Concepts for Stereoselective Acrylate Insertion

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.