Recoverable, Soluble Polymer-Supported Organic Catalysts
311
tion temperature at –78 °C and prolonging the reaction time to 60 h, the
top level of stereoselectivity obtained with the non-supported catalyst
could not be matched. PEG was considered to be responsible, at least
in part, for these results because of the following effects. By increasing
the polarity around the catalyst, PEG prevents the formation of a tight
ion pair between the enolate and the chiral ammonium salt, the formation of which is regarded as crucial for high stereocontrol. Moreover,
PEG enhances the solubility of the inorganic cation in the organic phase
leading to a competing non-stereoselective alkylation occurring on the
achiral cesium enolate. To check the validity of these hypotheses control
experiments were carried out by performing the reaction with the nonsupported catalyst in the presence of the bis-methylether of PEG 2000 .
The observed ee was 65%, a value that was in good agreement with
that observed with catalyst 16 but largely inferior to the >90% ee easily
achieved with the non-supported catalyst (Scheme 5).
4
Chiral Organic Catalysts
An organic catalyst has been recently defined as ‘organic compound of
relatively low molecular weight and simple structure capable of promoting a given transformation in substoichiometric quantity’ (Benaglia et
al. 2003). In this context chiral organic catalysts may be seen as a simplified version of enzymes, from which they are conceptually derived
and to which they are often compared. Even if they rarely display the remarkable selectivity peculiar of enzymes, in general metal-free catalysts
are more stable than bio-catalysts and show a larger field of application
under a variety of conditions unsustainable by enzymes; furthermore
organic catalysts may be readily immobilized on a support with the aim
of facilitating catalyst recovery and recycling.
4.1
Proline-Derived Catalysts
Enantioselective catalysis promoted by enantiomerically pure amines
(aminocatalysis) is the subject of considerable interest due to the ubiquitous presence and ready availability of these compounds in the chiral
pool. In this context amino acids have always played a key role. One of
the most successful and versatile chiral organic catalysts, proline, was
311
tion temperature at –78 °C and prolonging the reaction time to 60 h, the
top level of stereoselectivity obtained with the non-supported catalyst
could not be matched. PEG was considered to be responsible, at least
in part, for these results because of the following effects. By increasing
the polarity around the catalyst, PEG prevents the formation of a tight
ion pair between the enolate and the chiral ammonium salt, the formation of which is regarded as crucial for high stereocontrol. Moreover,
PEG enhances the solubility of the inorganic cation in the organic phase
leading to a competing non-stereoselective alkylation occurring on the
achiral cesium enolate. To check the validity of these hypotheses control
experiments were carried out by performing the reaction with the nonsupported catalyst in the presence of the bis-methylether of PEG 2000 .
The observed ee was 65%, a value that was in good agreement with
that observed with catalyst 16 but largely inferior to the >90% ee easily
achieved with the non-supported catalyst (Scheme 5).
4
Chiral Organic Catalysts
An organic catalyst has been recently defined as ‘organic compound of
relatively low molecular weight and simple structure capable of promoting a given transformation in substoichiometric quantity’ (Benaglia et
al. 2003). In this context chiral organic catalysts may be seen as a simplified version of enzymes, from which they are conceptually derived
and to which they are often compared. Even if they rarely display the remarkable selectivity peculiar of enzymes, in general metal-free catalysts
are more stable than bio-catalysts and show a larger field of application
under a variety of conditions unsustainable by enzymes; furthermore
organic catalysts may be readily immobilized on a support with the aim
of facilitating catalyst recovery and recycling.
4.1
Proline-Derived Catalysts
Enantioselective catalysis promoted by enantiomerically pure amines
(aminocatalysis) is the subject of considerable interest due to the ubiquitous presence and ready availability of these compounds in the chiral
pool. In this context amino acids have always played a key role. One of
the most successful and versatile chiral organic catalysts, proline, was