Bisoxazoline (BOX) Ligand-Metal Complexes:An Emerging Chiral Catalyst

Abstract

C2-symmetric chiral bisoxazoline (BOX) ligand-metal complexes have emerged as an effective catalyst for carrying out a wide range of enantioselective reactions. [1] The presence of a C2-symmetric axis in the bisoxazoline ligands minimizes the number of possible transition states in a particular reaction. [2] Of the various chiral bisoxazoline ligands used, ligands (1-7) with a one carbon bridging between the oxazoline rings are most frequently utilized. Several research groups have utilized these bisoxazoline ligands in combination with a wide range of mild Lewis acids as catalysts for carrying out different enantioselective reactions. [4-13] Recently, chiral BOX-metal(II) complexes covalently anchored to silica and mesoporous MCM-41 have been used as a new heterogenous catalyst for enantioselective Friedel-Crafts hydroxylation. [14] An intriguing feature of the BOX-metal(II) complexes is that the metal(II)-derived chiral Lewis acids shows pronounced counter ion [15a-c] as well as solvent dependence in context of yield, enantioselectivity, rate, and success of reaction. The selectivity observed is due to the fact that the bisoxazoline ligands form six membered metal chelates which are conformationally constrained and the chiral centers in these ligands are located in close proximity to donor nitrogen, thereby imposing a strong directing effect on the catalytic sites. [16]