Metal- and Oxidant-Free Alkenyl C?H/Aromatic C?H Cross-Coupling Using Electrochemically Generated Iodosulfonium Ions

Article abstract of DOI:10.1002/anie.201807592

A three-step transformation consisting of 1) addition of electrochemically generated iodosulfonium ions to vinylarenes to give (1-aryl-2-iodoethoxy)sulfonium ions, 2) nucleophilic substitution by subsequently added aromatic compounds to give 1,1-diaryl-2-

Full text of DOI:10.1002/anie.201807592

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201807592
German Edition: DOI: 10.1002/ange.201807592
À
C H Functionalization
À
À
Metal- and Oxidant-Free Alkenyl C H/Aromatic C H Cross-Coupling
Using Electrochemically Generated Iodosulfonium Ions
Ryutaro Hayashi, Akihiro Shimizu, Jonathan A. Davies, Yu Ishizaki, Chris Willis,
and Jun-ichi Yoshida*
Abstract: A three-step transformation consisting of 1) addition
of electrochemically generated iodosulfonium ions to vinyl-
arenes to give (1-aryl-2-iodoethoxy)sulfonium ions, 2) nucle-
ophilic substitution by subsequently added aromatic com-
pounds to give 1,1-diaryl-2-iodoethane, and 3) elimination of
HI with a base to give 1,1-diarylethenes was developed. The
transformation serves as a powerful metal- and chemical-
À
À
oxidant-free method for alkenyl C H/aromatic C H cross-
coupling.
À
À
C
H/C H cross-coupling serves as a powerful method for
À
making C C bonds and they are synthetically useful from
viewpoints of atom-[1] and step-economy.[2] In particular,
À
À
alkenyl C H/aromatic C H cross-coupling is useful for the
introduction of alkenyl groups to aromatic compounds. The
Fujiwara–Moritani reaction is an early example of a metal-
Scheme 1. Synthesis of diarylethylene.
catalyzed alkenyl C H/aromatic C H cross-coupling.[3] Later,
À
À
dehydrogenative Heck reactions were developed.[4] Recently,
À
À
À
À
intramolecular alkenyl C H/aromatic C H cross-coupling
synthesizing phenanthrene[5] is also reported using oxidized
iodine/iodide.[6] In these approaches, 1,2-diarylethylenes are
often obtained as major products from vinylarenes (Sche-
me 1a).
versatile method of alkenyl C H/aromatic C H cross-cou-
pling to make 1,1-diarylethylenes still remains a challenge.
Electrochemical oxidation serves as a powerful method
for generating reactive cationic species under mild conditions
without metal catalysts.[11] Various electrochemical C H bond
À
À
À
Because many biologically active compounds contain the
1,1-diarylethylene structure,[7] an efficient method for the
synthesis of 1,1-diarylethylenes has been highly desired. 1,1-
Diarylethylenes having an aryl or ester group at the 2-position
were synthesized by metal-catalyzed alkenyl C H/aromatic
C H cross-coupling of vinylarenes with aromatic compounds
transformations including C H/C H cross-coupling have
been developed using the reactive cationic species.[12]
Recently, we reported the stabilized cation pool method, in
which organic cations are electrochemically generated and
accumulated in solutions in the presence of stabilizing
agents.[13] This methodology has enabled us to develop
À
À
(Scheme 1b).[8] However, the method for the synthesis of 2-
unsubstituted 1,1-diarylethylenes is limited to the decarbox-
ylative cross-coupling of cinnamic acid derivatives and arenes
(Scheme 1c)[9] and cross-coupling of indolizine derivatives
and vinylarenes.[10] Therefore, the development of a more
benzylic C H/aromatic C H coupling. In this case electro-
oxidatively generated benzyl cations are stabilized by sulfil-
imines. We envisaged that stabilized cation pools can also be
generated by addition to alkenes. In fact, iodosulfonium ions
electrooxidatively generated from I2 and sulfoxides react with
À
À
vinylarenes
1
to give b-iodoalkoxysulfonium ions
2
(Scheme 2).[14] Considering the reaction mechanism of the
[*] Dr. R. Hayashi, Dr. A. Shimizu, Y. Ishizaki
Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University
Nishikyo-ku, Kyoto, 615-8510 (Japan)
J. A. Davies, Prof. Dr. C. Willis
School of Chemistry, University of Bristol
Bristol BS8 1TS (UK)
Prof. Dr. J. Yoshida
National Institute of Technology, Suzuka College
Suzuka, Mie, 510-0294 (Japan)
E-mail: j-yoshida@jim.suzuka-ct.ac.jp
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.201807592.
À
À
Scheme 2. Alkenyl C H/aromatic C H cross-coupling via b-iodo-
alkoxysulfonium ion. FG: a functional group.
