Phosphonium ion based salen derivatives for use in catalysis

Abstract

Schiff base ligands and their respective complexes are well established for use in asymmetric catalysis. With difficulty of separation of the catalyst from reaction mixtures being one of the more salient drawbacks to these systems, much recent work has been directed towards developing methods that allow easy recovery and reuse of the catalyst. The incorporation of phosphonium ionic cores into asymmetric salen ligands to generate a Task Specific Ionic Liquid (TSIL) could promote entrainment of the catalyst in an ionic liquid (IL). With the catalyst firmly immobilized in the ionic liquid layer, this forms a recyclable system whereby the desired product can be extracted, leaving the catalyst behind in the IL for reuse. The ligand, 5,5'-(1E,1'E)-(1R,2R)-(cyclohexane-1,2-diylbis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)bis(3-tert-butyl-4-hydroxy-5,1-phenylene)bis(methylene)bis(tributylphosphonium)hexafluorophosphate, as well its respective copper (II) and manganese (III) complexes were successfully synthesized and characterized. Aziridination reactions were catalysed by the copper complex with a phosphonium ion tagged ligand, in both 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF[subscript 6]], and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim][N(Tf)[subscript 2]] ionic liquids. The catalyst was recycled several times in both ILs without appreciable loss in product yield over successive runs. Recycling of the manganese complex in the epoxidation of styrene was performed in both [bmim][PF[subscript 6]] and [bmim][N(Tf)[subscrip t2]], with moderate to full conversion of substrate into product being observed for successive runs in each ionic liquid.