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thioamide does not greatly affect the overall yield of the reaction in most
cases. Only aryl bromide coupling partners were reported to participate in
the reaction.
6.5 Copper-Catalyzed Cross-Coupling Reactions of
Carbon-Sulfur Bonds
Although palladium provides the most general catalyst systems for cross-
coupling reactions of carbon-sulfur bonds, the development of more cost-
ecient copper catalysts for this process has increased. A primary limitation
with copper catalysts is their inability to effect cross-coupling reactions with
aryl chlorides and the lower reaction yields obtained with aryl bromides.
Representative copper-catalyzed cross-coupling reactions to form alkyl aryl
thioethers and diaryl thioethers are shown in Scheme 6.16.
75
The catalyst is
composed of copper iodide and excess ethanediol. Aryl iodides are partnered
with aryl thiols in high yields regardless of the functional group on the
aromatic ring of the iodide. However, lower yields were seen with more
hindered alkyl thiols.
Carbon disulfide has been used as a thiol equivalent in the copper-cata-
lyzed formation of symmetrical diaryl thioethers. Several examples of this
process are illustrated in Scheme 6.17.
76
The reaction proceeds with catalytic
5mol%CuI
2equiv HO(CH
2
)
2
OH
ArI
+
RSH
ArSR
K
2
CO
3
,iPrOH,80°C
Ar
R
Yield (%)
m
-BrC
6
H
4
C
6
H
5
92
o
-CO
2
HC
6
H
4
p
-CH
3
OC
6
H
4
88
o
-NH
2
CH
2
C
6
H
4
p
-CH
3
OC
6
H
4
93
3,5-(CH
3
)
2
C
6
H
3
cyclo-C
6
H
11
71
p
-NH
2
C
6
H
4
m
-CH
3
C
6
H
4
91
p
-CH
3
OC
6
H
4
Bn
91
Scheme 6.16
10 mol % CuI
S
ArI
+
CS
2
Ar
Ar
DBU, toluene, 100 °C
Ar
Yield (%)
C
6
H
5
85
o
-(CH
3
)
2
NC
6
H
4
78
o
-CH
3
OC
6
H
4
83
o
-OHC
6
H
4
81
o
-BrC
6
H
4
67
p
-ClC
6
H
4
70
Scheme 6.17
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