A mechanism of alkali metal carbonates catalysing the synthesis of β-hydroxyethyl sulfide with mercaptan and ethylene carbonate

Article abstract of DOI:10.1039/c9ob01816f

The reaction of β-hydroxyethylation is essential to the current practice of organic chemistry. Here, we proposed a new and green route to synthesize 2-hydroxyethyl n-alkyl sulfide with n-alkyl mercaptan and ethylene carbonate (EC) in the presence of alkali carbonates as catalysts and revealed the mechanism by experiments and theoretical calculations. The reaction reported proceeds rapidly with high yields when it is performed at 120 °C and the catalytic loading is ～1 mol%. This protocol is applicable to other mercaptans to synthesize the corresponding β-hydroxyethyl sulfide. Density functional theory-based calculations show the energy profile for the reaction pathway. The rate-determining step is the ring-opening of EC. A negatively charged O atom of alkali carbonates approaches the S atom of -SH under the influence of hydrogen bonds. An activated S atom that carries more negative charge serves as a nucleophilic reagent and assists in the ring-opening of EC by reducing the Mayer bond orders of the C1-O1 bond in EC. Alkali cations also contribute to the C1-O1 bond cleavage. The energy barrier for the ring-opening of EC decreases with the decrease of electronegativity of alkali cations. Subsequent transference of a H atom leads to the formation of β-hydroxyethyl sulfide, the dissociation of CO₂ and the reduction of K₂CO₃

Full text of DOI:10.1039/c9ob01816f

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A mechanism of alkali metal carbonates catalysing
the synthesis of β-hydroxyethyl sulfide with
mercaptan and ethylene carbonate†
Cite this: Org. Biomol. Chem., 2019,
17, 9367
a
Dongliang Liu, a Tiju Thomas,b Hong Gong,a Fei Li,a Qiang Li,a Lijuan Song,
Tamil Azhagan,b Heng Jiang*a and Minghui Yang*c
The reaction of β-hydroxyethylation is essential to the current practice of organic chemistry. Here, we
proposed a new and green route to synthesize 2-hydroxyethyl n-alkyl sulfide with n-alkyl mercaptan and
ethylene carbonate (EC) in the presence of alkali carbonates as catalysts and revealed the mechanism by
experiments and theoretical calculations. The reaction reported proceeds rapidly with high yields when it
is performed at 120 °C and the catalytic loading is ∼1 mol%. This protocol is applicable to other mercap-
tans to synthesize the corresponding β-hydroxyethyl sulfide. Density functional theory-based calculations
show the energy profile for the reaction pathway. The rate-determining step is the ring-opening of EC.
A negatively charged O atom of alkali carbonates approaches the S atom of –SH under the influence of
hydrogen bonds. An activated S atom that carries more negative charge serves as a nucleophilic reagent
and assists in the ring-opening of EC by reducing the Mayer bond orders of the C1–O1 bond in EC. Alkali
cations also contribute to the C1–O1 bond cleavage. The energy barrier for the ring-opening of EC
decreases with the decrease of electronegativity of alkali cations. Subsequent transference of a H atom
leads to the formation of β-hydroxyethyl sulfide, the dissociation of CO2 and the reduction of K2CO3.
