Mendeleev Commun., 2017, 27, 180–182
S
S S
Table 1 Current yields of cycloalkanethiols and anodic overpotentials (DE)
for reactions of hydrogen sulfide with cycloalkenes C5, C6 (t = 2 h).
– e
2a,b
– H+
n
n
n
Current yield (%)
3a,b
Method of H2S
activation
DE/Va
1a,b
2a
2b
S
S
H2S
I
28
36
28
18
29
39
35
38
35
23
33
77
0
– HS
II
0
a n = 1
b n = 2
n
n
n
n
III
IV
V
–
4a,b
–0.5
–0.8
–1.5
Scheme 3
VI
radicals occurs with high rate thus increasing the yield of organo-
sulfur compounds.12 Taking this into account, electrochemical
transformations of H2S in the presence of cycloalkenes and sulfur
in MeCN were studied. The direct activation of H2S (method II)
was accompanied by the formation of organic polysulfides in
8.1 and 18.8% yields for cyclopentene and cyclohexene, respec-
tively. Thus, the side reaction of radical copolymerization in the
presence of sulfur is more typical of cyclohexene.
Cathodic activation of H2S (method III) was carried out under
conditions of pulsed electrolysis at potential sweep in the range
from 0.5 to –1.7 V. The H2S is irreversibly reduced with forma-
tion of thiolate anion which is oxidized on anode (0.1 V) to thiyl
radical. The direct cathodic activation of H2S was previously used
in reaction of thiolation of cycloheptane at room temperature.13
Methods IV, V of redox-activation of H2S are indirect as they
use combined systems containing Pt-anode and chemical reagent
(oxidant or electromediator). For activation of H2S, in method
IV Q/QH2 redox pair was used. Earlier this system was applied
in the synthesis of cycloalkanethiols from cycloalkanes C5–C8
and H2S.14 The first stage of reaction is one-electron chemical
oxidation of H2S by quinone Q with formation of QH2. The
electrolysis of the cycloalkene + H2S mixture was performed
at potential of oxidation of QH2. It is characterized by the lower
value of E in comparison with methods I and II.
aDE was calculated relatively to the potential of oxidation of H2S for direct
method I.
For indirect generation of thiyl radicals, the combined (Pt-
anode + Et3N) system (method VI) was tested. Earlier this system
was used in reactions of SH-functionalization of furan, thiophene
and alkenes C6–C8 with hydrogen sulfide.15 In this case the poten-
tial of electrolysis significantly decreased due to deprotonation of
H2S with Et3N. The formed thiolate anion is oxidized on anode
to thiyl radical. Under these conditions, the reaction of H2S with
cycloalkenes 1 also affords cycloalkanethiols 2, disulfides 3 and
sulfides 4.
The current yields of cycloalkanethiols for different methods
of redox-activation of H2S are given in Table 1. In all cases, the
yieldofcyclohexanethiol2bishigherthanthatofcyclopentanethiol
2a. The results of electrochemical experiments correlate with the
data of quantum-chemical calculations. The calculated values of
DEHF for the reactions of H2S with cyclopentene and cyclohexene
indicate that the formation of thiols is energetically unhindered
(–87.6 and –64.7 kJ mol–1, respectively). As shown above, cyclo-
alkanethiols undergo further transformations. The calculated
values of DEHF for reactions of dimerization of cyclopentylthiyl
and cyclohexylthiyl radicals are –218.7 and –213.8 kJ mol–1,
respectively. However, the value of DEHF for dicyclopentyl sulfide
4a formation is significantly lower (–55.0 kJ mol–1) than the
corresponding value for dicyclohexyl sulfide 4b (2.7 kJ mol–1)
(see Scheme 3). This difference is probably the main reason for
observed lower current yield of cyclopentanethiol.
The current yields of cycloalkanethiols (see Table 1) depend
on the method of H2S redox-activation and, under conditions of
activation on electrodes (methods I, III), they are comparable.
Replacing the direct method of activation to indirect one is
attractive due to lower energy costs for carrying out the electro-
synthesis. Meantime, when using electromediators (methods
IV, V) the current yield of cycloalkanethiols decreases as a result
of side transformations. The combined system including Pt-anode
and Et3N proved to be the most promising, especially in terms of
DE. In the presence of this combined system, the current yield of
cyclohexanethiol 2b was as high as 77%. However, method VI is
characterized by formation of a large amount of elemental sulfur.
In conclusion, the possibility of electrosynthesis of cyclo-
alkanethiols from H2S and cycloalkenes C5, C6 with moderate
current yield at room temperature has been demonstrated.
Carrying out the reaction in the presence of Bu4NBr using
redox pair Br–/Br• as electromediator (method V) also allowed us
to obtain cycloalkanethiols along with the products of substrate
bromination (Scheme 4). At the same time, the decrease in the
anodic overpotential (DE) as compared with methods II and IV
was observed.
H2S
– e
+
Br–
Br
Br– + H2S
Pt-anode, MeCN
H+
SH
Pt-cathode,
+ e
1a,b
MeCN
1/2H2
2a,b + 1/2H2
Scheme 4
‡
The GC/MS was performed on a Shimadzu GCMS-QP2010 Ultra
instrument equipped with a mass spectrometric detector (EI, 70 eV).A capil-
lary column SPB-1 SULFUR (30 m×0.32 mm, Tmax = 320°C) was used.
The carrier gas was helium. The thermostat temperature was programmed
from 30 to 280°C. In mass spectra, the molecular ions of cyclohexane-
thiol, m/z: 116.05 [M]+ (calc. for C6H12S, m/z: 116.07) and cyclopentane-
thiol, m/z: 102.05 [M]+ (calc. for C5H10S, m/z: 102.05) were observed.
IR spectra were obtained in KBr pellets using a FSM-1211 IR Fourier
spectrometer. In IR spectra the valence vibrations of S–S (507 cm–1), C–S
(690 cm–1) and S–H (2356 cm-1) bonds were detected. Quantum-chemical
calculations were carried out in the WinGAMESS 07 software11 using
the density functional theory [B3LYP/6-31++G(d,p)]. The effect of solvent
(MeCN) was taken into account using the polarizable continuum model.
The energy effects of the studied reactions (DEHF) were calculated as the
difference between the total HF energies of products and substrates.
This work was supported by the Russian Science Foundation
(grant no. 14-13-00967).
References
1 J. Yoshida, K. Kataoka, R. Horcajada and A. Nagaki, Chem. Rev., 2008,
108, 2265.
2 Yu. N. Ogibin, M. N. Elinson and G. I. Nikishin, Russ. Chem. Rev., 2009,
78, 89 (Usp. Khim., 2009, 78, 99).
3 A. O. Terent’ev, O. M. Mulina, D. A. Pirgach, M. A. Syroeshkin, A. P.
Glinushkin and G. I. Nikishin, Mendeleev Commun., 2016, 26, 538.
– 181 –