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[5]
À
À
intramolecular alkenyl C H/aromatic C H cross-coupling,
if ions 2 react as stabilized benzyl cations with aromatic
compounds 3, the resulting 1,1-diaryl-2-iodoethanes 4 could
be easily transformed to 1,1-diarylethylene 5 by treatment
to an increase in the yield to 61%. Diphenyl sulfoxide was
more effective and gave 5aa in 82% yield (See Supporting
Information for the CV data of the electrochemical step). On
the other hand, the use of sulfilimine,[13] which is used for
À
À
À
À
with a base. Thus, alkenyl C H/aromatic C H cross-coupling
can be achieved. The concept works, and herein we report
benzylic C H/aromatic C H coupling, gave 5aa in only 18%
yield, although the reason is not clear at present. Therefore,
hereafter we used diphenyl sulfoxide. The use of lower
amount of 3a decreased the yield of 5aa. When 1, 2, and
5 equivalents of 3a were used, the NMR yield of 5aa were 66,
77, and 87%, respectively (Table S1 in the Supporting
Information). The present transformation can be performed
on a gram scale and 0.84 g of 5aa was obtained (Table 1,
entry 1).
À
À
metal- and oxidant-free alkenyl C H/aromatic C H cross-
coupling using electrogenerated iodosulfonium ions.
We first screened sulfoxides. Iodine was electrochemically
oxidized in the presence of various sulfoxides in dichloro-
methane to generate the corresponding iodosulfonium ions.
After the electrolysis was complete, 4-bromostyrene (1a) was
added to the anodic solution to generate b-iodoalkoxysulfo-
nium ion 2a (Scheme 3).
b-Iodoalkoxysulfonium ion 2a derived from 1a and
diphenyl sulfoxide was successfully characterized by mass
spectroscopy and H and 13C NMR analyses (See the Sup-
1
porting Information for details). There was a characteristic
lower chemical shift of the benzylic proton of 2a compared
with that of 1-(4-bromophenyl)-2-iodoethanol (6a)
1
(Figure 1). Variable-temperature H NMR analysis revealed
that 2a is stable at temperatures lower than À208C (Fig-
ure S3). Notably, 1,1-diaryl-2-iodoethane 4aa was isolated in
85% yield when DBU was not added.
Scheme 3. Screening of sulfoxides. Iodine (0.25 mmol) was electro-
chemically oxidized in the presence of 2.0 mmol of sulfoxide or
sulfilimine in a 0.3m solution of Bu4NBF4 in CH2Cl2 at À788C. After
3.0F of electricity was applied, 1a (0.4 mmol) was added. The resulting
solution was treated with 2a (2.0 mmol) and DBU (1,8-diazabicyclo-
[5.4.0]undec-7-ene) (2.0 mmol). Isolated yields based on the amount
of 1a used are shown.
Figure 1. 1H and 13C NMR chemical shifts of 2a and 6a.[18]
The reaction of 2a with 1,2-dimethoxybenzene (3a) gave
4aa presumably via a benzylic cation stabilized by intra-
molecular participation of iodine, indicating that the solution
of 2a serves as a stabilized cation pool for aromatic
compounds. The subsequent reaction with DBU[15] gave 1,1-
diarylethylene 5aa (Scheme 4a). When DMSO was used as
a sulfoxide, 5aa was obtained in 35% yield. In this case, the
corresponding epoxide 7a was also obtained in 13% yield
probably due to attack on the sulfur atom of 2a to give 6a,
which underwent cyclization by the action of DBU to give 7a
(Scheme 4b).[16,17] To suppress the epoxide formation, we
examined sulfoxides having bulky substituents. When tetra-
methylene sulfoxide was used, the yield of 5aa decreased to
23%. However, the use of methyl phenyl sulfoxide gave rise
The present one-pot transformation is applicable to other
vinylarenes bearing various functional groups as shown in
Table 1. Styrene (1b) and styrenes having methyl, ester, and
bromo groups on the aromatic ring gave the corresponding
1,1-diarylethylenes (entries 1–6). The reactions of substrates
with trisubstituted alkenes was interesting. The reaction of
(Z)-1g with 3a gave (Z)-5ga as a major product and (E)-5ga
as a minor product, while that of (E)-1g with 3a gave (E)-5ga
as a sole product. The present reaction can be applied to cyclic
alkenes. Indene (1h), a 5-membered ring alkene, gave the
corresponding 1,1-diarylethylene 5ha, while 1,2-dihydro-
naphthalene (1i), a 6-membered ring alkene, gave the
alkene 5ia’ instead of the corresponding diarylethylene. The
reaction of 1j with two equivalents of iododiphenylsulfonium
ion and 3a gave the double arylated alkene 5ja. The
regioselectivity of the reaction was very high. Only 1,1-
diarylethylenes were obtained.
Next, we examined the reactions of b-iodoalkoxysulfo-
nium ion 2a with various aromatic nucleophiles (Table 2). 1,2-
Dimethoxybenzene (3a) and 1,4-dimethoxybenzene (3c)
gave 5aa and 5ac, respectively in good yields, while 1,3-
dimethoxybenzene (3b) gave 5ab in a poor yield. Anisole
(3d) gave the 1,1-diarylethylene as a mixture of two isomers,
while 2-iodoanisole (3e) gave 5ae as a single isomer. Xylene
(3 f) and 1-methylnaphthalene (3g) gave the corresponding
Scheme 4. A plausible mechanism.
