Synthesis of New Phosphino Amino Alcohol Ligands via Ortho-Alkyllithiation Reactions. Versatile Coordination Behavior toward Copper(I) and Palladium(II)

Abstract

2-[Methyl(2-methylphenyl)amino]ethanol undergoes an ortho-alkyllithiation reaction with n -butyllithium to lead to a new mixed benzyllithium−lithium alkoxide. This organolithium species reacts with PPh 2 Cl, with selective P−C bond formation, to afford the ligand 2-[methyl(2-((diphenylphosphino)methyl)phenyl)amino]ethanol L 1 . The coordination of the ligand L 1 to copper(I) leads to the complex [Cu(L 1 ) 2 ](BF 4 ), whose structure has been determined by an X-ray diffraction study. In the solid state, one of the ligands acts as a monodentate phosphine while the other adopts a tridentate P,N,O coordination mode. A variable-temperature 31 P NMR study demonstrated the existence of an equilibrium between the two modes in solution, with a coalescence temperature of ca . 0 °C, indicating a double-hemilabile behavior for the nitrogen and the oxygen functions. L 1 reacts with [Pd(Me)(Cl)(COD)] to give a dinuclear complex in which the ligand appears to behave as a bridging anionic P,O ligand. Such a complex could serve as a model for a key intermediate in the proposed mechanism for the homogeneous catalysis of the methoxycarbonylation of propyne by certain palladium(II) complexes containing P,N ligands. L 1 can undergo a second ortho-alkylmetalation reaction with n -butyllithium which, after addition of PPh 2 Cl, provides the new ligand 2-{methyl[2-(bis(diphenylphosphino)methyl)phenyl]amino}ethanol (L 2 ) in high yield.