Copper-mediated oxysulfonylation of alkenyl oximes with sodium sulfinates: a facile synthesis of isoxazolines featuring a sulfone substituent

Article abstract of DOI:10.1039/c7cc00090a

A novel and efficient Cu(OAc)₂-mediated oxysulfonylation of alkenyl oximes with sodium sulfonates was developed. The reactions are easy to conduct, occur under mild conditions, and form a broad range of sulfone-substituted isoxazolines in good yields.

Full text of DOI:10.1039/c7cc00090a

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DOI: 10.1039/C7CC00090A
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Copper-Mediated Oxysulfonylation of Alkenyl Oximes with
Sodium Sulfinates: A Facile Synthesis of Isoxazolines Featuring a
Sulfone Substituent
Received 00th January 20xx,
Accepted 00th January 20xx
Li-Jing Wang*, Manman Chen, Lin Qi, Zhidong Xu and Wei Li*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel and efficient Cu(OAc)2-mediated oxysulfonylation of oxysulfenylation of enolates with sodium sulfinates to
alkenyl oximes with sodium sulfonates was developed. The construct sulfenylated cyclic ethers, which could be convered
reactions are easy to conduct, occur under mild conditions, and to the corresponding sulfone derivatives by the oxidation of m-
form a broad range of sulfone-substituted isoxazolines in good CPBA.8 Quite recently, Wang and co-workers developed an
yields.
iodine‐promoted cyclization of N‐propynyl amides via
sulfonylation with sulfonyl hydrazides to construct
sulfonyloxazoles. But when using N‐allyl amides, they can only
get thiooxazolines.9 These established methods, however,
Aryl alkyl sulfones are an important class of compounds that
find widespread applications as pharmaceuticals, bioactive
products, and materials.1 They are also versatile synthetic
intermediates in organic chemistry, including in fragment
coupling, in Ramberg–Bäcklund reaction, and in Julia
olefination.2 Therefore, the synthesis of organosulfone
compounds has received intensive attention and encouraged
the development of new synthetic strategies.3 Traditionally,
aryl alkyl sulfones are synthesized via the nucleophilic
substitution of carbon electrophiles with sodium sulfinates or
thiophenols followed by oxidation (Scheme 1a).4 These
methods suffers from the limitations of requiring multistep
sequences to preinstall a leaving group and that they are
incompatible with numerous functional groups. In 2015, a
more economical and efficient method for the synthesis of aryl
alkyl sulfones was developed by Shi and co-workers, which was
have disadvantages in that they need
a
two-step
thiolation/oxidation sequence to afford aryl alkyl sulfones.
Thus, methods for direct synthesis of sulfone-substituted
heterocycles from alkenes and sulfone reagents are indeed
desirable.
via a Pd(II)-catalyzed C(sp3)-H sulfonylation pathway (Scheme
5
1b).
In this reaction, the 8-aminoquinoline auxiliary,
stoichiometric silver salts and high reaction temperature were
essentially required.
Difunctionalization of unactivated alkenes has emerged as a Scheme 1. Synthesis of aryl alkyl sulfones
powerful tool for constructing diversity of chemical
structures.6 Compared with previously reported reactions for
Isoxazolines are an important class of heterocycles found in
constructing aryl alkyl sulfones between alkenes and sulfone
reagents,7 the synthesis of sulfone-substituted heterocycles
through a pattern of cyclization has not been well explored. In
2016, Jiang and co-workers reported a copper-catalyzed
several biologically active agents and versatile precursors in
synthetic chemistry.10 The modification of isoxazoline by
introduction of a sulfone group is of great interest both for
biology and organic synthesis research. In recent years,
difunctionalization of β,γ-unsaturated oximes with a functional
group,
such
as
dioxydation,11
oxy-
oxy-
Key Laboratory of Medicinal Chemistry, and Molecular Diagnosis of the Ministry of
Education, College of Chemistry & Environmental Science, HeBei University, 180
Wusi Donglu, Baoding 071002, P. R. China.
trifluoromethylation/trifluoromethylthiolation,12
fluorination,13 oxy-azidation,14 oxy-halogenation,15 and oxy-
amination16 has become a more straightforward way to
construct functionalized isoxazolines. Inspired by these results,
we decided to test whether β,γ-unsaturated oximes with
E-mail: wanglj0405@163.com; liweihebeilab@163.com
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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sulfone reagents could carry out a direct oxy-sulfonylation and Interestingly, increasing the amount of Cu(OAc)2 resulted in
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DOI: 10.1039/C7CC00090A
afford the corresponding sulfone-substituted isoxazolines. much higher yields of 50% and 64%, respectively (entries 11
Herein, we described a tandem preparation of aryl alkyl and 12). However, the reaction under an air atmosphere gave
sulfones through copper-mediated oxygen sulphone a lower yield (entry 13). The effect of bases was also examined
functionalization of unactivated alkenes (Scheme 1c).
