Electrochemical-Induced Hydrogenation of Electron-Deficient Internal Olefins and Alkynes with CH3OH as Hydrogen Donor

Article abstract of DOI:10.1002/adsc.202100022

Efficient hydrogenation of electron-deficient internal olefins and alkynes access to saturate ketone with CH₃OH as a single hydrogen donor under electrochemical conditions has been successfully developed. This hydrogenation strategy can be used to convert electron-deficient internal olefins and alkynes to saturate ketone under electrochemical conditions with exogenous-reductant and a metal catalyst. Mechanistic studies reveal that radical hydrogenation was involved in this transformation. Notably, various electron-deficient internal olefins and alkynes could be tolerated in such an electrochemical hydrogenation synthetic strategy and can be easily scaled up with good efficiency. (Figure presented.).

Full text of DOI:10.1002/adsc.202100022

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DOI: 10.1002/adsc.202100022
Electrochemical-Induced Hydrogenation of Electron-Deficient
Internal Olefins and Alkynes with CH3OH as Hydrogen Donor
Hongyun Qin,a Jianjing Yang,a,* Kelu Yan,a Yaxuan Xue,a Meichen Zhang,a
Xuejun Sun,a Jiangwei Wen,a,* and Hua Wanga
a
Institute of Medicine and Materials Applied Technologies, College of Chemistry and Chemical Engineering, Qufu Normal
University, Qufu, Shandong 273165, People’s Republic of China
E-mail: jjyang@whu.edu.cn; wenjy@qfnu.edu.cn
Manuscript received: January 7, 2021; Revised manuscript received: February 22, 2021;
Version of record online: March 4, 2021
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100022
Se, chlorosilane, or borane. Although various hydro-
genation or transfer hydrogenation methods have been
Abstract: Efficient hydrogenation of electron-defi-
cient internal olefins and alkynes access to saturate
established, traditional methods are usually limited by
ketone with CH3OH as a single hydrogen donor
the participation in transition metal catalysts or
under electrochemical conditions has been success-
activating reagents and harsh reaction conditions.
fully developed. This hydrogenation strategy can be
Therefore, it is important to develop a metal-free,
used to convert electron-deficient internal olefins
convenient, efficient, and environmentally friendly
and alkynes to saturate ketone under electrochem-
hydrogenation strategy to realize the hydrogenation of
ical conditions with exogenous-reductant and a
electron-deficient olefins or alkynes.
metal catalyst. Mechanistic studies reveal that
radical hydrogenation was involved in this trans-
Electrochemical synthesis uses electrons as a
powerful and sustainable reagent to activate substrates
formation. Notably, various electron-deficient inter-
by oxidation or reduction on the electrode surface.
nal olefins and alkynes could be tolerated in such an
This has attracted much attention from synthetic
electrochemical hydrogenation synthetic strategy
organic chemists.[5] In particular, electrochemical cath-
and can be easily scaled up with good efficiency.
ode-mediated reductive coupling or hydrogenation
reactions have attracted widespread attention from
synthetic organic chemists in recent years.[6] Among
Keywords: Electrochemical; Hydrogenation; Elec-
them, some elegant electrochemical hydrogenation
tron-deficient Internal Olefins and Alkynes; Methanol
strategies for olefins and alkynes have been
developed.[7] For example, Chen’s[7d,f] groups have just
disclosed the electrochemical hydrogenation of olefins,
The hydrogenation of alkenes and alkynes is regarded alkynes, and ketones with NH3 gas or D2O as hydrogen
as an essential step in the synthesis of important fine donors (Scheme 1b). Recently, Xia[7c] and Huang’s[7e]
chemicals, drugs, and natural products in industry and group have also successively realized the hydrogena-
academia.[1] Therefore, many efforts are devoted to the tion of chalcone and cinnamic acid derivatives with
development of convenient and efficient hydrogenation CH3OH/NH4Cl, DMSO, or H2O as hydrogen donors
strategies.[2–4] In the past few decades, the methods under the conditions of the electrochemical cathode
involving transition metal-catalyzed (Fe, Co, Pd, and (Scheme 1c). Moreover, Hertbert[7b] and co-workers
Ir, etc.) hydrogenation[2] with H2 or silicon hydrides as reported that electron-deficient-ketoester and benzox-
a hydrogen source, and/or transfer hydrogenation[3] azinone substrates can be selectively hydrogenated at
with formates, amine-borane, alcohols, and others as carbon electrodes using Brønsted acids. Despite several
hydrogen donor strategies of alkenes and alkynes have achievements made so far, it is still an urgent task and
been developed (Scheme 1a-I). Besides, the metal-free challenge to develop efficient, convenient, easy to
mediated hydrogenation reaction is recognized as an scale up, and environmentally friendly electrochemical
economical and environmentally friendly strategy that strategies for the selective hydrogenation of electron-
has attracted the attention of chemists (Scheme 1a- deficient internal olefins and alkynes to synthesize
II).[4] However, this type of conversion generally ketones. In the continuation of our efforts in the
requires the participation of activating reagents such as transfer hydrogenation of imidazopyridines with sec-
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Table 1. Optimization of the Reaction Conditions.[a]
Entry Deviation from standard conditions Yield of 2a[b](%)
1
2
3
4
5
6
7
8
Ref. 7c, 7e, and 7f conditions
None
Without current
Without KSCN
Without CH3OH
10 mA instead of 15 mA, 3 h
20 mA instead of 15 mA, 3 h
nBu4NClO4, nBu4NBF4, nBu4NPF6,
Et4NBr instead of Et4NBF4
0
89 (0)[c]
n. d.
