Chemistry Reference
In-Depth Information
concentration in the aqueous phase. Consequently,
the distribution coeffecient of PNO between the
phases is a significant factor in the overall rate equa-
tion [172]. The rate was determined to depend
strongly on the stirring rate up to 1200 rpm. The pH
is also a critical factor and when it exceeded pH 12
the process was complicated by competing hydroly-
sis of the acyl halide.
A related exchange reaction of benzoic and
chlorobenzoic anhydrides in the chloroform phase
also was catalysed by PNO [173].
Another reaction prone to inverse PTC is the acy-
lation of water-soluble amines (such as amino acids)
by acyl halides. Hippuric acid was synthesised from
benzoyl chloride and glycine in the presence of
DMAP catalyst [174]. An identical procedure was
applied for the protection of the amino group in
serine [175]. The mechanism of the DMAP- and
PNO-catalysed acylation of sodium glycinate [176]
and other amino acid salts [177] by benzoyl chloride
was studied by Jwo.
Sulfide-based inverse PTC was proposed by
Shaffer [178] in the polythioetherification of 1,8-
dibromooctane with sodium sulfide. A similar
mechanism was inferred by Takeishi [179] for the
thiocyanation of benzyl chlorides.
Cyclodextrins and modified cyclodextrins are
another notable family of inverse phase-transfer
catalysts. These were utilised mainly in biphasic
transition-metal-catalysed reactions. Metal-free
cyclodextrin-induced inverse PTC systems are rare
[180]. Trotta has demonstrated that lipophilic esters
are hydrolysed better using cyclodextrins than in the
normal phase-transfer system [181]. Sodium boro-
hydride reacted regioselectively in the ring-opening
reaction of epoxides in the presence of cyclodextrins
[182]. Oxidation of secondary [183] and benzylic
alcohols [184] and the haloform reaction [185] with
hypochlorites were claimed to take place effectively
in the presence of b-cyclodextrin. The latter, none-
theless, was found to be allied with the pH lowering
effect of the cyclodextrin rather than to the inverse
PTC mechanism [186].
The third mode of inverse PTC is based on water-
soluble surfactants. These allow the transport of the
lipophilic substrate into the aqueous phase via the
formation of micellar aggregates. This concept was
introduced by Roque
et al
., who have demonstrated
it in the sodium borohydride reduction of organo-
philic ketones [187] and in the hydrogen peroxide
epoxidation of unsaturated ketones under basic con-
ditions [188]. Optimal reaction conditions were
developed for the latter process [189]. Further
research of the mechanism resolved that there are
two competitive catalytic processes involved: an
interfacial catalysis and an inverse PTC. The first
process is predominant when a very low concen-
tration of surfactant (dodecyltrimethylammonium
bromide, DTAB) is used with rapid stirring and the
second process prevails with high surfactant concen-
tration and slow stirring. As expected, the rate of the
interfacial catalysis process is less sensitive to the sur-
factant concentration and it reaches a maximum at
40 mM (when the interface is apparently saturated
with DTAB), whereas in the inverse PTC process the
rate still increases even at 500 mM [190]. The poten-
tial of applying a normal phase-transfer catalyst in a
microemulsion system was examined by Hager
et al
.
[191].
4 Three Liquid Phases and
Triphase Catalysis
The formation of a third liquid phase has been
observed in the past in several PTC systems [192]. In
addition to unique catalytic effects observed upon
the presence of the third liquid phase, this phenom-
enon can be utilised for simple separation and recy-
cling of the catalyst. We have realised [193] that the
formation of a third phase is unique to the TBAB/
aqueous NaOH/apolar organic solvent mixture
and it is not observed with tetraethyl-, tetrapropyl-
or tetrapentylammonium bromides. In the toluene/
water/NaOH/TBAB system the third liquid phase
was assayed to be composed of (w/w) 44.4%
toluene, 2.2% water, 52.6% TBAB and only 50 ppm
OH
-
. In the elimination reaction of phenethyl
bromide to styrene in an aqueous NaOH/TBAB
mixture, a fivefold increase in the reaction rate was
observed at the point when the third phase was
formed. Conversely, when the third phase disap-
peared, e.g. at higher temperature or with more
concentrated base, a significant rate decrease was
noticed. Microscopic examination of a stirred three-
phase system revealed that the continuous phase is
the organic phase with aqueous phase droplets as the
dispersed phase surrounded by the third catalyst-rich
phase.
The ternary phase diagram toluene/40% aqueous
NaOH/toluene at 44°C is presented in Fig. 10.1