Received 21st August 2019,
Accepted 7th October 2019
DOI: 10.1039/c9ob01816f
rsc.li/obc
addition (Scheme 1c),7 group exchange (Scheme 1d)8 and
ethylene oxide addition (Scheme 1e).9 As for 2-hydroxyethyl
Introduction
The introduction of β-hydroxyethyl groups into N, O and S aryl sulfide, the common synthetic methods are the addition
atoms in organic compounds is a very important organic syn- reaction of aryl mercaptan with ethylene oxide,10 reaction of
thesis reaction.1 Among numerous β-hydroxyethyl alkyl sulfide aryl bromide mercaptan with β-halogenated ethanol,11 and
compounds, 2-hydroxyethyl n-octyl sulfide has been widely reaction of aryl with mercaptoethanol catalysed by NiBr2 or CuI.12
used in many fields.2 To date, one of its applications is its use
The protocols of synthesizing 2-hydroxyethyl alkyl (aryl)
as the only commercially available repellent,3 the commercial sulfide mentioned above have shortcomings, including un-
code of which is MGK R874 (Phillips Petroleum Company).4 At availability of reactants, by-products of halogen salts, harsh
present, 26 kinds of pesticide products mainly used by home- reaction conditions and isomeric by-products. Although the
owners contain 2-hydroxyethyl n-octyl sulfide as an active ethylene oxide addition method appears reasonable from an
ingredient because of its low toxicity and high repellent atom economy standpoint, the boiling point and flash point of
effect on insects. A variety of well-established methods for the ethylene oxide are so low that potential security risks may arise
synthesis of 2-hydroxyethyl n-octyl sulfide have been docu- in its storage, transportation and use. Ethylene oxide is apt to
mented, which include reduction of 2-(octylthio) acetic acid achieve further condensation with hydroxyl of the target
(Scheme 1a),5 nucleophilic substitution (Scheme 1b),6 olefin product because of its high activity, leading to the formation
of polyoxyethylene ether (homopolymers) as by-products. In
addition, ethylene oxide may also cause cancer.1e Clearly, the
aCollege of Chemistry, Chemical Engineering and Environment Engineering,
Liaoning Shihua University, Fushun 113001, China. E-mail: hjiang78@hotmail.com
bDepartment of Metallurgical and Materials Engineering, Indian Institute of
Technology Madras, Adyar, Chennai 600036, Tamil Nadu, India
cNingbo Institute of Materials Technology and Engineering, Chinese Academy of
Sciences, Ningbo 315201, P. R. China. E-mail: myang@nimte.ac.cn
†Electronic supplementary information (ESI) available: Copies of 1H and
13C NMR spectra. See DOI: 10.1039/c9ob01816f
development of efficient methods for the synthesis of 2-hydroxy-
ethyl alkyl (aryl) sulfide is still highly desired.
Ethylene carbonate (EC) has been found to be a good sub-
stitute for ethylene oxide for the synthesis of 2-hydroxyethyl
alkyl (aryl) sulfide (Scheme 1f). Compared with ethylene oxide,
the use of EC offers advantages. For example, melting and
boiling points of EC are 36 °C and 248 °C, respectively.
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Results and discussion
There is a wide range of commercial uses for 2-hydroxyethyl
n-octyl sulfide (2a). In light of this, the process of
β-hydroxyethylation reaction of n-octyl thiol (1a) with EC has
been investigated. Initially, we tried to use 10 mol% of K2CO3
catalyst and unexpectedly found that the reaction was very fast
(by observing the amount of released CO2). Inspired by this
result, the catalytic performance of various alkali metal car-
bonate catalysts in 1 mol% of M2CO3 is examined. The experi-
mental results are shown in Table 1. n-Octyl mercaptan does
not react with EC in the absence of any catalyst (entry 1,
Table 1). Li2CO3 shows no catalytic activity for the reaction
(entry 2, Table 1). The conversion of 1a is found to be only
65.6% when Na2CO3 is employed as a catalyst (entry 3,
Table 1). When Rb2CO3 and Cs2CO3 are used as catalysts, the
conversions of 1a are 91.0% and 99.8%, respectively (entries 5
and 6, Table 1). In both cases, the performance is slightly
higher than that observed with K2CO3 (entry 4, Table 1).
Therefore, according to the experimental results in Table 1, we
infer that as the radius of the alkali metal atom increases and
the electronegativity decreases gradually, the catalytic activity
is enhanced. The order of the catalytic activity of M2CO3 (from
low to high) is as follows: Li2CO3 ≪ Na2CO3 < K2CO3 < Rb2CO3
< Cs2CO3.
In order to depict the catalytic activity of M2CO3, the kinetic
curves associated with the release of CO2 are shown in Fig. 1.
It can be seen from Fig. 1 that the Na2CO3 catalysed reaction
rate is very slow. At the beginning of the reaction, Rb2CO3
shows higher activity than Cs2CO3, while the catalytic activity
of Cs2CO3 exceeds that of Rb2CO3 in the later stage of the reac-
Scheme 1 Methods for the synthesis of 2-hydroxyethyl n-octyl sulfide.
