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Ar-Cl
no reaction
elevated
temperature
i
Pr
Me
[RuCl
2
(
p
-cymene)]
2
Ru
MesCO
2
O
i
Pr
Me
O
N
4
Mes
Ru
MeO
O
2
CMes
N
cyclometallation
Figure 12.21 Mechanistic evidences towards pathway B.
N
N
R
[R u]
H
P
O
R
[Ru]
O
R
H
O
Figure 12.22 Probable transition states of ligand-assisted deprotonation.
workers
117
on a concerted metallation-deprotonation mechanism (CMD)
provide strong support for this fact.
98,151-153
Based on other literature pre-
cedents, in most cases cyclometallation is likely to proceed via an oxidative
addition. Unlike Ru
0
, cyclometallation with Ru
II
does not proceed via an
oxidative addition. The deprotonation of the ortho C-H bond facilitates the
metallocycle formation without any variation of the oxidation state.
The base-assisted C-H bond metallation proceeds via the formation of a five-
or six-membered ring (Figure 12.22).
The previously used tertiary phosphine or NHC were not compatible with
various scaffolds and also worked poorly.
154
In contrast, the Ru complex
derived from a bifunctional sterically hindered secondary phosphine oxide
(SPO)-based ligand was found to be more e
cient. Recent reports by Ack-
ermann and co-workers showcased carboxylates as potential co-catalysts
under phosphine-free reaction conditions.
154,155
12.5 Iridium-Catalyzed Reactions
Reports of Ir-catalyzed direct arylation via a C-H activation mechanism are
scarce. For ease of discussion, they can be classified into two groups.
12.5.1 C-H Arylation Without Selectivity
In the first report of non-selective C-H arylation, an Ir-hydride complex was
found suitable to catalyze the direct arylation of simple unsubstituted are-
nes.
156
Use of a strong base (KO
t
Bu) restricted the application of the method
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