Organometallics
Communication
In summary, a simple Pd(II) catalytic system has been
established for the coupling of electron-deficient arenes to
prepare biaryls, including nitrobenzene, (trifluoromethyl)-
benzene, and ethyl benzoate, which are among the most inert
arenes to C−H bond cleavage through the SEAr process.
Tuning the concentration of arene and TFA precisely can make
either homocoupling or cross-coupling reactions proceed
smoothly under the normal reaction conditions. Oxygen can
also play the role of an effective oxidant in most of these
catalytic reactions. Further research on the scope, mechanism,
and application of these coupling reactions is ongoing.
Scheme 3. Catalytic Cross-Coupling of Ethyl Benzoate with
Nitrobenzene or (Trifluoromethyl)benzene
a
a
1
Yield and selectivity determined by H NMR.
ASSOCIATED CONTENT
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S
* Supporting Information
6-position of the aromatic ring than did 2-chloro-1-nitrobenzene.
Although the current Pd(II) system is effective in the activation of
aryl C−H bonds on strongly electron-deficient arenes without a
CN, COR, or COOH group by controlling the loading of TFA,
the reasons for some low yields of couplings of these substrates,
especially ortho-substituted nitrobenzenes, are still unclear.
Text, figures, and tables giving experimental details and
characterization data for biaryl compounds. This material is
AUTHOR INFORMATION
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Corresponding Author
The cross-coupling reactions between these electron-
deficient arenes were also studied. Since ethyl benzoate showed
a very effective ortho-directing effect in the homocoupling
reactions, nitrobenzene and (trifluoromethyl)benzene were
selected as the partners, respectively, to couple with it, and
nearly all cross-coupling products in moderate yields were
obtained successfully. The major products were 2,3′-biaryls,
which means that the SEAr process was still involved (Scheme 3).
The more challenging work was the cross-coupling of two
different electron-deficient arenes without ortho-directing
groups. As mentioned before, in the catalytic system of
Pd(OAc)2/TFA for cross-coupling of two different simple
arenes, tuning the molar ratio of two arenes and the loading of
TFA can diminish or eliminate the homocoupling products and
thus increase the selectivity of cross-coupling reactions.6 After a
large number of tests, it was found that the reaction between
1,3-dinitrobenzene or 3-(trifluoromethyl)-1-nitrobenzene and
3-chloro-1-nitrobenzene could give the cross-coupling biaryls as
the major products, respectively, showing a moderate selectivity
in cross-couplings (Scheme 4). Homocoupling products could
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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We thank the National Natural Science Foundation of China
(Grant Nos. 20872089 and 21072129) for financial support.
REFERENCES
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Scheme 4. Cross-Coupling of 1,3-Dinitrobenzene or 3-
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a
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a
Selectivity: cross-coupling product/all coupling products. Coupling
1
products and selectivity determined by H NMR.
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not be avoided completely in these cross-coupling reactions,
because the reactivity and selectivity conflicted with each other
sharply in the activation of aryl C−H bonds on electron-
deficient arenes by an electrophilic metalation pathway with
TFA at elevated temperature.
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dx.doi.org/10.1021/om300114e | Organometallics 2012, 31, 2124−2127