Cation-triggered switchable asymmetric catalysis with chiral Aza-CrownPhos.

An aza-crown ether, modified phosphoramidite ligand, has been designed and synthesized. The ON/OFF reversible switch of catalytic activity for its rhodium catalyst was thoroughly investigated in the asymmetric hydrogenation of dehydroamino acid esters modulated by host-guest interactions. In the OFF state, the catalyst is almost inactive (less than 1% conversion) because of the formation of an intermolecular sandwich complex by two aza-crown ether moities and the cationic rhodium metal center. In using alkali-metal-cations as the trigger, the catalytic activity was turned ON and consequently resulted in full conversions and excellent enantioselectivities (up to 98% ee).

Podand-based dimeric chromium(III)-salen complex for asymmetric Henry reaction: cooperative catalysis promoted by complexation of alkali metal ions.

A new kind of podand-based dimeric salen ligand was synthesized, and its association with potassium cations was investigated by (1) H NMR spectroscopy. The corresponding Cr(III) -salen dimer was assembled by a supramolecular host-guest self-assembly process and was then used as a catalyst in highly efficient and enantioselective asymmetric Henry reactions. Regulation by KBArF (BArF =[3,5-(CF3 )2 C6 H3 ]4 B) led to remarkable improvements in yield (by up to 58 %) and enantioselectivity (for example, from 80 % ee to 96 % ee).