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

Abstract

Two air- and moisture-stable secondary phosphine oxides (SPOs), namely, 2-( tert -butylhydrophosphoryl)-1-(2-R-phenyl)-1 H -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 PdBr 2 ) yielded the corresponding monopalladium complex bis(2-( tert -butylhydrophosphoryl)-1-(2-R-phenyl)-1 H -imidazole)PdX 2 ( 5a, R = H, X = Cl; 5b, R = OMe, X = Br) in cis -form as well as dipalladium complex [(μ 2 -2-( tert -butylhydrophosphoryl)-1-(2-R-phenyl)-1 H -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.