3
co-workers reported the same annulation reaction of
benzoic acids with alkynes using an electron-deficient
Rh(III) complex under ambient conditions (at RT–40
o
C), using AgNTf
2
and Cu(OAc)
2
·H
2
O as the
additives.
[9]
In 2013, Ison and co-workers developed a
relatively mild reaction conditions (60 °C, 24 h in
CH
3
OH), for the same annulation reaction using
(Cp*IrCl
2
)
2
catalyst and AgOAc as the oxidant.
[10]
Very recently, Sundararaju and co-workers reported a
nontoxic and air stable Cp*Co(CO)I
2
-catalyzed
annulation reaction of benzoic and acrylic acids with
alkynes for the synthesis of isocoumarins and pyrones.
In this reaction CuO was used as an oxidant and
2,2,2-trifluoroethanol (TFE) was used as a solvent.
[11]
In 2012, Ackermann and co-workers demonstrated
how the in-situ generated cationic Ru(II) catalyst
could be used for the annulation reaction of alkynes
with anilines, 5-aryl-1H-pyrazoles and oximes for the
efficient synthesis of indoles, pyrazolo[5,1-
a]isoquinoline and isoquinolines respectively
(Scheme 5).
[12]
Scheme 5. Ru(II)-Catalyzed synthesis of indoles,
pyrazolo[5,1-a]isoquinoline and isoquinolines
Li and co-workers developed an unprecedented
Ru(II)-catalyzed intramolecular annulation reaction of
amides with alkynes which proceeds via formyl
translocation for the synthesis of 1H-indole-3-
carbaldehydes (Scheme 6).
[13]
The reaction is assumed
to proceed via oxidative addition of Ru(II) in to the
N–CHO bond, followed by intramolecular
nucleophilic cyclization and reductive elimination
Scheme 6. Ru(II)-Catalyzed synthesis of 1H-indole-3-
carbaldehydes
pathways.
Cheng and co-workers developed a Ru(II)-
catalyzed C-H activation and annulation reaction of
benzaldehydes, amines and alkynes for the
regioselective synthesis of isoquinolinium salts
(Scheme 7).
[14]
A similar three-component reaction for
the synthesis of isoquinolinium salts was already
reported using expensive catalyst [{RhCp*Cl
2
}
2
] (2.0
mol %).
[15]
In the Ru-catalyzed reaction, the cationic
Ru complex first coordinates with the nitrogen atom
of the in situ generated imine. Then ortho-C-H
activation, alkyne insertion and finally reductive
elimination of the metal affords the isoquinolinium
salts. The metal is reoxidized to the active Ru(II) by
Cu(BF
4
)
2
.
Scheme 7. Ru(II)-
Catalyzed synthesis of isoquinolinium salts
Ackermann and co-workers developed a Ru(II)-
catalyzed C-H activation and annulation reaction of 2-
arylindole s with alkynes in the presence of catalytic
amount of Cu(OAc)
2
and air as the oxidants for the
synthesis of indolo[2,1-a]isoquinolines. They
extended the scope of the reaction for the annulation
of 2-arylpyrroles with alkynes which afforded good
yield of pyrrolo[2,1-a]isoquinolines (Scheme 8).
[16]
A
similar oxidative coupling reaction of 2-phenylindoles
with alkynes was reported by Miura and co-workers
for the synthesis of indolo[2,1-a]isoquinolines using
expensive [(Cp*RhCl
2
)
2
].
[17]
Scheme 8. Ru(II)-
Catalyzed synthesis of indolo[2,1-a]isoquinolines and
pyrrolo[2,1-a]isoquinolines.
Ackermann et al. described a Ru(II)-catalyzed
oxidative annulation reaction of electron rich
enamines with alkynes for the synthesis of pyrroles
utilizing air as the ideal oxidant (Scheme 9).
[18a]
Guan
and co-workers developed a Pd(OAc)
2
-catalyzed (5
mol %) annulation reaction of aryl enamides with
alkynes to afford triaryl- substituted pyrroles. The
reaction was performed using Cu(OAc)
2
(1.0 equiv)
as the oxidant and xantphos (10 mol %) as the ligand.
The electron-rich enamides which are challenging
10.1002/adsc.201800755
Accepted Manuscript
Advanced Synthesis & Catalysis
This article is protected by copyright. All rights reserved.
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