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this salt is commercially available and relatively inexpensive.
Further applications are under investigation in our laboratory.
We are grateful to the National Science Foundation for
financial support (CHE-1401700). B.X. is grateful to the National
Science Foundation of China for financial support (NSFC-
21472018). We acknowledge Dr Zhou Li (University of Louisville)
for his helpful comments.
Notes and references
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Scheme 3 Effect of reaction promoter in silver, rhodium and ruthenium
˜
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In some cases (e.g., Scheme 3a), the acceleration effect of
KCTf3 was only marginally better than the control. We believed
that the effectiveness of a promoter depends on the proximity
between cationic metal and counterion in the reaction system,
and here solvents play an important role. The majority of
organic reactions are conducted in solvents of relatively low
dielectric constant (e.g. DCM, toluene, THF), where cationic
metals or their complexes exist as contact ion pairs or even
partially covalent in nature (e.g. L–Au–OTf).20 According to
recent studies by Macchioni and others,21 cationic gold catalysts
(e.g., [LAu+–alkyne]BF4À) exist as ion pairs in commonly used low
dielectric constant solvents like DCM or chloroform. Thus,
counterions are close to the reactive metal center and therefore
exert a strong influence on the reaction rate. In these cases, a
reaction promoter is expected to play a beneficial role. Most of
the reaction examples presented above belong to this category.
On the other hand, in reactions conducted in high dielectric
constant solvents (e.g., water, methanol or acetone), the majority
of ionic species will exist as dissociated ions.20 In these cases,
counterions will be far away from the reaction center and there-
fore will have minimum influence on the reaction rate. Hence, a
reaction promoter will not be useful. This point was demon-
strated with the results shown in Scheme 3a, which was con-
ducted in acetone (e = 20.7), a solvent with a much larger
dielectric constant than those used in other reactions (e.g.
chloroform, e = 4.8 or dioxane, e = 2.3). Another example of the
solvent effect discussed above is shown in eqn (1). This reaction
is similar to the reaction in Scheme 2c, except that it was
conducted in EtOH (e = 24.5) instead of AcOH (e = 6.2). In this
case, the use of KCTf3 led to only a negligible improvement.
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(1)
We have demonstrated that KCTf3 is a broadly applicable
promoter because it enhanced, in consistent and significant
manner, the reaction rates and the chemical yields of a wide
spectrum of ionic reactions, ranging from traditional Lewis acid
catalysis to transition metal catalysis, without modifying the
original reaction conditions. Our approach is practical because
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