Transition metal-catalyzed, “ligand free” P–C coupling reactions under MW conditions
Bianka Huszár1, Renáta Szolga1, Nóra Á. Szűcs1, Zoltán Mucsi1, György Keglevich1
1Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, e-mail: huszar.bianka@vbk.bme.hu
Transition metal-catalyzed, cross coupling reaction is a good choice to form a P–C bond. The first P–C coupling reactions were carried out between vinyl- or aryl halides and dialkyl phosphites in the presence of Pd(PPh3)4 catalyst by Hirao et al.. To replace the rather expensive Pd(PPh3)4, different types of Pd-precursors (e.g. Pd(OAc)2, PdCl2) or Pd-complexes (e.g. Pd(dba)2) were used together with P-ligands (e.g. PPh3, dppp). In these cases, the active catalyst was formed in situ. Later on, microwave (MW)-assisted reactions and several Ni- and Cu-catalyzed coupling reactions were also described.
Keglevich and co-workers developed a “P-ligand free”, Pd(OAc)2 or NiCl2-catalyzed, MW-assisted procedure, where no usual P-ligands, meaning cost, were added, instead, the excess of the >P(O)H-reagents served as P-ligands via there trivalent tautomeric form. The mechanism of the coupling reactions was explored by quantum chemical calculations. Furthermore, it was found that there is an inductive period, during which the active catalyst is formed. In our latest work, the Pd(OAc)2-catalyzed, “ligand free” method was extended to dihalogenobenzenes. After the Pd- and Ni-catalyzed accomplishments, the Cu(I) and Cu(II)-catalyzed “ligand free” P–C coupling reactions were also studied under MW conditions. These mechanisms were also explored by quantum chemical calculations.
