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

Abstract

The Hirao reaction involving the phosphinoylation or phosphonation of aryl halides by >P(O)H reagents is a P–C bond forming transformation belonging to the recently very hot topic of cross-couplings. The Pd- or Ni-catalyzed variations take place via the usual cycle including oxidative addition, ligand exchange, and reductive elimination. However, according to the literature, the nature of the transition metal catalysts is not unambiguous. In this feature article, the catalysts described for the Pd(OAc)2-promoted cases are summarized, and it is concluded that the “(HOY2P)2Pd(0)” species (Y = aryl, alkoxy) is the real catalyst. In our model, the excess of the >P(O)H reagent served as the P-ligand. During the less studied Ni(II)-catalyzed instances the “(HOY2P)(−OY2P)Ni(II)Cl−” form was found to enter the catalytic cycle. The newest conclusions involving the exact structure of the catalysts, and the mechanism for their formation explored by us were supported by our earlier experimental data and theoretical calculations.

Highly efficient and well-defined phosphinous acid-ligated Pd(ii) precatalysts for Hirao cross-coupling reaction

“…The major disadvantage of TM-catalysed Hirao coupling is the high-catalyst loading (often more than 5 mol%). 6,9 Although it seems that the low catalyst loading is detrimental to conversion to the desired product 3a (entries 7 and 8 in Table 1), to our delight, both high conversion and isolated yield were realized in Hirao coupling catalysed by precatalyst 1a (1 mol%) (Scheme 8). Moreover, the amount of di-p-tolylphosphine oxide employed in the reaction was raised to 1 mmol.…”

“…8 Furthermore, it seems that the use of 5 mol% or even more transition-metal catalyst is commonly practised in such reactions at elevated temperature using an extended reaction time. 6,9 The requirement for a large amount of transition-metal ( pre)catalyst is a major drawback of Hirao coupling. Consequently, it is important to develop effective and efficient precatalysts in such applications.…”

Nickel Complexes in C‒P Bond Formation

“…Taking into account the reaction conditions, costs and safety concerns, it can be concluded that the application of Pd(OAc) 2 is favorable, but the use of NiCl 2 can also be a good alternative. Moreover, the C-P coupling reactions which apply Ni(II) salts in the absence of reductants have been investigated earlier, including theoretical calculations [109,110]. These latest developments of Hirao coupling mean a big step forward to "P-ligand-free" catalytic reactions, since there is no need for sensitive and expensive P-ligands.…”

Microwave-Assisted Synthesis of Aryl Phosphonates and Tertiary Phosphine Oxides by the Hirao Reaction

“…It was found that 20% of CuBr was the most efficient catalyst precursor using with 2 equiv. of NEt 3 at 165 • C. In the next stage, efficient "ligand-free" nickel-catalyzed (e.g., NiCl2) syntheses were also elaborated by our group [14][15][16][17]. In these cases, surprisingly, Ni(II)→Ni(IV) oxidation happened instead of the Ni(0)→Ni(II) formation.…”