Regioselective and stereoselective sulfonylation of alkynylcarbonyl compounds in water

Article abstract of DOI:10.1039/c6gc01196a

With water as the reaction medium, an efficient protocol has been developed for the synthesis of Z-β-sulfonyl-α,β-unsaturated carbonyl products via water-promoted sulfonylation of alkynylcarbonyl compounds with sodium sulfinates. This strategy could effectively avoid the use of toxic organic solvents, catalysts and additives. This method features a wide substrate scope with a broad range of functional group tolerance, utilizes easily available starting materials, can be scaled-up, and is operationally simple.

Full text of DOI:10.1039/c6gc01196a

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Regioselective and stereoselective sulfonylation of
alkynylcarbonyl compounds in water†
Cite this: DOI: 10.1039/c6gc01196a
Wenyi Li, Guoxing Yin, Lei Huang, Yan Xiao, Zhimin Fu, Xiu Xin, Fang Liu, Zhizhang Li
and Weimin He*
With water as the reaction medium, an efficient protocol has been developed for the synthesis of Z-β-sul-
fonyl-α,β-unsaturated carbonyl products via water-promoted sulfonylation of alkynylcarbonyl compounds
with sodium sulfinates. This strategy could effectively avoid the use of toxic organic solvents, catalysts and
additives. This method features a wide substrate scope with a broad range of functional group tolerance,
utilizes easily available starting materials, can be scaled-up, and is operationally simple.
Received 28th April 2016,
Accepted 11th July 2016
DOI: 10.1039/c6gc01196a
www.rsc.org/greenchem
Introduction
β-Sulfonyl-α,β-unsaturated carbonyl compounds are valuable
1
structural units in biologically active molecules and serve as
2
versatile synthetic precursors for a diverse range of chemical
transformations that produce pharmaceutical and industrially
important organic compounds. New strategies for the facile
synthesis of stereo-challenging β-sulfonyl-α,β-unsaturated car-
bonyl compounds remain a considerable focus of research.
Selective access to either the E- or Z-isomer of a β-sulfonyl-
α,β-unsaturated carbonyl compound is often the decisive
factor in the success of any β-sulfonyl-α,β-unsaturated carbonyl
compound synthesis method. Among the routes for the syn-
thesis of β-sulfonyl-α,β-unsaturated carbonyl compounds, selec-
Scheme 1 Synthesis
compounds.
of
Z-β-sulfonyl-α,β-unsaturated
carbonyl
2c,3
tive preparation of E-isomers has been well established,
while the corresponding methods to access thermodynamically
less favorable Z-isomers remain a challenge. One method
involves the synthesisof Z-β-sulfonyl enoates through B(OH)
promoted sulfonylation of propargylic esters in the presence of have limited the application of this method.
irritant Bu NHSO as a phase transfer catalyst in aqueous-
Recently, significant progress has been made in the devel-
3
-
However, low stereoselectivity and the use of hazardous LiClO4
4
4
4
7
tetrahydrofuran. The other route involves the use of N-tosyli- opment of catalytic reactions in the aqueous phase. Water
mines to react with propargylic esters by using an expensive promotes or accelerates successfully these organic transform-
NHC catalyst, but only one example (ethyl 3-tosylacrylate) was ations and also serves as excellent supporting medium with
presented, and alkyl aldimines were not included in this numerous advantages including the ease of product isolation,
5
method (Scheme 1a). In the third route, Downey et al. treated nontoxicity, non-flammability, high heat capacity and redox
8
aryl and aliphatic thiols with propargylic esters to form stability. Several factors, such as hydrophobicity, hydrogen-
Z-β-thioenoates, which were then treated with m-CPBA and bonding, polarity and acidity contribute to the rate-enhancing
6
LiClO to form Z-β-sulfonyl enoate compounds (Scheme 1b).
