Transition-Metal-Free Cleavage of C-C Triple Bonds in Aromatic Alkynes with S8 and Amides Leading to Aryl Thioamides

Article abstract of DOI:10.1021/acs.orglett.8b00573

A novel transition-metal-free cleavage reaction of C-C triple bonds in aromatic alkynes with S₈ and amides furnishes aryl thioamides in moderate to excellent yields. The remarkable features of this thioamidation include the metal-free cleavage of C-C triple bond, mild reaction conditions, as well as wide substrate scope that is particularly compatible with some internal aromatic alkynes and acetamides.

Full text of DOI:10.1021/acs.orglett.8b00573

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Transition-Metal-Free Cleavage of C−C Triple Bonds in Aromatic
Alkynes with S8 and Amides Leading to Aryl Thioamides
Kai Xu,*,† Ziyi Li,† Fangyuan Cheng,† Zhenzhen Zuo,§ Tao Wang,† Mincan Wang,§
and Lantao Liu*,†,‡,§
†Henan Engineering Laboratory of Green Synthesis for Pharmaceuticals, College of Chemistry and Chemical Engineering, and
‡Henan Key Laboratory of Biomolecular Recognition and Sensing, College of Chemistry and Chemical Engineering, Shangqiu
Normal University, Shangqiu 476000, P. R. China
§College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China
S
* Supporting Information
ABSTRACT: A novel transition-metal-free cleavage reaction of C−
C triple bonds in aromatic alkynes with S8 and amides furnishes aryl
thioamides in moderate to excellent yields. The remarkable features
of this thioamidation include the metal-free cleavage of C−C triple
bond, mild reaction conditions, as well as wide substrate scope that
is particularly compatible with some internal aromatic alkynes and
acetamides.
he cleavage and the transformation of C−C triple bonds,
reactions9 have become an attractive alternative recently. In
Tcompared with those of C−C single bonds, are less detail, most reports involving three-component reactions
focused on the condensation of various substrates such as
benzylamines,9a nitriles,9b aldehydes,9c,g alkynes,9d,f or arylacetic
and cinnamic acids9e,h with amines or amides promoted by S8.
Among them, C(sp2)−C(sp3) bond cleavage was observed in
the Cu-catalyzed reactions of dimethylformamide with
arylacetonitriles and sulfur9b (Scheme 1, eq 1). In 2014,
explored and considered to be one of most challenging targets
in current organic synthesis.1 Except for the metathesis of
alkynes,2 previous methods for C−C triple bonds cleavage
often suffered from the use of stoichiometric amounts of
organometallic reagents or oxidants.3 In recent years, several
efficient and desirable transition-metal-catalyzed processes4
such as Rh-catalyzed cleavage of C−C triple bonds via
hydroiminoacylation4a or hydroacylation,4b Ru-catalyzed C−C
triple bonds cleavage of diynes through hydroamination,4c Ru-
catalyzed C−C triple bonds cleavage of ethynyl alcohols,4d Au-
catalyzed C−C triple bonds cleavage of (Z)-enynols with
molecular oxygen,4e Pd-catalyzed C−C triple bonds cleavage of
alkynes with molecular oxygen,4f Ni-catalyzed C−C triple
bonds cleavage of alkynes with molecular oxygen,4g Ag-
catalyzed C−C triple bonds cleavage and nitrogenation of
alkynes,4h Mn-catalyzed C−C triple bonds cleavage of alkynes
via oxidative rearrangement,4i Fe/Cu-catalyzed C−C triple
bonds cleavage of alkynones,4j and visible-light-induced Cu-
catalyzed C−C triple bond cleavage of terminal alkynes4k were
presented. Undoubtedly, the transition-metal-free approaches
developed very recently are more attractive, delivering very
useful building blocks such as acids, nitriles, esters, and amides.5
Despite these significant developments, transition-metal-free
approaches for C−C triple bond cleavage with inexpensive,
stable, and readily available reagents are still highly desirable.
Thioamides constitute a kind of useful intermediate for the
synthesis of heterocyclic compounds, protein, pharmaceuticals,
and materials.6 The usefulness of aryl thioamides has made
their synthesis the subject of extensive research.7 Apart from
the traditional methods such as thionation of cabonamides and
the Willgerodt−Kindler-type reactions,8 the strategies involving
the step-economic and easily handled three-component
Scheme 1. Thioamide Synthesis
Nguyen and co-workers presented an efficient three-compo-
nent synthesis of aliphatic thioamides from alkynes, elemental
sulfur, and amines9d (Scheme 1, eq 2). Meanwhile, elemental
sulfur has been used in many multicomponent reactions as well
as useful metal-free transformations.10 As far as we know, there
is no report on the synthesis of thioamides via C−C triple
bonds cleavage. Herein, we report a novel synthesis of aryl
thioamides by three-component reactions from alkynes,
Received: February 16, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00573
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
elemental sulfur, and amides via unexpected transition-metal-
free C−C triple bonds cleavage (Scheme 1, eq 3).
To our surprise, the reaction of phenylacetylene, sulfur
power and N,N-dimethylformamide (DMF) in the presence of
NaOtBu afforded N,N-dimethylbenzothioamide (Table 1, entry
Scheme 2. Substrate Scope
a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
base
solvent
yield (%)
1
no
NaOtBu
K2CO3
K3PO4
KOAc
