Manganese(III) complexes of novel chiral unsymmetrical BINOL-Salen ligands: Synthesis, characterization, and application in asymmetric epoxidation of olefins

Abstract

Several chiral unsymmetrical and C 2-symmetric Mn(III)-BINOL-Salen complexes have been designed, synthesized and applied to the asymmetric epoxidation of non-functionalized alkenes. Experimental results show these complexes are effective in the catalytic asymmetric epoxidation of alkenes. The catalyst 4c exhibited better enantioselectivity and reactivity than the catalysts 4b and 4a due to the steric effect of the ligands. To understand the synergistic effect of the two different chiral centers in the catalyst, the catalyst 6a has been investigated. By comparison of the enantioselectivity obtained by using 4c and 6a, respectively, the positive experimental results have proved that the chiral stereogenic centers in the diaminocyclohexane-derived catalysts played an important role in the current enantioselective epoxidation. Besides, the comparison of enantioselectivity displayed by 4c and 7a further demonstrates the significant influence through the cooperation of steric factors and chiral centers in catalyst.