Preparation of New Buchwald-Type Secondary Phosphine Oxide Ligands and Applications in Suzuki−Miyaura Reactions

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.