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6
N
N
N
N
N
N
Pd
6PF
6
N
N
N
N
N
N
550
Figure 4.25
Structure of palladium catalysts bearing carbene ligands tagged with
pyridinium cation.
Another significant contribution was reported by Hwang and co-workers.
114
Palladium catalysts bearing carbene ligands tagged with pyridinium
cations were developed (50, Figure 4.25). Their use permitted the
olefination of various aryl bromides and also chlorobenzene using NaOAc in
DMF or Cs
2
CO
3
in water. The catalytic system could be recycled up to
eight times.
4.3.2.3 Development of New Well-Defined Precatalysts
Shi and co-workers prepared binuclear palladium complexes bridged by
dicarbenes with linkers of different lengths (51, Figure 4.26).
115
The reaction
between styrene and bromobenzene was studied. Complexes featuring lin-
kers constituted of two or four carbons were the most ecient at promoting
the coupling. Huynh and Jothibasu reported the preparation of three cis-
chelating di-NHC-palladium complexes of formula [PdX
2
(di-NHC)],
116
where
X is an anionic co-ligand (52, X = I, SCN or CF
3
CO
2
, Figure 4.26). The reac-
tivities of the three complexes were examined and compared, notably in the
Heck coupling reaction. The results revealed an increasing eciency of the
precatalysts in the order SCN
o
I
o
CF
3
CO
2
. Blakemore et al. developed a
rare example of bidentate trans-chelating NHC ligands.
117
Four new com-
plexes were prepared and were found to be active in Heck coupling (53,
Figure 4.26). Nevertheless, their reactivity was poor for non- or deactivated
bromides and inecient for chlorides.
Lee and co-workers prepared new Pd(0) and Pd(II) complexes bearing
NHCs with amide functionalities (54, Figure 4.27).
118
The Pd(0) complexes
were found to be highly active in the coupling of activated aryl chlorides
using TBAB as the ionic liquid. For the coupling of deactivated aryl chlorides
and bulky aryl bromides, the cis-Pd(II) compounds were more effective
(Table 4.19).
In 2011, Dunsford and Cavell studied the influence of the ring size of the
NHCs in some Pd(0)-dvtms (dvtms = divinyltetramethyldisiloxane) com-
plexes (55, Figure 4.28).
119
The reactivities of eight complexes were compared
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