Chemistry Reference
In-Depth Information
N
X
+
X
-
1 - 2 mol% Pd/C
+
+
+ H
2
O
3
4
Zn
+
+
+ 3
+
ZnO
N
8 - 20 h
85 - 125
°
C
N
H
X = Cl, Br, I
1
2
3
conversion 68 - 75%
isolated yield
27 - 69%
Scheme 10.55
Pd(II)Cl
2
oxidised catalyst
2
Pd
0
reduced catalyst
Fig. 10.9
Tandem Oxidative and
Reductive Coupling of Benezene and
chlorobenzene.
2
Cl
nated with TBAB, prior to its addition to the reac-
tion mixture, significantly higher selectivity to biaryl
formation was observed [398].
Note that no monochlorobiphenyl was formed
in the coupling process. This suggests that attack of
phenyl radical on chlorobenzene is improbable and
that the aryl radicals couple because they are formed
on the catalyst surface. Again, this shows the impor-
tance of vacant Pd(0) sites. Remarkably, however,
we found that when a strong electron acceptor such
as pyridine is added to the system, cross-coupling
products are obtained in significant yields [399].
Interestingly, 2-phenylpyridine (Scheme 10.55)
was the only cross-coupling product obtained. As
could be expected, addition of a phase-transfer
catalyst to this system increased the conversion
but lowered the cross-coupling/homocoupling ratio
from 5 : 3 to 3 : 5.
Another important function of the phase-transfer
catalyst on palladium-catalysed reactions was real-
ised in exploring a related process where reductive
coupling of chlorobenzene catalysed by Pd(0) was
combined into a tandem reaction with the oxidative
coupling of benzene catalysed by Pd(II) (Fig. 10.9)
[400,401].
catalyst, higher hydrogen pressure would cause
hydride formation to predominate, thus favouring
the hydrogenolysis over coupling.
Phase-transfer agents may facilitate the removal of
HCl from the catalyst surface and transport of the acid
into the aqueous phase, where it is neutralised by the
base. Another conceivable function is the conse-
quence of the spontaneous adsorption of the quater-
nary ammonium or PEG derivatives to the carbon
support surface. This phenomenon, which was noted
by Tundo, creates an ionic (though lipophilic) thin
layer covering the palladium catalyst, which regulates
its microenvironment. This 'membrane' is permeable
to electron flow but limits the access of substrates or
solvent molecules (including hydrogen) to the palla-
dium metal. Thus, hydride formation is avoided and,
as a result, a higher coupling yield is observed. It is
conspicuous that the coupling reaction is feasible only
when the catalyst support is carbon (which is con-
ductive). With insulators such as silica or alumina
very poor yields were obtained. This unique role of
the phase-transfer agent was made known to the
authors recently by Professor C. Amatore.
Further support for this mechanism can be found
in the fact that when the Pd/C catalyst was impreg-
