302
M. Benaglia
2
General Considerations
on the Immobilization Process
The main goals of the immobilization of a catalyst on a support are the
simplification of the reaction work-up, and the recovery and hopefully
the recycling of the precious chiral catalyst. But besides the recovery
and the recycle of the catalyst, other reasons may lead to realize a supported version of a catalytic species.
Catalyst instability can be a problem that may be tackled by developing an immobilized catalyst. Organic catalysts do exist that slowly
decompose under the conditions necessary for their reaction and release
trace amounts of by-products that must be separated from the products.
For instance, in photooxigenation reactions catalysed by porphyrin the
release of highly coloured materials derived from the catalysts made
product purification a real problem. This problem can be solved by immobilization of the catalyst because the decomposed material is also
supported and can thus be removed from the reaction medium during
the process of catalyst recovery.
Among the different methodologies developed for the oxidation of
unsaturated compounds with singlet oxygen, dye-sensitized photooxidation of triplet oxygen offers the advantage of producing by-productfree reaction mixtures and uses a catalytic amount of a substance that
promotes oxygen to its singlet state. However, the presence of the sensitizer, or its decomposition derivatives, can contaminate the reaction
products and make the purification step really troublesome. A polymerbound catalyst can offer the possibility of simple recovery and recycling, and easy product purification and isolation. In this context, soluble polymers have recently been the subject of intense research activity.
By carrying out the reaction in homogeneous solution a higher chemical
and stereochemical efficiency could be achieved than by using insoluble
polymer.
Among the soluble polymeric matrixes used, poly(ethylene glycol)s
(PEG) are the most successful (Janda et al. 2002). These polymers with
M w > 2,000 Da are readily functionalized, commercially available, inexpensive supports that feature convenient solubility properties: they are
soluble in many common organic solvents and insoluble in a few other
solvents, such as diethyl ether, hexanes, t-butyl-methyl ether. Therefore,
M. Benaglia
2
General Considerations
on the Immobilization Process
The main goals of the immobilization of a catalyst on a support are the
simplification of the reaction work-up, and the recovery and hopefully
the recycling of the precious chiral catalyst. But besides the recovery
and the recycle of the catalyst, other reasons may lead to realize a supported version of a catalytic species.
Catalyst instability can be a problem that may be tackled by developing an immobilized catalyst. Organic catalysts do exist that slowly
decompose under the conditions necessary for their reaction and release
trace amounts of by-products that must be separated from the products.
For instance, in photooxigenation reactions catalysed by porphyrin the
release of highly coloured materials derived from the catalysts made
product purification a real problem. This problem can be solved by immobilization of the catalyst because the decomposed material is also
supported and can thus be removed from the reaction medium during
the process of catalyst recovery.
Among the different methodologies developed for the oxidation of
unsaturated compounds with singlet oxygen, dye-sensitized photooxidation of triplet oxygen offers the advantage of producing by-productfree reaction mixtures and uses a catalytic amount of a substance that
promotes oxygen to its singlet state. However, the presence of the sensitizer, or its decomposition derivatives, can contaminate the reaction
products and make the purification step really troublesome. A polymerbound catalyst can offer the possibility of simple recovery and recycling, and easy product purification and isolation. In this context, soluble polymers have recently been the subject of intense research activity.
By carrying out the reaction in homogeneous solution a higher chemical
and stereochemical efficiency could be achieved than by using insoluble
polymer.
Among the soluble polymeric matrixes used, poly(ethylene glycol)s
(PEG) are the most successful (Janda et al. 2002). These polymers with
M w > 2,000 Da are readily functionalized, commercially available, inexpensive supports that feature convenient solubility properties: they are
soluble in many common organic solvents and insoluble in a few other
solvents, such as diethyl ether, hexanes, t-butyl-methyl ether. Therefore,