Chemistry Reference
In-Depth Information
O
PTC
R
O
5%Pd/C (3 mol%)
R
X
H
2
(1 atm), 50
°
C, KOH
(aq)
no PTC
R
PTC = phase-transfer catalyst
Scheme 10.4
8
In another approach, palladium nanoparticles
encapsulated in amino-terminated polyamidoamine
dendrimers [380] were extracted into toluene as
ion pairs with dodecanoic acid [381]. These self-
assembled inverted micelles where shown to be
catalytically active in a model hydrogenation reac-
tion. The extraction could be reversed at pH 2.
A related PTC mechanism was concluded in
the hydrogen peroxide oxidation of alcohols catal-
ysed by combined RuCl
3
/dodecyldimethylammo-
nium bromide (DDAB) catalysts [382]. Using TEM
imaging we proved that the DDAB molecules formed
an organised vesicular structure in the organic phase
and that the Ru catalyst resided in the aqueous layer
of these vesicles. The alcohol substrate and the
hydrogen peroxide molecules reached the reaction
site by diffusion, which was established as the rate-
determining step in the overall process.
was altered under phase-transfer conditions. Thus,
p
-dichlorobenzene reacted five times slower than the
ortho
-isomer when the phase-transfer catalyst was
present, whereas the reduction rate of both isomers
was identical in the absence of the phase-transfer
catalyst. Interestingly, Raney nickel, which is nor-
mally inactive in hydrodehalogenation reactions,
became active upon the addition of quaternary salts
[386].
A unique phenomenon was noticed when
chloroarylketones were hydrogenated in the above
systems. Without PTC, the major product was the
corresponding alkylbenzene (complete reduction of
both the halide and the ketone). In the presence of
Aliquat 336 reduction of the ketone was retarded,
resulting in the selective formation of arylketones
[387]. Similar results were obtained when Pt/C was
used instead of Pd/C [388], with the formation of
arylalkylcarbinol as the single product under alkaline
conditions (pH > 13.5) and arylalkylketone at lower
pH (Scheme 10.48) [389].
Tundo proposed three functions for the quaternary
ammonium compounds in these systems: rapid
removal of HX from the catalyst surface; formation
of a lipophilic film at the catalyst surface, generating
a unique microenvironment for the reaction; and
Modification of the transition metal coordination
sphere due to the proximity of the Q
+
cation. Inter-
estingly, he also reported a visible change in the
behaviour of the supported metal catalyst when the
phase-transfer catalyst was added. Although initially
the catalyst tended to distribute evenly in both the
aqueous and the organic phases, when it was added
all the catalyst was transported into the organic
phase. Apparently the carbon particles actually were
'extracted' into the organic phase, possibly via inter-
action of the phase-transfer catalyst with carboxylic
acid groups on the carbon surface.
8.3 Phase transfer in heterogeneous catalysis
The utilisation of phase-transfer agents in solid
transition-metal-catalysed reactions was demon-
strated by Tundo. In a series of papers published
between 1993 and 1999, his group studied the effects
of phase-transfer agents on the heterogeneous Pd/C-
catalysed hydro-dehalogenation by hydrogen and
hydrogen donors. Thus 1,2,4,5-tetrachlorobenzene
was reduced rapidly to benzene at 50°C in the pres-
ence of Pd/C using sodium phosphite as the hydro-
gen donor [383]. Aliquat 336 was found to enhance
the reaction rate. Some intriguing observations were
made in systems where dihydrogen was used as
the reducing agent [384]. Chloroethylbenzenes (all
three isomers) were reduced 50 times faster in the
presence of phase-transfer catalysts. On the other
hand, bromotoluenes reacted five times
slower
upon
the addition of Aliquat 336 [385]. The selectivity also
