Phosphine-free rhodium-catalyzed hydroarylation of diaryl acetylenes with boronic acids

Article abstract of DOI:10.1016/j.tetlet.2008.05.140

Rh(CO)₂(acac) was found to be an efficient and simple catalyst in hydroarylation of diaryl acetylenes with boronic acids in the absence of phosphine ligand. Thus, triaryl-substituted olefins were prepared in good to excellent yields. No 1,4-rho

Full text of DOI:10.1016/j.tetlet.2008.05.140

Tetrahedron Letters 49 (2008) 5214–5216
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Phosphine-free rhodium-catalyzed hydroarylation of diaryl
acetylenes with boronic acids
*
Weiwei Zhang, Miaochang Liu, Huayue Wu, Jinchang Ding, Jiang Cheng
College of Chemistry and Materials Science, Wenzhou University, Wenzhou 325027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Rh(CO)2(acac) was found to be an efficient and simple catalyst in hydroarylation of diaryl acetylenes with
boronic acids in the absence of phosphine ligand. Thus, triaryl-substituted olefins were prepared in good
to excellent yields. No 1,4-rhodium shift was observed in the catalytic cycle.
Ó 2008 Published by Elsevier Ltd.
Received 23 February 2008
Revised 21 May 2008
Accepted 30 May 2008
Available online 4 June 2008
Recently, much attention has been paid to rhodium catalysts in
the formation of carbon–carbon bond since it represents a central
theme in organic synthesis.1 The addition of arylboronic acids to
unsaturated C–C bond catalyzed by transition-metal has drawn
considerable attention because organoboron reagents possess the
advantages of low toxicity, stability to air and/or moisture, and
good functional group tolerance.2 Recently, Hayashi and Miyaura
reported the rhodium-catalyzed 1,4-addition of organoboronic
of using a simple catalytic system for such transformation. Herein,
we report a phosphine-free rhodium-catalyzed hydroarylation of
alkynes with arylboronic acids, which did not involve a 1,4-rho-
dium shift.
Initial studies of the reaction conditions were conducted using
the addition of phenylboronic acid 2a to diphenyl acetylene 1a as
the model reaction. Since water was reported to enhance the reac-
tion rate in many rhodium-catalyzed addition reactions, we em-
ployed toluene/H2O = 10:1 as the co-solvents in the reaction. A
series of rhodium sources were tested under various reaction con-
ditions in the absence of any phosphine ligand. The screening re-
sults are summarized in Table 1.
acids to various
a
,b-unsaturated compounds.3 In these reactions,
only olefins activated by the presence of a conjugated electron-
withdrawing group could be used, which were generally carried
out in a mixture of organic solvent and water. The regio- and ster-
eoselective synthesis of multisubstituted olefins has been a chal-
lenge for synthetic organic chemists for years.4 In 2001, Hayashi
reported the rhodium-catalyzed hydroarylation of alkynes with
arylboronic acids via 1,4-shift of rhodium.5 Lautens described the
rhodium-catalyzed regioselective addition of arylboronic acids to
alkynes in water, albeit a pyridin-2-yl was required to be attached
in the C–C triple bonds.6 Genét also reported the rhodium-cata-
lyzed addition of arylboronic acids to alkynes, employing the water
soluble phosphines as ligands.7 In addition to the rhodium-cata-
lyzed methods,8 the palladium or nickel-catalyzed addition of
organoboron reagents to alkynes was also reported.9 In 2004, No-
lan described that NHC was an efficient ligand in the palladium-
catalyzed addition of arylboronic acids to alkynes.10 Although pro-
gress has been made for this reaction, additional ligands either
phosphine or NHC ligands are still needed. However, most ligands
are expensive and/or air-sensitive, which will result in the tedious
procedures. It still remains a continuing and urgent goal to develop
simple and versatile synthetic methods for this transformation.