Furthermore, EC is cheap and readily available. By using EC as tion and the total volume of CO2 released by Cs2CO3 catalysed
a reactant, the by-product formed is carbon dioxide, a gas at reaction is close to the theoretical value. Compared with
ambient conditions, which is easy to separate from the Rb2CO3, the activity of K2CO3 in the initial stage of the reaction
product. As far as we know, there have been no reports for the is lower. However, the catalytic reaction rate with K2CO3 is
synthesis of 2-hydroxyethyl alkyl sulfide with aliphatic mercap- almost close to that of Rb2CO3 in the later stage of the reac-
tan and EC. Carlson and Lissel have found that K2CO3 can be
used as a catalyst for the reaction of thiophenol with EC.13 The
yield of 2-hydroxyethyl phenyl sulfide was found to be ∼70%.
Table 1 Screening of catalysts for β-hydroxyethylation of 1a with ECa
Shivarkar and co-workers also reported that NaX zeolites could
catalyse the reaction of thiophenol with EC,1c but long reaction
time and high temperature are necessarily required. Alkali
salts, especially K2CO3, which are inexpensive and non-toxic,
play an important role as solid-base catalysts in organic syn-
thesis.14 In recent years, alkali carbonates have often been
used in catalysing the construction of the C–S bond and pro-
moting the conversion of mercaptan.15 Recently, our group
tried a new route for the synthesis of 2-hydroxyethyl n-octyl
sulfide with n-octyl mercaptan and EC catalysed by K2CO3. We
found that the efficiency of the catalytic reaction is high at
120 °C under solvent-free conditions and atmospheric
pressure; this is a rather surprising result. This prompted us to
examine the changes in the catalytic activity of various alkali
metal carbonates, the reactivity of normal aliphatic thiols and
their comparison with other thiols so as to understand the
catalytic reaction mechanism.
Time
(min)
CO2 vol.
(mL)
1a con.b
(%)
2a sel.b
(%)
Entry
Catalyst
1
2
3
4
5
6
7
8
9
Blank
180
180
0
0
0
0
0
0
Li2CO3
Na2CO3
K2CO3
184.5
41.7
39.2
35.8
177.5
51.2
38.6
1640
2240
2275
2470
1670
2170
2260
65.6
89.6
91.0
99.8
66.8
86.8
91.6
100
100
100
100
100
100
100
Rb2CO3
Cs2CO3
NaHCO3
c
c
KHCO3
KOHd
a Reaction conditions: 1a (112 mmol), M2CO3 (1 mol%), EC
(114 mmol) at 120 °C. b 1a conversion and 2a selectivity calculated by
GC. c MHCO3 (2 mol%). d KOH (2 mol%).
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Rb+ and Cs+, while the H of –SH is easier to combine with –O−
of CO32−. In other words, using Li2CO3 or Na2CO3 as a catalyst,
the equilibrium of the reaction tends to move to the left
(Scheme 2B), while the equilibrium tends to move to the right
when K2CO3, Rb2CO3 and Cs2CO3 are used. After that, RS−
which acts as a nucleophile species, attacks –CH2– in EC,
which is the electron-deficient center.17 This leads to the
easier cleavage of the C–O bond in EC. Thus, M2CO3 (M = K,
Rb, and Cs) lowers the activation energy of the process. The
above explanation is further verified by employing NaHCO3
and KHCO3 (entries 7 and 8, Table 1) as catalysts at the same
molar ratio of 1a to M+ (M/1a = 0.02). As expected, their cata-
lytic activity is close to that of the corresponding M2CO3, and
their kinetic curves of CO2 release are also reasonably similar
(Fig. S1 and S2†). These results strongly indicate that alkali
metal cations play a crucial role in the activation of RSH.
Meanwhile, KOH is also used as a catalyst for the reaction
Fig. 1 Kinetic curves of released CO2 produced by the synthesis of 2a
(the reaction conditions are the same, as shown in Table 1).
tion. Although the reaction time of K2CO3 is slightly longer, (entry 9, Table 1). It is known that the basicity of KOH is stron-
the prices of Rb2CO3 and Cs2CO3 are higher. Thus, K2CO3 is ger than that of K2CO3. However, its catalytic activity is com-
preferred from the standpoint of input costs. It is noteworthy parable to that of K2CO3 at the same molar ratio of 1a to M+
that the selectivity of 2a in the catalytic reaction is 100%. The (M/1a = 0.02). The possible explanation of this result is that
reaction is carried out in the absence of a solvent, and the CO2 is an acidic gas, produced by n-octyl mercaptan when it
molar ratio of EC to 1a is close to the theoretical value, which reacts with EC. It can react with KOH, which transforms KOH
are advantageous from the standpoint of organic synthesis.
into K2CO3 (or KHCO3). The resulting K2CO3 (or KHCO3) is the
The reason for alkali carbonates having different catalytic catalyst of the reaction rather than KOH. Next, for optimizing
activities is likely that the acidity of alkali metal cations varies. the reaction conditions, the effect of temperature and catalytic
Alkali metal cations are considered as Lewis acids (akin to H+), loading on the reaction is investigated (Table 2). The effects of
and their acidity decreases with the increase of ionic size.14f,16 temperature on the rate of hydroxyethylation are significant.
