Palladium-catalysed dimerization of vinylarenes using indium triflate as an effective co-catalyst

Article abstract of DOI:10.1039/b301280h

A palladium-indium triflate catalyst was found to be much more active for the dimerization of vinylarenes compared with generally used cationic palladium(II) catalysts.

Full text of DOI:10.1039/b301280h

Palladium-catalysed dimerization of vinylarenes using indium triflate as
an effective co-catalyst
Teruhisa Tsuchimoto, Susumu Kamiyama, Ryoju Negoro, Eiji Shirakawa* and Yusuke Kawakami
Graduate School of Materials Science, Japan Advanced Institute of Science and Technology, Asahidai,
Tatsunokuchi, Ishikawa 923-1292, Japan. E-mail: shira@jaist.ac.jp; Fax: 81 761 51 1631; Tel: 81 761 51
1631
Received (in Cambridge, UK) 3rd February 2003, Accepted 26th February 2003
First published as an Advance Article on the web 11th March 2003
A palladium–indium triflate catalyst was found to be much
more active for the dimerization of vinylarenes compared
with generally used cationic palladium(II) catalysts.
Palladium complexes are powerful catalysts for carbon–carbon
bond forming reactions because palladium(II) intermediates
generated by oxidative addition of organic electrophiles to
palladium(0) complexes can be utilized for wide scope of
transformations such as transmetalation and insertion of
unsaturated bonds.1 However, organic electrophiles that can
add oxidatively to palladium(0) complexes have been limited
mainly to aryl/alkenyl halides/triflates.1 Although the expan-
sion of the scope by enhancing electrophilicity of organic
compounds with Lewis acids has made a certain success, only
substrates having a carbon–heteroatom bond have been used.2,3
On the other hand, simple alkenes activated by Lewis acids are
widely known to accept addition of relatively weak nucleo-
philes such as arenes, i.e., the Friedel–Crafts alkylation.4 In this
context, we have reported that metal triflates are efficient
activators of alkynes and catalyse the Friedel–Crafts alkenyla-
tion of arenes.5 Thus we envisaged that metal triflates should
activate vinylarenes to accept nucleophilic attack of palla-
dium(0) complexes, giving oxidative adduct equivalents that
accept insertion of another vinylarene as shown in Scheme 1.
Here we report the palladium–indium triflate-catalysed dimer-
ization of vinlyarenes through nucleophilic attack of palla-
dium(0) complexes to vinylarenes activated by indium tri-
flate.
Scheme 2
Table 1 Dimerization of a-methylstyrene (1a) catalysed by Pd–Lewis
acida
Yield
Conv. (%) of
(%)b 2a + 3ab
Additive Lewis
(mol%)
Entry Pd cat.
Acid
1
2
3
4
5
6
7
8
9
10
11
12
13
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(0) complexc
[Pd(PPh3)2](BF4)2
none
In(OTf)3 > 99
78
86
83
92
41
< 1
< 1
67
< 1
< 1
< 1
< 1
30
PPh3 (1) In(OTf)3 > 99
PPh3 (2) In(OTf)3
dppp (1) In(OTf)3
PPh3 (2) In(OTf)3
91
92
70
< 1
< 1
99
none
none
[(p-C3H5)Pd(cod)]BF4 PPh3 (2) none
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
none
PPh3 (1) Zr(OTf)4
PPh3 (1) InCl3
PPh3 (1) AlCl3
PPh3 (1) TiCl4
PPh3 (1) none
11
< 1
< 1
< 1
46
none
In(OTf)3