Our interest in the development of transition metal-catalyzed
additions of arylmetallic reagents to the carbon–carbon or car-
bon-hetero unsaturated bonds11 led us to explore the possibility
During the screening process, it was observed that rhodium
source played an important role in the reaction. Rh(cod)(acac) gave
the desired product in 47% isolated yield with toluene and water as
solvent, and the yield was increased to 74% when Rh(cod)2Cl dimer
Table 1
Effects of rhodium sources and solvents on the rhodium-catalyzed addition of
phenylboronic acid to diphenyl acetylenea
Ph
Rh
Sol.
PhB(OH)2
+
Ph
Ph
Ph
Ph
1a
2a
3aa
Entry
Rhodium source
Solvent
Yieldb (%)
1
2
3
4
5
6
Rh(cod)(acac)
Rh(cod)2Cl dimer
Rh(CO)2(acac)
Rh(CO)2(acac)
RhCl3ꢀ3H2O
Toluene/H2O = 10:1
Toluene/H2O = 10:1
Toluene/H2O = 10:1
Toluene
Toluene/H2O = 10:1
Toluene/H2O = 10:1
47
74
90 (80)c
<5
<5
<5
Rh(PPh3)3Cl
a
All reactions were run in the presence of diphenyl acetylene (36 mg, 0.2 mmol),
phenylboronic acid (36 mg, 0.3 mmol), indicated rhodium source (5 mol %), and
solvent (3.3 mL) under reflux for 12 h.
b
Isolated yield.
Under room temperature.
* Corresponding author. Tel./fax: +86 577 88156826.
E-mail address: jiangcheng@wzu.edu.cn (J. Cheng).
c
0040-4039/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2008.05.140

W. Zhang et al. / Tetrahedron Letters 49 (2008) 5214–5216
5215
Table 2
D
D
D
D
Hydroarylation of symmetrical alkynes with boronic acidsa
D
D
D
D
D
D
D
D
D
D
D
D
D
D
Rh(CO)2(acac)
toluene/H2O
Ar
2a
+
D
D
Rh(CO)2(acac)
Ar1
Ar1
+
ArB(OH)2
Ar1
Ar1
toluene/H2O
H
Ph
1a'
1a
1
2
3
5, yield: 85%
Entry
Alkyne
Aryl
Product Yieldb (%)
Rh(CO)2(acac)
toluene/D2O
2a
+
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Ph 2a
3aa
3ab
3ac
3ae
3af
90
70
85
83
80
83
86
72
81
91
67
80
76
72
70
Ph
1a
Ph
4-MeO–C6H4-2b
4-Cl–C6H4-2c
2-Me–C6H4-2e
4-CF3–C6H4-2f
D
Ph
4, yield: 82%
trans-Ph–CH@CH-2g 3ag
Scheme 1. Labeling studies of the hydroarylation reaction.
4-Me–C6H4-2h
Ph 2a
4-Cl–C6H4-2c
Ph 2a
4-MeO–C6H4-2b
4-Cl–C6H4-2c
Ph 2a
3ah
3ba
3bc
3ca
3cb
3cc
3da
3dc
3ea
3-Tol
3-Tol
1b
4-Tol
4-Tol
1c
RhLnX
X = Cl, acac
2-nap
2-nap
2-thio
1d
4-Cl–C6H4-2c
1e Ph 2a
2-thio
RhLn(OH)
a
Reaction conditions: alkyne (0.2 mmol), boronic acid (0.3 mmol), Rh(CO)2(acac)
transmetallation
ArB(OH)2
A
(2.6 mg, 5 mol %), toluene/H2O (3.3 mL, 10/1), 110 °C, 12 h.
b
Isolated yield.
Ph
Ar
Ph
H
2
3
was employed (Table 1, entry 2). The Wilkinson catalyst was to-
tally ineffective in this transformation. To our delight, the yield
was dramatically increased to 90% in the presence of Rh(CO)2(acac)
(5 mol %) in toluene/H2O (Table 1, entry 3). In dry toluene,
Rh(CO)2(acac) did not produce the desired product.
hydrolysis
H2O
With the optimized conditions in hand, we then explored the
scope of the addition reaction in the presence of a variety of func-
tional groups of the substrates, as shown in Table 2.