Therefore, it can be inferred that the alkali metal salts with the The kinetic curves associated with the release of CO2 for each
same anion and different cations would exhibit different basi- catalyst at different reaction temperatures are shown in
cities owing to different atomic radius of alkali metal cations. Fig. S3–S5.† The highest 1a conversion is achieved at 120 °C.
Accordingly, as the strength of the Lewis acid (alkali metal
cations) increases, the basicity of the corresponding alkali
Table 2 Optimization of the reaction conditionsa
metal salts (Scheme 2A) decreases.
Besides, on the basis of the soft and hard acid–base theory,
K+, Rb+ and Cs+ are generally considered as soft acids because
of their large ionic radius and relatively high charge density,
while Li+, Na+ and Hδ+ of –SH are considered to be hard acids
owing to their small radius and low charge density. Sδ− of –SH
Amount
(mol%)
Time
(min)
1a con.b
(%)
2a sel.b
(%)
in n-octyl mercaptan has a large radius and high charge
Entry
Catalyst
density, and hence is considered to be a soft base. On the
other hand, O− of CO3 is considered to be a hard base
2−
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
1
1
1
0.1
0.01
1
1
1
0.1
0.01
1
1
1
1
0.1
0.01
41.7
96.7
116.7
79.9
120.9
39.2
48.9
60.5
77.7
90
35.8
53.5
70
77
70.6
104.6
89.6
83.2c
67.5d
83.4
74.6
91.0
86.4c
83.6d
89.4
77.6
98.8
91.5c
82.7d
78.8e
87.6
73.2
100
99.8
99.5
100
100
100
99.1
99.9
98.5
100
100
100
100
100
98.2
100
because of its smaller atomic radius and lower charge density.
Thus, K2CO3, Rb2CO3 and Cs2CO3 are more facile to activate
n-octyl mercaptan because S of –SH tends to combine with K+,
K2CO3
Rb2CO3
Rb2CO3
Rb2CO3
Rb2CO3
Rb2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
9
10
11
12
13
14
15
16
a Reaction conditions: 1a (112 mmol), EC (114 mmol) at 120 °C. b 1a
conversion and 2a selectivity determined by GC. c At 110 °C. d At
100 °C. e At 90 °C.
Scheme 2 Lewis acidity of alkali metal cations (A) and the proposed
activation mechanism of R–SH with M2CO3 (B).
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The time required for the catalytic reaction of K2CO3 at 120 °C, thiophenol is 6.61.18 Its relatively strong acidity is attributed to
110 °C and 100 °C, respectively, is 41.7 min, 96.7 min and the resonance effect, in which the negative charges of a S atom
116.8 min, and the conversion rates of 1a are 89.6%, 83.2% in C6H5S− are more inclined to be delocalized and enter the
and 67.5%, respectively (entries 1–3, Table 2). Rb2CO3 and aromatic ring, leading to the easier dissociation of a H atom
Cs2CO3 also exhibit the same patterns of change. For the same from the S–H bond with K2CO3. Thus, the energy barrier
reaction, the order of activity of the three catalysts is Cs2CO3 > required for K2CO3 to activate C6H5SH to C6H5S− is low, so
Rb2CO3 > K2CO3. Cs2CO3 exhibits the highest catalytic activity, rapid reaction can occur at 70 °C (entry 7, Table 3).
