochemical method in of the mono-deuterated linear alkylborate ester d1-3a with the
various aspects. Styrene was selected as the model substrate. deuterium incorporation at the benzyl position and the protio-
We concluded that our electrochemical method dominated in boronic ester 3a was obtained with a molar ratio of 84 : 16,
catalyst loading, reaction time and reaction steps (Fig. 2b–d). which was obtained from the analysis of the 1H NMR data.
2
The reaction time included the synthesis time of various The analysis of the D NMR data further confirmed the posi-
ligands and the catalytic reaction time of styrene with HBpin, tion of the deuterium atom in the product, which indicates
but excluded the post-treatment time to obtain the pure that the solvent CD3CN was involved in the hydroboration con-
product 3a. In addition, the summary standards of the reac- version reaction. d1-Pinacolborane (DBpin)43,44 was reacted
tion steps were the same as those of the reaction time. Gram- with styrene under the standard conditions (Scheme 2c), and
scale experiments catalysed by metal catalysts were not con- only a small amount of the mono-deuterium boronic ester d1-
ducted in previous literature and the results shown in Fig. 2e 3a was observed and the molar ratio was 16 : 84 with 3a deter-
are only magnifications of the original experimental amounts. mined from the analysis of the 1H NMR data. In addition, a
In addition, material costs only covered the costs of various moderate yield of the mono-protio-boronic ester d8-3a was syn-
substrates, and did not include the costs of solvent, reproces- thesised by employing d8-styrene and HBpin as the substrates
sing, human resources and other experimental conditions. (Scheme 2d). Accordingly, we speculated that the Bpin group
The gram-scale material cost of our electrochemical method of 3a originated from HBpin, whereas the H species at the
was slightly higher than that in the Cat. 1 system, but lower benzyl site of 3a originated from both HBpin and the CH3CN
than those of the other two catalytic systems. All in all, the solvent, and the latter was the primary hydrogen donor.
electrochemical hydroboration method explored in this paper
H2 detection experiments. Meanwhile, bubbles were
was simpler, more convenient, and economical in large-scale observed during the electrochemical hydroboration with
experiments, which provides a new platform for the appli- styrene and HBpin (ESI Video†). They were determined as H2
cation of electrochemical hydroboration in industry.
using GC (Fig. S5 in the ESI†). To determine the source of H2
and mechanism of production, the time dependence of the H2
production rates (mL min−1) was monitored in real time under
different conditions. The H2 production rates under the stan-
dard conditions (Fig. 4a) gradually decreased with time and
the amount of H2 released was approximately zero at 180 min,
which matched perfectly with the screened reaction time of
3 h. The H2 release values after 30 min under substrate-free
conditions (absence of styrene and HBpin, Fig. 4b) were sub-
stantially higher than those under the standard conditions,
which not only illustrates that the hydroboration reaction con-
Mechanism research
Deuterium-labelling tests. To further elucidate the trans-
formation process of the whole electrochemical system, a
series of deuterium-labelling experiments was conducted to
confirm the transfer of various groups. As shown in
Scheme 2a, if styrene reacted with HBpin in the mixed solvent
of CH3CN/THF-d8, no deuterium product was detected, which
Fig. 4 Time-dependent H2 production rates recorded under different
control conditions. The conditions shown in the legend are displayed
with reference to the standard electrochemical conditions.
Scheme 2 Deuterium-labelling experiments.
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sumes a massive amount of hydrogen ions, but also confirmed standard conditions) to 8% (1.0 equivalent of TEMPO). Whereas
that the mixed solvent system can produce large amounts of no product was detected when one equivalent of the galvinoxyl
H2 during the electrochemical process. Considering the radical was added. The above results demonstrated the radical
limited hydrogen evolution of 0.8 equivalent of DIEA45 and the nature of this electrochemical reaction. In addition, two poss-
outcome of the deuterium-labelling experiment, if CD3CN/THF ible intermediates existing in the electrochemical cycle were
was chosen as the solvent, we deduced that CH3CN is essential successfully captured by the radical scavengers and character-
for hydrogen evolution.
ised by mass spectrometry. One was the addition compound of
In addition, the overall stable curve depicted in Fig. 4c indi- the boryl radical and the galvinoxyl free radical, namely alkyl-
cates that CH3CN could be electrolysed on the surface of the galvinoxyl 8, with an m/z of 548.4141. The other was alkyl-
two platinum electrodes to generate H2, which corresponds to TEMPO 9, formed by the addition of a carbon radical at the
CH3CN as one of the main sources of hydrogen ions in the benzyl site with TEMPO, with an m/z of 387.3053.
