Electrochemical Hydroboration of Alkynes

Article abstract of DOI:10.1002/chem.202101132

Herein we reported the electrochemical hydroboration of alkynes by using B₂Pin₂ as the boron source. This unprecedented reaction manifold was applied to a broad range of alkynes, giving the hydroboration products in good to excellent yields without the need of a metal catalyst or a hydride source. This transformation relied on the possible electrochemical oxidation of an in situ formed borate. This anodic oxidation performed in an undivided cell allowed the formation of a putative boryl radical, which reacted on the alkyne.

Full text of DOI:10.1002/chem.202101132

Communication
doi.org/10.1002/chem.202101132
Chemistry—A European Journal
Electrochemical Hydroboration of Alkynes
Maude Aelterman, Morgane Sayes, Philippe Jubault,* and Thomas Poisson*
[a]
[b]
[a]
[a, c]
In parallel, the chemistry of boryl radicals was recently high-
lighted as a straightforward alternative to introduce a boron
residue. With that respect, Curran, Lacôte, Fensternbank and
Abstract: Herein we reported the electrochemical hydro-
boration of alkynes by using B Pin as the boron source.
[6]
2
2
This unprecedented reaction manifold was applied to a
broad range of alkynes, giving the hydroboration products
in good to excellent yields without the need of a metal
catalyst or a hydride source. This transformation relied on
the possible electrochemical oxidation of an in situ formed
borate. This anodic oxidation performed in an undivided
cell allowed the formation of a putative boryl radical, which
reacted on the alkyne.
Malacria explored the formation and reactivity of these radicals
from NHC-borane. They demonstrated the possibility to form
the radical using usual initiators, such as peroxides or AIBN, for
[7]
instance. Among the studied reactions, the addition of the
boryl radical to olefin and alkynes, pioneered by Lalevée and
[8]
Curran, has been less explored and the development of new
protocols is still highly desirable to extend the portfolio of
possible reactions. Very recently, the photocatalyzed formation
of boryl radicals using iridium catalysts has been reported
(Scheme 1). This new method was applied to the addition of
NHC-boron residues on fluorinated olefins and aryls, according
[
9]
In the realm of organic synthesis organoboron compounds play
to an addition/fluoride elimination sequence and to the
[1]
[10]
[11]
an important role to build up complex molecules. In addition,
boron-containing molecules found applications in medicinal
chemistry, which culminated in the approval of boron-contain-
hydroboration of imines,
alkenes
and electron poor
aromatic derivatives. Importantly, in contrast to the formation
of boryl radicals from NHC-borane, their formation from diboron
compounds remains scarce. Within the last five years organic
electrochemistry has known an impressive renewal of interest.
Indeed, electrochemical synthesis, by replacing hazardous
[12]
[2]
[13]
ing drugs (e.g. Dutogliptin). These key building blocks show-
case various and specific reactivities, which provide them a
pivotal role in the arsenal of organic chemists to design
[3]
retrosynthetic disconnections.
For instance, organoboron
compounds have been widely employed in allylation, aldolisa-
tion, or homologation reactions and it is important to notice
that the hydroboration and the Suzuki-Miyaura cross-coupling
reactions were celebrated by the Nobel prize given to H. C.
Brown and A. Suzuki in 1979 and 2010, respectively. Among
these synthetically useful reactions, the hydroboration of an
alkene or an alkyne is of paramount interest and has been
[4]
widely studied for almost half a century. These endeavors
[5]
gave birth to noble and non-noble metal catalyzed processes.
[
a] M. Aelterman, Prof. Dr. P. Jubault, Prof. Dr. T. Poisson
Normandie Université, INSA Rouen, UNIROUEN
CNRS, COBRA (UMR 6014)
76000 Rouen (France)
E-mail: philippe.jubault@insa-rouen.fr
thomas.poisson@insa-rouen.fr
[b] Dr. M. Sayes
Centre in Green Chemistry and Catalysis
Faculty of Arts and Sciences
Department of Chemistry, Université de Montréal
P.O. Box 6128, Station Downtown, Montréal, Québec H3 C3 J7 (Canada)
[c] Prof. Dr. T. Poisson
Institut Universitaire de France
1
rue Descartes, 75231 Paris (France)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202101132
©
2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-Com-
mercial NoDerivs License, which permits use and distribution in any medium,
provided the original work is properly cited, the use is non-commercial and
no modifications or adaptations are made.
