New phenantroline and bipyridine based macrocycles as ligands for asymmetric catalysis

In the last two decades, asymmetric catalysis has become the industrially most appealing method for the synthesis of enantiomerically enriched chiral molecules. For this reason, the interest of many research groups worldwide for asymmetric catalysis dramatically increased. Most of the asymmetric catalytic systems are metallic complexes of enan¬tiomerically pure chiral ligands. Among them, the nitrogen containing ligands play a major role. Recently, our group has synthesized some new chiral macrociclic ligands (see Figure 1 for three ex-amples) containing a phenantroline or a bipyridine moiety connected to a commercially or readily available chiral auxiliary by a spacer. The key-idea of this project is to modify the geometry of the macrocycles by changing the chiral auxiliary, the spacer unit or the linker bond, and thus modulate the stereoselectivity of a test-reaction catalysed by a ligand-metal complex. Figure 1 The new macrocycles have been tested as chiral ligands for the copper (I)-catalysed cyclopropana¬tion of styrene (figure 2) or of 1,1-diphenylethylene with an alkyl diazoacetate. Figure 2 Enantiomeric excess up to 67 % and yield up to 60 % were obtained. Interestingly, the ligand 2 showed the property of catalysing the cyclopropanation of styrene with diastereoselectivity in favour of the cis-isomer of the product. The procedure for macrocycles preparation and the results of their employment as chiral ligands for copper (I) catalysed cyclopropanation reactions will be presented in detail.