N-heterocyclic carbene-catalyzed regio- and stereoselective hydrothiolation reaction of alkynes

Article abstract of DOI:10.1080/00397911.2018.1468910

N-heterocyclic carbenes (NHCs) have been utilized as Br?nsted base to catalyze the hydrothiolation reaction between alkynes and thiols to produce the vinyl sulfides stereoselectively.

Full text of DOI:10.1080/00397911.2018.1468910

Synthetic Communications
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N-heterocyclic carbene-catalyzed regio- and
stereoselective hydrothiolation reaction of alkynes
Zi-Song Cong, Yang Zhang, Guang-Fen Du, Cheng-Zhi Gu & Lin He
To cite this article: Zi-Song Cong, Yang Zhang, Guang-Fen Du, Cheng-Zhi Gu & Lin He (2018):
N-heterocyclic carbene-catalyzed regio- and stereoselective hydrothiolation reaction of alkynes,
Synthetic Communications, DOI: 10.1080/00397911.2018.1468910
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https://doi.org/10.1080/00397911.2018.1468910
N-heterocyclic carbene-catalyzed regio- and stereoselective
hydrothiolation reaction of alkynes
Zi-Song Cong, Yang Zhang, Guang-Fen Du, Cheng-Zhi Gu, and Lin He
Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, School of Chemistry
and Chemical Engineering, Shihezi University, Xinjiang Uygur, Autonomous Region, People’s
Republic of China
ABSTRACT
ARTICLE HISTORY
Received 8 January 2018
N-heterocyclic carbenes (NHCs) have been utilized as Brønsted base
to catalyze the hydrothiolation reaction between alkynes and thiols
to produce the vinyl sulfides stereoselectively.
KEYWORDS
Hydrothiolation;
N-heterocyclic carbene;
vinyl sulfide
GRAPHICAL ABSTRACT
Introduction
Vinyl sulfides are significant structural motifs found in natural products, biologically
active compounds and functional materials.[1] The most atom-economical and straight-
forward method to synthesize vinyl sulfides is hydrothiolation reaction of alkynes.[2]
Over the past several decades, many different transition metal catalysts[3] have been
explored for the synthesis of vinyl sulfides. However, some of those methods suffer
some drawbacks like the use of expensive transition metals or low Z/E selectivity. Base-
mediated addition of thiols to alkynes was developed firstly by Truce[4] and co-workers,
more than half century ago, but the stoichiometric amount of a base was necessary.
Recently, Oshima and co-workers reported[5] that catalytic amount of cesium carbonate
can promote this hydrothiolation reaction efficiently. However, the radical inhibitor
TEMPO was needed and aryl thiols were unsuccessful for the addition. Despite great
progress made in this research, the development of more efficient and mild protocols
for the syntheses of vinyl sulfides is still of high importance.
CONTACT Guang-Fen Du
duguangfen@shzu.edu.cn
Laboratory for Green Processing of Chemical Engineering of
Xinjiang Bingtuan, School of Chemistry and Chemical Engineering, Shihezi University, Xinjiang Uygur Autonomous
Region 832000, People’s Republic of China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental material (1H and 13C NMR spectra and experimental details) can be accessed on the publisher’s website.
ß 2018 Taylor & Francis

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Z.-S. CONG ET AL.
As an important type of organic catalysts, NHCs have been utilized widely in different
transformations.[6] In addition to the classical benzoin condensation[7] and Stetter reac-
tion,[8] NHCs-catalyzed homoenolate transformation,[9] redox reaction,[10] cycloaddition,[11]
and other reactions[12] have been extensively studied in the past decades.
Those aforementioned reactions are mainly based on the Lewis basic properties of NHCs.
On the other hand, NHCs have unique Brønsted base characteristics. Nolan and Hedrick
independently developed NHC-catalyzed transesterifications.[13] The mechanism study
revealed that NHC acted as a Brønsted base to promote the transformation.[14] Using the
specific Brønsted base properties, NHCs-catalyzed Michael type additions between different
nucleophiles and Michael acceptors have been developed by Conquerel,[15] Scheidt,[16]
Zhang,[17] Huang,[18] and our group.[19] Very recently, we[20a,b] found that NHCs can cata-
lyze the reactions of thiols and active alkenes to form functionalized vinyl sulfides. Zhou,
Chen, and co-workers[20c] also reported an interesting method to synthesize this motifs via
NHC-catalyzed sulfenylation reaction of a,b-unsaturated aldehydes. Based on these works,
we envisioned that NHCs can function as a Brønsted base to catalyze the hydrothiolation
reaction of alkynes (Scheme 1). And herein, we wish to report this result.
Experimental
Our initial studies were carried out with the commercially available phenylacetylene 1a
and ethanethiol 2a as the model substrates (Table 1). Treatment of 1a with 1.5 equiv 2a
in the presence of the 10 mol% stable N-heterocyclic carbene A[21] in THF at room tem-
perature for 10 h afforded the adduct 3a in 57% yield with moderate Z/E selectivity. The
following studies showed that the reaction media has dramatic influence on the reaction
yield and Z/E selectivity. Only trace amount of the hydrothiolation adduct was obtained
in low polarity solvents (Table 1, entries 2–4). Conversely, high yield and Z/E selectivity
were obtained in high polarity solvents (Table 1, entries 5–7). Under similar conditions,
other NHCs derived from imidazolium salts can catalyze the addition efficiently to give
3a in good yield with high Z/E selectivity (Table 1, entries 8–10). NHC generated from
the saturated imidazolinium C showed relatively low activity, affording the product in
Scheme 1. NHCs-catalyzed synthesis of vinylsulfides.