Scheme 1. State of the art and present work.
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Chemistry—A European Journal
chemical redox reagents by electrons, avoids the generation of
wastes and if electricity is produced from renewable sources, it
and iii) no reaction occurred with MeONa in the absence of
current (entry 9), precluding a base assisted hydroboration
[14]
[17]
can be considered as a green process. As part of our program
to develop straightforward access to boron-containing
reaction. Then, to ascertain the versatility and to ensure the
reproducibility of our electrochemical hydroboration reaction, a
sensitivity assessment regarding the reaction parameters was
[15]
molecules,
we sought to use organic electrochemistry to
[18]
generate boryl radicals that would be used as an unprece-
dented reaction manifold in the hydroboration of alkynes.
Worth of note, during the preparation of this manuscript, Qing
and co-workers reported the electrochemical hydroboration of
styrene using HBpin as the boron source and DIPEA as
performed (Table 1). This hydroboration protocol was found
sensitive to the presence of water and to the temperature,
while the yield slightly decreased (c.a. 25% less) when the scale
[19]
of the reaction was increased.
Having optimized the reaction conditions, we explored the
scope of this catalyst- and hydride-free hydroboration reaction
(Scheme 2). First, a large panel of aryl substituted terminal
alkynes was tested. Aryls substituted with a methyl group
provided the corresponding vinyl boronate derivatives (2–4) in
good to excellent yields, whatever the substitution pattern.
Other electron donating groups like methoxy, phenyl, N,N-
dimethyl and non-protected amines were suitable affording the
products (6–11) in good to excellent yields. Interestingly, the
reaction was tolerant to the presence of a BPin group, a
benzylic alcohol as well as an acetal, furnishing the synthetically
useful hydroborated products 12, 13 and 14. Halogens and CF3
substituents on the aromatic did not affect the efficiency of the
reaction, offering an access to compounds 15–20 in good to
excellent yields. Regarding the presence of electron-withdraw-
ing on the aromatic, the aldehyde 21 and the ester 22 were
synthesized in moderate to good yields, at the cost of an
[16]
auxiliary. Herein, we report an original electrochemical hydro-
boration of terminal alkyne formation from the readily available
B Pin using electron as a green redox reagent.
2
2
As a model reaction we chose phenylacetylene as the
substrate and B Pin as the boron source. After extensive
2
2
optimization, we found that the open-air reaction performed in
MeOH using n-Bu NBF as the electrolyte (0.05 M), stainless steel
4
4
electrodes (SST) at both the cathode and anode along with a
À 1
constant current of 10 mA and a total charge of 2 Fmol were
required to ensure the formation of 1 in 84% yield (Table 1,
entry 1).
Indeed, the use of EtOH or i-PrOH did not afford 1 with
decent yields (entry 2, <25%), while the use of other electrodes
was deleterious for the reaction outcome (entries 3 and 4).
À 1
Then, a total charge of 1 Fmol or a decrease of the electrolyte
concentration gave lower yields (entries 5 and 6). Finally,
control experiments revealed that i) the reaction did not
occurred when B Pin was replaced by H-BPin (entry 7), ii) the
À 1
increase of the charge from 2.0 to 2.5 Fmol for 22. Similarly,
naphthyl derivatives 23 and 24 were readily obtained using
2
2
À 1
reaction did not occurred in the absence of current (entry 8)
2.5 Fmol . This methodology was applied to pyridine deriva-
tive, giving 25 in 63% isolated yield. Then, alkyl substituted
terminal alkynes were tested. The presence of a primary,
secondary and tertiary alcohols, as well as a phthalimide did
not affect the outcome of the reaction and the hydroborated
products 26–31 were isolated in moderate to excellent yields.
The reaction was selective to alkyne, since an enyne was
selectively functionalized on the alkyne residue (32). Interest-
ingly, the cyclopropyl derivative 33 was obtained with an
excellent 70% NMR yield and a modest 39% isolated yield, due
to a tedious purification. Surprisingly, the vinyl boronates 34–
Table 1. Optimization of the reaction and sensitivity tests.
[
a]
Entry
Variation from standard conditions
Yield [%]
[b]
1
2
3
4
5
6
7
8
9
none
84
3
9 were isolated as a mixture of α- and β-isomers in moderate
EtOH or i-PrOH instead of MeOH
graphite electrodes instead of SST
platinum electrodes instead of SST
1 Fmol instead of 2 Fmol
4 4
n-Bu NBF : 0.025M instead of 0.05M
H-BPin instead of B
no current
NaOMe (1 equiv.) & no current
<25
0
<30
51
75
0
[20]
yields, the β-isomer being the major one in all cases.
