K2PtCl4/AgOTf as a highly active catalyst for hydroarylation of propiolic acids with arenes

Article abstract of DOI:10.1246/cl.2005.1430

A new and efficient K₂PtCl₄/AgOTf catalyst for the hydroarylation of propiolic acid was found. The hydroarylation of propiolic acids gave predominantly (Z)-cinnamic acid derivatives in high yields. The K₂PtCl₄/AgOTf catalyst showed a high activity to less reactive benzene. Copyright

Full text of DOI:10.1246/cl.2005.1430

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Chemistry Letters Vol.34, No.10 (2005)
K PtCl /AgOTf as a Highly Active Catalyst for Hydroarylation
2
4
of Propiolic Acids with Arenes
ꢀ
Juzo Oyamada and Tsugio Kitamura
Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University,
Honjo-machi, Saga 840-8502
(Received August 10, 2005; CL-051037)
a
A new and efficient K2PtCl4/AgOTf catalyst for the hydro-
Table 1. Hydroarylation of propiolic acid with benzene
arylation of propiolic acid was found. The hydroarylation of
propiolic acids gave predominantly (Z)-cinnamic acid deriva-
tives in high yields. The K2PtCl4/AgOTf catalyst showed a high
activity to less reactive benzene.
The catalytic activation of unreactive C–H bond followed by
functionalization such as C–C bond formation is an important
reaction for the synthesis of useful chemicals and noticed as
1
an environmentally benign and atom-economy process. For
example, it can convert simple, unreactive, and cheap arenes
to aryl-substituted compounds directly, without pre-functionali-
zation like halogenation. Hydroarylation of alkynes via aromatic
C–H bond activation is one of the most atom-economic reactions
and has been studied widely by using various transition-metal
catalysts.2
–11
aReaction conditions: K2PtCl4 (0.05 mmol), AgOTf (0.10
mmol), 1a and 2a (2 mmol), and TFA (1 mL). Isolated yields
based on 2a. Z/E ratios were determined by H NMR. PtCl2
We reported that the Pd(OAc)2 catalyzed hydroarylation of
alkynes proceeded efficiently at room temperature to give aryl-
b
1
c
substituted alkenes.3
c,4a
However, the activity of Pd catalyst
d
(
mmol) was used instead of K PtCl /AgOTf.
0.05 mmol) was used instead of K2PtCl4. Pd(OAc)2 (0.05
for less reactive arenes such as benzene and p-xylene was low.
Moreover, the low selectivity was observed in the Pd-catalyzed
hydroarylation of ethyl propiolate. The reaction gave butadiene
derivatives along with the usual expected adduct, cinnamates.
Very recently, we reported that PtCl2/AgOTf catalyst was selec-
2
4
ing K2PtCl4, while the reaction using Pd(OAc)2 gave low yield
because of low selectivity although the reaction was faster than
Pt catalyst.
4
b
tive and effective for the hydroarylation of ethyl propiolate.
The reaction of ethyl propiolate gave the corresponding cinna-
This Pt-catalyzed reaction was very efficient to various ben-
zene derivatives (Table 2). The reactions of electron-donating,
methyl-substituted arenes such as pentamethylbenzene (1b)
and mesitylene (1c) proceeded smoothly even under milder con-
ditions compared with the reaction of benzene, affording the cor-
responding cinnamic acids in high yields (Entries 1 and 2). Even
the reaction of p-xylene (1d) afforded 3d in more than 90% yield
(Entry 3). Bromomesitylene (1e) having a bromine atom also
gave 3e in high yield although the reaction was conducted at
4
b
mate selectively, without butadiene derivatives. However,
the PtCl2 catalyst exhibited a low activity toward less reactive
aromatics and could not be applied to the hydroarylation with
a representative aromatic compound, benzene. In our continuous
study on Pt-catalyzed hydroarylation, we found that K2PtCl4
was a more active pre-catalyst than PtCl2. Here, we report the
