Abstract
Five monodentate aminophosphines were synthesised via a one-step condensation reaction between chlorodiphenylphosphine with a suitable secondary amine for use in Pd-mediated Suzuki-Miyaura cross-coupling reactions. The ligand design was based on systematically increasing the electron density and steric congestion around the phosphorus donor atom. The first class of the synthesised ligands were the butyl-based ligands which includes N,N-dibutyl-1,1-diphenylphosphinamine and N,N-di-iso-butyl-1,1-diphenylphosphinamine. The second class were the piperidine-based ligands which includes, 1-(diphenylphosphino)piperidine, 1-(diphenylphosphino)-2-methylpiperidine and 1-(diphenylphosphino)-2,6-dimethylpiperidine. Simple stability test showed the ligands to be air- and moisture-stable, in their isolated forms. Screening of class one ligands (N,N-dibutyl-1,1-diphenylphosphinamine and N,N-di-iso-butyl-1,1-diphenylphosphinamine) in the Pd-catalyzed Suzuki-Miyaura reaction of aryl bromides and chlorides produced slightly higher yields for N,N-di-iso-butyl-1,1-diphenylphosphinamine compared to N,N-dibutyl-1,1-diphenylphosphinamine. The catalytic activity of the superior ligand (N,N-di-iso-butyl-1,1-diphenylphosphinamine) was compared to that of the commercially available SPhos and PPh3 in coupling of aryl chlorides with boronic acids. N,N-di-iso-butyl-1,1-diphenylphosphinamine was further employed in coupling of sterically hindered and functionalized aryl chlorides as well as deactivated boronic acids. Yields ranging from 22 – 91% were obtained when the above-mentioned ligands were used in Pd-mediated Suzuki-Miyaura reactions of aryl chlorides and boronic acids. Attempts to use the cheaper nickel in combination with the ligands proved to be unsuccessful. An attempt to design and develop immobilized palladium catalyst systems, with the goal of recovering the costly palladium metal was made. Four ligands to be supported on the Merrifield resin were successfully synthesized. Catalytic evaluation and immobilization of these ligands forms part of the future work.
M.Sc. (Chemistry)