Pleasingly, the scope of this electrochemical hydroboration
reaction was extended to internal alkynes. Using one equivalent
of K CO as an additive, the boronic esters 40 and 41 were
À 1
À 1
2
3
2 2
Pin
0
0
isolated in 49% and 58% yield, respectively. Note that in the
case of 40, a 70:30 mixture of α- and β-isomers were obtained.
Finally, this methodology was applied to complexes molecules
to demonstrate the synthetic utility of the reaction. The steroids
derivatives 42 and 43 were obtained in 66% and 37% isolated
yields. Regarding 43, the low efficiency of the process was
explained by the poor solubility of the starting material in the
solvent of the reaction. Finally, the α-tocopherol derivative 44
was isolated in 27% yield.
To get further insight into the mechanism, the reaction was
carried out in the presence of CD OD instead of CH OH
Reaction conditions: phenylacetylene (0.2 mmol), B
2
Pin
2
(0.4 mmol), MeOH
3
3
[19]
(4 mL), r.t. [a] NMR yield. [b] Isolated yield. C: concentration, T: temperature,
(Scheme 3, Eq. (1)).
This control experiment furnished the
S: electrodes surface.
deuterated product [D]-1 in 42% yield with a complete
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1
Scheme 2. Exploration of the scope of the electrochemical hydroboration of alkynes. All data are the average of two runs. [a] Yield determined by H NMR by
À 1
À 1
1
using DMF as an internal standard. [b] 2.5 Fmol instead of 2 Fmol . [c] Determined by H NMR on the crude reaction mixture. [d] 1 equiv. of K
added.
2 3
CO was
incorporation of deuterium, which demonstrates that the
hydrogen atom comes from the solvent. Then, to determine if
the H abstraction results from the cleavage of the CÀ H or OÀ H
5.75 was measured in a 1:1 mixture of CH OH/CD OD,
3 3
suggesting that the hydrogen abstraction from the solvent after
the addition of the boryl radical might be the rate determining
step (Scheme 3, Eqs. (4) & (5)). Finally, to ascertain the reaction
does not result from the presence of metallic salts released
from the electrodes, the electrochemical reaction was stopped
bond of the solvent, reactions were carried out with CD OH and
3
CH OD, independently (Scheme 3, Eqs. (2) & (3)). These results
3
clearly highlight that the H results from the cleavage of the
OÀ H bond, despite its higher BDE compared to the CÀ H bond.
Finally, we studied the kinetic isotopic effect (KIE). Parallel
reactions in CH OH or CD OD revealed a KIE=2, while a KIE of
À 1
after 32 minutes (1 Fmol ) and the reaction mixture was stirred
within the electrosynthesis cell for an additional 24 h
(Scheme 3, Eq. (6)). Pleasingly, we did not observe an increase
3
3
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Scheme 3. Mechanistic study: control experiments, CV measurements. nBu
4
NBF
4
was used as the electrolyte for the cyclic voltammetry measurements (see
Supporting Information for details). All potentials are reported vs. SCE.
of the reaction yield after this additional stirring. This observa-
tion precluded a background hydroboration reaction catalyzed
by metallic salts released from the SST electrodes. Then, since
we surmised a possible oxidation of a transient borate species,
we carried out cyclic voltammetry (CV) experiments to depict a
possible reaction mechanism. The CV measurements of B Pin
2
2
(
orange), NaOMe (yellow), B Pin and NaOMe (blue), phenyl-
2 2
acetylene (grey) and the mixture of B Pin , NaOMe and phenyl-
2
2
[19]
acetylene (green) in MeOH were carried out. To mimic the
formation of the alkoxide, which is unlikely during the CV
measurement due to the tiny surface of the anode, NaOMe was
added to the B Pin species. The CV analysis of the mixture
2
2
B Pin /NaOMe (blue) demonstrated an irreversible oxidation
2
2
wave at +1.2 V vs. SCE, that is not observed on the CV
measurement of the B Pin (orange). Moreover, a similar
2
2
Scheme 4. Plausible Mechanism.