and the conversion rate of 1a reaches ∼78.8% even at 90 °C
Due to the electron-donating effect of the alkyl group, the
(entry 14, Table 2). As shown in Table 2, at a reaction tempera- acidity of normal aliphatic thiols decreases as the carbon
ture of 120 °C, the conversion rate of 1a shows a downward chain increases. The pKa values of methyl mercaptan, ethyl
trend as the amount of M2CO3 is lowered. The kinetic curves mercaptan and n-propyl mercaptan are 10.30, 10.50 and 10.66,
associated with the release of CO2 are shown in Fig. S6–S8.† respectively.19 It is reasonable to expect that high carbon fatty
When the amount of M2CO3 is 0.1 mol%, the conversion rate thiols are less acidic. It can be seen from Table 3 that the reac-
of 1a is more than 80%. Even when the amount of M2CO3 is tion time and the conversion of n-hexyl mercaptan at 120 °C
lowered to 0.01 mol%, the conversion rate of 1a is over 70% and 110 °C is almost the same (entries 1 and 2, Table 3), while
(entries 5, 10 and 16, Table 2), and the corresponding reaction the reaction time of n-octyl mercaptan at 110 °C is more than
time is not significantly prolonged or shortened.
2 times longer than that at 120 °C (entries 3 and 4, Table 3).
In addition, to demonstrate the generality of the protocol, This may indirectly prove that the acidity of n-octyl mercaptan
we investigate the reactive activity of other mercaptans, and is weaker than that of n-hexyl mercaptan. Also at a reaction
the results are summarized in Table 3. As expected, all selected temperature of 120 °C, the reaction time of n-dodecyl mercap-
mercaptans are amenable for the reaction. The reactive rate tan is relatively longer than that of n-hexyl mercaptan and
with thiophenol is the fastest, and the minimum temperature n-octyl mercaptan (entry 5, Table 3). This may be indirectly
required for the reaction to occur is ∼70 °C at a high conver- due to the fact that the acidity of normal aliphatic mercaptans
sion of 1d and high selectivity of 2d (entry 7, Table 3). In con- decreases as the carbon chain increases.
trast, normal aliphatic mercaptans have difficulty in the con-
The pKa value of 2-mercaptoethanol ranges from 9.43 to
version to 2a–2c (entries 1–5, Table 3), which is efficient only 9.75.19 Obviously, the acidity of 2-mercaptoethanol is between
at 120 °C. As for furyl mercaptan, its reactive activity is higher thiophenol and high carbon (≥C6). The relatively strong acidity
than that of 2-mercaptoethanol by comparing conversion and of 2-mercaptoethanol can be attributed to the long-term elec-
selectivity under the same conditions (entries 8–12, Table 3).
tron withdrawing of –OH groups.20 As can be seen from
The difference in the reactivity of different molecular struc- Table 3, the reaction time of the reaction at 120 °C and 100 °C
tures of thiols may be attributed to the difference in the acidity is almost the same (entries 8 and 9, Table 3), although the use
of thiols. The pKa value (in water at 25 °C, the same below) of of 2-mercaptoethanol results in slightly lower reaction rates at
80 °C (entry 10, Table 3). The activity of furfuryl mercaptan is
higher than that of 2-mercaptoethanol, and the conversion of
furfuryl mercaptan reaches 88.4% at 80 °C (entry 12, Table 3).
Although it is impossible to obtain the pKa literature data of
furfuryl mercaptan, it can be predicted that the pKa value of
Table 3 Reactive ability of various mercaptans with EC catalysed by
K2CO3
a
furfuryl mercaptan must be slightly less than 2-mercapto-
ethanol, which is in turn higher than that of thiophenol. This
could not only be due to the long-range electron absorption of
oxygen atoms, but also due to the aromaticity of the furan
ring.
Based on the above results, a tentative mechanism is pro-
posed, as in Scheme 3. The whole reaction can be divided into
three steps. Firstly, RSH couples with K2CO3 to give the inter-
mediate product RS–K+ (IP-1) through a transition state (TS-1)
and releases KHCO3.21 Secondly, IP-1 as a nucleophilic reagent
attacks –CH2– of EC, resulting in the formation of IP-2.22
Finally, the reaction of KHCO3 and IP-2 produces CO2, 2-hydro-
xyethyl sulfide, and K2CO3 by way of TS-3.
To gain deeper insights into the mechanism, n-octyl mer-
captan is selected as a model mercaptan and DFT calculations
are performed to simulate the aforementioned three steps.
