A surprising mechanism lacking the Ni(0) state during the Ni(II)-catalyzed P-C cross-coupling reaction performed in the absence of a reducing agent - An experimental and a theoretical study

The Hirao reaction, i.e. the P-C coupling between a bromoarene and a >P(O)II reagent performed in most cases in the presence of a Pd(0) complex incorporating a P-ligand may also be carried out applying a Ni(II) catalyst precursor with or without Zn or Mg as the reducing agent. The Ni catalysts may include P- or N-ligands. B3LYP/6-31G(d,p)//PCM(MeCN) quantum chemical calculations suggested that the mechanism of the NiX2 catalyzed (X=Cl or Br) P-C couplings performed in the absence of a reducing agent, and in the excess of the >P(O)H reagent serving as the P-ligand (via its tautomeric >POH form) is completely different from that of the Pd(OAc)(2) promoted version, as no reduction of the Ni(II) occurs. In the two variations mentioned, the active catalyst is the dehydrobrominated species derived from primary complex [(HO)Y2P](2)Ni(II)Br-2, and the [(HO)Y2P](2)Pd(0) complex itself, respectively. Both species undergo temporary oxidation (to Ni(IV) and Pd(II), respectively) in the catalytic cycle. During the catalysis with P2Ni(II)X-2, one of the P-ligands serves the >P(O)H function of the ArP(O)H < product. The consequence of this difference is that in the Ni(II)-catalyzed case, somewhat less >P(O)H-species is needed than in the Pd(O)-promoted instance. Applying 10% of the Pd(OAc)(2) or NiX2 precursor, the optimum quantity of the P-reagent is 1.3 equivalent and, in the first approach, 1.1 equivalent, respectively. Preparative experiments justified the new mechanism explored. The ligation of Ni(II) was also investigated by theoretical calculations. It was proved that the bis-complexation is the most favorable energetically as compared to the mono-, tri- and tetra-ligation.