effective hydroarylation of propiolic acids with the aid of a
new and efficient catalyst, K2PtCl4/AgOTf.
ꢁ
40 C (Entry 4). Bromine is tolerable under the Pt catalysis.
K2PtCl4 is one of the most readily available platinum salts
and has been used in organic synthesis. Therefore, we used
K2PtCl4 as a pre-catalyst in the hydroarylation of propiolic acid
The reaction with 2-naphthol (1g) gave coumarin 3g (Entry 6).
In addition, a substituted propiolic acid, phenylpropiolic acid
(2b, R ¼ Ph), can be applied as an alkyne in the reaction. The
reaction of 2b with 1c gave the corresponding cinnamic acid
3h in high yield (Entry 7). The reaction of 2b with 3-methoxy-
phenol (1h) afforded coumarin 3i similar to the reaction of 2a
with 1g (Entry 8).
In conclusion, we have demonstrated a new and efficient
K2PtCl4/AgOTf catalyst for the hydroarylation of propiolic
acids. The hydroarylation of propiolic acids gave predominantly
(Z)-cinnamic acid derivatives in high yields. Noteworthy is a
high activity of K2PtCl4/AgOTf catalyst toward less reactive
benzene. Further studies on the scope of this hydroarylation
using K2PtCl4/AgOTf are now in progress.
1
2
(2a) with benzene (1a). The results are listed in Table 1. The
hydroarylation of 2a with 1a proceeded by using K2PtCl4/
AgOTf catalyst to give (Z)-cinnamic acid selectively although
1
3
the yield was low (Entry 1). The yield was improved by elon-
gation of reaction time (Entry 2). Using 6 equivalents of 1a in-
creased the yield (Entry 3). Furthermore, the elevation of tem-
perature was effective, to give the best yield 61% in shorter re-
action time (Entry 6). However, further increase of the amount of
1a did not improve the yield (Entry 7). Entries 3 to 5 clearly
show the effectiveness of K2PtCl4 among Pd(OAc)2, PtCl2,
and K2PtCl4. The reaction using PtCl2 was slower than that us-
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.10 (2005)
1431
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1
1
1
2 General Procedure for the K2PtCl4/AgOTf catalyzed hydro-
arylation of propiolic acid with arenes: A mixture of K2PtCl4
aReaction conditions: K2PtCl4 (0.05 mmol), AgOTf (0.10
mmol), arene 1 (6 mmol), propiolic acid 2 (2 mmol), and TFA
(
(
0.05 mmol) and AgOTf (0.10 mmol) in trifluoroacetic acid
TFA) (1 mL) was stirred at room temperature for 10 min.
b
c
(
1 mL). Isolated yield based on 2. 1b (3 mmol) was used.
d
An arene and propiolic acid were added to the mixture. Then,
the mixture was stirred at the desired temperature. After a
certain period, the reaction mixture was poured into water
(20 mL), neutralized by NaHCO3, and washed with Et2O
CH2Cl2 (0.25 mL) was added. CH2Cl2 (0.5 mL) was added.
e
f
1
f (4 mmol) was used. Cl(CH2)2Cl (0.75 mL) was added. 1g
g
(
4 mmol) was used. CH2Cl2 (0.75 mL) was added. 1h (4 mmol)
was used. CH2Cl2 (0.5 mL) was used.
(
20 mL). Then, the ethereal layer was extracted with aq
References and Notes
1
NaOH (10 mL ꢂ 3). The combined aqueous layer was wash-
ed with Et2O (20 mL), acidified by aq HCl (ca. 36%) and
extracted with CH2Cl2 (20 mL ꢂ 3). The organic layer
was dried over anhydrous Na2SO4 and concentrated in
vacuo, affording cinnamic acids.
a) V. Ritleng, C. Sirlin, and M. Pfeffer, Chem. Rev., 102,
1
731 (2002). b) G. Dyker, Angew. Chem., Int. Ed. Engl.,
8, 1698 (1999). c) F. Kakiuchi and N. Chatani, Adv. Synth.
3
Catal., 345, 1077 (2003). d) A. E. Shilov and G. B. Shul’pin,
Chem. Rev., 97, 2879 (1997). e) C. Jia, T. Kitamura, and
Y. Fujiwara, Acc. Chem. Res., 34, 633 (2001). f) F. Kakiuchi
13 The use of 4 equivalents of AgOTf shortened the reaction
time but did not improve the yield of products.
Published on the web (Advance View) September 17, 2005; DOI 10.1246/cl.2005.1430