Copper-Catalyzed Geminal Difunctionalization of Terminal Alkynes by Splitting Sulfonyl Hydrazones into Two Parts

Article abstract of DOI:10.1021/acs.orglett.8b02268

A copper(I)-catalyzed geminal difunctionalization of terminal alkynes was developed via a carbene migratory insertion and an addition of a sulfonyl anion to the triple bond at the same time under mild reaction conditions, providing a variety of vinyl sulfones with good yields and excellent stereoselectivities.

Full text of DOI:10.1021/acs.orglett.8b02268

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Geminal Difunctionalization of Terminal Alkynes
by Splitting Sulfonyl Hydrazones into Two Parts
Qi Sun, Linge Li, Liyan Liu, Qianqian Guan, Yu Yang, Zhenggen Zha, and Zhiyong Wang*
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry & Center for
Excellence in Molecular Synthesis of Chinese Academy of Sciences, Collaborative Innovation Center of Suzhou Nano Science and
Technology & School of Chemistry and Materials Science in University of Science and Technology of China, Hefei, Anhui 230026,
P. R. China
S
* Supporting Information
ABSTRACT: A copper(I)-catalyzed geminal difunctionaliza-
tion of terminal alkynes was developed via a carbene
migratory insertion and an addition of a sulfonyl anion to
the triple bond at the same time under mild reaction
conditions, providing a variety of vinyl sulfones with good yields and excellent stereoselectivities.
structural transformation.5 In recent years, typical methods of
ifunctionalization of terminal alkynes has long served as a
1
Dpowerful strategy to access multisubstituted olefins. In direct sulfonyl radical additions have been extensively
investigated, in which the anti-Markovnikov products were
obtained exclusively owing to the Kharasch effect.6 In order to
synthesize α-substituted vinyl sulfones under the transition-
metal-free and visible light conditions, a novel radical
Markovnikov addition was proposed by Lei’s group.7 Recently,
a one-step procedure was developed in the presence of
DABCO·2SO2 (DABSO) via an insertion of sulfur dioxide.8
Another one-step strategy by ultilizing benzenesulfonyl
chloride as the sulfonyl radical source was developed by
Nevado and co-workers to prepare β,β-disubstituted vinyl
sulfones.9 Besides, nonradical processes were also employed for
the synthesis of vinyl sulfones by using sulfonyl hydrazides,10
sodium sulfinates,11 and sulfinic acids12 as the sulfonyl anion
sources. However, these reported methods suffer from harsh
conditions or toxic reagents. Low atomic economical efficiency
also limited their synthetic utility since only the sulfonyl anion
was used in the previous methods.
Sulfonyl hydrazones as a safe precursor for diazo
compounds13 have been extensively explored to access various
skeletons by inter- or intramolecular carbene insertions.14−17
However, in the generation of diazos, sulfonyl anions as a
byproduct are commonly discarded,18 which significantly
restricts the atom economy of the transformations (Scheme
2). Herein, we proposed that sulfonyl hydrazones could be
split into two fragments, a carbene and a sulfonyl equivalent.
Indeed, a copper(I)-catalyzed geminal difunctionalization of
terminal alkynes was achieved with this difunctional reagent. In
this efficient protocol, an allene species, which was generated
by the carbene insertion of alkyne, was identified as the key
intermediate to access trisubstituted vinyl sulfones (Scheme 2).
In comparison with the previous synthetic routes of vinyl
sulfones, this method is not only a multifunctional reaction
most of the previous strategies, the difunctionalization of
terminal alkynes was achieved by electrophilic additions or
radical additions. As a common reaction mode for the addition
to C−C multiple bonds, these methods almost exclusively
result in 1,2-difunctionalization (vicinal) regardless of the
difference in the synthetic routes.2 In contrast, the reports on
the geminal difunctionalization of terminal alkyne are scarce.
Recently, a remarkable breakthrough of the 1,1-diboration of
alkynes was achieved by Sawamura’s group and Chirik’s
group.3 However, a general one-step approach or strategy for
1,1-difunctionalization (geminal) of terminal alkynes is still
highly demanded (Scheme 1). In this context, we envisioned
that a strategy involving carbene chemistry might be a solution
for this long-standing problem.
