Synthesis and Application of Air-Stable Secondary Phosphine Oxide, Oxime, and Amine-Phosphine Palladium(II) Complexes in Transfer Hydrogenation and Cross-Coupling Reactions
Catalysis plays an important role in industry as over 90% of chemical processes involve catalysts in at least one step. Historically, heterogeneous catalysis has been more commonly employed in industry as these systems tend to be more economically friendly and environmentally benign. However, improved procedures for homogeneous catalysis using organocatalysts and organometallic compounds has generated increased interest in homogeneous systems. Organometallic compounds offer higher specific activities and selectivity in comparison to traditional heterogeneous systems. These qualities make homogeneous catalysis particularly attractive for the production of fine chemicals, pharmaceuticals, and natural products. Ligand design is critical for the development of homogeneous catalysts. For late-transition-metal-catalyzed cross-coupling reactions that rely on C-X or C-H oxidative addition, strongly electron-donating and sterically demanding ligands afford the most efficient systems. Several privileged classes of ligands have been identified and employed in palladium-catalyzed reactions. These systems are believed to promote their respective reactions through a monoligated palladium(0) species. Our work focused on the development of precatalysts with an established 1:1 L:Pd ratio. Specifically, we were interested in the synthesis of palladium(II) precatalysts with the general formula (R3P)Pd(amine)Cl2, using di-tert-butylneopentylphosphine and trineopentylphosphine. Under optimized conditions, the precatalysts were effective for the Buchwald-Hartwig amination of aryl bromides and aryl chlorides. We discovered that the identity of the amine ligand significantly impacts catalyst efficiency with linear primary alkyl amines providing the most active catalysts. A comparison study between the air-stable precatalysts and in situ generated catalysts showed improved activity for the precatalysts. Typically, the aforementioned privileged classes of ligands behave as spectators during the bond activation process. However, ligands containing functional groups that introduce acidic/basic sites proximal to the metal center can participate through acid-base interactions with the substrates. Multifunctional ligands have proven to be efficient in numerous transition-metal-catalyzed processes. We focused on synthesizing secondary phosphine oxide and oxime ligands that contain a basic hydroxyl group(s) upon complexation. The synthesis of oxime-derived palladacycles was facile and efficient. These catalysts efficiently facilitated the transfer hydrogenation of benzophenone. The active species is believed to be a three- or four-atom palladium cluster resulting from decomposition of the palladium-oxime complex.
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