Concepts for stereoselective acrylate insertion

Abstract

Various phosphinesulfonato ligands and the corresponding palladium complexes [{((P^O)PdMeCl)-μ-M}_n] ([{(^X1-Cl)-μ-M}_n], (P^O) = κ²-P,O-Ar₂PC₆H₄SO₂O) with symmetric (Ar = 2-MeOC₆H₄, 2-CF₃C₆H₄, 2,6-(MeO)₂C₆H₃, 2,6-(iPrO)₂C₆H₃, 2-(2',6'-(MeO)₂C₆H₃)C₆H₄) and asymmetric substituted phosphorus atoms (Ar¹ = 2,6-(MeO)₂C₆H₃, Ar² = 2'-(2,6-(MeO)₂C₆H₃)C₆H₄; Ar¹ = 2,6-(MeO)₂C₆H₃, Ar² = 2-cHexOC₆H₄) were synthesized. Analyses of molecular motions and dynamics by variable temperature NMR studies and line shape analysis were performed for the free ligands and the complexes. The highest barriers of ΔG_‡ = 44–64 kJ/mol were assigned to an aryl rotation process, and the flexibility of the ligand framework was found to be a key obstacle to a more effective stereocontrol. An increase of steric bulk at the aryl substituents raises the motional barriers but diminishes insertion rates and regioselectivity. The stereoselectivity of the first and the second methyl acrylate (MA) insertion into the Pd–Me bond of in situ generated complexes ^X1 was investigated by NMR and DFT methods. The substitution pattern of the ligand clearly affects the first MA insertion, resulting in a stereoselectivity of up to 6:1 for complexes with an asymmetric substituted phosphorus. In the consecutive insertion, the stereoselectivity is diminished in all cases. DFT analysis of the corresponding insertion transition states revealed that a selectivity for the first insertion with asymmetric (P^O) complexes is diminished in the consecutive insertions due to uncooperatively working enantiomorphic and chain end stereocontrol. From these observations, further concepts are developed.