As one of the functionalized products of terminal alkynes,
vinyl sulfones display special physiological properties and
biological activities,4 such as the inhibitors of SrtA, inhibitors
of cysteine protease, and a potential treatment for Chagas’
disease. At the same time they are frequently employed as
building blocks in organic synthesis due to their versatility in
Scheme 1. Vicinal or Geminal Difunctionalization of
Terminal Alkynes
Received: July 20, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02268
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Letter
With the optimal reaction conditions in hand (entry 3,
Scheme 3), we then explored the scope of the substrates. As
Scheme 2. Sulfonyl Hydrazones As a Precursor of Carbene
in Transformations
a b
,
Scheme 3. Reaction Scope of Sulfonyl Hydrazones
(including sulfonyl radicals, sulfur dioxide reagent, or sulfonyl
anions) but also a highly efficient reaction (excellent
functional-group tolerance, mild conditions, and high stereo-
selectivity).
To find the optimal reaction conditions, phenylacetylene 1a
and N-tosylhydrazone 2a were initially selected as the model
substrates (Table 1). The expected product 3aa could be
a
The reaction was carried out with 1a (0.2 mmol), 2 (0.4 mmol), CuI
(10 mol %), and Et3N (0.6 mmol) in DCM (1 mL) at room
temperature. Isolated yield.
b
a
Table 1. Optimization of the Reaction Conditions
shown in Scheme 3, N-tosylhydrazones, bearing various
functional groups at the para-position of the phenyl ring,
reacted smoothly with phenylacetylene 1a to afford the desired
products. Both electron-donating (3aa, 3ac, Scheme 3) and
electron-withdrawing (3ag, 3ah, Scheme 3) groups on the
phenyl ring could be tolerated under this condition. Never-
theless, the electron-withdrawing groups had a negative
influence on this reaction (3ad−3ah, Scheme 3) while
electron-donating groups favored this reaction slightly more
(3aa, 3ac, Scheme 3). On the other hand, when
naphthylsulfonyl hydrazone was employed as the substrate,
the corresponding product can be obtained efficiently (3ai,
Scheme 3). Pleasingly, butylsulfonyl hydrazone can also be the
reaction substrate to give the desired product with a moderate
yield (3aj, Scheme 3). Moreover, N-(4-sulfo-benzyl)-hydra-
zone can be employed as the substrate and the reaction was
carried out smoothly to yield the product in 79% yield (3ak,
Scheme 3). The R2 on the ester of the substrate 2 can also be
varied. When the substrate 2l and 2n were employed in this
reaction, the desired products 3al and 3am can be obtained
with yields of 80% and 76%, respectively. The substituent on
the carbon of the double bond of hydrazone can be changed to
give the desired product with good yield (3an, Scheme 3).
Next, we turned our attention to assess the scope of the
alkynes. A series of terminal alkynes (1b−1l, Scheme 4) were
employed to react with N-tosylhydrazone 2a under the
optimized reaction conditions. The electronic effect of
substituents on the aryl ring of the alkynes was examined
first. Aromatic alkynes bearing either electron-donating groups
(3ba−3da, 3ha, Scheme 4) or electron-withdrawing groups
(3ea−3ga, 3ia, Scheme 4) on the aryl ring could afford the
corresponding products in good yields. Furthermore, the
phenylacetylene with an ortho- or a meta-substituent on the
phenyl ring could be efficiently converted to the corresponding
products with good yields (3ja, 3ka, Scheme 4). All aromatic
alkynes underwent the reaction to give the desired products in
good yields. Moreover, the high selectivity could be maintained
regardless of the properties and the positions of the
b
entry
catalyst
CuCl
CuBr
solvent
base
yield (%)
1
2
3
4
5
6
7
8
DCM
DCM
DCM
DCM
DCM
MeCN
toluene
DMF
CHCl3
DCM
DCM
DCM
DCM
DCM
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
K2CO3
LiOtBu
DBU
Et3N
Et3N
11
17
89
CuI
Cu(MeCN)4PF6
Cu(MeCN)4BF4
n.d.
n.d.
n.d.
n.d.
n.d.
78
n.d.
n.d.
n.d.
75
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
9
10
11
12
c
13
d
14
82
a
The reaction was carried out with 1a (0.2 mmol), 2a (0.4 mmol),
catalyst (10 mol %), and base (0.6 mmol) in solvent (1 mL) at room
temperature for 11 h. The yields refer to the isolated products. 0 °C.
b
c
d
50 °C.
obtained in 11% isolated yield under catalysis of CuCl in the
presence of Et3N (entry 1, Table 1). Various copper salts were
studied in this reaction, and CuI was found to be the best
catalyst with a yield of 89% of the desired product 3aa (entries
2−5, Table 1). The screening of solvents indicated that DCM
was the ideal solvent (entries 6−9, Table 1). The effect of
bases was then examined, and Et3N was found to give the
optimal result (entries 10−12, Table 1). Temperature
screening showed that the reaction temperature had a great
impact on this reaction. For instance, when the reaction was
carried out at 0 °C, only a low reaction yield was obtained
(entry 13, Table 1). The higher temperature did not improve
the yield either (entries 13−14, Table 1).
