Microwave-Assisted Palladium Acetate-Catalyzed C-P Cross-Coupling of Arylboronic Acids and >P(O)H Reagents in the Absence of the Usual Mono- and Bidentate P-Ligands: Mechanistic Insights.

A less-studied halogen-free variation of the Hirao reaction involving the coupling of arylboronic acids with >P(O)H reagents, such as diarylphosphine oxides, diethyl phosphite, and ethyl phenyl--phosphinate, was investigated in detail using Pd(OAc) as the catalyst precursor and applying some excess of the P-reagent to supply the ligand via its trivalent tautomeric (>P-OH) form. The optimum conditions (1.2 equiv of the P-reagent, 135-150 °C, and air) were explored for the synthesis of diaryl-phenylphosphine oxides, aryl-diphenylphosphine oxides, diethyl arylphosphonates, ethyl diphenylphosphinate, and two bisphosphinoyl derivatives. In the reaction of 4-chlorophenyl- or 3-chlorophenylboronic acid with PhP(O)H, triphenylphosphine oxide was also formed as a byproduct. Theoretical calculations suggested that the catalytic cycle of the P-C coupling of PhB(OH) with PhP(O)H is different from that of the usual cross-coupling reactions. It comprises the addition of a phenyl anion and then the tautomeric form >P-OH of the >P(O)H reagent to the Pd catalyst complex. This is then followed by reductive elimination affording PhPO that is accompanied with the conversion of Pd to Pd. There is a need for a subsequent stoichiometric oxidation of Pd(0) by molecular oxygen. The spontaneous formation of the self-assembling ligands around the Pd center from the >P(O)H reactant plays a crucial role in the mechanism and promotes the efficiency of the catalyst.