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 NiCl2/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. M06-2X/6-31G(d,p)//PCM(MeCN) calculations for the reaction of Ph2P(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 Ni2+ 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 NiCl2/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.
Highlights
The Hirao reaction that is a P−C coupling between vinyl and aryl halides and >P(O)H reagents, such as dialkyl phosphites, alkyl phenyl-H-phosphinates, and secondary phosphine oxides, is in the focus these days
The coupling of bromobenzene or iodobenzene with diphenylphosphine oxide served as the model reaction
By applying the conditions (a 1 day heating at 70 °C) described by Zhao et al.,[23] and following the workup suggested, triphenylphosphine oxide (TPPO) was obtained in a low yield of 5% that could be confirmed after a few repetitions (Table 2, entry 1)
Summary
The Hirao reaction that is a P−C coupling between vinyl and aryl halides (mostly bromides) and >P(O)H reagents, such as dialkyl phosphites, alkyl phenyl-H-phosphinates, and secondary phosphine oxides, is in the focus these days. Hirao described the application of tetrakis(triphenylphosphine)palladium as the catalyst,[3−5] and different Pd(0) precursors, for example, Pd(II) salts, were used together with mono- or bidentate P-ligands.[1,2,6] There is an agreement that the originally present or the in situ formed Pd(0) undergoes oxidative addition with the vinyl- or aryl bromide. This step is followed by a ligand exchange connecting the >P(O)H reagent to the central Pd(II) ion.
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