The synthesis of aryl phosphonates as important pharmaceutical, and pesticide intermediates, catalyst ligands precursors and that of tertiary phosphine oxides is a widely studied area in organophosphorus chemistry. One way of forming P–C bond uses transition metal catalyzed cross coupling reactions. The first examples of the P–C coupling reaction between vinyl- and aryl halides and dialkyl phosphites were carried out in the presence of Pd(PPh3)4 catalyst by Hirao et al. To replace the air- and moisture sensitive and rather expensive Pd(PPh3)4, different types of Pd-precursors (e.g. Pd(OAc)2 or PdCl2) were used together with mono- or bidentate P-ligands. This method was more user friendly, due to the active catalyst was formed in situ. Later on, several cheaper Ni- and Cu-complexes were also applied in the coupling reactions. Eventually, the microwave technology became an attractive method to realize the Hirao reaction. Keglevich and co-workers developed a MW-assisted procedure in the presence of Pd(OAc)2 or NiCl2 precursors. These methods were called a “P-ligand free” P–C coupling reactions, as no usual, expensive P-ligands were added, instead, the >P(O)H-reagents were used in an excess. They explored the complete mechanism of the coupling reactions by quantum chemical calculations.
In our latest work, we studied the reactivity of various aryl derivatives (e.g. PhI, PhBr) towards diphenylphosphine oxide and diethyl phosphite in the Pd(OAc)2-catalyzed, “P-ligand free” P–C coupling reactions under MW condition. Furthermore, we found that there is an inductive period, during which the active catalyst may be formed. After palladium and nickel, the less known copper(I)-promoted reaction was investigated experimentally using iodobenzene and a few secondary phosphine oxides. The mechanism was investigated by quantum chemical calculations.
