Study of the recycling possibilities for azabis(oxazoline)–cobalt complexes as catalysts for enantioselective conjugate reduction

Abstract

Azabis(oxazoline)–cobalt(II) complexes have been tested in multiphasic catalytic systems for enantioselective conjugate reduction of (E)-3-phenylbut-2-enoate with NaBH₄. Immobilization by electrostatic interactions with laponite clay leads to excellent results in the first run, as a consequence of the role of the clay as an anion. However most of the reaction takes place in solution due to leaching of active complex, and the solids are not reusable. Covalent immobilization to Merrifield's resin does not allow the high enantioselectivity obtained in solution to be reached, and heterogeneous catalysts are again not reusable. Only the use of biphasic liquid systems allows an efficient recovery of the catalyst. The combination of a new ditopic ligand and 1,3-bis(2,2,2-trifluoroethoxy)propan-2-ol as a solvent for the catalyst phase is optimal to recycle the catalytic system with 90–96% ee for 5 runs.