2
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Table 1: Scope of alkenes.
Table 2: Scope of nucleophiles.
I2 (0.25 mmol) was electrochemically oxidized in the presence of
2.0 mmol of diphenyl sulfoxide in a 0.3m solution of Bu4NBF4 in CH2Cl2
at À788C. After 3.0F of electricity was applied, 1a (0.4 mmol) was added
to the resulting solution, followed by the treatment with 3 (2.0 mmol)
and DBU (2.0 mmol). Isolated yield based on 1a used is shown. [a] After
the addition of 3 f (5 mL), the mixture was stirred at À208C for 42 h then
at 258C for 1 h. The following compounds were also examined as
nucleophiles, but they were not effective: N-methyl indole and N-methyl
acetanilide (1a was recovered), 2-methylfuran (very low NMR yield of ca.
35%), and benzofuran (a mixture of two compounds, which were not
separable and were not fully characterized, was obtained).
I2 (0.25 mmol) was electrochemically oxidized in the presence of
2.0 mmol of diphenyl sulfoxide in a 0.3m solution of Bu4NBF4 in CH2Cl2
at À788C using a divided cell under constant current conditions. After
3.0F of electricity was applied, 1x (0.4 mmol) was added to the resulting
anodic solution. Then, 3a (2.0 mmol) was added and the resulting 4xa
was treated with DBU (2.0 mmol). Isolated yields based on 1 are shown.
[a] I2 (2.5 mmol), diphenyl sulfoxide (20 mmol), 1a (4.0 mmol), 3a
(20 mmol), and DBU (20 mmol) was used. [b] Yields were determined by
GC. [c] After addition of DBU, the mixture was stirred at 358C for 3 h.
[d] 10 equiv of DBU was used, and the mixture was stirred for 68 h. [e] 1j
(0.2 mmol) was used.
ion which can be easily generated from I2 and diphenyl
sulfoxide using the electrochemical method. Notably, the
present transformation can be performed in one-pot without
isolating the intermediates. We hope that the present method
will be applied to synthesis of various 1,1-diarylethylenes
which serve as useful building blocks for making biologically
interesting compounds.
1,1-diarylethylenes 5af and 5ag, respectively. Heterocyclic
compounds such as N-tosylpyrrole (3h) and bromothiophene
(3i) gave the corresponding 1,1-diarylethylenes 5ah and 5ai,
respectively.
À
In conclusion, we have developed metal-free alkenyl C
À
H/aromatic C H cross-coupling using iododiphenylsulfonium
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Experimental Section
In the anodic chamber of an H-type divided cell were placed
iodine (63.4 mg, 0.25 mmol), diphenyl sulfoxide (404 mg, 2.00 mmol),
and 0.3m Bu4NBF4/CH2Cl2 (10.0 mL). In the cathodic chamber were
placed trifluoromethanesulfonic acid (80 mL), 0.3m Bu4NBF4/CH2Cl2
(10.0 mL). The constant current electrolysis (8.0 mA) was carried out
at À788C with magnetic stirring until 3.0F of electricity was
consumed. To the anodic chamber was added a solution of styrene
substrates 1 (0.40 mmol) in CH2Cl2 (0.5 mL), and to the cathodic
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1 h at 258C. Then DBU (300 mL) was added to both the anodic and
cathodic chambers, and the resulting mixture was stirred for addi-
tional 30 min. The solution in the anodic chamber was collected and
the solvent was removed under reduced pressure. The crude product
was purified by flash chromatography and GPC to obtain the coupling
product 5xa.
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Acknowledgements
This work was supported by the JSPS KAKENHI Grant
Number JP26220804, JP16K14057. We also thank the EPSRC
Chemical Synthesis Centre for Doctoral Training (EP/
L015366/1) for a studentship to J.A.D.
Conflict of interest
The authors declare no conflict of interest.
À
À
À
Keywords: benzyl cations · C H functionalization · C H/C
H cross-coupling · electrochemistry · oxidation
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Manuscript received: July 2, 2018
Revised manuscript received: August 6, 2018
Version of record online: && &&, &&&&
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Communications
À
C H Functionalization
A three-step transformation consisting of
1) addition of electrochemically gener-
ated iodosulfonium ions to vinylarenes,
2) nucleophilic substitution by subse-
quently added aromatic compounds, and
3) elimination of HI with a base enables
R. Hayashi, A. Shimizu, J. A. Davies,
Y. Ishizaki, C. Willis,
J. Yoshida*
&&&& — &&&&
À
À
Metal- and Oxidant-Free Alkenyl C H/
metal- and oxidant-free alkenyl C H/
À
À
Aromatic C H Cross-Coupling Using
aromatic C H cross-coupling. This one-
Electrochemically Generated
Iodosulfonium Ions
pot transformation provides an easy
access to various 1,1-diarylethylenes.
6
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Angew. Chem. Int. Ed. 2018, 57, 1 – 6
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