Rhodium(III)-catalyzed ortho -alkenylation through C-H bond cleavage directed by sulfoxide groups

Article abstract of DOI:10.1021/ol5000605

The rhodium-catalyzed ortho-alkenylation of phenyl sulfoxides using alkenes or alkynes as substituent sources proceeds efficiently through sulfoxide group directed C-H bond cleavage to produce the corresponding o-alkenylphenyl sulfoxides. The products rea

Full text of DOI:10.1021/ol5000605

Letter
pubs.acs.org/OrgLett
Rhodium(III)-Catalyzed Ortho-Alkenylation through C−H Bond
Cleavage Directed by Sulfoxide Groups
Kazunori Nobushige,† Koji Hirano,† Tetsuya Satoh,†,*,‡ and Masahiro Miura*,†
†Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
‡JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
S
* Supporting Information
ABSTRACT: The rhodium-catalyzed ortho-alkenylation of
phenyl sulfoxides using alkenes or alkynes as substituent
sources proceeds efficiently through sulfoxide group directed
C−H bond cleavage to produce the corresponding o-
alkenylphenyl sulfoxides. The products readily undergo
interrupted Pumerer cyclization as well as reduction to afford
benzothiophenes and o-alkenylphenyl sulfides.
ransition-metal-catalyzed aromatic C−H functionalization
Thas been recognized to be a highly important synthetic
tool because it provides step- and atom-economical routes to
functionalized aromatic molecules.1 With the aim of the
regioselective cleavage of C−H bonds existing ubiquitously in
organic molecules, directing groups are usually employed.
Coordination of such groups toward a transition-metal center
leads to ortho-selective C−H bond cleavage and functionaliza-
tion.2 A variety of oxygen- and nitrogen-containing directing
groups such as hydroxyl, carboxyl, carbonyl, amide, imino, and
pyridyl groups have been utilized in a wide range of reactions.
Recently, phosphorus-containing functions have also come into
general use.3
Cu(OAc)2·H2O (0.5 mmol) as catalyst and oxidant, respec-
tively, in chlorobenzene (PhCl) (3 mL) at 120 °C for 6 h under
N2. As a result, an ortho-alkenylated product, butyl (E)-3-(2-
(methylsulfinyl)phenyl)acrylate (3a), was formed in 39% yield
(based on the amount of 2a used) (entry 1 in Table 1). In
other solvents such as t-AmOH, α,α,α-trifluorotoluene
Table 1. Reaction of Methyl Phenyl Sulfoxide (1a) with
a
Butyl Acrylate (2a)
Meanwhile, it is generally considered that sulfur-containing
functional groups such as sulfides and sulfoxides that have lone
pairs on the sulfur coordinate metal centers too strongly to
suppress their catalytic activities. However, mainly under
palladium catalysis, sulfoxide group directed C−H functional-
ization has recently been developed.4,5 During our continuous
studies on Rh(III)-catalyzed C−H functionalization,6,7 we
succeeded in finding that phenyl sulfoxides undergo ortho-
alkenylation using alkenes and alkynes as substituent sources.
The products, o-alkenylated phenyl sulfoxides, are of
importance because they can be readily converted to the
corresponding benzothiophenes via deoxygenative cyclization
(interrupted Pummerer cyclization).8 The sulfoxide groups are
also known to be transformed to other functional groups such
as sulfides by reduction treatment.9 Besides such utilities for
further derivatization, o-alkenylphenyl sulfoxides themselves are
applicable as useful ligands for transition metals.10 Herein, the
new findings concerning the ortho-alkenylation of phenyl
sulfoxides and preliminary examinations for further trans-
formation of the alkenylated products are described.
b
entry
oxidant (mmol)
solvent
yield of 3a (%)
1
2
3
4
5
Cu(OAc)2·H2O (0.5)
Cu(OAc)2·H2O (0.5)
Cu(OAc)2·H2O (0.5)
Cu(OAc)2·H2O (0.5)
Ag2CO3 (0.25)
PhCl
39
t-AmOH
PhCF3
diglyme
PhCl
34
32
26
69
c
6
Ag2CO3 (0.25)
PhCl
0
d
7
Ag2CO3 (0.25)
PhCl
41
e
8
Ag2CO3 (0.25)
PhCl
80 (77)
89 (77)
(72)
e
9
Ag2CO3 (0.25)
PhCF3
PhCF3
f
10
Ag2CO3 (2.5)
a
Reaction conditions: 1a (0.5 mmol), 2a (0.25 mmol), [Cp*Rh-
(MeCN)3][SbF6]2 (0.02 mmol) in solvent (3 mL) at 120 °C for 6 h
under N2, unless otherwise noted. GC yield based on the amount of
b
2a used. Value in parentheses indicates yield after purification.
c
d
e
Without [Cp*Rh(MeCN)3][SbF6]2. With 1a (0.25 mmol). With
f
1a (0.75 mmol). With 1a (7.5 mmol), 2a (2.5 mmol), an
[Cp*Rh(MeCN)3][SbF6]2 (0.2 mmol), in solvent (30 mL).
