Preparation of new buchwald-type secondary phosphine oxide ligands and applications in Suzuki-Miyaura reactions

Article abstract of DOI:10.1021/om101132t

Two air- and moisture-stable secondary phosphine oxides (SPOs), namely, 2-(tert-butylhydrophosphoryl)-1-(2-R-phenyl)-1H-imidazole (3a, R = H; 3b, R = OMe) were prepared and characterized. A tautomeric equilibrium exists between the secondary phosphine oxide 3a (or 3b) and its corresponding phosphinous acid 3a′ (or 3b′). The reaction of phosphinous acid 3a′ (or 3b′) with Pd(COD)Cl₂ (or PdBr₂) yielded the corresponding monopalladium complex bis(2-(tert-butylhydrophosphoryl)-1-(2-R- phenyl)-1H-imidazole)PdX₂ (5a, R = H, X = Cl; 5b, R = OMe, X = Br) in cis-form as well as dipalladium complex [(μ₂-2-(tert- butylhydrophosphoryl)-1-(2-R-phenyl)-1H-imidazole-N,P)Pd(I)X]₂ (6a, R = H, X = Cl; 6b, R = OMe, X = Br). The phosphinous acids 3a′ and 3b′ act as P,N-chelating ligands in the formation of 6a and 6b, which feature direct Pd(I)-Pd(I) bonds. Good to excellent reactivities were observed by applying these ligand-chelated palladium complexes in Suzuki-Miyaura coupling reactions.