Fig. 2 and Fig. S9† show the pathway for the synthesis of
2-hydroxyethyl n-octyl sulfide by the reaction of n-octyl mercap-
tan and EC catalysed by alkali carbonates. All the initial reac-
Time
(min) (mL)
CO2 vol. 1a–1f
2a–2f
Entry
R
con.b (%) sel.b (%)
1
2
3
4
5
6
7
8
n-C6H13 – (1b)
43.2
55.4
41.5
96.7
74.4
8.1
2300
92.0
88.4
89.6
83.2
88.8
99.6
96.8
99.6
99.0
88.0
92.6
88.4
100
100
100
100
100
100
100
100
100
100
100
100
2210c
2240
n-C8H17 – (1a)
2080c
2220
n-C12
C6H5 – (1d)
H25 – (1c)
2490e
2420g
2490
19.4
HOCH2CH2 – (1e) 17.4
9
22.7
73.7
18.7
66.7
2475d
2200f
2315d
2210f
10
11
12
Furfuryl – (1f)
a Reaction conditions: 1a–1f (112 mmol), EC (114 mmol), K2CO3
(1 mol%) at 120 °C. b 1a–1f conversion and 2a–2f selectivity calculated
by GC. c At 110 °C. d At 100 °C. e At 90 °C. f At 80 °C. g At 70 °C.
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where a S atom from RS− acts as a nucleophile assisting the
ring-opening of EC. The first step would be considered as the
activation of a S atom from mercaptan, which is a facile
process with an activation free energy of 2.0 kcal mol−1. The
distance between the O− of alkali carbonates and H atom of
–SH in R-1 is investigated and is shown in Table S2.† This con-
firms the existence of hydrogen bonds,23 so the H atom would
be pulled closer to the O atom, resulting in the cleavage of the
S–H bond and the formation of KHCO3. During the next step,
RS− approaches EC. Thus, the C1–O1 bond in EC can be wea-
kened by the coordination of RS−, which can be verified by the
Mayer bond orders of the C1–O1 bond (see Table S3†). This
weakness of the C1–O1 bond can lead to the cleavage of EC. By
comparing difference among catalysts, it can be obviously
observed that, when using alkali carbonates as catalysts, the
Mayer bond orders decrease.24 After that, RS− would combine
with open-loop EC to form IP-2. The final step is the transfer-
ence of a H atom on KHCO3 to the O2 atom on IP-2; this is
relatively easy since its energy barrier is just 9.3 kcal mol−1
This is followed by the dissociation of CO2.
.
Scheme 3 Proposed mechanism for the synthesis of β-hydroxyethyl
sulfide with mercaptan and EC catalysed by alkali carbonates.
In Fig. 2, the reaction path without any catalysts is also
investigated to specify the differences of the catalytic reaction
tants, intermediate products, transition states and products pathway by alkali carbonates. This is likely a two-step reaction
along the reaction pathway have been investigated, and their and the rate-controlling step is still the process of the ring-
optimized structures are shown in Fig. 3 (see Fig. S10–S13† for opening of EC with a free energy barrier of 68.9 kcal mol−1
.
other alkali carbonates). As shown in Fig. 2, the ring-opening Therefore, alkali carbonates catalyse the reaction by changing
of EC in the presence of K2CO3 is the rate-determining step the reaction pathway and decreasing difficulty in the ring-
with an energy barrier of 37.2 kcal mol−1 for the reaction, opening of EC.
Fig. 2 Calculated relative potential energy profiles for the synthesis of 2-hydroxyethyl n-octyl sulfide with n-octyl mercaptan and EC catalysed by
alkali carbonates.
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Fig. 3 Optimized structures of the species involved in the low-energy synthesis of 2-hydroxyethyl n-octyl sulfide with n-octyl mercaptan and EC
catalysed by K2CO3. The purple ball is the K atom. The red ball is the O atom. The grey ball is the C atom. The white ball is the H atom. The yellow
ball is the S atom.
From above, it is reasonable to state that alkali carbonates
catalyse the reaction by activating the S atom of mercaptan.
However, the energy barrier for the ring-opening of EC with
different alkali carbonates varies. Further investigation is
attempted to determine whether alkali cations have influence
on the C1–O1 bond cleavage. Table S4† shows the number of
the Hirshfeld charges of C1, O1 and alkali cations in IP-1 and
R-2. To our surprise, the number of the Hirshfeld charge
changes of both O1 and alkali cations correlates with the energy
barrier for the ring-opening of EC, as shown in Fig. 4. Thus, it
can be deduced that there are electrons transferring from O1 to
alkali cations when RS− acts as a nucleophilic reagent, attacking
C1 from EC. Meanwhile, the number of transferred electrons
increases with the increase of electronegativity of alkali cations,
and the larger the number of transferred electrons, the larger
the energy barrier for C1–O1 cleavage in EC.