a The reaction was carried out in 1,4-dioxane (1.0 mL) at 20 °C for 3 h using
1a (1.0 mmol) in the presence of a palladium catalyst (10 µmol), an additive
and a Lewis acid (50 µmol). b Determined by GC. c A palladium(0) complex
generated in situ by the reaction of [(p-C3H5)PdCl]2 with NaCH(CO2Me)2
was used.8
The Pd(OAc)2/PPh3/In(OTf)3 (1/1/5) catalyst was success-
fully applied to the dimerization of various vinylarenes (Scheme
3 and Table 2). In addition to simple vinylarenes such as styrene
(1b) and 2-vinylnaphthalene, styrenes bearing an electron-
withdrawing group at the para position also dimerized to give
1,3-diaryl-1-butenes in high yields (entries 1–6). A styrene
having a carboxyl group underwent the dimerization without
protection (entry 4). Even aryl and benzyl halides, which are
known to add to palladium(0) complexes, were tolerated
(entries 5–7).1 For the reaction of an electron-rich vinylarene,
higher dilution that may prevent polymerization was required
(entry 8). o-Divinylbenzene (4) underwent an intramolecular
reaction to give 3-methylindene (5) in 73% yield, where
CH3CN was a superior solvent to 1,4-dioxane (Scheme 4).
We had assumed that 1a is more susceptible than 1b to
activation by Lewis acids because its methyl group would
stabilize the resulting benzyl cation, whereas cationic H–Pd
complexes, active species in the cationic palladium(II)-cata-
lysed dimerization of alkenes,9 prefer less bulky 1b to 1a. Thus,
we examined the reaction with a 1+1 mixture of 1a and 1b using
a Pd–In(OTf)3 catalyst or a cationic Pd(II) catalyst and found
that 1a dimerized predominantly in the presence of the Pd–
In(OTf)3 catalyst, whereas a cationic complex catalysed the
reaction of 1b exclusively (Scheme 5). The different substrate
preference in addition to the result that a palladium(0) is as
effective as Pd(OAc)2 (entries 3 and 5 of Table 1) should imply
that a palladium(0) complex is included as an active species10 in
Scheme 1
We first examined the catalytic activity of Pd–In(OTf)3 for
the dimerization of a-methylstyrene (1a) (Scheme 2 and Table
1). Treatment of 1a with 1 mol % of Pd(OAc)26 and 5 mol % of
In(OTf)3 in 1,4-dioxane at 20 °C for 3 h gave a mixture of (E)-
4-methyl-2,4-diphenyl-2-pentene (2a) and 4-methyl-2,4-diph-
enyl-1-pentene (3a) in 78% yield with complete consumption of
1a (entry 1).7 Use of a catalytic amount of a phosphine such as
PPh3 or 1,3-bis(diphenylphosphino)propane (dppp) raised the
yields in compensation for the reaction rate (entries 2–4). An in
situ generated palladium(0) complex8 coordinated by 2 equiva-
lents of PPh3 was also effective, whereas cationic Pd(II)–PPh3
complexes, which are known to catalyse the dimerization of
alkenes,9 were totally inactive (entries 5–7). The use of
Zr(OTf)4 instead of In(OTf)3 slightly lowered the yield,
whereas metal chlorides failed to catalyse the dimerization
(entries 8–11). Lack of a metal triflate or a palladium complex
resulted in no or slow reaction, respectively (entries 12 and
13).
852
CHEM. COMMUN., 2003, 852–853
This journal is © The Royal Society of Chemistry 2003

