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Br
O
O
Pd(OAc)
2
(5 mol%)
PhDav-Phos (10 mol%)
R
2
R
1
N
N
+
H
PivOH (0.2 equiv.)
K
2
CO
3
(1.5 equiv.)
toluene, 25
S
R
3
R
3
S
R
3
R
2
°
C
Br
Pd(OAc)
2
(5 m ol%)
t
Bu
3
P.H BF
4
(5 mol%)
O
O
R
1
R
1
N
N
R
2
R
2
+
K
2
CO
3
(1.5 equiv.)
toluene, 70
S
S
°
C
H
R
3
R
3
Br
O
O
Pd (OAc)
2
(5 mol%)
PPh
3
(15 mol%)
H
N
N
R
2
+
R
2
K
2
CO
3
(1.5 equiv.)
to lue ne, 11 0
S
R
1
S
R
1
°
C
R
3
Scheme 12.35 C2, C4 and C5 arylations of azole N-oxides.
Br
Pd(OAc)
2
,Pcy
3.
HBF
4
PivOH, K
2
CO
3
+
H
DMA, 100
°
C
X
R
X
X=O,S,N
17-72h
R
Scheme 12.36 C2 arylation of pyrrole, furan and thiophene.
Br
Pd(OAc)
2
(0.01 - 0.5 mol%
)
+
R
2
R
2
H
R
3
KOAc or K
2
CO
3
N
N
R
3
R
1
DMAc, 150 °C, 17h
R
1
Scheme 12.37 C2 arylations of pyrroles.
Also in 2009, Roger and Doucet reported another protocol for the site-
selective arylation of N-substituted pyrroles without using any external lig-
and.
67
In comparison with the previous work, this reaction consisted a
bimolecular pathway without involving any pyrrolyl salt and minimized
metal salt waste. The reaction was effective for a variety of N-substituted
pyrrole substrates and functionalized aryl bromide coupling partner
(Scheme 12.37). The reaction furnished a C2 arylated product whereas in
case of substitution at C2, C5 arylation occurred. Notably, the reaction re-
quired high-boiling DMAc as a solvent, which is relatively non-toxic.
Fenner and co-workers implemented cheap and readily available aryl
tosylates and mesylates, instead of aryl bromide, as the coupling partner for
heterocyclic arylation (Scheme 12.38).
65
Doucet and co-workers later developed a green protocol for heterocycle
arylation using diethyl carbonate as solvent. The protocol was compatible
with a number of benzoxazole, benzothiazole furan, thiophene and pyrrole
derivatives (Scheme 12.39).
63
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