163
5.1.2.2 Aromatization of Aromatic C-H Bonds
Biheteroaryl skeletons are widely distributed in many natural products, functional
materials, and dyes. In general, biheteroaryl compounds are synthesized by transition metal-catalyzed cross-coupling of heteroaryl halides or pseudohalides with
organometallic reagents. However, these methods require prefunctionalization of
substrates and produce stoichiometric amounts of hazardous by-products. Very
recently, Zhang’s group developed an H 2 -evolving cross-coupling between the
β-site of indoles/pyrrole and the α-site of N-heteroarenes, which could efficiently
generate N-containing biheteroarenes using an iridium (Ir) catalyst without the need
for an external oxidant [8] (Scheme 5.13).
N
N
R 2
R 1
R 3
+
R 4
Cu(OAc) 2
. H 2 O(0.3 equiv)
N
N
R 2
R 1
R 3
R 4
+ H 2
33
34
35
[Cp*Rh(CH 3 CN) 3 ](SbF 6 ) 2
(5 mol %)
CH 3 COOH, CH 3 CN, 100 °C
N
N
H 3 C
O
N
N
N
Cl
O
N
N
N
NC
O
N
N
N
O
N
N
N
O
N
Et
N
N
O
NH 2
Et
75 %
45 %
35 %
74 %
68 %
81 %
Scheme 5.12 ortho-Alkenylation between N-aryl-substituted 7-azaindoles and alkenes
5 Fourth-Generation Oxidative Cross-Coupling Reactions