Iodine-mediated regioselective synthesis of vinyl triflones from styrenes with CF3SO2Na

Article abstract of DOI:10.1039/c6ob01567k

An unexpected regioselective synthesis of vinyl triflones was developed. This iodine-mediated C-H triflylation of styrenes with CF₃SO₂Na occurred at room temperature affording various vinyl triflones, which could be used for the prep

Full text of DOI:10.1039/c6ob01567k

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Iodine‐mediated regioselective synthesis of vinyl triflones from
styrenes with CF SO Na
3
2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a,b
*a
*a,b
Li‐Na Hua, Huan Li, Feng‐Ling Qing, Yangen Huang and Xiu‐Hua Xu
DOI: 10.1039/x0xx00000x
www.rsc.org/
An unexpected regioselective synthesis of vinyl triflones was
developed. This iodine‐mediated C―H triflylation of styrenes with
CF SO Na occurred at room temperature affording various vinyl
previous work
R
R1
SO2CF3
R1
SO2CF3 (a)
R
SO2CF3
3
2
R2
triflones, which could be used for the preparation of other CF SO ‐
3
2
containing compounds.
SO CF
2
3
base
OSO2CF3
(b)
(c)
R1
OH
( )n
R1
The wide application of fluorinated compounds in
pharmaceuticals, agrochemicals, and materials have resulted
( )n
1
this work
in increasing interest in fluorine‐containing groups and
fluorinating methods. Among various fluorine‐containing
2 3
SO CF
2
I2, CuCN (cat.)
NaHCO3, CH3CN, rt
+
CF3SO2Na
Ar
Ar
groups, the trifluoromethanesulfonyl (triflyl, SO CF , Tf) group
2
3
Scheme 1. Methods for the preparation of vinyl triflones
is the strongest electron‐withdrawing group with high
3
lipophilicity. The CF SO ‐containing compounds are frequently
3
2
4
used as structural units in bioactive compounds, catalysts or
ligands, and advanced functional materials. In this context,
substantial efforts have been devoted to the development of
13,14
5
6
Recently, a variety of transition‐metal‐free protocols,
especially iodine‐catalyzed/mediated direct sulfonylation of
14
2f
alkene and alkyne derivatives have been developed for the
efficient synthesis of vinyl sulfones. Inspired by these
achievements, we were dedicated to exploring the direct
triflylation of alkenes under transition‐metal‐free conditions.
Herein, we report our progress in the iodide‐mediated
regioselective synthesis of vinyl triflones from styrenes with
CF SO Na (Scheme 1c). To the best of our knowledge, this
3 2
work represents the first example of direct C―H triflylation of
alkenes.
new synthetic methods of CF SO ‐containing compounds,
3
2
7
especially for aryl trifluoromethanesulfones (triflones). In
contrast, much less attention has been paid in the preparation
of vinyl triflones, despite these compounds are attractive in
organic synthesis and pharmaceutical industry. Normally, vinyl
triflones are prepared by transformation from alkyl triflones
or alkynyl triflones (Scheme 1a). The intramolecular triflyl
8
9
rearrangement, reported by the groups of Okauchi, Maas, and
1
0
ours, provides an alternative approach to vinyl triflones
Scheme 1b). However, the direct incorporation of triflyl into
We embarked on our studies with the repetition of the
iodine‐mediated synthesis of vinyl sulfones by the reaction
(
1
4e
alkenes has never been reported.
of 1‐methyl‐4‐vinylbenzene 1a and PhSO Na (Scheme 2a).
2
Pd‐ or Cu‐catalyzed cross‐coupling reactions of vinyl halides,
vinyl boronic acids, or alkynes with sulfinate salts provides a
1
1
routine and direct approach to vinyl sulfones. However, the
use of CF SO Na as a nucleophilic coupling partner remains to
3
2
be a challenge, because the strong electron‐withdrawing
3
12
character of the triflyl group reduces its nucleophilicity.
a.
College of Chemistry, Chemical Engineering and Biotechnology, Donghua
University, 2999 North Renmin Lu, Shanghai 201620, China.
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Science, 345 Lingling Lu, Shanghai 200032, China.
E‐Mail: hyg@dhu.edu.cn or xuxiuhua@sioc.ac.cn
b.
†
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/
2
Scheme 2. I ‐mediated synthesis of vinyl sulfones
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After obtaining vinyl sulfone 2a in 86% yield, we then decided Scheme 3. Iodine‐mediated regioselective synthesis of vinyl
View Article Online
DOI: 10.1039/C6OB01567K
a
to switch PhSO Na to CF SO Na for the analogous synthesis of triflones
2
3
2
vinyl triflones (Scheme 2b). In contrast to the high yield of vinyl
sulfone 2a, the conversion of 1a to vinyl triflone was low.
Noteworthily, a novel unknown vinyl triflone 4a, not the
15
expected product 3a
, was obtained.
The above interesting result encouraged us to optimize the
reaction conditions for improving the yield of vinyl triflone 4a
(