Ph
Ar
Ph
Ar-RhLn
B
RhLn
C
As expected, all substrates reacted smoothly under the reaction
conditions and provided the desired hydroarylation products in
good to excellent yields. Furthermore, electron-withdrawing aryl-
boronic acids generally reacted with alkynes easily and gave
triaryl-substituted olefins in much higher yields, while the elec-
tron-donating analogues provided the desired products with rela-
tively lower yields. The hindrance in the aromatic ring of the
boronic acids had little influence on the reaction. For example,
comparing with 3aa, 3ae was produced in slightly decreased 83%
yield (Table 2, entry 4). Heteroarylboronic acids were not proper
substrates in the procedure. This may be at least partly because
the heteroatoms in the heteroaryl boronic acid may coordinate to
the transition metal. But interestingly, the di-heteroarylacetylene
1e worked well in the procedure, and 3ea was prepared in 70%
yield (Table 2, entry 15). It was noteworthy that trans-Ph–
CH@CH–B(OH)2 1g was still a good reaction partner in the proce-
dure and delivered the conjugated diene 3ag in 83% yield (Table
2, entry 6). The reaction could run smoothly at room temperature,
despite the yield of 3aa was slightly decreased to 80% (Table 2,
entry 1). However, the feasibility of employing the dialkyl or aryl
alkyl acetylenes and dimethyl but-2-ynedioate analogues in the
procedure was failed.
Ph
Ph
1
insertion
Scheme 2. Plausible mechanism.
duced; and (4) intermediate C directly hydrolyzes13 to deliver the
hydroarylation product 3 and the true catalytic species A is
regenerated.
In conclusion, we developed a phosphine-free rhodium-cata-
lyzed hydroarylation of alkynes with arylboronic acids. The addi-
tion reaction was proved to be an efficient, simple and versatile
method for the synthesis of triaryl-substituted ethene derivatives.
Furthermore, the reaction did not involve a 1,4-rhodium shift.14
Acknowledgement
We thank the National Natural Science Foundation of China
(No. 20504023) for financial support.
Supplementary data
To further understand the mechanism, diphenyl alkyne-d10 1a0
was prepared,12 and labeling studies were conducted (Scheme 1).
This result indicated that the reaction did not involve a 1,4-rho-
dium shift, which was firstly reported by Hayashi.5 Further labeling
studies clearly showed that the H attached to the C@C bond was
derived from the solvent (Scheme 1).
Based upon the above experimental results, a plausible mecha-
nism is outlined in Scheme 2. The catalytic cycle may contain four
steps: (1) RhLnX undergoes hydrolysis to form RhLn(OH) A; (2)
ArB(OH)2 transmetallates with A to form intermediate B; (3) inter-
mediate B inserts into the triple bond, and intermediate C is pro-
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.05.140.
References and notes
1. Fagnou, K.; Lautens, M. Chem. Rev. 2003, 103, 169.
2. (a) Suzuki, A. Acc. Chem. Res. 1982, 15, 178; (b) Miyaura, N.; Suzuki, A. Chem.
Rev. 1995, 95, 2457; (c) Suzuki, A. J. Organomet. Chem. 1999, 576, 147; (d)
Darses, S.; Genét, J. P. Eur. J. Org. Chem. 2003, 4313.
3. (a) Otomaru, Y.; Senda, T.; Hayashi, T. Org. Lett. 2004, 6, 3357; (b) Kina, A.;
Iwamura, H.; Hayashi, T. J. Am. Chem. Soc. 2006, 128, 3904; (c) Shintani, R.;

5216
W. Zhang et al. / Tetrahedron Letters 49 (2008) 5214–5216
Ueyama, K.; Yamada, I.; Hayashi, T. Org. Lett. 2004, 6, 3425; (d) Hayashi, T.;
Ed. 2003, 42, 805; (c) Zeng, H.; Hua, R. J. Org. Chem. 2008, 73, 558. For Ni-
catalyzed reactions, see: (d) Shirakawa, E.; Takahashi, G.; Tsuchimoto, T.;
Kawakami, Y. Chem. Commun. 2001, 2688.