(entries 14–16). The yield of 3a was increased to 68% when KF
was used (entry 16). The loading of the KF was evaluated as
well (entries 17-19). The yield was further increased to 76%
when using 2.0 equiv of KF (entry 18). In addition, no reaction
occurred without KF (entry 20). The screening of the loading of
sodium p-toluenesulfinate 2a indicated that 2.0 equiv of 2a is
the best choice for the reaction (entries 21 and 22). The
subsequent control experiments revealed that the Cu(OAc)2
was crucial to this transformation (entry 23).
Table 1. Optimization of reaction conditionsa
entry
[Cu] (equiv)
base (equiv)
oxidant
yield
(%)b
35
24
24
26
0
0
0
0
31
45
(2.0 equiv)
1
2
3
4
5
6
7
8
9
Cu(OAc)2 (0.2)
CuI (0.2)
Cu(CF3SO3)2 (0.2)
CuBr2 (0.2)
Cu2O (0.2)
CuCl (0.2)
Cu(OAc)2 (0.2)
Cu(OAc)2 (0.2)
Cu(OAc)2 (0.2)
Cu(OAc)2 (0.2)
NaOAc (1.2)
NaOAc (1.2)
NaOAc (1.2)
NaOAc (1.2)
NaOAc (1.2)
NaOAc (1.2)
NaOAc (1.2)
NaOAc (1.2)
NaOAc (1.2)
NaOAc (1.2)
-
-
-
-
-
-
K2S2O8
PhI(OAc)2
DTBP
Mn(OAc)3·
10
2H2O
11
12
13c
14
15
16
17
18
19
20
21d
22e
23
Cu(OAc)2 (1.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
-
NaOAc (1.2)
NaOAc (1.2)
NaOAc (1.2)
t-BuOK (1.2)
NEt3 (1.2)
KF (1.2)
KF (0.2)
KF (2.0)
KF (2.5)
-
-
-
-
-
-
-
-
-
-
-
-
-
50
64
55
52
57
68
67
76
66
0
aAll reactions were carried out by using 1 (0.2 mmol), 2a (2.0 equiv), Cu(OAc)2
(2.0 equiv), KF (2.0 equiv) and DMSO (2 mL) under argon and stirred at room
temperature for 36 h, except as noted. Yields refer to isolated yields.
-
Scheme 2. Substrate scope of β,γ-unsaturated oximesa
KF (2.0)
KF (2.0)
KF (2.0)
80
80
0
With the optimized conditions in hand (Table 1, entry 20), we
then examined the scope of this transformation. As shown in
Scheme 2, various β,γ-unsaturated oximes were subjected to
the standard conditions. First, a series of aromatic oximes with
a para-substituent on the phenyl ring including some with
electrondonating groups (Me, isopropyl) and some with
electronwithdrawing groups (Cl, Br) underwent the oxy-
sulfonylation smoothly and provided the desired products with
good yields (3b and 3d-f). However, compounds 1c and 1g
having a methoxyl or a nitro group only gave the desired
product in yields of 25% and 46%, respectively. Next,
substrates with a 2-substituted phenyl were tested. Conversely,
1i with a methoxy group gave the corresponding products in
good yield, while 1h with a methyl group and 1j with a
chlorine group both gave a lower yield. We have also
investigated the effect of meta-substituted phenyl ring. To our
delight, 1k-n gave the corresponding products in good to
excellent yields. The oxime bearing a naphthyl group was a
compatible substrate and gave the product in good yield (3o).
Thiophene-containing oxime (1p) was also transformed into
the desired product in 68% yield. We were pleased to find that
the aliphatic oximes also provided good conversion to the
desired product 3q under the stated conditions. Then the
aAll reactions were carried out by using 1a (0.2 mmol), 2a (1.5 equiv),
copper salts (2.0 equiv), bases (2.0 equiv), oxidants (2.0 equiv) and DMSO
(2 mL) under argon and stirred at room temperature for 36 h, except as
noted. Isolated yield. under air. d2.0 equiv of 2a was used. e 2.5 equiv of
b
c
2a was used.