KF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
78
80
84
57
68
77
75
84
84
82
81
83
80
66
<5
2
no
3
4
no
no
5
no
6
no
Cs2CO3
NaOH
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
no
7
no
8
no
DMF/toluene (1:1)
9
FeCl3
NiCl2
no
DMF
DMF
DMF
DMF
DMF
DMF
DMF
10
c
11
d
12
no
e
13
no
f
14
no
15
no
a
Reagents and conditions: phenylacetylene (0.4 mmol), S8 (9.6
mmol), base (1.2 mmol), and the catalyst (10 mol %) in solvent (2
b
mL) at 115 °C under air for 20 h. Isolated yield based on the amount
c
d
e
of phenylacetylene. Under Ar. Using sublimated sulfur powder. At
f
90 °C. S8 (4.8 mmol).
1). The selected results for reaction condition optimization are
summarized in Table 1. In general, the bases tested could
deliver the target product (Table 1, entries 1−7), and K3PO4
(Table 1, entry 3) was found to be the best with an isolated
yield up to 84% comparable with the yield of 80% given by
K2CO3 (Table 1, entry 2). Regarding to the effects of different
solvents, the use of DMF without combination of another
solvent exhibited a favorable result (Table 1, entry 3 vs 8). In
particular, when the reaction proceeded in a mixed solvent of
DMF/H2O (1:1), a messy conversion was observed. Some
control experiments were also explored (Table 1, entries 9−
15). The addition of metal salts, performing the reaction under
Ar in place of air, the use of sublimated sulfur powder, or a
slight decrease in the temperature had little influence on the
yield (Table 1, entries 9−13). However, the yield dropped
obviously with a decreasing amount of sulfur powder (Table 1,
entry 3 vs entry14). Note that no target product was isolated in
the absence of base (Table 1, entry 15).
The substrate scope was studied under optimized reaction
conditions (Scheme 2). Overall, several aromatic alkynes and
amides could be amenable to the reaction and afford the
corresponding products. When terminal aromatic alkynes were
treated with DMF and S8, electron-neutral and electron-rich
ones displayed higher reactivity than the electron-deficient ones
(3a1, 3b, 3c1, 3d, 3e vs 3g1, 3h, 3i). Surpringly, the
substituents NH2 and CF3 on the aromatic alkynes could also
survive in this transformation (3f and 3g). While ethynyl-2-
methylbenzene could not deliver the corresponding product,
ethynyl-3-methylbenzene reacted smoothly to generate 3d in
excellent yield, presumably due to the effect of steric hindrance.
a
Reagents and conditions: aromatic alkyne (0.4 mmol), S8 (9.6
mmol), and K3PO4 (1.2 mmol) in amide (2 mL) at 115 °C under air
for 20 h. Yield of isolated product based on the amount of aromatic
alkyne.
The secondary thioamide 3j was generated in 93% yield when
N-methylformamide was subjected to the reaction conditions.
In addition, replacement of DMF with formylmorpholine led to
the formation of thioamides 3k to 3n in good yields (3k−n).
Both N,N-dimethylacetamide and N,N-dimethylpropionamide
could take part in the reaction producing the same product 3a
as DMF did, albeit in lower yields (3a1 vs 3a2, 3a3), which
indicated that the amides were more likely to undergo scission
of the C(sp2)−N bond in a basic environment9g,h offering the
N(Me)2 moiety. Remarkably, internal aromatic alkynes were
also tolerant to the reaction, which provided 3a, 3c, 3g, 3l, 3m,
and 3n, respectively (3a4, 3a5, 3c2, 3g2, 3l2, 3l3, 3m2, 3n2).
In addition, in N-substituted acetamides, some aromatic alkynes
worked well to afford the corresponding aryl thioamides in
moderate yields from 40% to 60% (Table 1, see the Supporting
Information (SI) for more details).
In particular, 1-methyl-4-(phenylethynyl)benzene was
equally split into 3a and 3c in the presence of DMF and S8
(Scheme 3, eq 1). Moreover, when 1-octyne was added to the
standard conditions, N,N-dimethyloctanethioamide (3o), the
B
DOI: 10.1021/acs.orglett.8b00573
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
amides. Further mechanistic studies and synthetic applications
of these methodologies are currently underway in our
laboratory.
Scheme 3. Thioamidation of Alkynes by DMF and S8
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00573.
Experimental procedures; spectroscopic data for new
compounds (PDF)
product without C−C triple bond breaking, was obtained in
47% yield (Scheme 3, eq 2).
Some possible intermediates including aryl methyl ketones
exemplified as 1-(4-methylphenyl)ethanone, benzoic acid,
phenylglyoxal monohydrate, and 2-oxo-2-phenylacetic acid
were subjected to the standard conditions, but none of them
gave cracked products in good yield, which ruled out their
formation to a large extent in the reaction process (Scheme 4,
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xuk1008@163.com.
*E-mail: liult05@iccas.ac.cn.
ORCID
Mincan Wang: 0000-0002-3817-3607
Lantao Liu: 0000-0002-8025-7222
Notes
Scheme 4. Mechanism Investigation
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21572126, 21202095), Program for
Science & Technology Innovation Talents in Universities of
Henan Province (14HASTIT016), Technicians Troop Con-
struction Projects of Henan Province (No. C20150030),
Program for key scientific research project in Universities of
Henan Province (17A150017), and Program of Science and
Technology Innovation Talents of Henan Province
(2018JQ0011).
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