We are grateful to the Ministry of Education, Sports, Culture,
Science, and Technology, Japan Government, for Grant-in-Aid
for Scientific Research, No. 13750794 for financial support.
T.T. thanks the Teijin Ltd. for financial support through Award
in Synthetic Organic Chemistry, Japan.
Scheme 3
a
Table 2 Dimerization of vinylarenes catalysed by Pd–In(OTf)3
Entry
Ar
Temp (°C)
Time (h)
Yield (%)b
Notes and references
1 J. Tsuji, Palladium Reagents and Catalysts, John Wiley & Sons,
Chichester, 1995, p. 125.
1
2
3
4
5
6
7
8c
Ph
2-Naphtyl
20
50
50
20
50
20
50
20
3
3
3
24
2.5
3
14
2
98
79
86
70
96
70
73
85
2 Lewis acids have been utilized for the palladium-catalysed carbon–
carbon bond forming reactions of heteroatom-containing compounds.
BEt3 for allylic alcohols: Y. Tamaru, Y. Horino, M. Araki, S. Tanaka
and M. Kimura, Tetrahedron Lett., 2000, 41, 5705; Y. Horino, M. Naito,
M. Kimura, S. Tanaka and Y. Tamaru, Tetrahedron Lett., 2001, 42,
3113; M. Kimura, Y. Horino, R. Mukai, S. Tanaka and Y. Tamaru, J.
Am. Chem. Soc., 2001, 123, 10401; Ti(O-i-Pr)4 for allylic alcohols: K.
Itoh, N. Hamaguchi, M. Miura and M. Nomura, J. Chem. Soc., Perkin
Trans. 1, 1992, 2883; SnCl2 for allylic alcohols: J. P. Takahara, Y.
Masuyama and Y. Kurusu, J. Am. Chem. Soc., 1992, 114, 2577; BX3 (X
= F, C6F5) or AlX3 (X = Cl, Me) for a,b-unsaturated carbonyl
compounds: S. Ogoshi, T. Yoshida, T. Nishida, M. Morita and H.
Kurosawa, J. Am. Chem. Soc., 2001, 123, 1944; RZnI (R = Me, Et) for
thioimidates: I. Ghosh and P. A. Jacobi, J. Org. Chem., 2002, 67,
9304.
4-CF3-C6H4
4-HOCO-C6H4
4-Cl-C6H4
4-Br-C6H4
4-ClCH2-C6H4
4-CH3-C6H4
a The reaction was carried out in 1,4-dioxane (1.0 mL) using 1 (1.0 mmol)
in the presence of Pd(OAc)2 (10 µmol), PPh3 (10 µmol) and In(OTf)3 (50
µmol). b Isolated yield. c 1,4-Dioxane (5.0 mL) was used.
3 The palladium-catalysed carbon–heteroatom bond forming reactions
with a Lewis acid have been reported. ZnCl2 for a,b-unsaturated
carbonyl compounds: E. Keinan and N. Greenspoon, J. Am. Chem. Soc.,
1986, 108, 7314; Ti(O-i-Pr)4 for allylic alcohols: S.-C. Yang and C.-W.
Hung, J. Org. Chem., 1999, 64, 5000; S.-C. Yang and W.-H. Chung,
Tetrahedron Lett., 1999, 40, 953; Me3SiOTf for a,b-unsaturated
carbonyl compounds or arenecarbaldehydes: S. Ogoshi, S. Tomiyasu,
M. Morita and H. Kurosawa, J. Am. Chem. Soc., 2002, 124, 11598.
4 G. A. Olah, R. Krishnamurti and G. K. S. Prakash in Comprehensive
Organic Synthesis, ed. B. M. Trost, I. Fleming and G. Pattenden,
Pergamon Press, New York, 1991, vol. 3, ch. 1.8, p. 293.
5 T. Tsuchimoto, T. Maeda, E. Shirakawa and Y. Kawakami, Chem.
Commun., 2000, 1573.
Scheme 4
6 It is well-known that palladium(II) complexes are reduced to palla-
dium(0) species in the presence of an alkene. For example, see p 37 of
reference 1.
7 The reproducibility on the ratio of 2a and 3a was poor.
8 T. Hayashi, M. Kawatsura and Y. Uozumi, Chem. Commun., 1997, 561;
M. Kawatsura, Y. Uozumi and T. Hayashi, Chem. Commun., 1998,
217.
9 [Pd(PPh3)2](BF4)2: Z. Jiang and A. Sen, Organometallics, 1993, 12,
1406 and references therein; [(p-C3H5)Pd(solvent)2]BF4–PPh3: A. Sen
and T.-W. Lai, Organometallics, 1983, 2, 1059 and references
therein.
10 No palladium(0) species is thought to be involved in the catalytic cycle
of the previously reported palladium(II)-catalysed dimerization of
alkenes. See reference 9.
11 There might be a possibility that a complex having a Pd–In bond
catalyses the reaction. For late transition metal–indium bonds, see: R. A.
Fischer and J. Behm, J. Organomet. Chem., 1991, 413, C10; R. A.
Fischer and J. Weib, Angew. Chem. Int. Ed., 1999, 38, 2830.
Scheme 5
the present reaction not only in use of a palladium(0) complex
as a precursor but also with Pd(OAc)2.11
In conclusion, we have disclosed that the novel catalyst
system consisted of a palladium complex, PPh3 and In(OTf)3 is
quite effective for the dimerization of vinylarenes. Studies on
the details of the reaction mechanism as well as applications of
the catalyst to various unreactive compounds are in progress in
our laboratory.
CHEM. COMMUN., 2003, 852–853
853