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R'
R'
X
R''
70a
(0.15 - 5 mol%)
B
B
R
R''
R
+
CsOAc
DMA, 125°C
N
N
A
A
SEM
SEM
X=Br,I
COOMe
NC
N
SEM
N
SEM
N
SEM
X=Br,[Pd]=5mol%:58%
X = I , [P d] = 0.1 5 m ol%: 9 1%
X=I,[Pd]=1.5mol%:82%
X=I,[Pd]=5mol%:22%
NC
NC
N
SE M
N
SEM
N
X = I, [Pd] = 5 mol%: 59%
X = I, [P d] = 5 mol% : 49%
Scheme 4.47 Direct arylation of SEM-protected azoles using 70a.
a)
70a
(5 mol%), CsOAc
DMA, 125°C, 16 - 20 h
X
R
+
R
NN
SEM
or
b) Pd(OA c)
2
(5 mol% )
P(nBu)Ad
2
(7.5 mol%)
K
2
CO
3,
DMA, 1 20°C, 18 h
NN
SE M
X=Cl,Br,I
a) X = Br, I: 29 - 61%
b) X = Cl, Br: 56 - 7 2%
Scheme 4.48 Precatalyst 70a versus Pd(OAc)
2
/P(nBu)Ad
2
for the direct arylation of
SEM-protected imidazoles.
Pd ( II) (5 mol%)
I
BF
4
+
AcOH, 25°C
N
N
Pd(OAc)
2
:49%,5min
[Pd(IMes)(OAc)
2
]: 86%, 1 8h
Scheme 4.49 C2-arylation of indoles using hypervalent iodine salts.
In 2010,
¨
zdemir et al. started to develop a wide range of new well-defined
Pd-NHC complexes for the direct arylation of heterocycles (71-74,
Figure 4.33). First they studied the C5 direct arylation of furans, thiophenes
and thiazoles using 71, 72 and 73.
149
Deactivated bromides were found to be
suitable coupling partners. Activated aryl chlorides were also successfully
used for the first time in a reaction involving a Pd-NHC complex and a
heteroaromatic substrate. The corresponding heterocycles were sub-
sequently isolated in moderate to good yields (Scheme 4.50). The various
precatalysts did not show significant differences in terms of reactivity in the
coupling reaction.
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