Recoverable, Soluble Polymer-Supported Organic Catalysts
309
parison with that of their non-supported soluble counterparts. Since the
1960s soluble polymeric supports have been envisaged as possible alternatives to their insoluble counterparts for catalyst immobilization.
A quaternary ammonium salt was easily synthesized on a modified
MeOPEG, and this supported catalyst was shown to be an efficient and
recoverable promoter of several reactions carried out under PTC conditions. Catalyst 13 showed a catalytic activity that was similar to, or even
better than that of the non-supported catalysts (Benaglia et al. 2000).
Remarkably, the benzylation of phenol and pyrrole required only
0.01 equivalents of catalyst 13 to occur in –95% yield. Dichloromethane, in which the catalyst is readily soluble, was found to be the
organic solvent of choice, but the reaction could satisfactorily be carried out also in the absence of organic solvent. Generally the use of
solid/liquid conditions led to higher yields than those observed
under liquid/liquid conditions. The PEG-supported ammonium salt
could be recovered by precipitation and filtration, and recycled three
times to run the same or even a different reaction without any appreciable loss of catalytic activity. It is also worth mentioning that compound
13 compares favourably as catalyst to other quaternary ammonium salts
immobilized on insoluble polystyrene supports. The use of these catalysts generally required higher reaction temperatures and/or longer reaction times than those used here. In addition, solid supported phasetransfer catalysts required a preliminary, long conditioning time (up to
15 h) to ensure bead swelling and optimum accessibility of substrate and
reagent to the catalytic site. Finally, the high stirring rate necessary with
these catalysts resulted in extensive mechanical degradation of the polymer beads, that were difficult to recover by filtration. It seems possible
that the catalyst benefits from the involvement of the support, because
the polyethylenoxy chain of PEG can complex the alkaline cation of the
base helping the transfer of the HO
– counterion in the organic phase.
In order to increase the number and a proper spatial arrangement of
the catalytic sites the loading expansion of PEG was carried out exploiting the principles of dendrimer chemistry and led to the synthesis
of the PEG-supported tetrakis ammonium salt 14 (Tocco et al. 2002).
This catalyst displayed a higher catalytic efficiency than 13, while retaining the solubility properties peculiar of the PEG support, that allowed simple catalyst recovery and recycling by precipitation and filtra-
309
parison with that of their non-supported soluble counterparts. Since the
1960s soluble polymeric supports have been envisaged as possible alternatives to their insoluble counterparts for catalyst immobilization.
A quaternary ammonium salt was easily synthesized on a modified
MeOPEG, and this supported catalyst was shown to be an efficient and
recoverable promoter of several reactions carried out under PTC conditions. Catalyst 13 showed a catalytic activity that was similar to, or even
better than that of the non-supported catalysts (Benaglia et al. 2000).
Remarkably, the benzylation of phenol and pyrrole required only
0.01 equivalents of catalyst 13 to occur in –95% yield. Dichloromethane, in which the catalyst is readily soluble, was found to be the
organic solvent of choice, but the reaction could satisfactorily be carried out also in the absence of organic solvent. Generally the use of
solid/liquid conditions led to higher yields than those observed
under liquid/liquid conditions. The PEG-supported ammonium salt
could be recovered by precipitation and filtration, and recycled three
times to run the same or even a different reaction without any appreciable loss of catalytic activity. It is also worth mentioning that compound
13 compares favourably as catalyst to other quaternary ammonium salts
immobilized on insoluble polystyrene supports. The use of these catalysts generally required higher reaction temperatures and/or longer reaction times than those used here. In addition, solid supported phasetransfer catalysts required a preliminary, long conditioning time (up to
15 h) to ensure bead swelling and optimum accessibility of substrate and
reagent to the catalytic site. Finally, the high stirring rate necessary with
these catalysts resulted in extensive mechanical degradation of the polymer beads, that were difficult to recover by filtration. It seems possible
that the catalyst benefits from the involvement of the support, because
the polyethylenoxy chain of PEG can complex the alkaline cation of the
base helping the transfer of the HO
– counterion in the organic phase.
In order to increase the number and a proper spatial arrangement of
the catalytic sites the loading expansion of PEG was carried out exploiting the principles of dendrimer chemistry and led to the synthesis
of the PEG-supported tetrakis ammonium salt 14 (Tocco et al. 2002).
This catalyst displayed a higher catalytic efficiency than 13, while retaining the solubility properties peculiar of the PEG support, that allowed simple catalyst recovery and recycling by precipitation and filtra-