At the outset of our studies, we chose oxime 1a as the
substrate, 1.5 equiv of sodium p-toluenesulfinate 2a as sulfone
reagent, 20 mol% Cu(OAc)2 as catalyst, 1.2 equiv of NaOAc as
the base and DMSO as solvent. The reaction was stirred at
room temperature for 36 h under argon atmosphere and gave
the desired product 3a in 35% yield (Table 1, entry 1), the
structure of which was unambiguously confirmed by X-ray
analysis.17 Subsequently, We tested other solvents (see the
Supporting Information), such as DMF, CH3CN, and toluene. It
was found to be less effective for this reaction. Other copper
salts were also examined in the reaction: CuI, Cu(CF3SO3)2, and
CuBr2 led to lower yields; Cu2O, and CuCl were completely
ineffective for this reaction (entries 2–6). We then turned to
oxidizing agents for a trial: PhI(OAc)2 and K2S2O8 failed to
generate the desired products (entries 7 and 8); Di-tert-butyl
peroxide(DTBP) gave
Mn(OAc)3·2H2O produced a higher yield (45%, entry 10);
a
lower yield (31%, entry 9);
2 | J. Name., 2012, 00, 1-3
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present method was successfully applied to construct the 84% yield. But when electron-deficient substrate V2iegwwAraticsleuOsnelidne,
DOI: 10.1039/C7CC00090A
desired product (3r) containing a quaternary carbon center. the oxy-sulfonylation was not found to proceed under
Furthermore, to expand the scope of this reaction, we also standard conditions.
investigated α-phenyl oxime 1s. However, it couldn’t work To have an insight into the mechanism of the chemical
under standard conditions.
reaction, several control experiments were conducted. When
the reaction was performed in the presence of a radical
scavenger such as TEMPO, the yield of 3a was decreased
significantly to 17% and 1a was recovered in 26% yield. In
addition,
4 was detected by ESI-HRMS measurement of the
crude reaction mixture. Increasing the amount of TEMPO to
4.0 equiv, no product was detected and 1a was recovered in
99% yield (eq 1). The results implied that a radical pathway
might be involved in the reaction. However, when we tried to
use 1,1-diphenylethylene to trap the radical in the presence or
absence of oximes, the yields of the radical coupling products
were very low (eq 2 and eq 3). Therefore, we could not
overlook a cation pathway in this transformation. On the basis
of the above results and literature reports, we propose the
following reaction mechanism involving two possible pathways
for this transformation (Scheme 5).12, 14, 18
aAll reactions were carried out by using 1a (0.2 mmol),
2 (2.0 equiv),
Cu(OAc)2 (2.0 equiv), KF (2.0 equiv) and DMSO (2 mL) under argon and
stirred at room temperature for 36 h, except as noted. Yields refer to
isolated yields.
Scheme 3. Substrate scope of sodium sulfinatesa
To further demonstrate the synthetic potential of this method,
various sodium sulfinates were employed in this oxy-
sulfonylation reaction (Scheme 3). As expected, sodium
benzenesulfinate was worked well under standard conditions
(3ab). Substrates with electron-donating substituents gave the
Scheme 5. Proposed mechanism.
Initially, intermediate
A is generated by the reaction of oxime
1a and KF. And according to the literature19, intermediate
A
can be converted to the corresponding oxime radical
B.
Meanwhile, the sodium sulfonate is oxidized by CuII to give
sulfonyl radical
cation . In path a, the alkene
electrophilic sulfonyl cation to form intermediate
followed by 5-exo-trig cyclization to give the desired
isoxazoline . In path b, sulfonyl radical coupled with oxime
radical to give the desired product
D
, and a second oxidation can deliver sulfonyl
is activated by the
, which is
E
A
E
F
3
D
B
3.
Scheme 4. Mechanistic studies
corresponding products
(3ae, and 3ag) in good yields.
Conclusions
However, substrates with electron-withdrawing groups only
gave the desired products (3ac, 3ad, and 3ah) in medium
yields. Compound 2f with a chlorine group at the o- position
even failed to obtain the corresponding product. Sodium
methanesulphinate (2i) could react easily with 1a to give 3ai in
In conclusion, we have developed an efficient and practical
copper-mediated oxysulfonylation reaction of alkenes with
sodium sulfinates for the synthesis of sulfonylated isoxazolines.
This reaction proceeds readily at room temperature and is
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We thank the Hebei Province Science Foundation for
Key Program (No. B2016201031), Hebei Province Higher
School Science Foundation for High-level personnel (No.
GCC2014014), and Hebei university Science Foundation for
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