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)Cl-2 (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 [(mu(2)-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.