Investigation of the behavior of arenediazonium salts with olefins in BmimPF6

Article abstract of DOI:10.1016/j.tetlet.2004.02.023

The palladium-catalyzed reactions of olefins with arenediazonium salts in ionic liquids were investigated. For methyl acrylate and methyl acrylonitrile, normal Heck cross-coupling products are obtained in good yields. However, highly selective dimerization products are formed in excellent yields for styrenes. The catalyst system can be recycled.

Full text of DOI:10.1016/j.tetlet.2004.02.023

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2775–2777
Investigation of the behavior of arenediazonium salts with
olefins in BmimPF6
George W. Kabalka,* Gang Dong and Bollu Venkataiah
Departments of Chemistry and Radiology, The University of Tennessee, Knoxville, TN 37996-1600, USA
Received 23 January 2004; accepted 6 February 2004
Abstract—The palladium-catalyzed reactions of olefins with arenediazonium salts in ionic liquids were investigated. For methyl
acrylate and methyl acrylonitrile, normal Heck cross-coupling products are obtained in good yields. However, highly selective
dimerization products are formed in excellent yields for styrenes. The catalyst system can be recycled.
Ó 2004Published by Elsevier Ltd.
The palladium-catalyzed Heck reaction is a powerful
and versatile tool for carbon–carbon bond formation.1
For economic reasons, the recovery and recycling of the
expensive palladium catalyst is of importance. This issue
has been addressed through the use of polymer2 or
inorganic3 supported palladium complexes in organic
solvents and by using special media such as PEG4 or
ionic liquids.5 Ionic liquids have attracted increasing
attention because of their compatibility with transition
metal catalysts and their limited miscibility with com-
mon organic solvents (which simplifies recycling).
was found to be 1-butyl-3-methylimidazolium hexaflu-
orophosphate (BmimPF6). Only traces of product were
obtained using either 1-butyl-3-methylimidazolium
bromide (BmimBr) or 1-butyl-3-methylimidazolium
tetrafluoroborate (BmimBF4).
After establishing the reaction conditions, a number of
arenediazonium salts were examined (Table 1, entries 1–
9). Both bromo and iodo groups were tolerated. The
catalytic system can be recycled at least four times
without loss of activity (entries 1–4). The reactions of
other olefins with diazonium salts were then examined
(entries 10–13). Methyl acrylonitrile gave satisfactory
results but higher temperatures and longer reaction times
were required (entries 10 and 11). Vinyl ether and vinyl
ester derivatives were unreactive (entries 12 and 13).
The Heck reactions of aryl halides in ionic liquids have
been studied extensively.6 However, to the best of our
knowledge, the reactions of arenediazonium salts in
ionic liquids have not been investigated. Diazonium
salts have several advantages over aryl halides including
the accessibility of precursor anilines and superior
reactivity. In continuation of our studies on ionic liq-
uids,7 we investigated reactions of arenediazonium tetra-
fluoroborate salts with various olefins in 1-butyl-
3-methylimidazolium hexafluorophosphate (BmimPF6).
We wish to report the results of this study.
Surprisingly, when styrene was used as the olefin, the
reaction produced the dimerization product, (E)-1,3-di-
phenyl-1-butene, in high yield instead of the expected
Heck cross-coupling product. Transition-metal cata-
lyzed dimerizations of aliphatic olefins have been
investigated extensively,8 however, dimerizations of
aromatic olefins are not well known and generally give
mixtures of trans-, cis-, and cyclic isomers in low yields.9
Recently Shirakuwa and co-workers reported the
indium triflate promoted, palladium-catalyzed dimeriza-
tion of styrene.10 However, the expensive palladium
catalyst cannot be recycled and an indium triflate co-
catalyst is required.
The reaction of methyl acrylate with phenyldiazonium
tetrafluoroborate was investigated first. A variety of
ionic liquids were used as solvents and the most effective
Keywords: Ionic liquids; Diazonium salts; Heck reaction; Dimeriza-
tion; Palladium catalyzed.
At first glance, it would appear that the diazonium salt is
not involved in the dimerization reaction. However, in
* Corresponding author. Tel.: +1-865-974-3260; fax: +1-865-974-2997;
e-mail: kabalka@utk.edu
0040-4039/$ - see front matter Ó 2004Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2004.02.023

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G. W. Kabalka et al. / Tetrahedron Letters 45 (2004) 2775–2777
Table 1. Palladium-catalyzed Heck reaction of arenediazonium saltsa
N2BF4
X
Pd(OAc)2 2 mol %
BmimPF6
X
+
R
R
Entry
R
X
Temperature (°C)
Time (h)
Isolated yield (%)
1
H
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CN
rt
3
76
76
2b
3c
4d
5
H
rt
3
H
rt
3
73
72
H
rt
3
p-Br
p-Cl
rt
3
82
78
32 (75)e
6
rt
3
7
p-I
rt
3
8
o-CN
p-OCH3
p-Cl
rt
2
80
72
9
rt
3
10
11
12
13
50
50
50
50
470
468
5
p-OCH3
p-OCH3
p-OCH3
CN
OBn
0
0
OAc
5
a Reaction conditions: methyl acrylate or methyl acetonitrile (1 mmol), arenediazonium salt (1.2 mmol), Pd(OAc)2 (2 mol %) at rt or 50 °C, 2–4h.
b Second cycle of catalyst.
c Third cycle of catalyst.
d Fourth cycle of catalyst.
e GC yield in parenthesis, product decomposed on silica gel.
the absence of the diazonium salt, no trace of dimer was
detected. We found that 1 mol % of diazonium salt
was sufficient to catalyze the dimerization. The reaction
was examined using a variety of substituted styrenes
(Table 2). No differences in reactivity with respect to the
electronic and steric effects of the substituent were
observed. Halogens are tolerated (entries 2–5) and the
palladium catalyst can be recycled at least four times
(entries 6–9). Though the role of the diazonium salt in
the reaction is still not clear, a free radical mechanism
has been ruled out by the fact that no dimerization
product formed when AIBN was used in place of the
diazonium salt.
In summary, reactions of olefins with arenediazonium
salts in ionic liquids were investigated. The results
depend on the olefin used: methyl acrylate and methyl
acrylonitrile yield normal Heck cross-coupling products,
whereas styrenes produce dimerization products in
excellent yields in the presence of a catalytic amount of
diazonium salt. The catalytic system can be recycled.
Acknowledgements
We would like to thank the US Department of Energy
and the Robert H. Cole Foundation for support of this
research.
Table 2. Palladium catalyzed, diazonium salt promoted dimerization
reactions of styrenesa
Pd(OAc)2 2 mol %
Diazoniumsalt 1 mol%
References and notes
R
BmimPF6, r . t.
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b Second cycle of catalyst.
c Third cycle of catalyst.
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