Fig. 4 Hirshfeld charge change of O1 and M+ from IP-1 to R-2 (M = Li+,
Na+, K+, Rb+, and Cs+).
Fig. 5 shows the relationship between the experimentally decrease of the energy barrier, and the higher the energy
observed data in Table 1 and computational energy barrier for barrier is, the longer the reaction time is. To sum up, the
the ring-opening of EC in the presence of the corresponding theoretical calculation results are consistent with the experi-
alkali carbonates. The conversion of 1a increases with the mental results.
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N2 flow as the carrier gas). Infrared (IR) spectra are obtained
as KBr window samples using a PE FT-IR Spectrum GX.
General procedure
A 100 mL round-bottomed flask is charged with mercaptan
(1a–1f, 112 mmol), EC (114 mmol) and M2CO3 (1 mol% based
on mercaptan). The mixture is then heated while stirring the
solution; it is then heated at a constant temperature until the
reaction is finished. The release of CO2 is monitored by a drai-
nage gas collection method. At the end of the reaction, the
round-bottom flask is taken out and cooled to room tem-
perature. M2CO3 catalyst is removed by water washing. The
conversion of 1a–1f and the selectivity of 2a–2f are determined
by gas chromatography. The product is finally obtained by
simple distillation (horizontal) under vacuum. 2c and 2f were
further purified by silica gel column chromatography using
petroleum ether/ethyl acetate as an eluent.
Fig. 5 1a Conversion and reaction time of the synthesis of n-octyl mer-
captan and EC with alkali carbonates in 1 mol% at 120 °C.
Conclusions
Computational details
In summary, we have developed a simple, environmentally
friendly and highly efficient method for synthesizing 2-hydroxy-
ethyl n-octyl sulfide. The synthesis from n-octyl mercaptan
and EC is performed under solvent-free conditions in the pres-
ence of alkali carbonates. High yields are observed at 120 °C.
Here, we have demonstrated that alkali carbonates except
Li2CO3 have catalytic activity for the reaction. Reaction rates
and conversion of n-octyl sulfide increase with the increase in
ionic size of alkali cations. This protocol is applicable to
β-hydroxyethylation of other mercaptans as well. Density func-
tional theory (DFT) calculations successfully delineated the
potential energy profile for each of the reaction pathways. The
ring-opening of EC is the rate-determining step. S–H bond
cleavage in the first step is in part because of the impact of the
hydrogen bond between a H atom from –SH; the negatively
charged O atom of alkali carbonates is also responsible for
this being the rate-determining step. The activated S atom
decreases the Mayer bond orders of the C1–O1 bond in EC,
leading to the easier ring-opening of EC. Alkali cations are also
proven to contribute to C1–O1 bond cleavage, and the energy
barrier decreases with the decrease of electronegativity of
alkali cations. The conclusion obtained from theoretical calcu-
lation is consistent with the experimental results. The catalytic
ability of alkali carbonates is in the sequence Li2CO3 < Na2CO3
< K2CO3 < Rb2CO3 < Cs2CO3.
All the DFT calculations are performed by applying the Dmol3
program in Materials Studio 5.5 package of Accelrys Ltd. The
exchange and correlation interactions are modelled using the
revised Perdew–Burke–Ernzerhof functional (RPBE) within the
generalized gradient approximation (GGA).25 Double-numeric
quality base set with the polarization functions (DNP) is
applied. A thermal smearing of 0.002 Ha is used to improve
the computational performance. All SCF tolerances are set to
fine.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural
Science Foundation of China (grant no. 61971405). Tiju
Thomas thanks the Department of S & T (DST) of the
Government of India for help rendered via project nos:
(i) MET1617146DSTXTIJU, (ii) MET1718170DSTXTIJU and
(iii) SOL1819001DSTXHOCX. Ministry of Electronics and
Information Technology also supported Tiju Thomas via
ELE1819353MEITENA.
Experimental section
General information
Notes and references
All the chemicals were commercially available and used
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