effect of water. Continuing with our interest in alkyne chem-
istry, we focused our attention to the development of a new
4
9
method for the synthesis of Z-β-sulfonyl-α,β-unsaturated carbo-
nyl compounds, and we now report the results of our investi-
gation, water-mediated sulfonylation of alkynylcarbonyl
compounds with sodium sulfinates (Scheme 1c). This
approach offers a valuable alternative compared to the above-
mentioned methods: (i) sodium sulfinates are versatile and
Key Laboratory of Comprehensive Utilization of Advantage Plants Resources in
Hunan South, College of Chemistry and Bioengineering, Hunan University of Science
and Engineering, Yongzhou 425100, China. E-mail: weiminhe2016@yeah.net
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6gc01196a
1
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readily available intermediates in organic synthesis; (ii) no yield, rate or selectivity (entry 10). Conducting the reaction in a
catalysts or hazardous reagents are used. A mechanistic study Schlenk tube did not affect the reaction yield (entry 11). We
suggests that the sulfonylation reaction proceeds via a water- were pleased to observe that all of the conditions that we
promoted anti-selective addition of alkynylcarbonyl com- tested resulted in the formation of Z-tosylacrylate in high
pounds, which leads to high Z-selectivity.
stereoselectivity, establishing the feasibility of our proposed
sulfonylation process. Mild reaction conditions, shorter reac-
tion times, cost-effectiveness, operational simplicity, excellent
yields and high stereoselectivity make this transformation an
alternative method for the straightforward preparation of
numerous Z-β-sulfonyl enoates. Within the set of previous
methods highlighted herein, a solution was offered to this
problem in the form of a novel, mild and green alternative for
this key transformation.
With the optimal system in hand, we examined the scope of
sodium sulfinate substrates with ethyl propiolate 2a as illus-
trated in Table 2. To our delight, the current reaction system
was suitable for a wide range of aryl sodium sulfinates
Results and discussion
To test the possibility of our envisioned method, commercially
available 4-methylbenzenesulfinate 1a was selected as a SO2
−
source due to the ease of converting sodium 4-methyl-
benzenesulfinate into sulfonyl anions in water. When 1a was
treated with 1 equiv. of ethyl propiolate 2a in water at room
temperature under air, a 14% yield of 3-tosylacrylate 3aa based
on 16% conversion of the starting material 1a was obtained
after 24 h. We found that the formation of 3aa was more favor-
able at elevated temperatures (Table 1, entries 2 and 3).
However, increasing the temperature to 80 °C has a negative
impact on the reaction, resulting in fast hydrolysis of ethyl pro-
piolate 2a and 3-tosylacrylate 3aa (entry 4). Significantly,
employment of an excess amount of 2a was found to provide
the desired sulfonylated product 3aa in good yield within a
very short reaction time (entries 5–7). A nearly quantitative sul-
fonylation yield (98%) was obtained when the amount of ethyl
propiolate reached 3.5 equiv. (entry 7). Concentration screen-
ing (entries 8 and 9) revealed that when the reaction was
carried out at a concentration of 0.2 M, the reaction time could
be shortened to 20 min and the yield increased to 100%
(
Table 2, 3aa–3pa). No matter whether the benzene ring of aryl
sodium sulfinate was substituted with either a sterically hin-
dered, electron-donating or electron-withdrawing group, all of
them delivered the desired products in good to excellent yields
and with an excellent range of Z/E ratios from 12 : 1 to 28 : 1
1
(
based on the analysis of H NMR spectra). Importantly, a
variety of functional groups, such as methyl, alkoxyl (OMe and
OCF ), amide, halide (F, Cl, Br and I), trifluoromethyl, cyano
3
and nitro groups were all well tolerated. The reactions of di-
and tri-substituted sodium benzenesulfinates gave the desired
products in 89% (3oa, Z/E 10 : 1) and 95% (3pa, Z/E 12 : 1)
yields, respectively. Polycyclic and heteroaromatic substituted
sodium sulfinates could also be transformed into the corres-
ponding products in good to excellent yields as well as high
Z/E ratios (3qa and 3ra). When aliphatic sodium sulfinate sub-
strates were employed for the transformation, the corres-
ponding products were obtained in 81–83% yields (3sa–3ua).