oxidation wave was observed on the mixture of all reagents
green). Finally, no oxidation nor reduction of B Pin or phenyl-
(
2
2
acetylene was observed, while a completely different oxidation
wave was witnessed with NaOMe (yellow). Hence, with these
experimental data in hand, we suggested the following
plausible mechanism (Scheme 4). First, we surmised the for-
mation of the boryl radical from the oxidation of an in situ
undergo an addition reaction onto the alkyne to form the
corresponding highly reactive vinyl radical D. Then, taking into
account the above-mentioned results from the labelling experi-
ments, a plausible pathway to explain the abstraction of the H
[21]
[22]
formed borate A. Indeed, the coordination of an alkoxide,
here generated by the reduction of the solvent at the cathode
from the stronger OÀ H bond is suggested. The coordination
of a boron species, which can act as a Lewis acid, to MeOH
might decrease the OÀ H bond dissociation energy and allow
the H abstraction by the vinyl radical. Then, the resulting O
radical could be reduced at the cathode to complete the redox
(
i.e. MeOH), to the empty orbital of a boron atom of B Pin
2 2
[17]
might gave birth to this species. Then, the anodic oxidation
of the latter would generate B, that would quickly collapse into
[23]
*
the boryl radical C (i.e. BPin) and MeOBPin. This boryl radical is
cycle. Alternatively, the methoxide anion, generated at the
cathode, might act as a hydrogen donor as well. Although its
concentration in the reaction media is much lower than MeOH
probably a 7-electrons centered one, rather than a less stable 5-
electrons, thanks to the coordination of the solvent to the
[6b]
[24]
empty orbital of the boron atom. This radical would then
this pathway can be productive as well.
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In conclusion, we developed the first electrochemical hydro-
boration of alkynes. This catalyst- and hydride-free method-
ology provides a greener alternative for the synthesis of
borylated alkenes. Furthermore, the use of stainless steel
electrodes makes the process cost-effective. According to a
possible anodic oxidation of an in situ formed borate species, a
putative boryl radical derived from the cleavage of the BÀ B
bond of B Pin was added to a large panel of alkynes, including
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Keywords: alkynes
electrochemistry · hydroboration
· anodic oxidation · boryl radical ·
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a) D. H. Geske, J. Phys. Chem. 1959, 63, 1062–1070; b) D. H. Geske, J.
Phys. Chem. 1962, 66, 1743–1744; c) S. B. Beil, S. Möhle, P. Enders, S. R.
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Baumann, P. Spieß, A. Plantefol, T. C. Jagau, D. Didier, J. Am. Chem. Soc.
2020, 142, 4341–4348; e) A. N. Baumann, A. Music, J. Dechent, N. Müller,
T. C. Jagau, D. Didier, Chem. Eur. J. 2020, 26, 8382–8387; f) H. Yan, M.
[
[
Chem. Eur. J. 2021, 27, 1–7
www.chemeurj.org
5
© 2021 The Authors. Published by Wiley-VCH GmbH
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These are not the final page numbers!

Communication
doi.org/10.1002/chem.202101132
Chemistry—A European Journal
Chen, D. Zhao, J. Xu, C. Li, C. Lu, Angew. Chem. Int. Ed. 2021, 60, 7838–
[24] W. J. Boyle, J. F. Bunnett, J. Am. Chem. Soc. 1974, 96, 1418–1422.
7
844; Angew. Chem. 2021, 133, 7917–7923.
À 1
À 1
[
[
22] H-CH
2
OH, BDE=96 kcal.mol and CH
3
OÀ H BDE=104 kcal.mol
.
23] The reduction of the vinyl radical D into the corresponding vinyl anion
followed by protonation to afford the hydroborated product is unlikely,
since no reduction has been witnessed during the CV measurement
with all reaction partners, see Scheme 3B.
Manuscript received: March 30, 2021
Accepted manuscript online: May 4, 2021
Version of record online: ■■■, ■■■■
Chem. Eur. J. 2021, 27, 1–7
www.chemeurj.org
6
© 2021 The Authors. Published by Wiley-VCH GmbH
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These are not the final page numbers!

COMMUNICATION
M. Aelterman, Dr. M. Sayes, Prof. Dr. P.
Jubault*, Prof. Dr. T. Poisson*
1
– 7
Electrochemical Hydroboration of
Alkynes
The electrochemical hydroboration
droboration products in good to
excellent yields without the need of a
metal catalyst or a hydride source. In
addition, the mechanism of this trans-
formation was study.
of alkynes by using B Pin as the
2
2
boron source is reported. This
reaction manifold was applied to a
broad range of alkynes, giving the hy-