Chemistry Reference
In-Depth Information
Cl
Cl
Cl
+
2.3 mol% THAC
+
2.8 mol% PdCl
2
+
AcOH/AcONa, 80-120
°
C
Cl
Selectivity
Conversion 35-50%
67%
12%
21%
Scheme 10.56
2C
6
H
6
C
6
H
5
-C
6
H
5
Pd
0
(homogeneous)
PdCl
2
(homogeneous)
We have examined the reaction of chlorobenzene
with benzene in acetic acid at 80-120°C in the pres-
ence of PdCl
2
and tetra-
n
-hexylammonium chloride
(THAC) catalysts (Scheme 10.56) [402]. The main
function of the ammonium salt was claimed to be
stabilisation of small palladium clusters and, conse-
quently, retardation of catalyst deactivation caused
by aggregation in the course of reduction of Pd(II)
to Pd(0) (Fig. 10.10). Indeed, stabilisation of Rh(0)
[403], Pd(0) [404] and Pd/Pt [405] nanoclusters by
quaternary ammonium salts has been discussed at
length and their role in regenerating PdCl
2
from
Pd(0) complexes using 1,2-dichloroethane has been
studied [406,407].
To summarise, the role of PTC in the Pd-catalysed
arene coupling reactions may be threefold: rapid
removal and neutralisation of HCl formed on the
catalyst; formation of a lipophilic ionic membrane
on the catalyst surface; stabilisation of catalytically
active Pd(0) nanoparticles.
A stimulating family of reactions combining
copper metal and cyclodextrin catalysis was reported
by Hirai
et al.
These authors developed a regioselec-
tive carboxylation of aromatic systems using carbon
tetrachloride and aqueous NaOH. A typical illu-
stration was the selective double carboxylation of
naphthalene (Scheme 10.57) to yield naphthalene
2,6-dicarboxylic acid [408].
The regioselectivity was ascribed to the conforma-
tion of the b-cyclodextrin complex with naphthalene
and with the intermediate 2-naphthalenecarboxylic
acid. The structure of the latter in aqueous alkaline
solution was determined by nuclear magnetic reso-
clustering
C
6
H
5
-C
6
H
5
2C
6
H
5
Cl
Pd
0
clusters
- stabilised by QX
aggregation
Pd
0
aggregates (inactive)
Fig. 10.10
Mechanism of the Palladium Catalyst Dectivation by
Aggregation.
nance spectroscopy using homonuclear Overhauser
enhancement. In a similar fashion, biphenyl was
carboxylated to 4,4¢-biphenyldicarboxylic acid [409].
Benzoic acid [410] and even benzene [411] were
transformed into terephthalic acid.
Another copper-catalysed reaction, the Ullmann
synthesis of triarylamines, was improved substan-
tially by employing a phase-transfer catalyst. Thus,
18-crown-6 was combined with copper powder and
solid KOH in xylene to catalyse the reaction of 4,4¢-
diiodobiphenyl with bis(3-methylphenyl)amine
(Scheme 10.58) [412]. The product
N
,
N
¢-diphenyl-
N
,
N
¢-di(3-tolyl)-
p
-benzidine (TPD) is a charge trans-
