Synthesis and application of sterically flexible and water-soluble phosphine ligands in palladium catalysis

The need for more environmentally benign and sustainable chemical processes is currently a widely recognized motivation in chemical research. One area that has been of particular interest to our research group is the development of biphasic and aqueous phase systems for palladium catalysis. My research at The University of Alabama has focused on the design and synthesis of water-soluble phosphines for application in aqueous and biphasic palladium catalysis. Our group, through collaboration with the Dixon and Rogers groups, seeks also to better understand how phosphine ligand structure influences catalysis in order to design more effective systems. My work began with the study of the influence of flexible, electron-rich phosphine substituents such as the neopentyl and benzyl groups on catalyst activity, particularly for the Suzuki coupling of sterically hindered substrates. I have also synthesized two new water-soluble phosphine ligands that utilize two sterically hindered, electron-rich alkyl groups (di-tert-butyl or di-1-adamantyl) in conjunction with a flexible benzyl moiety, made water soluble through addition of an anionic sulfonate group. Both of the new ligands have shown moderate activity in Suzuki and Sonogashira coupling reactions. Through collaboration with the Hartman group, the ability to recycle a water-solubilized palladium-phosphine catalyst in biphasic microflow systems is being studied. Additionally, work has begun to extend the application of water-soluble phosphine ligands to other palladium catalyzed reactions, such as direct arylations.