5In spite of the inherent advantages of reuse and stability of immobilized chiral catalysts, they are frequently not enough to justify the effort necessary to immobilize the chiral complexes, unless additional advantages are found. When chiral catalysts are immobilized on solids with surface and pores of well controlled size, shape and dimensionality, the solid may act as a nano-reactor reducing the free movement of reagents, intermediates and, mainly, transition states. In this way 10 relative energies of the transition states leading to the different stereoisomers may be modified and, as a consequence, the stereochemical result of the reaction is modified when compared with the obtained in solution. The use of support effects to improve and/or change the stereochemical result of enantioselective reactions is currently emerging as an area of interest, but still with a limited number of examples. A better comprehension of the support influence will be needed in 15 order to allow, in the near future, the design of chiral ligands better adapted to this strategy.
Chiral bis(oxazolines) are readily dialkylated in the methylene bridge, opening the way to immobilization at that position, keeping the C 2 symmetry of the chiral ligand. Bis(oxazolines) functionalized with two allyl or vinylbenzyl groups are easily grafted onto mercaptopropylsilica. Another approach to immobilization is the polymerization of the ligands bearing vinylbenzyl groups to yield insoluble polymers. The Cu(OTf) 2 complexes of the immobilized ligands promote the enantioselective cyclopropanation reaction between styrene and ethyl diazoacetate. The results depend on the nature of the support and the method of immobilization. With regard to the type of solid, the best results, which are similar to or even better than those obtained with the corresponding dibenzylated homogeneous catalysts, are obtained with homopolymers. With regard to the bis-(oxazoline), that bearing indan groups leads to good results both onto silica and polymers, whereas with the ligand bearing tert-butyl groups good enantioselectivities are only obtained with homopolymeric catalysts. Some of the heterogeneous catalysts can be easily recovered and reused, as much as five times, with the same yield and stereoselectivities.
Bis(oxazoline)-copper complexes, immobilised by electrostatic interactions on Laponite and Nafion/silica nanocomposites, have been tested as catalysts in the cyclopropanation of styrene with ethyl diazoacetate. Several factors play a decisive role in the outcome of the reaction. Firstly, the nature of the solid counterion is important, with perfluorosulfonic solids being the best in this regard. Secondly, the nature of the solvent used has a marked influence as some solvents cause the solid support to modify the stereochemical course of the reaction. Finally, the nature of the chiral ligand is also very important, in particular the strength of its copper complex, in order to avoid the formation of non-chiral catalytic copper centres. This particular effect has been highlighted by using a ligand that is able to give strong complexation, namely an iminobis(oxazoline) ligand, and this leads to the best enantioselectivities being obtained for reactions using electrostatically immobilised catalysts.
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