Air-stable palladium(II) precatalysts: synthesis, properties, and applications in cross-coupling reactions

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Palladium catalyzed cross-coupling reactions are a versatile tool in organic chemistry to produce small molecules. Current research interests involve the development of new catalyst systems for various palladium catalyzed reactions. Previous work has suggested the active species for palladium-catalyzed cross-coupling reactions to be a mono-ligated palladium(0) species when sterically demanding ligands are used. Therefore, the my work focused on the synthesis of precatalysts with an established 1:1 L:Pd. Specifically, we were focused on the synthesis of mono-ligated palladium(II) precatalysts of general formula [(R_3P)PdCl_2]_2, using di-tert-butylneopentylphosphine (DTBNpP), tert-butyldineopentylphosphine (TBDNpP), and trineopentylphosphine (TNpP) as ligands. Under optimized conditions, both [(DTBNpP)PdCl_2]_2 and [(TNpP)PdCl_2]_2 were effective precatalysts for the Suzuki cross-coupling of a wide range of aryl bromides. Comparison studies with the air-stable precatalysts versus the in-situ generated catalyst showed the precatalysts to have improved conversions and higher rates of reaction under both inert and ambient atmospheres. The precatalyst activation pathways were investigated by 31P NMR spectroscopy. The spectra obtained from 31P NMR experiments revealed a side reaction resulting in a catalytically inactive palladacycle species. The results obtained from the mechanistic investigations led to further optimization of reaction conditions to decrease the amounts of catalytically inactive side products formed.

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