More importantly, the synthesis of 3aa was conducted on a
(Table 1, entry 8). The reaction of 1a with 2a in the presence of
catalytic amounts of acetic acid had no significant effect on
a
Table 1 Optimization of reaction conditions
∼2.41 g scale at a higher concentration (95% yield on a
b,c
b
a
Entry
Equiv. of 2a
Temp.
Time
Yield
E/Z
Table 2 Reaction scope of sodium sulfinate
1
2
3
4
5
6
7
1
1
1
1
2
3
3.5
3.5
3.5
3.5
3.5
rt
24 h
12 h
1 h
1 h
35 min
25 min
20 min
20 min
20 min
20 min
20 min
14
18
30
27
72
95
98
100 (96)
97
99
7 : 1
7 : 1
8 : 1
8 : 1
11 : 1
13 : 1
13 : 1
13 : 1
13 : 1
12 : 1
13 : 1
40
60
80
60
60
60
60
60
60
60
d
8
9
e
d, f
1
1
0
1
d,g
100
a
Unless otherwise specified, the reactions were carried out in a sealed
b
tube in the presence of 1a (0.1 mmol), 2a and water (1 ml). Estimated
by H NMR using diethyl phthalate as an internal reference. The
1
c
d
number in parentheses shows isolated yield. 0.5 ml water was used.
e
f
2
ml water was used. 0.01 mmol acetic acid was used as the catalyst
g
and 0.5 ml water was used as the solvent. The reaction was carried
out in a Schlenk tube in the presence of 1a (0.1 mmol), 2a (0.35 mmol)
and water (0.5 ml). rt = room temperature.
a
All reactions were carried out in a sealed tube in the presence of 1
(0.2 mmol), 2 (0.7 mmol) and water (1 ml); isolated yields are reported.
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0 mmol scale, 0.4 M) without any problem, which would with electron-withdrawing groups, including amide (3aq and
potential economic production on a commercial scale. 3ar), acetyl (3as) and azine (3at) groups, which gave the corres-
We next turned our attention to the scope of propargylic ponding sulfonylation products in moderate to good yields.
carboxylate substrates (Table 3). We were pleased to find that β-Sulfonyl enoates are not only useful building blocks in
the readily available propargylic carboxylate did smoothly react organic synthesis but also important structure motifs com-
with sodium 4-methylbenzenesulfinate (1a) to produce a monly found in drugs. Moreover, late-stage modification is a
greatly widened range of tosylacrylates, which clearly exhibited valuable strategy for medicinal chemistry research. Therefore,
the power of this sulfonylation strategy. Propargylic carboxy- several complex bioactive molecule derivatives were subjected
lates with various chain lengths and isomeric structures did to the standard reaction conditions (Table 4). Natural alcohol
not significantly affect the product yields and Z-selectivity derivatives containing tosylpropiolates, such as indanol (4aa),
(
3ab–3aj). High functional group compatibility was exhibited, perillyl (4ab), benzoin (4ac) and farnesol derivatives (4ad)
tolerating alkyl (3ab–3ad, 3af), Ph (3ae), cyclopentyl (3ag), worked well in the current transformation, generating the
cyclopropylmethyl (3ah), free OH (3ai), and Br (3aj) moieties. corresponding tosylacrylates in 56–82% yields and with Z/E
The internal propargylic carboxylates, which were usually very ratios ranging from 5 : 1 to 13 : 1. These results demonstrate
less reactive than the terminal propargylic carboxylates, were that the present protocol could be applied in late stage bio-
also tested. Sterically demanding phenyl-substituted ethyl pro- active compound modification.
