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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.
Adv. Synth. Catal. 2021, 363, 2104–2109
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