No abstract

The mechanism of the copper(I)-catalyzed cyclopropanation reaction has been extensively investigated for a medium-size reaction model by means of B3LYP/6-31G(d) calculations. The starting ethylene complex of the N,N'-dimethylmalonaldiimine--copper (I) catalyst undergoes a ligand exchange with methyl diazoacetate to yield a reaction intermediate, which subsequently undergoes nitrogen extrusion to generate a copper--carbene complex. The cyclopropanation step takes place through a direct carbene insertion of the metal--carbene species to yield a catalyst--product complex, which can finally regenerate the starting complex. The stereochemical predictions of a more realistic model (by considering a chiral bis(oxazoline)--copper (I) catalyst) have been rationalized in terms of steric repulsions, showing good agreement with experimental data.

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.