In an initial attempt, methyl phenyl sulfoxide (1a) (0.5
mmol) was treated with butyl acrylate (2a) (0.25 mmol) in the
presence of [Cp*Rh(MeCN)3][SbF6]2 (0.02 mmol) and
Received: January 7, 2014
Published: February 12, 2014
© 2014 American Chemical Society
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Letter
(PhCF3), and diglyme, the product yield slightly decreased
(entries 2−4). To our delight, the 3a yield was significantly
improved by using Ag2CO3 (0.25 mmol) as oxidant (entry 5).
The control experiment without Rh-catalyst did not give 3a at
all (entry 6). Decreasing the amount of 1a to 0.25 mmol caused
the reduction of the 3a yield (entry 7), while using 0.75 mmol
of 1a enhanced it up to 80% (entry 8). In PhCF3, the reaction
also proceeded smoothly to give 3a in the same isolated yield
(entry 9). It should be noted that the present reaction could be
readily scaled up to 10 times. Thus, from 1a (7.5 mmol) and 2a
(2.5 mmol), 3a was obtained in 72% isolated yield (0.48 g,
entry 10).
A plausible mechanism for the reaction of phenyl sulfoxide 1
with alkene 2 is illustrated in Scheme 1. Coordination of the
Scheme 1. Plausible Mechanism for the Reaction of 1 with 2
With the optimized conditions in hand, the reactions of
variously substituted phenyl sulfoxides with alkenes were
examined next (Table 2). A series of acrylates, iso and tert-
a
Table 2. Reaction of Phenyl Sulfoxides 1 with Alkenes 2
sulfoxide group11 of 1 to a RhIII center and subsequent
cyclorhodation on the phenyl group of a resulting intermediate
A take place to form a five-membered rhodacycle intermediate
B. Then, alkene insertion to form C and β-hydrogen
elimination may occur to release 3 and HX. The resulting
RhI species seems to be oxidized by AgI to regenerate RhIII.
For providing additional mechanistic information, a
deuterated substrate, methyl d5-phenyl sulfoxide (1a-d5), was
treated with 2a under standard conditions (see the Supporting
Information). After 3 h, no significant D−H exchange at the
ortho positions of recovered 1a-d5 as well as at the 6-position of
produced 3a-d4 was observed. This result indicates that the
initial C−H(D) bond-cleavage step to form B is likely
irreversible. Next, an intermolecular competition reaction of
1a-d0/1a-d5 with 2a was examined. As shown in Scheme 2, the
Scheme 2. Plausible Mechanism for the Reaction of 1 with 2
a
Reaction conditions: 1 (0.75 mmol), 2 (0.25 mmol), [Cp*Rh-
(MeCN)3][SbF6]2 (0.02 mmol), Ag2CO3 (0.25 mmol), in PhCl (3
mL) at 120 °C for 6 h under N2, unless otherwise noted. Isolated
yield based on the amount of 2 used. In PhCF3. At 100 °C. At 140
°C.
b
c
d
e
alkenylated products 3a-d0 and 3a-d4 were obtained in a ratio of
5:1 (kH/kD), suggesting that the rate-determining step involves
the C−H(D) bond cleavage at the ortho-position of 1a (A to B
in Scheme 1). Two parallel reactions using 1a-d0 and 1a-d5
were also performed separately and showed a similar KIE value
of 5.7 (see the Supporting Information).
