Phosphonium ionic liquid based salen ligands for use in catalysis

Abstract

Salen ligands and their corresponding complexes are well known in the field of asymmetric catalysis due to their success in stereoselective reactions. A method that easily allows for recycling of the catalyst without degradation of catalytic activity would make them more industrially viable. The generation of a Task Specific Ionic Liquid (TSIL) by attaching tributylphosphonium moieties to Jacobsen’s ligand could promote the entrainment of the catalyst in an ionic liquid (IL). This would potentially create a recyclable system where the desired product can be extracted, leaving the IL/catalyst layer behind ready for reuse. The racemic and asymmetric versions of the ligand, 5,5'-(1E,1'E)-(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) hexafluoridophosphate, as well as their respective copper complexes were successfully synthesized and characterized. These were applied to aziridinations of styrene, using tosyliminophenyliodinane (PhINTs) as a nitrene source, in both acetonitrile and three different ILs. Acetonitrile generated yields of up to 88%, however no product was extracted from 1-butyl-3-methylimidazolium hexafluoridophosphate, tetradecyl(trihexyl) phosphonium hexafluoridophosphate and chloride.