deuterium-labelling experiment and the redox behaviour of
CH3CN reported in the previous literature.46,47 In particular,
Proposed electrochemical cycle
the amount of H2 released shown in Fig. 4b was higher than
that shown in Fig. 4e, demonstrating the vital role of DIEA in
Based on the above results and literature reports,46,47,54 a plaus-
ible mechanism for the electrochemical hydroboration of
styrene is presented in Scheme 4. In the first step, DIEA is pre-
ferentially oxidised to an N-centred radical cation A through a
single electron transfer on the anode surface. The lower oxi-
dation peak of DIEA compared to CH3CN and HBpin validated
the presumption (Fig. S111 in the ESI†). Then intermediate A
extracts electrons from CH3CN and HBpin and regenerates to
DIEA, accompanied by the generation of intermediates B and C.
Next, B and C released hydrogen ions to produce the carbon-
centred radical D and boron-centred radical E. Simultaneously,
the partial produced hydrogen ions are protonated by THF and
DIEA. Afterwards, radical E is added to styrene, which results in
the formation of the carbon-centred radical intermediate F.
Then F is further reduced to the boronate alkyl intermediate G
on the cathode surface. Finally, G reacts with a hydrogen ion, to
produce the target compound 3a. Meanwhile, the remaining
hydrogen ions are reduced at the cathode surface and formed
assisting the electrolysis of CH3CN. Referring to the litera-
ture,40 we speculated that DIEA could be a sacrificial electron
donor to pull the continuous electrolysis of CH3CN and com-
plete the entire electrochemical cycle. In addition, the release
rates of H2 shown in Fig. 4e were slightly lower than those
shown in Fig. 4c without THF. On the one hand, this result
indicated that THF was not the source of hydrogen ions in the
electrochemical reaction, corresponding to the outcome of the
THF-d8 experiment. On the other hand, the reduction of H2
production rate in the presence of THF might be explained by
the protonation of THF,48–51 which facilitated the transfer of
the hydrogen ions to the target compounds.
Radical capture experiments. Details relating to the origin
and transfer of H and Bpin groups were determined through
deuterium-labelling and H2 detection experiments. But in what
form did HBpin react and what was the explicit mechanism of
this electrochemical borylation reaction? To further explore the
reaction mechanism and capture the possible intermediates,
the electrochemical hydroboration of styrene and HBpin was
performed in the presence of the radical scavenger 2,2′,6,6′-tet-
ramethyl-1-piperidinyloxy (TEMPO) or the galvinoxyl free
radical, respectively.52,53 As shown in Scheme 3, with the
increase of TEMPO loading added to the system, the yield of
electrochemical product 3a decreased sharply from 72% (the
•
H2. As for the radical CH2CN (D), the electron paramagnetic
Scheme 3 Radical inhibition experiments.
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Scheme 4 A possible mechanism of electrochemical hydroboration.
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Table 2 Selective synthesis of diboronate estersa
resonance (EPR) signal of the carbon-centred radical (g =
2.0052, AN = 14.47 G, AH = 20.82 G) trapped by 5,5-dimethyl-1-
pyrroline N-oxide (DMPO) was recorded (Fig. S113 in the ESI†),
and the corresponding adduct DMPO–CH2CN 10 was also suc-
cessfully detected by mass spectrometry.55,56 This confirms the
presumption that CH3CN is involved in the formation of H2. In
the whole electrochemical cycle, the ability of CH3CN and
HBpin to produce hydrogen ions assisted by DIEA and the
ability of THF and DIEA to combine hydrogen ions are crucial
for the transformation as it provides sufficient hydrogen
protons to participate in this conversion.
