Chemistry Reference
In-Depth Information
I
OMe
OMe
Pd(OAc)
2
(5%)
K
2
CO
3
, QX, H
2
O, 50
+
°
C, 2 h
O
O
Scheme 10.47
A further development in the field of transition
metal biphasic catalysis was re-extraction of the
hydrophilic metal complex from the aqueous to the
organic phase using conventional PTC techniques.
This idea originated at Ruhrchemie [334] for the
separation of sulfonated phosphines from the sul-
fonation mixture using a fatty amine/toluene extrac-
tion system. It was realised later that the new
multi-ion-paired phosphines could be the basis for a
new family of organophilic trifunctional ligands.
These ligands then could affect the hydroformylation
of higher olefins [335]. An additional advantage of
these large-size 're-immobilized ligands' was that
they could be separated from the reaction mixture
using membrane technology. A pilot experiment
demonstrating the double hydroformylation of dicy-
clopentadiene using the Rh/TPPTS/distearylamine
catalyst system followed by membrane separation of
the catalyst has been described by Bahrmann [336].
A natural extension of this technology is the devel-
opment of chiral polysulfonated phosphines [337],
which were applied in asymmetric hydroformylation
[338] and hydrogenation [339] reactions. Phos-
phines modified with carbohydrates also were pro-
posed as enantioselective catalysts [340].
Rhodium-catalysed hydroformylation is by far the
major field where water-soluble metal catalysts were
applied. Other aqueous catalytic reactions are the
Wacker oxidation [341], cleavage of allylic substrates
[342], hydrocarboxylation of higher a-olefins [343]
catalysed by Pd/cyclodextrins and hydrogenation
catalysed by Ru/cyclodextrins [344]. Some recent
studies ascertained the molecular recognition
between the cyclodextrins and the water-soluble
phosphane ligand [345] and also between the
cyclodextrins and the organic substrate. In the latter
case, the deprotection of allyl carbonates was ac-
celerated 300-fold when a specifically modified
cyclodextrin was used as a co-catalyst [346].
One of the major contributions of PTC to organic
synthesis is the dramatic effect that quaternary
ammonium salts had on the Heck reaction. This phe-
nomenon was reported first by Jeffery in 1984 [347].
A typical example can be found in the arylation
of methyl acrylate by iodobenzene catalysed by
Pd(OAc)
2
under basic conditions (Scheme 10.47).
Although stoichiometric quantities of ammonium
salts usually are needed, the impact is remarkable.
The presence of phase-transfer agents allowed for
the application of water as the sole solvent for the
reaction and spares the need for phosphine ligands,
which are otherwise critical for stabilisation of the
palladium catalyst [348]. Thus, under the conditions
described in Equation 10.47, yields of 92% of methyl
cinnamate were obtained with TBAB or TBAC (70%
yield with TBAHS).
This acceleration of Heck-type reactions was
attributed by Jeffery to the assistance of tetraalky-
lammonium salts in the regeneration of the zerova-
lent palladium catalyst via reductive elimination of
HX (a key step in the catalytic cycle in all the various
routes suggested for the Heck reactions) [349]. Three
possible PTC mechanisms were proposed for this
dehydrochlorination in the presence of a base:
extraction; interfacial; and nucleophilic, based on
hydrogen bonding [350]. The appropriate selection
of Pd/base/QX combination was found to direct
the arylation of 2,3-dihydrofuran [351] and of
vinyltrimethylsilane [352].
Jeffery's conditions were extended by Nguefack to
an interesting modification of the Heck reaction—
namely, the coupling of aryl halides with terminal
acetylenes [353]. Arylation of thiophenes also was
carried out using the same protocol [354,355].
Poly(ethylene glycols) also were used as phase-
transfer agents in the Heck reaction [356].
A more reactive catalytic system was developed
by Reetz [357], who combined Ph
4
PCl with
PdCl
2
(MeCN)
2
to affect the coupling of 4-
chlorobenzaldehyde with styrene in very high yields.
In a later work from the same group, phosphane-free
palladium nanoparticles were stabilised by quater-
nary ammonium salts and applied in Heck and
Suzuki type reactions [358]. The authors claimed
that palladium colloids were the actual catalysts
whenever Jeffery's conditions applied.
