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[Rh(coe)
2
Cl
2
]
2
(2.5 mol%)
[p-(CF
3
)-C
6
H
4
]
3
P(15mol%)
CsOPiv (1.4 equiv)
dioxane, 120 °C, 18-36h
+
ArI
1.2 equiv
Ar
H
H
R
R
Me
H
Piv
N
H
N
H
H
Me
0%
82%
78%
Scheme 12.64 Rh-catalyzed arylation of free NH-indole.
Ph
L
PivO
Rh
OPiv
H
slow step
H
HOPiv
H
PhI
CsOPiv
L
L
Ph
OPiv
[Rh(coe)
2
Cl]
2
Rh OPiv
Ph
Rh
L
-CsI
PivO
H
3
L
CsI
Ph
PhI
CsOPiv
RhL
2
(OPiv)
H
Figure 12.18 Proposed mechanism of the Rh-catalyzed arylation of free NH-indole.
According to the proposed mechanism, [Rh(coe)
2
Cl
2
]
2
, in the presence of
aryl iodide and cesium pivalate, generates complex 3, which forms a
p-complex with indole. Pivalate assisted the C-H bond dissociation of indole
and the arylated indole product was formed after a subsequent reductive
elimination (Figure 12.18).
92
Kinetic studies revealed a first-order depend-
ence on complex 3 and indole and an inverse first-order dependence with
respect to the ligand. A significant kinetic isotope effect was observed
(k
H
/k
D
¼
3.0), which suggests that the C2 metallation is likely to be the rate-
limiting step (Figure 12.18).
92
Nyori and co-workers reported the first Rh-catalyzed bis-heteroarene for-
mation through direct C-H activation.
93,94
They developed an Rh catalyst
having p-accepting ligands, which reduces the electron density on the metal,
facilitating electrophilic metallation of electron-rich heteroarenes. This
protocol involves (hetero)aryl iodides with simple heterocycles as coupling
partners (Scheme 12.65). Based on some preliminary observations, an Rh
I
/
Rh
III
catalytic cycle was proposed.
Rh-catalyzed C-H arylation through a radical pathway was reported by
Proch and Kempe in 2007.
95
The active catalyst was generated in situ from
[bis(2-pyridyl)amino]diphenylphosphane and 0.5 equiv. of [Rh(COD)Cl]
2
precursor (Scheme 12.66).
95
This bimetallic Rh catalyst was found to be
effective in arylating unactivated simple arenes with aryl iodides, bromides
and even chlorides.
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