Chemistry Reference
In-Depth Information
that the local conformation of the L-Pro residue suffers a dramatic change
going from nonaggregated states in which this residue is participating in a
strong intramolecular H-bond to aggregates and gels where the proline lone
pairs are oriented outwards available and accessible to form the enamines.
Remarkably, the conformational change associated to the self-assembly is
paralleled with an inversion of the stereoselectivity of the reaction compared to
a solution of analogue 69.
97
The replacement of organic solvents by water is highly convenient both from
an environmental as well as an economic point of view. Remarkably important
is also the fact that water is the biological solvent being the hydrophobic effect
the most important noncovalent interaction in biocatalysis. In this sense, the
study of L-proline and related catalysts in water is receiving increasing
attention.
98-101
Recently, we designed an amphiphilic hydrogelator derived from L-proline
(44).
102
This compound self-assembled in water forming hydrogels that
were tested for the direct aldol reaction between cyclohexanone and
4-nitrobenzaldehyde (Figure 5.13). Reagents were easily added on top of the gel
dissolved in toluene and the reaction was quantitatively completed after 24 h at
5 1C with high stereoselectivity (anti : syn 92 : 8, 88% ee). Moreover, the
catalytic hydrogel could be reused after decanting of the toluene phase for at
least three times with the same eciency and stereoselectivity. Remarkably, in
this system the hydrophobic effect plays a dual role: first as the driving force for
gelator self-assembly and, secondly, conducting reagents to the hydrophobic
catalytic sites.
In summary, molecular gels offer great possibilities in the field of catalysis.
Several of the features of these materials are very convenient from a practical
point of view: reversibility and sensitivity to external stimuli that allow control
d
n
1
r
3
n
g
|
7
3
.
Figure 5.13
Application of hydrogels of compound 44 as catalysts for the direct aldol
reaction.