Senda, T.; Takaya, Y.; Ogasawara, M. J. Am. Chem. Soc. 1999, 121, 11591; (e)
Hayashi, T.; Senda, T.; Ogasawara, M. J. Am. Chem. Soc. 2000, 122, 10716; (f)
Takaya, Y.; Ogasawara, M.; Hayashi, T.; Sakai, M.; Miyaura, N. J. Am. Chem. Soc.
1998, 120, 5579.
10. Viciu, M. S.;Stevens, E. D.;Petersen, J. L.; Nolan, S. P. Organometallics2004, 23, 3752.
11. (a) Qin, C.; Wu, H.; Cheng, J.; Chen, X.; Liu, M.; Zhang, W.; Su, W.; Ding, J. J. Org.
Chem. 2007, 72, 4102; (b) Qin, C.; Chen, J.; Wu, H.; Cheng, J.; Zhang, Q.; Zuo, B.;
Su, W.; Ding, J. Tetrahedron Lett. 2008, 49, 1884; (c) Zhang, Q.; Chen, J.; Liu, M.;
Wu, H.; Cheng, J.; Qin, C.; Su, W.; Ding, J. Synlett 2008, 935.
4. (a) Denmark, S. E.; Amburgey, J. J. Am. Chem. Soc. 1993, 115, 10386; (b) Creton,
I.; Marek, I.; Normant, J. F. Synthesis 1996, 1499; (c) Brown, S. D.; Armstrong,
R. W. J. Am. Chem. Soc. 1996, 118, 6331; (d) Organ, M. G.; Cooper, J. T.; Rogers, L.
R.; Soleymanzadeh, F.; Paul, T. J. Org. Chem. 2000, 65, 7959.
5. Hayashi, T.; Inoue, K.; Taniguchi, N.; Ogasawara, M. J. Am. Chem. Soc. 2001, 123,
9918.
12. Zhang, W.; Wu, H.; Liu, Z.; Zhong, P.; Zhang, L.; Huang, X.; Cheng, J. Chem.
Commun. 2006, 4826.
13. The ligand feedback effect of CO is stronger than that of dppe. Thus, the electron
density of R–Rh(CO)2 is higher than that of R–Rh(dppe). The intermediate C
directly underwent hydrolysis rather than the 1,4-rhodium shift, which may at
least partly be caused by the weaker Lewis acidity of R–Rh(CO)2 than that of R–
Rh(dppe). Example of rhodium-catalyzed hydroarylation of alkynes with
boronic acids that did not involve 1,4-rhodium shift, see: Ref. 4.
14. General procedure: Under air, a reaction tube was charged with diaryl alkynes
(0.2 mmol), boronic acids (0.3 mmol), Rh(CO)2(acac) (2.6 mg, 5 mol %), and
toluene/H2O (3.3 mL, 10/1). The mixture was heated under air at 110 °C for
12 h, then cooled to room temperature. The mixture was concentrated in vacuo
and the residue was purified by flash column chromatography on a silica gel to
give the desired product.
6. (a) Lautens, M.; Yoshida, M. Org. Lett. 2002, 4, 123; (b) Lautens, M.; Yoshida, M.
J. Org. Chem. 2003, 68, 762.
7. (a) Gernin, E.; Michelet, V.; Genét, J.-P. Tetrahedron Lett. 2004, 45, 4157; (b)
Gernin, E.; Michelet, V.; Genét, J.-P. J. Organomet. Chem. 2004, 689, 3829.
8. (a) Miura, T.; Shimada, M.; Murakami, M. Tetrahedron 2007, 63, 6131; (b)
Tomoya, M.; Motoshi, Y.; Masahiro, M. Synlett 2007, 2029; (c) Arsin, ö.; Dege,
N.; Artok, L.; Türkmen, H.; Çetinkaya, B. Chem. Commun. 2006, 3187; (d)
Kurahashi, T.; Shinokubo, H.; Osuka, A. Angew. Chem., Int. Ed. 2006, 45,
6336.
9. For palladium-catalyzed reactions, see: (a) Gupta, A. K.; Kim, K. S.; Oh, C. H.
Synlett 2005, 457; (b) Oh, C. H.; Jung, H. H.; Kim, K. S.; Kim, N. Angew. Chem., Int.