trace
n. d. (<10)[c]
43
62
�50
9
CH3OH instead of CH3CN/CH3OH 62
10
11
12
EtOH instead of CH3OH
Carbon rod as cathode
Ni as cathode
16
trace
60
Scheme 1. Hydrogenation of Alkenes and Alkynes.
[a] Standard conditions A: C anode, Pt (1.5×1.5 cm) cathode,
constant current = 15 mA, 1 a (0.25 mmol), KSCN
(0.5 mmol), Et4NBF4 (2.0 equiv.), MeCN/CH3OH (10.0 mL,
v=5/5), r.t., N2, 4 h. n. d.=not detected.
[b] Isolated yields.
ondary amine as hydrogen donor[8] and functionaliza-
tion of alkenes,[9] here, we wish to report highly
selective hydrogenation of electron-deficient internal
alkenes and alkynes access to the saturated ketone with
inexpensive methanol serving as a proton donor.
KSCN and Ph2S were selected as the anode parallel
paired electrolysis reagents for hydrogenation of
[c] Yield of 2a’.
ketene dithioacetals and ynones, which can effectively electrolytes, KSCN as the anode parallel paired
inhibit the side reactions caused by oxidation of the electrolysis reagent in the mixture solvent of CH3CN/
substrate.
CH3OH., The desired product 2a was obtained in 89%
Overall, compared with the previous work, the yield while the thiocyanation of 1a was not observed
present methods have the following merits: (1) various under optimal conditions (Table 1, entry 2). The CV
ketene dithioacetals and ynones can be applied to the experiment was carefully executed to explain this
present protocol with excellent yields; (2) no hydrogen superior selectivity. Figure S5b confirms that the
or NH3 gas, additional catalysts, redox mediators are oxidation potential of KSCN (Eonset =0.62 V vs Ag/
required; (3) The presence of parallel paired electrol- AgCl) is significantly lower than 1a (Eonset =1.47 V vs
ysis reagents can effectively inhibit the side reactions Ag/AgCl), thus KSCN should be preferentially oxi-
caused by oxidation of the substrate to increase the dized on the surface of the anode. Moreover, a lower
yield of hydrogenated products (4) outstanding gram- electrode potential was exhibited when electrolysis of
scale yields can be expected.
1a was performed in the mixed solvent of CH3CN/
The electrochemical hydrogenation of ketene di- CH3OH compared to pure CH3CN (Figure S5a). There-
thioacetals was selected as a benchmark system fore, the hydrogenation of 1a is preferentially per-
(Table 1). Initially, 2-(1,3-dithiolan-2-ylidene)-1-phe- formed on the surface of the cathode rather than
nylethan-1-one (1a) was chosen as the substrate to thiocyanation. The control experiments demonstrate
obtain the desired hydrogenation products 2a under that current, KSCN, and methanol play key roles in
the previous reaction conditions (Table 1, entry 1). this transformation (Table 1, entries 3–5). Our inves-
However, substrate 1a was retained under all the tigations reveal that the yield of the desired product 2a
previous optimal conditions, indicating that the pre- is influenced by the current chosen for the constant
vious protocols were not compatible with the hydro- current experiments (Table 1, entries 6 and 7). Sub-
genation of ketene dithioacetals. By optimizing key sequently, the desired product could obtain an excellent
reaction parameters, the optimal condition was ob- yield when Et4NBF4 was used as the electrolyte, while
n
n
tained by performing the electrolysis at room temper- other ammonium salts such as Bu4NClO4, Bu4NBF4,
ature under a constant current of 15 mA in an nBu4NPF6, Et4NBr instead of Et4NBF4 afforded lower
undivided cell equipped with a carbon rod as the anode yields (Table 1, entry 8). The desired product can be
and platinum plate as the cathode, conducting obtained with a yield of 62% when methanol instead
ammonium tetraethyl tetrafluoroborate (Et4NBF4) as of the mixed solvent of CH3CN/CH3OH (Table 1,
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entry 9). Moreover, it was found that using ethanol In particular, various substituted saturate ketones such
instead of methanol as a hydrogen donor can only give as F, Cl, Br, and CF3 can be obtained with excellent
the product in a low yield (Table 1, entry 10). Finally, yield, which plays a crucial foundation for further
the effects of the electrode materials were also modification to synthesize other important molecules
investigated. The platinum electrode of the cathode (2d–2g). Moreover, the substrates with electron-
was replaced with a carbon rod or Ni, and the desired donating and electron-withdrawing groups in ο- or m-
product could be obtained in trace amounts and 60% position of the aromatic lead to products in moderate
yield, respectively (Table 1, entries 11 and 12). This to good (2h–2l). To our delight, the ketene dithioace-
indicates that the presence of the metal electrode is tals substituted by the steric hindrance and heterocyclic
more conducive to the reduction and protonation of the groups can be smoothly compatible in the established
ketene dithioacetal on the cathode surface.