piolate (2n) provided the corresponding product in lower yield
than the alkyl-substituted substrates (3l and 3m), indicating onylation process, the following competitive experiments were
that steric congestion around the alkynyl decreases the reac- performed. First, the sulfonylation of 2a in D O (0.5 mL)
To gain insight into the possible mechanism of this sulf-
2
tion efficiency. The electron-withdrawing substituted ethyl pro- afforded deuterated product D-3aa with a small quantity of
piolate showed inferior reaction efficiency to that of the D-4a (Scheme 2a), which indicated that the α-hydrogen atom
electron-donating ones (3ao and 3ap vs. 3al). Ethyl 3-(tri- of ethyl 3-tosylpropiolate was originated from water. In addition,
methylsilyl)propiolate (2p) could well react with 1a to give a small quantity of D-4a was detected by MS. The preformed 2l
2
(Z)-ethyl 3-tosylacrylate (3aa) in 87% yield. Apparently, the 2p was subjected to the reaction with 1a in D O, and the expected
proceeds via an intermolecular nucleophilic substitution reac- D-4l was also detected by MS (Scheme 2b). These results were
tion with concomitant release of the TMS group. Furthermore, consistent with those obtained for the sulfonylation reactions.
the method could also be applied to other acetylenes substituted An excess of ethyl propiolate was critical to obtain a high yield
of 3aa (see Table 1, entries 4–6), which suggested that pro-
pargylic acid was essential for the success of this transform-
a
Table 3 Reaction scope of propargylic carboxylate
ation. Subsequently, radical trapping experiments were
performed (Scheme 2c). The addition of radical scavengers such
as TEMPO and BHT (2.0 equiv.) did not retard the sulfonylation
of 2a under standard conditions, which indicated that the reac-
tion might not proceed through a radical pathway.
On the basis of the results described above and previous
reports, a plausible mechanism is proposed (Scheme 3). First,
a
Table 4 Reaction scope of late-stage modification
a
a
All reactions were carried out in a sealed tube in the presence of 1
All reactions were carried out in a sealed tube in the presence of 1
(0.2 mmol), 2 (0.7 mmol) and water (1 ml); isolated yields are reported. (0.2 mmol), 2 (0.7 mmol) and water (1 ml); isolated yields are reported.
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room temperature. H NMR spectra were recorded at 400 MHz
1
3
and C NMR spectra were recorded at 100 MHz by using a
Bruker Avance 400 spectrometer. Chemical shifts were
reported in parts per million (δ) relative to tetramethylsilane
(
TMS). The following abbreviations were used to describe peak
splitting patterns when appropriate: s = singlet, d = doublet,
t = triplet, q = quartet, m = multiplet and brs = broad singlet.
IR spectra were recorded on a Nicolet iS10 FT-IR spectrometer
and only major peaks were reported in cm− . Mass spectra
were recorded on a spectrometer operating on ESI-TOF.
1
Scheme 2 Control experiments.
General procedure for the synthesis of Z-β-sulfonyl enoates
In a sealed tube were placed sodium sulfinate (0.2 mmol),
alkynylcarbonyl compound (0.7 mmol) and H O (1 mL), and
2
then the contents were stirred at 60 °C. The progress of the
reaction was monitored by TLC. The reaction typically took
20 min. Upon completion, the reaction mixture was cooled
down to room temperature, mixed with silica gel and concen-
trated under vacuum. The residue was purified by column
chromatography on silica gel (eluent: hexanes/ethyl acetate) to
afford 3–4.
Scheme 3 Proposed mechanism.
1
1
Acknowledgements
an ethenium intermediate I is formed through electrophilic
addition of a proton to an activated alkyne moiety, which is
generated from the hydrolysis of ethyl propiolate under
heating conditions. Then, the sulfonyl anion anti-attacked the
less steric hindered carbon of intermediate I to produce
Z-β-sulfonyl enoates 3.
We are grateful for financial support from the National Natural
Science Foundation of China (no. 21302048), the Hunan Pro-
vincial Natural Science Foundation of China (no. 14JJ7028 and
2016JJ4035), the Scientific Research Hunan Provincial Edu-
cation Department (no. 15B095), the Science and Technology
Innovative Research Team in Higher Educational Institutions
of Hunan Province (2012-318) and the Construct Program of
the Key Discipline in Hunan Province.
Conclusions
In conclusion, we report a direct C–S coupling reaction which
was promoted by water for the efficient synthesis of various
Z-β-sulfonyl-α,β-unsaturated carbonyl compounds under Notes and references
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the sulfonylation reaction; (3) complete regioselectivity and
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