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?

Abstract

It was found by us that the P–C coupling reaction of >P(O)H reagents with PhX (X = I and Br) in the presence of NiCl 2 /Zn as the precursors for the assumed Ni(0) complexant together with 2,2′-bipyridine as the ligand took place only with PhI at 50/70 °C. M06-2 X /6-31G(d,p)//PCM(MeCN) calculations for the reaction of Ph 2 P(O)H and PhX revealed a favorable energetics only for the loss of iodide following the oxidative addition of PhI on the Ni(0) atom. However, the assumed transition states with Ni(II) formed after P-ligand uptake and deprotonation could not undergo reductive elimination meaning a “dead-end route”. Hence, it was assumed that the initial complexation of the remaining Ni 2+ ions with 2,2′-bipyridine may move the P–C coupling forward via a Ni(II) → Ni(IV) transition. This route was also confirmed by calculations, and this mechanism was justified by preparative experiments carried out using NiCl 2 /bipyridine in the absence of Zn. Hence, the generally accepted Ni(0) → Ni(II) route was refuted by us, confirming the generality of the Ni(II) → N(IV) protocol, either in the presence of bipyridine, or using the excess of the >P(O)H reagent as the P -ligand. The results of the calculations on the complex forming ability of Ni(0) and Ni(II) with 2,2′-bipyridine or the P -reagents were in accord with our mechanistic proposition.

Secondary Phosphine Oxide-Activated Nickel Catalysts for Site-Selective Alkene Isomerization and Remote Hydrophosphination

“…With reference to Fig. 1b, we speculated that a ligated Ni(0) species was generated in situ by reduction of a Ni(II) complex 59,60 prior to activation by L1. However, isomerization was inefficient with Ni(cod)2 as pre-catalyst, and adding catalytic (or stoichiometric) quantities of DME as a coordinating ligand did not improve results substantially (entry 5).…”

Microwaves as “Co-Catalysts” or as Substitute for Catalysts in Organophosphorus Chemistry

“…This surprising finding was also proved to be true for earlier Ni-catalyzed instances [ 164 ] carried out originally in the presence of Zn or Mg reductants [ 165 , 166 , 167 ]. Hence, the Ni(II) → Ni(IV) conversion may be of more general value instead of the earlier assumed Ni(0) → Ni(II) protocol [ 168 ].…”

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

“…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 ]. However, surveying the literature, the Ni(II) → Ni(IV) transition is well-known during the interconversion of different Ni complexes [ 73 , 74 , 75 ].…”