Cyclodextrin-[RuCl2(Arene)]2 conjugates: another way to enhance the enantioselectivity of aromatic ketones reduction by aromatic ligands' volume

Eight amino alcohol-modified β-CDs CD-1-CD-8 have been synthesized in acceptable yields and were employed to form artificial metalloenzymes with [RuCl2(Benzene)]2 and [RuCl2(Mesitylene)]2, respectively. All the conformations of CD-1-CD-8, the complexes between CD-1-CD-8 and [RuCl2(Arene)]2, and the inclusion complexes between CD-1-CD-8 and acetophenone were characterized by UV, 1H NMR, 1H ROESY NMR, and quantum calculation. The catalytic activity of the formed artificial metalloenzymes in the asymmetric hydrogenation of aromatic ketones, especially the effect of the aromatic ligands' volume on the enantioselectivity were investigated in detail, in which it was obvious that the enantioselectivity increased as the increase in the aromatic ligands' volume. For the best artificial metalloenzyme constructed from the complex between CD-8 and [RuCl2(Mesitylene)]2, which not only exhibits a good tolerance to a wide range of substrates but also demonstrates some substrate selectivity, 76.39% ee was obtained for acetophenone and 79.67% ee for 2-acetylnaphthalene. A strategy to improve the enantioselectivity in the asymmetric reactions catalyzed by the artificial metalloenzymes based on CDs has been provided.