Table 1). First, a variety of iodine sources were screened. It
was found that molecular iodine was the most effective (entry
). NIS or PhI(OAc) /KI gave lower yields, while ICl could not
1
2
promote this reaction (entries 2‐4). Considering the fact that
HI would be generated from the reaction system, the addition
of a base should benefit this reaction. Indeed, higher yield of
4a was observed in the presence of an organic or inorganic
base (entries 5‐8). The reaction performed with two
equivalents of NaHCO3 gave the best result (entry 8).
Subsequently, the effect of catalytic metal salt was evaluated
(
entries 9‐12). Among the common metal salts, CuI could
result in slightly higher yield (entry 9). Other metal salts, such
as FeCl , were ineffective. Further screening of copper salts
2
revealed CuCN was optimal, giving 4a in 57% yield (entry 12).
Finally, different solvents including DCE, THF, DMF, and DMSO
were investigated (entries 13‐16). However, none of them
gave better result than that of CH CN. It was noteworthy that
3
lowering or elevating reaction temperature also resulted in
lower yields (entries 17 and 18).
a
Table 1. Optimization of reaction conditions
a
Reaction conditions: 1 (0.5 mmol), CF
mmol), NaHCO (1.0 mmol), CuCN (0.05 mmol), CH
under air, rt, 12 h, isolated yields.
3
SO
2
Na (1.5 mmol), I
2
(1.5
b
entry iodine source Base
metal solvent
yield (%)
3
3
CN (2.0 mL),
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
I
2
─
─
─
─
DBU
NaOAc
K2CO3
NaHCO3
─
─
─
─
─
─
─
─
CH
CH
3
CN
CN
18
14
11
0
NIS
3
PhI(OAc) /KI
ICl
I2
CH CN
2
3
CH CN
With the optimized reaction conditions in hand, the
substrate scope of this protocol was then examined. As shown
3
CH CN
20
22
37
40
45
33
38
57
trace
7
16
28
41
28
3
I2
CH CN
3
in Scheme 3, a range of styrenes (1a
‐1o) participated in the
I2
I2
I2
I2
I2
I2
I2
I2
I2
I2
I2
CH CN
3
reaction giving vinyl triflones 4a 4o in moderate yields along
‐
CH CN
3
with the recovery of the starting materials in low yields. In
general, the electron‐donating substituent such as alkyl,
alkoxyl, or aryl group was essential for this transformation. The
aromatic fluoride (1m), chloride (1n), and bromide (1o) were
tolerated under the standard reaction conditions, thus
providing opportunities for additional transformations. In the
NaHCO3 CuI
CH CN
3
0
1
2
3
4
5
6
7
8
NaHCO3 FeCl2 CH CN
3
3
NaHCO3 CuTc CH CN
NaHCO3 CuCN CH CN
3
NaHCO3 CuCN DCE
NaHCO3 CuCN THF
NaHCO3 CuCN DMF
NaHCO3 CuCN DMSO
cases of
styrenes bearing strong electron‐withdrawing
c
NaHCO3 CuCN CH CN
3
substituents including ester, nitro, and cyano, low conversion
was obtained. Unfortunately, neither the sterically hindered
ortho‐substituted styrenes nor the unactivated alkenes were
suitable substrates. It was noteworthy that benzofuran‐ or
d
I2
NaHCO3
3
CuCN CH CN
a
Reaction conditions: 1a (0.2 mmol), CF
iodine source (0.6 mmol), base (0.4 mmol), metal salt (0.02
mmol), solvent (1.0 mL), under air, rt, 12 h. Yields determined
by F NMR spectroscopy using trifluoromethoxybenzene as an
3
SO
2
Na (0.6 mmol),
b
thiophene‐derived styrenes (1p, 1q) were applicable to the
1
9
reaction, affording the vinyl triflones 4p and 4q in moderate
yields. The structure of product 4h was further confirmed by
HMQC and DEPT135 NMR spectra (see ESI).
c
o
d
o
internal standard. 50 C. 0 C.
2
| J. Name., 2012, 00, 1‐3
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Scheme 4. Transformation of compound 4g
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DOI: 10.1039/C6OB01567K
Scheme 6. Proposed reaction mechanism
In conclusion, we have developed an iodine‐mediated direct
C―H triflylation of styrenes with CF SO Na. This reaction
3
2
offers a new route for the regioselective synthesis of vinyl
triflones, which can undergo Michael addition by different
nucleophiles to afford
a
series of CF SO ‐containing
3 2
compounds. Further investigations of the mechanism and the
application of this method are ongoing in our laboratory.
We are grateful for the financial support from National
Natural Science Foundation of China (21502215, 21421002,
2
1332010, 21272036), National Basic Research Program of
Scheme 5. Gram‐scale preparation of 10 from 1g
China (2012CB21600), Strategic Priority Research Program of
the Chinese Academy of Sciences (XDB20020000), Youth
Innovation Promotion Association CAS (No. 2016234), and
Shanghai Pujiang Program (No. 15PJ1410300).
Notes and references
The hitherto unknown vinyl triflones
4
are compounds with
1
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3
2
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14a,b,e,g‐i
yielding vinyl triflones
4.
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3
3
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