Abstract
The duel objective of the research described in this dissertation, was the optimisation of reaction conditions for various palladium(0)-catalysed Heck-type reactions, and the development and application of bidentate iminophosphine ligands for some of these reactions.
The efficiency of Heck and Heck-type reactions was evaluated by comparison of various parameters, e.g. solvent, palladium salt, reaction temperature and ligands for these palladium salts. In most cases, DMF was used as solvent, and palladium acetate served as a source of catalyst. Reactions were mostly carried out at 110 ºC, and triphenylposphine was used as the bench mark monodentate ligand, while 1,3-bis(diphenylphos-phino)propane (dppp) was applied as a bench mark for the performance of bidentate ligands. Apart from the Heck reaction between aryl halides and olefins, palladium-catalysed esterification reactions, transmetallation reactions and intramolecular reactions were investigated.
A series of iminophosphine ligands was synthesised via the condensation of 2-(diphenylphosphino)benzaldehyde with primary amines. Although these ligands are thermally stable, they are extremely susceptible towards oxidation. This problem of oxidation, together with their unstability on silica, led us to the development of specialised techniques to isolate and purify the ligands. Distillation of these ligands under vacuum, together with the simultaneous subjection of the distillation apparatus to an inert argon atmosphere, resulted in the isolation of pure, unoxidised iminophosphine ligands.
Application of the synthesised iminophosphine ligands to the optimised Heck reaction, indicated that these ligands were catalytically active. A series of Heck reactions employing different aryl halides, was chosen to test the efficiency of the newly synthesised ligands. In the cases of iodobenzene and o-nitroiodobenzene, similar results were obtained for triphenylphosphine, the iminophosphine ligands, and dppp. However, in the case of p-bromobenzonitrile, where the oxidative addition of palladium to the C-Br bond is slower than for corresponding C-I bonds, the efficiency of the iminophosphine ligands could clearly be seen in the higher yields that were obtained in comparison to the bench mark ligands. Further development of bidentate P-N ligands with different electronic characteristics, and application of these ligands with palladium and other transition metals for catalysis, is a field of research that should not be left unexpored.
Prof. D.B.G. Williams