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.

Automated summary

We synthesized a palladium complex (1a) with two different P-donors and proved its effectiveness as a Pd(ii) precatalyst for carbon-phosphorus bond formation. The complex exhibited efficient catalytic activity for the Hirao coupling reaction under environmentally benign conditions. Mechanistic studies were conducted to investigate the generation of plausible Pd(0) active species. The use of bulky di(1-adamantyl)phosphine oxide as a preligand and less bulky di-p-tolylphosphine oxide as the substrate in the coupling reaction was also demonstrated.