A practical synthesis of biaryls and aromatic acetylenes by stille coupling in room-temperature ionic liquids

Article abstract of DOI:10.1080/00397911.2011.558233

The Stille cross-coupling reactions of aryl halides with aryl or alkynylstannanes have been achieved under mild conditions in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), affording the corresponding biaryls and aromatic acetylenes in good yields. Use of this solvent allows for facile recycling of the solvent and catalyst system, which can be used at least five times without loss of activity.

Full text of DOI:10.1080/00397911.2011.558233

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A Practical Synthesis of Biaryls and
Aromatic Acetylenes by Stille Coupling in
Room-Temperature Ionic Liquids
Wenyan Hao a , Zhiwen Xi a & Mingzhong Cai a
a Department of Chemistry , Jiangxi Normal University , Nanchang ,
China
Accepted author version posted online: 15 Dec 2011.Published
online: 14 May 2012.
To cite this article: Wenyan Hao , Zhiwen Xi & Mingzhong Cai (2012) A Practical Synthesis of
Biaryls and Aromatic Acetylenes by Stille Coupling in Room-Temperature Ionic Liquids, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
42:16, 2396-2406, DOI: 10.1080/00397911.2011.558233
To link to this article: http://dx.doi.org/10.1080/00397911.2011.558233
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Synthetic Communications1, 42: 2396–2406, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.558233
A PRACTICAL SYNTHESIS OF BIARYLS AND
AROMATIC ACETYLENES BY STILLE COUPLING IN
ROOM-TEMPERATURE IONIC LIQUIDS
Wenyan Hao, Zhiwen Xi, and Mingzhong Cai
Department of Chemistry, Jiangxi Normal University, Nanchang, China
GRAPHICAL ABSTRACT
Abstract The Stille cross-coupling reactions of aryl halides with aryl or alkynylstannanes
have been achieved under mild conditions in 1-butyl-3-methylimidazolium hexafluoropho-
sphate ([bmim][PF6]), affording the corresponding biaryls and aromatic acetylenes in
good yields. Use of this solvent allows for facile recycling of the solvent and catalyst system,
which can be used at least five times without loss of activity.
Keywords Alkynylstannane; aromatic acetylene; biaryl; ionic liquid; Stille coupling
INTRODUCTION
The palladium-catalyzed cross-coupling of organostannanes with organic
halides and triflates, known as the Stille reaction, is one of the most important,
powerful, and versatile tools for the formation of carbon–carbon bonds.[1] This
coupling reaction has been widely applied in organic synthesis[2] because a variety
of functionality can be tolerated on either partner. The yields of coupled products
are often excellent, and the organotin reagents can be readily synthesized, purified,
and stored. However, the reaction generally proceeds in the presence of a homo-
geneous palladium catalyst such as Pd(PPh3)4, PdCl2(PPh3)2, and PdCl2(MeCN)2,
which makes the recovery of the expensive metal tedious, if not impossible, and
might result in unacceptable palladium contamination of the product. Ideally, one
would be able to recover and recycle the entire catalyst system but avoid the chal-
lenges presented by either mounting the catalyst on a solid support or by preparing
designer ligands for use in aqueous or fluorous biphasic systems.
Received December 19, 2010.
Address correspondence to Mingzhong Cai, Department of Chemistry, Jiangxi Normal University,
Nanchang 330022, China. E-mail: caimzhong@163.com
2396

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STILLE COUPLING IN RTILs
2397
Recently, a new alternative solution for catalyst recycling has been reported.
This involves the use of room-temperature ionic liquids (RTILs), in essence salts that
are liquid at or below room temperature.[3] RTILs, especially those based upon the
1,3-dialkylimidazolium cation, have attracted growing interest in recent years.[4]
They offer an alternative and ecologically sound medium compared to conventional
organic solvents, as they are nonvolatile, recyclable, thermally robust, and excellent
solvents for a wide range of organic and inorganic materials. Furthermore, their
good compatibility with transition-metal catalysts and limited miscibility with com-
mon solvents enables easy product and catalyst separation with the retention of the
stabilized catalyst in the ionic phase.[5] These and related ionic liquids have been suc-
cessfully applied to hydrogenations,[6] alkene dimerizations,[7] Friedel–Crafts reac-
tions,[8] Diels–Alder reactions,[9] Heck reactions,[10] Bechmann condensations,[11]
Suzuki reactions,[12] Baylis–Hillman reactions,[13] and Sonogashira reactions.[14]
Handy and Zhang reported that the Stille coupling reaction of aryl and vinyl iodides
with vinyl- or arylstannanes could be performed in 1-butyl-3-methylimi-dazolium
tetrafluoroborate ([bmim][BF4]): however, the use of CuI as cocatalyst and highly
toxic Ph3As was required.[15] Chiappe et al. reported ligandless Stille cross-coupling
of aryl iodides with vinyl and alkylstannanes in ionic liquids.[16] However, to the best
of our knowledge, there have been no reports on the Stille coupling reaction of aryl
halides with alkynylstannanes in ionic liquids to date. In this article, we report a new
palladium catalytic system made from 1-butyl-3-methylimidazolium hexafluoro
phosphate ([bmim][PF6]) and Pd(PPh3)4 for the Stille coupling of aryl halides with
aryl or alkynylstannanes to give a variety of biaryls and aromatic acetylenes in good
yields without the use of Ph3As and CuI.
RESULTS AND DISCUSSION
Biaryls are important building blocks of numerous agrochemicals, pharmaceu-
ticals, natural products, conducting materials, and asymmetric catalysts.[17] Because
of their widespread applications, the development of straightforward and environ-
mentally friendly methods for the preparation of biaryls has aroused attentions.[18]
The Stille cross-coupling of aryl halides with arylstannanes is one of the effective
methods. Our choice of solvent was the readily prepared 1-butyl-3-methylimidazo-
lium hexa fluorophosphate ([bmim][PF6]). The Stille coupling of a variety of aryl
halides with arylstannanes in [bmim][PF6] was investigated (Scheme 1), and the
experimental results are summarized in Table 1. As shown in Table 1, the Stille coup-
ling reactions of a variety of aryl iodides with different arylstannanes proceeded
smoothly in [bmim][PF6] at 80 ꢀC, giving a variety of unsymmetrical biaryls in good
yields (entries 1–12). The reactions of sterically hindered 2-iodoanisole and bulky
1-iodonaphthalene with phenylstannane also provided good yields of the desired
Scheme 1. Synthesis of unsymmetrical biaryls.