Synthesis of chiral salen-MnIII complexes tagged with ionic liquid as recoverable homogeneous catalysts for the oxidative kinetic resolution of (?)-secondary alcohols

New chiral salen ligands tagged with imidazolium based ionic liquids (ILs) were synthesized from the 5-bromopentyl benzoate functionality at C-5 position of salicylaldehyde moiety, N-methylimidazole, and (1R,2R)-1,2-diphenylethane-1,2-diamine/(1R,2R)-cyclohexane-1,2-diamine in 81-92% yields. The chiral salen li-gands were further reacted with Mn(OAc)2.4H2O in dry methanol under N2 atmosphere, followed by air oxidation with LiCl to produce the IL tagged chiral salen-MnIII complexes 1(a-c) and 2(a-c) in 79-86% yields. The catalytic activities of these complexes were evaluated in the oxidative kinetic resolution of (+/-)-1-phe-nylethanol (8a) with N-bromosuccinimide (NBS) as an oxidizer in biphasic solvent system [H2O -organic solvent (2: 1, v/v)] at 0 degrees C. The enantioselective oxidation of various (+/-)-secondary alcohols catalyzed by complex 1c (1 mol%) afforded the chiral secondary alcohols with up to 99% enantiomeric excess and 29.6 krel (selectivity factor) value. The IL tagged chiral salen-MnIII complex 1c was easily recovered with hexane and subsequently reused for five recycles with no significant loss in conversion of (+/-)-1-phenylethanol (8a) and ee of the resolved product.

Automated summary

New chiral salen ligands with imidazolium based ionic liquids were synthesized and reacted with Mn(OAc)2.4H2O to form IL tagged chiral salen-MnIII complexes. These complexes exhibited excellent catalytic activity in the oxidative kinetic resolution.