conditions (2m–2n). Altering the dithioalkyl moiety to
With the optimized conditions in the hand, the À S(CH2)3SÀ could also give the desired product with
scope and limitations of the electron-deficient internal excellent yield (2o–2p). Interestingly, 3-(methylthio)-
alkene substrate were explored, the results as shown in 1-phenylpropan-1-one (2q) can be obtained with 85%
Table 2. The electron-deficient internal alkenes con- yield when 3,3-bis(methylthio)-1-phenylprop-2-en-1-
taining a variety of electron-donating or electro-with- one was performed in the established conditions. The
drawing substituents on the aryl groups are compatible desired product 4a could be obtained with a yield of
in the present protocol with excellent yields (2a–2g). 80% when chalcone was placed in standard condition
A. Unfortunately, alkyl dithioacetals, enamides, and
styrene did not yield the desired products under the
standard conditions (2r–2t).
Table 2. Scope of Ketene Dithioacetals.[a]
By simply adjusting the above optimum conditions,
the hydrogenation of ynones can be selectively
performed to form saturated ketones by using the Ph2S
as the anode parallel paired electrolysis reagent, carbon
rods as the cathode (Table 3). First, various ynones
with different electron-donating or electron-withdraw-
ing group substituents at p, m, o-positions of 1-aryl
rings can be compatible in the present protocol, and
the corresponding products can be obtained with
excellent yield and selectivity (4a–4j). Subsequently,
the functional group tolerance of the 3-positions of the
aryl groups was investigated, and it was found that
ynones with Me, F, Cl, and Br substituents at p-, m-, or
o-positions corresponding products can be obtained
with excellent yields (4k–4q). Moreover, cinnamoni-
trile can be obtained with a yield of 61% when 3-
phenylpropiolonitrile was implemented in the present
protocol (4r). However, it was found that activated
internal alkynes containing Br, amide, or ester sub-
stituents were incompatible in the present protocol
(4s–4u).
To demonstrate the practical utility of this electro-
chemical protocol, the 1a and 3d were selected as
substrates for gram-scale experiments. As shown in
Scheme 2, the reaction can afford 2a and 4d in 72%
and 68% yields, respectively. The results indicate that
this electrochemically-induced hydrogenation of elec-
tron-deficient internal olefins and alkynes is a practical
and efficient strategy with CH3OH as the proton donor.
To gain further insights into the transformation
mechanism, control experiments, and cyclic voltamme-
try (CV) experiments were carried out. Firstly, deutera-
tion experiments demonstrate that CH3OH serves as a
proton donor in the present electrochemical hydro-
genation of electron-deficient internal alkenes and
alkynes (Scheme 3a–b, Figure S1–2). Subsequently, no
[a] Standard conditions A: carbon rods (d: 6 mm) anode, Pt
(1.5 × 1.5 cm) cathode, constant current = 15 mA, 1
(0.25 mmol), KSCN (0.5 mmol), Et4NBF4 (2.0 equiv.),
MeCN/CH3OH (10.0 mL, v=5/5), r.t., N2, 4 h. n. d.=not
detected.
[b] Isolated yields.
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Table 3. Scope of Alkynes.[a]
Scheme 3. Control Experiments.
D are reasonable intermediates (Scheme 3c–d). To our
delight, alkene radical intermediate B where p-
CF3C6H5 served in place of Ph could be captured by
TEMPO to afford 5 in a yield of 65% (Figure S3–4).
