Therefore, the diarylation of olefins catalysed by the
PdCl2–3–NaOAc catalyst system with 0.2 mol% palladium
loading was investigated (Table 4).
Acknowledgements
We thank the Natural Science Foundation of China for
financial support (20402006).
In sharp contrast to the monoarylation, electronic factors
from the aryl bromides showed few effects on the diarylation
of olefins catalysed by the PdCl2–3–NaOAc system. Both
electron-rich and -poor aryl bromides reacted similarly giving
diarylation products in high yields (Table 4, entries 1, 5, 7 and
8), indicating that oxidative addition of aryl bromides to
palladium(0) species was no longer the rate-determining step
in the catalytic cycle. The activated aryl chloride, 4-chloro-
acetophone, also reacted smoothly (Table 4, entry 9). How-
ever, steric hindrance from aryl halides affected the diarylation
markedly. Only a trace amount of diarylation product was
detected by GC-MS for the reactions of 2-bromotoluene and
2-chlorobenzonitrile with butyl acrylate (Table 4, entries 6 and
10). The major products of these reactions consisted of
monoarylation products, trans-cinnamates, as the major com-
ponent along with some aryls and biaryls from dehalogenation
and homocoupling of aryl halides, respectively. Diarylation of
styrene and N,N-dimethyl acrylamide with bromobenzene also
proceeded smoothly giving trisubstituted olefins in good yields
(Table 4, entries 3 and 4). It is noteworthy that almost no
diarylation was observed before the terminal olefins, such as
acrylate, were consumed in all cases. Thus the mono- and di-
arylation of terminal olefins catalysed by the PdCl2–3–NaOAc
system is well controlled by simply changing the substrate
ratio. These results implied that the insertion of the C–C
double bond of 1,2-disubstituted olefins, such as cinnamate,
to the Pd–CAr bond should determine the rate of the diaryl-
ation reaction.
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Conclusions
In summary, the Heck reaction of aryl bromides and activated
aryl chlorides is efficiently catalysed by (benzimidazolylidene)-
palladium generated in situ from benzimidazolium and PdCl2
in TBAB. Remarkable electronic effects from the benzimida-
zolylidene were observed, leading to the development of a
highly active benzimidazolium–palladium–NaOAc catalyst
system based on the electron-rich benzimidazolium, 5,6-dibu-
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without steric hindrance. The electronic factors from aryl
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808 | New J. Chem., 2006, 30, 803–809 This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006