It was proved by our experiments that on microwave irradiation, the mono- or bidentate phosphorus ligands generally applied in the palladium(II)-catalyzed P-C coupling reaction of aryl bromides and dialkyl phosphites or secondary phosphine oxides may be substituted by the excess of the >P(O)H reagent that exists under a tautomeric equilibrium. Taking into account that the reduction of the palladium(II) salt and the ligation of the palladium(0) so formed requires 3 equivalents of the P-species for the catalyst applied in a quantity of 5-10%, all together, 15-30% of the P-reagent is necessary beyond its stoichiometric quantity. In the coupling reaction of diphenylphosphine oxide, it was possible to apply diethyl phosphite as the reducing agent and as the P-ligand. The reactivities of the diethyl phosphite and diphenylphosphine oxide reagents were compared in a competitive reaction. The mechanism and the energetics of this new variation of the Hirao reaction of bromobenzene with Y2P(O)H reagents (Y=EtO and Ph) was explored by quantum chemical calculations. The first detailed study on simple reaction models justified our assumption that, under the conditions of the reaction, the trivalent form of the >P(O)H reagent may serve as the P-ligand in the palladium(0) catalyst, and shed light on the fine mechanism of the reaction sequence. The existence of the earlier described bis(palladium complex) {[H(OPh2P)2PdOAc]2} was refuted by high level theoretical calculations. This kind of complex may be formed only with chloride anions instead of the acetate anion. The interaction of palladium acetate and Y2P(O)H may result in only the formation of the [(HO)Y2P]2Pd complex that is the active catalyst in the Hirao reaction. The new variation of the Hirao reaction is of a more general value, and represents the greenest protocol, as there is no need for the usual P-ligands. Instead, the >P(O)H reagent should be used in an excess of up to 30%. Hence, the costs and environmental burdens may be decreased.
Keywords: cross-coupling; energetics; microwave heating; palladium catalyst; phosphorus ligands; reaction mechanisms.