Chemistry Reference
In-Depth Information
H
2
2H
+
Pd
0
Pd
0
Pd
0
Pd
+2
C
6
H
5
Cl
fast
disproportionation
-
metastable
clusters
Cl
Pd
+
Pd
0
radical anion
Pd
+
Pd
+
-
.
C
6
H
5
Cl
Cl
-
.
+ 2Cl
-
+ 2Cl
-
2
Cl
2
radical anion
Pd
0
Pd
+2
Pd
0
Pd
+2
Pd
0
Pd
+2
Fig. 10.
8
Mechanism of Pd/C catalysed
Biaryl Formation.
>110
°
C, P
H2
< 1 bar
PTC, high catalyst loading, high basicity
Cl
Pd/C
Base, H
2
<50
°
C, P
H2
> 10 bar
low catalyst loading, low basicity
PTC = phase-transfer catalyst
Scheme 10.52
H
2
+ Pd
0
Pd
+2
(H
-
)
2
H
2
+ Pd
+2
Pd
0
+ 2H
+
Pd
+2
(H
-
)
2
+ C
6
H
5
Cl
Pd
+2
(H
-
)(Cl
-
) + C
6
H
6
Scheme 10.53
Scheme 10.54
However, hydrogen also can adsorb on the
reduced palladium to form palladium hydride which
in turn may reduce chlorobenzene to benzene
(hydro-dehalogenation) (Scheme 10.54).
The resulting PdCl
2
may release HCl or reduce
another chlorobenzene molecule. Addition of a base
should enhance the reactions in which the products
were Pd(0) and HCl. The significance of this lies in
the greater sensitivity of the coupling reaction to the
effective concentration of available Pd(0) sites. The
coordination of a single chlorobenzene molecule to
one catalytic site should suffice for hydrogenolysis,
whereas coordination of two chlorobenzene mole-
cules to two neighbouring sites is required for the
coupling reaction. The fact that higher Pd/C loading
increased the coupling yield also supports this argu-
ment. Similarly, it could be asserted that although
hydrogen is necessary for regeneration of the Pd(0)