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Table 1. Screening of reaction conditionsa.
Entry
Conditions
Time (h)
Yield (%)b
Z/Ec
1
2
3
4
5
6
7
8
A (10 mol%), THF
A (10 mol%), DCM
A (10 mol%), toluene
A (10 mol%), DCE
A (10 mol%), CH3CN
A (10 mol%), DMSO
10
10
10
10
10
10
10
10
10
10
10
24
57
trace
trace
trace
73
87
66
64
70
5:3
/
/
/
17:1
17:1
17:1
14:1
17:1
11:1
14:1
17:1
A (10 mol%), DMF
B1, Cs2CO3(10 mol%), DMSO
B2, Cs2CO3(10 mol%), DMSO
B3, Cs2CO3(10 mol%), DMSO
C, Cs2CO3(10 mol%), DMSO
A (5 mol%), DMSO
9
10
11
12
71
44
50
a1a (1 equiv., 0.2 mmol), 2a (2.0 equiv., 0.4 mmol), NHCs (10 mol%), solvent (2.0 mL).
bIsolated total yield of Z/E stereoisomers.
cZ/E was determined by 1H NMR analysis of the crude products.
moderate yield with high Z/E selectivity (Table 1, entry 11). Reduction NHC loading to
5 mol% led to a dramatic decrease of the reaction yield (Table 1, entry 12).
Results and discussion
With the optimal conditions in hand (Table 1, entry 6), we examined the scope and
generality of the reaction. As shown in Table 2, primary, secondary, and the sterically
hindered tertiary thiols can react with phenyl acetylene 1a to produce the desired prod-
ucts in good to excellent yields with high Z/E selectivities (Table 2, entries 1–7). More
interestingly, the free hydroxy group of 2-(2-mercaptoethoxy)ethanol can be tolerated
for the reaction, producing 3h in 90% yield with >99:1 Z/E selectivity (Table 2, entry 8).
Intriguingly, thiophenol, which was proved to be unsuccessful in cesium carbonate-
catalyzed hydrothiolation reaction,[5] reacted with phenyl acetylene smoothly to deliver
3i in 52% yield, but with opposite stereoselectivities (Table 2, entry 9). We concluded
that NHC can react with the acidic sulfhydryl group of thiophenol to form the free
thioxy anion species, which further reacted with phenyl acetylene to give the desired
product with E-selectivity. The scope of alkynes was next examined for the reaction.
Electron-withdrawing groups substituted terminal alkynes reacted with thiols smoothly,
producing the desired adducts in high yield and Z/E selectivity (Table 2, entries 10–12).
Moreover, different positions of the substituents can be well tolerated for the reaction

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Z.-S. CONG ET AL.
Table 2. Evaluation of the substrate scopea.
Entry
1
Product
Yield (%)b
Z/Ec
3a, 87%
17:1
2
3
4
5
6
3b, 86%
3c, 83%
3d, 70%
3e, 88%
3f, 95%
17:1
14:1
6:1
17:1
20:1
7
8
3g, 92%
3h, 90%
28:1
>99:1
9
3i, 52%
3j, 88%
1:4
10
32:1
11
12
13
3k, 93%
3l, 89%
3m, 93%
10:1
8:1
6:1
(continued)