Moreover, the desired product 4a can obtain a 57%
yield when chalcone was performed under standard
conditions B, which indicates that chalcone is a key
intermediate for hydrogenation of ynones. Besides,
cyclic voltammetry experiments were carried out to
further explain the selectivity hydrogenation of the
electron-deficient internal alkynes. As shown in Fig-
ure S5a, the reduction electrode potential of 3a in
CH3CN/CH3OH (À 1.35 V vs Ag/AgCl) is lower than
that in CH3CN (À 1.43 V vs Ag/AgCl), which indicates
that using CH3CN/CH3OH as a solvent is easier to
perform the electrochemical hydrogenation of ynones.
As shown in Figure S5b, 3a (Eonset =1.89 V vs Ag/
AgCl) exhibits a higher oxidation potential than
diphenyl sulfide (Eonset =1.42 V vs Ag/AgCl). These
results indicate that the hydrogenation of ynones
should be preferentially performed on the surface of
the cathode, and diphenyl sulfide will be anodized to
diphenyl sulfoxide.
[a] Standard conditions B: carbon rods (d: 6 mm) anode, Pt
(1.5 × 1.5 cm) cathode, constant current = 12 mA, 3
(0.25 mmol), Ph2S (0.5 mmol), Et4NBF4 (2.0 equiv.),
MeCN/CH3OH (10.0 mL, v=5/5), r.t., N2, 6 h. n.d.=not
detected.
[b] Isolated yields.
[c] Ref. 7c, 7e, 7f conditions.
Base on the above preliminary experimental results
and previous reports,[6,7c,e,f,9a] a possible mechanism was
outlined and described in Scheme 4. For the electro-
chemical hydrogenation of ketene dithioacetals: Ini-
tially, 1a is reduced and protonated on the surface of
the cathode to generate the intermediate radical A. The
radical intermediate A was reduced and then proto-
nated yielding the desired product 2a. At the same
Scheme 2. Gram-scale Experiment.
desired product was observed when 2,2,6,6-tetrameth- time, KSCN as the anode parallel paired electrolysis
yl-1-piperidinyloxy (TEMPO) or 2,6-di-tert-butyl-4- reagent could be oxidized to (SCN)2. For the selective
hydroxytoluene (BHT) was added to the reaction hydrogenation of ynones: Firstly, 3a undergoes reduc-
system, suggesting that a radical such as A, B, C, and tion and protonation to obtain the alkene radical
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CH2Cl2 (10 mL×3). The organic layers were combined, dried
over Na2SO4, and concentrated. The pure product was obtained
by flash column chromatography on silica gel.
Acknowledgments
This work is supported by the National Natural Science
Foundation of China (No. 21902083), Natural Science Founda-
tion
of
Shandong
Province
(No.
ZR2020QB130,
ZR2018MB014), Shandong, P. R. China. College Students
Innovation and Entrepreneurship Training Project of Shandong
Province (No. S202010446066). This work is also supported by
the Talent Program Foundation of Qufu Normal University
(NO. 6132 and 6125).
Scheme 4. Postulated Reaction Pathway.
References
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and protonation to produce the benzyl radical inter-
mediate D, which finally underwent the same sequence
to obtain the saturate ketone 4a. Similarly, diphenyl
sulfide acts as an anode parallel paired electrolysis
reagent that was oxidized to diphenyl sulfoxide on the
surface of the anode (Figure S6).
In conclusion, the electrochemical-induced selec-
tive hydrogenation of electron-deficient internal olefins
and alkynes with CH3OH as a proton donor has been
disclosed. The established method can simultaneously
realize the electrochemical hydrogenation of electron-
deficient internal alkenes and alkynes to synthesize
saturated ketone derivatives. Mechanism studies con-
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tion strategy of electron-deficient internal alkenes and
alkynes underwent the radical hydrogenation process.
Moreover, this electrochemical hydrogenation strategy
can be easily amplified with excellent efficiency and
functional group tolerance. Besides, employing a
similar hydrogenation strategy to synthesize other
biologically active compounds is currently underway
in our laboratory.
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Experimental Section
In an oven-dried undivided three-necked flask (25 mL)
equipped with a stir bar, ketene dithioacetal 1 or 3 (0.25 mmol),
KSCN, or Ph2S (0.5 mmol), and Et4NBF4 (0.5 mmol, 108.5 mg)
were combined and added. The undivided three-necked flask
was equipped with a carbon rod (d: 6.0 mm) anode and
platinum plate (1.5×1.5 cm) or a carbon rod as the cathode and
was then charged with nitrogen. Under the protection of
nitrogen, CH3OH (5.0 mL) and CH3CN (5.0 mL) were slowly
injected into the reaction flask. The reaction mixture was stirred
and electrolyte at a constant current of 15 mA or 12 mA under
room temperature for 4–6 h. When the reaction was finished,
the reaction mixture was washed with water and extracted with
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