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copper iodide as catalyst, with DBU, at near reflux temperatures of toluene.
The reaction is currently limited to aryl iodides.
6.6 Atypical Metal Catalysts for Cross-Coupling
Reactions of Carbon-Sulfur Bonds
As is the case for most transition metal-catalyzed cross-coupling reactions,
palladium is the metal of choice. This also holds true for metal-catalyzed
reactions to form carbon-sulfur bonds However, several groups have re-
placed palladium catalysts with catalysts composed of manganese, cobalt,
nickel and rhodium.
6.6.1 Manganese- and Cobalt-Catalyzed Formation of Diaryl
and Alkyl Aryl Thioethers
Catalytic manganese chloride in the presence of phenanthroline ligands
has been reported for the formation of carbon-sulfur bonds.
77
While only aryl
iodides were successfully coupled, both aryl and alkyl thiols could be employed.
Some examples of diaryl thioether syntheses are shown in Scheme 6.18. The
yields of aryl thioethers were typically much higher than those for alkyl aryl
thioethers. Similarly, a cobalt catalyst in the presence of stoichiometric zinc has
been shown to couple carbon-sulfur bonds of aryl iodides and thiols.
78
Nearly
all reported thiol substrates were aromatic, with only a few examples of re-
actions with alkyl thiols, and representative examples are shown in Scheme 6.19.
6.6.2 Nickel-Catalyzed Formation of Diaryl or Alkyl Aryl
Thioethers
The use of nickel catalysts to promote the formation of diaryl of alkyl aryl
thioethers has advanced in recent years and numerous nickel complexes
20 mol % MnCl
2
20 mol %
27
Cs
2
CO
3
, to
lue
ne, 135 °C
S
+
Ar
1
SH
ArI
Ar
1
Ar
N
N
27
Ar
1
Ar
Yield (%)
m
-CH
3
C
6
H
4
C
6
H
5
99
o
-CH
3
C
6
H
4
C
6
H
5
91
p
-CH
3
OC
6
H
4
C
6
H
5
99
o
-CH
3
OC
6
H
4
C
6
H
5
80
p
-CH
3
OC
6
H
4
p
-CH
3
OC
6
H
4
88
o
-(HOCH
2
)C
6
H
4
p
-ClC
6
H
4
76
Scheme 6.18
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