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Table 2. Continued
Entry
Product
Yield (%)b
Z/Ec
14
3n, 99%
13:1
15d
Z-3o, 74%
Z-3p, 67%
E-3o, 93%
E-3p, 85%
1.3:1
2.5:1
16d
17e
18e
1:1.8
1:2.5
8:1
19
3q, 99%
3r, 98%
3s, 68%
20
50:1
21d
17:1
a1 (0.2 mmol), 2 (0.3 mmol), NHC A (10 mol%), DMSO (2.0 mL), rt 10 h.
bIsolated total yield of Z/E isomers.
cZ/E was determined by 1H NMR analysis of the crude products.
d1 (0.2 mmol), 2 (0.6 mmol), rt 24 h.
eConducted the reaction at 80 ꢀC.
(Table 2, entries 13 and 14). However, when electron-donating groups substituted
alkynes were used for the reaction, the hydrothiolation products can be formed in good
yields but with dramatically decreased Z/E selectivity (Table 2, entries 15 and 16).
More interestingly, when the reactions were conducted at 80 ꢀC, E-vinyl sulfides were
obtained as the main isomer (Table 2, entries 17 and 18). We concluded that the
electron-donating substituents lowered the electrophilicity of alkynes and lead to the
increase in the amount of the thermodynamic stable E-isomer. Notably, heteroaryl sub-
stituted alkynes were proven to be competent substrates, producing the corresponding
products 3q and 3r in excellent yields and high Z/E selectivities, respectively (Table 2,
entries 19 and 20). Diphenylacetylene is a very good candidate for the addition, furnish-
ing 3s in 68% yield with high Z/E ratio (Table 2, entry 21).
When the more active methyl ethynyl ketone and methyl propiolate were also tested
for the reaction, the double sulfa-Michael products 3t and 3u were obtained in good
yields (Scheme 2).

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Z.-S. CONG ET AL.
Scheme 2. Double hydrothiolation reaction.
Scheme 3. Proposed mechanism.
Based on the pioneering work on NHC-catalyzed transesterification and C–S bond
forming reactions,[13,20] a plausible mechanism was proposed as depicted in Scheme 3.
NHC functions as a Brønsted base to interact with a thiol to form NHC-thioxy species
I, which undergoes nucleophilic addition with the triple bond of alkyne to form anion
II, and after protonation to produce the final product with releasing of free NHC.
Conclusions
An organocatalytic hydrothiolation reaction of alkynes has been described. The mild
and transition metal free conditions, simple procedure and generally high yield and Z/E
selectivity provide a new protocol for the synthesis of vinyl sulfide products.
Experimental
General section
All reactions were conducted under the nitrogen atmosphere in oven-dried glassware
1
with magnetic stirring bar. H NMR (400 MHz, CDCl3), 13C NMR (101 MHz, CDCl3),
and 19F NMR (376 MHz, CDCl3) spectra were recorded using deuterated chloroform as
a solvent, with tetramethylsilane as an internal standard and reported in ppm (d).
Melting points were measured on a WRS-1B melting point apparatus and were uncor-
rected. Thiols and other chemicals were obtained from Adamas-beta and used without
purification. Anhydrous THF and toluene were distilled from sodium and

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benzophenone. DMF, CH2Cl2, and CH3CN were distilled from calcium hydride. DMSO
was distilled from calcium hydride under vacuum. 1, 2-Dichloroethane was distilled
from calcium chloride.
Typical procedure for NHC-catalyzed hydrothiolation reaction of alkyne
NHC A (7.8 mg, 10 mol%) was dissolved in 2.0 mL dry DMSO, followed by the addition
of alkyne 1a (20.4 mg, 0.2 mmol), and thiol 2a (18.6 mg, 0.3 mmol) at ambient tempera-
ture. The reaction mixture was stirred at the same temperature for 10 h. Then, the mix-
ture was diluted with EtOAc (30 mL), washed with water and dried over MgSO4. The
solvent was removed in vacuo and the crude product was purified by flash column chro-
matography on silica gel (PE) to give the desired product 3a[22] as a colorless oil in 87%
1
yield (28.5 mg); an inseparable 17:1 Z/E mixture; Data for Z-3a: H NMR (400 MHz,
CDCl3) d 7.450–7.45 (m, 2H), 7.37-7.32 (m, 2H), 7.26–7.15 (m, 1H), 6.45 (d,
J ¼ 10.9 Hz, 1H), 6.25 (d, J ¼ 10.9 Hz, 1H), 2.80 (q, J ¼ 7.4 Hz, 2H), 1.35 (t, J ¼ 7.4 Hz,
1
3H). Selected data for E-3a: H NMR (400 MHz, CDCl3) d 6.73 (d, J ¼ 15.6 Hz, 1H),
6.47 (d, J ¼ 16.2 Hz, 1H). Data for Z-3a: 13C NMR (100 MHz, CDCl3) d 137.0, 128.6,
128.2, 127.2, 126.6, 125.5, 29.7, 15.4.
Funding
This work was supported by the National Natural Science Foundation of China [No. 21662029],
the Young Scientists Foundation of Shihezi University [Nos. RCZX201546, 2015ZRKXJQ05] and
the Excellent Young Teachers Plan of Bingtuan [CZ027203].
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