Highly efficient and well-defined phosphinous acid-ligated Pd(II) precatalysts for Hirao cross-coupling reaction

We report the synthesis of a new palladium complex (1a) bearing two different P-donors, di(1-adamantyl)phosphinous acid and triphenylphosphine. A heteroleptic complex with a phosphinous acid ligand has rarely been reported. With phenyl bromide and di-p-tolylphosphine oxide as reagents, PPh₃-stabilised 1a was proved to be a noteworthy Pd(II) precatalyst for carbon-phosphorus bond formation. 1a-catalysed Hirao coupling could be efficiently made in environmentally benign ethanol. Reacting for 10 to 120 minutes, aryl bromides equipped with electron-donating or electron-withdrawing groups were successfully catalysed. Nucleophile-sensitive 2-bromopyridine, 2-bromothiophene, and 4-bromobenzonitrile were applicable in toluene/ethylene glycol (EG) (9/1). 1a-catalyzed Hirao coupling was successfully applied to the synthesis of a host material in an organic light-emitting diode (OLED) and precursor of biarylphosphines. A mechanistic study regarding how plausible Pd(0) active species are generated was jointly investigated by means of DFT calculation, ESI mass spectroscopy, and experiment. Interestingly, we demonstrated a proof of concept that bulky di(1-adamantyl)phosphine oxide is a useful preligand and less bulky di-p-tolylphosphine oxide is the substrate in the Hirao coupling.