Although the possible reaction mechanism has been pro-
posed, there are still several points worthy of further discus-
sion, such as the dynamic tracking and detection of free rad-
icals generated in the hydroboration reaction. So far, it is
difficult to explore suitable testing means to monitor and
analyse the real-time state of free radical production and trans-
formation, because of their own instability, sensitivity to air
and humidity, the nature of coupling, and binding to itself or
other compounds.57–60 The first issue is the stability of the free
Bpin radical. In many related literature studies, the free boryl
radicals can be stabilized by DMA,61,62 DMF or Et3N.63 In 2019,
the Mo group performed density functional theory (DFT) calcu-
lations on the C–B bond formation steps using the free Bpin
Entry
1
Substrate
Yield of 3b (%)
Yield of 7b (%)
6
71
2
3
3
2
84
88
4c
5c
26
48
41
43
6c
59
20
a Reaction conditions: 1 (1.0 mmol), 2 (4.4 mmol), DIEA (80 mol%),
nBu4NBF4 (20 mol%), Pt(+)|Pt(−), constant current (I) = 20 mA,
radical.64 In our system, we can only detect the free Bpin CH3CN : THF = 4 : 1 (v/v), the total volume of the solvent is 10 mL, N2,
rt, 4 h. b Isolated yields are shown. c The amount of 2 is increased to
radical qualitatively but not quantitatively, maybe they are
stabilized by DIEA or THF with certain alkenes, but it was
8.8 mmol.
difficult to find direct experimental evidence. The second issue
is the lifetime of the benzyl radical F. Although the migration
of persistent radicals between two electrodes is common in the
traditional paired electrolysis, few works presented a detailed
elaboration on the study of lifetimes of radicals during the
Fig. S131 in the ESI.† As depicted in Table 2, if 1-vinylnaphtha-
lene and 2-isopropenylnaphthalene were chosen as the sub-
strates, the corresponding diboronate esters (7s and 7t) were
also smoothly isolated in 88% and 84% yields, respectively. To
obtain more structural information, compound 7t was deter-
mined by X-ray diffraction analysis. The solid-state structure
was confirmed by X-ray crystallography (Fig. S6 in the ESI†),
which indicated that 7t contained a linear and a branched
migration. Compared to the proposed reaction mechanism of
the hydrosilylation reaction in the reported reference,65 the
hydroboration reaction mechanism shown in our system
seems feasible. More detailed work should be conducted to
disclose the reaction details, which will help to improve the
reaction efficiency and lay the foundation for industrialization.
benzylic boron substituent. In addition, we considered
whether diborylation could be expanded to substrates contain-
ing one single benzene ring when reacting with excess HBpin.
As a result, compared to the systems containing naphthalene
rings, the yields of diboronate esters with different substitu-
ents (3a, 3b and 3g) were slightly inferior. In addition, to use
styrene as the model substrate, a detailed proportion screening
Reaction expansion
Compared to monoboronate esters, diboronate esters are more
attractive, because the chemical transformations of diboron-
groups to target-oriented synthesis are more diverse.66
Surprisingly, if 2-vinylnaphthalene was chosen as the sub-
strate, even in the presence of 1.1 equivalents of HBpin, a
partial diboronate ester 7r was detected. Therefore, we specu-
lated whether it was possible to selectively produce a diboro-
nate ester by increasing the proportion of HBpin. 2-vinyl-
1
of styrene and HBpin and in situ H NMR monitoring data are
provided in Table S7 and Fig. S107 in the ESI,† respectively.
Derivatisation of diboronate esters
naphthalene was selected as the model substrate to explore the Diboronate esters are versatile synthetic intermediates, which
optimal proportions of alkene and HBpin to form the diboro- could be converted into various functional groups. Here, to
nate compound. Further exploration (Table S6 in the ESI†) further demonstrate the potential synthetic utility of this
indicated that if 4.4 equivalents of HBpin were added into the electrochemical borylation reaction, several transformations of
system, 2-vinylnaphthalene was completely transformed and the diboronate ester 7t were examined. As illustrated in
the diboronate ester 7r was obtained in 71% isolated yield. A Scheme 5, firstly, a gram-scale reaction using 10 mmol 1t was
plausible mechanism for the electrochemical hydroboration successfully conducted under a constant current (45 mA) for
with 2-vinylnaphthalene as
a substrate is presented in 12 h to produce compound 7t in 85% isolated yield. For the
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Innovation Funding (DICP-RC201801) and the LiaoNing
Revitalization Talents Program (XLYC1802109). Dr Zhang
appreciates the valuable discussions with Prof. Dr Guoxiong
Wang and Dunfeng Gao in DICP. The authors thank the
Technique Supporting Platform for Clean Energy Research
Division-DICP for technical support.
Notes and references
Scheme 5 Multi-functional transformations of diboronate esters.
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