Chemistry Reference
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boronic acid couplings were carried out using the SPhos-containing
palladacycle 11.
121
It has also been demonstrated that a combination of
palladacycle with added free ligand may sometimes improve the reactions
compared with the use of the palladacycle alone.
122,123
Despite the potential
of the second-generation palladacycles, a couple of drawbacks limit their
scope for use in coupling reactions: first, they cannot be generated using
extremely bulky ligands, such as t-BuBrettPhos; and second, they decompose
in solution with extended periods of time.
The most recent addition to the palladacycle class of precatalysts is a
third-generation palladacycle that contains an OMs group in place of a
chloride (Scheme 3.40).
124
This improved family of catalysts display greater
solution stability than the chloride-containing catalysts.
The most recently reported palladacycles containing a mesylate ligand in
place of the chloride have been used in a number of applications, such as
cyanation of heteroaryl halides,
125
synthesis of aryl ethers,
126,127
monoaryl-
ation of ammonia with a wide range of aryl- and heteroaryl halides,
128
aryl-
ation of primary amides
127
and Negishi coupling reactions (Scheme 3.41).
129
H
2
N
Pd
OMs
Pd(OAc)
2
MsOH
Ligand
L
NH
2
NH
3
OMs
NH
2
Pd
OMs
IPAc, rt
99 %
PhMe, 65 °C
96 %
THF, rt
2
L=L1
12
13
L=L2
L=L3
14
OMe
OMe
OMe
Me
Me
MeO
P(
t
-Bu)
2
i
-Pr
Me
P(
t
-Bu)
2
i
-Pr
Me
PCy
2
i
-Pr
i
-Pr
i
-Pr
i
-Pr
i
-Pr
i
-Pr
L1
L2
L3
i
-Pr
Scheme 3.40
Synthesis of third-generation palladacycles.
Me
2
N
N
NH
2
3mol%
12
,3mol%
L1
Me
2
N
N
Cl
+NH
3
(3 equiv)
rt, dioxane
N
83 %
N
OMe
H
2
NO
MeO
1mol%
13
Cl
H
N
+
MeO
K
3
PO
4
,
t-
BuOH, 100 °C
N
H
MeO
OMe
O
N
H
90 %
OMe
O
2mol%
14
S
Cl
HO
S
+
N
N
Cs
2
CO
3
,PhMe,90°C
96 %
Scheme 3.41 Examples of monoarylation of ammonia, amide arylation and syn-
thesis of aryl ethers using palladacycle precatalysts.
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