Next, the ortho-alkenylation of phenyl sulfoxides 1 with
alkynes 43g,12 was examined under various conditions (see the
Supporting Information). Consequently, it was found that 1a
(0.75 mmol) coupled with diphenylacetylene (4a) (0.25 mmol)
efficiently in the presence of [Cp*Rh(MeCN)3][SbF6]2 (0.02
mmol), Cu(OAc)2·H2O (0.025 mmol), and 1-adamantanecar-
boxylic acid (1-AdCO2H) (1 mmol) in diglyme (1 mL) at 120
°C under N2 to produce (E)-1-[2-(methylsulfinyl)phenyl]-1,2-
diphenylethene (5a) stereoselectively in 68% yield (entry 1 in
Table 3). In the absence of the copper cocatalyst, the product
butyl, cyclohexyl, and ethyl acrylates (2b−e) coupled with 1a to
produce the corresponding o-alkenylated phenyl sulfoxides 3b−
e (entries 1−4). In the case using sterically hindered ester 2c, a
better result was obtained in PhCF3 (entry 2). The reaction
with relatively volatile 2e was conducted at 100 °C (entry 4).
On the other hand, p-methyl- and p-methoxyphenyl sulfoxides
1b and 1c underwent the reaction with 2a in PhCF3 to form 3f
and 3g, respectively (entries 5 and 6). The reaction of electron-
deficient 1d was found to be sluggish. Thus, even at 140 °C, the
product yield was moderate (entry 7). Diphenyl sulfoxide (1e)
reacted with 2a smoothly under standard conditions to give 3i
in 62% yield (entry 8).
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a
sulfoxide (1f) coupled with 4a in similar ways to give 5k and 5l
(entries 12 and 13).
Table 3. Reaction of Phenyl Sulfoxides 1 with Alkynes 4
Further derivatization of ortho-alkenylated phenyl sulfoxides
was then examined. Treatment of 5a and 5f with TfOH
induced interrupted Pummerer cyclization to produce 2,3-
diphenylbenzothiophenes 6a,b in good yields (eq (a) in
Scheme 3).8a Meanwhile, treatment of 5a under reductive
Scheme 3. Transformation of o-Alkenylphenyl Sulfoxides
conditions using Zn/AcOH on silica gel9a gave the correspond-
ing o-alkenylphenyl sulfide 7 (eq (b) in Scheme 3). The o-
alkenylphenyl sulfide structures can be seen in a range of
bioactive compounds.13
In summary, we have demonstrated that the rhodium-
catalyzed alkenylation of phenylsulfoxides occurs smoothly
through a sulfoxide group directed C−H bond cleavage.
Produced o-alkenylphenyl sulfoxides can be transformed to the
corresponding sulfide as well as benzothiophenes via reduction
and cyclization, respectively.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, additional results, and character-
ization data of products. This material is available free of charge
via the Internet at http://pubs.acs.org.
a
Reaction conditions: 1 (0.75 mmol), 4 (0.25 mmol), [Cp*Rh-
(MeCN)3][SbF6]2 (0.02 mmol), Cu(OAc)2 (0.025 mmol), 1-
AdCO2H (1 mmol), in diglyme (1 mL) at 120 °C under N2, unless
AUTHOR INFORMATION
Corresponding Authors
■
b
c
otherwise noted. Isolated yield based on the amount of 4 used. With
1a (7.5 mmol), 4a (2.5 mmol), [Cp*Rh(MeCN)3][SbF6]2 (0.2
mmol), Cu(OAc)2 (0.25 mmol), 1-AdCO2H (10 mmol), in diglyme
*E-mail: satoh@chem.eng.osaka-u.ac.jp.
*E-mail: miura@chem.eng.osaka-u.ac.jp.
Author Contributions
d
(10 mL). At 100 °C.
All authors have given approval to the final version of the
manuscript.
Notes
was contaminated by an unidentified, inseparable impurity. A
10-fold scaled-up reaction using 1a (7.5 mmol) and 4a (2.5
mmol) could be conducted smoothly to form 5a in 60% yield
(0.48 g, entry 2). Methyl-, chloro-, trifluoromethyl-, and
methoxy-substituted diphenylacetylenes 4b−e also reacted
with 1a under similar conditions to give products 5b−e
(entries 3−6). In the case with 4e, a better result was obtained
at 100 °C (entry 6). Interestingly, the reactions of unsym-
metrical alkynes 4f and 4g proceeded stereo- and regioselec-
tively to produce 5f and 5g as single major products (entries 7
and 8). The reaction of a dialkylacetylene, 4-octyne, was
sluggish to form the corresponding ortho-alkenylated product in
a low yield (∼20%), along with inseparable unidentified
byproducts. Treatment of a terminal alkyne, phenylacetylene,
gave only a trace amount of coupling product.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was partly supported by Grants-in-Aid from MEXT,
JSPS, and JST, Japan.
■
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