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H
R
COOH
COOH
RM
Pd
II
RM
Pd
II
COOH
R
1
R
2
COOH
R
1
R
2
H
R
O
Me
M=B(OH)
2
,BF
3
K,
B
Me
O
Scheme 12.12 Carboxyl functionalities as directing groups.
Pd(OAc)
2
(10 mol%),
Benzoquinone (0.5 equiv)
Ag
2
CO
3
(1equiv), K
2
HPO
4
t
BuOH, 100 °C, 3h
COOH
O
B
COOH
Me
+
Ph
R
R
Me
O
Ph
H
Scheme 12.13 Ortho-arylation of benzoic acids.
Pd(OAc)
2
(10 mol%)
Benzoquinone, K
2
HPO
4
O
2
(20 atm), 100 °C
t
BuOH, 24h
COOH
COOH
+
R
Ar BF
3
K
R
H
Ar
Scheme 12.14 Ortho-arylation of benzoic acids using Ar-BF
3
K.
R
1
R
1
R
2
R
2
Pd(OAc)
2
(10 mol%)
Benzoquinone, K
2
HPO
4
O
2
(20 atm), 110 °C
t
BuOH, 48h
OH
OH
+
Ar BF
3
K
R
R
O
O
Ar
H
Scheme 12.15 Coupling of substituted arylacetic acids with-Ar-BF
3
K.
found to be beneficial for C-H activation, but in the presence of Ag
+
the
catalytic system was found to be deactivated through competitive binding
with the carboxylate anion. This revised protocol was applicable to different
substituted phenylacetic acids (Scheme 12.15). a-Substituted phenylacetic
acids resulted in reasonable mono-selectivity due to steric crowding. In
addition, strongly electron-donating and -withdrawing substituents were
found to be compatible with these conditions, although the scope of this
protocol needed to be improved.
In general, direct arylation reactions have several elementary steps such as
C-H activation, transmetallation, reductive elimination and reoxidation,
which can be improved by using appropriate ligands.
31
Yu's group improved
the catalytic eciency with the help of specific ligands, which made the
reaction faster, afforded an improved scope of the reaction and gave higher
yields compared with the previous methods (Scheme 12.16).
According to proposed catalytic cycle,
31
the substrate first coordinates to
the Pd center, followed by C-H cleavage, which is facilitated due to the
bound amino acid ligand (Figure 12.6). Transmetallation and reductive
elimination give the desired product and generate Pd
0
, which is oxidized
back to Pd
II
(active species) in the presence of Ag
I
or O
2
/BQ.
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