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R'
R'
R
R
NN
NN
R
R'
4
NN
N
N
Cl
Pd
Cl
Cl
Pd
Cl
N
Cl
Pd
Cl
N
R
R
Pd
N
N
N
N
N
R'
R
R
R'
Br
Br
Cl
71
72
73
74
Figure 4.33
Structures of
the precatalysts studied for the direct arylation of
heterocycles.
Y
X
771
,
772
or
773
(1 mol%)
Y
+
R'
R'
R
Z
R
KOAc
DMA , 150°C, 20 h
Z
X=Cl,Br
COOMe
OAc
nBu
O
O
O
MeO
MeO
O
2
N
X = Br: 5 8 - 79%
X = Br: 46 - 76%
X=Cl:52 - 78%
N
nBu
nPr
S
S
Me
2
N
NC
X = Br : 21 - 69%
X=Cl:37-41%
Scheme 4.50 Direct arylation of furans, thiophenes and thiazoles using precatalysts
71, 72 and 73.
Later, they studied the activity of complexes 74 for the C2 arylation of
benzothiazole.
150
Complex 74a was identified as an ecient precatalyst,
affording the desired 2-arylated benzothiazoles in good yields (Scheme 4.51).
Finally, in a more recent contribution,
151
the arylation of pyrroles was
investigated. New complexes of the type 71 were prepared (Figure 4.33),
allowing the coupling of deactivated aryl chlorides using KOAc in DMA
at 150 1C. Various functionalized pyrroles were prepared in good yields
(Scheme 4.52). Interestingly, despite the harsh reaction conditions, alde-
hyde, ketone, cyano and ester functionalities were well tolerated.
Another significant contribution was made by Lee and co-workers.
152
Mixed phosphine-NHC palladium complexes were prepared for the direct
C5 arylation of imidazoles (Scheme 4.53). Complex 75 bearing the electron-
rich PCy
3
ligand exhibited the highest catalytic activity in the reaction.
Notably, the coupling of deactivated 4-chloroanisole was successfully
achieved in that case. Microwave irradiation was discovered to reduce the
reaction times considerably, from 18 to 2 h.
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