Abstract
In this study polar ligands that would be suitable for use in homogenous catalysis in aqueous-organic and/or ionic liquid systems were designed and synthesised.
Initially an attempt was made to design a synthetic route that would incorporate an imidazolium into a phosphorus compound which would then allow the variation of the alkyl group on the imidazole or of the anion, thereby producing a series of ionic ligands with wide range of anticipated chemical and physical properties. This approach was partially successful: phosphines containing imidazolium moieties were synthesised by incorporating N-methyl imidazolium into bi- and tri-aryl phosphine compounds. As these ligands contain imidazolium residues, the retention of the catalyst in the ionic liquid to be used in the catalytic transformation would be enhanced.
Water-soluble ligands that are suitable to aqueous-organic systems were also synthesised. The ligands were synthesised by first performing a Michael type reaction with diphenyl phosphine and a Michael acceptor, in this case, malonate esters. These were reduced and subsequently reacted with 1,4-butanesultone. The addition of butane sultone to these phosphorus compounds made them sufficiently water-soluble and therefore ideal for catalysis in aqueous biphasic or ionic solutions.
Several water-soluble/ionic ligands were synthesised by incorporating dimethylphosphoryl chloride into di- and tri-aryl phosphine ligands in high yields followed by subsequent hydrolysis of the phosphonate groups.
All the ligands produce were tested in transition metal-catalysed reactions, namely the Heck reaction, the Suzuki cross coupling and the Hydroformylation reaction. Test reactions were carried out under conditions selected to show the effectiveness of the ligands in the reaction, and they were performed in organic and aqueous-organic systems, as well as in ionic liquids for the Heck reaction. The results of these reactions were compared to the benchmark triphenylphosphine and it was found that the new ligands produced results that were similar to the triphenylphosphine but there were numerous instances where the new ligands produced better results.
This study, therefore allowed for the synthesis for a range of polar phosphine ligands that would be suitable for homogenous catalysis using aqueous and/or ionic liquid systems, and these ligands were shown to influence transition metal-catalysed reactions.
Prof. D.B.G. Williams