Chemistry Reference
In-Depth Information
Ar
S
Ar
oxidative
addition
reductive
elimintion
I
L
n
Ir
I
Ar
Ar
S
Ir
III
I
L
n
Ir
III
L
n
Ag
2
CO
3
AgHCO
3
AgI
Ar
Ar
Ir
III
O
L
n
O
L
n
S
Ag
Ir
II I
O
O
H
S
O
H
O
Ag
Figure 12.24 Proposed CMD mechanism for C-H arylation.
[Ir(ppy)
2
(dtbpy)]PF
6
(5 mol%)
Pd(NO
3
)
2
(10 mol%)
26 W lightbulb
MeOH, 15 h, rt
DG
DG
[Ar
2
I]BF
4
+
Ar
H
19 examples
upto 94% yield
O
R
O
O
N
DG :
Me
Me
N
Me
N
H
H
2
N
O
O
O
MeO
MeO
N
Me
N
N
H
Me
Me
Scheme 12.91 Different directing group-assisted Ir/Pd co-catalyzed direct C-H
arylation.
arylation reactions were sluggish, typically requiring high temperatures (80-110
1C) over extended periods of time (8-24 h), likely due to an ionic mechanism
involving 2e
oxidation of a palladacycle intermediate by Ar
2
I
+
. In this new
approach, the Ir photocatalyst reverses the mechanism into a radical path-
way, leading to a high-yielding transformation under mild conditions by
generating Ar
from Ar
2
I
+
in situ. The major advantage of this method is that
the same conditions can be applied to a wide variety of directing groups at
room temperature.
Through a series of experiments with radical scavengers, the radical na-
ture of this reaction has been established with the involvement of Pd
II/IV
and
Ir
III/IV
cycles. Initially, Ir
3+
undergoes photoexcitation by visible light and the
resultant Ir
3+
* can generate Ar
from Ar
2
I
+
. The aryl radical (Ar
) can par-
ticipate in the catalytic cycle by oxidizing the cyclopalladated complex. Then
Ir
4+
further oxidizes the Pd
3+
center to Pd
4+
, from where reductive elimin-
ation affords the biaryl products with regeneration of Pd
2+
(Figure 12.25).
162
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