Chemistry Reference
In-Depth Information
the metal catalyst) from the aqueous to the organic
phase or from the organic to the aqueous phase.
Other options include transfer from the organic
phase into other immiscible phase, e.g. fluorous
phase [288], supercritical CO
2
[289] or ionic liquids
[290].
In the aqueous to organic transfer process the
phase-transfer agents play a dual role, extracting
both the metal complex anion
and
the reagent
(either anion or neutral) into the organic phase
where the reaction takes place. Numerous reactions,
such as hydrogenation, oxidation, carbonylation,
isomerisation, cyanation, desulfurisation and vinyla-
tion, were catalysed in this manner. The subject was
surveyed thoroughly by Goldberg [291] and Amer
[292]. The opposite phase-transfer process, namely
organic to aqueous transfer (sometimes referred
to as 'inverse PTC' or 'counter PTC' [293]), is based
on water-soluble ligands that, upon coordination,
solubilise a transition metal catalyst in water. This
facilitates the separation of the catalyst from the
reaction products. This concept was the basis of the
Ruhrchemie/Rhone Poulenc propene hydroformyla-
tion process catalysed by HRh(CO)(TPPTS)
3
(TPPTS
= triphenylphosphine trisulfonate) [294]. The area of
aqueous biphasic catalysis has been reviewed exten-
sively by Cornils [295], Joo [296], Kalck [297],
Herrmann [298], Sinou [299], Hanson [300],
Nomura [301], Bertoux [302] and Driessen-Holscher
[303]. Sulfonated triphenylphosphines are the most
common transfer agents/ligands [304] but other
hydrophilic ligands also are reported. Typical ex-
amples are carbohydrate-substituted phosphines
[305], b-cyclodextrin-modified phosphines [306],
xanthene-based diphosphines [307], dibenzofuran-
based phosphines [308], aminomethylated phos-
phines [309], carboxylated phosphines [310] and
alkylated surface-active phosphines [311]. Other
instances include polymeric hydrophilic ligands
[312] such as polyethylene glycol [313], polyacrylic
acid [314], PEG-polystyrene [315], polyethyl-
eneimine [316], polypentenoic acid combined with
bis[2-(diphenylphosphino)ethyl]amine [317] and
even human serum albumin [318].
A major shortcoming of this aqueous biphasic
catalysis was the low solubility of most organic sub-
strates in the aqueous phase where the catalyst
abides. The method therefore was particularly attrac-
tive to water-soluble or partially soluble substrates
[319]. Numerous techniques were proposed in
the literature to enhance the mass transport of
organophilic substrates into the aqueous phase or,
alternatively, to force the catalyst to reside near the
interface. For example, the addition of small
amounts of triphenylphosphine ('promoter ligand')
to the RhCl
3
/TPPTS system maintained the active
catalyst at the interface, enhancing the reaction rate
by a factor of 10-50 [320]. Cyclodextrins [321], sur-
factants [322], or co-solvents such as ethanol [323],
ethylene glycol [324], and others [325] affected the
transport of organophilic substrates to the aqueous
phase. Modification of the phosphane backbone by
attaching it to various ampiphilic groups (as above)
also was beneficial. Two typical recent examples
were the phosphacalix[4] arene ligands [326] pro-
posed by Shimizu and the surface-active xantphos
derivatives introduced by van Leeuwen [327]. Both
types of compounds functioned as inverse phase-
transfer catalysts in addition to their role as ligands,
the first via the formation of cage compounds and
the second through the formation of vesicles that
enhance the solubility of the substrate in the
aqueous phase. Both papers reported high yields of
the hydroformylation of 1-octene. Supporting the
aqueous phase on a solid carrier such as silica [328]
or glass beads [329] also had a beneficial effect on
the reaction rate.
Of particular interest are the non-ionic ampiphilic
phosphine ligands of the general formula P[
p-
C
6
H
5
O(CH
2
CH
2
O)
n
H]
3
(PETPP), with
n
~6, which
were prepared by the ethoxylation of tris(
p
-
hydroxyphenyl)phosphine. The complex PETPP/
RhCl
3
was insoluble in solvents such as toluene at
room temperature, but upon heating to a critical
solution temperature the complex dissolved and the
system became homogeneous. Upon cooling, the
catalyst precipitated and was separated readily by
decantation. This concept was termed by Fell [330]
and Jin [331] as 'thermoregulated phase-transfer
ligands and catalysis' (TRPTC). It was applied mainly
in hydroformylation of higher olefins, which are
insoluble in water [332]. In the hydroformylation of
1-dodecene a conversion of 95.8% and a yield of
93.7% were reported [333].
Note that each of the above methods introduced
technical complications, which rendered it impracti-
cal for industrial applications. Currently, the only
industrial application is the RC/RP hydroformylation
of propene using Rh/TPPTS catalyst in a biphasic
system.
