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

Abstract

The Hirao reaction, i.e. the P–C coupling between a bromoarene and a >P(O)H 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]2Ni(II)Br2, and the [(HO)Y2P]2Pd(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)X2”, 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(0)-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.

Experimental and Theoretical Study on the “2,2′-Bipiridyl-Ni-Catalyzed” Hirao Reaction of >P(O)H Reagents and Halobenzenes: A Ni(0) → Ni(II) or a Ni(II) → Ni(IV) Mechanism?

“…We decided to return to the Ni(II) → Ni(IV) → Ni(II) protocol, which was proved to be a realistic route applying P -ligands. 32 The catalytic process starting with Ni(II)-bipyridyl complex II was also evaluated by the M06-2 X /6-31G(d,p)//PCM(MeCN) method. The II → VII → VI → VIII → 11 → 12 → 13 → ( 14 + 3a ) route proposed is summarized in Figure 5 , while the energetics are listed in Table 7 .…”

“…The MW-assisted reactions were performed according to our earlier methods. 31 , 32 To a MW glass vessel was added 0.0065 g (0.050 mmol) of nickel chloride, 0.052 mL (0.50 mmol) of bromobenzene, 0.12 g (0.60 mmol) of diphenylphosphine oxide ( 2a ) or 0.090 mL (0.60 mmol) of ethyl phenyl- H -phosphinate ( 2b ), 0.16 g (0.50 mmol) of cesium carbonate, and 1 mL of acetonitrile. In one case ( Table 2 , entry 15), 0.039 g of (0.6 mmol) zinc powder was also added.…”

Focusing on the Catalysts of the Pd- and Ni-Catalyzed Hirao Reactions

“…It follows that, as a result, Ni(II) gets in the Ni(IV) state. This was indeed a fully surprising experience, but quantum chemical calculations confirmed this [ 71 ]. It was another surprise that in the “reductive” variation, again a Ni(II) → Ni(IV) oxidation was substantiated by the preparative experiments and calculations [ 72 ].…”

“…In our case, the excess of the >P(O)H reagent served as the P-ligand. Preparative experiments suggested that two equivalents of the P-reagent to the NiCl 2 was the optimum quantity [ 47 , 71 ]. Then, theoretical calculations suggested the following mechanism for the formation of the active catalyst entering in the Hirao reaction ( Scheme 11 ) [ 71 ].…”

“…Preparative experiments suggested that two equivalents of the P-reagent to the NiCl 2 was the optimum quantity [ 47 , 71 ]. Then, theoretical calculations suggested the following mechanism for the formation of the active catalyst entering in the Hirao reaction ( Scheme 11 ) [ 71 ]. Two units of the >P(O)H reagent are attached to the central Ni atom of NiCl 2 in the trivalent tautomeric form in a stepwise manner via intermediate 22 .…”