159
product 16 and cobalt species 17 were then generated through elimination. The following aza-Michael addition of 16 gave the spirocyclic product 11. Finally, H 2 gas
was released by the reaction of species 17 with HOTf, and the active cobalt species
13 was regenerated.
5.1.1.2 Formation of O-Containing Heterocyclic Compounds
The benzofuran ring is an important structural motif in bioactive molecules and
natural products. Existing strategies to synthesize benzofurans are limited in their
scope and sustainability. In 2016, Liu’s group reported a dehydrogenative coupling
of intramolecular C(sp
2
)-H bonds and phenolic hydroxyl groups through palladium
metal (Pd(0))-catalyzed C-H bond functionalization, which could synthesize benzofurans from ortho-alkenylphenols in one step along with the release of H 2 gas as the
only by-product (Scheme 5.8) [5].
N
H
H
R 2
R 1
[MnBr(CO)5]
NH 2
R 2
R 1
Br
CO
(detected)
R 1
[Mn(CO)4]
N
R 2
R 3 R 4
H
1
6
R 1
Mn(CO)4
N
R 2
H
R 4
R 3
5
CO
4
[HMn(CO)4]
7
1
N
[Mn(CO)4]
H
R 2
R 1
H
8
H 2
(detected)
R 1
Mn
(CO)4
N
R 2
H
3
CO
R 3
R 4
2
isolated
H
Scheme 5.5 Proposed mechanism of the annulation between imines and alkynes
Scheme 5.6 Ru-catalyzed annulation of imines and alkynes
5 Fourth-Generation Oxidative Cross-Coupling Reactions