B
DOI: 10.1021/acs.orglett.8b02268
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Letter
ab
a b
,
Scheme 4. Reaction Scope of Terminal Alkynes
Scheme 5. Scale-up Reaction
a
The scale-up reaction was carried out with 1a (5.0 mmol), 2a (10.0
mmol), CuI (10 mol %), and Et3N (15.0 mmol) in DCM (20 mL) at
b
room temperature. Isolated yield
conditions, 3aa was obtained smoothly although the yield
was decreased slightly [Scheme 6, eq 1]. These results
a b
,
Scheme 6. Control Experiments
a
The reaction was carried out with 1 (0.2 mmol), 2 (0.4 mmol), CuI
(10 mol %), and Et3N (0.6 mmol) in DCM (1 mL) at room
b
temperature. Isolated yield.
substituents. To our delight, an aliphatic alkyne could also be
employed as the substrate to afford the desired product with
moderate yield (3la, Scheme 4). More importantly, the
absolute configuration of the product 3ac was confirmed by
X-ray crystal diffraction (Figure 1, CCDC 1852379).
a
b
TsNa = sodium p-toluenesulfinate. Isolated yield
excluded the possibility of a radical process. Furthermore,
when compounds D and E were prepared and reacted with
sodium p-toluenesulfinate in the presence of Et3N, 3aa was
generated in moderate yields [Scheme 6, eqs 2 and 3].
However, no product could be obtained when D reacted with
p-toluenesulfinate in the absence of Et3N [Scheme 6, eq 4]. In
order to figure out which might be the intermediate in this
reaction, D was employed to react with Et3N, followed by
addition of sodium p-toluenesulfinate after 2 h. It was found
that 3aa could be obtained in 49% yield [Scheme 6, eq 5].
These results implied that E should be quickly formed from D
in the presence of Et3N. With increasing time, E was consumed
gradually and the product formed; that is, E should be the real
intermediate for this transformation.
Figure 1. X-ray structure of product 3ac.
Inspired by these results and previous reports,13,14,19,20
a
This developed methodology features high efficiency, simple
manipulation, and a broad scope. Encouraged by these
features, we then scaled-up the reaction to examine the
synthetic utility. When 5.0 mmol of phenylacetylene 1a were
reacted with 10.0 mmol of N-tosylhydrazone 2a, the
corresponding product 3aa can be obtained with a satisfactory
yield of 81%, demonstrating great potential in pharmaceutical
synthesis (Scheme 5).
Several control experiments were conducted to unravel the
reaction mechanism. First, when the radical scavenger 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) or tert-butylhydroxy-
toluene (BHT) was added to the reaction under the
plausible mechanism was postulated to account for the current
copper(I)-catalyzed coupling (Scheme 7). Diazo 2a′ is
generated from N-tosylhydrazone 2a in the presence of
Et3N, and reacts with copper acetylide A which is formed
from phenylacetylene 1a, leading to the construction of
coppercarbenoid B. Migratory insertion of alkynyl group to
the carbenic carbon atom gives the intermediate C, and the
formation of D is accompanied by the protonation at the
carbon atom attached the copper center; then, the released
copper can sustain the next catalytic cycle. Finally, E can be
formed rapidly under the promotion of Et3N to then react with
the sulfonyl anion generated in situ from 2a. It is noteworthy
C
DOI: 10.1021/acs.orglett.8b02268
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Letter
Scheme 7. Mechanistic Proposal
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the Natural
Science Foundation of China (21432009, 21672200,
21472177, and 21772185) and for the assistance of the
product characterization from the Chemistry Experiment
Teaching Center of University of Science and Technology of
China. This work was supported by the Strategic Priority
Research Program of the Chinese Academy of Sciences, Grant
XDB20000000.
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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.8b02268.
Experimental procedures, characterization data, copies of
1H NMR, 13C NMR of new compounds, and crystallo-
graphic data for compound 3ac (PDF)
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: zwang3@ustc.edu.cn.
ORCID
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Zhiyong Wang: 0000-0002-3400-2851
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