Organophosphorus Chemistry by György Keglevich (Ed.)

Author:György Keglevich (Ed.)
Language: eng
Format: epub
Publisher: Walter de Gruyter
Published: 2018-05-27T04:00:00+00:00

Figure 8.31: Another Pd(OAc)2-catalyzed “P-ligand-free” Hirao reaction. Figure 8.32: An MW-assisted P-ligand-free desulfitative phosphonation. Figure 8.33: P–C couplings using NiCl2 as the catalyst without added P-ligands under MW conditions. Figure 8.34: NiCl2-catalyzed Hirao reactions of bromoarenes with diethyl phosphite. Both Pd- and Ni-catalyzed “P-ligand-free” reactions are suitable for the coupling of bromoarenes and various >P(O)H species. Considering the reaction conditions, costs and safety issues, one can conclude that the use of Pd(OAc)2 is favorable, but the application of NiCl2 may be a good alternative too. These recent developments and extensions of the Hirao reaction mean a big step further, since there is no need for expensive and sensitive P-ligands. The authors of this chapter proved that in the “P-ligand-free” accomplishment, the excess of the >P(O)H reagent existing under a tautomeric equilibrium may serve as the P-ligand. The excess of the >P(O)H species may also promote the reduction of Pd(II) before entering the catalytic cycles.

Finally, a new catalyst-free method was developed for the reaction of halobenzoic acids and secondary phosphine oxides in aqueous medium (Figure 8.35) [103]. 4-Iodo-, 3-bromo- and 4-bromobenzoic acids were reacted with diaryl phosphine oxides in the presence of K2CO3 at 180°C under MW conditions. The only limitation of this green P–C coupling reaction is the low water solubility of the secondary phosphine oxides.

Figure 8.35: Catalyst-free P–C coupling reaction in water.

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