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STILLE COUPLING IN RTILs
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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.
Hao, Wenyan
